Sludge Solids Concentration Research Articles

Characteristics and aging of microplastics in waste activated sludge under persulfate and hydrothermal co-treatment: Impact of solid content and temperature

Activated persulfate and hydrothermal treatment (HTT) are often employed to treat waste activated sludge, which can improve the efficiency of subsequent sludge treatment and change the distribution of pollutants in the sludge. However, the impact of sludge solid content and temperature on the occurrence and aging of microplastics (MPs) during HTT remains poorly understood. This study investigated the effects of persulfate-HTT (SPS-HTT) co-treatment on the migration, occurrence, and aging of MPs in sludge with different solid contents (2% and 5% solid content). The results indicated that SPS-HTT co-treatment triggers both the disruption of sludge flocs and the melting deformation of MPs at high temperatures, leading to variations in the increasing trend of MP concentration in the solid-liquid phase at different solid contents. 5% solid content sludge showed a weak release of MPs from the solid phase. The proportion of fiber MPs first increased and then decreased with increasing temperature, while no significant changes were observed in the color and type of MPs. Higher temperature and solid content induced the melting deformation of MPs, exacerbated the aging of polypropylene MPs, and resulted in rough surfaces, higher carbonyl index, and variations in crystallinity. Moreover, the correlation between the carbonyl index and aging indicators increased with increasing solid content. The MP-derived dissolved organic matter under HTT primarily comprised soluble microbial by-products and humic acid-like substances. These findings underscore the significance of sludge solid content in affecting the migration and aging of MPs during HTT, and offer novel insights into the application of HTT to MP management in sludge treatment.

Effect of low-temperature thermal-alkali pre-treatment on waste activated sludge solubilisation

The study was aimed to examine the effects of alkaline and thermal-alkaline pre-treatment on sewage sludge solubilisation with different alkali and its doses ranging from 40 mg alkali/g TS to 120 mg alkali/g TS. Initially, the effect of alkali pre-treatment was monitored with four alkali at room temperature, to further enhance soluble chemical oxygen demand (sCOD) and reduce treatment time, thermal-alkaline pre-treatment was performed with varying temperature from 40 to 70 °C. To explore and optimise the effects of temperature, alkali, and alkali doses on COD solubilisation, response surface methodology was employed. Statistical analysis of the variance was conducted to identify the relationship between experimental data and the proposed model. The thermal-alkaline process was optimised for maximum COD solubilisation with optimal conditions achieved at 65.61 °C of 66.57 mg alkali /g TS for NaOH and 27 °C of 20 mg/g TS for KOH. The maximum COD solubilisation achieved at these optimised conditions for NaOH and KOH was 21.28 % and 16.13 %, respectively. The results demonstrated a significant improvement in protein and carbohydrate concentration and a 16–20 % reduction in sludge solid content after thermal-alkali pre-treatment.

The development of an efficient precipitation-flotation-dewatering (PFD) process for acid mine drainage treatment

Acid mine drainage (AMD) stands as a prevalent environmental challenge across numerous mining sites, often addressed through the high-density sludge (HDS) process, a typical neutralization and sedimentation process. However, despite its widespread use, the HDS process requires prolonged treatment durations and presents challenges in sludge dewatering and site relocation, thus falling short of an optimal solution. In this study, an efficient precipitation-flotation-dewatering (PFD) process was developed and optimized as a viable alternative to the HDS process, offering efficient AMD treatment. The experimental results indicate that over 90 % of ions were successfully precipitated by employing CaO as a neutralizing agent to adjust AMD pH to 10.0. Flotation was then conducted with a low-profile flotation column after adding an optimal concentration of 10 mg/L of A-100 flocculant and 400 mg/L of NaOL. Compared with original AMD, the final removal of toxic and hazardous ions of Fe, Al, Cd, Sn, Co, Ni, Zn, and Mn were 97.61 %, 98.85 %, 98.21 %, 97.78 %, 97.22 %, 99.34 %, 99.89 %, and 99.14 %, respectively. Additionally, the solid content of produced sludge was 27 %. Mechanistic inquiries revealed that A-100 and NaOL both chemically adsorbed on the particle surface and respectively enhanced the collision and attachment probabilities between precipitate particles and bubbles, thereby augmenting the separation efficiency. Furthermore, shortening the flotation column reduced the particle flotation path, thereby improving the flotation efficiency. The efficient PFD process exhibits significant advantages of rapid and effective AMD treatment, high sludge solid content, small footprint, and high relocatable convenience, demonstrating satisfactory cost effectiveness and significant potential as a replacement of the HDS process for AMD treatment.

