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Article

Circular Economy in Wastewater Treatment Plants—Potential Opportunities for Biogenic Elements Recovery

1
Department of Industrial Products Quality and Chemistry, Gdynia Maritime University, ul. Morska 81-87, 81-225 Gdynia, Poland
2
Wastewater Treatment Plant ‘Swarzewo’, ul. Wladyslawowska 84, 84-100 Swarzewo, Poland
*
Author to whom correspondence should be addressed.
Water 2023, 15(21), 3857; https://doi.org/10.3390/w15213857
Submission received: 19 September 2023 / Revised: 29 October 2023 / Accepted: 3 November 2023 / Published: 6 November 2023

Abstract

:
Technologies used in municipal wastewater treatment plants (WWTPs) allow the recovery of energy and valuable elements (phosphorus, nitrogen, and organic carbon) for the soil. This article presents, in schematic form, the carbon, nitrogen, and phosphorus cycling in a WWTP with a load of 70,000 Population Equivalent and develops a spreadsheet to estimate their recovery. Biogas generation enables the recovery of 1126 Mg of organic carbon per year and the generation of 12.6 GWh of energy. The most rational form of organic waste recycling is the production of compost with fertilizing parameters, but efforts should be made to reduce iron compounds in its composition. It has been estimated that compost production provides the recovery of 30% of carbon, 98% of phosphorus, and 18% of nitrogen from the streams of these elements entering the WWTP. The possibility of partially replacing the iron coagulants used to precipitate phosphorus with waste magnesium salt is presented, leading to the precipitation of struvite, which is well absorbed by plants. The article presents the advantages of combining sewage treatment with organic waste management in WWTPs. The developed spreadsheet allows for the control of energy recovery through the quantitative selection of organic waste for fermentation.

