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Sludge Management at the Kraków-Płaszów WWTP—Case Study

Faculty of Environmental Engineering and Energy, Cracow University of Technology, 31-155 Kraków, Poland
Doctoral School, Cracow University of Technology, 31-155 Kraków, Poland
Author to whom correspondence should be addressed.
Sustainability 2022, 14(13), 7982;
Received: 8 June 2022 / Revised: 26 June 2022 / Accepted: 27 June 2022 / Published: 30 June 2022
(This article belongs to the Section Environmental Sustainability and Applications)


Municipal wastewater treatment plants are good examples of facilities where the concept of a circular economy model can be effectively implemented by the recovery of energy as well as secondary and natural materials. That is why anaerobic co-digestion has become one of the most appealing renewable energy pathways and takes a key position within sludge-handling processes. This research looked into the feasibility of the utilization of water sludge from a water treatment plant in anaerobic co-digestion with sewage sludge. The experiments confirmed that anaerobic digestion of sewage sludge together with water sludge significantly improved fermentation gas (biogas) production. The best results were observed when water treatment sludge constituted 30% of the mass of sewage sludge (as volatile solids, VS). At this ratio, approximately 20% more biogas was produced in laboratory experiments compared to the biogas production from sewage sludge only. The results, once confirmed on a semi-technical scale, will help to develop a sequence of processes which would enhance biogas production. Both the technology and the final product offer a comprehensive solution for waste generated at water and wastewater treatment plants. The innovative approach allows for the use of various waste streams and their combined processing following the principle of the circular economy.

1. Introduction

The sludge generated during water treatment processes is a specific municipal waste that has been well-recognized in terms of physical and chemical characteristics. Therefore, more and more often such sludge is treated as a valuable source of micro/macronutrients and organic substances [1]. Its specific composition and properties determine the choice of appropriate methods of sludge handling and disposal. Typically, sludge from water treatment is used, e.g., for production of coagulants (recovery of raw materials), in the production of bricks or cement, and in agriculture and forestry.
The water treatment sludge can also be used in the production of bricks, roof tiles, tiles, and ceramic pipes [2]. Cement supplemented with ash, obtained after burning water sludge rich in aluminum compounds and silicates, is more durable and resistant to sulfate corrosion, as proved by Zdeb et al. and Luo et al. [3,4,5]. Sludge with iron coagulant changed the color of the produced bricks—they had an intense red color [6]. Furthermore, research showed that the coagulant can be recovered from the water treatment sludge by extraction with sulfuric acid [7].
Sludge may also be used in wastewater technology. However, these methods are expensive and in many cases difficult to implement [8,9,10]; sludge constituency, especially possible contaminants, should be recognized before a decision for practical use [11].
Water treatment sludge may take part in wastewater treatment processes. Mainly sludge with aluminum coagulant can be used to intensify the removal of chemical oxygen demand (COD) from sewage; a 15–20% removal of COD was observed at the water sludge dose of 18–20 mg Al dm−3 [11]. When the sludge with PAC (aluminium polychloride) coagulant at a dose of 15 g dm−3, was used in the UASB reactor, a 74% reduction in COD was observed [12]. Water treatment sludges can be used both at municipal and industrial wastewater treatment plants, as in [9,10]. They help to remove pollutants and heavy metals from wastewater. They were found effective in removing contaminants such as phosphorus, hydrogen sulfide, boron, fluorides, perchlorates, glyphosate, mercury, arsenic, lead, and selenium [13].
On the other hand, sludge from municipal wastewater treatment plants has high moisture and comes in various physical forms, and also the conductive materials from these sludges in anaerobic digestion processes can improve methane production by reducing lag phases, contributing to a more stable operation of the systems [14]. The properties of sewage sludge depend on the type of sewage and the treatment technology used [15], and the addition of water treatment sludges may increase the overall efficiency of the process by supplementation of necessary trace elements [16]. These improvements have no adverse effect on the combustibility of sludge [17]. Currently, anaerobic digestion is the most approachable sludge disposal process at large wastewater treatment plants; however, it requires case-by-case studies on technical applicability [18,19,20], especially with respect to potential contamination by, e.g., metals [21]. Biogas may play a significant role as an energy carrier as it is flexible in use and it is storable [22], so it is supposed to play vital role in a renewable energy castor, even in regions rich in fossil fuels [23,24]. This process enables energy release and recovery providing a good source of clean energy (biogas) [25]. In recent years, biogas installations, where waste is stabilized in anaerobic conditions, have also become more and more popular. Various products, so called “co-substrates”, are used in these installations. The co-fermentation process proceeds in the same way as a classic fermentation, i.e., it comprises four stages: hydrolysis, acidogenesis, acetateogenesis, and methanogenesis [26,27,28,29,30]. The course of a fermentation process depends on the type of substrate, temperature, pH, time, presence of toxic substances, substrate loading, concentration of easily biodegradable components, and appropriate conditions for biomass growth. Chandra R. also stated that anaerobic co-fermentation, as a complex process, depends on the quantitative and qualitative characteristics of the substrate [31].
So far, the co-fermentation of water and sewage sludge has not been thoroughly investigated. The tests have been performed in both laboratory conditions and on a semi-technical scale. According to the literature data, conditioning of sewage sludge with water treatment sludge may be a potentially promising solution, although detailed recommendations and procedures enabling use of this sludge-handling method on a wider scale still have to be developed. In general, it can be stated that a higher percentage of water treatment sludge in a sewage sludge sample improves its dewatering properties [9,10,32,33]. Therefore, the authors tried to study a possible utilization of sludge from the water treatment processes together with sewage sludge in anaerobic digestion [34,35,36]. Sludge from water treatment (with aluminum salt and with iron salt) was used in primary sludge digestion. The results showed that the efficiency of the digestion process decreased by about 19% with the use of iron coagulant (dose range 10–40% of dry solids (TS)) and by 45–55% with the use of aluminum coagulant (dose 40% of TS) [32]. Additionally, fermentation studies confirmed that at 20 and 80% volumetric fractions of water sludge, a low reduction in organic solids and low biogas production were achieved; at an 80% share, the process simply ceased [37]. Other studies have investigated biogas production at an aluminum concentration of 100 mg·dm−3; at this value, biogas production decreased by 10% [38]. On the other hand, during the co-digestion of water sludge and food waste (water sludge dose of 6.6 mg∙dm−3), an increase in the biogas volume by 58–68% was obtained compared to the control sample.
The main goal of the work was to develop a technology for an integrated treatment of the sludge produced in a system with sustainable water and sewage management, i.e., water treatment sludge and sewage sludge.

