Influence of physical, chemical and dual sewage sludge conditioning methods on the dewatering efficiency

Influence of physical, chemical and dual sewage sludge conditioning methods on the dewatering efficiency

Powder Technology 344 (2019) 96–102 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec In...

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Powder Technology 344 (2019) 96–102

Contents lists available at ScienceDirect

Powder Technology journal homepage: www.elsevier.com/locate/powtec

Influence of physical, chemical and dual sewage sludge conditioning methods on the dewatering efficiency Marta Wójcik ⁎, Feliks Stachowicz Rzeszow University of Technology, Faculty of Mechanical Engineering and Aeronautics, Department of Materials Forming and Processing, Powstańców Warszawy 8, 35-959 Rzeszów, Poland

a r t i c l e

i n f o

Article history: Received 9 July 2018 Received in revised form 16 November 2018 Accepted 1 December 2018 Available online 03 December 2018 Keywords: Sewage sludge Conditioning Dewatering Biomass ash Polyelectrolyte Powder materials

a b s t r a c t This paper presents the influence of different methods of sewage sludge conditioning on the effectiveness of dewatering. In the laboratory research, three conditioning methods were tested: (1) with the use of biomass ash, (2) chemical conditioning by means of polyelectrolyte and (3) dual conditioning with the application of biomass ash in conjunction with polyelectrolyte. The specific resistance to filtration, the moisture content and characteristics of filtrates were measured. The influence of sludge conditioning on capillary suction time and the initial pH was also examined. The obtained results showed that MC and SRF decreased after sludge conditioning. Among all tested methods, the dual conditioning influenced the sludge dewaterability to the highest extent. Due to their effectiveness, physical and dual conditioning methods might be a promising alternative for the application of polyelectrolytes. The reduction of polyelectrolyte consumption has also economic benefits, which was confirmed in a previous economic analysis. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Sewage sludge is a residue of wastewater treatment, which consists of water and solid particles [2]. Due to the development of sewerage system, the amount of generated sewage sludge is steadily growing every year [11]. In Europe, the annual production of sewage sludge is approximately 11 million tons of sewage sludge dry mass (DM) [3]. Among all European countries, most of sewage sludge is produced in Germany (approximately 1.9 million tons per year) [17]. On the other hand, Cyprus and Malta are characterized by the lowest production of wastewater sludge [25]. Excessive production of sludge is also a significant problem outside Europe. According to Qiao et al. [21], approximately 14 million tons of dewatered sewage sludge was produced in China in 2007. In Unites States, the annual production of biosolids from stabilized sewage sludge is at the level of 8 million tons of dry mass [20]. Due to specific characteristics, especially the content of heavy metals, organic compounds and microorganisms, sewage sludge should be properly processed. The inappropriate sludge treatment might result in secondary pollution [9]. The typical sewage sludge treatment in wastewater treatment plants usually includes: thickening, stabilization, conditioning, dewatering and utilization [10]. Because sewage sludge is not ‘typical’ waste, the selection of efficient treatment requires individual approach [19]. ⁎ Corresponding author. E-mail address: [email protected] (M. Wójcik).

https://doi.org/10.1016/j.powtec.2018.12.001 0032-5910/© 2018 Elsevier B.V. All rights reserved.

Among all the processes of sewage sludge treatment, the most important, as well as the most expensive one, is dewatering. According to Wang and Wang [30], the annual cost of sludge dewatering in the USA is approximately 5 billion dollars. Sludge dewatering is a key process which results in the reduction of its moisture content (MC) and its volume. Depending on the susceptibility of sewage sludge to dewatering, the sludge moisture might change from 95 to 99% to 65–85% [5,21]. As reported by Schaum and Lux [24], the volume of sewage sludge with initial solids content of approximately 5% might decrease by even 90%. It contributes to reduction of costs associated with transport and utilization of sewage sludge. The effectiveness of sewage sludge dewatering depends on different factors, including: the wastewater treatment technology, the type of sewage sludge, the composition of sludge and the method of dewatering [26]. Generally, sewage sludge is characterized by the low dewaterability, which is connected with strong hydrophylicity of sludge particles [40]. According to Chen et al. [4] and Wójcik et al. [34], raw sewage sludge is characterized by the negative charge and it creates a stable system with a low sedimentation and low dewatering capacity. In order to improve the dewaterability of sewage sludge, a change in its structure is necessary. The dewatering process might be enhanced by sludge conditioning. In general, there are three main sewage sludge conditioning methods: chemical, biological and physical. The sewage sludge dewatering might be also improved by the application of microwaves, ultrasounds or thermal methods [12,23]. In laboratory tests, unconventional methods, such as Fenton's process are also examined [7].

