Evolution and Prospects in Managing Sewage Sludge Resulting from Municipal Wastewater Purification

Abstract

Municipal sewage sludge is the residual material produced as a waste of municipal wastewater purification. It is a sophisticated multi-component material, hard to handle. For many years, it has been landfilled, incinerated, and widely used in agriculture practice. When unproperly discharged, it is very polluting and unhealthy. The rapidly increasing global amount of municipal sewage sludge produced annually depends on urbanization, degree of development, and lifestyle. Some diffused traditional practices were banned or became economically unfeasible or unacceptable by the communities. In contrast, it has been established that MSS contains valuable resources, which can be utilized as energy and fertilizer. The objective of the review was to prove that resource recovery is beneficially affordable using modern approaches and proper technologies and to estimate the required resources and time. The open sources of information were deeply mined, critically examined, and selected to derive the necessary information regarding each network segment, from the source to the final point, where the municipal sewage sludge is produced and disposed of. We found that developed and some developing countries are involved with ambitious and costly plans for remediation, the modernization of regulations, collecting and purification systems, and beneficial waste management using a modern approach. We also found that the activated sludge process is the leading technology for wastewater purification, and anaerobic digestion is the leading technology for downstream waste. However, biological technologies appear inadequate and hydrothermal carbonization, already applicable at full scale, is the best candidate for playing a significant role in managing municipal sewage sludge produced by big towns and small villages.

1. Introduction

Few numbers suffice to quantify the relevance of the topic of the review:

• Municipal wastewater (MWW) means the spent water of a community. From the standpoint of source, it may be a combination of the liquid- and water-carried waste from residences, commercial buildings, industrial plants, and institutions, together with any groundwater, surface water, and stormwater that may be present [1,2];
• Sewage sludge is the residue resulting from the treatment of wastewater released from various sources, including homes, industries, medical facilities, street runoff, and businesses [3,4]. Municipal sewage sludge (MSS) is the residual material produced as a by-product of MWW treatment plants (WWTPs);
• Municipal sewer systems refers to all sewer mains within the limits of the public right of way, or easements, including maintenance holes and appurtenances [5,6].

In 2020, the volume of MWW generated annually worldwide was estimated to be 360–380 cubic kilometers, predicting a 24% increase by 2030 and a 51% increase by 2050. A little more than half is treated in about sixty thousand WWTPs operating worldwide, while the remaining is released directly into the environment [7,8].

Five years ago, the annual global MSS production rate was estimated to be 45 million tons as dry solid (DS) [9]. The result was obtained considering that two billion people were connected to MWW sanitation systems having secondary treatment facilities and quantifying the amount of sludge produced per capita as 70 g of DS per day. In terms of sludge (i.e., the wetted organic and inorganic matter resulting from the treatment of MWW), the amount is from 5 to 10 times higher than in terms of DS [10].

Presently, the total length of the sewer network across the EU has been estimated at around 3.3 million kilometers [10]; thus, its total length worldwide is close to one order of magnitude higher (i.e., almost a thousand times the circumference of the Earth). It is also worth mentioning that the EU sewer network length has increased by about 10% during the last four years [11].

The main reason for such a challenging and relevant situation is urbanization, for which there is currently no universal definition. The UN reports figures based on nationally defined urban shares. Figure 1 shows how the urban and rural populations have changed globally during the last 30 years and will predictably change during the next 30 years [12].

As can be seen, the urban population surpassed the rural population in 2006, has continued to grow, and is expected to grow at least until 2050. In contrast, the rural population will tend to decline. Presently, the global urban population amounts to 4.54 billion people, to rise to 6.61 billion people in 2050. The present global rural population amounts to 3.42 billion people, to decline to 3.11 billion people in 2050. The already mentioned source of data [12] presents the share between urban and rural populations for several individual countries, including developing and fast-developing highly populated countries, as a function of time. Figure 2 shows the relationship between urbanization and gross domestic product per capita (GDPpc) for 10 key countries or geopolitical regions.

The strict (although not unique) relationship between urbanization and the degree of development enables estimation of how the global MWW production rate and associated MSS will evolve, along with the dimensions of the sewer network. Figure 3 shows the results obtained assuming that the evolution in developed countries will focus on modernizing existing infrastructure without significantly affecting the length of sewers and the global production rate of MWW and MSS.

The results reported in Figure 3 highlight the infrastructural effort that these countries will have to face in the next 30 years to prevent or reduce the environmental and health impacts deriving from the uncontrolled disposal of increasing quantities of MWW and or MSS.

To account for different legislation, culture, and natural vocations, it is necessary to separately analyze some of the world’s most significant geopolitical areas.

Presently, the EU geopolitical area is the world’s third largest economy in terms of GDP (being recently surpassed by China), but it remains clearly second in terms of GDPpc. However, it is worth considering that the degree of development of the European member states is significantly unequal. Consequently, it is predictable that urbanization will increase along with the number and the quality of the sewer systems and WWTPs, thus producing a higher amount of purified water and less MSS of better quality [13,14,15,16,17,18,19,20,21].

In the EU geopolitical area, including Switzerland, the primary reference standards for MWW, MSS, and related topics are shown in

Table 1. Overview of EU legislation relating to MWW and MSS.

	Name	Number	Ref.
Collection	Urban Waste Water Treatment Directive (UWWTD)	91/271/EEC	[22]
Processing	UWWTD	-	-
	Waste Framework Directive (Waste FD)	2008/98/EC	[23]
	Industrial Emissions Directive (IED)	2010/75/EU	[24]
Transporting	Waste FD	-	-
Outlets	Sewage Sludge Directive (SSD)	86/278/EEC	[25]
	Waste FD	-	-
	IED	-	-
Environmental protection	SSD	-	-
	Water Framework Directive and the daughter Directives	2000/60/EC	[26]
	Nitrate Directive	91/676/EEC	[27]

.

In 2009 the EU established the European Environmental Agency (EEA) which, among others, coordinates the European Environment Information and Observation Network (Eionet) [28]. One of the main tasks of the EEA is to provide the European Commission, the European Parliament, and the Council with the necessary technical and scientific support. Furthermore, the European Federation of National Associations of Water Service (EurEau), representing national drinking and wastewater service providers from 29 countries, was established on 21 March 1975 and extended its work to include wastewater in 1998. Annually, EEA, EurEau, and the Statistical Office of the European Union (Eurostat) make many detailed data available regarding the evolution of legislation, the amount of MWW and MSS production and composition, connected people, sewer systems, sanitation, the recovery of chemicals, the final destination of MSS, etc. A further source of information is the reports of the many R&D projects supported by the EU.

It is also worth mentioning that, according to the EU list of waste (LoW) [29], MSS is identified with the six-digit code 19.08.05 and classified as non-hazardous waste. The per capita annual amount of MSS as DS is about 15–25 kg [10], corresponding to an annual total of about 14 million tons, accounting for the whole served EU population equivalent, produced by 20–30 thousand WWTPs [7,30]. The recovery and recycling of nutrients such as phosphorous during the MWW treatment cycle is strongly encouraged, because phosphate rock has been entered on the list of critical raw materials [31]. Furthermore, a debate is ongoing in the EU regarding the ban on removing MSS from WWTPs or using them for agronomic and land application purposes without prior heat treatment to drastically reduce pathogenic microorganisms; some member states such as the Netherlands, Switzerland, and Germany, to some extent, have already legislated to that effect.

