Waste Activated Sludge Conditioning Research Articles

Optimization of biological enzymes combined with Fe2+-activated advanced oxidation process for waste activated sludge conditioning using the response surface method

Waste activated sludge (WAS) conditioning is crucial for enhancing sludge dewaterability and effectively reducing the management cost. The feasibility of bio-enzymes with Fe2+-activated advanced oxidation processes (AOPs) for WAS conditioning was investigated in this study. Two representative bio-enzymes, α-amylase and neutral protease, and two typical AOPs, the Fenton system and Fe2+/Ca(ClO)2 system, were chosen. The medical dosages of AOPs were first optimized by single-factor experiments. On this basis, the dosages of bio-enzyme, ferrous ion, and oxidant were optimized using the response surface method (RSM). Results showed that neutral protease combined with the Fe2+/Ca(ClO)2 system performed better in CST and SRF reduction rates while reducing the medical dosages of AOPs. Under the optimal conditions of neutral protease dosage of 20.9 mg/g TSS, Fe2+ dosage of 0.64 mmol/g TSS, and Ca(ClO)2 dosage of 0.54 mmol/g TSS, the reduction in sludge CST and SRF reached 84.81 % and 94.06 %, respectively. The neutral protease with the Fe2+/Ca(ClO)2 system was a potential method for improving sludge dewatering performance.

Dewaterability improvement and environmental risk mitigation of waste activated sludge using peroxymonosulfate activated by zero-valent metals: Fe0 vs. Al0

The stabilization and dewaterability of waste activated sludge (WAS) are essential factors for downstream disposal or reuse. Herein, two types of zero-valent metals, zero-valent iron (Fe0) and zero-valent aluminum (Al0), were compared for their ability to activate peroxymonosulfate (PMS) during the WAS conditioning process, with the effects of PMS activation by these two metals on WAS dewaterability and the potential environmental risks evaluated. Results showed that compared to Al0/PMS treatment, Fe0/PMS treatment achieved superior WAS dewaterability and reduced operational costs. Using PMS combined with Fe0 and Al0 treatments under optimal conditions, the water content (Wc) of dewatered sludge decreased to 55.7 ± 2.7 wt% and 59.4 ± 1.3 wt%, respectively. Meanwhile, application of the Fe0/PMS treatment system reduced the total annual cost by approximately 33.1%, compared to the Al0/PMS treatment. Analysis of the dewatering mechanism demonstrated that in the Fe0/PMS treatment, Fe3+/Fe2+ flocculation played an important role in the enhancement of WAS dewatering, while sulfate radical (SO4•−) oxidation was the dominant factor for WAS dewaterability improvement in Al0/PMS treatment. The greater enhancement of WAS dewaterability by Fe0/PMS treatment, was mainly attributed to more efficient reduction of hydrophilic extracellular polymeric substances (EPS) and an increase in surface charge neutralization. Environmental risk evaluation results indicated that Fe0/PMS and Al0/PMS treatments both effectively alleviated the environmental risks of heavy metals and faecal coliforms in dewatered sludge. Overall, this study proposes a novel perspective for the selection of an optimal PMS activator in sludge treatment.

Using thermal-acidic-modified kaolin as a physical-chemical conditioner for waste activated sludge dewatering

In this study, the effect of thermal-acidic-modified kaolin (TAMK) on waste activated sludge (WAS) conditioning and dewatering was investigated for the first time. Using analyses of XRD, XRF, FT-IR, porosity and specific surface area, structural changes of kaolin were evaluated before and after modification. Conditioning with TAMK reduced the specific resistance to filterability (SRF) and time to filter (TTF), capillary suction time (CST) of WAS to 91.2%, 73.1% and 70.8% respectively. WAS conditioning with TAMK resulted in reducing compressibility and increasing filtration yield, the porosity of WAS, that indicating its function as a physical conditioner (skeleton builder). TAMK increased the WAS particle size significantly (from 47.8 μm to 451 μm) and improved their resistance against shear force. TAMK decreased the bound water (58.1%), negative zeta potential (from −31.4 mV to −7.2 mV) and EPS (extracellular polymeric substances) of WAS, indicating its function as a chemical conditioner. This study showed that TAMK acts as a physical-chemical conditioner. Also, the estimated costs of WAS conditioning with TAMK is 35.22 $/ton dry solids (DS). In general, TAMK is an affordable, inexpensive and effective option for sludge conditioning and dewatering.