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Chemical conditioning with the use of polyelectrolytes is commonly applied in wastewater treatment plants. The addition of cationic polyelectrolyte results in neutralization of the sludge charge and bridging of particles. Chemical conditioning eliminates repulsion of particles across the created of short-term forces and enables the connection of fine particles into larger agglomerates [6,22]. The effectiveness of chemical conditioning was proved by many researchers. Jin et al. [13] used cationic polyacrylamide in order to improve sludge dewatering. Kuglarz et al. [14] proved that the application of Praestol 610BC cationic polyelectrolyte in the dosage of 2.5 g/kg DM resulted in the decrease of moisture and the specific resistance to filtration (SRF), respectively, by approximately 4 and 81%. However, chemical conditioning indicates some limitations. According to Wu et al. [38], conditioned sewage sludge might be more compact during filtration. It can influence the limitation of further dewatering. Other disadvantages are related with high dosages of reagents and the relatively high cost of polyelectrolytes. According to Stachowicz et al. [27], the monthly cost of the acquisition of polyelectrolyte is approximately EURO 90 for a treatment plant which produces 560 tons of sewage sludge DM per year. Apart from chemical conditioning, physical conditioners called ‘skeleton builders’ are applied. Recently, the addition of different fractions of waste into sludge has been tested. In a laboratory research, the usefulness in sewage sludge treatment was indicated by, for example: rice biochar, rice powder, lignite, gypsum, wood chips and wheat bran [8,15,28,39,41,43]. The impact of energetic waste on the effectiveness of sludge dewatering is also under examination, but in some cases, physical conditioners do not improve sludge dewatering in a significant way. Physical conditioners are inert substances which only influence the change of mechanical strength and the permeability of sludge during filtration [38]. In order to improve the efficiency of sludge conditioning, skeleton builders are often modified with the use of chemical reagents. The dual conditioning methods by means of physical and chemical conditioners are also applied. Chen et al. [4] examined the effectiveness of sewage sludge dewatering after the application of coal fly ash modified with the sulphuric acid. Kuglarz et al. [14] also proved the positive impact of dual conditioning with the use of coal fly ash and Praestol 610 BC cationic polyelectrolyte on the sewage sludge dewaterability. The main mechanism of sewage sludge conditioning with the use of skeleton builders after chemical modification includes: charge neutralization, adsorption, bridging and agglomeration of flocs [16]. The literature review confirms that dual conditioning methods are superior in comparison to the single method. Chen et al. [4] proved that the coal fly ash after chemical modification can decrease the moisture content to a greater degree in comparison to raw ash. Wu et al. [38] also showed that the rice husk biochar after modification with the use of FeCl3 can decrease the sludge moisture content and SRF, respectively, by approximately 25 and 91%. In comparison, raw rice husk biochar decreased the aforementioned parameters, accordingly by 4 and 60%. In literature, there are only some papers concerning the influence of biomass ashes on the sewage sludge dewatering. In our previous studies, the sludge conditioning with wheat straw, beech wood and willow tree ashes were examined [35,36]. Results of Wójcik et al. [35] showed that the addition of biomass ash into wastewater sludge could decrease the moisture by approximately 10–25%, depending on the method of dewatering. Due to the increasing consumption of biomass in the energy sector, the ash is an inexpensive and easily-obtained material, which can be used as a physical conditioner. However, the dual sewage sludge conditioning with the application of biomass ash and cationic polyelectrolyte was not examined. Due to this fact, the effectiveness of the aforementioned method is not known. In this paper, physical conditioning with the use of biomass ash was examined. The dual conditioning with the application of biomass ash as well as cationic polyacrylamide was also assessed. The influence of conditioning methods on selected parameters of sewage sludge, for example, the moisture content, pH and specific resistance to filtration, was determined. The effectiveness of the aforementioned methods was

