The Influence of the Addition of Cement and Zeolite on the Increase in the Efficiency of Sewage Sludge Dewatering in the Pressure Filtration Process ^†

Abstract

The process of removing water from sewage sludge is particularly important due to its high content in the raw sludge. This translates into problems with the transport and storage of sediments. Additionally, high water content reduces the calorific value of the sludge. The methods for selecting the appropriate parameters for sewage sludge conditioning and filtration, based on the experimental data presented in this work, may allow for the optimization of sludge dewatering lines in small and large sewage treatment plants. The optimization of the dewatering process has a significant impact on the environmental and economic benefits, which consequently results in a decrease in the power costs of the devices used, flocculants, and sludge processing, and, above all, it contributes to the reduction in the negative impact on the environment. The use of mineral substances in the preparation of sewage sludge improves the effects of its dewatering in the pressure filtration process, as expressed in the obtained values of the final hydration and process efficiency. The use of polyelectrolytes alone significantly improves the effects of sewage sludge dewatering. In this work, the polyelectrolytes were supported by the addition of cement or zeolite. The conditioning of sewage sludge in combined methods using C-494 polyelectrolyte and minerals made it possible to reduce the compressibility coefficient to the range of 0.24–0.47 and, at the same time, to achieve the best results of sludge dewatering in the filtration process. The lowest hydration of 74.9% was achieved when polyelectrolyte and cement were added to the sludge, and this hydration was 6.5 percentage points lower compared to that of the non-filtered sludge.

1. Introduction

A key stage in sewage sludge processing is dewatering, which can minimize the amount of sludge, facilitate transport, and even reduce the amount of leachate from sludge landfills [1,2]. However, sludge is a heterogeneous colloidal system in which small sludge particles form a stable suspension in water and are very difficult to separate from the aqueous phase if only mechanical pressure is applied [3,4,5]. To improve the dewaterability of sludge and achieve a deep dewatering effect, technological processes have been extensively researched, developed, and improved. Over the last three decades, much research has been carried out on the development of the technical mechanisms of various sludge dewatering processes, such as advanced oxidation, flocculation, precipitation, and electric dewatering methods and sludge framework processes [6,7,8,9,10,11,12,13]. In most cases, prior to mechanical dewatering, disintegration techniques are necessary, such as the mechanical methods using ultrasonics, mills or homogenizers, thermal hydrolysis, and freezing and thawing, and the biological method usually performed by enzymes [2,12,14,15]. Generally, despite conditioning, the moisture content of the cake remains very high, usually higher than 80% after dewatering by centrifugation and vacuum filter, which cannot meet the requirement of economical drying or incineration of the filter cake. The dry matter content of the sludge should be greater than 40% according to some guidelines adopted by various countries. Therefore, most sewage treatment plants use plate, frame, chamber, or belt filter presses in the sludge dewatering process. To increase the effectiveness of sludge dewatering using filter presses, the research on the process of building the sludge framework was intensified. The process of sediment framework construction has attracted attention due to its ability to directly modify the physical properties (compressibility and permeability) of the sediment cake through the addition of inert rigid materials. With in-depth research, scientists have found that these materials show greater potential in terms of preparation and transportation costs, environmental friendliness, and waste recycling [16]. Thanks to this, frame construction has gained significant development in recent years. Despite the previous efforts, a comprehensive overview of the progress and perspectives of sewage sludge treatment framework builders is still lacking; this makes it difficult for researchers to obtain a clear and comprehensive understanding of their research progress.

The following article contains new insights into the engineering applications of conditioning methods and mechanical dewatering technologies for sewage sludge in the pressure filtration process. Polyelectrolytes, cement, and zeolites were used as conditioning agents for the builders of the sludge framework, and the changes in the content of total organic carbon in leachates were also examined, while taking into account the load returned to the sewage treatment process. The results presented in the article are a continuation and extension of research on increasing the efficiency of the separation of pollutants from sewage sludge by the pressure filtration process [17].

The primary goal of the research was to maintain the high efficiency of the filtration process and to inhibit the increase in the resistance of the filter baffle as well as the cake layer (the phenomenon of collimation) by changing the compressibility coefficient, which leads to more efficient drainage of the water contained in the sludge.

