Coupled heating/acidification pretreatment of chemical sludge for dewatering by using waste sulfuric acid at low temperature

Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Sludge from wastewater treatment processes with high water content and large volume has become an inevitable issue in environmental management. Due to the challenging dewatering properties of sludge, current mechanical dewatering methods are no longer sufficient to meet the escalating water content standards of sludge. This paper summarizes the characteristics of various sludge and raises reasons for the their dewaterability differences. Affected by extracellular polymeric substances, biological sludge is hydrophilic and negatively charged, which limits the dewatering degree. The rheological properties, flocs, ionic composition, and solid phase concentration of the sludge also influence the dewatering to some extent. For these factors, the chemical conditioning measures with simple operation and excellent effect improve its dewaterability, which mainly include flocculation/coagulation, acid/alkali treatment, advanced oxidation, surfactant treatment and combined treatment. There is a growing necessity to explore the development of new chemical conditioning agents, even though traditional agents continue to remain widely used. However, the development of these new agents should prioritize finding a balance between various factors such as efficiency, effectiveness, ease of operation, environmental safety, and cost-effectiveness. Electrochemical dewatering enhances solid-liquid separation, and its coupling with chemical conditioning is also an excellent means to further reduce water content. In addition, the improvement of press filter is an effective way, which is influenced by pressure, processing time, sludge cake thickness and pore structure, filter media etc. In general, it is essential to develop new conditioning agents and enhance mechanical filtration press technology based on a thorough understanding of various sludge properties. Concurrently, an in-depth study of the principles of mechanical pressure filtration will contribute to establishing a theoretical foundation for effective deep sludge dewatering and propel further advancements in this field.

Treatment of industrial ferric sludge through a facile acid-assisted hydrothermal reaction: Focusing on dry mass reduction and hydrochar recyclability performance

Large amounts of Fenton sludge and waste activated sludge (WAS) are mixed as ferric sludge (FS) in most industrial wastewater treatment plants. The treatment of such waste represents a challenge and quantity-dependent cost, so that a reliable way for FS waste reduction is required. In this study, we develop a facile acid-assisted hydrothermal treatment (HT) for the cost-efficient treatment of hazardous FS waste. Sulfuric acid was dosed at 0.25 mL/g dry solid (DS) to the HT process, which significantly increased the total solid mass reduction (TMR) by 25.1 % and dry mass reduction (DMR) by 104.4 %. The participation of sulfuric acid during the HT process changed the HT reaction pathway from dehydration to demethylation based on the analysis of the derivative thermogravimetric and Van Krevelen diagram. The addition of sulfuric acid improved the release of Fe from FS by 52.9 %, which contributed to the DMR. During the acid-assisted HT, Fe(III) was effectively reduced to Fe(II) within the produced hydrochar, which can be recycled for the Fenton reaction during the degradation of actual industrial wastewater such as pharmaceutical wastewater. Moreover, Sulfuric acid facilitated the generation of sulfonated hydrochar, which was efficient as an adsorbent for the complete removal of some metals such as Cu(II) - cation metal (98.8 %) and Cr(VI) - anion metal (99.9 %). This study firstly provides a novel and reliable approach for hazardous FS reduction and pointed out the recycling of hydrochar as the supplement for the Fenton reaction and adsorbents for some hazardous heavy metals.

Principles and potential of thermal hydrolysis of sewage sludge to enhance anaerobic digestion

Sewage sludge is rich source of carbon, nutrients, and trace elements and can be subjected to proper treatment before disposal to fulfill government legislation and protect receiving environments. Anaerobic digestion (AD) is a well-adopted technology for stabilizing sewage sludge and recovering energy-rich biogas and nutrient-rich digestate. However, a slow hydrolysis rate limits the biodegradability of sludge. In the present study we have attempted to explain the potential of thermal hydrolysis to enhance anaerobic digestion of sewage sludge. Thermal pretreatment improves biodegradability and recycling of the sludge as an excellent energy and nutrients recovery source at reasonable capital (CAPEX) and operational (OPEX) costs. Other pretreatments like conventional (below/above 100 °C), temperature-phased anaerobic digestion (TPAD), microwave and chemically mediated thermal pretreatment have also been accounted. This review provides a holistic overview of sludge's characterization and value-added properties, various techniques used for sludge pretreatment for resource recovery, emphasizing conventional and advanced thermal pretreatment, challenges in scale-up of these technologies, and successful commercialization of thermal pretreatment techniques.