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compared with the chemical conditioning by means of cationic polyacrylamide alone. 2. Materials and methods 2.1. Materials Sewage sludge was derived from a thickening tank in a municipal wastewater treatment plant (WWTP) in Świlcza-Kamyszyn (Podkarpackie region, Poland). It is a physical and a biological treatment plant with a capacity of 1940 m3/d. The samples were transported to the laboratory in a plastic container at the temperature below 10 °C and in a way that limited the access of light. In order to achieve good reproducibility, all laboratory tests were completed within 3 days from 16 to 18 August 2017. Between tests, sewage sludge was stored in the temperature of approximately 4 °C. Some characteristics of sewage sludge were: the moisture content – (97.94 ± 0.27)%; the dry solid content – (20.56 ± 2.67) kg/m3; pH – (6.90 ± 0.37); SRF – (2.19 ± 0.31) ∙ 1012 m/kg. The biomass ash used in laboratory tests was derived from the Thermal Power Station in Arłamów (Podkarpackie region, Poland). It is a residue of biomass combustion at the temperature of approximately 900 °C. In this power plant, woody biomass is mainly fired. Because the material was not sieved before the laboratory research, particles with a diameter in the range of 0.01–600.00 μm were present. It was an intentional action. By means of that, the usefulness of unprocessed ash was tested. The chemical composition of biomass ash, which was analyzed by means of the X-Rays Fluoresces method (XRF), is presented in Table 1. Before the laboratory research, ash was dried at 105 °C for 3 h. In the chemical and dual conditioning of sewage sludge, the SEDIFLOC 1050CMMW cationic polyacrylamide was used (Kemira Company, Poland). The reagent was applied as a 0.5% solution after 2 h after the preparation. The detailed product specification is shown in Table 2. 2.2. Sewage sludge conditioning The sewage sludge conditioning was carried out for three different methods: [1] the physical conditioning with the application of biomass ash alone; [2] the chemical conditioning by means of cationic polyacrylamide; [3] the dual conditioning with the use of biomass ash in conjunction with polyelectrolyte. The sewage sludge conditioning was carried out as follows. Firstly, sewage sludge was poured into laboratory beakers of the volume of 1 dm3. The appropriate dosages of biomass ash or/and polyacrylamide were added into wastewater sludge (Table 3). The doses of materials and the conditions of this process were determined on the basis of the literature review and the initial research. After that, samples were stirred at the speed of 250 rpm for 1 min and then, they were mixed with at the speed of 50 rpm for 15 min. The identical methodology was used for all tested methods of sewage sludge conditioning. Raw sewage sludge (RS) was used as a control sample. The effectiveness of sewage sludge conditioning was assessed by means of the capillary suction time (CST) value. The CST was determined with the use of Baskerville and Gaulle method and was measured using the CST instrument (ProLabTech Company, Poland) equipped with 32 and 45 mm diameter funnels. Due to the pozzolanic activity of the biomass ash and its impact on the alkalinity, the pH and the moisture content after sludge conditioning were determined. The MC was

Table 1 The chemical characteristic of biomass ash used in the laboratory test. Constituent [%] CaO

SiO2

K2O

P2O5

MgO

SO3

Al2O3

Fe2O3

MnO

Other

49.48

19.76

9.53

4.76

3.92

3.69

3.17

2.66

1.10

1.93

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Table 2 The characteristic of SEDIFLOC 1050CMMW polyelectrolyte. Parameter

State of matter

Colour

Kind

Density at 25 °C [kg/m3]

Freezing point [oC]

Flashpoint [oC]

Viscosity at 25 °C [mPa/s]

Value

Liquid

White, opalescent

Strong cationic

1.0–1.1

−18.0

N 93.0

1560.0

calculated according to the formula presented in part 2.3. The pH of sludge was measured with the application of HQ40d pH-meter (HACH Company, United States).

The research was carried out in triplicate. The obtained results were determined as an average value from these series. For all the tested parameters, raw sewage sludge was used as a control sample.