Techno-economic comparison of flocculation combined with dissolved air flotation versus sedimentation for microalgae harvesting

Microalgae are a promising new source of biomass but the harvesting method needs to optimised to reduce the production cost. In a two-step harvesting process, the bulk water is removed via flocculation (first step) followed by complete dewatering using centrifugation (second step). Flocs can be separated from the bulk water using either gravity sedimentation or dissolved air flotation (DAF). DAF requires a more complex setup and demands more energy than sedimentation, but it is faster and generates a sludge with a higher dry matter content. This study aims to compare the performance of DAF and sedimentation in laboratory experiments and then estimates the cost for a 100-hectare farm using sedimentation versus DAF for harvesting (step-1) and using a decanter centrifuge (step-2) for dewatering. Chlorella vulgaris was the model species and poly(dimethylaminoethyl methacrylate) (pDMAEMA) was synthesised and used as the flocculant. In bench-scale testing, sedimentation and DAF achieved maximum harvesting efficiencies of 87 % and 98 %, respectively. Sedimentation was slow and generated a sludge with low solids content (1.1 %). DAF achieved a fast separation and generated a sludge with 3× higher solids content of 3.4 %. When estimating the costs of biomass harvesting at 100-hectare scale, DAF was found to have higher capital and operating expenditure than sedimentation. However, when combining harvesting with the dewatering step in a two-stage harvesting-dewatering system, the total capital and operating expenditure was projected to be 20–38 % and 10–18 % cheaper, respectively, than sedimentation. This is due to the faster separation, higher sludge solids content, and greater separation efficiencies obtained via DAF than sedimentation. Hence, DAF is recommended over sedimentation for harvesting microalgae. Importantly, this study showcases that contrary to conventional belief, flocculation–DAF is cheaper than flocculation–sedimentation for large-scale algal harvesting.

Optimization of biogas production from thermal-alkali pre-treated sludge using response surface methodology and random forest regressor model

BackgroundUnderstanding the intricate relationships between input variables and methane potential is crucial for optimizing anaerobic digestion process after thermal-alkali pre-treatment. This study aimed to optimize low-temperature thermal alkali pre-treatment, focusing on dosage based on the total solids (TS) content of raw sludge. A comparative analysis was conducted to optimize input parameters related to biogas generation potential as the primary response. MethodsBiomethane potential data were collected following the pre-treatment of sludge within the temperature range of 40–70 °C, using various doses of NaOH, KOH, Mg(OH)2, and Ca(OH)2 ranging from 40 mg alkali/g TS to 120 mg alkali/g TS. The optimization of biogas and methane production was conducted through Response Surface Methodology (RSM) and the Random Forest Regressor (RFR) model considering dosage and temperature as input variables. Additionally, sensitivity analysis was performed using the permutation feature importance function to gain insights into the influence of alkali types on biogas and methane production. FindingsThe results obtained from the RFR model showcased a notably high accuracy in predicting optimal solutions. NaOH addition was the most effective pre-treatment with an optimal dose of 65 mg/g TS and at a temperature of 56 °C. NaOH pre-treatment increased methane yield to 414.40 mL/g VS, compared to 117.08 mL/g VS for the untreated sludge. The sensitivity analysis results indicated that the temperature had a more significant effect on biogas potential than the alkali dosage.

