Solutions for Energy and Raw Material Recovery from Sewage Sludge Within the Concept of Circular Economy

Abstract

Wastewater treatment plants traditionally dispose of sludge using the method of landfilling and incineration, with both being carbon-intensive and environmentally harmful. Converting sludge into energy or reusable materials avoids landfills or incineration, helping reduce the volume of waste and associated pollution. Sludge treatment with energy recovery can offset fossil fuel use, further reducing the carbon footprint of sewage treatment processes. This research explores ways to recover energy from sewage sludge, a byproduct of wastewater treatment that is often considered waste. Transforming sludge into valuable resources aligns with the principles of the circular economy, where waste streams are repurposed, minimizing environmental impact and enhancing resource efficiency. In this paper, a method is presented to reduce the volume of wastewater sludge by drying it in a hot flue gas stream at 700 °C. The energy of the exhaust gas is recovered in an organic Rankine cycle system, which powers the wastewater treatment facilities themselves, making them more self-sustaining.

1. Introduction

1.1. Motivation and Purpose

Currently, the effect of human activity on the environment is marked by profound extreme climatic manifestations, generating multifunctional imbalances across the entire planet. “The 2024 state of climate report: Perilous times on planet Earth” [1] is a manifesto document that draws attention to the planet’s irreversible climate disaster. The year 2024 will go down in history as the year with the hottest three days in July ever recorded (Figure 1). The phenomenon of deep drought is manifesting itself in more and more regions of the world. The infrastructure and ecosystem damage generated by extreme weather phenomena are increasing year by year [1,2].

Added to this is population growth, which, although it will have an annual rate of 0.1% (much lower than the 2015 rate of 2.1%), is forecasted to reach 11.2 billion people by 2100 (Figure 2) [3,4]. It generates an increase in water demand for both domestic and economic activity. Along with the increase in water consumption, the volume of water created as waste and the amount of wastewater generated by humans through domestic and economic activity increases also.

The times when wastewater was returned to nature without decontamination have been gone for more than a century [5]. The role of wastewater treatment plants is to return wastewater to the natural circuit at the chemical parameters of the effluent [6]. From the point of view of the circular economy model, wastewater treatment plants (WWTPs) can be adapted to meet the requirements of the three green Rs (recover, reuse, and recycle).

The water resulting from the technological process of wastewater treatment can be used in human activity (agriculture, industry, and even for other human uses, if it undergoes tertiary disinfection treatment). The sludge resulting from the wastewater treatment process can be transformed into an energy resource or useful materials for other economic sectors (e.g., agriculture, cement industry). So, a modern WWTP is not only an infrastructure of wastewater treatment but also a recovery facility for both energy and useful byproducts [7].

According to Eurostat data, Romania produced in 2021, in urban treatment plants and disposal, an amount of 264.34 × 10³ metric tons of sewage sludge (Figure 3).

The management of municipal sewage sludge with energy valorization has become increasingly significant in the context of a circular economy, where waste is transformed into valuable resources rather than being treated as refuse. As populations grow, water consumption rises, and the volume of sewage sludge produced poses environmental challenges. Efficient treatment of this byproduct can mitigate the negative effects on the environment. There are solutions that facilitate the recovery of energy, raw materials, and nutrients from the sewage sludge, contributing to an integrated waste management system in the context of the circular economy, promoting sustainability and resource efficiency in urban environments. In this context, paper [9] refers to the sustainability of renewable energy and calculates the composite index RESI (renewable energy sustainable index), which indicates yearly the macroeconomic state of each country. These solutions can reduce landfill usage and greenhouse gas emissions, reducing environmental impacts.

1.2. Sewage Sludge Treatment Methods and Management

The management of sewage sludge is a complex process with technical, economic, environmental, and legal aspects that interact with each other. Even though sewage sludge treatment and its disposal represent about 20 to 60% of the operating costs of a wastewater treatment plant [10], the sludge issue is often underrated in the activities of management. Sludge treatment consists of thickening, stabilization, dewatering, disinfection, and final disposal. The main sludge contaminants are metals, organic compounds, and pathogenic organisms. The sludge stabilization process is conducted by using the methods of biological stabilization (anaerobic digestion and aerobic digestion), chemical stabilization, and thermal stabilization. To reduce the sludge volume, the fluid properties of the sludge are improved by a thickening procedure followed by dewatering. The methods that are recommended for removing the sludge’s pathogens are sludge disinfection, composting, auto-thermal aerobic digestion, alkaline stabilization, pasteurization, and thermal drying. The main types of sludge transformation and disposal are thermal drying, wet air oxidation, incineration, and disposal in landfills [10].

Properly treated sludge limits the spread of harmful contaminants, such as pollutants, heavy metals, and pathogens, which could harm the environment. Within a circular economy, the dual objectives of waste minimization and energy recovery can lead to a more resilient and sustainable environment [11].

Municipal sewage sludge management is an environmental challenge due to the large volumes produced and the presence of potentially hazardous substances. Many researchers have explored specific methods for energy recovery, their advantages and disadvantages, associated problems, and potential solutions. Current strategies emphasize optimizing sludge treatment processes to enhance the production of biogas and biochar, facilitating their use as renewable energy sources and a valuable soil conditioner. For instance, paper [12] outlines some methodologies for sludge optimization that not only reduce operational costs but also explore the local market potential for byproducts. Additionally, there is a growing emphasis on integrating innovative technologies that can recover multiple resources from wastewater treatment plants [13].