2.3. Sewage sludge dewatering

3. Results and discussion

The influence of different sludge conditioning methods on its dewaterability was determined by means of the vacuum filtration. The filtration was carried out as follows: 50 cm3 of the conditioned sewage sludge was poured into a 9 cm Buchner funnel. The process was carried out for 15 min under the vacuum value of 0.02 MPa. As the main criteria to evaluate the effectiveness of sewage sludge dewatering, the MC was taken under consideration. After dewatering, the samples of raw and conditioned sewage sludge were dried at 105 °C in a laboratory drying oven. The MC was calculated with the following equation [1] [35]:

3.1. The influence of sludge conditioning on its characteristics

MC ¼ ½ðw−dÞ=w  100%

ð1Þ

where w is the weight of a sample before drying (g) and d is the weight of a sample after drying (g). The filterability of sewage sludge was assessed by means of the specific resistance to filtration (SRF) [2] [4]:   SRF ¼ 2  b  P  A2 =ðμ  cÞ

ð2Þ

where b is the slope of filtrate discharge curve (s/m6); P is the pressure of filtration (N/m2); A is the filter area (m2), μ is the viscosity of the filtrate (Ns/m2) and c is the weight of cake solids per the volume of filtrate (kg/m3). For filtrates obtained after vacuum filtration, the volume and the total suspension (TS) was measured. TS was determined by means of the weight method and was calculated according to the following formula [3] [33]: TS ¼ 1000  ða−bÞ=V

ð3Þ

where a is the mass of the folded filter paper after filtration (g); b is the mass of the folded filter paper before filtration (g); V is the volume of the filtrate (cm3).

The influence of different methods of sewage sludge conditioning on its pH, CST and the initial MC is presented in Fig. 1. Raw sewage sludge was characterized by the pH of approximately 6.9. Both physical and dual conditioning methods influenced the increase of the aforementioned parameter with the increase of the biomass ash dosage. With 7.5; 15 and 30 kg/m3 dosages of ash, the pH increased by approximately 1, 3 and 5 units, respectively. The physical and dual conditioning of sewage sludge influenced the pH in a similar way. On the other hand, the application of polyacrylamide did not affect the change of the pH of sewage sludge. The increase of pH is closely related with the content of alkaline ions, especially Ca2+ in biomass ashes, and their high leachingability [29]. As reported by Wang and Virraraghavan [31], the alkaline ions are transported from ash into sewage sludge and the increase of pH is observed. Because pH is one of the main factors which influence the sludge higienization, biomass ash might be also considered as an alkalizing substance. The sewage sludge conditioning resulted in the reduction of its CST. Depending on the method of sludge conditioning, the obtained results were varied. In comparison to raw sludge, a significant change of CST was observed for 7.5 kg/m3 dosage of ash, for both physical and dual conditioning. The lowest value of the aforementioned parameter was noted for 30 kg/m3 dosage of biomass ash and for 30 kg/m3 dosage of biomass ash in conjunction with polyelectrolyte in the dosage of 1.0 g/kg DM, namely 20.21 s and 18.52 s. Overall, the trend of CST reduction for sludge after physical conditioning was similar as for the sludge after dual conditioning. However, slightly better results were achieved by the other method. It is closely related with the mechanism of sludge conditioning. The application of physical conditioners only results in the improvement of strengths and permeability of sludge. For this reason, better effectiveness was indicated by the dual methods with the application of both physical and chemical substances [38]. It was also found from the CST measurement that the sludge after conditioning with the

Table 3 Details concerning the dosages and procedure of sewage sludge conditioning. Symbol

Method of conditioning

RS B1 B2 B3 P1 P2 P3 BP1 BP2 BP3

– Physicalb Physical Physical Chemicalc Chemical Chemical Duald Dual Dual

a b c d

Dosages in the adjusted sewage sludge dry mass [g/kg DMa]

Volume dosages [kg/m3]

Biomass ash

Polyelectrolyte

Biomass ash

Polyelectrolyte

– 293.0 576.7 1132.6 0 0 0 293.0 576.7 1132.6

– 0 0 0 1.0 1.5 2.0 1.0 1.0 1.0

– 7.5 15.0 30.0 0 0 0 7.5 15.0 30.0

– 0.0 0.0 0.0 0.03 0,05 0.06 0.03 0.03 0.03

Dry mass. Physical method with the application of biomass ash. Chemical method with the application of SEDIFLOC 1050CMMW cationic polyacrylamide. Dual conditioning by means of biomass ash in conjunction with SEDIFLOC 1050CMMW cationic polyacrylamide.