2. Materials and Methods

2.1. Substrate

2.1.1. Digested Sludge

The samples of digested sludge were collected at the municipal wastewater treatment plant Płaszów-Kraków. The samples were immediately transferred to the laboratory of the Cracow University of Technology (Krakow, Poland) and stored at 4 °C.
The Płaszów Wastewater Treatment Plant (Płaszów WWTP) combines mechanical and biological treatment as well as a sewage sludge processing line with biogas production. It was designed for 680 thousand PE. According to the water permit, the average daily wastewater flow in a dry weather period is 165,000 m3·d−1. Pre-treatment takes place at the fine screens where large solids carried by wastewater are removed and then in aerated grit chambers, where a removal of sand and floating solids takes place. After the grit chambers, wastewater goes to primary settling tanks, where remaining suspended solids settle down (primary sludge) and are transported to the sludge processing line. The wastewater from primary settling tanks passes to the biological stage comprising five biological reactors. In biological reactors, biological treatment with so-called activated sludge is carried out; activated sludge is an active biomass full of microorganisms that remove organic compounds as well as nitrogen and phosphorus from wastewater. Next treatment units are secondary settling tanks, where the activated sludge is separated from wastewater. The sludge settles down and is partly returned to the reactor (recirculation) and partly discharged, as an excess sludge, to the sludge handling line. The effluent from secondary settling tanks is discharged to the receiving body—the Drwina river. Both primary and excess sludge are anaerobically digested in fermentation chambers (WKF). The fermentation product (biogas) is collected from the WKF and fed to the biogas line. The digested sludge passes to the dewatering station and then to the STUO, i.e., the sludge thermal treatment station. The thermal utilization technology in the PYROFLUID® fluidized bed reactor was used for the thermal utilization of wastewater sludge. The design of the PYROFLUID® reactor utilizes the technology of a fluidized bed. The sludge fed to the reactor is introduced to the sand bed, where the temperature reaches 750 °C. Due to turbulence within the sand bed and a high temperature, water evaporates and the remaining dry mass of the solids is distributed over the entire surface of the bed. The thermal sludge conversion process begins in the fluidized bed itself, but it also continues in the combustion chamber. The temperature of the exit gases in the chamber exceeds 850 °C, while the residence time exceeds 2 s. Under such process conditions, the organic fraction of the sludge is completely oxidized and its content in the remaining ashes stays below 3%.