1. Introduction

The circular economy (CE) aims to transform waste into resources that can be reused or reintroduced into the production process [1,2]. It introduces new directions for the development of many sectors of the economy, including the wastewater treatment sector. There are strong relations between the developmental goals of this sector and the CE concept [3,4]. They mainly concern sustainable development, saving resources and energy, the use of renewable energy sources, as well as preventing the progressive eutrophication of waters. The implementation of a CE in municipal wastewater treatment plants (WWTPs) can lead to the recovery of nutrients and the production of green energy. Thus, wastewater can be considered not as a waste but as a valuable non-conventional resource containing nutrients such as nitrogen, phosphorus, and organic carbon matter [5,6,7,8]. Recovering components from wastewater and waste for reuse has both economic and environmental benefits. By increasing the circularity of the use of waste streams, these trends also fit into the principles of the bioeconomy [9,10].
One of the challenges facing the closed-loop economy is the need to design numerical indicators to quantify efficiency in waste recycling. For this purpose, closed-loop economy indicators are useful [11,12]. In terms of wastewater treatment plant processes, this is more difficult than in industrial production. For this reason, most publications only present concepts for implementing the circular economy in WWTPs [13,14,15,16,17]. On the other hand, works presenting quantification of the degree of approximation of the wastewater treatment process to the circular economy mode are few [18].
The Ellen MacArthur Foundation presented a graphic form of the CE that distinguishes between technical resources, which can be recycled through closed loops, and biological resources, which can be recycled through the biosphere [19]. The biological cycle focuses on, among others, the recycling of nutrients into the biosphere through composting or anaerobic digestion, leading to biogas production. The proper circulation of nutrients plays a key role in this cycle. Otherwise, shortages may occur near the biomass source, and there may be an excess of nutrients in the ecosystems where the biomass is used or consumed. Nutrient deficiencies reduce soil fertility and impede healthy plant growth, while nutrient excess causes environmental problems as a result of the release of nutrients into groundwater and surface water [20,21]. The lack of circularity for phosphorous is associated with the limited resources of phosphate rocks. This raw material has been identified by the European Committee as critical in view of its high economic importance and high risk of supply [22]. Equally important is the correct circulation of nitrogen and sustainable management, based on reducing high-energy processes in its cycle [23,24]. At present, nitrogen management is not energy-efficient. Biological mineralization of ammonium compounds in WWTPs is part of the nitrogen cycle, the final product of which is the emission of gaseous nitrogen into the atmosphere. This is accompanied by a significant energy input, just as in the synthesis of nitrogen fertilizers, for which the reverse process occurs—nitrogen uptake from the atmosphere. In addition, the release of reactive nitrogen into the environment affects water and air quality, the greenhouse gas balance, ecosystems, biodiversity, and soil quality [25,26]. By looking for ways to recover nitrogen from wastewater and digestate it, energy consumption can be reduced, with significant environmental benefits [27].
The wastewater sector also has great potential for the recovery of organic carbon. This most common biogenic element is characterized by its significant availability in organic compounds. However, it should be noted that it is very important that organic carbon compounds are returned to the soil in a form that enriches it and improves its structure. In the pursuit of sustainability, it is also important to reduce the carbon footprint, which defines the cumulative greenhouse gas emissions generated by WWTPs, and offsetting the carbon footprint was found to be an appropriate strategy for the WWTPs studied to enable the transition to low-carbon operations [28]. WWTPs could reduce their carbon footprint with practices such as selling electricity and fertilizers. Using sewage sludge to produce compost could also be an effective way to sequester carbon, storing it in the soil instead of releasing it into the air. Composting can increase water holding capacity, restore soil health, and improve air quality. Once compost is applied, soil health increases as microorganisms grow and become more numerous. These microorganisms sequester carbon in the soil through photosynthesis.
In WWTPs, organic waste and municipal sewage sludge can be treated together via anaerobic digestion [29,30,31]. The sustainability, environmental friendliness, and economic benefits of this process should be noted. These benefits arise from both the generation of green energy through biogas production and the recovery of important nutrients, such as phosphorus or nitrogen, from the treated matter [5,32]. Thus, the implementation of the sludge co-fermentation process with organic waste can be considered an important element capable of closing material cycles in modern water and wastewater management.