The authors of paper [14] evaluated the feasibility and utility of pretreated sewage sludge in cement kiln co-processing, highlighting its potential for significant environmental benefits and resource recovery. Co-processing in cement kilns involves using pretreated sewage sludge as an alternative fuel, aiding in energy recovery and reducing the carbon footprint of cement production.

Paper [15] discusses the classification of pelletized dried sewage sludge, identifying its key properties as a renewable material and its potential for environmentally non-harmful energy utilization.

Energy valorization technologies with existing wastewater treatment infrastructure require careful planning and optimization. Developing modular and scalable technologies can facilitate smoother integration. This approach can be found in paper [16], which explores the drying performance of a combined solar greenhouse dryer, highlighting innovative approaches to improving sludge management efficiency.

The references emphasize that the sewage sludge management must consider the size and location of the wastewater treatment plant. This aspect is also important for choosing the energy recovery method of the sludge treatment and disposal process.

Anaerobic digestion of 20–25 kgDM (dry matter)/PE of sewage sludge produces 5.8–7.3 Nm³/PE biogas, resulting 12 to 16 kWh/PE (electricity) and 19 to 24 kWh/PE (thermal energy), but the incineration method, in addition to the advantage of significantly reducing the volume of sewage sludge, has the advantage of emitting 58% fewer greenhouse gas emissions compared to natural gas and 80% compared to coal [17,18,19].

The kinetics of sewage sludge combustion in variable oxygen and carbon dioxide atmospheres leads to an optimization of the energy recovery process from sewage sludge, with consequences for the development of “waste-to-energy” applications [20].

From the methods and technologies analyzed for energy recovery from sewage sludge (anaerobic digestion, combustion, pyrolysis, and gasification), paper [21] emphasizes the need to apply treatments to increase the calorific value of sewage sludge and gives an example of using combined combustion of sludge with fossil fuels or biomass. The same aspect is also mentioned by [22], who combine sewage sludge with agricultural residues, which optimizes the anaerobic digestion process and biogas production.

The research has shown that by using anaerobic membrane bioreactors (AnMBR), the energy recovery obtained from sewage sludge is much better compared to the conventional anaerobic digestion (CAD) process [23].

By integrating pyrolysis with combustion, together with scrubbing technology, nutrient and energy recovery from sewage sludge can be optimized. Thus, by pyrolysis, 12,000 t/year of biochar with 500 t phosphorus and 150 t nitrogen, and by combustion, 120 GWh/year of heat and 9700 t/year of ash with 500 t phosphorus, can be obtained from 65,000 t/year of sewage sludge [24].

Another integrated system for treating sewage sludge is also promoted in [25]. The authors use anaerobic digestion and pyrolysis to generate energy from sewage sludge and food waste.

Optimizing the anaerobic digestion process of sewage sludge has shown that by applying a chemically improved primary treatment to sewage sludge, a sustainable technical solution for energy recovery is obtained in [26]. Also, by using hydrothermal carbonization (HTC), municipal sludge with high ash content can be transformed into solid fuel with improved energy characteristics [27,28]. The direct conversion of chemical energy from sludge into electricity is a method that attracts the attention of specialists, even if it is in the experimental phase [28]. The research in [29] showed that the hydrothermal liquefaction (HTL) process is a viable method for recovering energy and nutrients from sewage sludge.

In the authors’ previous research in paper [30], a method was proposed to neutralize the sludge from the wastewater treatment plant. As a follow-up, the authors intend to recover the waste thermal energy from sewage sludge treatment with an ORC system. This recovered energy powers the wastewater treatment facilities themselves, making them more self-sustaining.

By transforming sludge into resources, the research minimizes the carbon emissions associated with disposal. Sludge treatment with energy recovery can offset fossil fuel use, further reducing the carbon footprint of sewage treatment processes.

This research integrates waste recovery into the sewage treatment process, embodying the principles of a circular economy, where materials are continuously cycled through use and reuse. This model reduces reliance on linear processes that produce waste, helping industries transition into sustainable, closed-loop systems. It supports the development of sustainable infrastructure in cities, where sewage treatment plants can act as resource hubs, recovering energy.

Figure 4 presents the most common methods used for sewage sludge treatment, including the thermodynamic cycle for cogenerating electricity and heat, in addition to the drying process of sewage sludge. The principle of the circular economy is highlighted.

1.3. Constanta North Wastewater Treatment Plant

The Constanța North Wastewater Treatment Plant (WWTP) (Figure 5) is one of the treatment plants that operates with recirculated activated sludge, processing approximately 40% of the wastewater volume, designed to serve a population of over 255,000 inhabitants [31].

The Constanța North WWTP was built in 1967 and was initially designed with one primary treatment stage—the mechanical stage. In 2010, it underwent a first phase of renovation, when it became a new and modern wastewater treatment plant, equipped with advanced mechanical–biological treatment technology.

Due to the important geographical position, located at the exit of the municipality of Constanța and the entrance to the famous Mamaia resort, a second stage of modernization of the wastewater treatment plant was necessary. During this stage, completed in 2013, all bioreactors and secondary decanters, as well as their distribution chambers, were covered together with the connecting channel between the two distribution chambers.

Also, in the second stage of modernization, the Poiana Sludge Dewatering Station and the sludge transport pipeline from the Constanța Nord WWTP to Poiana (Constanța County), over a distance of approximately 6 km, were built. The sludge transport pipeline is fed by the sludge pumping station built for this purpose.

