Abstract
The article presents the results of an experimental study of the effect of high-temperature thermal pretreatment on the specific resistance to filtration (SRF) of the sewage sludge (SS) from the Lviv wastewater treatment plant (WWTP), which is a combination of primary sludge and excess-activated sludge collected in primary sedimentation tanks. The kinetics of SRF reduction over time at temperatures of 140 − 150 °С are described by simple exponents, while at temperatures of 160 − 170 °С, they are described by modified two-parameter exponents. The study analyzed the dimensionless optimization function, which is the product of the final relative SRF of the sludge and the dimensionless time of thermal pretreatment. An optimal dimensionless thermal pretreatment time of 4.1 tr.0/2 was determined, and an empirical exponential equation for the time of SRF reduction by twice tr.0/2 was derived. Based on the analysis, it was found that the highest efficiency in reducing the SRF of Lviv WWTP SS occurs at a temperature of 170 °C and an optimal duration of thermal pretreatment of 55 min.
HIGHLIGHTS
Thermal pretreatment is an effective method for decreasing specific resistance to filtration (SRF) of sewage sludge (SS).
Two-parameter exponents are used as trendlines for SRF reduction during thermal pretreatment.
An optimization function is proposed for the thermal pretreatment of SS.
Optimal parameters for the thermal pretreatment of SS from the Lviv wastewater treatment plant are determined.
INTRODUCTION
At typical municipal wastewater treatment plants (WWTPs), as a result of sewage treatment, a mixture of sewage sludge (SS) is formed in the amount of 1–1.5% of the volume of treated sewage. The disposal of SS is a complex and costly process; however, SS is a source of carbon, nutrients, and trace elements and can be effectively disposed of (Gahlot et al. 2022; Kelessidis & Stasinakis 2012; Liew et al. 2021). An important step in SS utilization is a dewatering process, which allows a significant reduction of the SS volumes (Wu et al. 2020; Cao et al. 2021). Depending on the susceptibility of the sludge to mechanical dewatering, the water content in the SS can range from 95–99 to 65–85% (Skinner et al. 2015; Wójcik & Stachowicz 2019).
One of the main quantitative parameters of SS dewatering properties is specific resistance to filtration (SRF). SRF varies within very wide limits for types of sludge and different WWTPs. Thus, the values of SRF for sludge from primary settling tanks at different WWTPs are in the range of (60–3,000) × 1011 m/kg (Smollen 1986; Wojciechowska 2005; Liu et al. 2012), which can be explained by differences in the composition of wastewater at the inlet of WWTP, primarily due to the influence of industrial wastewater. The activated sludge of typical WWTP has low dewatering properties, its SRF is in the range of (1,700–11,500) × 1011 m/kg, and it increases sharply with increasing the total solids (TS) content (Ng & Hermanowicz 2005; Xiao et al. 2022).
On the other hand, SS from primary clarifiers and activated sludge of WWTPs have less SRF compared to anaerobically digested sludge. The SRF of the digested SS depends not only on the type of sludge and the mode of its digestion but also on the accepted loading dosage and the method of SS mixing in the digester, and SRF decreases slightly with prolonged fermentation. Anaerobically digested activated sludge has higher SRF compared with the primary sludge fermented in the same conditions (Liu et al. 2021).
A key issue in the efficiency of sludge dewatering is the selection of an appropriate conditioning method according to the physicochemical properties of SS (Barber 2016; Wu et al. 2020). Zhang et al. (2022) analyzed the advantages and deficiencies of the different SS pretreatment methods, as well as their mechanisms. Most often, SS pretreatment is performed by adding the flocculants (Wójcik & Stachowicz 2019; Hu et al. 2021; Qi et al. 2011), by the sonochemical method (Pilli et al. 2011), and by thermal hydrolysis (Haug et al. 1978; Lin & Shien 2001; Feng et al. 2014; Wang & Li 2015). In order to reduce the consumption of chemical reagents, the sludge conditioning process is modified, and new methods of sludge pretreatment are proposed and studied, for example, different combined methods (Bień & Bień 2020, 2021; Ngo et al. 2021; Nguyen et al. 2021).
Thermal pretreatment of SS is a process of heating the sludge to 60–180 °C in a hermetic environment under appropriate excess pressure (Bougrier et al. 2008; Deng et al. 2019; Kim et al. 2020). The process of thermal hydrolysis of SS has been used since the 1930s and has reached its maximum spread in the last 2–3 decades (Gavala et al. 2003; Hidaka et al. 2022; Neyens & Baeyens 2003; Wang & Li 2015). Thermal pretreatment of SS is a simple and effective method of reducing its filtration resistance (Neyens & Baeyens 2003; Bonu et al. 2023). However, it is important to note that thermal treatment is associated with high specific energy consumption (Ruffino et al. 2015). Therefore, the cost-effectiveness of this method in each specific case must be carefully evaluated through the development of an appropriate feasibility study. This highlights the significance of optimizing thermal pretreatment techniques.