2. Materials and Methods

Preliminary sewage sludge and mixed sludge (containing 50% preliminary sewage sludge and 50% excess sludge) were selected for the study. The sewage sludge came from a municipal treatment plant operating in a mechanical–biological treatment system. The process line consisted of mechanical screens, a grit chamber, an aerated skimmer, and radial pre-sludge traps. Multifunctional biological reactors of activated sludge were used; these reactors are where oxidation of the organic compounds, nitrification, denitrification, and biological defosfation takes place. The activated sludge was settled and thickened in secondary settling tanks; then, the treated wastewater was discharged to a receiving water body. The grease fraction and the resulting sludge were discharged to separate digesters.

Table 1. Characteristics of the examined sludge.

	Unit	Primary Sludge	Mixed Sludge
Initial hydration	%	97.1–98.4 ± 0.19	97.5–98.1 ± 0.15
Dry mass of sludge	g/L	29.0–16.0 ± 0.2	25.0–19.0 ± 0.18
The content of organic substances	g/L	18.5–10.2 ± 0.16	16.7–13.2 ± 0.18
The content of mineral substances	g/L	10.5–5.8 ± 0.13	8.3–5.8 ± 0.10
pH	-	6.8–7.1 ± 0.10	6.9–7.0 ± 0.10
CST (capillary suction time)	s	112–128 ± 8.0	206–238 ± 19.0

shows the characteristics of the sludge.

The research undertaken was intended to demonstrate the influence of the changes in the compressibility coefficient of sewage sludge conditioned with polyelectrolytes and mineral substances (cement, zeolite) on the efficiency of their dewatering in the pressure filtration process. The filtration process was carried out in accordance with the research model shown in Figure 1.

Before conditioning began, the following was determined:

• The doses of mineral substances, i.e., cement, zeolite, and polyelectrolyte;
• The pressure filtration process parameters—variable pressure.

Conditioning substances used:

–

Polyelectrolyte—Kemira Superfloc^® (Kemira, Helsinki, Finland) polyelectrolytes from the C series were used for chemical preparation of sewage sludge: weakly cationic C-494. The gel works by exchanging a charge between the polyelectrolyte chain and the suspension. As a result of this action, the suspension loses stability and becomes capable of coagulation or the formation of flocs. In order to improve the properties and ability to release water, polyelectrolyte was added to the tested sludge at a dose of 2.5 mg/g d.m., regardless of the type of sludge.

–

Cement—Portland type, a combination of ground cement clinker and gypsum. Cement clinker is obtained by firing a mixture of ground raw materials containing limestone and aluminosilicates at 1450 °C. The chemical composition of clinker includes allite, tricalcium silicate (50–65% by weight of clinker), belite, dicalcium silicate (about 20% by weight of clinker), brownmillerite, a compound of calcium oxide, aluminum oxide and iron (III) oxide (about 10% by weight of clinker), tricalcium aluminate (about 10% by weight of clinker), and other compounds of aluminum, calcium, and magnesium. Gypsum or a mixture of gypsum and anhydrite, a setting time regulator, and up to 5% of other ingredients (limestone, slag, and pozzolan) were added to the clinker produced from the above mixture.

–

Zeolite 13X—synthetic adsorber type 13X. The unit cell of X-type zeolite contains 192 tetrahedra (Si,Al)O₄. The basic element of the structure are cuboctahedrons composed of 14 SiO₄ tetrahedra and 10 AlO₄ tetrahedra. Zeolite 13X has a pore diameter of approximately 1.3 μm and a ball size of 1.3 to 2.3 mm. The tests used doses of 0.8 and 1.6 g/L of sewage sludge.

The tests were conducted on the pressure filtration device shown in Figure 2. The device consists of a pressure filter, in which the filter cloth is placed; air is supplied at a preset pressure. Measurements are taken of the volume of filtrate per unit time until the pressure on the filter drops.

Sewage sludge is a material that is compressible. Resistivity usually increases with increasing pressure during the filtration process. The solid phase, or rather its particles, undergoes deformations that vary depending on the set pressure and fill the pores inside the resulting cake. This relationship can be expressed by the equation:

$$r=r_0·p^s$$

(1)

where

• r—specific resistance of the sludge under pressure p,
• r₀—constant representing the resistivity of the incompressible sludge cake,
• p—filtration pressure,
• s—compressibility coefficient.