The Performance and Mechanism of Sludge Reduction by the Bioaugmentation Approach

Simple Summary

Activated sludge-based wastewater treatment process is one of the most popular adopted systems in wastewater treatment plants around the world. Excess sludge is an inevitable byproduct of the process, and the enormous quantity has brought a significant burden on operational costs. Various physicochemical and biological methods have been developed. Biological-based methods are promising because of less chemical consumption and low operation cost comparing to physicochemical methods. Hence, the present study is aimed at searching for functional bacteria that could reduce sludge, enhance the performance of sludge reduction through optimization, and try to unveil the underlying mechanism during sludge reduction. A total of 19 strains that belong to Firmicutes, Proteobacteria, and Actinobacteria were successfully isolated and identified. Subsequently, the performance of sludge reduction by pure culture or mix-cultures was validated. In total, 21.2% and 13.9% of total suspended and volatile suspended solids were reduced within 48 h after optimization via response surface methodology. The three-dimensional excitation-emission matrix fluorescence spectrum and hydrolases test results revealed that the sludge reduction might be promoted by the strain mainly through hydrolysis via proteinase and amylase. The results obtained from the study could help us to find an effective and economical way to resolve the sludge issue.

Abstract

Millions of wastewater treatment plants (WWTPs) based on the activated sludge process have been established worldwide to help to purify wastewater. However, a vast amount of sludge is inevitably generated, and the cost of sludge disposal could reach over half of the total operation cost of a WWTP. Various sludge reduction techniques have been developed, including physicochemical, biological, and combinational methods. Micro-organisms that could reduce sludge by cryptic growth are vital to the biological approach. Currently, only limited functional bacteria have been isolated, and the lack of knowledge on the underlying mechanism hinders the technique development. Therefore, the present study is aimed at isolating sludge-reducing bacteria and optimizing the sludge reduction process through response surface methodology. Nineteen strains were obtained from sludge. The mix-cultures did not show a higher sludge reduction rate than the pure culture, which may be ascribed to the complicated interactions, such as competition and antagonistic effects. In total, 21.2% and 13.9% of total suspended and volatile suspended solids were reduced within 48 h after optimization. The three-dimensional excitation-emission matrix fluorescence spectrum and hydrolases test results revealed that the sludge reduction might be promoted by the strain mainly through hydrolysis via proteinase and amylase. The results obtained from the study demonstrate the potential of using micro-organisms for sludge reduction through cryptic growth.

Freeze-thaw vacuum treatment of landfill sludge: Mechanism of uneven frost heaving and dewatering performance

The method of freeze-thaw combined with vacuum pretreatment for landfill sludge (LS) has attracted extensive attention. However, most of the existing approaches are based on small-scale laboratory testing, and further testing studies must be performed to realize in situ treatment. To enhance the practicality of such approaches, the range of temperature effects on LS was analysed after field freeze-thaw model testing. After the freeze-thaw model test, samples were transported to the laboratory for unidirectional oedometer tests, and the remaining samples were retained in the field to continue vacuum model testing for exploring the differences in the consolidation and drainage effect of the LS. Results show that temperature changes during freeze-thaw process affect the distribution of sludge and water in the model boxes, resulting in frost heave and the appearance of "extrusion rings". In addition, the coefficient of consolidation obtained from the unidirectional oedometer test shows that the consolidation coefficient is generally larger near the freezing tubes at a lower temperature. The settlement determined from the field vacuum preloading test shows that the subsequent vacuum consolidation settlement is larger at the position with a lower elevation of the frozen sludge surface. The comparison indicates that the consolidation and drainage effect in the field is not as significant as that in the laboratory. The findings can provide reference to optimize the field conditions during the in situ engineering practice of sludge treatment.

