Energy demand in sludge dewatering

Conversion of biomass waste to solid fuel via hydrothermal co-carbonization of distillers grains and sewage sludge

A synergistic process was proposed to prepare hydrochar by hydrothermal co-carbonization (HTcoC) of waste distillers grains with sewage sludge, focusing on hydrochar properties and combustion behavior under different mixing ratios. Results show that the co-hydrochar from HTcoC exhibited excellent synergistic characteristics with relatively high synergistic coefficients (0.1-1.2% for hydrochar yield, 4.8-8.0% for higher heating value (HHV), 8.0-12.6% for organic retention, and 2.2-4.0% for carbon retention, respectively), partially evidenced by FTIR data. And the co-hydrochar showed a higher fuel ratio of 0.09-0.13 with the fixed carbon increased to 8.3-10.0 at an remarkably enhanced coalification degree. Moreover, thermal analysis showed that the co-hydrochar exhibited improved combustion efficiency and a more stable flame. As a result, the HTcoC process with 13.0-22.5% increase in biofuel recovery rate and 25.6-47.7% increase in net energy gain may provide an effective approach for the conversion of both biomass wastes into clean biofuel.

Performance and stability of biogas recirculation-driven anaerobic digestion system coupling with alkali addition strategy for sewage sludge treatment

Wastewater treatment plants are particularly challenging with the treatment and disposal of sewage sludge produced from the treatment units due to its high costs and environmental hazards. In this study, a biogas recirculation-driven anaerobic digestion (AD) system was developed with upward shear force being provided by biogas recirculation coupled with the alkali addition strategy, targeting biogas upgrading, sludge stabilization, and sludge flocculation simultaneously, thus reducing the sludge management costs. Compared to the conventional AD system, the novel biogas recirculation-driven AD system could achieve biogas upgrading with 10% higher CH₄ content. Besides, the combination of NaOH and Ca(OH)₂ addition strategy obviously improved sludge settleability and dewaterability compared to the single NaOH addition strategy. Owing to the attraction between negatively charged sludge particles and Ca²⁺ ions, the available Ca²⁺ in the former AD system may facilitate the re-flocculation and P immobilization in solid digestate, fix partial CO₂ with less CO₂ emission, and bridge with some sludge flocs. Moreover, 12.6% lower net cost for sludge management was achieved by this biogas recirculation-driven AD system together with the combination alkali addition strategy, which is regarded as a promising integrated multi-purpose system for sludge treatment.

Anaerobic co-digestion of hydrolysate from anaerobically digested sludge with raw waste activated sludge: Feasibility assessment of a new sewage sludge management strategy in the context of a local wastewater treatment plant

Sustainable sewage sludge management is a worldwide issue in wastewater treatment plants (WWTPs). This work developed a new strategy for sewage sludge treatment involving the integration of hydrothermal treatment (HT) with anaerobic co-digestion (AcoD), particularly on the feasibility of mesophilic AcoD of anaerobically digested sludge (DS) hydrolysate and waste activated sludge (WAS). Results show that AcoD of DS hydrolysate from HT 170℃ for 30 min with WAS achieved the highest CH₄ production of 205.39 mL CH₄/g-VS_fed. By adopting the new AD-HT170-AcoD strategy, 61.88 mL CH₄/g-tVS_fed higher CH₄ yield and 22.2% more total solids (TS) reduction were obtained in addition to much better sludge settleability and 7.6% wt. less sludge cake production compared to the conventional mono AD. Although negative energy gain was obtained, the proposed AD-HT170-AcoD strategy is promising, economically feasible, and sustainable when the final sludge disposal of WAS is concerned in the context of whole WWTP.

Mechanism analysis to improve sludge dewaterability during anaerobic digestion based on moisture distribution

Previous studies have demonstrated that anaerobic digestion (AD) enhances sludge dewaterability. However, the mechanism of AD influence on digested sludge dewaterability is still not well understood. In this study, moisture distribution and bond energy were used to evaluate the influence of AD on sludge dewaterability. The change from free moisture (FM) to mechanically bound moisture (MBM) was analysed, along with degradation of organic fractions in extracellular polymeric substances (EPSs) of the sludge during AD. Results indicated that the AD process, with a 41.9% reduction in volatile solids (VS) and cumulative methane production of 283.2 mL/g VS, improved sludge dewaterability by disintegrating macro-molecular organic matter into micro-molecular matter. MBM reduction reached 7% in 40 days. Correlations between FM/MBM and extracellular protein (extra-PN) were significant (R = -0.861, p < 0.01; R = 0.869, p < 0.01). Therefore, it can be concluded that the degradation of protein in the extra-microcolony polymer (EMPS) layer results in the destruction of the bond between organic fractions and moisture. As a result, the release of mechanically bound moisture is accelerated. In addition, the moisture content, as well as the extra-PN, continued to vary when AD entered the stabilisation stage, that is, the destruction of protein in EMPS stimulated mechanically bound moisture release during AD. The results of this work provide a better understanding on the effect of AD on sludge reduction.