Figure 6 presents the simplified technological process of the plant [32].

The wastewater is filtered at the entrance to WWTP through a series of grates that retain the existing solid elements (sand, pebbles, plastic, metals, etc.), which are directed to the landfill. The filtered water containing fine suspensions is biologically treated in aeration tanks, after which it is directed to decanters.

The fraction of impurities is deposited and forms the sewage sludge, and the decanted and treated water is pumped into the effluent. The resulting sewage sludge passes into the dehydration stage, in which, by centrifugation, sludge with 85% moisture is obtained to be stored, or, through modern technological solutions, transformed into energy and/or raw materials for other industries [31].

The dry mass content of sewage sludge before mechanical dewatering is 2–8%, meaning that the moisture content of the sludge is 92–98% [33]. High water content affects the efficiency of thermal processes like incineration and pyrolysis. Dewatering is essential for reducing transport costs and enhancing the energy recovery potential [34]. After mechanical dewatering, the water content in the sludge drops to 85%. Above these values, forced drying is required, a process addressed/proposed in this paper.

2. Materials and Methods

2.1. Sample Analysis Methods

Municipal sewage sludge is a heterogeneous material whose composition varies depending on the source and treatment processes.

Samples of sewage sludge were collected after technological treatment described above from the wastewater treatment plant in Constanta.

The key components analyzed in this research include water content, volatile matter, ash content, and fixed carbon, which are critical for determining its energy recovery potential and environmental impact.

Proximate analyses of both sewage sludge by-products, resulted from centrifugal dewatering and thermal drying, were performed.

Moisture content (W_t) was determined by drying the samples in a POL-EKO drying oven SLN 53 ECO with natural convection at 105 °C for 1 h.

Volatile matter (V) was determined by the gravimetric method using a muffle furnace Nabertherm Model: LE 14/11/B150, heating the sample at 550 ± 5 °C for 1 h. Ash content (A) was also determined by the gravimetric method, heating the sample at 550 ± 5 °C for 4 h in the same muffle furnace according to the standards. The content of fixed carbon (C_f) was determined using the formula:

$$C_f=100-W_t-V-A,$$

(1)

The research method regarding laboratory drying was carried out in an oven according to the standards. This methodology reduces the quantity subject to analysis to two or three grams of ash. For the analysis of the ash composition, a large quantity was required (500 g), which forced us to apply a drying method by layer combustion in a high-powered oven, with the reproduction of the conditions in the laboratory oven.

The sample of sewage sludge harvested from Constanța North WWTP was delivered in a 5-L canteen, weighing 4.114 kg. Its initial density was 823 kg/m³.

It was then subjected to a two-stage dehydration and thermal-drying process. In the first stage, the primary moisture was removed by exposing the contents to a unidirectional thermal radiant flow in an open external environment (Figure 7a). At intervals, the mixture was homogenized using a spatula.

This was followed by thermal processing at a constant temperature in a boiler furnace under the action of the flame generated by a light liquid fuel burner (Figure 7b). The result is shown in (Figure 7c). The cumulative duration of the thermal treatment was 4 h.

Following this treatment, the mass of the sludge became equal to 0.650 kg. The analysis of ash that resulted after thermal dehydration was performed with a Thermo Fisher Scientific spectrometer, using Uniquant software, a quantitative analysis method [35].

2.2. Drying Installation Description and Related Technology

The sewage sludge neutralization system is derived from wastewater treatment processes. For the solution proposed in this research (Figure 8), the drying occurs within a rotary kiln, which is supplied with combustion gases. At the kiln’s inlet, gas temperatures range between 700 °C and 750 °C, facilitating the release of volatile matter and the ignition of combustible components.

During the drying process, the heat produced is transferred to the sludge, leading to significant water removal from the sewage sludge at the rotary kiln’s outlet. This process yields a neutralized residue with a reduced volume. The resulting material can be reused as an auxiliary resource for various applications, such as supplementary material for the cement industry and construction, or as an agricultural additive.

At the rotary kiln’s exit, the combustion gases, containing water vapor, reach a temperature of 200 °C, enabling the integration of a basic ORC-EG system (organic Rankine cycle coupled with an electric generator) that will be presented in Section 2.4.

2.3. Energy Balance of the Plant

The heat balance equation on the technological assembly is:

$$Q_{CG}+Q_A=Q_{DA}+Q_V+Q_{Cf}+Q_{DS}+Q_{EG},$$

(2)

The sludge inlet temperature and thermal effect of rotary drum were neglected.

$Q_{CG}$—the amount of heat of the combustion gases, [kJ/kg];

$Q_A$—the amount of air heat, [kJ/kg];

$Q_{DA}$—the amount of heat of the drying agent, [kJ/kg];

$Q_V$—the amount of heat produced by volatiles burning, [kJ/kg];

$Q_{Cf}$—the amount of heat produced by fixed carbon burning, [kJ/kg];

$Q_{DS}$—the amount of heat of the discharged sludge, [kJ/kg];

$Q_{EG}$—the amount of heat of the exhaust gases, [kJ/kg].

The calculation of the amount of drying agent is conducted for two working cases: (case 1) where is no combustion heat in the dryer and (case 2) with heat produced by volatiles and fixed carbon burning (case 2).