Co-utilization of lake sediment and blue-green algae for porous lightweight aggregate (ceramsite) production

Lake sediment and algal sludge with large output posed significant environmental risks. In this work, an idea of co-utilization of both solid wastes for the production of ceramsite (a sort of porous lightweight aggregates as building materials) was proposed and validated for the first time. The treatment process contained a dewatering step by a flocculation-pressure filtration method, and a sintered ceramsite preparation step. Effects of flocculant type and dosage on the dewatering performance were studied in the first step. An environmental-friendly amphoteric starch flocculant with a dosage of 12 mg/(g dried sample) was found to achieve the best dewatering performance. Effects of raw material mass ratio, sintering temperature and time in the second step were investigated. Under the optimal conditions (60 wt% of dewatered sediment; 20 wt% of dewatered algal sludge; 20 wt% of additives (fly ash: calcium oxide: kaolin = 2:1:2); sintering temperature: 1100 °C; time: 35 min), the obtained ceramsite met the Chinese National Standard as a qualified building material, with reliable environmental safety according to the leaching results for both heavy metals and microcystins. Both environmental and economic benefits of the proposed treatment were assessed. The process completely followed the rules of "reduction, harmlessness and resource utilization" for solid waste treatment and disposal; Meanwhile, the profit of the proposed ceramsite production could be more than 2.3 US dollar/m³. The co-utilization method in this work acted as a good example for the comprehensive management of solid wastes in water-rich areas.

Enhanced dewatering of activated sludge by acid assisted Heat-CaO2 treatment: Simultaneously removing heavy metals and mitigating antibiotic resistance genes

High water content and accumulation of heavy metals and antibiotic resistance genes (ARGs) in sewage sludge limit its application. Fenton process has been widely used in sludge dewatering, but the use of hydrogen peroxide (H₂O₂) and generation of acid sludge are the main drawbacks. Here, a novel method of heat-CaO₂ treatment was proposed to enhance sludge dewatering. Results showed that CaO₂ (12.5 mg/g dry solids (DS)) combined with heat at 60 °C significantly improved the sludge dewaterability, e.g. the water content decreased from 79.9% to 69.2% and the specific resistance to filtration (SRF) decreased from 9.21 × 10¹³ to 1.51 × 10¹³ m/kg. At 62.5 mg CaO₂/g DS, the final pH of filtrate was close to neutral and the good dewatering performance was still achieved. The improvement of sludge dewaterability was closely correlated with the decomposition of tightly-bound extracellular polymeric substances (EPS), lysis of the sludge cells, and increased particle size of the flocs. The distribution of bacterial community in the sludge has changed, leading to the decreases in the percentage of some ARGs. The concentrations of typical heavy metals wrapped in the sludge colloid network dramatically reduced. Economic analyses showed that the heat-CaO₂ treatment was a promising method for sludge disposal.

Agricultural and non-agricultural directions of bio-based sewage sludge valorization by chemical conditioning

This literature review outlines the most important-agricultural and non-agricultural-types of sewage sludge management. The potential of waste sludge protein hydrolysates obtained by chemical sludge conditioning was reported. The discussed areas include acidic and alkaline hydrolysis, lime conditioning, polyelectrolyte dewatering and other supporting techniques such as ultrasounds, microwave or thermal methods. The legislative aspects related to the indication of the development method and admission to various applications based on specified criteria were discussed. Particular attention was devoted to the legally regulated content of toxic elements: cadmium, lead, nickel, mercury, chromium and microelements that may be toxic: copper and zinc. Various methods of extracting valuable proteins from sewage sludge have been proposed: chemical, physical and enzymatic. While developing the process concept, you need to consider extraction efficiency (time, temperature, humidity, pH), drainage efficiency of post-extraction residues and directions of their management. The final process optimization is crucial. Despite the development of assumptions for various technologies, excess sewage sludge remains a big problem for sewage treatment plants. The high costs of enzymatic hydrolysis, thermal hydrolysis and ultrasonic methods and the need for a neutralizing agent in acid solubilization limit the rapid implementation of these processes in industrial practice.

An enhanced disintegration using refinery spent caustic for anaerobic digestion of refinery waste activated sludge

Alkali-mediated disintegration is efficient to improve the anaerobic digestion of waste activated sludge (WAS). In the present study, the role and potential of refinery spent caustic (RSC), an alkaline hazardous waste, in enhancing the disintegration of refinery waste activated sludge (RWAS) was investigated. The high alkalinity and free ammonia of RSC destroyed the microbial cell wall and promoted the release of intracellular substances. The contents of N-acetylglucosamine and proteins in the disintegrated liquid greatly increased to 0.41 mg/L and 1147 mg/L, respectively, relative to no disintegration (0.04 mg/L and 3.3 mg/L). The methane production (66.1 mL/g-VS) from RWAS anaerobic digestion increased by 226% compared to without disintegration (20.3 mL/g-VS). This study provides a newly developed "wastes-treat-wastes" management approach of refinery wastewater using combined treatment processes for RWAS and RSC using a cost-efficient and environmentally friendly disintegration of RWAS.