The process of thermal pretreatment before anaerobic digestion improves sludge dewatering, increases the biodegradability of excess-activated sludge, ensures sludge decontamination, and causes a positive energy balance compared to other sludge pretreatment methods (Haug et al. 1983; Pinnekamp 1989). In different studies of thermal pretreatment of activated sludge at temperatures of 160–180 °C and a duration of 30–60 min, a reduction in the time of SS anaerobic digestion, an increase in the specific yield of biogas, a higher degree of decomposition of organic substances and better dewaterability of the obtained digestate were obtained (Pilli et al. 2015).
Lab-scale thermal pretreatment of the activated sludge of the industrial WWTP of a cellulose manufacturing plant (Aanekoski, Finland) at the temperature of 180 °C and the duration of 30 min caused the SRF decrease from (200 − 800) × 1011 to 5 × 1011 m/kg (Kyllönen et al. 1988).
In previous studies, a lack of information was identified regarding the kinetics of SS SRF reduction based on the duration of high-temperature pretreatment. The experimental data provided by Everett (1972) only cover the temperature range of 180–220 °C, which raises concerns about energy efficiency. On the other hand, there is also a notable gap in optimizing the thermal pretreatment of SS, which is particularly relevant given the current rise in electricity and other energy costs.
The objective of this study is to elaborate on the method of optimizing the thermal pretreatment of SS, with a specific focus on the temperature and duration of the pretreatment process. This study hypothesizes that it is possible to determine the optimal temperature and duration for the thermal pretreatment of SS by analyzing the kinetics of SRF reduction using lab-scale experiments. The significance of this study lies in its potential to reduce energy costs associated with the thermal pretreatment of SS in other WWTPs by applying the developed method.
MATERIALS AND METHODS
Tested SS
An experimental lab-scale study of the effect of high-temperature pretreatment on the dewatering properties of SS from the Lviv municipal WWTP was performed. In the city of Lviv (Ukraine), a combined sewerage system is used. The total nominal capacity of the Lviv WWTP is 490,000 m3/day. The SS of the Lviv WWTP is a mixture of primary sludge and excess-activated sludge that is collected from the primary settling tanks. Excess-activated sludge is added to primary clarifiers in order to increase the sedimentation efficiency of primary sludge, acting as a biocoagulant. The composition of SS from the Lviv WWTP exhibits consistent stability over time. This stability can be attributed to the diversion of stormwater runoff during periods of heavy rainfall. Rather than being directed to the WWTP, the stormwater is channeled through the overflow spillway into the Poltva River, which is part of the Baltic Sea basin. This practice is implemented to ensure compliance with permissible environmental impacts.
The dry matter (DM) content in the samples of SS from the Lviv WWTP was determined to be 40 g/L using standard laboratory techniques. Additionally, the volatile solids (VS) were found to account for 70% of the DM. Following the thermal pretreatment process and subsequent cooling to a temperature of 20 °C, the samples were diluted with tap water to achieve a lower DM content of 8 g/L. This dilution was done to enhance the accuracy of the filtration stage results.
Thermal pretreatment experimental unit
Experimental lab-scale unit for SS thermal pretreatment: 1 – high-pressure reactor; 2 – electrical transformer; 3 – air supply pipeline; 4 – control valve; 5 – thermometer; 6 – manometer.
Experimental lab-scale unit for SS thermal pretreatment: 1 – high-pressure reactor; 2 – electrical transformer; 3 – air supply pipeline; 4 – control valve; 5 – thermometer; 6 – manometer.
The sludge in reactor 1 was heated to a specified temperature, which was controlled by a thermometer 5. After some time of exposure at a given temperature, the heater was turned off, and reactor 1 was cooled to room temperature. Then the pretreated sludge was poured out of reactor 1 for the SRF measurement.
High-pressure reactor: 1 – reduction valve; 2 – cover; 3 – gasket; 4 – sleeve with oil for thermometer; 5 – heating device; 6 – body of reactor; 7 – nozzle for connecting a pressure gauge; 8 – fitting for connecting the air supply pipeline.
High-pressure reactor: 1 – reduction valve; 2 – cover; 3 – gasket; 4 – sleeve with oil for thermometer; 5 – heating device; 6 – body of reactor; 7 – nozzle for connecting a pressure gauge; 8 – fitting for connecting the air supply pipeline.