In the United States of America (USA), the federal government’s response to the developing urban wastewater management issue was to enact the Water Pollution Control Act (WPCA) of 1948. The legislation provided comprehensive planning, technical services, research, financial assistance, and enforcement. After several extensions and amendments to the original text, the WPCA became permanent legislation. In 1972, the unprecedented goal was to eliminate all water pollution by 1985 and authorize relevant expenditures in research and construction grants [32]. Since the end of 1970, the United States Environmental Protection Agency (EPA) has enforced federal clean water and safe drinking water laws, provided support for municipal wastewater treatment plants, and participated in pollution prevention efforts to protect watersheds and sources of drinking water. In particular, in the review competition, the EPA regulates the discharge and treatment of wastewater under the Clean Water Act (CWA). The National Pollutant Elimination System (NPDES) covers all wastewater traps and treatment plants. Such permits set specific discharge limits, monitoring and reporting requirements, and may also require such facilities to take extraordinary measures to protect the environment from harmful pollutants. The EPA monitors stormwater and sewer discharge through NPDES, guiding municipalities, states, and federal licensing authorities in achieving stormwater pollution control goals as flexibly and cost-effectively as possible. The EPA sets numerical limits and management practices that protect public health and the environment from the reasonably anticipated adverse effects of chemical and microbial pollutants during the use or disposal of MSS. Additionally, the agency reviews MSS (biosolids) regulations every two years to identify any additional pollutants that may be present in biosolids and then sets regulations for those pollutants if sufficient scientific evidence shows they can harm human health or the environment. A recent study published by the University of Michigan Center for Sustainable Systems [33] updated the state of MWW, MSS, and related topics in the USA. Currently, approximately 14,748 publicly owned treatment plants (POTWs) in the United States provide urban wastewater collection, treatment, and disposal services to more than 238 million people, producing approximately 14 million tons of MSS as DS annually. However, much of the municipal wastewater infrastructure, including collection systems, treatment plants, and equipment, has deteriorated and requires repair or replacement. For example, about 15% of the treatment plants have already exceeded their treatment capacity, while the rest operate on average at 81% of their maximum capacity [34].

Similar information is available in the literature for other developed countries, such as Japan [35,36], Canada [2], the United Kingdom [37], and Australia [38]. An overview of the MWW, MSS, and related topic legislation for other developed countries worldwide have been developed, among others, by Christodoulou et al. [39].

According to the Organisation for Economic Co-operation and Development (OECD) [40], the urban population connected to MWW sanitary systems is over 80% in developed countries and close to 100% in several cases. The historical debate between centralized and decentralized systems regarding cost-effectiveness and environmental impact is still open, but municipal or public centralized systems are prevalent. The Waste Water Assessment Programme Report of the United Nations [41] stated that high-income countries treat about 70% of the generated wastewater. The ratio drops to 38% in upper-middle-income countries, 28% in lower-middle-income countries, and 8% in low-income countries. This adds up to around 20% of the wastewater being treated globally.

China is a rapidly developing country, and is the most populous in the world. In China, the key data sources on WWTP distribution and sludge production in the different provinces are the statistics and official websites of the Ministry of Housing and Urban-Rural Development (MOHURD). In 2019, 5476 urban WWTPs annually produced around 10 million tons of MSS as DS, resulting in a significant increase from 10 years earlier [42]. As of January 2020, China’s 10,113 MWWTPs treat sewage for 95% of municipalities and 30% of rural areas. In 2020, an additional 39,000 WWTPs were set up. China plans to build or renovate 80,000 km of sewage collection pipeline networks, thus increasing the annual sewage treatment capacity by 7.3 cubic kilometers between 2021 and 2025. In 2020, the annually collected MWW amounted to 41 cubic kilometers; the Water Action Plan, released in 2015, requires 90% of cities to improve their sludge treatment capacity by the end of 2023. Presently, China is the largest producer of MSS, which will reach 18 million tons per year as DS by 2025. In the next few years, China plans support and investment which will focus on sewage pipeline maintenance, black and odorous water body treatment, and wastewater treatment facility construction in second- and third-tier cities [43].

Other studies have depicted the significant evolution of MWW, MSS management, and environmental remediation in China [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57].

Notably, a recently published paper [49] on the emerging trends and prospects for MWW management in China, claimed that substantial progress was achieved providing wastewater service in urban areas in China over the past decades, resulting in essentially complete coverage for wastewater collection and treatment. At the same time, the lack of attention to collection systems has led to their poor condition, leading the characteristics of the wastewater to negatively affect the performance of the process and opportunities for the recovery of resources. Two emerging water management paradigms, integrated water management and resource recovery, are being implemented in China through “The Concept Wastewater Treatment program and Sponge Cities”. Wastewater management in China is progressing from the implementation of essential treatment to the adoption and development of new technologies and the implementation of integrated solutions aimed at achieving a more comprehensive range of results, including improving water quality and resource recovery and excellent livability.

India is developing and is the second-most populous country globally, predicted to become the first by 2030. A recent study depicted the state of wastewater and associated management strategies in India [58]. A recent inventory [59] reported that the MWW generated annually in India amounted to more than 10 cubic kilometers, corresponding to a potential annual amount of 100–200 million tons of MSS as DS. In contrast, the present MWW treatment capacity is 20–25%, producing 10–15 million tons per year of MSS as DS, which can be improved by about 6%, accounting for the annual treatment capacity under the planning or construction stage [60]. It is worth pointing out that the Indian government has planned the National Mission for a Clean Ganga (NMCG), focusing on pollution reduction interventions, including the interception, diversion and treatment of wastewater flowing through open drains. Aims to reduce pollution through bioremediation, in situ MSS treatment using state-of-the-art technologies, the best technologies for wastewater treatment plants (STPs), effluent treatment plants (ETPs) and water remove dirt and sludge sedimented by the river (Namami Ganges) [61,62].

Similar information is available in the literature for other important developing, emerging, and underdeveloped countries such as Russia (the largest country in the world) [63], the Middle East and North Africa (MENA) geopolitical area [64], Africa [6,65], and Latin America and the Caribbean (LA&C) [66,67,68].

The book edited by Pay Drechsel et al. [69] and other recently published books [70,71] provide the most comprehensive overview of MWW and associated MSS production and management worldwide.

In summary, the unprecedented and growing scale of the sewers and corresponding amount of collected sewage in WWTPs is attracting increasing attention because of the health and environmental impacts and treatment costs of the resulting waste. Presently, the final disposal of MSS amounts to 20–60% of WWTP’s total operating costs [60]. However, it is also true that the potential chemical energy held in typical municipal wastewater is five times higher than that needed for the operation of a conventional activated sludge (CAS) process [72]. MSS contains most polluting materials but, on the other hand, there are several beneficial compounds, i.e., fertilizers, volatile fatty acids (VFAs), extracellular polymeric substances (EPSs), and single-cell proteins (SCPs) [72]. Economically affordable MSS management requires changing the paradigm that considers MSS as waste. Instead, it is a vast renewable source for producing energy, agricultural nutrients and soil improvers, and chemicals for making valuable materials, accounting for economic feasibility, sanitary security, and environmentally friendly circular economy. This concept is well established in the scientific community [73], governments, environmental agencies, intergovernmental and humanitarian organizations, and stakeholders. All these entities produce thousands of articles [74], patents [75], reports, and reviews annually. Some recent articles and reviews surveyed as part of this study are reported here to complement the information on the continuous evolution [69,73,76,77,78,79,80,81,82,83]. Others will be reported in the paragraphs where we discuss traditional technologies, emerging ones, and those in the research and development phase to manage MWW and associated MSS.

As already documented, rapidly developing countries have planned and initiated ambitious programs to expand urban sewer networks and repair previous environmental damage related to improper MWW discharge. The experience gained using the first generation of urban sewerage networks, WWTPs, and related products has resulted in a continuous revision of the legislation that considers the most severe constraints relating to protecting the environment and human health. Technological evolution has witnessed the possibility of satisfying these constraints and enhancing the energy efficiency and enhancement products of waste treatment plants. We have found that the most recent published documents and reviews focus on particular aspects instead of considering the entire urban network that has generated the MSS to be managed. Furthermore, they have neither considered nor underestimated the potential of emerging strategies and technologies already applicable on a large scale for the fruitful management of sludge produced by urban WWTPs.

The first purpose of this additional review is to update the state of the art to identify and evaluate the new options already applicable and those under development for the beneficial management of MSS. We also focused on the potential of hydrothermal carbonization as mature technology for managing MSS produced by big towns and small villages and on its hybridization with anaerobic digestion to obtain the best from the management of MSS produced by large-sized WWTPs.