Full-scale thermophilic aerobic co-composting of blue-green algae sludge with livestock faeces and straw

A high incidence of harmful algal bloom in eutrophic surface waters causes many environmental problems. Thermophilic aerobic composting enables effective treatment and disposal of algal sludge that remains after the dewatering of algae slurries, and provides a value-added organic fertiliser. Previous studies have either only dealt with the composting of a single waste component or were conducted at a lab-/pilot-scale; however, this work is a comprehensive assessment of full-scale mechanized thermophilic aerobic co-composting of algal sludge and other typical biomass-based wastes, including chicken faeces and rice straw, in a water-rich rural area in the Tai lake basin, China. With the optimised feedstock material mass ratio (6.0:1.8:1.0 for straw:algae:faeces; initial C/N ratio of 20; and initial moisture of 60 wt%), the co-composting process effectively achieved the reduction, harmlessness, and reuse of waste. The moisture content (28.36 wt% of wet weight), organic matter content (57.91 wt% of dried weight), total nutrient content (6.59 wt% for TN + TP + TK of dried weight), and heavy metal contents as well as the pH of the final product fully met the Chinese National Agricultural Organic Fertiliser Standard requirements. The reduction rates of microcystin and toxic volatile fatty acid contents were higher than 99.5%, and the seed germination index of the product was 114.5%. A notable economic benefit with a gross profit margin of 167-434% of the process was highlighted. Investigation of the associated mechanisms, including statistical analysis, spectral characterisation, micro-morphological observation, and microbial community analysis, revealed that a decreased particle sizes with a looser structure and an efficient humification effect, resulting from the work of several identified dominant microbial species, contributed to the high product quality. The current study provided a demonstration of the promising full-scale co-composting technology for comprehensive management of the environment in water-rich rural areas and the construction of a sustainable watershed.

Bioelectrochemical system for the enhancement of methane production by anaerobic digestion of alkaline pretreated sludge

An increasing interest is devoted to combined microbial electrolysis cell-anaerobic digestion (MEC-AD) system which could convert waste activated sludge into biogas. In this study series tests were initially conducted to study the effect of alkaline pretreatment on AD system and the results showed that alkaline pretreatment could promote the dissolution of organic matters in the sludge and thus improve the methane production. Then, the methane production in combined MEC-AD system fed with alkaline-pretreated sludge was investigated. The results indicated that the methane productions increased by 37% and 42% when applied voltage was 0.5 V and 0.8 V. The microbial electrochemical system strongly promoted the growth of Euryarchaeota (Methanosaeta and Methanobacterium). Meanwhile, the abundance of Paraclostridium increased from 17.9% to 38.5% when applied voltage was 0.8 V, suggesting an enhanced fermentation and acetogenesis process. The results of energy balance estimation indicated that MEC-AD system at 0.5 V could achieve higher net energy output.

Fe(II) catalyzing sodium percarbonate facilitates the dewaterability of waste activated sludge: Performance, mechanism, and implication

In this work, Fe(II) catalyzing sodium percarbonate (Fe(II)/SPC) was managed to facilitate waste activated sludge (WAS) dewatering for the first time. The results showed that after WAS was treated by 20 mg/g total suspended solids (TSS) Fe(II) and 50 mg/g TSS SPC, the water content of sludge cake (WC_SC) by press filtration and capillary suction time (CST) dropped from 90.8% ± 1.6% and 96.1 ± 4.0 s (the control) to 55.6% ± 1.4% and 30.1 ± 2.5 s, respectively. The mechanism investigations indicated that four intermediates or products (i.e., •OH, H₂O₂, Fe(II), and Fe(III)) generated in the Fe(II)/SPC process were responsible for the improved WAS dewaterability, and •OH and Fe(III) were the two major contributors. It was found that •OH collapsed and fragmented extracellular polymeric substances, damaged cell wall and permeabilized cytoplasmic membrane, and transformed conformation of the extracellular proteins secondary structure via both affecting the hydrogen bond maintaining α-helix and cracking disulfide bond in cysteine residues while Fe(III), the oxidization product of Fe(II), decreased the surface electronegativity and water-affinity surface areas of WAS flocs. As a result, the bound water release, flocculability, surface hydrophobicity, drain capability, and flowability of WAS flocs were strengthened whereas the compact surface structure, colloidal forces, network strength, gel-like structure, and apparent viscosity of WAS flocs were weakened. In addition, Fe(II)/SPC process also reduced the recalcitrant organics and fecal coliforms in sludge, which facilitated land application of dewatered sludge. The findings acquired in this work not only deepens our understanding of Fe(II)/SPC-involved WAS treatment process but also may guide engineers to develop both effective and promising strategies to better condition WAS for dewatering in the future.