A comparative study of waste activated sludge conditioning with Fe(II)-peroxymonosulfate oxidative process

Improving sludge dewaterability is a key step of waste sludge recycling. The effect and mechanisms on two kinds of waste activated sludge (WAS) with Fe(II)-peroxymonosulfate (PMS) oxidation were compared. For WAS-A, the optimum capillary suction time (CST) and specific resistance to filtration (SRF) reduction rates were achieved (56.5% and 89.3%, respectively) with PMS 0.1 g/g TSS and Fe(II) 0.05 g/g TSS. For WAS-B, the optimal results were PMS 0.1 g/g TSS and Fe(II) 0.1 g/g TSS, and the reduction in CST and SRF was 64.6% and 90.1%, respectively. After conditioning, the particle size decreased slightly while the zeta potential increased dramatically. The proteins and polysaccharides analysis indicated the destruction in extracellular polymeric substances (EPS) and microbial cells resulted from the sulfate radicals. Microtopography showed that the original structure was changed with irregular pores formed on the surface, thus facilitating the passage of internal water and further improving the sludge dewaterability.

Waste activated sludge conditioning in a new Fe2+/persulfate/tannic acid process: Effectiveness and optimization study to enhance dewaterability

Fe2+/persulfate process can be used to improve sludge dewaterability significantly. However, a large dose of chemicals was always needed and it will cause filter cloth clogging, equipment corrosion and sludge calorific value reduction in practice. In this study, a new Fe2+/persulfate/TA (tannic acid) process was put forward, which could reduce the use of inorganic reagents and keep the outstanding conditioning capability. A Box-Behnken experimental design method based on response surface methodology (RSM) was selected to assess the roles of independent variables including SPS (sodium persulfate), Fe2+ and TA, on sludge dewaterability characterized by capillary suction time, specific resistance of filtration, and water content of dewatered sludge cake. The optimized conditions were SPS dosage of 0.43 mmol/gTS (total solid), Fe2+ dosage of 0.58 mmol/gTS and TA dosage of 0.14 mmol/gTS, at which the water content of dewatered sludge cake showed (75.89 ± 0.01)% highly close to the predicted minimum water content of dewatered sludge cake using the fitted equation. Moreover, in the new process, sludge properties altered distinctly with the elimination of extracellular protein, supernatant viscosity reduction and particle size increase. Further investigations were needed to explore the underlying mechanism and practical application for a better and comphrensive understanding of Fe2+/SPS/TA process.

High-level waste activated sludge dewaterability using Fenton-like process based on pretreated zero valent scrap iron as an in-situ cycle iron donator

A novel, recyclable, and rapid pre-ultrasound-thermal-acid-washed zero valent scrap iron/hydrogen peroxide (UTA-ZVSI/H2O2) method has been developed to effectively enhance waste activated sludge (WAS) dewaterability. The effects of UTA ultrasound densities, UTA temperature, newly generated iron solution, H2O2 concentrations, and WAS conditioning time on the WAS dewaterability were investigated using a bench-scale system. Results indicated that the UTA-ZVSI/H2O2 treatment significantly improved the WAS dewaterability. The water content of the dewatered cake decreased to 44.15 ± 0.98 wt% during optimal operational conditions, which was significantly lower than that achieved using Fenton-based processes. Based on this outcome, a three-step treatment mechanism involving UTA-ZVSI/H2O2 has been developed, including iron flocculation, hydroxyl radical oxidation, and skeleton building. The dewatering efficiencies of three types of representative WAS were consistently effective in the UTA-ZVSI/H2O2 reactor for up to 15 cycles. Efficiencies levels were significantly higher than those achieved with Fenton-based processes. Economic analysis illustrated that the developed UTA-ZVSI/H2O2 system was the most cost-effective among other WAS dewatering treatments. In addition, the treatment system significantly alleviated toxicity of heavy metals and phytotoxicity in the dewatered sludge. This supported subsequent agricultural use. In summary, this study provided a comprehensive and useful basis for improving WAS dewatering and subsequent disposal.

