Impact of Conditioning Substances and Filtration Pressure on Dewatering Efficiency of Sewage Sludge

Sludge dewatering is a key element in sludge’s processing and management. Solid-phase separation, sludge volume reduction and the separation of a high-quality supernatant are the main tasks of the dewatering devices. The effectiveness of this task, in turn, results from the selection of the dewatering method and conditioning of the sludge before dewatering. The aim of the research was to determine the influence of conditioning substances (polyelectrolyte, fly ash and zeolite) on the efficiency of dewatering at variable filtration pressure. The use of polyelectrolyte in sludge conditioning was a solution that increased the efficiency of the filtration process. On the contrary, the use of structure-forming substances (fly ash and zeolite) slows the process but reduces the hydration of the filter cake. The same trends were obtained in dual conditioning combining dosing of polyelectrolyte and ash or zeolite. It was also shown that the dewatering efficiency of conditioned sludge with these two different substances (ash and zeolite) was the same. The use of zeolite in independent or dual conditioning was important for the quality of the supernatant.


Introduction
Municipal sewage sludge is a compressible material whose property can deteriorate the effects of drainage filtration. Laheij et al. proved that, at the filtration pressure of 300-400 kPa, it is possible to obtain 35-40% of the dry matter content in the filter cake [1]. It is also possible, depending on the type and characteristics of the sludge, that better dewatering effects can be obtained by increasing the pressure and filtration time [2]. Using variable pressure in the process of pressure filtration, a fundamental difference in final water content was obtained already at the stage of dewatering of raw sludge. Between the lowest (0.25 MPa) and the highest (0.75 MPa) applied pressure, a water content difference of up to 3.7% (80.4% for 0.25 MPa pressure and 76.7% for 0.75 MPa pressure) was obtained. It was also noted that, with increased pressure, filtration resistance decreased slightly from 1.13 × 10 13 m/kg (0.25 MPa) to 0.82 × 10 13 m/kg (0.75 MPa) [3]. A two-fold increase in filtration efficiency was also observed at the highest pressure. Zhao and Bache [2] found that, depending on the type and properties of the sludge, there is an optimal pressure and filtration time. This is confirmed by the studies of Wu et al. [4], which indicate the need to determine the threshold filtration pressure, beyond which the dewatering efficiency deteriorates.
The use of chemical additives, especially polyelectrolytes, for the conditioning of sewage sludge to ensure the proper functioning of many processes of separation of the solid phase from hydrated sludge is now of key importance. The type of polyelectrolyte used and its doses may vary during the examinations of the process. These changes may lead to its improper use. However, progress in understanding the processes of flocculation of suspensions and sludge allows for the development of a scientific strategy for optimization of these procedures. Polyelectrolytes have been used in the conditioning of sewage sludge due to two major mechanisms: neutralization of the surface charge and formation of intermolecular bridges [5]. Control of the polyelectrolyte dose is required for sludge conditioning, since overdosing increases costs and reduces the effects of sludge dewatering. The optimal dose of a polyelectrolyte is usually associated with a decrease in electrokinetic potential and the formation of large flocs of sludge [6]. There is no practical algorithm related to the properties of polyelectrolytes in order to calculate the most favorable dose at which the best dewatering effects are obtained. The dose is chosen experimentally. This is confirmed by the studies of other researchers [7][8][9]. When using polyelectrolytes to condition sewage sludge, important factors include the polyelectrolyte preparation method, concentration and flocculation time. Sorensen et al. [10] found that the high compressibility of the sludge conditioned with polyelectrolyte has an inhibitory effect on the filtration rate that is strongly correlated with the filtration pressure. Therefore, the main task in the dewatering of biological sludge is the reduction of compressibility, which enables the reduction of the process time and the increase of dry matter in the dewatered sludge. This task can be achieved by adding structure-forming substances, for example, cement, to the sludge before dewatering [3].
The efficiency of mechanical dewatering of sewage sludge mainly depends on sludge properties, e.g., the amount of extracellular polymeric substances, water content, size of sludge particles, the content of dry mass, etc. Before mechanical dewatering, sludge is subjected to preparation in order to change its properties and, consequently, improve conditions of water release. In practice, chemical conditioning is most often used. The role of the chemical mechanism in sewage sludge conditioning is to destroy the colloidal system of sludge and sludge flocculation by the addition of chemical substances such as polyelectrolyte, iron chlorides, etc. Chemical reagents used in sewage sludge conditioning are expensive and contribute significantly to the overall cost of sludge management. Therefore, it is critical to find a cost-effective and efficient way to condition sewage sludge. In the present study, the requirements are met by such chemical substances as fly ash and zeolite. Fly ash particles from hard coal combustion are characterized by their spherical and rough structure. The particle size of fly ash ranges from 0.01 to 0.1 mm. Large contents of SiO 2 and Al 2 O 3 from hard coal combustion present in ashes lead to the formation of active spots on the particle surfaces. Substantial specific area of fly ash particles leads to the adsorption of sludge particles and, consequently, the formation of greater aggregates and the increase in the efficiency of sewage sludge dewatering. A high amount of negatively charged sludge particles leads to the repulsion of particles, due to the electrostatic effect and formation of a stable system that impacts on low efficiency during dewatering of sewage sludge. Dosing the fly ashes to sewage sludge with negative electrokinetic potential leads to the increase in the value of 0 mV. A negative charge of sewage sludge is destabilized by positive charges of aluminum, iron or calcium silicates contained in the fly ash. Consequently, this leads to the destruction of stability of colloidal particles and their approaching to each other. With the intermolecular effect of Van der Waals forces, colloidal particles are clustered into greater aggregates, thus significantly improving the dewatering capacity of the conditioned sewage sludge and the clarity of the obtained supernatant.
As a result of the mechanical dewatering of sludge, a highly polluting supernatant is formed, especially with an organic substance. The supernatant from sludge treatment is recirculated, mainly without pretreatment, to the technological wastewater-treatment line. This has an adverse effect on the operating conditions of biological reactors by increasing the load of activated sludge. There are also unfavorable proportions between biologically degradable organic compounds and the content of nutrients [11]. The amount of supernatant is about 0.4-12% of the volume of wastewater flowing into the treatment plant [12][13][14], and even 20% [15,16]. In addition, the supernatant quality is highly variable and depends on the sludge characteristics and treatment methods [16][17][18][19][20][21][22]. For example, the supernatant from the primary sludge thickening is characterized by a high concentration of the dissolved fraction above 20% of total chemical oxygen demand (COD) of raw wastewater [23,24]. Supernatant from mechanical thickening of excessive sludge is characterized by a significant share of non-biodegradable fraction in the range from 69 to 87% of total COD [25].
The aim of the study was to compare the effect of polyelectrolyte and structureforming substances-conditioners (fly ash and zeolites)-on the efficiency of the pressure filtration process. In the study, we tested independent and dual methods of conditioning and the effect of pressure on the separation efficiency of solid phase and the quality of the supernatant.