2. Methodology

The production of MSS is a story of the last 50 years after the United States began to legislate on environmental matters. To reconstruct the evolution that has taken place in the management of the MSS, we therefore started from the establishment of the EPA and the tasks subsequently delegated to it for the containment of environmental damage deriving from the need to remove MWW from the cities and to initially dispose of the purification residues, called biosolids. The initiatives of the USA have been followed by those undertaken by other developed countries and, more recently, by extensive and populous developing countries such as China and India. This reconstruction made it possible to understand the interactions between the different segments of the complex system that start from the numerous and various sources of MWW production and end with the management or final disposal of the MSS with the highlighting of the strengths and criticalities. One example is the long debate over source purification as an alternative to piping MWW into purification plants where MSS is produced. Other examples concern the type of sewerage networks and the location, size, management, and type of plants. Almost one thousand documents have been extracted from the technical–scientific literature, critically analyzed, and selected, privileging the most recent and those produced by reliable sources. The evolution of urbanization (the primary cause of MSS production) and the evolution of technologies, R&D activities, and practices of the use and disposal of MSS were first reconstructed. The collection and critical analysis of bibliographic material and the evolution of regulations were updated with the documents published during the drafting of the review to make the forecasts of future developments, both short- and long-term, more reliable. Emphasis was given to hydrothermal carbonization for the review.

3. Sewer Networks and WWTPs

Typical MWW consists of more than 99% water, and the remaining 1% is a vast variety of organic and inorganic pollutants. Organic matter consists of many compounds, and it is practically impossible to identify and quantify all of them; therefore, it is usually represented by the biochemical oxygen demand (BOD), which is the amount of oxygen required to biologically decompose the organic matter, and by the chemical oxygen demand (COD), which is the amount of oxygen required to chemically decompose the organic matter. MWW also contains many bacteria and viruses, some pathogenic [84,85,86]. Figure 4 shows the quantities of MWW produced, collected, and treated in various regions around the world [87].

3.1. MWW Collection

The characteristics of an MWW from a community depend on many factors, and flows typically vary consistently across days, weeks, seasons, and years [88]. In order to talk about MWW collection and treatment systems, it is necessary to distinguish between the different forms of urbanization. Usually, in developed and developing countries, it is possible to distinguish:

• Towns and big cities
• Small towns or villages;
• Urban centers;
• Isolated houses.

For each of these cases, there are essentially two types of collection and treatment systems that can be applied: at source, and collective management. In the first case, wastewater is accumulated in a septic tank or similar, and we refer to non-collective sanitation, also called individual sanitation. It is considered to be a suitable solution when the collective sewerage is more than 200 m from the reference structure, and is typical of scattered houses or isolated hotels. Maintenance indeed remains the primary concern of individual sanitation [89]. This solution is widespread in low-population-density countries. For example, approximately 21% of American homes use on-site sewage disposal systems [90].

In collective management systems, MWW is collected through a sewerage network and discharged into one or more WWTPs. Except for isolated houses, this solution is the most commonly used. The type of sewerage is influenced by numerous factors: soil conditions, topography, type of effluent, and even the construction sequencing. MWW is usually transported by gravity in reinforced concrete or corrugated metal pipes or by implementing the use of pumping stations in situations where gravity flow is not possible. Pumping stations are avoided as much as possible, because large pumping capacities would be required with the associated energy consumption. Concerning the collection and conveying of MWW and urban runoff to WWTPs, the first-generation combined sewer networks (CSNs) were designed and built to collect and convey the whole dry and wet weather flow. In separate sewer networks (SSNs), each type of water is conveyed by independent pipes [91].

CSNs are designed to transport significant amounts of water and are equipped with relief points to prevent flooding. One of the main problems related to CSNs occurs during significant rainfall events; if a large amount of water cannot be treated, it is discharged into the river or sea without any treatment. For this reason, CSNs can be designed with temporary storage areas or swirl concentrators [91]. Sanitary wastewater and atmospheric water are treated in combined systems, and this combined treatment requires the bulk dimensions of treatment plants with a nonconstant flow. In addition, pollution load is nonconstant: it is higher when there are periods with a lack of precipitation, and lower during periods of higher precipitation due to dilution [92].

Stormwater flow varies considerably during the day, and for this reason, in SSNs, pipes of stormwater collection systems are designed to dislodge the maximum flow to the recipient. However, because stormwater does not mix with wastewater, the dimensions of treatment plants for sanitary wastewaters are smaller, with the flow and the pollution load more constant [92].

Presently, in high-income countries with a high-tech industries and robust systems, such as Germany, the share of SSNs and CSNs are in favor of SSNs [11]. Several studies have compared and discussed the advantages and disadvantages of SSNs and CSNs [13,14,15,16,17,18,19,20,21,32], enabling us to predict the future increases in SSNs when rebuilding obsolete CSNs or expanding the sewer services. The investment cost for two separate sewers is significantly higher and, especially during the reconstruction phase, challenging to achieve. However, SSNs allow for the optimization of the dimensions and treatments in the WWTPs and to obtain better results in terms of the quality of the purified water and the quantity and usability of the sludge produced [93].

3.2. WWTPs

Urban wastewater treatment is fundamental to ensuring public health and environmental protection. The individual methods are usually classified as physical, chemical, and biological unit processes, although these processes occur in various combinations in treatment systems [94]. There are three levels of wastewater treatment:

• Preliminary and primary treatment;
• Secondary treatment;
• Advanced tertiary and quaternary treatment.

Preliminary treatment includes screening, grinding, grit removal, flotation, equalization, and flocculation. Screens, grinders, and grit removal are provided to protect other equipment in the treatment plant [95,96,97].

Primary treatment aims to reduce BOD and the fraction of suspended solids, often reduced by gravity settling. Sedimentation tanks are the most common form of primary wastewater treatment. The suspended solids settle on the bottom of the settling tanks, are scraped at a central point, and then removed by gravity or a pump. The foam formed, mainly oil and fat, floats to the surface, where a mechanical arm collects it and periodically sucks [84,94]. Approximately 25–50% of the incoming BOD, 30–60% of the total suspended solids, and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, phosphorus, and heavy metals associated with solids are also removed during primary sedimentation, although colloidal and dissolved constituents are not affected [98,99].

The purpose of secondary treatment is the removal of remaining pollutants from preliminary and primary treatment. The aim is mostly achieved through activated sludge processes (ASPs). In ASPs, the microorganisms convert organic substances in wastewater into carbon dioxide and new microorganisms. This process usually takes place inside reactors which can be suspended biomass or attached biomass.

In ASPs, oxygen is usually supplied through mechanical aerators. The bacteria must then be separated from the wastewater. Most secondary processes involve a sedimentation tank needed to deposit the flocculated cell mass. The effluent, which constitutes the overflow course of the sedimentation tank, continues downstream of the treatment processes or is discharged into a receiving water body for final disposal. The sludge is returned from the sedimentation tank to the aeration tank, which maintains a vital concentration of bacteria to metabolize the incoming organic material; this is called return activated sludge. Some of the sludge must be removed and not returned to the aeration tank to prevent sludge concentrations from rising too high. The removed sludge is called excess activated sludge or activated waste sludge.

Table 2. Most common ASP configurations.

Type of ASP	Ref.
CAS	[100,101,102,103]
Step aeration	[100,101,102]
Modified aeration (with or without primary settling)	[100,101,102]
High-rate ASP	[100,101,102,104]
Extended aeration	[105,106,107]
Hatfield and Krauss processes	[108,109,110]
Contact stabilization	[111,112,113,114]
Membrane bioreactor (MBR)	[115,116,117]
Sequencing batch reactor (SBR)	[118,119,120,121]

shows different types of ASPs.

The microorganisms partly feed on and partly retain various inorganic compounds; for example, modern activated sludge plants manage to retain about 90% of the phosphorus; even in the case of wastewater with a high phosphorus content (about 15–20 mg/L), the component value is below 2 mg/L, which is the maximum value allowed in many developed countries [122]. Furthermore, these microorganisms retain a good percentage of different heavy metals present in MWW [123].

Linked growth systems are similar to suspended growth systems. However, in this case, the reactor is filled with an inert medium (stone or plastic) on which the bacteria grow. The primary effluent is sprayed onto the medium, and the bacteria extract oxygen directly from the air.

There are also other variants of aerobic treatment processes, such as “aerobic” lagoons. Aerobic lagoons are often used in developing countries. They are natural systems where oxygen is provided by the algae that develop in the lagoon and generate oxygen through the photosynthesis process [84,85].

The anaerobic treatment process is characterized by two different phases: an acidification phase and a methane production phase. Initially, anaerobic microorganisms break down complex organic compounds into simpler molecules. Anaerobes synthesize organic acids to form acetate, hydrogen gas, and carbon dioxide; finally, the microorganisms act on these newly formed molecules, producing methane gas and carbon dioxide during methanogenesis. These by-products can be recovered and used as fuel, whereas wastewater can be channeled for further treatment or discharge [96]. Among the central anaerobic wastewater treatment systems are anaerobic lagoons, anaerobic sludge blanket reactors, and anaerobic filter reactors.