Breakage–reflocculation implemented by two-stage shear for enhancing waste-activated sludge dewaterability: Effects of shear condition and extracellular polymeric substances

ABSTRACTThe conditioning of waste-activated sludge (WAS) before dewatering is crucial for enhancing sludge dewaterability. The breakage–reflocculation that was implemented by two-stage shear (drastic first-stage shear for breakage and moderate second-stage shear for reflocculation utilizing the bioflocculation function) which was proposed as a novel WAS conditioning method with several advantages (simple operation, lower cost, and none added reagent) compared to traditional methods. Effects of the shear condition and extracellular polymeric substances (EPS) on breakage–reflocculation were orderly investigated. Two equations were developed by response surface methodology for predicting breakage–reflocculation conditioning performance. Analysis of variance (ANOVA) indicated that individual effects of first-stage shear rate (G1), second-stage shear rate (G2), second-stage shear time (t2), and interactive effect of G1G2 were significant. More compact WAS flocs with better dewaterability and larger floc size formed through breakage–reflocculation. This was reflected in that the capillary suction time decreased by 16.9% and mean floc size increased by 24% under the optimum shear condition. In addition, the loosely bound EPS was revealed to be closely negatively correlated with breakage–reflocculation conditioning performance, indicating its adverse role in breakage–reflocculation. The breakage–reflocculation could be used as an independent conditioning method with low cost or a part of combined method.

Enhanced short chain fatty acids production from waste activated sludge conditioning with typical agricultural residues: carbon source composition regulates community functions.

Background A wide range of value-added by-products can be potentially produced from waste activated sludge (WAS) through anaerobic fermentation, among which short-chain fatty acids (SCFAs) are versatile green chemicals, but the conversion yield of SCFAs is usually constrained by the low carbon-to-nitrogen ratio of the original WAS. Conditioning of the WAS with cellulose-containing agricultural residues (ARs) has been reported to be an efficient and economical solution for balancing its nutrient components. However, contributions of different ARs to SCFAs production are still not well understood.ResultsTo optimize SCFAs production through carbon conditioning of WAS, we investigated the effects of two typical ARs [straws and spent mushroom substrates (SMSs)] on WAS hydrolysis and acidification in semi-continuous anaerobic fermentation. Straw-conditioning group showed a threefold increase in short-chain fatty acids yield over blank test (without conditioning), which was 1.2-fold higher than that yielded by SMS-conditioning. The maximum SCFAs yield in straw-conditioning groups reached 486.6 mgCOD/gVSS (Sludge retention time of 8 d) and the highest volumetric SCFAs productivity was 1.83 kgCOD/({text{m}}_{text{reactor}}^{3} cdot {text{d}}) (Sludge retention time of 5 d). In batch WAS fermentation tests, higher initial SCFAs production rates were achieved in straw-conditioning groups [49.5 and 52.2 mgCOD/(L·h)] than SMS-conditioning groups [41.5 and 35.2 mgCOD/(L·h)]. High-throughput sequencing analysis revealed that the microbial communities were significantly shifted in two conditioning systems. Carbohydrate-fermentation-related genera (such as Clostridium IV, Xylanibacter, and Parabacteroides) and protein-fermentation-related genus Lysinibacillus were enriched by straw-conditioning, while totally different fermentation genera (Levilinea, Proteiniphilum, and Petrimonas) were enriched by SMS-conditioning. Canonical correlation analysis illustrated that the enrichment of characteristic genera in straw-conditioning group showed positive correlation with the content of cellulose and hemicellulose, but showed negative correlation with the content of lignin and humus.ConclusionsCompared with SMSs, straw-conditioning remarkably accelerated WAS hydrolysis and conversion, resulting in higher SCFAs yield. Distinct microbial communities were induced by different types of ARs. And the communities induced by straw-conditioning were verified with better acid production ability than SMS-conditioning. High cellulose accessibility of carbohydrate substrates played a crucial role in enriching bacteria with better hydrolysis and acidification abilities.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0369-x) contains supplementary material, which is available to authorized users.