Materials and Methods
Mixed sludge (primary and excess sludge in the volume ratio about 1:1) from the municipal wastewater treatment plant in Poland was used in the research. The average daily wastewater inflow to the treatment plant is 40,000 m 3 /d. The mixture was taken from a buffer tank before mechanical-dewatering-station belt presses operating in parallel.
Cationic Superfloc C-496 polyelectrolyte, zeolite 13X-B1 and fly ash were used as the conditioning substances. Fly ash, in sludge conditioning, is classified as a structureforming substance. Zeolite is classified similarly, bearing in mind that the final use of the waste zeolite is possible based on its physical properties and not the sorption capacity of commercial zeolite. The fly ash was derived from the combustion of hard coal (coal-fired power plant in Poland) ( Table 1). Zeolite 13X-B1 (JALON) is an adsorbent with large pores and high capacity, and it is especially effective for removing air pollutants ( Table 2). Superfloc C-494 high cationic polyelectrolyte (Brenntag) was used at a concentration of 0.1%. The optimal doses of polyelectrolyte C-496 were determined by the capillary suction time test and were 4.0 g/kgDS ± 0.2 g/kgDS (CSK = 38 s). Mixing of the tested sludge with conditioning agents was carried by using a paddle stirrer in a glass vessel. The sludge was stirred with the C-494 dose for 20 s and 250 rpm (G~600 s −1 ).
After sludge conditioning, mechanical dewatering was carried out in the pressure filtration process, for four different pressures, namely 0.25, 0.5, 0.75 and 1.0 MPa. The tested samples of conditioned sludge had a volume of 100 mL. Ten main combinations were tested in the study, taking into account a variable dose of conditioning substances, dual conditioning and variable filtration pressure (Table 3).
Dry solids (DSs), volatile solids (VSs), capillary suction time (CST) and filtration efficiency were measured according to standard methods [26]. In the supernatant, after sludge filtration, the soluble chemical oxygen demand fraction (COD S ) was measured. Hach test tubes and a spectrophotometer Hach Lange DR 5000 were used to measure COD S . The supernatant samples, before COD S measurement, were centrifuged for 15 min at 12,100 rcf and filtered through a membrane filter (0.45 µm, cellulose acetate).
In terms of mathematical analysis, the R2 coefficient of determination of linear regression (of the tested parameters as a function of pressure change) was calculated. In terms of statistical analysis, ANOVA was performed (Statistica 13.3). Combinations 1-10 were tested. Since the ANOVA was negative, that is, not all sets (combinations) were created equal, the Tukey's post hoc test was performed. The adopted alpha significance level was 0.05. Combinations that differed in the effects of conditioning are marked with letters (a, b, c, d and e) in the graphs.