Methodology

250 rpm for 1 min → 50 rpm after 15 min

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use of SEDIFLOC 1050CMMW cationic polyacrylamide showed better dewaterability than other methods. Remarkable reduction of CST was observed for the lowest tested dosage of polyacrylamide (approximately 33% in comparison to raw sludge), but the best result was obtained for the dosage of 2.0 g/kg DM. For the aforementioned dosage, CST decreased by approximately 93% to the value of 9.77 s. The CST reduction was approximately 50% higher than for the addition of biomass ash alone in the dosage of 30 kg/m3 and approximately 7% higher than for the dual conditioning with the use of kg/m3 dosage of ash. The effectiveness of chemical conditioning is related with the change of the sludge structure and the increase of spaces between solid particles [37]. As a result, more water might be released from sludge. Due to the pozzolanic properties of most ashes, including those from biomass combustion, the change of sludge water after conditioning was noted. Raw sewage sludge was characterized by MC of approximately 98%. Application of the biomass ash affected the decrease of MC with the increase of its dosage. After sludge conditioning with the use of ash in the dosage of 30 kg/m3, MC decreased by approximately 4%, from 97.94% to 93.64%. The dual conditioning resulted in the maximum decrease of the sludge moisture by approximately 3.5% for the highest tested dosage of ash (30 kg/m3). The chemical conditioning of sludge

Fig. 1. The influence of different conditioning methods on sludge characteristics; CST (a), pH (b) and MC (c).

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by means of SEDIFLOC 1050 CMMW cationic polyacrylamide did not affect the change of the sludge MC in a significant way. The knowledge of the influence of different conditioning methods on the initial sewage sludge moisture might be helpful in evaluating the effectiveness of sludge dewatering.

3.2. The effectiveness of sewage sludge dewatering after previous conditioning The MC of the conditioned sewage sludge after vacuum filtration is presented in Fig. 2. Raw sewage sludge indicated the poor filterability and MC amounted approximately 87% after vacuum filtration. It is associated with the low permeability of non-conditioned sludge which might deform under pressure. For this reason, removal of water from sewage sludge is impeded [4]. On the basis of the obtained results it was stated that MC decreased as the dosage of the conditioner increased. The aforementioned relationship was observed for all the tested conditioning methods. For sewage sludge after conditioning with the use of biomass ash in the dosages of 7.5–30 kg/m3, MC was 83.97; 82.50 and 80.80%, respectively. The aforementioned parameter decreased by approximately 13.2; 13.3 and 13.7%, respectively. The addition of ash forms a rigid and permeable structure of sludge. By means of the formation of special channels, the sewage sludge cake remains permeable under pressure and the water can be easily removed [40]. However, only slight improvement of sludge dewaterability was noted. As stated by Zhang et al. [40] and Zhu et al. [42], single physical conditioners cannot improve the effectiveness of sludge conditioning to the same extent as chemical flocculants. Physical conditioners are neutral materials which only increase the strength of flocs and improve the permeability of sludge [38]. The application of SEDIFLOC 1050 CMMW cationic polyacrylamide improved the dewaterability of sewage sludge to a higher extent in comparison to physical conditioning. With dosages from 1.0 to 2.0 g/kg DM, the MC was approximately 77.28; 76.53 and 74.70%, respectively. Chemical conditioning could reduce the MC by approximately 12; 22 and 24%, accordingly. The main mechanism of sludge conditioning with the use of polyacrylamide is based on charge neutralization and bridging of particles [18]. In this laboratory research, the best results of sludge dewatering were achieved for the dual conditioning method. The MC of sewage sludge after the conditioning by means of polyelectrolyte and biomass ash in the dosages of 7.5–30 kg/m3 was 79.07; 70.89 and 66.13%, respectively. The aforementioned method could decrease the MC by approximately 19; 27 and 30%, accordingly. Higher effectiveness of dual conditioning methods is closely related with the synergic effect of chemical and physical conditioners. According to Zhu et al. [42,43], the main mechanism of dual conditioning is based on charge neutralization, bridging of particles and improvement of structure. Superior

Fig. 2. The influence of different conditioning methods on sewage sludge moisture content.