The compressibility coefficient is determined on the basis of the specific filtration resistance test according to several tests carried out for the same sample but at different pressure values. By plotting the specific resistance of the filtration as a function of pressure (logarithmic coordinate system), we obtain the relationship log r = f (log p), which will be a straight line. The value of the compressibility coefficient was determined on the basis of the following equation, using the analytical and graphic method. The value of the compressibility coefficient of sewage sludge expresses the tangent of the angle of the straight line, which can be seen in the formula and Figure 3.

$$S=\mathrm{tg}\propto =\frac{\log r_2-\log r_1}{\log P_2-\log P_1}$$

(2)

where

• S—compressibility coefficient,
• r₁—specific resistance under pressure p₁,
• r₂—specific resistance under pressure p₂.

Figure 3. Determination of the sludge compressibility coefficient.

Figure 3. Determination of the sludge compressibility coefficient.

3. Results

The aim of using of sewage sludge conditioning is to change the physico-chemically and physically bound water into gravity-free water, which is the easiest to remove from the sludge. Conditioning sludge with mineral substances (e.g., cement and zeolite) involves adding them in appropriate proportions to reduce the compressibility of the sludge. However, if the compressibility of the sewage sludge is too high, it leads to inhibition of the dewatering process; therefore, it is advisable to condition the sludge before dewatering. Figure 4 shows the results obtained after the pressure filtration process of primary sludge with the addition of C-494 polyelectrolyte and cement at doses of 0.8 g and 1.6 g per 1 dm³ sample. The addition of the cement + polyelectrolyte C 494 complex significantly reduced the compressibility coefficient of sewage sludge for all the considered pressures of the filtration process.

The decrease in the compressibility coefficient was noticeable for both doses of cement, i.e., 0.8 g and 1.6 g. It should be noted that a larger dose of cement in the presence of polyelectrolyte translated into a greater decrease in the compressibility coefficient. The difference in the value of the discussed parameter between the raw sludge and the sludge with the addition of cement at a dose of 1.6 g and polyelectrolyte at the highest process pressure used was 0.36 (Figure 4). The value of the sludge compressibility coefficient decreased with the increase in pressure. The addition of cement and C-494 polyelectrolyte resulted in a significant decrease in the hydration of the final sludge compared to the unconditioned sludge sample. A decrease in water content in the sludge occurred for both doses of cement and polyelectrolyte. The best sludge dewatering effects were obtained at the highest applied pressure of 0.6 MPa, and a higher cement dose used in the method combined with the polyelectrolyte and amounted to 74.9% (Figure 5).

The addition of zeolite and C-494 electrolyte resulted in a decrease in the compressibility coefficient of the sewage sludge, regardless of the pressure used in the filtration process. The lowest value (0.31; Figure 6) was obtained at a pressure of 0.6 MPa for the sediment conditioned with zeolite at a dose of 1.6 g in combination with a polyelectrolyte. The decrease in the compressibility coefficient was noticeable for both doses of zeolite. The differences in the values of the discussed parameter between the doses of 0.8 g and 1.6 g were relatively small, reaching up to 13% (Figure 6). As with the previous measurement series, the sediment compressibility coefficient decreased with the increasing pressure. The reduction in the compressibility coefficient translated into a reduction in the hydration of sediments after the filtration process (Figure 7). The addition of zeolite and polyelectrolyte C-494 to the sludge resulted in a decrease in the water content in the sludge cake for both of the doses used. Its effectiveness increased with the increase in the pressure used. A zeolite dose of 1.6 g resulted in lower final hydration (75.1%, Figure 7).

The addition of cement and C-494 polyelectrolyte caused a significant decrease in the compressibility coefficient of the mixed sediments compared to the raw sediments (Figure 8). In the case of a cement dose of 0.8 g, the compressibility coefficient was 0.47–0.36, depending on the pressure used. This meant a decrease in the value of the compressibility coefficient from 0.12 to 0.23 compared to the mixed sediments not subjected to conditioning (Figure 8). The percentage difference was 20–40%. In the case of a cement dose of 1.6 g with polyelectrolyte, the value of the compressibility coefficient decreased from 0.34 to 0.24. The percentage difference in the coefficient value compared to sludge without additives ranged from approximately 43 to 60.0%. The decrease in the compressibility coefficient after the addition of cement and polyelectrolyte C-494 was associated with a significant decrease in the final hydration of mixed sludge. A decrease in water content occurred for both cement doses at each of the pressures used during filtration. In the case of a dose of 0.8 g of cement and polyelectrolyte, the final hydration values ranged from 79.9–77.2%. The decrease compared to mixed sludge without additives was 4.0–4.9 percentage points. An improvement in the final hydration reduction (79.2–76.4%; Figure 9) was achieved by using an ash dose of 1.6 g.