2.1.2. Water Treatment Sludge

Water treatment sludge was collected at the municipal water treatment plant; the current water production is 100,000 m3·d−1. The source of raw water is a reservoir with a capacity of 127 million m3 and aluminum salts, under the brand name PAX XL 19F (polyaluminium chloride), are used as a coagulant. Sludge samples were collected with a bucket, directly from the sludge drying beds The sludge samples were stored in similar conditions as the sewage sludge samples.

2.2. Experiments

2.2.1. Laboratory Experiments—Sample Analysis for Biomethane Potential Tests

The impact of water sludge (collected on a seasonal basis) on methane fermentation and sewage sludge dewatering properties was analyzed for the following mixing ratios: 1:1, 1:0.5, 1:0.3, 1:0.2, and 1:0.1 (1.0 g VS of sludge per x g of VS of water sludge). For comparison, some respirometric tests were also carried out on the digested sludge samples (control samples). The study on methanogenic activity of biomass lasted about 21 days and followed 21 days of the laboratory tests.
Batch (respirometric) tests are one of the tools developed to estimate methanogenic activity. They are used to measure methane production potential of the tested substrate and to estimate the rate of substrate degradation [39,40]. Biochemical activity is also a good tool to assess the possible impact of toxic components of the substrate on the fermentation process.
A unit biogas production and methanogenic activity (AKT) were determined using batch tests; they were verified based on the volume of methane produced in the tested samples. Biogas production was expressed in m3 of methane per 1 kg of dry organic solids (VS) removed during anaerobic digestion. On the other hand, AKT values were expressed as g of CODCH4 (broken down to methane), and they were related to 1 g of VS per 1 day.

2.2.2. Pilot Studies

In general, pilot studies were conducted to verify the laboratory tests. They took place in an installation (Figure 1) similar to the one located at the existing municipal wastewater treatment plant, where a full-scale sewage sludge fermentation process is in operation. The studies lasted 14 days. The installation of a capacity of 1300 L was located in the existing building with a full access to hot water and substrates. It was designed to duplicate the real conditions and dynamics that occur in full-scale processes. To guarantee an efficient and stable biogas production, some parameters of the methane fermentation process had to be maintained, i.e., reactor volumetric loading, retention time, and uniform mixing of a fermentation liquid. Initially, only sewage sludge was tested at the pilot installation; these were control samples needed to check installation operation conditions and also provide information necessary to compare the fermentation efficiency of the digested sludge and the sludge mixture. The actual sludge mixture comprised sewage sludge and water sludge fed to the installation at a certain ratio determined in laboratory tests. Once the pilot installation started its operation, the fermentation process temperature, and readings from the gas meter (volume of biogas gas produced) were recorded three times a day.

3. Results

3.1. Characteristics of Water Treatment Sludge

The water treatment sludge was analyzed during each of four seasons because the characteristic of raw water changes on a seasonal basis. The sludge showed a high variability of the organic content (VS); it varied in the range of 21–45% of dry solids (TS). Higher levels of organic matter in the sludge were observed in summer, while lower levels were observed in winter.
Figure 2 shows the XRF analysis of the water sludge. The following elements were detected in samples in all four seasons: Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, and Zn. The dominant elements in the water treatment sludge are silicon (36.9–47.9%), aluminum (25–33% of TS), and iron (12.2–15.1% of TS). The remaining elements were detected in trace amounts only.
The phase composition of the water treatment sludge was investigated using XRD diffraction. Figure 3 shows the diffraction patterns of the water treatment sludge. The sludge did not show an amorphous structure; it underwent the syneresis process (sludge aging), which may have worsened its filtration properties. In the sludge samples collected in spring and summer, iron phosphates (FePO4) and silicon oxides (SiO2) in crystalline form were detected. The compounds formed a hexagonal system. In autumn, additionally, calcium carbonate (CaCO3) in the form of crystalline hexagonal pyramids was observed, apart from FePO4 and SiO2. On the other hand, in winter, the sludge from water treatment contained silicon oxide (SiO2) and kaolinite (Al2Si2O5(OH)4), both in a crystalline form.