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3.3. Changes in filtrates after sewage sludge dewatering

effectiveness of dual conditioning than for the addition of only physical conditioners was also reported by Kuglarz et al. [14] and Weichao et al. [32]. The SRF of raw sewage sludge after vacuum filtration was 2.19·1012 m/kg, which confirms its poor filterability (Fig. 3). It was noted that all conditioning methods influenced the decrease of the SRF value. After sludge conditioning with the application of biomass ash alone, in the dosages of 7.5, 15 and 30 kg/m3, SRF decreased, respectively, by 15, 29 and 43%. Zhu et al. [43] reported that the addition of raw rice husk decreased the SRF by only 9% in comparison to raw sludge. Weichao et al. [32] showed that application of cinder in the dosage of 24 kg/m3 caused the SRF reduction by 30%. Slightly better results were achieved for the application of SEDIFLOC 1050 CMMW cationic polyacrylamide. Chemical conditioning resulted in the decrease of SRF by approximately 17, 32 and 46% for the dosages of 1; 1.5 and 2 g/kg DM. For the dosage of 2 g/kg DM, Wolny [37] achieved 70% reduction of SRF. However, Wolny [37] applied another polyelectrolyte in her research. The lowest SRF values were achieved for sewage sludge after previous conditioning by means of the dual method. The SRF decreased with the increase of biomass ash dosage. The application of ash in a dosages of 7.5–30 kg/m3 in conjunction with polyelectrolyte in the dosage of 1 g/kg DM could decrease SRF by 42; 46 and 53%, respectively. Kuglarz et al. [14] reported that the dual conditioning method with the use of polyelectrolyte and coal fly ash decreased the SRF value by approximately 47%, but they used other reagents in their tests. The results of the research showed that the biomass ash is an effective conditioner in comparison to chemical reagents. Additionally, the high improvement of dewatering might be achieved by the reduction of the polyelectrolyte dosage and its replacement by the addition of biomass ash.

The filtrate volume after vacuum filtration was shown in Fig. 4. After filtration of raw sewage sludge, 42 cm3 of filtrate was obtained. After conditioning with the use of biomass ash alone, the filtrate volume was in the range of 42.5–44 cm3, depending on the dosage of ash. It was noted that the volume increased with the increase of the biomass ash, but changes were not significant. When dual conditioning with the use of biomass ash and polyelectrolyte was applied, the filtrate volume increased to 46–47 cm3. Similarly to physical conditioning, the amount of filtrate increased with the increase of the dosage of biomass ash, but the changes were not significant. Weichao et al. [32] also reported small changes in the filtrate volume after conditioning with the use of raw and chemically modified cinder. It is closely related with the specific properties of combustion by-product. According to Behera (2010), most of ashes have hygroscopic nature and for this reason, the small change of filtrate volume after sludge conditioning was observed. For 1; 1.5 and 2 g/kg DM dosages of SEDIFLOC 1050CMMW cationic polyacrylamide; 46, 47 and 48 cm3 of filtrate were obtained. Although the filtrate volume did not change significantly after sludge conditioning, the filtration speed was higher than for raw sludge. In order to obtain 10 cm3 of filtrate from raw sludge, approximately 120 s were needed. It was observed that this time decreased with the increase of the reagent doses. For the highest tested dosage of the ash in physical and dual conditioning methods, the filter duration was reduced to 40 and 20 s, respectively. Similar observations were noted by Wójcik et al. [35] for sewage sludge after conditioning with the use of biomass ash. Weichao et al. [32] also observed the shortening of filter duration after the addition of cinder. The application of SEDIFLOC 1050CMMW cationic polyacrylamide in the dosage of 2 g/kg DM resulted in reduction of filter duration from 120 to 30 s. Physical and dual conditioning of sewage sludge also influenced chemical characteristics of the obtained filtrates. The pH of the filtrate and the total suspension (TS) after vacuum filtration are shown in Fig. 5 and in Table 4. When raw sewage sludge was filtered, pH and TS were 7.72 and 0.00 kg/m3 respectively. After chemical conditioning with the use of SEDIFLOC 1050CMMW cationic polyacrylamide, the TS did not change. The pH after the addition of chemical conditioner decreased insignificantly. However, physical and dual conditioning methods affected the increase of pH and TS. The highest value of pH, namely 10.55 and 10.57, was achieved for 30 kg/m3 dosage of ash for both methods. The increase of the pH of filtrates is closely related with leaching of alkaline ions from biomass ashes. For the lowest tested dosage of biomass ash (7.5 kg/m3), the TS did not change. However, the application of higher dosages of ash influenced slow increase of TS in filtrates. With the addition of biomass ash alone in the dosages of 15 and 30 kg/m3, TS was 0.15 and 0.24 g/cm3, respectively. After dual conditioning with the use of biomass ash and polyelectrolyte, the TS was 0.11 and 0.19 g/cm3 for 15 and 30 kg/m3