By changing the mixed sludge conditioning factor to zeolite and adding it in combination with a polyelectrolyte, a decrease in the compressibility coefficient was also achieved. This decrease occurred with both zeolite doses (0.8 and 1.6 g) and for each of the filtration pressures used. In the case of a dose of 0.8 g of zeolite and polyelectrolyte, the compressibility coefficient ranged from 0.38 to 0.34 (Figure 10). Compared to the mixed sludge not subjected to conditioning, the decrease was from 0.21 to 0.23. With a dose of 1.6 g of zeolite, the value of the sludge compressibility coefficient was from 0.37 to 0.30; as can be seen, this was not a significant difference compared to the use of a dose of 0.8 g of zeolite. As the compressibility coefficient decreased, the moisture content in the mixed sludge after the filtration process decreased. A decrease in final hydration occurred with both zeolite doses and with each filtration pressure value used in the experiment. With a zeolite dose of 0.8 g, the final hydration of the sludge ranged from 79.1–77.8% (Figure 11). For both doses, the final hydration decreased with increasing filtration pressure. At the same time, it was observed that the increase in pressure translated into a greater increase in the effectiveness of the 0.8 g dose (Figure 11). In the case of a pressure of 0.6 MPa, the final hydration of the sludge differed for both zeolite doses by only 0.6 percentage points.

The remaining parameters of the filtration process were analyzed—filtration efficiency and speed, as well as filtration resistance (

Table 2. Changes in selected parameters of the filtration process of primary and mixed sludge with the addition of cement and C-494 polyelectrolyte.

		Primary Sludge			Mixed Sludge
Conditioning Method	Pressure MPa	Efficiency kg/m2h	Speed cm3/s	Specific Resistance to Filtration, m/kg × 1013	Efficiency kg/m2h	Speed cm3/s	Specific Resistance to Filtration, m/kg × 1013
raw sludge	0.3	2.24 ± 0.015	0.14 ± 0.009	4.24 ± 0.06	1.51 ± 0.015	0.14 ± 0.009	5.59 ± 0.06
	0.4	2.31 ± 0.010	0.16 ± 0.007	4.12 ± 0.08	1.87 ± 0.010	0.15 ± 0.007	5.48 ± 0.08
	0.5	3.01 ± 0.017	0.20 ± 0.003	4.67 ± 0.08	2.12 ± 0.017	0.11 ± 0.003	5.06 ± 0.08
	0.6	3.71 ± 0.014	0.27 ± 0.010	4.91 ± 0.04	2.56 ± 0.014	0.19 ± 0.010	5.51 ± 0.04
raw sludge + cement 0.8 g + polyelectrolyte (C-494)	0.3	8.97 ± 0.033	0.38 ± 0.017	2.02 ± 0.02	7.32 ± 0.022	0.34 ± 0.014	2.12 ± 0.02
	0.4	9.54 ± 0.039	0.40 ± 0.019	2.96 ± 0.04	8.48 ± 0.020	0.39 ± 0.016	2.34 ± 0.03
	0.5	9.12 ± 0.033	0.41 ± 0.019	2.54 ± 0.04	7.45 ± 0.026	0.31 ± 0.015	2.68 ± 0.03
	0.6	9.79 ± 0.039	0.43 ± 0.021	2.59 ± 0.03	9.02 ± 0.023	0.47 ± 0.019	2.94 ± 0.03
raw sludge + cement 1.6 g + polyelectrolyte (C-494)	0.3	9.17 ± 0.034	0.41 ± 0.019	2.15 ± 0.04	7.48 ± 0.032	0.43 ± 0.017	3.16 ± 0.05
	0.4	9.59 ± 0.039	0.49 ± 0.023	3.63 ± 0.06	8.86 ± 0.033	0.41 ± 0.019	3.28 ± 0.05
	0.5	11.23 ± 0.043	0.45 ± 0.022	2.41 ± 0.05	9.41 ± 0.035	0.44 ± 0.021	3.69 ± 0.06
	0.6	11.94 ± 0.046	0.53 ± 0.025	3.02 ± 0.06	9.31 ± 0.037	0.49 ± 0.023	3.56 ± 0.07

and

Table 3. Changes in selected parameters of the filtration process of primary and mixed sludge with the addition of zeolite and C-494 polyelectrolyte.