3.2. Laboratory Experiments

In order to study the impact of water treatment sludge on biogas production, the results from the 3-year research period were analyzed. They included the tests done at the following mixing ratios 1:0.1, 1:0.2, 1:0.3, 1:0.5, and 1:1. The test results were compared with the results for the sewage sludge itself (0). Since the paper focuses mostly on the pilot studies, the laboratory tests were only used to select the mixing ratio for these studies. Figure 4 shows the distribution of the unit biogas production in m3 per 1 kg of VS removed. The average biogas production for individual water sludge samples was in the range of 0.23–0.32 m3·kg−1 VS rem. The highest value was observed for a dose of 0.3 g VS, while the lowest one was for 1.0 g VS. The difference between the control sample and 1:0.3 sample was significant—for the 1:0.3 sample it was 28% higher, on average.
Similar conclusions can be drawn from the distribution of methanogenic activity in the mixed samples (Figure 5). The average methanogenic activity of biomass for the samples with water sludge was in the range of 0.23–0.32 m3·kg−1 VS rem. The highest value was observed for 0.3 g VS, and the lowest was for 1.0 g VS. The difference between the control sample and 1:0.3 sample was approximately 22% (in favor of the 1:0.3 sample). Moreover, it can be concluded that the values for all ratios were close, which proves the consistency of the test results.
Therefore, the laboratory tests confirmed that addition of water sludge to sewage sludge may improve the efficiency of the sewage sludge fermentation process. In general, Figure 4 shows that there is a maximum dose of water sludge (with the highest volume of biogas gas) beyond which biogas production decreases; it is 0.3 g VS of water sludge mixed with 1.0 g VS of sewage sludge. It was also found that the water sludge improves the methanogenic activity of the sludge (Figure 5). However, the doses over 0.3 g VS of water sludge per 1.0 g VS of sewage sludge reduced this parameter.

3.3. Pilot Studies

The laboratory results helped to develop the pilot plant operation regime. The pilot studies were conducted for the samples mixed with water treatment sludge (mixing ratio 1:0.3) and for the digested sludge only (control sample); each test was repeated three times. The parameters measured during the anaerobic process are summarized in the Table 1.
Figure 6 shows the biogas production during methane fermentation in the pilot installation for both the control sample and the mixed sample over 14 days. The sample with the mixed sludge produced more biogas. The highest volume of biogas for both samples was recorded on the first day of testing. Biogas production was also similar in both analyzed samples during the first two days of the process. Then, in following days, more biogas was produced in the mixed sludge samples.
Figure 7 shows the overall biogas production for the samples. The total volume of biogas produced over the 14 days of the operation with the mixed sludge sample was recorded. The fermentation curves for different substrates take different shapes. The biogas production for the control sample was the highest in the first two days and accounted for approx. 50% of the total biogas production over the 14 days. On the other hand, the curve presenting production of biogas for mixed sludge showed that daily increases of production were similar. The addition of water treatment sludge increased the efficiency of the anaerobic process by 21%. After 14 days of fermentation, the biogas production was 0.63 m3·kg−1 VS removed for the mixed sample and 0.52 m3·kg−1 VS removed for the control sample.

4. Discussion

The results show that it is possible to use water treatment sludge in sewage sludge processing to improve process efficiency. During the combined digestion of water and sewage sludge, beneficial energy effects were obtained, such as higher recovery of a calorific gas (methane) as a product of the methane fermentation process. The proposed method may be a good starting point for the development of procedures for the use of sludge co-digestion.
Xie, et al. [34] used sludge from water treatment (with aluminum salt and with iron salt) in primary sludge digestion. The results showed that the efficiency of the digestion process decreased by about 19% with the use of iron coagulant and by 45–55% with the use of aluminum coagulant [34]. Some researchers have also tried to use water sludge in the recovery of iron nanoparticles (sludge with iron salts); the nanoparticles were then used in co-digestion with slaughterhouse waste. It has been proved that a dose of iron nanoparticles of 9 mg·dm−3 increased the biogas yield by 37.6% compared to the control sample [41,42,43].
Therefore, it should be noted that there is no common ground between the researchers regarding the actual impact of water sludge on the anaerobic digestion process. On the one hand, sludge from water treatment processes contains some organic compounds and various micro- and macro elements, which are likely to increase the efficiency of anaerobic processing of sewage sludge. On the other hand, many scientists point out the possible opposite effect caused by high concentrations of aluminum and iron salts in water treatment sludge, which may have a toxic effect on the anaerobic process.
The literature review classifies processing of sludge from water treatment plants as a complex process and states that its current understanding is not as broad as in the case of wastewater sludge. Selection of an appropriate method of water sludge disposal involves both economic and technical decisions, so there is a need for technologies that provide an explicit solution to this problem.