Results
Research on the process of pressure filtration of sewage sludge was carried out for a number of selected combinations of sewage sludge conditioning. During the analysis of the results of the pressure filtration process, the greatest attention was focused on the parameter of filtration efficiency and the sludge dewatering effect directly related to efficiency. The diagrams below present a summary of the most favorable values achieved for individual methods of sludge conditioning.
The analysis of the results presented in the graph in Figure 1 reveals that the highest efficiency (ca. 9 kg/m 2 h) occurred for the sludge conditioned with polyelectrolyte dewatered at the highest filtration pressure. With sludge prepared by using polyelectrolyte, followed by the filtration process, the process efficiency increased with the increasing pressure. A similar tendency was observed when analyzing other samples subjected to filtration. Generally, the factor determining the increase in the filtration-process efficiency for each combination of sludge conditioning was the pressure used. The lowest filtration efficiency was documented for raw sludge samples (from 0.8 kg/m 2 h for the pressure of 0.25 MPa to 3.08 kg/m 2 h for the pressure of 1.0 MPa). Statistical analysis showed, that for all conditioning methods, except for polyelectrolyte conditioning (group b), the filtration efficiency was the same (group a). The graph in Figure 2 compares the results of the process efficiency where comb methods were used to condition sewage sludge before the filtration process, using conditioning factors: polyelectrolyte and zeolite or ash. Supporting zeolite conditioning with the previous application of polyelectrolyt sulted in an increase in filtration efficiency of sludge prepared in this way. During fi tion at the pressure of 1.0 Mpa of these sludge samples (polyelectrolyte + zeolite 0.6 g increase in process efficiency was obtained, from 4.01 to 5.54 kg/m 2 h. A similar tend was observed in the case of sludge conditioning with ash, preceded by polyelectro The graph in Figure 2 compares the results of the process efficiency where combined methods were used to condition sewage sludge before the filtration process, using two conditioning factors: polyelectrolyte and zeolite or ash. The graph in Figure 2 compares the results of the process efficiency where combin methods were used to condition sewage sludge before the filtration process, using t conditioning factors: polyelectrolyte and zeolite or ash. Supporting zeolite conditioning with the previous application of polyelectrolyte sulted in an increase in filtration efficiency of sludge prepared in this way. During fil tion at the pressure of 1.0 Mpa of these sludge samples (polyelectrolyte + zeolite 0.6 g) increase in process efficiency was obtained, from 4.01 to 5.54 kg/m 2 h. A similar tende was observed in the case of sludge conditioning with ash, preceded by polyelectrol dosing (polyelectrolyte + ash 1.2 g), with the growth ranging from 3.19 to 5.68 kg/m 2 h. T linear trend of increasing the process efficiency depending on the filtration pressure w Supporting zeolite conditioning with the previous application of polyelectrolyte resulted in an increase in filtration efficiency of sludge prepared in this way. During filtration at the pressure of 1.0 Mpa of these sludge samples (polyelectrolyte + zeolite 0.6 g), an increase in process efficiency was obtained, from 4.01 to 5.54 kg/m 2 h. A similar tendency was observed in the case of sludge conditioning with ash, preceded by polyelectrolyte dosing (polyelectrolyte + ash 1.2 g), with the growth ranging from 3.19 to 5.68 kg/m 2 h. The linear trend of increasing the process efficiency depending on the filtration pressure was also maintained. In the case of the dual-conditioning polyelectrolyte-forming substance, significant differences occurred in the case of using polyelectolite + zeolite 1.2 g (group d). The other dual conditioning combinations did not change the filtration efficiency (group c).
The effects of sewage sludge dewatering are presented in the diagrams in Figures 3 and 4. The highest water content of 93.2% (Figure 3; pressure used in the process, 0.25 MPa) was found for dewatering raw (unprepared) sludge. Lower water content was found for increasing the process pressure, with a final water content of 90.2% obtained for the highest pressure of 1.0 MPa. Using polyelectrolyte for conditioning as an independent conditioning factor, at the highest applied pressure, we found that the final water content was at the level of 84.2%. Significantly better dewatering effects were observed when 0.6 g of ash was added to the samples. Compared to water content in the raw sludge after the process (90.2%), the obtained water content of 81.3% was reduced by ca. 9 percentage points. Increasing the addition of ash (1.2 g) to the sludge before the filtration process led to the decrease of the final water content to the level of 79.6%. A similar pattern was observed in the case of addition of zeolite to the sludge before dewatering in the amount of 6 and 1.2 g. The final water content after the filtration process was 82.1 and 80.1%, respectively ( Figure 3). The