When a high degree of water purification is required, tertiary treatment is required. The tertiary treatment removes inorganic compounds, bacteria, viruses, and parasites. The product of this final stage of the WW treatment process is practically pure water, which can be destined for multiple uses or safely released into the environment. In most cases, tertiary treatment consists of filtration followed by further disinfection treatments. It may include other methods dedicated to the biological removal of nutrients or the removal of nitrogen and phosphorus. Filtration and disinfection can be performed using different technologies.

Table 3. Technologies for filtration and disinfection of tertiary treatment.

Filtration	Disinfection
Bag filters	Dichlorination
Drum filters	UV treatment
Disc filters	Ozone treatment
Membrane	Advanced ozone oxidation treatment

summarizes the most common approaches.

About 50% of global MWW is treated in heterogeneous plants in terms of size and technological level [124]. As already mentioned, the quantity of water to be treated and the country’s degree of development significantly affect the type of plant and the degree of water purification. Figure 5 shows the situation in different EU countries [125]. The situation is very heterogeneous within Europe, in countries such as Turkey, Serbia, Bosnia and Herzegovina, and Albania, where the percentage of connected but untreated wastewater is still very high (>20% of the population). The opposite situation occurs in countries such as Switzerland, Sweden, Holland, Germany, Finland, Estonia, Denmark, and Austria, where the degree of wastewater purification is so high that it can almost exclusively be found in plants with treatments up to the tertiary level.

During the design of WWTPs, the choice of technologies and type of plant strongly depends on the number of population equivalent (PE) that must be connected to that future plant.

According to the OECD Glossary of Statistical Terms, PE in wastewater monitoring and treatment refers to the amount of oxygen-demanding substances whose oxygen consumption during biodegradation equals the average BOD of the wastewater produced by one person. For practical calculations, it is assumed that one unit equals 54 g of BOD per 24 h [40]. However, 60 g in the EU [22,126] and 80 g in the USA [127] are the most commonly used values. Additionally, a new approach to the PE concept through a detailed characterization of grey and black waters has been proposed [128].

Certain purification technologies become economically sustainable only over a specific volume of processed water. When considering small plants (a few hundred PE), we mean plants where there is mainly only a primary type of treatment, rarely followed by aerobic stabilization.

A significant difference arises when moving from medium-sized plants (<100,000 PE) to large plants (>100,000 PE). The discriminating technology is certainly anaerobic digestion (AD). It is now a common opinion that an ASP followed by anaerobic treatment characterizes the plant solution with the most significant advantages. In these cases, AD stabilizes the sludge, leaving the ASP section and reducing its water content. However, this solution is not applicable under a certain amount of treated water because AD requires significant investment costs and excellent skills in managing the plant, leading to economic sustainability only for large plants, which treat about 40,000 tons/a of MSS.

For medium-sized plants, stabilization usually takes place under aerobic conditions. This requires energy (for the supply of air to the system) and does not allow a significant reduction in the water content in the sludge, as occurs for AD. The residual moisture of this type of sludge is greater than 80%.

4. MSS Disposal and Nutrient and Chemical Recovery

The annual per capita amount and the quality of MWW vary significantly depending on geographical and lifestyle variables. The associated MSS specific amount and characteristics also depend on the dimension and the kind of WWTP. The pollution load of MSS is considerably variable in the samples analyzed in different part of the world.

As already mentioned, the main focus of most municipal WWTPs is on the biological removal of dissolved and suspended organic and inorganic pollutants by ASP [97], followed by sedimentation, which gives rise to raw sewage sludge (RMSS). The ASP section follows the upstream operations and feeds the RMSS to the downstream section. Conventionally, upstream operations are referred as preliminary and primary treatments; ASP is a secondary treatment; and downstream operation is tertiary and quaternary treatment [129]. Most of these treatments happen inside the WWTP, others outside.

In the case of on-site treatments in an efficient WWTP, MSS contains almost all of the polluting compounds because the purified water must meet stringent modern regulations concerning the concentration of the residual pollutants.

Presently, the MSS as DS produced annually for more than 100,000 WWTPs operating worldwide amounts to a minimum of 45 [130], although a more realistic estimate is 75–100 million tons. According to Shanmugam et al. [131], global sewage sludge production is predicted to reach 127.5 million tons as DS by 2030. However, when considering RMSS, the annual global production has been estimated at 1.3 billion tons, which could almost double in 30 years [132].

Table 4. Annual production of MSS as DS for different countries.

Country/Region	MSS [Million tons/a]	Ref.
EU	12–14	[30,60,133]
USA	14	[33,134]
China	12–18	[42,43]
India	10–15	[59]
Canada	0.7–1.2	[2]
Australia	0.3	[38]
Japan	2.3	[35,36]
Russia	5–15	[135,136]
South Africa	1.2–2	[137]
UK	1.6–1.8	[37]

reports an updated census for the annual production of MSS as DS as part of the review.

Notably, unlike a few years ago, China and India significantly contribute to MSS production along with established developed countries and geopolitical entities. However, the quality and the composition of MSS are different depending on the lifestyle, expertise, and the skill in managing technologically sophisticated WWTPs, regulations, and others. The concentration of pollutants in the RMSS is about 2–3 orders of magnitude higher than that in MWW, although the variety of pollutants is very wide, including both organic and inorganic materials containing several compounds. Additionally, RMSS contains micro-polluting substances such as bacteria, viruses, pharmaceuticals, residual care products, microplastics, carbohydrates of several molecular dimensions, surfactants, and many others.

The final disposal of RMSS is a very challenging, inevitable, and costly task affordable by several strategies and technologies which must meet the increasingly stringent regulations and, finally, the acceptability of communities. On the other hand, the availability of such a considerable amount of renewable material enables, at least in principle, the production of energetic substances and a variety of chemicals in the context of a circular economy [138,139,140].

4.1. On-Site Treatments

4.1.1. Stabilization and Thickening

Activated sludge treatments [141] of municipal sewage followed by thickening and sanitation enable the discharge of more than 99% purified water. A volume reduction of approximately 30–80% can be reached with RMSS thickening. The most commonly used thickening processes include gravity thickening, dissolved air flotation, and rotary drum thickening. Centrifuge thickening is also becoming more common. The type of thickening selected is usually determined by the size of WWTP, its physical constraints, and the planned further downstream operations. Thickening commonly produces sludge solids concentrations in the 3% to 5% range, whereas the point at which sludge begins to have the properties of a solid is between 15% and 20% solids concentration.

As consequence of the natural fermentation and the presence of pathogenic organisms, RMSS gives rise to offensive odors and health problems. Stabilization is the on-site downstream process that prevent these problems and is usually achieved by chemical, biological, and thermochemical treatments.

The typical chemical stabilization is alkaline stabilization, almost always performed by adding lime, or lime and waste solid materials, to the thickened liquid sludge, to raise the pH to greater than 12 and maintain the resulting materials under these conditions for several days [142,143,144,145]. The resulting solid and significantly stable product can be easily and safely transported outside the WWTP for further treatment or final disposal. Recently, ozonation and other advanced oxidation processes (AOPs), such as Peroxone, have attracted attention for the chemical stabilization and reduction in RMSS and as a pretreatment prior to the biological stabilization of RMSS [146,147,148,149,150,151,152,153,154].

Biological stabilization employs microorganisms which develop naturally within the sludge under reactor conditions. It can be conducted under aerobic [102,155,156,157,158,159] or anaerobic [160,161,162,163,164,165,166,167,168,169] conditions, namely, aerobic stabilization (AS) or AD. In the last case, when the C/N ratio is too high, nitrogen can be added in an inorganic form (ammonia) or in an organic form such as livestock manure, urea, or food waste [129]. According to the WEF Manual of Practice [170], AD is generally applied for WWTPs, treating wastewater incoming flows greater than about twenty million liters per day or about 60,000 to 100,000 PE, whereas AS is typically applied in smaller WWTPs. Both AD and AS significantly reduce the amount of raw activated sludge and produce temporarily stabilized wet solid material. However, the energy balance of AS and AD is quite different, because the composition of the resulting solid phase and its dehydration are different. The other relevant difference between AS and AD are investment and management costs, ease of use, and greenhouse gas emissions. Of note is the energy balance which, unlike in AS, a significant amount of biogas composed of 45–60% methane is produced in AD. As a result, AD provides a significant part of the energy required by the wastewater purification process.