Effect of citric acid on extracellular polymeric substances disruption and cell lysis in the waste activated sludge by pH regulation

This paper investigated the effects of citric acid (CA) on extracellular polymer destruction and cell lysis in sludge at different initial pH by measuring capillary suction time (CST), extracellular polymeric substances (EPS) and intracellular bound water. The results indicated that under CA concentration at 0.05 g/g suspended solids (SS) and initial pH 4, the CST value decreased from 175.5 s to 112.3 s, slime extracellular polymeric substances (S-EPS) and loosely bound EPS (LB-EPS) content respectively to increase from 4.92 to 41.43, 2.27 to 5.49 mg/g volatile suspended solids (Vss), while tightly bound EPS (TB-EPS) content to decrease from 12.35 to 5.01 mg/g (Vss), which suggested CA could disrupt outer EPS effectively. Intracellular bound water content decreased from 1.23 g/g to 0.41 g/g dry solid (DS). As a result, CA could release intracellular bound water effectively, thereby improving sludge dewatering degree.

A one-step acidification strategy for sewage sludge dewatering with oxalic acid

Sewage sludge dewatering is an efficient approach to reduce the volume of sludge for the subsequent disposal. In this study, a novel one-step acidification sludge dewatering method was developed with using oxalic acid as a conditioner. In laboratory-scale experiments with the dosage of 200 mg/g dry solid (DS), the normalized capillary suction time and the specific resistance to filtration were respectively decreased by 78.7% and 60.0% after 30 min of oxalic acid conditioning, much more efficient than those conditioned with sulfuric acid and hydrochloric acid at the same pH value. This superior dewatering performance was attributed to two factors. One was that oxalic acid could more efficiently promote the hydrolysis of polysaccharide, especially pectins, to release bound water. The other was that OA could dissolve more Fe³⁺ and Al³⁺, as well as form precipitate with Ca²⁺ in sludge, which may act as flocculants or co-precipitator for the subsequent sludge particles coagulation. In pilot-scale experiments, the water content of oxalic acid conditioned sludge cake was reduced to 60% under the optimum conditions, while the reagent cost was as low as 110.0 USD/t DS. This work provides a cost-effective and easy-operated sewage sludge disposal technique, and also sheds light on the potential of oxalic acid in environmental waste treatment.

Effect of extracellular polymeric substances (EPS) conditioned by combined lysozyme and cationic polyacrylamide on the dewatering performance of activated sludge

Extracellular polymeric substance (EPS) and the water within it account for about 80% of the total sludge mass, significantly correlated with sludge charge properties, particle size, and dewaterability, while their relation is still ambiguous. To clarify the effect of EPS characteristics on the activated sludge dewaterability, the sludge conditioned by cationic polyacrylamide (CPAM) and lysozyme alone or in combination was comparatively investigated by the means of the SPSS17 software incorporation with chemical analysis. According to the results, the combined conditioning increased both dewatering extent and dewatering rate with the water content as low as 57.79%. It was mainly attributed to the destruction of microbial cell wall and EPS structure by enzymatic conditioning, beneficial for the release of protein (PN) and polysaccharide (PS), as well as the conversion of intracellular water and some bound water into free water. Additionally, Pearson's correlation and factor analysis confirmed the significant influence of EPS properties on sludge dewaterability and explored their detailed relationship. It was indicated the most crucial factors consisted of PNS (PN in S-EPS), PNL (PN in LB-EPS), PNT (PN in TB-EPS), and PST (PS in TB-EPS) accounted for 72.83% in all of the total variance for the contribution to the dewatered water content. Moreover, the high concentration of PNS and PNL led to the zeta potential rising to -9.74 mV, and the destruction of EPS structure was favorable for sludge to form smaller particle size and compact floc structure. All the results were confirmed by the microstructure changes of the sludge flocs.