Methane and Hydrogen Sulfide Production from the Anaerobic Digestion of Fish Sludge from Recirculating Aquaculture Systems: Effect of Varying Initial Solid Concentrations

Recirculating aquaculture systems (RAS) are efficient at solid waste capture and collection but generate a concentrated waste stream. Anaerobic digestion (AD) could be one potential treatment option for RAS facilities. However, the concentration of organic matter in the sludge can significantly affect the biogas quality from AD. This study evaluated the effect of fish sludge (FS) solid concentration on biogas quality. Three FS treatments consisted of different initial total solid concentrations (1.5%, 2.5%, and 3.5%) from a mixture of sludge produced by Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Methane (CH4) production was measured, quantified, and normalized on a volatile solids (VS) basis. The highest solid concentration treatment produced 23% more CH4 than the lowest solid concentration (519 mL/g VS versus 422 mL/g VS, respectively). Peak CH4 production occurred on Day 7 for the lowest FS concentration (78.2 mL/day), while the highest FS concentration peaked on Day 11 (96 mL/day). Peak hydrogen sulfide (H2S) concentrations ranged from 1803–2074 ppm across treatments, signifying the requirement of downstream unit processes for H2S removal from biogas. Overall, this study demonstrated that increasing the FS concentration can significantly enhance CH4 production without affecting the stability of the digestion process.

Gravimetric selection of activated sludge for settling properties improvement and granular sludge formation – full scale case study

Abstract With an increasing frequency of extreme weather events wastewater treatment plant operators face a need to increase plant hydraulic capacity while continuing to comply with effluent quality requirements. An economically attractive way to meet this challenge is to significantly improve secondary settling tank performance and therefore assure safe wet weather plant operation. The objective of this study was to evaluate gravimetric selection technology conducted in hydrocyclones as a way for improving activated sludge settling characteristics in continuous flow BNR with a long sludge retention time and assess its ability to form granular sludge. Long term operational data and sludge morphology monitoring data were analysed. A significant drop in sludge volume index (SVI) (to values under 50 ml/g) and increase in return activated sludge solids concentration (to values above 20 g/L) was observed. Hydrocyclone installation consistently, selectively retained denser, larger flocs in the system and after a start-up phase large granules begin to appear. Gravimetric selection did not prevent the seasonal filamentous biomass outgrowth and temperature drop at the beginning of winter resulted in rapid rise of SVI and SST sludge blanket height especially after heavy rain. Technology under study proved to be effective under certain process conditions but it needs further research to consistently maintain low SVI values throughout the whole year.

Fundamental Studies on CO2 Sequestration of Concrete Slurry Water Using Supercritical CO2

To prevent drastic climate change due to global warming, it is necessary to transition to a carbon-neutral society by reducing greenhouse gas emissions in all industrial sectors. This study aims to prepare measures to reduce the greenhouse gas in the cement industry, which is a large source of greenhouse gas emissions. The research uses supercritical CO2 carbonation to develop a carbon utilization fixation technology that uses concrete slurry water generated via concrete production as a new CO2 fixation source. Experiments were conducted using this concrete slurry water and supernatant water under different conditions of temperature (40 and 80 °C), pressure (100 and 150 bar), and reaction time (10 and 30 min). The results showed that reaction for 10 min was sufficient for complete carbonation at a sludge solids content of 5%. However, reaction products of supernatant water could not be identified due to the presence of Ca(HCO3)2 as an aqueous solution, warranting further research.