The amount of drying agent (natural gas and air) at the entrance to the sludge neutralization technological cycle, for the first case, can be calculated with the formula [36]:

$$g_{DA,i}={\displaystyle \frac{∆W·\left(r_{H2O}+r_{DA,i}·t_{DA,i}-r_{CG,i}·t_{CG,i}\right)+(1-∆W)·c_{CG}^{anh}\left(t_{DA,e}-t_{CG,i}\right)}{c_{DA,i}·t_{DA,i}-c_{DA,i}·t_{DA,e}}},$$

(3)

$$g_{DA,i}={\displaystyle \frac{∆W·\left(2491+1.88·t_{DA,i}-4.19·t_{CG,i}\right)+(1-∆W)·c_{CG}^{anh}\left(t_{DA,e}-t_{CG,i}\right)}{1.29·(t_{DA,i}-t_{DA,e})}},$$

(4)

where:

$g_{DA,i}$—the amount of drying agent at the inlet [Nm³/kg];

$∆W$—the sludge humidity at the entrance [kg/kg];

$r_{H2O}$—the heat of vaporization of water [kJ/kg]; $r_{H2O}$ = 2491 kJ/kg was considered;

$t_{DA,i}$—the temperature of the drying agent at the inlet $t_{DA,i}=750 °\mathrm{C}$;

$r_{DA,i}$—the specific heat of the drying agent [kJ/Nm³·K⁻¹];

$t_{CG,i}$—the inlet fuel temperature $t_{CG,i}=20 °\mathrm{C}$;

$r_{CG,i}$—the specific heat of the drying agent at the exit of the plant [kJ/Nm³·K⁻¹];

$c_{CG}^{anh}$—the mass heat of the fuel relative to the anhydrous mass; $c_{CG}^{anh}=1.5$ kJ/kg·K⁻¹ was considered;

$t_{DA,e}$—the temperature of the drying agent at the exit; the allowed value for the technological assembly is $t_{DA,e}=200 °\mathrm{C}$;

$c_{DA,i}$—the mass heat of the drying agent at the inlet; $c_{DA,i}=1.29$ kJ/kg·K⁻¹ was considered.

The amount of drying agent at the entrance to the sewage sludge neutralization technological cycle, assuming the combustion of volatiles and fixed carbon, can be calculated with the formula:

$$g_{DA,i}^v={\displaystyle \frac{∆W·\left(r_{H2O}+r_{DA,i}·t_{DA,i}-r_{CG,i}·t_{CG,i}\right)+\left(1-∆W\right)·c_{CG}^{anh}\left(t_{DA,e}-t_{CG,i}\right)-\left(1-∆W\right)·[V·H_V+C_f·H_{C_f}]}{c_{DA,i}·t_{DA,i}-c_{DA,i}·t_{DA,e}}},$$

(5)

where:

$V$ is the volatiles content [kg/kg];

$H_V$ is the low heating value of the volatiles [kJ/kg];

$C_f$ is the fixed carbon content [kg/kg];

$H_{C_f}$ is the low heating value of fixed carbon [kJ/kg].

The values from the equations are 2491 kJ/kg—latent heat of water vaporization; 4.19 kJ/kg·K⁻¹—specific heat of water; and 1.88 kJ/Nm³·K⁻¹—the specific heat of the drying agent.

The following values were admitted for the calculations: $H_V$ = 12,000 kJ/kg, respectively, and $H_{C_f}$= 30,000 kJ/kg

Considering the case of the volatiles’ combustion, the following calculation model of their ignition process in the entrance area of the rotating kiln was made (Figure 9).

The ignition limit concentrations for the gaseous components in volatiles vary as follows: H₂ = 4.1% ÷ 73%, CO = 12.5% ÷ 75%, CH₄ = 5.3% ÷ 13.0% [36].

The volatile composition measured on the sludge from the Constanța Nord wastewater treatment plant is H₂ = 28%; CO = 40%; CH₄ = 2%; and N₂ = 30% [30]. For the concentration of volatiles at the exit from the rotary kiln, the zero value was adopted.

For a mixture of n combustible gases with r participation, the lower limit $\left(x^i\right)$ and upper limit $\left(x^s\right)$ of the ignition concentration is given by the relationship [36]:

$$x^{i,s}={\displaystyle \frac{\sum r_n}{\sum \frac{r_n}{{\left(x^{i,s}\right)}_n}}},$$

(6)

With the above data, $x^i=6.7\mathrm{\%}$ and $x^s=65.8\mathrm{\%}$.

For both concentrations of volatiles in the drying agent, at average values, $x^i=14\mathrm{\%}$ and $x^s=17\mathrm{\%}$ are within ignition limits.

The power at the exit of the technological drying cycle is given by the formula:

$$P_t={\displaystyle \frac{g_{DA,i}·M_{sludge}}{3600}}·1.29·\left(t_{DA,i}-t_{DA,e}\right),$$

(7)

where $M_{sludge}$ [kg/h] is the sludge mass flow rate that is entering into the preheater.

Considering the efficiency of the thermal installation of $\eta$ = 90%, the natural gas flow rate used in the installation is calculated:

$$B_{NG}={\displaystyle \frac{P_t}{\eta ·H_{NG}}},$$

(8)

where $H_{NG}=\mathrm{35,700}$ kW/Nm³ is the low heating value of natural gas.