Effect of sonolysis on waste activated sludge solubilisation and anaerobic biodegradability

Activated sludge processes are key technologies in wastewater treatment. These biological processes produce huge amounts of waste activated sludge (WAS) or otherwise biosolids. Mechanical, thermal, and/or chemical WAS conditioning techniques have been proposed to reduce the sludge burden. Among the WAS treatments, the pre-treatment with ultrasound (US) is one of the most innovative processes. In many anaerobic digestion processes for the treatment of the sludge produced in wastewater treatment plants, the hydrolysis of the organic matter has been identified as the rate limiting step. This study is focused on the effect of US pre-treatment of WAS to the anaerobic digestion. Particle size reduction, Chemical Oxygen Demand (COD) solubilization and biodegradability by anaerobic digestion were monitored in order to find the optimal dose in US pre-treatment. The results show the better sonolysis conditions (US density, sonication time, specific energy) which can significantly improve the COD solubilisation and the anaerobic biodegradability.

Ultrasonic treatment of waste activated sludge

AbstractActivated sludge processes are key technologies to treat wastewater. These biological processes produce huge amounts of waste activated sludge (WAS), now commonly called biosolids. Mechanical, thermal, and/or chemical WAS conditioning techniques have been proposed to reduce the sludge burden. The ultrasonic treatment of WAS is quite novel.The present paper reports on extensive investigations using an ultrasonic treatment of WAS, to study its potential to meet one or all of four objectives: (1) reduce WAS quantities; (2) achieve a better dewaterability; (3) provoke a release of soluble chemical oxygen demand (COD) from the biosolids, preferably transformed into biodegradable organics; and (4) possibly destroy the filamentous microorganisms responsible for sludge bulking. Although meeting these objectives would help to solve the problems cited, the energy consumption could be a considerable drawback: the paper will thus assess whether all or some objectives are met, and at what operational cost.A literature survey defines the occurring phenomena (cavitation) and the important operation parameters [such as frequency, duration, specific energy input (SE)].The experiments are carried out in a batch reactor of volume up to 2.3 L. The ultrasonic equipment consisted of a generator, a converter, and a sonotrode, supplied by Alpha Ultrasonics under the brand name of Telsonic. Three different kinds of sludge were tested, with different concentrations of dry solids (DS) between approximately 3.5 and 14 g DS/L WAS. Ultrasonic energy was introduced in a continuous manner (against possible pulsed operation). The major operational parameters studied include duration of the ultrasonic treatment and specific energy input. The applied frequency was set at 20 kHz.The release of COD from the WAS phase into the filtrate phase is a function of the specific energy input with yields of nearly 30% achievable at SE values of 30,000 kJ/kg DS. A major fraction of the COD is transformed into biodegradable organics (BOD). The reduction in DS fraction of the sludge is proportional to the COD release rates.Although the DS content is reduced, the dewaterability of the sludge is not improved. This reflects itself in increased filtration times during vacuum filtration and in increased values of the capillary suction time (CST). This more difficult dewaterability is the result of considerably reduced floc sizes, offering an extended surface area: more surface water is bound (CST increases) and the filterability decreases as a result of clogging of the cake. To reach the same dryness as for the untreated cake, the required dosage of polyelectrolyte is nearly doubled when the SE of the ultrasound treatment is increased from 7500 to 20,000 kJ/kg DS.The ultrasonic reduction of filamentous WAS organisms is not conclusive and very little effect is seen at low intensities and short treatment durations. Microscopic analysis of the WAS identified the dominant presence of Actynomyces.The release of soluble COD and BOD certainly merit further research. © 2006 American Institute of Chemical Engineers Environ Prog, 2006

Waste activated sludge conditioning by direct slurry freezing

It has been widely observed and noted that waste sludges subjected to freezing undergo a conditioning effect that greatly enhances subsequent dewatering. However, wide-spread application of freezing for conditioning purposes has never been practiced because of the high operating costs associated with conventional indirect freezing systems. Furthermore, all reported efforts involved the solid freezing of the experimental sludge. The purpose of this research was to evaluate the technical and economic feasibility of waste activated sludge conditioning for dewatering using a direct slurry freezing system. Liquid butane was used as the refrigerant and it was bubbled through the liquid sludge to form an ice-sludge slurry. Economic investigation included butane recovery techniques. The results of the investigation showed that the dewaterability of waste activated sludge can be greatly improved by slurry freezing with liquid butane. Slurry freezing results in better conditioning and a better quality supernatant than does solid freezing because fewer cellular solids are destroyed by the freezing pressure. The conditioning effect is also comparable to or better than conditioning by widely used processes such as heat treatment. Furthermore, recovery of butane for subsequent reuse reduces the cost of slurry freezing to less than the cost of nearly all conditioning processes in current use.