Evaluation of impact of sludge types and solids content on sludge treatment using microwave enhanced advanced oxidation process

ABSTRACT The microwave enhanced advanced oxidation process (MW-AOP) has been applied to pre-treat different sludge types and high solids content. Secondary sludge not only had the highest solids and nutrient content but also yielded higher treatment efficiency than primary or mixed sludge. In the case of secondary sludge with 4% total solids (TS), the total suspended solids (TSS) concentration was reduced by 32% while soluble chemical oxygen demand concentration increased from 1% to 40% after treatment at 110°C. A high level of nutrient release was also achieved; about 65% total phosphate (TP) solubilized at 110°C. The degree of secondary sludge disintegration was dictated by temperature and hydrogen peroxide dosage. The optimal operating temperature for the system was 110°C, and sludge containing TS up to 8% was treated effectively. Secondary sludge with 8% TS had a TSS reduction of 41% after treatment at 110°C while COD solubilization was about 45%; about 55% TP was solubilized at 10 min holding time. Treatment of sludge with higher solids content would allow for handling larger amounts of sludge at a given period and reduce heating cost per unit of treated sludge. The inter-relationship between the degree of sludge disintegration and changes in chemical and physical properties was also clearly demonstrated here. The treated sludge would be an ideal substrate for anaerobic digestion or phosphorous recovery processes. High levels of nutrients (phosphorus and nitrogen) and metal release, and solids disintegration from sludge containing high solids content would make subsequent resource recovery processes more effective and economical.

Physiochemical, Instrumental and thermal characterization of the post coagulation sludge from paint industrial wastewater treatment

Most sludge generated from water treatment plants is toxic. One of such sludge as focused on this work is paint wastewater treatment sludge from coagulation-flocculation process. Due to the problem of sludge disposal, post coagulation sludge was characterized via physical, instrumental and thermal characterization to suggest possible treatment methods to avert disposal of toxic wastes into the environment. Post coagulation sludge recovered after jar-test experiment has high settleability index with Sludge Volume Index ˂ 80. The best sludge volume index of 11 was obtained at sludge solid concentration of 2264.3 mg/l. Post coagulation sludge has pH of 6.83, Volatile Solid of 77% of the total solid (2210 mg/l) and Carbon: Nitrogen value of 26:1. The Biological Oxygen Demand - Chemical Oxygen Demand ratio of 0.7 shows that post coagulation sludge is moderately toxic. CHNX analysis shows that post coagulation sludge contains majorly calcium (35.54%), oxygen (30.2%), carbon (9.25%) and sodium (7.74 (aluminum (3.05%), silicon (3.14%) and titanium (3.69%)) and some trace elements. The prominent functional groups include =C–H in-plane bending of alkene aliphatic compound at 1403 cm−1, carboxylic group at 3000 cm−1, cellulose –OH stretching vibration group at 3476 cm−1,NH stretching of amine group within 3300–3500 cm−1 among others in trace forms. 10% reduction in weight was observed via thermo-gravimetric analysis (TGA). The mid-point glass temperature was 45 °C, while the melting point was 175 °C. In addition, PCS turns to ashes above 300 °C. Based on the results; Post coagulation sludge can be managed via, biological treatment methods, anaerobic digestion and by thermal degradation.

Microwave-alkali treatment with water elutriation to enhance waste activated sludge solubilization for carbon sources and nutrients recovery

• The dissolution degree of COD was 12.38 ± 0.20% with MW treatment alone, 54.68 ± 0.28% by MA, and 55.58 ± 0.68% by MAW. • A VSS/TSS ratio reduction was 10.50% by alkali treatment, and 22.60% by MA. • Water elutriation after MA treatment improved TP and TN release more significantly. • High MW power inputs, long MW heating time, or high sludge solids concentration could cause gelatinization and carbonization. Effects of MW treatment, combined MW-alkali (MA) treatment, and combined MA-water elutriation (MAW) treatment on sludge solubilization were studied; this work aims at the recovery of carbon sources and nutrients. Effects of MW power input, MW heating time, NaOH dosages, alkali treatment durations, and sludge solids concentration on sludge solubilization were explored. These treatments produced good chemical oxygen demand (COD) dissolution degree outcomes, with optimal values of 12.38 ± 0.20% (MW treatment), 54.68 ± 0.28% (MA treatment), and 55.58 ± 0.68% (MAW treatment), respectively. Besides, MW treatment could disrupt sludge flocs and biomass cell membranes, with a 120 mg/L volatile fatty acids (VFAs) release (280 W for 5 min). A reduction of VSS/TSS ratio was 10.50% by alkali treatment, and 22.60% by MA treatment. Compared with MA treatment only, water elutriation after MA resulted in outstanding nutrients release, with optimal dissolution degrees increasing from 67.87 ± 0.52% to 81.87 ± 0.60% (TP), and 56.08 ± 0.94% to 73.73 ± 1.04% (TN), respectively. MW power inputs, MW heating duration, alkali treatment time, and especially NaOH dosages had significant effects on sludge solubilization, but solids concentration had a minor effect on sludge solubilization. Moreover, higher MW power input, longer MW heating time, and greater sludge solids concentration could cause gelatinization and even carbonization.