Aerobic stabilization resembles the conventional activated sludge process, but excludes wastewater feed and employs longer solids retention times (SRTs). There is no new supply of organic material from the wastewater; therefore, the activated sludge biota begins to die and is used as a substrate by saprotrophic bacteria. This stage of the process is known as endogenous breathing. It has the task of drastically reducing the number of pathogenic organisms and the concentration of organic solids in the sludge, solidifying and temporarily stabilizing the RMSS, which can easily be transported safely outside the IDA for final disposal with or without deriving benefits. An interesting variant of AS is the so-called autothermal aerobic thermophilic aerobic digestion (ATAD) based on the natural phenomenon of the exothermic oxidation of organic carbon with the addition of forced air or oxygen. If the reactor is well insulated, the heat produced is not lost, and the temperature rises to 70–75 °C, with some favorable implications [171,172]. Comparisons between AS and AD and between AS and lime stabilization have been reported [159,173].

4.1.2. Dewatering

MSS temporarily stabilized aerobically or anaerobically needs further on-site treatment and dehydration to separate the solid material from an aqueous stream, usually directed to the inlet of the WWTP. The solid resembles a cake, not sliding, and instead forming lumps that can only be transported by a conveyor belt, mechanical earth-moving equipment, or a spade. The first difference between thickening and dewatering is that the former removes free water, increasing the DS content to 4–6%, whereas the latter removes significantly more water content of the sludge to produce a concentrated sludge product with a concentration of SD, generally between 15% and 25%. The second difference is in the way the process is carried out. Dehydration can only be achieved by applying significant mechanical force in a filtration or centrifugation system. Alternatively, the liquid can be allowed to evaporate in ambient conditions (sludge drying lagoons) or either evaporate or drain by gravity through a porous medium on which the sludge is located (sludge drying bed) [174,175,176]. Both mechanically dewatered aerobically and anaerobically stabilized MSS contain 75–85% of intracellular, interstitial, and surface water. However, when the treatment capacity of the WWTP is sufficiently high, the AD section can be optimized by pre-treating the thickened raw primary and excess activated sludge by thermal hydrolysis (TH), which obtains better results in terms of biogas production and enhanced dewaterability of the digestate, as discussed by Akajis et al. [177], among others. TH disintegrates organics, allowing for more fermentable organic substances and interstitial water to become free water within the digestate. Typically, the TH reactor operates at 6 bar, 400–440 K, for about half an hour of retention time, followed by treatment in a flash tank operating at atmospheric pressure. The steam produced when flashing was returned for heat recovery by direct contact, preheating the feed to the TH reactor.

4.2. Off-Site Treatments

Stabilized MSS is usually removed from WWTPs for further treatment before beneficial use or for final disposal.

4.2.1. Application to Agricultural Land and Reclamation Sites

In 1991, the U.S. EPA introduced the term “biosolids” for stabilized solid material leaving the municipal WWTPs or POTWs [39]. Australia and New Zealand use the same terminology, which is now familiar and recognizable worldwide. Unlike in the EU, countries that use the term biosolids in their formal documents do not regard this material as waste, but instead as a by-product to be eventually post-treated for recycling. Traditionally, semi-solid and solid materials produced by municipal WWTPs have been widely used around the world as fertilizers and soil improvers, being rich in nutrients, mainly compounds of phosphorus and nitrogen that are bioavailable for vegetation and crop cultivation and carbon-rich substances. However, basic biosolids and stabilized and dehydrated MSS contain various micropollutants, regardless of terminology and regulations. As of 2020, the U.S. EPA [134] classified sewage sludge (biosolids) into Class A—exceptional quality (EQ)—or Class B, based on the specific treatment requirements for pollutants and pathogens, as well as management practices. Minimum requirements for meeting Class A and Class B biosolids are determined by federal regulation 40 CFR Part 503. A 2019 census of the EPA based on the operating POTWS reported that about 51% of produced biosolids found beneficial use as applications to agricultural land and reclamation sites. Similar disposal shares of MSS are maintained worldwide. Despite the increasingly stringent regulations for using biosolids in agriculture and land applications [178], a recent report [179] predicted an increase in this disposal method. However, recent studies [74,180,181,182] have stated that the direct use of sewage sludge and compost containing sewage sludge should no longer be considered a direct source of nutrients and organic matter in agriculture because of their pollutant contents. Agricultural and land applications of biosolids are controversial. We believe that the forecast made by the already mentioned market report is likely correct, considering the ongoing evolution of technologies for purifying excessive concentrations of heavy metals [183], microplastics, and other phytotoxic substances [184,185,186]. The practice of land applications for good-quality biosolids remains the most economically feasible and environmentally friendly, and is in harmony with the circular economy and the conservation of non-renewable resources. This practice requires the availability of land not excessively far from the places of production, and is more practicable in countries with a low population density. Composting raw biosolids is the most widely applied technology to enhance the quality of dewatered MSS when used as fertilizer and soil, improving fertility. It is a well-known and globally applied bioprocess allowing the biodegradation of various substrates carried out by a microbial community composed of various populations in aerobic conditions and in a solid state. The exothermic process starts with a mesophilic phase (ambient to 40 °C), proceeds with a thermophilic phase up to about 65–70 °C, and ends with a maturation phase. Completing the first two phases requires about one week, whereas maturation requires up to six months. The main factors that affect the biology of composting are moisture, temperature, pH, nutrient concentration, and oxygen supply. The optimum moisture content is 50–60%; less than 40% moisture may limit the decomposition rate, whereas with an initial mixture higher than 60%, proper structural integrity will not be achieved, and substrates will not decompose well. Temperature rise is important to destroying pathogens, although most mesophiles are killed when it exceeds 70 °C. The optimum pH range for the growth of most bacteria is between 6 and 7.5, and between 5.5 and 8 for fungi. The pH varies throughout the composting steps, and is essentially self-regulating. Carbon and nitrogen are required as energy sources for the growth of microorganisms. The most desirable carbon-to-nitrogen ratio should be 25–35 in the composting mixture by weight. Oxygen concentrations in the composting mass should be maintained between 5% and 15% by volume of gas mass, and should be well distributed. Excessive aeration will compromise the required temperature profile during the process. Composting can be performed in different ways (batch or continuous), using very simple (pile) or sophisticated and automatically controlled reactors (bio cells), depending on the potentiality of the composting plant [187]. As mentioned, aerobically and anaerobically stabilized and mechanically dewatered MSS retain interstitial water. Consequently, the moisture content is higher than that required for composting. For this reason, before composting, woody material is added as sawdust and wood chips that can be recovered and recycled before the maturation phase. In this way, the aeration process is also favored as the porosity of the mass increases. As far as nutrients are concerned, when the C/N ratio is too low, sawdust is added, while the availability of P is always satisfied. The pH value, on the other hand, is within the optimal range for starting the process and, as already mentioned, tends to self-regulate.

4.2.2. Thermal Treatments (TT)

There are a variety of off-site thermal treatments of stabilized and dewatered MSS based on well-consolidated and new technologies which allow for the final disposal with or without beneficial use.

TT1: Drying

Drying is a well-known and widely applied unit operation that removes the mixture from a solid substance. For example, a wet solid such as wood can be dried by evaporation of the mixture either in a gas stream or without the benefit of gas to carry away the vapor, although the mechanical removal of such a mixture by expression or centrifugation is not ordinarily considered drying. Analogously, a solid dissolved into an aqueous solution can be dried by spraying it in fine droplets into a hot, dry gas, resulting in the liquid’s evaporation. However, evaporation of the solution by boiling it in the absence of gas to carry away the mixture is not ordinarily considered a drying operation. A relevant example is the production of whey protein concentrate (WPC) and whey protein isolate (WPI) by spry-drying the retentate of whey ultrafiltration. Drying can be classified according to the method of operation, i.e., batch or continuous, according to the method of supplying the heat necessary for evaporation of the mixture, i.e., direct driers when the heat is supplied entirely by contact of the gas with the substance, or indirect driers when the gas and the substance are separated by a wall having good conductivity property; and solar drying, i.e., when the heat is supplied entirely by exposing the substance to solar radiation.