Organic matter, which can be potentially processed in WWTP and should be subject to organic recycling, is processed in three main waste streams: (i) organic matter that originally constitutes food and after digestion ends up in wastewater; (ii) kitchen waste (from the municipal waste group); and (iii) disposed waste from green areas or vegetable wastes, constituting a margin of agricultural production. Among them, the least developed segregation system has biodegradable organic waste generated in households. They account for 30–40% of all municipal waste and are only marginally segregated at source and treated with soil matter recovery. Between 2017 and 2021, the amount of separately collected kitchen biowaste in Poland remained at a very low level of 7–12% [33,34]. In addition to reducing the amount of waste sent to landfills, efficient separate waste collection also contributes to the recovery of recyclable materials and reduces transportation costs. The kitchen waste stream, currently largely going to mixed waste and incinerated, in combination with the wastewater stream represents an untapped source of organic matter. Increased recycling of organic biodegradable waste, especially kitchen waste, can be ensured through sustainable regional waste management, utilizing the potential of WWTPs for its management [35,36].
The technology used in WWTP allows the recovery of valuable elements for the soil (phosphorus, nitrogen, and organic carbon). Sandy soils cover approximately 900 million ha globally. Agriculture on these soils often requires high nutrient inputs, especially for high-yielding crops [37]. Recycling of organic matter is made possible by the WWTP’s production of organic fertilizer from composted mixtures of sewage sludge and plant waste. The application of such fertilizers improves soil structure and enriches it with biogenic elements. The inherent condition for organic recycling in WWTP is waste segregation at the source of its generation and an appropriate sludge quality standard. It eliminates the cost of separating plastics or other contaminants that would affect the quality and safety of the resulting compost or digestate. The possibility of contamination of the sludge treated in WWTPs with toxic substances (e.g., pharmaceuticals, microplastics, or heavy metals) should also be taken into account [38,39,40]. Due to the lack of sufficient cleanliness in the segregation of solid organic waste from residents, WWTPs usually refrain from processing the biodegradable kitchen waste stream. Thus, it limits the achievement of high organic matter reduction rates in the region. Consequently, the continuous education of residents and employees of industrial plants in order to promote pro-ecological behavior plays an important role [8].
To achieve the goals of the CE, proper circulation of biogenic elements is required. In WWTPs, the technologies enabling phosphorus recovery are also being increasingly implemented. Digested, or composted, sewage sludge effectively improves the soil, as it can improve the physical and nutritional properties of the soil, enrich it with biogenic elements, and reduce agriculture’s reliance on synthetic fertilizers. However, these benefits depend on the quality of the sludge added to the soil. Increasingly, the content of micropollutants and toxic substances in treated sludge is being analyzed [41,42,43,44]. However, little attention is paid to the bioavailability of the phosphorus compounds they contain. Standard analyses of compost to assess its fertilizer suitability do not distinguish between biologically active phosphorus and hard-soluble forms of inorganic phosphorus (e.g., iron phosphate). This problem was more extensively reported in the work of Torri et al. [45]. Overflow aspects are also monitored in soil science articles [46]. A pro-ecological solution to this problem may be to replace some of the iron coagulant with magnesium compounds. One method that allows both phosphorus and nitrogen to be recovered is the precipitation of ammonium magnesium phosphate (struvite) from the fermentation leachate [47,48,49,50,51,52]. Due to the content of two biogenic elements and their slow release into the soil, the precipitated crystals have desirable fertilizing features [53,54]. Moreover, the presence of magnesium makes struvite very useful for some types of plants [55].
The paper proposes a novel approach to balancing the circulation of biologically active matter in a wastewater treatment plant. The work presented here includes, for the first time, an estimate of the nutrient content of each WWTP stream. The purpose of this work is the following: (i) to create a model to estimate the rate of nutrient recovery in wastewater treatment plants using co-digestion of surplus sludge with organic waste and composting; (ii) to develop a spreadsheet to determine the content of nutrients (C, P, and N) in wastewater and biomass at each stage of the wastewater treatment plant process line; and (iii) to estimate the amount of organic matter rich in readily available nutrients that can be returned to the environment as organic fertilizer.