Fig. 4. The influence of different conditioning methods on the filtrate volume after vacuum filtration.

Fig. 5. The influence of different conditioning methods on pH of filtrate after vacuum filtration.

Fig. 3. The influence of different conditioning methods on SRF of sewage sludge after vacuum filtration.

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5. Conclusions

Table 4 The influence of sludge conditioning on the TS of filtrates. Sample

TS [g/cm3]

RS B1 B2 B3 P1 P2 P3 PB1 PB2 PB3

0.00 0.00 0.15 0.24 0.00 0.00 0.00 0.00 0.11 0.19

dosages of ash, respectively. This can be explained by the fact that biomass ash particles are relatively loosely bounded to the floc matrix and might be easily removed under stress [33].

4. Economic analysis of different conditioning methods Before application of a new method of sludge conditioning, the initial economic analysis is necessary. In the previous economic analysis (Table 5), the cost of conditioners was included. The price of SEDIFLOC 1050 CMMW cationic polyacrylamide is EURO 1580 per ton. The cost of biomass ash included only its transport from a power plan. The method is economically viable for treatment plants which are located closer than 50 km from a power plant. The price for 1 km is approximately EURO 1. Depending on a dosage, the total cost of sewage sludge conditioning with the use of biomass ash is in the range of EURO 0.29–1.14 per ton of dry mass. The total cost of chemical conditioning is in the range of EURO 1.58–3.16 per ton of dry mass. The dual conditioning method with the application of polyelectrolyte in the dosage of 1 g/kg DM and biomass ash confirms the improvement of sludge dewatering with a lower cost than in the case of chemical conditioning. Depending on the dosage of biomass ash, the cost of dual conditioning is in the range of EURO 1.87–2.27 per ton of dry mass. The reduction of the highest dosage of polyacrylamide by a half and the application of biomass ash in the dosages of 15 and 30 kg/m3 can decrease the total cost by 31 and 14% while providing better results of dewatering. Wang and Virraraghavan [31] noted that the application of coal fly ash instead of lime can decrease the operation cost of treatment plant by 80%. Stachowicz et al. [27] also proved that sewage sludge conditioning with the use of biomass ash can reduce the cost of dewatering by even 90% in comparison to the addition of polyelectrolyte. Therefore, conditioning methods based on the use of biomass ashes are a promising alternative in sewage sludge management.

Table 5 Economic analysis for different conditioning methods. Method of Dosages conditioning Biomass ash [kg/m3]

Polyelectrolyte [g/kg DM]

B1 B2 B3 P1 P2 P3 PB1 PB2 PB3

– – – 1.0 1.5 2.0 1.0 1.0 1.0

7.5 15.0 30.0 – – – 7.5 15.0 30.0

Cost of conditioners [EURO/ton]

Total cost [EURO/ton DM]

1.0

0.29 0.58 1.14 1.58 2.37 3.16 1.87 2.16 2.72

1580.0

1580.0 (polyelectrolyte); 1.0 (biomass ash)