Biostimulating Effects of Disintegrated Waste Activated Sludge on Soil Enzyme Activities

Sludge disintegration methods are used in a variety of applications to enhance sludge characteristics or bring about a reduction in solid content of sludge. Hydrodynamic cavitation is one of the mechanical disintegration methods used on waste-activated sludge, and its efficiency has been investigated in recent years. In this study, the possible use of waste-activated sludge disintegrated by hydrodynamic cavitation and chemically assisted hydrodynamic cavitation as a soil amendment was evaluated by an incubation procedure. Possible biostimulating effects were assessed by monitoring the variations in soil enzyme activities. The obtained results indicated that sludge solubilization was effectively performed with respect to soluble chemical oxygen demand, soluble total Kjeldahl nitrogen and soluble total phosphorus. According to the incubation study, the biostimulating/bioremediating properties of waste-activated sludge were significantly improved by the applied methods. Especially after a 60-day incubation period, the levels of urease, alkaline phosphatase, β-glucosidase and dehydrogenase activities in soils applied with a high dose of disintegrated sludge were significantly higher than the levels in raw sludge-amended soils. Consequently, it is thought that mechanically and chemically disintegrated waste-activated sludge with reduced numbers of indicator bacteria can be considered as a way of soil amendment to rehabilitate the quality of environmentally stressed soils.

Sludge thickening in a wastewater treatment plant using a modified hydrocyclone

The main application of hydrocyclones in most industry sectors is the separation of particles from liquids. For thickening, however, the separation properties of the cyclone should be adjustable, which requires a different operating mode. Therefore, we investigated an add-on thickening module, consisting of a conical plug and a secondary vortex finder in the underflow of the hydrocyclone. Laboratory scale experiments are carried out to investigate the function of the concept of annular gap modification by the use of the thickening module. In the second step, we transferred this concept to industrial scale for field tests at a municipal wastewater treatment plant. In the field tests, geometrical properties (annular gap modification) and operational parameters (volume flow rate, sludge type) were investigated. The results show that thickening in terms of increasing the total solids content of sludge is possible with the modified hydrocyclone. In the field tests, a thickening factor (total solids content of thickened sludge related to feed sludge) of 1.31 (average) was reached for digested sludge. Hence, a hydrocyclone could be a promising addition in wastewater treatment plants for dewatering of sludge.

Supercritical water oxidation for treatment of wastewater sludge and recalcitrant organic contaminants

Abstract This study investigated the SCWO (supercritical water oxidation) treatment of wastewater sludge and degradation of p-tert-butylcatechol (TBC) under a range of operating conditions including sludge solids content (2%–12%), TBC concentration (0.1%–1%), reactor temperature (400–550 ° C), reactor pressure (18–28 MPa), oxygen excess (n=1-8) and residence time (1–30 min).​ Treatment performance was evaluated by measuring the chemical oxygen demand (COD) and using Fourier Transform Infrared Spectroscopy (FTIR) The best operating conditions for sludge treatment were found to be T=550 °C, P=25 MPa, n=5, and t=10 min. SCWO was very effective for sludge treatment, and over 99% COD removal could be achieved under these conditions. The addition of TBC decreased the COD removal efficiency, but still, 98% COD removal with final COD values less than 50 mg/L could be achieved.