Evaluation of acidification and oxidation of sludge to improve the effect of a starch-based flocculant on the dewaterability of sewage sludge

Conditioning is essential for achieving effective sludge dewatering and easier disposal. In this study, a combined pretreatment of acidification and oxidation using potassium permanganate (KMnO₄) as oxidant was conducted to improve the effect of a cationic starch-based flocculant (St-WH) on the dewaterability of sewage sludge. Synergetic dewatering mechanisms by acidification, oxidation, and flocculation are discussed in detail according to the analysis of the changes in bound water content, extracellular polymeric substances (EPS) fractions and components, zeta potentials, floc size, and surface microstructures of sludge cakes in the dewatering process. Acidification and oxidation could destroy the sludge flocs, thereby causing the degradation of EPS and formation of fine particles. Original loosely and tightly bound EPS partially converted to soluble EPS, resulting in release of trapped water, which can be reflected by the significant correlation between loosely bound EPS and filter cake moisture content (FCMC) (R_s = 0.83, P < 0.05). Those fine particles, simultaneously produced, were adverse to filtration efficiency. In addition to enhancing the oxidation effect of KMnO₄, acidification treatment could still compress the protein-like materials in soluble EPS due to protonation effect, which was positively related to specific resistance to filtration (SRF) (R_s = 0.74, P < 0.05). The following flocculation using St-WH efficiently aggregated those fine particles and restrained the released EPS to bind with free water through charge neutralization and bridging effects, thereby resulting in improved filtration performance and enhanced removal of bound water (R_s = 0.88, P < 0.01). Response surface methodology was also applied to achieve an optimal condition and evaluate the effects of various environmental factors.

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

A cost-effective approach for pretreatment of chemical sludge for further dewatering, based on the idea of "using waste to treat waste", is provided. It is a coupled heating/acidification pretreatment method, where waste sulfuric acid is employed and relatively low temperatures (<100 °C) are applied. Effects of reaction time, temperature, and dosage of waste acid on dewatering performance (both dewatering speed and degree) are studied. Under the optimal conditions (reaction time: 30 min; temperature: 90 °C; waste acid dosage: 0.175 g/(g dried sludge)), the method of this work demonstrates three advantages compared to the conventional method using lime+polyacrylamide: lower moisture content of treated sludge; higher calorific value for incineration process; and lower cost. Detailed mechanism of the pretreatment for dewatering is investigated via characterizations and statistical analyses of various parameters, among which zeta potential, particle size, protein and polysaccharide contents, soluble chemical oxygen demand (SCOD), reduction of combined water and volatile suspended solid (VSS), are associated with dewatering performance. Both heating and acidification generate disintegration of cells in sludge, giving rise to two phenomena: more organic matters are released into solution and more bound water turns into free water. Meantime, the released organic polymers flocculate sludge particles, further accelerating the solid-liquid separation process.

Relationship between enhanced dewaterability and structural properties of hydrothermal sludge after hydrothermal treatment of excess sludge

Hydrothermal treatment is an effective method to enhance the deep dewaterability of excess sludge with low energy consumption. In this study, an insight into the relationship between enhanced dewaterability and structural properties of the produced hydrothermal sludge was presented, aiming at better understanding the effect of hydrothermal process on excess sludge dewatering performance. The results indicated that hydrothermal effect induced the transformation of surface water to interstitial and free water by lowering the binding strength between adjacent water and solid particles and that free water became the main form for moisture existence in hydrothermal sludge as temperature was higher than 180 °C. Increase in temperature of hydrothermal treatment generated a significant size reduction of sludge flocs but treated sludge with a higher rigidity, which not only strengthened the network of hydrothermal sludge but also destroyed the binding of EPS with water. Hydrothermal process caused crevice and pore structures of excess sludge to disappear gradually, which was a main driving force of water removal as temperature was below 150 °C. With the temperature of hydrothermal treatment exceeding 180 °C, the morphology of hydrothermal sludge became rough which linked closely to the solid precipitation of condensation polymerization, and further became smooth at higher temperature (210 °C) due to the coal-like structures with higher aromaticities, indicating that hydrothermal reaction pathways began to play a main role in enhanced dewaterability. Hydrothermal treatment led to more alkyl and aromatic carbon, but lower O-alkyl, carboxyl and carbonyl carbon.