2.4. Energy Recovery by an ORC System

After the rotary kiln, the combustion gases exit with a temperature of 200 °C. Usually, this hot stream of combustion gases is released into the atmosphere, and the available thermal energy is lost. The recovered heat can be used to obtain hot water for heating or domestic needs or to obtain electricity via an organic Rankine cycle (ORC) system coupled with an electric generator (EG). In this paper, the second option will be explored. Thus, a basic ORC-EG system will be used to recover the heat available in the combustion gases to convert it into work and finally into electricity.

A basic configuration of an ORC-EG system is further detailed. The schematic of the basic ORC-EG system and its corresponding thermodynamic cycle are presented in Figure 10a and 10b, respectively.

As described in Figure 10a, the basic ORC-EG system is composed of four main parts: pump, evaporator, expander, and condenser. The working fluid of the basic ORC-EG system enters the pump having thermodynamic state 1, having a condensing temperature t_con and a condensing pressure p_con. From Figure 10b, one can notice that, during process 1–2_r, which takes place in the pump, the pressure increases from p_con to the evaporating pressure p_evap. The working fluid is discharged from the pump in thermodynamic state 2_r. Thermodynamic state 2_r is obtained by assuming an isentropic efficiency of the pump of η_P = 0.8, which leads to a higher entropy in state 2_r compared to state 2 at the exit of the pump. Process 1–2 is the theoretical one, while process 1–2_r is closer to the real process occurring in the pump. This will lead to higher work input in process 1–2_r compared to the theoretical process 1–2. Having thermodynamic state 2_r, the working fluid enters the evaporator. In the evaporator, based on the heat rejected by the combustion gases coming from the rotary kiln, the temperature of the working fluid increases from t_2r to t₃ during heating process 2_r–3. Heating process 2_r–3 is followed by evaporating process 3–4 and finally by superheating process 4–5. On the combustion gases side, the temperature decreases from t₈ to t₁₁. After the superheating process, the working fluid having state 5 enters the expander, where expansion process 5–6_r takes place. During the expansion process, the pressure drops from p_evap to p_con, and thus, work is obtained. The expander is coupled with the electric generator (EG), and electricity is generated. The thermodynamic state 6_r is obtained by assuming an isentropic efficiency of the expander, η_D = 0.7, which leads to a higher entropy in state 6_r compared to state 6 at the exit of the expander. Process 5–6 is the theoretical one, while process 5–6_r is closer to the real process occurring in the expander. This will lead to lower work production in process 5–6_r compared to theoretical process 5–6. The working fluid enters the condenser at state 6_r, where it cools from state 6_r to state 7 during desuperheating process 6_r–7. In state 7, condensing process 7–1 begins. In the present work, a water-cooled condenser is considered. The cooling water enters the condenser having temperature t₁₂ and exists at temperature t₁₄. With state 1, the working fluid enters the pump, and the thermodynamic cycle repeats. In Figure 10a, a liquid receiver has been added in case variable speed pumps are used in order to adapt the system to partial operational loads [37,38].

To evaluate the electricity output when a basic ORC-EG system is used to recover the heat from combustion gases coming from the rotary kiln of a wastewater treatment plant, a thermodynamic study is carried out. The steps needed to carry out a thermodynamic study involving waste heat recovery by means of a basic ORC system have been presented by the authors in papers [37,38], where the thermodynamic studies are carried out considering waste heat recovery from the exhaust gases of internal combustion engines. The principles applied in the present paper to evaluate the heat available in the combustion gases and the electricity output of the basic ORC-EG system are similar to those found in papers [37,38]. For the present study, the authors have developed a program using Engineering Equation Solver Software (EES) [39].

The input data needed to develop the thermodynamic study in EES is presented in

Table 1. Input data for the thermodynamic study.

No. crt	Quantity	Symbol	Value	Unit
1.	Temperature of the combustion gases at the evaporator inlet	t8	200	°C
2.	Pressure of the combustion gases	p8	1.1	bar
3.	Volume fraction of CO2 from combustion gases	rCO2	0.09	-
4.	Volume fraction of N2 from combustion gases	rN2	0.67	-
5.	Volume fraction of H2O from combustion gases	rH2O	0.24	-
6.	Critical temperature	tcr	Depending on the working fluid	°C
7.	Temperature difference between the critical temperature and the evaporating temperature	Δtv	20	°C
8.	Isentropic efficiency of the pump	ηP	0.8	-
9.	Isentropic efficiency of the expander	ηD	0.7	-
10.	Pressure of condenser cooling water	pw	1.1	bar
11.	Temperature of the water at the condenser inlet	t12	25	°C
12.	Temperature difference between t13 and t12	Δtw	5	°C
13.	Temperature difference between t7 and t13	Δtw,con	5	°C

.

The first step of the thermodynamic study is to evaluate the waste heat available in the combustion gases, which are considered to be an ideal gas mixture. In order to do so, the partial pressures of the CO₂, N_2, and H₂O components of the combustion gases have been evaluated. The temperature of the combustion gases at the evaporator outlet has been determined depending on the saturation temperature of the water vapor, which depends on its partial pressure. For the present study, the resulting saturation temperature of the water vapor is 66.1 °C. This value is the lowest acceptable limit for the combustion gases temperature at the evaporator outlet. In order to avoid condensation inside the evaporator on the combustion gas side in different operation conditions, the temperature of the combustion gases at the exit of the evaporator, t₁₁, has been set to 100 °C.