Dewaterability and energy consumption model construction by comparison of electro-dewatering for industry sludges and river sediments

Electro-dewatering (EDW) is an emerging technology for improved sludge/sediment dewatering enabling subsequent cost effective treatment for toxicity and pathogenic reduction if required and/or disposal, but the effects of sediment/sludge properties on the efficacy of EDW remain unclear. Here we investigate EDW in the absence of chemical conditioning which can result in secondary pollution. The influence of sediment/sludge volatile solids content (VS), electrical conductivity (EC), pH and zeta potential (ζ), on mechanical and electrical behaviors determining dewaterability and energy consumption (PE) was investigated. Optimization of EDW parameters increased the final solids content (DSf) from 40 wt% to more than 55 wt% for river sediment, while the solids content in municipal sludge was only increased from 10 wt% to 15–20 wt%. Multiple linear regression and statistical analysis showed that electro-dewatering performance is primarily affected by VS and PE is mainly affected by EC. A theoretical basis for engineering design and selection of operational parameters for sludge/sediment electro-dewatering is provided by this study.

Impact of methodological artifact on digested sludge flow curve measurement

Inconsistent experimental procedures have been used to characterize sludge rheology in literature. This often has resulted in proposing different rheological models for sludge as well as non-comparable data. Any collected rheological data needs to be interpreted considering the methodology used for its collection because otherwise they cannot be used by engineers for design and troubleshooting. This paper intends to shed light on the influential parameters during data collection procedure to produce a reliable and reproducible data.This paper systematically investigates the impact of different geometries, preshear, equilibrium, rest and storage time on flow curve measurement for digested sludge in the range of 2.3 to 6% total solid and recommends a reliable procedure for reproducible sludge flow curve measurements. While the magnitude of impacts is different, we found all factors are significantly dependent on the sludge solid concentration. Besides, the method of the development of the protocol can be utilized to develop appropriate protocols for rheological characterization of any other sludge. The customization highlights are:•Selecting the geometry according to the sludge solid concentration.•Allocating an equilibrium time at each point of flow curve according to the sludge solid concentration.•Flow curve data requires to visually inspected for instabilities.

Effects of Solid Content and Substrate Concentration on Bioleaching of Heavy Metals from Sewage Sludge Using Aspergillus niger

The presence of heavy metals in sewage sludge not only affects the performance of sludge anaerobic digestion process but also restricts the land application of treated sewage sludge. Therefore, a fungi-mediated bioleaching process for simultaneous metal leaching and sludge digestion by Aspergillus niger was developed to treat the sewage sludge containing heavy metals in this study. The effects of two important parameters, sludge solid content and substrate (sucrose) concentration, on the performance of fungal bioleaching were investigated in this study. The results showed that the rate of pH reduction increased with increasing sludge solid contents and sucrose concentrations. In this study, the efficiency of metal removal decreases in the order of Mn > Zn > Ni > Pb. The efficiencies of metal leaching and solid degradation (SS and VSS) were found to be decreased with an increase of sludge solid content and a decrease of sucrose concentration. At 2 days of reaction time, the maximum efficiency of metal solubilization was 95, 56, 21 and 13% for Mn, Zn, Ni and Pb, respectively.

Electro-dewatering of sewage sludge: Influence of combined action of constant current and constant voltage on performance and energy consumption