SFR of Lviv WWTP SS after the thermal pretreatment at temperatures of 140 − 170 °С and the corresponding exponential trend lines (4), (5), (7) and (8) (indicated bars of relative errors ±10%).
SFR of Lviv WWTP SS after the thermal pretreatment at temperatures of 140 − 170 °С and the corresponding exponential trend lines (4), (5), (7) and (8) (indicated bars of relative errors ±10%).
SRF of the SS
During the mechanical dewatering of SS, mainly free water is released, so Equation (1) for determining the SRF is fully applicable to the SS. The duration of filtration depends on the rate of sludge dewatering. To obtain a sufficient number of data, as usual, it does not exceed 20 min. The parameter b was defined as the coefficient of the linear trend of the experimental data in the axes V–t/V. Knowing the volumes of filtrate V1, V2, V3, … Vn, at the time t1, t2, t3, … tn, respectively, parameter b was found for each tested sludge.
Sludge SRF was determined at a vacuum pressure of 66.7 kPa (500 mmHg). Buchner funnel with a diameter of 80 mm and a filtration area of 50.24 cm2 was used. The viscosity of the filtrate was assumed equal to the viscosity of water at the temperature of 20 °C (η = 0.001 Pa·s). TS content was equal to C = 80 kg/m3. Thus, the SRF of the raw, untreated SS of the Lviv WWTP was equal to 686.5 × 1011 m/kg.
RESULTS AND DISCUSSION
Kinetics of the decreasing of SS SRF
High-temperature pretreatment of samples of the SS from the Lviv WWTP was performed at four different temperatures, namely 140, 150, 160, and 170 °С. The control values for the duration of the thermal pretreatment were equal to 30, 60, and 90 min.
Experimental duration of Lviv WWTP SS SRF decreasing to (100–400) × 1011 m/kg depending on the temperature of the thermal pretreatment.
Experimental duration of Lviv WWTP SS SRF decreasing to (100–400) × 1011 m/kg depending on the temperature of the thermal pretreatment.
Thus, an increase in the pretreatment temperature of the SS from T1 = 140 °C to T2 = 150 °C leads to an increase in the constant k from 0.0216 to 0.0252 min−1 or by 16.7%.
The experimental duration of SRF decreasing to typical round values in the range of (100 − 400) × 1011m/kg for temperatures of 140 − 170 °С is shown in Figure 4.
Parameters of trend lines (9) and (10) for the thermal pretreatment of the SS of the Lviv WWTP at temperatures of 140–170 °С
SRF (×1011 m/kg) . | Logarithmic Equation (9) . | Linear Equation (10) . | ||||
---|---|---|---|---|---|---|
C1 . | k1 . | R2 . | C2 . | k2 . | R2 . | |
100 | 1397.8 | 264.1 | 0.9428 | 333.0 | 1.720 | 0.9543 |
150 | 1107.6 | 209.4 | 0.9775 | 262.8 | 1.361 | 0.9852 |
200 | 867.6 | 163.8 | 0.9897 | 206.8 | 1.062 | 0.994 |
300 | 523.6 | 98.1 | 0.9953 | 127.65 | 0.636 | 0.9977 |
400 | 287.8 | 53.1 | 0.9944 | 72.26 | 0.344 | 0.9968 |
SRF (×1011 m/kg) . | Logarithmic Equation (9) . | Linear Equation (10) . | ||||
---|---|---|---|---|---|---|
C1 . | k1 . | R2 . | C2 . | k2 . | R2 . | |
100 | 1397.8 | 264.1 | 0.9428 | 333.0 | 1.720 | 0.9543 |
150 | 1107.6 | 209.4 | 0.9775 | 262.8 | 1.361 | 0.9852 |
200 | 867.6 | 163.8 | 0.9897 | 206.8 | 1.062 | 0.994 |
300 | 523.6 | 98.1 | 0.9953 | 127.65 | 0.636 | 0.9977 |
400 | 287.8 | 53.1 | 0.9944 | 72.26 | 0.344 | 0.9968 |
Decreasing of the dimensionless SRF of the SS of the Lviv WWTP at temperatures of 140 − 170 °C (results of the authors' research) and of activated sludge at a temperature of 180 °C (Everett 1972).
Decreasing of the dimensionless SRF of the SS of the Lviv WWTP at temperatures of 140 − 170 °C (results of the authors' research) and of activated sludge at a temperature of 180 °C (Everett 1972).
Generalized dimensionless kinetics of the Lviv WWTP SS SRF decreasing after thermal pretreatment at temperatures of 140 − 170 °С depending on the dimensionless time t/tr.0/2 (deviations ±0.05 are specified).
Generalized dimensionless kinetics of the Lviv WWTP SS SRF decreasing after thermal pretreatment at temperatures of 140 − 170 °С depending on the dimensionless time t/tr.0/2 (deviations ±0.05 are specified).