Furthermore, classification can consider the nature of the substance to be dried: rigid solid, flexible material, granular solids and crystals, thick paste, thin slurry, or a solution. The residual mixture content after drying is typically 8–10% [188]. The water industry has practiced the drying of stabilized and mechanically dewatered MSS for many years to further reduce the volume of the product, making its storage, transportation, packaging, and retail easier. Commercial and legal pressures have also focused on the final product and its use, i.e., for producing EQ biosolids and as a pretreatment for further intensive thermal process [189,190,191,192,193].

TT2: Torrefaction

Torrefaction is a thermochemical process performed at 200–300 °C, at atmospheric pressure, in an inert or low-oxygen-content flue gas or superheated steam conditions, and at a relatively low residence time. Dry torrefaction transforms biomass into a superior handling, milling, co-firing, and clean renewable energy into a carbonaceous and hydrophobic solid [194]. Torrefaction enhances feedstock’s properties in several ways to enable its use as a direct fuel: reduction in moisture, increase in energy density, reduction in the O/C and H/C ratio, increase in heating value, and improved ignitability and reactivity of the processed fuel. Historically, development of the torrefaction process only began with coffee production in the late 19th century, as documented in the first patents by Thiel (1897) and Offrion (1900). Additional historical information is available in the literature [194,195]. The most recent efforts and current research and development on torrefaction are extensive, with more than two thousand papers assessed as part of the review, published in the last ten years. The main product of the torrefaction process is a coal-like solid, while liquids and gases are produced as by-products. Torrefied sewage sludge is recognized as a better solid fuel than the dry sewage sludge, and it is also beneficial for subsequent thermochemical processes and for producing soil improver and adsorbent material [196,197,198,199,200,201].

TT3: Hydrothermal Carbonization and Hydrothermal Liquefaction

Hydrothermal carbonization (HTC) produces carbonaceous material, hydrochar (HC), from very wet biomass, such as AS and AD sewage sludge. HTC had fallen into relative obscurity after the initial investigations and first description by Friedrich Bergius in 1913. Since then, research activity was slowly initiated to understand natural coal formations, until recent studies on HC chemistry and applications in innovative materials and soil quality improvements revived interest [202]. As described by the review’s authors in a recently published paper [203], HTC is performed at 180–250 °C, approximately the vapor pressure of water under the operating temperature. Typical water-to-biomass ratios range from 5 to 10, and the residence time ranges from 0.25 to 2 h. It works in batch or continuous mode. In addition to hydrochar, HTC produces a large volume of process water (PW), plus a small amount of a CO₂-rich gas phase. Water acts as a solvent and reaction medium, and the mechanisms mainly involve decarboxylation, dehydration, and polymerization (FZ). During the process, the ionic product of water increases, whereas the dielectric constant decreases. Consequently, water acts more as a non-polar solvent [204]. Furthermore, it has been found that, whatever the biomass, the time course of electrical conductivity follows a unique law, unquestionably corresponding to the evolution of solid-phase carbon content [205]. HTC is still in an evolutionary phase, even though in European, many pilot plants and some full-size plants have been built and managed, and a few years ago in China, a full-size plant was built with German technology and is operational for the final disposal of MSS with the recovery of commercial materials. HTC technology is emerging for MSS management, with several favorable implications. The primary and excess sludges, well thickened or slightly dehydrated, can be hydrothermally carbonized directly in situ, i.e., inside the WWTP, due to the favorable ratio between water and organic material. The most favorable conditions apply to small- to medium-sized WWTPs, where an AD section is economically impractical, if not difficult to manage. In this case, the integration of WWTP with an HTC section was drastically reduced [203] the quantity of sludge, i.e., the mechanically dewatered and aerobically stabilized sludge, and which must be disposed of definitively outside the WWTP. Without the HTC section, the sludge to be disposed of has a mixture of about 80%. Considering that the HC yield as DS after the HTC treatment is in the range of 55–65% [206] and the residual mixture content of the mechanically dewatered HC is 30–35% [207,208], it amounts to 16–20% of the solid waste that a small–medium WWTP must dispose of paying, an average of EUR 200 per tons. The possibility of obtaining a solid residue with such a low moisture content depends on the granulometric and hydrophobic characteristics of the HC, which appears as a very hard and finely divided brown solid. HC is sterilized and free of organic volatiles compounds, its moisture content can be further reduced by temporary thin-layer exposure to air or by using warm exhaust air, if available, in a free drying unit. To better quantify the advantages of the direct on-site treatment of RMSS by HTC, we took 4000 T/a of AS and mechanically dewatered MSS produced by the “Ponte Rosarolo” WWTP in L’Aquila, Italy, as a case study [209]. A saving of between 80% and 84% was obtained, i.e., EUR 640,000–672,000/a, corresponding to about 50% of the overall purification costs of the WWTP. The press-filtered HC obtained by the HTC of primary and excess sludge produced by the activated sludge process exhibited significant favorable properties, being biologically stable (sterilized).

Furthermore, HTC degrades microplastics (MPs) [210], pharmaceutical and personal care products (PPCPs), and other persistent organic pollutants such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) [211], thus giving a significantly better material in comparison with the digestate. Figure 1 schematically shows the on-site continuous HTC process of the MSS produced by a typical WWTP. Notably, a double heat-exchanger enables the recovery of about 70% of the heat required to warm the feed from ambient temperature up to 200 °C, required inside the well-insulated pressurized reactor. The remnants are partially supplied by the exothermic reactions and by injecting a saturated vapor stream at about 2 MPa (about 0.55 T/d per 5.5 T/d of raw material with 10% DS). The saturated vapor stream entering the reactor helps with mixing and rapidly heating. The steam added for final heating has a marginal effect on the quantity of circulating fluid and on the economic savings deriving from reducing the quantity of solid residue that must eventually be disposed of outside the WWTP.

Table 5. Material and energy balances of the process schematically shown in Figure 1.

		1	2	3	4	5	6	7	8	9
Temperature	°C	25	25	25	25	150	50	30	30	25
Pressure	bar	1	1	1	1	20	10–15	1	1	1
Mass Flow
Total	kg/h	1000	2.0	9.8	3.9	5.9	5.9	4.5	1.4	999
Water	kg/h	999	1.9	8.8	2.9	4.9	4.9	4.5	0.4	999
Solid	kg/h	1	0.02	0.98	0.98	1	1	-	1	-
Mass Fraction
Water	-	0.999	0.99	0.9	0.75	0.83	0.83	1	0.3	1
Solid	-	0.001	0.01	0.1	0.25	0.17	0.17	-	0.7	-

summarizes the material and energy balances of the process shown in Figure 6.