5. Conclusions

  • In the experimental studies, mixing and co-fermentation of water and sewage sludge had a positive effect on biogas production. Efficiency of the anaerobic stabilization of the sludge mixture was evaluated by sludge methanogenic potential. It was determined by a volume of the fermentation gas produced and the methanogenic activity of microorganisms. The laboratory analysis proved that the highest production of biogas was observed in the sludge sample with 30% water sludge (as sludge VS); in the samples with a higher water sludge proportion, a decrease in the process efficiency was observed.
  • The pilot studies also confirmed that anaerobic stabilization of sewage sludge mixed with water sludge improved the process efficiency (more biogas produced). The pilot scale experiments copied the real conditions and dynamics of the full-scale process. The research enabled us to determine the parameters of the methane fermentation process, i.e., unit biogas production or reduction of an organic content in the sludge. This way proper parameters of co-fermentation in a technical scale can be selected.
  • Application of water sludge in the processes of sewage sludge handling may have a positive impact on the management of waste generated in municipal water and sewage facilities (e.g., changes in sludge recovery and disposal technology).

Author Contributions

The authors (M.C.-R., J.G., D.P.), in equal parts, contributed to the creation of the paper in the following areas: conceptualization by methodology, software, validation, formal analysis, investigation, resources, data curation, writing-original draft preparation, writing-review and editing, visualization up to supervision, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript.


This research and the APC were funded by: NCBiR, grant number POIR.04.01.02-00-0032/17 and the Polish National Agency for Academic Exchange within the framework of the grant: E-mobility and sustainable materials and technologies EMMAT, grant number: PPI/APM/2018/1/00027/U/001.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.


The research was carried out as part of the project no. POIR.04.01.02-00-0032/17 “Innovative technologies of waste recovery and processing and revitalization of contaminated areas in the municipal circulation economy system”—co-financed by the European Union from the funds of the European Regional Development Fund and the National Center for Research and Development and also as a part of project of the Polish National Agency for Academic Exchange within the framework of the grant: E-mobility and sustainable materials and technologies EMMAT, grant number: PPI/APM/2018/1/00027/U/001.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationship that could have influenced the work reported in this paper.


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Figure 1. Pilot plant for anaerobic studies at the municipal wastewater treatment plant.
Figure 1. Pilot plant for anaerobic studies at the municipal wastewater treatment plant.
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Figure 2. XRF analysis of water treatment sludge.
Figure 2. XRF analysis of water treatment sludge.
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Figure 3. Diffractogram of water treatment sludge samples.
Figure 3. Diffractogram of water treatment sludge samples.
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Figure 4. Distribution of unit biogas production in m3·kg−1 of VS removed (for individual mixing ratios).
Figure 4. Distribution of unit biogas production in m3·kg−1 of VS removed (for individual mixing ratios).
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Figure 5. Distribution of methanogenic activity of biomass in g COD for individual mixing ratios.
Figure 5. Distribution of methanogenic activity of biomass in g COD for individual mixing ratios.
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Figure 6. Daily production of fermentation gas.
Figure 6. Daily production of fermentation gas.
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Figure 7. Unit production of fermentation gas in the analyzed samples.
Figure 7. Unit production of fermentation gas in the analyzed samples.
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Table 1. Physical and chemical parameters measured during anaerobic tests obtained during pilot studies.
Table 1. Physical and chemical parameters measured during anaerobic tests obtained during pilot studies.
ParametersUnitControl SampleMixed Sludge
VS% TS62.767.359.262.055.755.7
Volatile fatty acidsmg/dm310614236241001516445
Ammonium nitrogen% TS3.596.715.553.695.426.07
Total phosphorus% TS2.442.752.582.812.762.99
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Górka, J.; Cimochowicz-Rybicka, M.; Poproch, D. Sludge Management at the Kraków-Płaszów WWTP—Case Study. Sustainability 2022, 14, 7982.

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Górka J, Cimochowicz-Rybicka M, Poproch D. Sludge Management at the Kraków-Płaszów WWTP—Case Study. Sustainability. 2022; 14(13):7982.

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Górka, Justyna, Małgorzata Cimochowicz-Rybicka, and Dominika Poproch. 2022. "Sludge Management at the Kraków-Płaszów WWTP—Case Study" Sustainability 14, no. 13: 7982.

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