Applying the specific thermodynamic equations for ideal gas mixtures, the component mass fractions, mass-specific heat at constant pressure, and finally, the heat available for recovery from the combustion gases have been determined. The results obtained, considering 1 kg of sludge dried in the rotary kiln, are presented in Section 3.3.

3. Results and Discussion

3.1. Analysis of Sewage Sludge

The proximate analysis for the sewage sludge, in terms of total water content (W_t), volatile matters (V), ash content (A) and fixed carbon content (C_f), for the samples collected after the technological process described in Section 1.3 and for the residue obtained after thermal-drying process, are presented in

Table 2. Technical analysis of samples before and after drying process.

Sample	Wt (%)	A (%)	V (%)	Cf (%)
Sewage sludge before thermal-drying process	73.92	11.40	14.05	0.63
Solid residue after thermal-drying process	2.85	56.78	30.14	10.23

.

The results for the first sample show that combustion or fermentation technologies are not recommended, and that thermal neutralization with flue gases is the only effective method.

The variation between the moisture content at the time of sample collection from the wastewater treatment plant (85%) [31] and the one determined in the laboratory is due to transportation and storage time before laboratory analysis. Performing the experiment in the laboratory, the drying process was assumed in a fixed layer, which inhibited the air–gas thermo-diffusion. As a result, the analysis of the residue obtained indicated an average value for ash of 56.78% and 10.23% for fixed carbon content.

Even for these values for the fixed carbon content, the residue has no fuel potential, and it is recommended to be used as a raw material in construction. High ash content reduces the calorific value of sludge and impacts thermal processing efficiency [40]. Ash usually includes silica, aluminum, iron oxides, and other minerals, which influence its suitability for reuse in construction materials or as a soil amendment.

After the drying process, the mass of the sewage sludge is reduced by 84%. Fixed-bed drying led to agglutination of the residue as granules (Figure 7c), that’s why it is recommended for industrial facilities to use a rotary kiln for this process.

The results of chemical analysis of the ash are presented in

Table 3. Chemical analysis of residue after thermal treatment.

Element	Weight [%]
Fe	31.56
Ca	21.14
Si	18.18
Px	8.75
K	7.43
Al	4.19
Sx	1.66
Ti	1.66
Mg	0.797
Zn	0.599
Sr	0.439
Cl	0.315
Ba	0.298
Mn	0.267
Br	0.243
Cu	0.233
Zr	0.227
Mo	0.0723
Pb	0.0595
Cr	0.0528
Ni	0.0367
V	0.0354
Cd	0.0237

.

Several elements, such as Cu, Zn, Fe, Mn, Mo, Ni, Mg, Ca, and B, are classified as essential mineral nutrients for plant growth and productivity. At relatively low concentrations, these elements can enhance specific cellular functions in plants. The elements that are frequently found to contaminate agricultural soils and cause toxic effects at elevated levels in plants include Cd, Pb, Cr, As, Hg, Ni, Cu, and Zn [41,42].

Our measured data reveal a quantum of 1% of the sum of these toxic elements, meaning that the ash can be used as fertilizer in agriculture without danger for public health.

Meanwhile, another industry that can use the sewage sludge ash (SSA) is the cement industry, which can use the SiO₂, Al₂O₃, and Fe₂O₃ in cement clinker production, whilst its CaO content may also lead to minor reductions in CO₂ emissions by lowering the calcareous material content [43].

Although the chemical analysis presented in Table 3 does not highlight the chemical compounds mentioned above, the share of the elements Fe, Ca, Si, Al, and Mg in the ash amounts to 75.9%, which recommends it as a raw material for the manufacture of cement.

3.2. Gas Flow Rate and Power Calculated for Different Sludge Flow Rates

Using the formulas from Section 2.4, the gas flow rate required for three sludge flow rates, respectively, $M_{sludge}=\left(5000;\mathrm{10,000};\mathrm{20,000}\right)$ kg/h, is determined. The moisture content of the sludge has values in the range of $∆W=0.6–0.7$. The results for the cases considered are presented in

Table 4. The natural gas flow rate and power calculated for case 1.

${\mathit{M}}_{\mathit{s}\mathit{l}\mathit{u}\mathit{d}\mathit{g}\mathit{e}}$ [kg/h]	$∆\mathit{W}$ [kg/kg]	${\mathit{g}}_{\mathit{D}\mathit{A}.\mathit{i}}$ [Nm3/kg]	${\mathit{P}}_{\mathit{t}}$ [kW]	${\mathit{B}}_{\mathit{N}\mathit{G}}$ [Nm3/h]
5000	0.6	3.380	3331	373.22
	0.65	3.630	3577	400.82
	0.7	3.880	3824	428.42
10,000	0.6	3.380	6662	746.44
	0.65	3.630	7155	801.64
	0.7	3.880	7647	856.84
20,000	0.6	3.380	13,324	1492.88
	0.65	3.630	14,309	1603.28
	0.7	3.880	15,295	1713.68

(case 1) and

Table 5. The natural gas flow rate and power calculated for case 2.

${\mathit{M}}_{\mathit{s}\mathit{l}\mathit{u}\mathit{d}\mathit{g}\mathit{e}}$ [kg/h]	$∆\mathit{W}$ [kg/kg]	${\mathit{g}}_{\mathit{D}\mathit{A}.\mathit{i}}$ [Nm3/kg]	${\mathit{P}}_{\mathit{t}}$ [kW]	${\mathit{B}}_{\mathit{N}\mathit{G}}$ [Nm3/h]
5000	0.6	2.433	2398	268.65
	0.65	2.802	2761	309.32
	0.7	3.170	3124	349.99
10,000	0.6	2.433	4795	537.29
	0.65	2.802	5521	618.64
	0.7	3.170	6247	699.98
20,000	0.6	2.433	9591	1074.58
	0.65	2.802	11,043	1237.27
	0.7	3.170	12,495	1399.96

(case 2).