2. Methods

2.1. Model Wastewater Treatment Plant

The conditions for the model municipal wastewater treatment plant (MWWTP) were developed on the basis of the technology used at the ‘Swarzewo’ WWTP (54°77′ N, 18°41′ E). Municipal wastewater flows into the treatment plant at a rate of 2,500,000 m3/year. The average annual number of inhabitants served by the ‘Swarzewo’ WWTP is 68,000. These data take into account both permanent residents and people staying in the area during periods of increased tourism. The process sequence in the wastewater treatment section and the section related to the treatment of sludge and organic waste is as follows:
Mechanical partraw wastewater (stream 1) is pre-treated in the mechanical part of the treatment plant, from which the preliminary sludge (stream 2) is sent for biogas production in closed digestion chambers.
Biological partwastewater after mechanical pre-treatment (stream 11) flows into the SBR-type biological reactors enriched with secondary settling tanks, where biological decomposition of organic compounds takes place with the production of excessive sludge (stream 13), which is directed to the digestion chamber. Simultaneous precipitation of phosphorus compounds with an iron coagulant also takes place in the SBR reactors. Therefore, the excessive sludge leaving the biological section contains hardly soluble iron phosphate.
Closed digesters—accept the following waste for processing: primary sludge (stream 2), excessive sludge (stream 13), sludge from other WWTPs, and wastes generated from the pretreatment of wastewater in local food industry plants (stream 3). The biogas separated from the digesters (stream 16) is burned in cogeneration units, producing electricity (with an average efficiency of 2.7 kWh/m3 of biogas) and heat (with an average efficiency of 3 kWh/m3 of biogas).
Composting plant—accepts digestate from closed digesters (stream 5), mechanically dehydrated to 20% dry matter (stream 7), and organic solid waste in the form of straw, branches, and leaves (stream 4). The composting plant produces organic fertilizer in thermal prisms (stream 9). The leachates from the digesters (stream 6), flowing directly from the digestion sludge dewatering station, are treated in the main line of the biological part of the MWWTP.
The scheme of MWWTP also included modern solutions not present in the ‘Swarzewo’ WWTP process line, allowing the recovery of phosphorus and nitrogen in the form of struvite from leachates (stream 10). It was also estimated how the enrichment of organic waste processed in methane fermentation with kitchen waste (stream 3) would affect the individual streams of the nutrient cycle (and the amount of biogas produced).

2.2. Circulation of Biogenic Elements in the MWWTP

An analysis of data related to the circulation of nutrients (nitrogen, phosphorus, and organic carbon) in ‘Swarzewo’ WWTP for the years 2017–2020 was carried out. A spreadsheet balancing the circulation of nutrients in the MWWTP was created on the basis of data obtained from measurements taken at 13 points in the ‘Swarzewo’ WWTP, supplemented with literature data.
The analysis of mass fluxes occurring in the treatment plant included the following parameters: mass flows, total dry matter (t.d.m.), volatile organic matter (v.m.), dry mineral matter (d.m.), total phosphorus (Ptot), total nitrogen (Ntot), organic carbon (OWO), the content of methane (CH4), and carbon dioxide (CO2) in the biogas.