The on-site HTC of RMSS results in a very efficient stabilization and sanitation treatment. After dewatering, the cost for the eventual disposal of the HC with the residual mixture content is much lower than the cost for the disposal of the AS and mechanically dewatered SS. For the potentiality of the already mentioned WWTP “Ponte Rosarolo”, a reactor of two cubic meters enabled a residence time of almost two hours, more than required to account for unperfected mixing. In addition, three pumps, a double heat exchanger, a small boiler for steam production, and piping had to be installed in place of the aerobic stabilization tank with its ventilation system. An investment of EUR 1.5–2 million has been estimated, including design, construction supervision, and assembly, corresponding to a pay-back time of about four years. In contrast, AD is not economically feasible. We did not find any report, paper, or review discussing similar findings. Small–medium WWTPs are numerous, but their full treatment capacity is significantly smaller than that of WWTPs serving big cities. Only little information is available on treating AS and mechanically dewatered sewage sludge, which is usually performed in large-scale facilities for final disposal. However, HTC has been considered to stabilize further and dewater digested sludge other than facilitating phosphorous recovery and as an on-site pretreatment or post-treatment of AD to improve the methane potential of the primary and excess sludge [212,213,214,215,216,217]. We also found a laboratory research paper on the direct application of HTC to the natural mixture of human faces and urines [218]. Hydrothermal carbonization is effective in the disinfection of sewage sludge [219]. Therefore, it attracts attention to the treatment of any biologically derived MSS before their beneficial use or final disposal in the environment. Many companies, researchers, stakeholders, and other entities involved in MSS management are considering the design, development, and construction of high-potentiality plants for the hydrothermal carbonization of AS and AD MSS produced by all operating WWTPs in a given area, as an alternative to more conventional treatments [220,221]. In such a circumstance, the PW cannot be recycled to the inlet of a WWTP and needs to be purified. Economic feasibility is only achievable for high potentialities. The necessary authorization for the construction and operation of this system is still difficult to obtain, considering that the regulations in force in the EU and other countries do not yet explicitly include HTC among the applicable technologies. However, many projects concerning research and technological development for applying HTC to treat MSS have received significant public funding. When the operating temperature of the hydrothermal treatment is in the range of 300–360 °C under pressurized water with enough time (0.25–1 h), the treatment is known as hydrothermal liquefaction (HTL), or direct liquefaction, because it enables the conversion of most of the biomass into a liquid bio-oil by breaking down the solid biopolymeric structure into a mainly liquid components. When HTL is applied to MSS [222,223,224,225,226], the yield in bio-oil is 35–50% of the MSS as DS, depending on the severity of the treatment. It is considered a promising green method for the beneficial conversion of sewage sludge. If the operating temperature is higher than the critical temperature of the water, i.e., 374.1 °C, and the operating pressure is higher than the critical pressure of water, i.e., 21.83 MPa, the treatment is usually termed supercritical water gasification (SCWG). It is accepted as a promising technology for sustainable MSS beneficial disposal [227,228,229,230,231] because it eliminates the need for costly water reduction and drying processes before disposal by conventional methods. Under typical pressure (25–30 MPa) and temperature (450–550 °C) operating conditions, the decomposition of organic and metallo-organic compounds takes place in a few minutes, giving rise to a well-mixed gaseous mixture rich in hydrogen and methane. In fact, in such conditions, water behaves as a completely non-polar solvent, capable of solubilizing organic substances but not inorganic substances. From this point of view, SSCWG produces a solid residue similar to that produced by combustion, i.e., a very high phosphorus concentration. Notably, modern activated sludge processes remove 90% of phosphorus in the treated water by accumulating it in the sludge, and the HTC of AS and AD-stabilized MSS transfers 93–97% of the phosphorus to HC in the form of organic and inorganic phosphorus (ARB). In these circumstances, the concentration of phosphorus in the solid inorganic residue produced by the SCWG is comparable with that type of phosphate rock. Consequently, SCWG coupled with HTC could represent an interesting alternative to incineration when phosphorous recovery must be achieved. The catalytic gasification of sewage sludge in supercritical water enhanced phosphorous recovery and hydrogen production [232,233]. The final and complete destruction of the organic pollutants in the gaseous mixture has also been considered as an alternative, using supercritical water oxidation (SCWO) technology. Since 2001, several commercial-scale SCWO plants have been installed to treat MSS with solid-containing rates of 7–8 wt.%. Nevertheless, the three key problems concerning corrosion, plugging triggered by salt precipitation, and high running cost still exist, even making some commercial-scale SCWO plants inactive [234].

TT4: Pyrolysis

The production of charred matter always involves a thermochemical conversion process. The decomposition of organic material under the influence of heat in a gaseous or liquid environment, without the involvement of further reactants (oxygen), is called pyrolysis: from the Greek words ‘pyr’, meaning fire, and ‘lysis’, meaning dissolution [202]. Typically, the pyrolysis or carbonization of woody biomass is carried out at atmospheric pressure by heating the material to 281 °C to cause the breakdown of heavy organic molecules and trigger a series of exothermic reactions capable of obtaining further heating of the mass up to 500–600 °C. Under these conditions, the main product is anhydrous carbon with a high energy density. At the same time, a gas is produced that contains all the water initially present in the feed, the volatile and less volatile organic substances resulting from the thermal splitting of the larger molecules into coal, and compounds which, at room temperature, have a bituminous appearance [235,236]. When the moisture content of the feedstock is not excessively high, i.e., less than 35%, the combustion of the gaseous phase may be sufficient to support the endothermic phase of the pyrolysis process. At the same time, it supports the elimination of a gaseous effluent that should otherwise be re-condensed to avoid atmospheric pollution. A more general and current definition is that pyrolysis refers to the decomposition of organic matter at elevated temperatures (300–1300 °C) in the absence of oxygen and under substantially dry conditions. The operating pressure is usually atmospheric, although vacuum pyrolysis (0.1–0.2 bar) and pressurized pyrolysis (5–20 MPa) have been studied and reported. In all cases, oxygen-free conditions are maintained [174]. The pyrolysis of digested and mechanically dewatered MSS has been widely studied and described in the literature [237,238,239,240,241,242,243]. However, such MSS must be further dehydrated by drying before pyrolysis treatment to avoid the need for recondensation with the formation of a highly polluting organic bituminous phase, which is very difficult to handle due to its unfavorable rheological properties. In contrast, pyrolysis can be applied to mechanically dehydrated HCs because the concentrations of bituminous and other organic compounds in the gaseous phase are about double that of water; consequently, it can be burned, obtaining a combustion gas that can be used to sustain the endothermic phase of the process entirely before it discharges safely into the atmosphere [244]. In summary, the on-site HTC of AS and mechanically dewatered MSS could be followed by the off-site pyrolysis of the HC produced by all the WWTPs of a given area in order to minimize further the unit amount of the solid by-product, charcoal, which can be further processed, as discuss in the following sections. It would be interesting to compare the on-site AD of primary sludge and that in excess with the on-site HTC of the same material followed by off-site pyrolysis in a plant serving WWTPs operating in a specific area. The quantity of coal is much lower than that of the mechanically dehydrated digestate and the HC with its residual content of the mixture, although the quality is much higher for the subsequent beneficial waste-to-energy and waste-to-energy treatments.

TT5: Gasification

As already mentioned, biomass can be gasified in the absence of oxygen by pyrolysis or in a supercritical water environment. However, typical gasification is a process that converts carbonaceous organic or fossil-based materials at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen or vapor, into carbon monoxide, hydrogen, and carbon dioxide. Carbon monoxide reacts with water to form carbon dioxide and hydrogen via a water–gas displacement reaction (CO + H₂O → CO₂ + H₂). Modern membranes can separate hydrogen from carbon dioxide and small amounts of impurities [245]. In general, biomass does not gasify as rapidly as coal, producing other hydrocarbon compounds in the gas mixture leaving the gasifier; this is especially true when no oxygen is used. Consequently, a further step must be taken to reform these hydrocarbons with a catalyst to produce a clean syngas mixture of hydrogen, carbon monoxide, and carbon dioxide. Thus, just like in the gasification process for hydrogen production, a displacement reaction step converts carbon monoxide into carbon dioxide. The hydrogen produced is then separated and purified. Gasification is a well-known technology and has been considered for the beneficial treatment of MSS. The results have been reported and discussed in detail in several articles [245,246,247,248,249]. It is worth emphasizing that its economically feasible applicability is only obtained for high potentiality. Furthermore, some pretreatments, such as drying, torrefaction, pyrolysis, or hydrothermal treatments of raw material, are usually applied before final gasification.

TT6: Incineration

Incineration is a combustion process aiming to destroy or minimize waste, particularly that produced in urban areas. The destruction is a consequence of burning the organic fraction of the waste with an oxygen concentration well above the stoichiometric one to ensure the reaction is complete. It is a typical waste-to-energy (WTE) operation, converting the waste into ash (primarily inorganic compounds), flue gas to be cleaned before discharging in the atmosphere, and heat, often used to generate electric power. The first incineration facility for waste disposal was built in the United Kingdom about 150 years ago [250]. Since then, incinerators have been built and operated in the United States and other developed and developing countries, mainly for WTE from municipal solid waste (MSW) and for destroying hazardous waste. The incinerator’s operating temperature must be above 850 °C, and large plants’ treatment capacities can reach about three thousand tons per day. However, much smaller and mobile incinerators are in operation in some cases. One particular case of incineration uses a cement kiln where the ash is incorporated into the mass of a Portland cement clinker [251]. The effect of phosphorus and iron on the composition and property of the product has recently been quantified and discussed [252]. The application of incineration for the final disposal of MSS has been widely considered worldwide, with the exclusion of undeveloped countries and some recent adverse opinions [253]. It has been reported that sludge with a moisture content of no more than 40% can stably burn in a fluidized bed without any auxiliary fuel input [254]. Sewage sludge incineration coupled with phosphorous recovery is the most preferred MSS treatment in Germany, Japan, and many other industrialized and wealthy countries lacking phosphorous mineral deposits. A recent report by the German Environment Agency [255] accurately analyzed technical, economic, environmental, and health issues related to sewage sludge disposal. Notably, the report considered the unavoidable problems created by land applications of only biologically treated sewage sludge containing ingredients of concern such as pharmaceutical residues, hormonally active substances, nanoscale substances, microplastics, and various pathogens. These substances accumulate in sludge and can only be destroyed by proper thermal treatments. About ten years earlier, Bauerfeld et al. [256] stated that with the planned new restrictions, sewage sludges might not be suitable for agricultural reuse any longer. As a 2020 update, Schnell et al. [257] highlighted Germany’s mono-incineration situation, the disadvantages of co-incineration, and possible alternative thermal technologies. The choices made by Germany on the use of the MSS are only partially applicable to other EU member states, and are different from those in other countries with a high rate of socio-economic development. In the case of developing countries, these are not applicable due to a lack of resources and limited technical capacity to manage complex plants. The fact remains that biological treatments alone and subsequent insertion or spreading in or on the soil is not sufficient to avoid irreversible environmental damage and severe sanitation problems.