		Primary Sludge			Mixed Sludge
Conditioning Method	Pressure MPa	Efficiency kg/m2h	Speed cm3/s	Specific Resistance to Filtration, m/kg × 1013	Efficiency kg/m2h	Speed cm3/s	Specific Resistance to Filtration, m/kg × 1013
raw sludge	0.3	2.24 ± 0.015	0.14 ± 0.009	4.24 ± 0.06	1.51 ± 0.015	0.14 ± 0.009	5.59 ± 0.06
	0.4	2.31 ± 0.010	0.16 ± 0.007	4.12 ± 0.08	1.87 ± 0.010	0.15 ± 0.007	5.48 ± 0.08
	0.5	3.01 ± 0.017	0.20 ± 0.003	4.67 ± 0.08	2.12 ± 0.017	0.11 ± 0.003	5.06 ± 0.08
	0.6	3.71 ± 0.014	0.27 ± 0.010	4.91 ± 0.04	2.56 ± 0.014	0.19 ± 0.010	5.51 ± 0.04
raw sludge + zeolite 0.8 g + polyelectrolyte (C-494)	0.3	6.52 ± 0.031	0.38 ± 0.018	2.04 ± 0.02	5.78 ± 0.017	0.32 ± 0.013	2.34 ± 0.02
	0.4	7.16 ± 0.033	0.34 ± 0.016	2.24 ± 0.03	7.02 ± 0.020	0.32 ± 0.015	2.06 ± 0.03
	0.5	7.34 ± 0.033	0.33 ± 0.019	2.36 ± 0.04	7.61 ± 0.021	0.33 ± 0.014	1.89 ± 0.02
	0.6	8.04 ± 0.034	0.32 ± 0.017	2.19 ± 0.03	7.98 ± 0.030	0.38 ± 0.017	2.08 ± 0.03
raw sludge + zeolite 1.6 g + polyelectrolyte (C-494)	0.3	7.98 ± 0.034	0.34 ± 0.019	2.24 ± 0.03	7.02 ± 0.027	0.32 ± 0.016	2.01 ± 0.02
	0.4	8.29 ± 0.035	0.34 ± 0.020	2.04 ± 0.02	7.48 ± 0.025	0.34 ± 0.016	2.24 ± 0.03
	0.5	8.87 ± 0.035	0.42 ± 0.022	2.19 ± 0.03	8.61 ± 0.034	0.40 ± 0.020	2.94 ± 0.04
	0.6	9.15 ± 0.036	0.41 ± 0.024	2.67 ± 0.04	8.39 ± 0.038	0.44 ± 0.026	2.76 ± 0.03

). For both the primary and the mixed sludge, the addition of cement with C-494 polyelectrolyte at a dose of 0.8 g resulted in an increase in the efficiency of the filtration process to values ranging from 8.97 to 11.94 kg/m²h (primary sludge) and from 7.32 to 9.41 kg/m²h (sludge mixed). The changes in relation to the primary and unconditioned mixed sludge at a pressure of 0.3 MPa accounted for approximately 410% (primary sludge) and 500% (mixed sludge) of the increase in efficiency values. The speed of penetration of the sludge cake by the liquid fraction also increased. After adding 0.8 g of cement and C-494 polyelectrolyte, the breakthrough speed increased from 0.27 to 0.43 cm3/s and from 0.19 to 0.47 cm³/s at a pressure of 0.6 MPa. Compared to the primary and unconditioned mixed sludge at a pressure of 0.3 MPa, the velocity increase was approximately 300%. A decrease in the filtration resistance of the sludge conditioned using the combined cement and polyelectrolyte method compared to that of the raw sludge was observed. The best results (reduction of 49% in primary sludge, 63% in mixed sludge) were obtained when conditioning the sludge with 0.8 g of cement and polyelectrolyte and a process pressure of 0.3 MPa (Table 2).

By changing the conditioning factor to zeolite and polyelectrolyte C-494 at a dose of 0.8 g, an increase in the efficiency of the filtration process to the values of 9.15 and 8.39 kg/m²h was also achieved (in every range of the filtration pressure used) for both the primary and the mixed sludge. Compared to the unconditioned sludge, the process efficiency increased by 350%. With the discussed sludge conditioning method, the speed of the filtration process was increased. At a pressure of 0.6 MPa and a dose of 1.6 g of zeolite and polyelectrolyte, the filtration speed increased to 0.41 cm³/s for primary sludge and 0.44 cm³/s for mixed sludge (Table 3). In the analysis of the changes in filtration resistance, a tendency was observed which was similar to that in the case of using a conditioning agent such as cement. For both the primary and mixed sediments, a reduction in filtration resistance of up to 60% was recorded (Table 3).