other dual conditioning combinations did not change the filtration efficiency (gro c). The effects of sewage sludge dewatering are presented in the diagrams in Figure  and 4. The highest water content of 93.2% (Figure 3; pressure used in the process, 0 MPa) was found for dewatering raw (unprepared) sludge. Lower water content w found for increasing the process pressure, with a final water content of 90.2% obtained the highest pressure of 1.0 MPa. Using polyelectrolyte for conditioning as an independ conditioning factor, at the highest applied pressure, we found that the final water cont was at the level of 84.2%. Significantly better dewatering effects were observed when g of ash was added to the samples. Compared to water content in the raw sludge after process (90.2%), the obtained water content of 81.3% was reduced by ca. 9 percenta points. Increasing the addition of ash (1.2 g) to the sludge before the filtration process to the decrease of the final water content to the level of 79.6%. A similar pattern was o served in the case of addition of zeolite to the sludge before dewatering in the amount 6 and 1.2 g. The final water content after the filtration process was 82.1 and 80.1%, resp tively ( Figure 3).  Considering the effects of dewatering of sewage sludge conditioned by a combined method in different variations by using polyelectrolyte and then adding zeolite or ash, it was found that lower water content was obtained in each case compared to dewatering of sludge conditioned by using independent factors (Figure 4). The lowest final water content of 78.2% was obtained by dewatering of sludge samples conditioned by means of a combined method, using polyelectrolyte and 1.2 g of ash (Figure 4). Considering the effects of dewatering of sewage sludge conditioned by a combin method in different variations by using polyelectrolyte and then adding zeolite or ash was found that lower water content was obtained in each case compared to dewatering sludge conditioned by using independent factors (Figure 4). The lowest final water co tent of 78.2% was obtained by dewatering of sludge samples conditioned by means o combined method, using polyelectrolyte and 1.2 g of ash ( Figure 4).
Regardless of the independent or dual conditioning, the addition of structure-for ing substances did not have a statistically significant effect on the change in final hyd tion (group c).
In the supernatant after sludge dewatering, regardless of the filtration pressure, the were no clear changes in the concentration of organic compounds expressed as CODS 4 ( Figures 5 and 6). Conditioning of sludge with polyelectrolyte/zeolite/ash caused a d crease in CODS, along with an increase in the pressure used in the filtration process ( Figures 5 and 6). Through the use of zeolite for sludge conditioning, the best quality supernatant after dewatering was obtained. The CODS for raw sludge (non-conditione was on average 550 mgO2/L. The sludge conditioning by addition of zeolite at a dose 12 g/L resulted in a two-fold decrease in CODS to an average of 380 mgO2/L ( Figure 5). using dual conditioning polyelectrolyte + zeolite or ash (Figure 6), a very similar CO was obtained as in the case of using the single method ( Figure 5). The most advantageo combination of conditioning was the use of polyelectrolyte and zeolite (12 g/L), where a filtration pressure of 0.75 MPa, a CODS of 348 mg O2/L was obtained. Regardless of the independent or dual conditioning, the addition of structure-forming substances did not have a statistically significant effect on the change in final hydration (group c).
In the supernatant after sludge dewatering, regardless of the filtration pressure, there were no clear changes in the concentration of organic compounds expressed as COD S 4 ( Figures 5 and 6). Conditioning of sludge with polyelectrolyte/zeolite/ash caused a decrease in COD S , along with an increase in the pressure used in the filtration process ( Figures 5 and 6). Through the use of zeolite for sludge conditioning, the best quality of supernatant after dewatering was obtained. The COD S for raw sludge (non-conditioned) was on average 550 mgO 2 /L. The sludge conditioning by addition of zeolite at a dose of 12 g/L resulted in a two-fold decrease in CODS to an average of 380 mgO 2 /L ( Figure 5). By using dual conditioning polyelectrolyte + zeolite or ash ( Figure 6), a very similar COD S was obtained as in the case of using the single method ( Figure 5). The most advantageous combination of conditioning was the use of polyelectrolyte and zeolite (12 g/L), where, at a filtration pressure of 0.75 MPa, a COD S of 348 mg O 2 /L was obtained.
Statistically, the type and dose of the forming substance influenced the quality of sludge liquids. Generally, the use of zeolites in the independent or dual conditioning was significantly different from other methods, and the effect on the quality of the supernatant was different.  Statistically, the type and dose of the forming substance influenced the quality of sludge liquids. Generally, the use of zeolites in the independent or dual conditioning was significantly different from other methods, and the effect on the quality of the supernatant was different.