TT7: Process integration and biorefinery

Process integration which involves the interconnection of two or more unit operations to treat complex raw materials, e.g., MWW and related MSS, is an attractive strategy because, theoretically, it could recover energy and separate all the components. The technologies currently available and those under development [258] could enable reaching the objective, but the limit lies in the economic feasibility deriving from the need to recover the necessary investments and bear the operating costs with the commercial value of the products. A biorefinery implies process integration, but used when the aim is the recovery of chemicals to be further used as renewable raw materials for synthesizing commercial products. There are numerous contemporary examples of consolidated on-site process integration and biorefinery activities applied at full scale within large WWTPs. In addition, numerous laboratory studies and research and development activities at pilot plants are ongoing worldwide, along with some advanced projects for converting WWTPs into water resource recovery facilities (WRRFs). The hybrid ASP–AD process with electrical energy and heat production is the most relevant example of process integration. Nitrogen (N), phosphorus (P), carbon (C), and related compounds are the most interesting for quantity and usability [10]. The recovery and upgrading of dewatered digestate or AS (biosolids) usable for agricultural and land application are significant examples of first-generation biorefinery activities. However, this practice is questionable with the evolution of legislation. Some EU member states have already banned or drastically limited the use of biosolids for agricultural purposes, and others are expected to follow this route. However, as already mentioned, the EU is heavily focused on recovering P from MSS because of its uneven distribution across Earth, and its use in various applications. Notably, P concentrations in MWW range from 5 to 20 mg/L, whereas in purified water, P concentrations must be much lower, usually less than 1 mg/L. The difference is accumulated almost entirely into the MSS-derived HC or the digestate. It may be calculated that the present potential for the annual recovery of phosphorous on a global basis amounts to 5.6 million tons. This annual amount could reasonably rise to more than 9 million tons in thirty years, more than one-third of the current production from non-renewable mineral reserves. On-site pre- and post-treatment of sludge produced by ASPs by chemical (ozonization or similar) or thermochemical (thermolysis, torrefaction, pyrolysis, and HTC) processes for enhancing their methane potential are examples of contemporary full-scale applications of process integration or biorefinery. The ongoing conversion of Avedøre WWTP to a WRRF by 2025 (a VARGA project in Copenhagen, Denmark) [259] is a significant example of the ongoing development of a full-scale biorefinery. The research and development activities discussed by Dufour et al. [260] and other researchers [83,261,262] are examples of future predictable second-generation biorefinery applications. In addition to technological integration, smart cities will also provide opportunities for the integration of raw materials and the adoption of more virtuous behavior by citizens [263]. Smart cities are a fascinating and stimulating subject, although analyzing this topic is beyond the scope of this review.

The main technologies presented in this chapter are summarized and compared in

Table 6. Comparison between the main applicable technologies on RMSS management.

RMSS Management	Energetic Performance	Mass and Volume Reduction	Required Skill	Development Degree
Anaerobic Digestion	WTE technology	Significant	High	Consolidated and diffused Medium–large plants
Composting	Aeration is energy-demanding	Moderate	Low	Consolidated and diffused
Application on agricultural land and reclamation sites	-	-	Low	Consolidated and diffused Product often out-specification Conflicting with new regulations
Drying	Energy-intensive	Significant	Low	Consolidated
Pyrolysis	Dehydration pretreatment is energy-intensive	Significant	From low to high depending on operating parameters	Consolidated
Gasification	Influenced by the initial mixture content	High	High Severe operating conditions	Consolidated Large plants
Incineration	WTE technology but dehydration pretreatment or co-incineration is required	High	High Severe operating conditions	Consolidated Large plants
HTC and HTL	Directly applicable to RMSS Almost self-sufficient	High	Low Mild operating conditions	Ready for full-scale applications
Process integration and biorefinery	Hybrid ASP–HTC–AD is a WTE technology	High	High	Ready for full-scale applications

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5. Conclusions

This review highlights and discusses the evolution from the past to the present, and the reasonable future expectable changes in each segment of the chain, starting with the collection and purification of MWW and ending with the beneficial final disposal of MSS. It has been confirmed that urbanization and the degree of development are related. However, Latin America is a significant exception, and many big cities already existed in the historically rural world. Urbanization did grow significantly in recent years and will predictably grow in the coming 30 years, primarily due to the evolution of very populous developing countries. The growth of urbanization has led to and will entail the need to create new sewer networks and WWTPs, simultaneously with the final disposal of larger quantities of MSS. China has practically connected all key cities and is currently addressing the problem of urban sparsely populated areas. Sewer networks and WWTPs implemented so far need to be improved, and there is the additional problem of the remediation of the sites and the bottom of contaminated surface waters. India is very active in research and development, although does not have the necessary economic resources to adequately equip what will become the most populous country in the world in a few years. The great powers of the last century, the United States and Russia, along with many other industrialized countries, have the problem of rebuilding often obsolete and crumbling infrastructure (WWTPs and sewers). About half of the EU member states have problems similar to or even worse than that of India and China. Furthermore, they must account for the incoming severe restrictions on the final disposal of MSS. Underdeveloped countries could only participate in this evolution with the intervention of developed countries because the former lack expertise and the massive amount of unavailable endogenous investment. It is reasonably predictable that new sewer networks and many of the CSNs damaged will be developed as SSNs in the following years. Many large WWTPs will be optimized and integrated with further unit operations for material and energy recovery on-site, while minimizing the final solid residue to be processed off-site. Better materials and new technologies will be involved in the modernization process using the experience accumulated managing the first and, in some cases, the second-generation infrastructure. Medium and small WWTPs will probably be enhanced by introducing new practical and economically feasible technologies, such as thermolysis and HTC. Although the share of biosolids produced by only biological treatments and used as low-cost fertilizers, soil improvers, and land applications is still very high almost worldwide, their phytotoxicity, polluting properties, and unhygienic conditions cannot be ignored longer. On the other hand, incineration is costly, and co-incineration is only valuable for Portland cement plants when added at a low percentage. The leading role of bio-based technologies, which reproduce natural phenomena in a confined space and perform much better and faster, appears to be irreversible. However, bio-based technologies alone started to become unsatisfactory. Here, ASP and AD are untouchable, although composting could be replaced by some mild thermal technology to achieve the complete sanitation and demolition of microplastics and other organic polluting compounds resistant to microorganisms. Material recovery is practically restricted to phosphorous recovery as a consequence of its very unevenly distribution, along with the inability to do without it. We believe that the on-site treatment of RMSS with HTC in significant ASP-based WWTPs followed by the AD of the PW and the leaching of HC will be the preferred solution for the beneficial management of most of the MSS produced annually worldwide. Such a hybrid process significantly enhances the methane potential and drastically reduces the amount of solid residue, which is a sterilized HC with a phosphorous concentration comparable to that of the natural phosphate rocks. The HC can be used as a raw material for producing phosphate fertilizers, or leached to partially remove the heavy metals to be used as soil improvers. For consortiums of smaller WWTPs, the mechanically well-dewatered HC produced off-site is further treated by simple technologies and finally disposed of by land applications or used as a secondary raw material for phosphorous recovery. However, in this case, PW cannot be recycled back to the ASP section and must be purified by membrane filtration and oxidizing agents before discharging.