2.3. Analytical Methods and Measurement

  • Phosphate concentrations were measured using commercial assays from Merck (Spectroquant® 114543). All colorimetric analyses were performed using a Spectroquant Vega 400 spectrophotometer (Merck, Darmstad, Germany).
  • Nitrate and ammonium concentrations were measured using the AN-ISE sc Ammonium and Nitrate ion-selective probe (Hach Lange, Dusseldorf, Germany).
  • The total dry matter (t.d.m.), volatile organic matter (v.m.), and dry mineral matter (d.m.) of samples were determined according to standard methods [56].
  • Mass flows were measured using flow meters installed at WWTP ‘Swarzewo’: electromagnetic flow meters for sewage and thickened sludge (type DMA20 DMA20-AAABA1Z) and biogas flow meters with simultaneous determination of composition (type Proline Prosonic Flow B 200) (Endress &Houser, Frankfurt, Germany).

3. Results

On the basis of the collected measurement data and the sequence of wastewater and waste treatment processes at the ‘Swarzewo’ WWTP, a scheme of biogenic element cycling was drawn up for the MWWTP. The scheme is presented in Figure 1.
The basis for calculating the amount of biodegradable organic matter that can be utilized at ‘Swarzewo’ WWTP was an analysis of the potential amount of organic solid waste generated by the agglomeration’s residents. In Poland, it is estimated that the amount of biodegradable waste generated by one person per year is 87 kg/year, but the collection efficiency is only 35% (for collective housing) to 65% (for single-family housing) [25]. It was assumed that the MWWTP serves 70,000 inhabitants and processes externally delivered organic waste, including sewage sludge from other treatment plants, straw, grease, and municipal waste, including kitchen waste, acquired through a system of detailed segregation. It has been assumed that 5000 Mg of kitchen waste generated in the region can be treated annually at the MWWTP using co-digestion with sludge and composting processes, due to the collection efficiency. Detailed mass flow calculations for carbon (C), phosphorus (P), and nitrogen (N) made on the basis of data recorded at the ‘Swarzewo’ WWTP in 2021 are shown in Table S1 attached in Supplementary Materials. The masses of nutrients outgoing from the WWTP (shown in Figure 1 with stream numbers 8, 9, 14, 15, and 17) calculated for the input masses (streams 1, 3, and 4), taking into account the different quantities of kitchen waste extracted for digestion, are summarized in Table 1.
The results of the parameters obtained in the ‘Swarzewo’ WWTP in 2021 are summarized in Part A of Table 1. The table also shows the recovery of organic matter in a closed loop, achieved through the production of fertilizer (stream 9) and biogas (stream 17). Organic recycling at the ‘Swarzewo’ WWTP included 55.7 Mg of nitrogen and 67 Mg of phosphorus through the production of full-grade compost with fertilizer properties (Table 1, Part A). Organic carbon recovery was 1708.4 Mg (in the form of biogas and compost combined). The 1,503,182 m3 of biogas generated in the digesters at ‘Swarzewo’ WWTP ensures its self-sufficiency in terms of energy. And even more than that, it allows 665 MWh of electricity to be sold to the energy network. The production of fertilizer-certified compost ensures organic recycling of 18% of the nitrogen, 98% of the phosphorus, and 30% of the organic carbon contained in the substrates treated at the WWTP. However, it should be taken into account that a significant (approximately 75%) part of the phosphorus present in the compost is bonded in the form of iron phosphate, which is not assimilable by plants. Only 25% of the phosphorus remains in the prisms in the form of organic compounds from the sludge and organic waste.
The results obtained in the ‘Swarzewo’ WWTP do not take into account co-digestion of kitchen waste or struvite precipitation. These processes are not currently used in the process line of this treatment plant. For the MWWTP, a modification to the spreadsheet was made, increasing the mass of digested waste by 5000 Mg of kitchen waste and introducing values for struvite (stream 8), precipitated from digestate leachate (Table S2, presented in Supplementary Materials). The predicted nutrient masses for the aforementioned streams are shown in Part B of Table 1. Management of kitchen waste from the entire agglomeration and recovery of nutrients through struvite precipitation allow the recovery of 73.8 Mg of nitrogen, 82.6 Mg of phosphorus, and 2110.9 Mg of organic carbon, respectively. The 2,103,182 m3 of biogas that could be obtained (Table 1, Part B) allows the generation of approximately 12.6 GWh of energy. In a target model MWWTP, using struvite precipitation, the plant-available mineral phosphorus content of the organic fertilizer can be increased to 42%, reducing the proportion of practically insoluble iron phosphate.