Discussion
Due to the specific properties of chemical substances added to sewage sludge that lead to destabilization of the colloidal systems of the sludge, mineral particles act as a core for aggregation of sludge flocs on them. Adding fly ash and zeolite led to the formation of microzones with sludge flocs concentrated around particles (cores) of the added chemical substances. This resulted in an increase of the floc size and the bridging of the remain-   Statistically, the type and dose of the forming substance influenced the quality of sludge liquids. Generally, the use of zeolites in the independent or dual conditioning was significantly different from other methods, and the effect on the quality of the supernatant was different.

Discussion
Due to the specific properties of chemical substances added to sewage sludge that lead to destabilization of the colloidal systems of the sludge, mineral particles act as a core for aggregation of sludge flocs on them. Adding fly ash and zeolite led to the formation of microzones with sludge flocs concentrated around particles (cores) of the added chemical substances. This resulted in an increase of the floc size and the bridging of the remaining microspheres, leading to the formation of large reinforced aggregates of sludge flocs

Discussion
Due to the specific properties of chemical substances added to sewage sludge that lead to destabilization of the colloidal systems of the sludge, mineral particles act as a core for aggregation of sludge flocs on them. Adding fly ash and zeolite led to the formation of microzones with sludge flocs concentrated around particles (cores) of the added chemical substances. This resulted in an increase of the floc size and the bridging of the remaining microspheres, leading to the formation of large reinforced aggregates of sludge flocs which were resistant to structural damage caused by high pressure during filtration. Apart from the destabilization of sludge floc systems, conditioning of sewage sludge with chemical substances used in the above examinations leads to the formation of their structure, which is a permeable and rigid structure that remains porous under high pressure and also changes sludge compressibility. Furthermore, the rough surface of fly ash and zeolite particles is likely to ensure more microchannels that stop solid particles in sewage sludge and enhance the release of free water, thus enhancing the dewatering effect [3].
At all times, the disposal of sewage sludge is a problem for sewage-treatment plants. Regardless of the adopted direction of sludge disposal, combustion or natural use, the most important thing is quick and effective dehydration, which allows the energy demand for water evaporation to be reduced. Theoretically, any waste that does not generate an additional pollutant load on the sludge can be used as a conditioner. The advantage of using energy waste for sludge conditioning is the possibility of recycling by-products of combustion (fly ash), as well as flue gas cleaning (waste zeolites), which, in countries based on coal energy, poses problems with proper disposal. The solution of adding structureforming substances also seems appropriate in the case of highly contaminated sludge, for which the only disposal option is incineration. A more advantageous solution that can be used for mineralized sludge (by adding fly ash or zeolites) is its use in the construction industry, as an environmentally safe method of sludge disposal [27]. It is estimated that, in the production of building materials such as bricks, ceramic materials and lightweight aggregates, it is possible to replace up to 20% of natural materials [28].

Conclusions
The use of polyelectrolyte in sludge conditioning and dewatering was the most advantageous solution in terms of the efficiency (process speed) of sludge pressure filtration. Much lower filtration efficiency was achieved during the dewatering of sludge conditioned with fly ash or zeolite. Dual conditioning based on dosing of polyelectrolyte and ash or zeolite also failed as a method of increasing the efficiency of the process. On the contrary, by the use of structure-forming substances, it was possible to increase the efficiency of solid-phase separation and to reduce the hydration of the filter cake. This phenomenon was correlated with an increase in filtration pressure from 0.25 to 1.0 MPa, as well as with a higher dosage of the forming substances. On the other hand, the zeolite-conditioning method made it possible to reduce contamination of the jet liquid from dewatering that results from the sorption capacity of this material. On the other hand, the zeolite-conditioning method made it possible to reduce the contamination of the supernatant, which results from the sorption capacity of this material. Finally, it should be stated that the use of forming substances in sludge conditioning (especially dual conditioning) intensified the removal of water from the sludge, but the process becomes slower, i.e., more time-consuming.
Author Contributions: M.K. was responsible for conceptualization, methodology, software, investigation, validation, data curation, writing-original draft, writing-review and editing, visualization and supervision. T.K. was responsible for investigation, validation, data curation, resources and writing-review and editing. All authors have read and agreed to the published version of the manuscript.

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.