The research was supplemented with measurements of the change in total organic carbon in leachates after the filtration process to check whether the additions of mineral substances added to sewage sludge could act as organic carbon adsorbents. For primary sludge, the addition of cement and C-494 polyelectrolyte resulted in a decrease in the organic carbon content in leachates compared to sludge without additives at a filtration pressure higher than 0.3 MPa (Figure 12). Decreases in the value of total organic carbon for primary sludge reached up to 20%. For mixed sediments, the reduction in the content of the parameter in question, by up to 15%, was irregular and no change tendency was observed. When filtering sludge with zeolite and polyelectrolyte, the content of total organic carbon in the leachate decreased in each range of pressures used (Figure 13). For both zeolite doses, for both primary and mixed sludge, a similar decrease in the total organic carbon content of up to 30% was observed.

4. Discussion

When conducting research on sludge dewatering by pressure filtration, it was found that the applied pressure is an important parameter. Increasing the efficiency and speed of the process is not always related to the use of high process pressure. A high dry matter content of 35–40% in dewatered sludge can be achieved by applying a pressure of 0.3–0.4 MPa during filtration, but 60% dry matter content can be achieved at a pressure of 6–10 MPa, as demonstrated by Laheij et al. in their study [18]. The variable pressure applied in the pressure filtration process leads to different final hydration levels as early as the raw sludge dewatering stage. Increasing the pressure by 0.3 MPa leads to a difference in hydration that reaches 2.2% (82.6%—0.3 MPa pressure and 80.4%—0.6 MPa pressure). With an increase in the applied pressure, the filtration resistance increased slightly from 4.24 × 10¹³ m/kg (0.3 MPa) to 4.91 × 10¹³ m/kg (0.6 MPa). Zhao and Bache [19] included pressure and filtration time as well as polymer dosage in their study to investigate the effect on the dewatering of aluminous sludge. The dewatering effects can be increased by increasing the pressure and filtration time, as they demonstrated in their study. When a certain optimum point was reached, further increases in pressure and filtration time did not increase the effects. In conducting their research, Wu et al. [20] established a pressure threshold; exceeding the threshold significantly worsened the pressure filtration process. When sludge was conditioned with polyelectrolyte followed by the addition of ash, a marked increase in efficiency was observed at each applied pressure, while at the same time the filtration resistance decreased. The highest process efficiency of 11.94 kg/m²h (0.6 MPa) and a filtration resistance of 3.56 × 10¹³ m/kg at the same pressure were recorded. By preparing the sludge with polyelectrolyte and cement, the highest concentration of dry matter in the dewatered sludge was obtained at 25%, at a pressure of 0.6 MPa. The literature reports that a much higher pressure of 28 MPa is able to provide about 45% dry matter concentration in the dewatered sample [21]. Chemical reactants (e.g., polyelectrolytes) used in sludge conditioning are expensive and contribute significantly to the overall cost of sludge management at wastewater treatment plants. Research is still being carried out with the aim of finding a cheap and effective way of conditioning. In this work, which is a continuation and extension of already conducted and published research [17], the requirements are met by the chemicals used in the study, such as cement and zeolite.

5. Final Conclusions

• The compressibility coefficient, taking values in the range of 0.25–0.30, results in a significant increase in efficiency and velocity and affects the reduction in final hydration in the pressure filtration process.
• The value of the sediment compressibility coefficient decreases as the pressure of the filtration process and the dose of mineral substances increase.
• By conducting the pressure filtration process using combined methods of sludge conditioning with an organic reagent (polyelectrolyte) and mineral substances (cement, zeolite), it is possible to achieve a reduction in the compressibility coefficient of sludge, which results in an increase in the efficiency and speed of the process and, at the same time, makes it possible to dewater sludge to a value of 74.9%.
• The use of the highest filtration process pressure of 0.6 MPa leads to the best results in terms of final hydration, efficiency, and filtration speed.
• The addition of zeolites and polyelectrolyte resulted in a decrease in total organic carbon content as the applied pressure increased.