4. Discussion

The data in Table 1 indicate a significant role of organic waste input on biogas production volume. Analyzing the results obtained, it can be concluded that the use of kitchen waste in the co-digestion process will increase biogas production by about 120 m3 per year from each Mg of such waste (400 m3 from 1 Mg of dry matter). In Poland, 13.1 million Mg of municipal waste was generated in 2020, of which approximately 30% was kitchen waste, which is a free, biodegradable source of energy [33,34]. Selectively separating them and subjecting them to co-digestion makes it possible, hypothetically, to produce 470 million m3 of biogas, or 1270 GWh of electricity nationwide. Due to the ban on landfilling waste whose calorific value exceeds 6 MJ/kg, kitchen waste is a fraction for which municipalities must find alternative disposal methods. Using the technological facilities of a MWWTP to dispose of them would increase the level of recycling achieved in the municipality, with a concomitant gain in the form of energy production and useful fertilizer. However, it should be noted that the use of large quantities of organic waste entails changes in the treatment plant process line. Pre-treatment of the waste (e.g., maceration) is necessary, and a separate line for the production of organic pulp from the waste is desirable. The increased concentration of phosphorus and nitrogen compounds present in the leachate should also be taken into account. This requires increased aeration of the effluent (for nitrogen oxidation) and the use of a higher dose of coagulant (for phosphorus precipitation). To reduce this energy-intensive process, it is very beneficial to use the struvite precipitation process on digestate leachate, as it is a salt that binds both phosphorus and ammonium nitrogen. In parallel with the precipitation of a ton of phosphorus, 0.5 tons of ammonium nitrogen are also precipitated. This results in significant energy savings, consumed in the WWTP for aeration (the oxidation of 1 Mg of ammonia nitrogen requires 4.5 Mg of oxygen, the generation of which consumes 2.25 MWh). Achieving energy savings at the WWTP is one of the objectives of the realization of a closed-loop economy. In WWTPs, reductions in energy consumption are most often achieved using energy-saving installations or sludge combustion [57,58,59]. However, over the last few decades, particular emphasis has been placed on solutions that obtain energy and nutrients through processes such as recycling, reuse, and recovery [12,17]. The implementation of struvite precipitation in the leachate stream fits into such a trend.
The use of digester-generated sludge (from organic waste and sewage sludge) for compost production can significantly improve the organic waste reduction balance in the municipality. Maintaining clean ‘at source’ segregation of waste creates the opportunity to turn compost into product-like fertilizer, thereby increasing the level of CE.
If possible, care should be taken to limit the nitrogen losses that occur during composting. In the compost prisms, a high temperature of up to 75 °C is generated, at which most organic compounds decompose. Proteins in the ammonification process break down into mineralized ammonium nitrogen compounds, which are released into the atmosphere in the absence of sufficient oxygen. The nitrogen loss from the volatilization of ammonia into the atmosphere (stream 8 N) can be up to 60% of the nitrogen input to the treatment plant. In order to retain the nitrogen in the fertilizer produced and to reduce odor nuisance, the prisms must be intensively aerated so that the ammonium nitrogen compounds are oxidized to nitrates, thus interrupting the loss of nitrogen in the volatile compounds. Nitrogen retention in the compost can also be aided by the use of biochar. It is a sludge-derived, carbon-rich by-product formed by pyrolysis. It is also used as a fertilizer, carbon stabilizer sorbent, or mesoporous catalyst [60,61,62]. High temperatures also sterilize the sludge and reduce the water content of the pile. Within a short period of time (6 to 8 weeks), the pile takes on an earthy form, the water content drops from 70% to 45%, and the composting process stops. During composting, there is a loss of carbon to the atmosphere in the form of CO2 and nitrogen in the form of ammonia (streams N8 and C8). It is estimated that the loss of carbon in this process reaches 25% of the total stream [63].
An important issue in the production of organic fertilizer is the bioavailability of the nutrients it contains. For fertilizer produced in WWTPs, the way in which phosphorus is bound in the sludge is of particular importance. Phosphorus removal is most often carried out in WWTPs by biological incorporation into the sludge biomass, assisted by a chemical precipitation process. Chemical precipitation ensures an effective reduction in phosphate concentration, but at the same time generates the production of a chemical deposit of little use, which is discharged together with excessive sludge (stream P13 in Figure 1). Using iron compounds as a precipitating agent, the resulting sludge binds phosphorus in the form of iron phosphate, which is difficult for plants to access [45]. When this compound enters the compost with the digestate (stream P7) and is subsequently converted to fertilizer (stream P9), it passes into the soil, increasing the accumulation of permanently bound phosphorus in the soil [64,65,66]. The insoluble inorganic compounds can be present in the soil for many years but are not available to plants and have very little effect on soil fertility. Thus, the presence of iron phosphate in the compost produced limits its fertilizer usefulness [46]. Phosphorus precipitation in the form of ammonium-magnesium phosphate (struvite), which is a compound with very good fertilizing properties, is increasingly used in wastewater treatment plants [48,67]. Advanced struvite precipitation technologies allow simultaneous 90% phosphorus reduction and 40% nitrogen reduction, thus increasing the recycling rate of both of these nutrients. Effective struvite precipitation requires a high concentration of phosphorus and nitrogen in solution, provided by the composition of the digestate leachate (streams N6 and P6) [68]. In the MWWTP concept proposed in Figure 1, which includes both digesters and a composting plant, the precipitated struvite can be mixed with compost, increasing the plant-available phosphorus, nitrogen, and magnesium content of the resulting fertilizer (streams N9 and P9). The use of a struvite precipitation and co-digestion facility for 5000 Mg of kitchen waste in the MWWTP technologies (Table 1 Part B) increases the recovery of biogenic elements in the compost to values of 89.9 Mg N/year, 82.6 Mg P/year, and 1198 Mg C/year. This represents an increase of 33% in nitrogen, 23% in phosphorus, and 32.7% in organic carbon compared to the results obtained at the MWWTP before the changes (Table 1).
Increasingly, attention is being paid to the presence of heavy metal ions in the compost and also to the possibility of their binding in struvite [69]. Therefore, the presented solution is most favorable for wastewater treatment plants located in a region of low uptake, for which the burden of heavy metals is insignificant. The composting process is very important in sludge treatment due to the production of structure-forming material for the soil, but it also serves to reduce the leaching of heavy metals from the sludge [70]. Recently, more and more attention has been paid to biochar-based fertilizers, which are also characterized by a slow release of nutrients. Biochar shows numerous benefits from its use as a fertilizer; among other things, it helps improve soil quality [71,72,73]. It can also be used to absorb biogens from leachate [74].
At the ‘Swarzewo’ WWTP, 460 Mg of coagulant in the form of hydrated iron sulphate is used annually to remove phosphorus. The annual purchase cost of the coagulant is PLN 326,000. If kitchen waste is included in the digestion process, these figures will increase significantly. Low-cost waste material can be used to remove phosphorus in the form of magnesium ammonium phosphate (struvite), thus reducing the use of iron coagulant without increasing current expenditure. For example, magnesium chloride, which is a waste product from the extraction of table salt and is currently used in large quantities as road salt, can be used. Its effectiveness has been confirmed in previous studies, both on laboratory and semi-technical scales [75]. It was estimated that the use of a magnesium substitute would increase the percentage of bioavailable phosphorus in the compost from 25% to 42%. The complete elimination of iron coagulant is not advisable as the phosphate concentration in the main treatment stream (aeration chamber) is too low to use magnesium compounds. However, magnesium salt can be used to remove phosphorus from digestate leachates. It has been estimated that such a modification would reduce iron coagulant consumption by 50–80% (on a process-wide basis). High struvite precipitation efficiency can be achieved by increasing the phosphate concentration in the leachate, e.g., by increasing the concentration of sludge dry matter in the digester, reducing the dilution of the digested waste, or increasing the thickening of excess and pre-sludge. The magnesium dosing process must be preceded by a separate analysis for each WWTP, as excess magnesium in the wastewater circuit may result in the crystallization of struvite in undesirable locations (e.g., in pipes discharging digested sludge).