It is noticed that the consumption of natural gas is lower in the presence of volatiles for the same amount of waste sludge at the same moisture concentrations. The consumption of natural gas increases with the increase in sludge water content.

For the considered sludge flow rates, a rotary kiln from the cement industry was chosen for drying (its combustion installation was abandoned). The cement rotary kiln, without the combustion plant, that turned into a rotary dryer has the following characteristics presented in

Table 6. The characteristics of the rotary kiln proposed [44].

	Capacity [kg/h]
	5000–10,000	>20,000
Specification [m]	Φ2.8/2.5 × 44	Φ3 × 48
Thrust roller type	mechanical	mechanical or hydraulic
Kiln rotation speed [rpm]	0.445–2.22	0.676–3.38
Motor
Model	ZSN4-225-21	Z2-101
Rated power [kW]	55	100
Speed regulation range [rpm]	~1000	~1500
Reducer
Model	ZS145-1-1	ZS145-1-1
Overall speed ratio	49–63	50

.

3.3. Results Regarding Energy Recovery by the ORC-EG System

In

Table 7. Specific computed data for the combustion gases considering 1 kg of sludge dried in the rotary kiln.

No. crt.	Quantity	Symbol	Value	Unit
1.	Volume of combustion gases for 1 kg of sludge dried in the rotary kiln	V	3	m3
2.	Partial pressure of CO2 from combustion gases	pCO2	0.099	bar
3.	Partial pressure of N2 from combustion gases	pN2	0.737	bar
4.	Partial pressure of H2O from combustion gases	pH2O	0.264	bar
5.	Mass fraction of CO2 from combustion gases	gCO2	0.147	-
6.	Mass fraction of N2 from combustion gases	gN2	0.694	-
7.	Mass fraction of H2O from combustion gases	gH2O	0.159	-
8.	Mass-specific heat at constant pressure for CO2	cp,CO2	0.958	kJ/(kg·K)
9.	Mass-specific heat at constant pressure for N2	cp,N2	1.047	kJ/(kg·K)
10.	Mass-specific heat at constant pressure for H2O	cp,H2O	1.931	kJ/(kg·K)
11.	Mass-specific heat at constant pressure for the combustion gases	cp,g	1.175	kJ/(kg·K)
12.	Mass of combustion gases for 1 kg of sludge dried in the rotary kiln	mg	2.268	kg
13.	Density of combustion gases	ρg	0.756	kg/m3
14.	Heat recovered from combustion gases for 1 kg of sludge dried in the rotary kiln	Qrec	266.5	kJ/kg of sludge

, the mass-specific heat at constant pressure for the components has been determined, considering the medium temperature between the inlet (t₈) and outlet (t₁₁) of the combustion gases from the evaporator.

The second step of the thermodynamic study is to evaluate the net power output, the thermal efficiency, and the electricity production of the basic ORC-EG system. In this regard, two working fluids have been considered: R600a and R1336mzz(Z). These two working fluids have been chosen as an example based on the Regulation (EU) 2024/573 of the European Parliament and the Council [45], and the main selection criteria were the environmental ones related to the global warming potential (GWP). The working fluid R600a has a GWP of 0, while working fluid R1336mzz(Z) has a GWP of 2.08. The thermodynamic properties of these two working fluids are available in the EES software and are not in focus in the present work. Information about the thermodynamic properties and safety issues for working fluids R600a and R1336mzz(Z) can be found in paper [46]. The main goal of the thermodynamic study is to calculate the amount of electricity that can be obtained by using a basic ORC-EG system to recover the waste heat available in the combustion gases after the rotary kiln in the case of a wastewater treatment plant. The results of the thermodynamic study, in terms of heat flux recovered from the combustion gases (${\dot{Q}}_{rec}$), superheating degree (ΔT_sh), working fluid mass flow rate (${\dot{m}}_{wf}$), net power output of the basic ORC system (P_net), thermal efficiency of the basic ORC system (η_t), condenser cooling water mass flow rate (${\dot{m}}_w$), and electricity production (E_p), have been generated considering three different sludge flow rates of 5000 kg/h, 10,000 kg/h, and 20,000 kg/h, respectively, and are presented in

Table 8. Results of the thermodynamic study.

Working Fluid	Sludge Mass Flow Rate [kg/h]	${\dot{\mathit{Q}}}_{\mathit{r}\mathit{e}\mathit{c}}$ [kW]	ΔTsh [°C]	${\dot{\mathit{m}}}_{\mathit{w}\mathit{f}}$ [kg/s]	Pnet [kW]	ηt [-]	${\dot{\mathit{m}}}_{\mathit{w}}$ [kg/s]	Ep [kWh/day]
R600a	5000	370.2	23	0.788	38.84	0.1049	3.601	913.4
	10,000	740.4		1.577	77.68		7.201	1826.8
	20,000	1480.7		3.154	155.36		14.402	3653.3
R1336mzz(Z)	5000	370.2	1	1.483	46.86	0.1266	3.531	1102.3
	10,000	740.3		2.966	93.73		7.063	2204.3
	20,000	1480.7		5.932	187.46		14.127	4409.0

.