The duration of the SRF halving of the Lviv WWTP SS depending on the pretreatment temperature T (deviations ±10% are specified).
The duration of the SRF halving of the Lviv WWTP SS depending on the pretreatment temperature T (deviations ±10% are specified).
Optimization of the high-temperature thermal pretreatment of SS
Since the generalized kinetics of the SRF decreasing of the Lviv WWTP SS can be well described by a two-parameter exponent, according to which the SRF decreases asymptotically with increasing time to a non-zero final value rlim, the optimization problem, which consists in determining the optimal time of high-temperature thermal pretreatment, is thus relevant.
Optimum parameters of thermal pretreatment of the Lviv WWTP SS at temperatures of 140–170 °С
T (°C) . | tr.0/2 (min) . | topt (min) . | r (×1011 m/kg) at topt . | r/r0 . | topt × r/r0 (min) . |
---|---|---|---|---|---|
140 | 34.9 | 142.8a | 31.4 | 0.046 | 6.53 |
150 | 25.4 | 103.7a | 50.3 | 0.073 | 7.60 |
160 | 18.4 | 75.3 | 79.6 | 0.116 | 8.73 |
170 | 13.4 | 54.7 | 71.4 | 0.104 | 5.69 |
T (°C) . | tr.0/2 (min) . | topt (min) . | r (×1011 m/kg) at topt . | r/r0 . | topt × r/r0 (min) . |
---|---|---|---|---|---|
140 | 34.9 | 142.8a | 31.4 | 0.046 | 6.53 |
150 | 25.4 | 103.7a | 50.3 | 0.073 | 7.60 |
160 | 18.4 | 75.3 | 79.6 | 0.116 | 8.73 |
170 | 13.4 | 54.7 | 71.4 | 0.104 | 5.69 |
aExtrapolated values.
Optimization parameters of the Lviv WWTP SS SRF decreasing after the thermal pretreatment: F(t′) – optimization function; r′(t′) is a dimensionless curve of SRF decreasing; t′*, t′** are local extrema.
Optimization parameters of the Lviv WWTP SS SRF decreasing after the thermal pretreatment: F(t′) – optimization function; r′(t′) is a dimensionless curve of SRF decreasing; t′*, t′** are local extrema.
Thus, the optimal technical and economic decreasing of the SRF of the Lviv WWTP SS corresponds to the temperature of 170 °C and the duration of the thermal pretreatment of about 55 min (Table 2). Concerning the wide variability of the composition and SRF values of SS at different WWTPs, it is advisable to carry out similar lab-scale studies to optimize thermal pretreatment for each individual type of SS using the method presented above.
CONCLUSIONS
High-temperature thermal pretreatment of SS is a reagent-free, simple, and effective method of decreasing the SRF of the SS at WWTP, while also ensuring the simultaneous disinfection of the sludge and improving the parameters of its anaerobic digestion.
An experimental study of the effect of high-temperature pretreatment on the SRF was performed for the Lviv WWTP SS, which is a mixture of primary sludge and excess-activated sludge deposited in primary sedimentation tanks. The main parameters of the studied SS before the thermal pretreatment are TS content – 8 g/L; VS – 70%, and the SRF – 686.5 × 1011 m/kg.
Dependences of SRF were obtained for the SS thermal pretreatment in the temperature range of 140 − 170 °С and the duration from 30 to 90 min. SRF kinetics at temperatures of 140 − 150 °С are described by simple exponential Equations (4) and (5), while at temperatures of 160 − 170 °С using the two-parameter exponents (7) − (8).
A dimensionless optimization function is introduced as a product of the exit dimensionless SRF of the SS and the dimensionless time of the thermal pretreatment. The optimal dimensionless thermal pretreatment time 4.1 × tr.0/2 is found, where the time of the SRF halving tr.0/2 is an empirical exponential function (13) of the temperature. According to the optimization function analysis, the highest efficiency in decreasing the SRF of the Lviv WWTP SS corresponds to the temperature of 170 °C and the thermal pretreatment duration of about 55 min.
The presented optimization method can be applied to determine the optimal time and temperature for thermal pretreatment of SS at various WWTPs. To achieve this, a comprehensive laboratory study should be conducted using 3–4 sludge samples collected under diverse weather and operational conditions over a minimum period of 6 months. For each SS sample, kinetic curves depicting the dependence of SRF on the duration of thermal hydrolysis should be obtained at a minimum of four temperature values within the range of 140–180 °C. A recommended approach is to conduct thermal hydrolysis for up to 120 min, with intermediate SRF measurements taken every 30 min to find the parameters of the maximum efficiency of the thermal pretreatment.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.