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In this study, chemical, physical, as well as dual conditioning methods were compared. After chemical conditioning with the application of cationic polyacrylamide in different dosages, CST, MC and SRF decreased by approximately 69–93%, 21–24% and 17–46%, respectively. Depending on the dosage of biomass ash, sludge conditioning by means of biomass ash could decrease the aforementioned parameters by approximately 34–86%, 13–14% and 15–43%, accordingly. CST, MC and SRF decreased with the increase of a biomass ash dosage. The significant improvement of sludge dewaterability was also achieved for the dual conditioning method with the addition of biomass ash and polyelectrolyte. The application of cationic polyacrylamide in a dosage of 1 g/kg DM in conjunction with the biomass ash could decrease CST, MC and SRF by approximately 37–87%, 18–30% and 42–52%, depending on the dosage of ash. The influence of biomass ash on the growth of pH of sewage sludge was also noted. The sludge conditioning did not affect the growth on the filtrate volume in a significant way, but the filtration was more rapid in comparison to raw sludge. As a result, the filter duration was reduced. The physical and dual conditioning methods also influenced the increase of suspended solids and pH of filtrates. The addition of polyacrylamide did not affect the change of the characteristics of filtrate in a significant way. Among all tested conditioning methods, the application of polyelectrolyte in conjunction with biomass ash was characterized by the highest effectiveness. The dual conditioning method did not improved the sludge dewatering in a significant way in comparison to the use of polyelectrolyte, but the reduction of a polyacrylamide dosage by a half and its replacement by the use of biomass ash can decrease the cost of sludge dewatering by even 30%. Due to the aforementioned aspects, the sludge conditioning methods with the use of biomass ash alone or in conjunction with the chemical conditioners might be a promising solution in waste management. Acknowledgements Funding: This work was financially supported by Research Funds “The analysis of the application of biomass ashes in sewage sludge management” [No. BT.17.003], Rzeszow University of Technology, Poland. References [1] R.K. Bahera, Characterization of Fly Ash for their Effective Management and Utilization, 2010, National Institute of Technology. Orissa, 2010. [2] S. Baroutian, N. Eshtiaghi, D.J. Gapes, Rheology of a primary and secondary sewage sludge mixture: dependency on temperature and solid concentration, Bioresour. Technol. 140 (2013) 227–233. [3] A. Bianchini, L. Bonfiglioli, M. Pellergini, C. Saccani, Sewage sludge management in Europe: a critical analysis of data quality, IJEWM 18 (3) (2016) 226–238. [4] C. Chen, P. Zhang, G. Zeng, J. Deng, Y. Zhou, H. Lu, Sewage sludge conditioning with coal fly ash modified by sulphuric acid, Chem. Eng. J. 158 (2010) 616–622. [5] Z. Chen, M.T. Afzal, A.A. Salema, Microwave drying of wastewater sewage sludge, JOCET 2 (3) (2014) 282–286. [6] Q. Chen, Y. Wang, Influence of single- and dual-flocculant conditioning on the geometric morphology and internal structure of activated sludge, Powder Technol. 270 Part A (2015) 1–9. [7] M. Dębowski, M. Zieliński, M. Krzemieniewski, Efficiency of sewage sludge conditioning with the Fenton's method, Environ. Poll. Control 30 (2) (2008) 43–47 (in Polish). [8] A. Ding, F. Qu, S. Guo, Y. Ren, G. Xu, G. Li, Effect of adding wood chips on sewage sludge dewatering in a pilot-scale plate-and-frame filter press process, RSC Adv. (47) (2014) 24762–24768. [9] E. Eriksson, N. Christensen, J.E. Schmidt, A. Ledin, Potential priority pollutants in sewage sludge, Desalination 226 (2008) 371–388. [10] V. Feodorov, Modern Technologies of Treatment and Stabilization for Sewage Sludge from Water Treatment Plant, Agric. Agric. Sci. Procedia 10 (2016) 417–430. [11] Z. Hao, B. Yang, D. Jahng, Combustion characteristics of biodried sewage sludge, Waste Manag. 72 (2018) 296–305. [12] S.M. Hong, J.K. Park, N. Teeradej, Y.O. Lee, Y.K. Cho, C.H. Park, Pretreatment of sludge with microwaves for pathogen destruction and improved anaerobic digestion performance, Water Environ. Res. 78 (2006) 76–83.

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