Roles of iron species and pH optimization on sewage sludge conditioning with Fenton's reagent and lime

Conditioning sewage sludge with Fenton's reagent could effectively improve its dewaterability. However, drawbacks of conditioning with Fenton's reagent are requirement of acidic conditions to prevent iron precipitation and subsequent neutralization with alkaline additive to obtain the pH of the filtrate close to neutrality. In this study, roles of pH were thoroughly investigated in the acidification pretreatment, Fenton reaction, and the final filtrate after conditioning. Through the response surface methodology (RSM), the optimal dosages of H2SO4, Fe(2+), H2O2, and lime acted as a neutralizer were found to be 0 (no acidification), 47.9, 34.3 and 43.2 mg/g DS (dry solids). With those optimal doses, water content of the dewatered sludge cakes could be reduced to 55.8 ± 0.6 wt%, and pH of the final filtrate was 6.6 ± 0.2. Fenton conditioning without initial acidification can simplify the conditioning process and reduce the usage of lime. The Fe(3+) content in the sludge cakes showed a close correlation with the dewaterability of conditioned sludge, i.e., the water content of sludge cakes, SRF (specific resistance to filtration), CST (capillary suction time), bound water content, and specific surface area. It indicated that the coagulation by Fe(3+) species in Fenton reaction could play an important role, compared to traditional Fenton oxidation effect on sludge conditioning. Thus, a two-step mechanism of Fenton oxidation and Fe(III) coagulation was proposed in sewage sludge conditioning. The mechanisms include the following: (1) extracellular polymeric substances (EPS) were firstly degraded into dissolved organics by Fenton oxidation; (2) bound water was converted to free water due to degradation of EPS; (3) the sludge particles were disintegrated into small ones by oxidation; (4) Fe(3+) generated from Fenton reaction acted as a coagulant to agglomerate smaller sludge particles into larger dense particles with less bond water; (5) finally, the dewatered sludge cakes were obtained, with less small pores (1-10 nm) that contributed to water affinity, but with more large pores (>10 nm) that contributed to a permeable, rigid lattice structure. Morphology of the Fenton-conditioned sludge cake exhibited a porous structure. The estimated cost of the composite conditioner, Fenton's reagent and lime, is USD$ 43.8/t DS, which is less than that of ferric chloride and lime (USD$ 54/t DS). Furthermore, pH of the final filtrate using this composite conditioner is about 6.6. Comparatively, that using ferric chloride and lime is as high as 12.4.

Dewatering in biological wastewater treatment: A review

Biological wastewater treatment removes organic materials, nitrogen, and phosphorus from wastewater using microbial biomass (activated sludge, biofilm, granules) which is separated from the liquid in a clarifier or by a membrane. Part of this biomass (excess sludge) is transported to digesters for bioenergy production and then dewatered, it is dewatered directly, often by using belt filters or decanter centrifuges before further handling, or it is dewatered by sludge mineralization beds. Sludge is generally difficult to dewater, but great variations in dewaterability are observed for sludges from different wastewater treatment plants as a consequence of differences in plant design and physical-chemical factors. This review gives an overview of key parameters affecting sludge dewatering, i.e. filtration and consolidation. The best dewaterability is observed for activated sludge that contains strong, compact flocs without single cells and dissolved extracellular polymeric substances. Polyvalent ions such as calcium ions improve floc strength and dewaterability, whereas sodium ions (e.g. from road salt, sea water intrusion, and industry) reduce dewaterability because flocs disintegrate at high conductivity. Dewaterability dramatically decreases at high pH due to floc disintegration. Storage under anaerobic conditions lowers dewaterability. High shear levels destroy the flocs and reduce dewaterability. Thus, pumping and mixing should be gentle and in pipes without sharp bends.

Coagulation-membrane filtration of Chlorella vulgaris

Filtration-based separation of Chlorella vulgaris, a species with excellent potential for CO(2) capture and lipid production, was investigated using a surface-modified hydrophilic polytetrafluoroethylene (PTFE) membrane. Coagulation using polyaluminum chloride (PACl) attained maximum turbidity removal at 200 mg L(-1) as Al(2)O(3). The membrane filtration flux at 1 bar increased as the PACl dose increased, regardless of overdosing in the coagulation stage. The filtered cake at the end of filtration tests peaked in solid content at 10 mg L(-1) as Al(2)O(3), reaching 34% w/w, roughly two times that of the original suspension. Differential scanning calorimetry (DSC) tests demonstrate that the cake with minimum water-solid binding strength produced the driest filter cake. Coagulation using 10 mg L(-1) PACl as Al(2)O(3), followed by PTFE membrane filtration at 1 bar, is an effective process for harvesting C. vulgaris from algal froth.

Deodorization and dewatering of biosolids by using dimethyl ether

We proposed a method for the deodorising and dewatering of biosolids. In the proposed method, liquefied dimethyl ether (DME) was used as an extractant for odorous components and water. We developed a bench-scale experiment to almost completely deodorize and dewater biosolids by using liquefied DME at room temperature. The deodorized and dewatered biosolids have sufficient caloric density and can be used as a carbon neutral fuel.