The superheating degree presented in Table 8 is the superheating degree for which the maximum thermal efficiency is achieved. The superheating degree is defined as the temperature difference between t₅ and t₄ (Figure 10b). As mentioned in papers [37,38], the superheating degree and the maximum thermal efficiency are given by the fluid properties and not as a result of a mathematical optimization process. Not all fluids display this behavior. Details regarding the subject can be found in paper [47]. In order to evaluate the electricity production Ep, a conversion factor from work to electricity at the electric generator level of 0.98 has been assumed, and the wastewater treatment plant operates, theoretically, 24 h out of 24.

From Table 8, one can notice that as the sludge mass flow rate increases and so does the heat flux recovered from the combustion gases. The working fluid’s mass flow rate increases with the increase in the sludge mass flow rate, and its value is highly affected by the working fluid type. The net power production of the basic ORC system increases with the increase of the sludge mass flow rate. The same applies to the condenser’s cooling water mass flow rate. The thermal efficiency of the basic ORC system is not affected by the sludge mass flow rate, and it depends on the selected working fluid. Finally, the electricity production (E_p) is revealed. The values obtained for the E_p are not negligible at all. The electricity obtained from the basic ORC-EG system could be used locally or injected into the national grid. One of the main disadvantages of ORC systems in general is their low thermal efficiency. Nevertheless, when applied for waste heat recovery, the ORC-EG technology can lead to higher overall efficiencies and, why not, to financial advantages. The Ep obtained with the basic ORC-EG system could be subtracted from the overall energy demand of the wastewater treatment plant, lowering the carbon footprint and increasing its sustainability.

At the exit of the evaporator, the combustion gases have a temperature of t₁₁ = 100 °C. Their temperature is high enough to obtain hot water for local use in the wastewater treatment plant. This action could increase even more the overall efficiency of the wastewater treatment plant. The subject of overall efficiency of the wastewater treatment plant will be approached in future work.

4. Conclusions

In conclusion, the management and energy valorization of municipal sewage sludge represent critical components in advancing a circular economy. The integration of innovative technologies for resource recovery can address the environmental challenges posed by traditional wastewater treatment methods.

The research drives innovation in sludge treatment technologies, making it more efficient by using the ORC system for energy recovery after the drying process. These results can be applied to other industries with similar waste challenges, broadening the impact of circular economy practices.

The paper presents the motivation for the subject developed in the context of the circular economy. The direct link between climate change as a result of anthropogenic action on the environment is emphasized through the increase in temperatures that affect the technological processes in wastewater treatment plants; the decrease in water resources from natural sources as a result of the extension of drought phenomenon; and the increase in the population that requests larger quantities of water but also the increase in the volumes of processed wastewater and sewage sludge. In these conditions, it is normal for the technology of wastewater treatment plants to respond to the conditions of sustainable development in which the circular economy paradigm leads to a reduction in energy consumption, the reuse of water by adding a tertiary treatment of wastewater with the possibility of making it potable, and the recycling of sewage sludge, as well as its transformation into useful materials for other economic sectors.

The methods of treatment and management of sewage sludge are presented, highlighting the results obtained and disseminated by the authors of the various works discussed in the bibliographic references. A case study was conducted on the Constanta Nord sewage treatment plant in Romania designed to process municipal wastewater for 250,000 inhabitants.

The material analyzed in the study is the sludge obtained after the centrifugation stage of the technological process at WWTP Constanta Nord. The samples were dehydrated and thermally dried. The industrial installation proposed for the technological process uses, as drying equipment (compared to current methods), a rotary kiln similar to those in the coal or cement industry.

Through the thermal-drying process proposed, the moisture content of the analyzed material decreased from 73.92% to 2.85%, and its mass was reduced by 84%. In the technological scheme presented at the exit of the sludge-drying installation, an organic Rankine cycle (ORC) thermodynamic system is integrated, coupled with an electric generator whose role is to recover the thermal energy from the drying process and transform it into electrical and thermal energy. Using the calculation methodology, for 20,000 kg/h of sludge with the characteristics of the sludge from WWTP Constanta Nord and the humidity considered to be 0.7, a power of 12,495 kW is obtained with a gas consumption of ~1400 Nm³/h in the situation where volatiles in the drying process are also considered. Also, the energy recovered in the ORC-EG for 20,000 kg/h of sewage sludge is 4409 kWh/day if the temperature variation ${∆T}_{ab}$ is 1 °C and the working fluid is R136mzz(z).

Another research path was represented by the chemical analysis of the solid byproduct resulting from neutralization, in order to select the future use of a raw material in industry and agriculture. The results were encouraging, highlighting the valuable elements that could enhance plant growth (as fertilizers) or be used as raw materials in the construction industry. By closing the resource loop, minimizing waste, and reducing pollution, the research not only reinforces the sustainability of sewage treatment but also encourages a broader shift towards a resource-efficient, low-impact economy that aligns with global sustainability initiatives.

In essence, by integrating energy recovery into sewage treatment processes, this research supports a transition to a circular, sustainable society that values waste as a resource, minimizes environmental impacts, and fosters resilience.

For further research development, a full energy balance and techno-economic analysis of the proposed system for neutralizing the sewage sludge need to be conducted for some common existing wastewater treatment plants.