5. Conclusions

In an era of depleting natural resources and increasing environmental pollution, it is becoming imperative to implement clean, closed-loop technologies that take into account the recovery of raw materials from waste. Drawing up a diagram of the biogenic element cycle in the MWWTP enables the proper selection of waste for co-digestion. It is presented that the process of co-fermentation of kitchen waste obtained from residents can result in an increase in biogas and therefore green energy production by 30%. Estimating the changes in nutrient concentrations in leachate resulting from the different compositions of the digestion charge facilitates the control of the treatment processes of the generated post-fermentation leachate. The analyses carried out made it possible to develop a spreadsheet to calculate and estimate changes in nutrient flow rates in the process line of a MWWTP. The spreadsheet allows an assessment of the extent to which nutrients are recovered in a closed loop, depending on the composition of the wastewater, the wastes used in co-digestion and the phosphorus precipitating reagent (coagulant). Thus, the developed spreadsheet can be a useful tool used to develop closed-loop indicators. Monitoring the flow of individual elements made it possible to determine the level of the content of a biologically unavailable form of phosphorus in the compost produced at WWTP. Partial replacement of iron coagulant with waste magnesium salt may lead to an even two-fold increase in the content of available phosphorus compounds in the compost.
On a national scale, the use of separately separated kitchen waste and the inclusion in waste management plans of cooperative biogas plants and local sewage treatment plants can have a very positive effect on the levels of organic recycling achieved. Comprehensive management of local waste also makes it possible to develop both processes for obtaining energy from bio-waste and an economy geared towards the regeneration of natural resources, especially soil.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15213857/s1, Table S1. Parameters of wastewater, organic waste (exluding kitchen waste and struvite precipitation) and biogas at the different stages of the MWWTP process line. Table S2. Parameters of wastewater, organic waste (including 5000 Mg kitchen waste and struvite precipitation) and biogas at the different stages of the MWWTP process line.

Author Contributions

A.D. and S.C.—conceptualization, methodology, formal analysis, investigation, resources, data curation, writing—original draft preparation, review and editing, visualization, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

The paper presents results developed in the scope of the research project “Monitoring and analysis of the impact of selected substances and materials in terms of environmental protection”, supported by Gdynia Maritime University (project grant no. WZNJ/2023/PZ/10).

Data Availability Statement

The data are available on request from the first or corresponding author.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

Ctotstream of total carbon
CEcircular economy
CH4methane
CO2carbon dioxide
CODchemical oxygen demand
d.m.mineral dry matter
DSdissolved substances
MWWTPmodel wastewater treatment plant
Ntotstream of total nitrogen
OWOorganic carbon
Ptotstream of total phosphorus
SBRanaerobic/aerobic sequencing batch reactor
t.d.m.total dry matter
v.m.volatile organic matter
WWTPwastewater treatment plant

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Figure 1. Diagram of the nutrient cycle in a model wastewater treatment plant.
Figure 1. Diagram of the nutrient cycle in a model wastewater treatment plant.
Water 15 03857 g001
Table 1. Nutrient masses entering and leaving the municipal wastewater treatment plant [Mg].
Table 1. Nutrient masses entering and leaving the municipal wastewater treatment plant [Mg].
Part A. Kitchen Waste Load 0 Mg
INPUTOUTPUT
Stream13489141516
N282.58510.5378131.455.7251660378
P41.824.32.268.30671.30068.3
C976.71166.2440.62583.580774237.5191.7805.32583.5
Recovery of agricultural fertilizer [Mg]5761Biogas recovery [m3]1,503,318
Part B. Kitchen Waste Load 5000 Mg
INPUTOUTPUT
Stream13489141516
N282.5107.513.9403.9139.373.825165.80403.9
P41.839.32.883.9082.61.30083.9
C976.71691.2584.53252.4886984.237.5217.81126.73252.4
Recovery of agricultural fertilizer [Mg]7642Biogas recovery [m3]2,103,318
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Dereszewska, A.; Cytawa, S. Circular Economy in Wastewater Treatment Plants—Potential Opportunities for Biogenic Elements Recovery. Water 2023, 15, 3857. https://doi.org/10.3390/w15213857

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Dereszewska A, Cytawa S. Circular Economy in Wastewater Treatment Plants—Potential Opportunities for Biogenic Elements Recovery. Water. 2023; 15(21):3857. https://doi.org/10.3390/w15213857

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Dereszewska, Alina, and Stanislaw Cytawa. 2023. "Circular Economy in Wastewater Treatment Plants—Potential Opportunities for Biogenic Elements Recovery" Water 15, no. 21: 3857. https://doi.org/10.3390/w15213857

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