In this study, mechanically-dewatered sludge was used to investigate the effect of electro-dewatering (EDW) under two electrical modes, which are constant current mode followed by constant voltage mode (CV-EDW), and constant voltage mode followed by constant current mode (VC-EDW). The effect of current and voltage changes on dewatering efficiency and energy consumption of sludge electroosmosis under CV-EDW and VC-EDWs was evaluated The results show that compared with constant current mode (C-EDW), CV-EDW can improve the final dry solids content and reduce the heating rate, and the final dry solids content and unit energy consumption increase with the decrease of current and the increase of voltage. Under CV-EDW, when the dry solids content is 32%, the energy consumption can be reduced by changing to the constant voltage stage, and the energy consumption is 0.093–0.113 kWh/kgwater. Compared with constant voltage mode (V-EDW), VC-EDW significantly improves sludge dewatering rate. Under VC-EDW, the final dry solids content of sludge increases with the decrease of current and voltage. When the voltage is decreased by 10 V, the unit energy consumption is reduced by 27.15 ± 1.77% on average, and the energy consumption is 0.132–0.163 kWh/kgwater. Compared with CV-EDW, the dehydration rate of VC-EDW is increased by 72.9% on average. However, the unit energy consumption required for dehydration increases by 43.09% when the dry solids content is less than 45%.

Comparative study on hydrothermal treatment as pre- and post-treatment of anaerobic digestion of primary sludge: Focus on energy balance, resources transformation and sludge dewaterability

Hydrothermal treatment (HTT) has been recognized as a highly efficient technology for organics decomposition and energy/nutrients recovery from waste biomass. Up to now, however, the efficiency of HTT as the pre- and post-treatment of anaerobic digestion (AD) has not been well compared and documented. In this study, the effects of HTT as the pre- (strategy I, HTT + AD) and post-treatment (strategy II, 1st AD + HTT + 2nd AD) of AD of primary sludge were evaluated based on comparative experiments in regards to energy balance, nutrients transformation, and sludge dewaterability. Results show that the optimal HTT temperature was 130 °C for strategy I and strategy II according to the maximum methane production rate (μ) estimated from Gompertz model and the net energy gain (ΔQ) calculated. Although HTT as the post-treatment of AD achieved higher total methane yield and solids reduction, the increment of methane yield was found to be similar through both strategies compared to their control counterparts (no HTT). The decomposition of insoluble organic carbon was also similar via both strategies. Insoluble nitrogen fraction was detected to be the lowest (6.8%) after HTT at 190 °C in strategy II, comparable to that after HTT at 210 °C (∼10.0%) using strategy I. The proportion of bioavailable phosphorus was found to slightly decrease with the increase of HTT temperature in strategy II, probably due to the alkaline pH in the treated digestate. The sludge dewaterability indicated by specific resistance to filtration (SRF) showed a similar trend after AD under the tested conditions of both strategies, which was remarkably improved when HTT was performed at temperatures higher than 170 °C. HTT as the pre-treatment (strategy I) was found to be more energy efficient in comparison to HTT as the post-treatment (strategy II) of AD of primary sludge. In addition, a positive energy efficiency could be achieved when the sludge solids content ≥2.2%.

Pretreatment of waste activated sludge by peracetic acid oxidation for enhanced anaerobic digestion

This article studied the sludge reduction and supernatant changes during peracetic acid (PAA) oxidation and investigated subsequent anaerobic biodegradation. PAA oxidation treatment effectively leads to a disintegration of sewage sludge. The results suggested that the sludge solid concentration and volatile solid concentration decreased by 24.5 and 39.0% after 120 min, respectively, when the PAA dose was 30 mg PAA/g SS. The VSS to SS ratio dropped from 0.85 to 0.71. The concentration of supernatant soluble chemical oxygen demand, protein, total phosphorus, polysaccharide, and ammonia nitrogen increased by 530, 177, 166, 165, and 222%, respectively. Biogas production was enhanced when PAA was employed as a pretreatment. Compared with the control system, the cumulative biogas production increased by 20.0% after 28 days of anaerobic digestion (AD). The biogas production rate in digester with pretreated sludge reached the maximum on day 8 at 87mL/d, but the peak value was obtained on day 11 at 74mL/d in digester with the raw sludge, which indicated that PAA pretreatment could decrease the retention time of AD and increase biogas yield. © 2018 American Institute of Chemical Engineers Environ Prog, 37: 1901–1907, 2018