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Comparison of different thickening methods for active biomass recycle for anaerobic digestion of wastewater sludge
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The effect of returning solids to the digester, after one of three thickening processes, on volatile solids reduction (VSR) and gas production was investigated. Three different thickening methods were compared: centrifugation, flotation and gravitational sedimentation. The amount and activity of retained biomass in thickened recycled sludge affected the efficiency of digestion. Semi-continuous laboratory digesters were used to study the influence of thickening processes on thermophilic sludge digestion efficiency. Centrifugation was the most effective method used and caused an increase of VSR from 43% (control) up to 70% and gas generation from 0.40 to 0.44 L g(-1) VS. Flotation and gravitational sedimentation ways of thickening appeared to be less effective if compared with centrifugation. These methods increased VSR only by up to 65 and 51%, respectively and showed no significant increase of gas production. The dewatering capacity of digested sludge, as measured by its specific resistance to filtration, was essentially better for the sludge digested in the reactors with centrifugated and settled recycle. The VS concentration of recycle (g L(-1)), as reflecting the amount of retained biomass, appeared to be one of the most important factors influencing the efficiency of sludge digestion in the recycling technology.
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... Recuperative thickening is a process for intensifying anaerobic digesters, by removing water from digestate, and returning the high-solids concentrate back to the digester. It thereby increases effective solids retention time over hydraulic retention time (Reynolds et al., 2001; Vanyushina et al., 2012). It is particularly suited to primary sludge, which has a relatively high degradability extent, but which is also relatively slow to degrade, and generally poorly suited material with a limited degradability extent, since this will result in the recycling of large amounts of inert material (see model provided supplementary information SI). ...
... Both recuperative thickening and final dewatering can be regarded as comparable processes. In the case of recuperative thickening, digester sludge is thickened to 6%e12% through a range of techniques, including centrifuges and belt filter presses (Tchobanoglous et al., 2003), dissolved air flotation (Vanyushina et al., 2012), gravity belt thickeners (Reynolds et al., 2001), and rotary drum thickeners (Tchobanoglous et al., 2003 ). These can be generally classed by the DS achieved and the potential impact on the biology as shown inTable 1. Final dewatering depends on high-g and standard centrifuges, or belt filter presses to produce a final cake at >12% DS (Albertson et al., 1987 ) whereas recuperative thickening technologies target lower DS. ...
Dewatering methods for recuperative thickening and final dewatering can potentially impact methanogenic activity and microbial community. This influences both the feasibility of recuperative thickening to increase solids residence time within a digester, and the utilisation of dewatered digestate as inoculum for new digesters. Thickening technology can reduce methanogenic activity through either air contact (rotary drum, DAF, or belt filter press), or by lysing cells through shear (centrifuge). To assess this, two plants with recuperative thickening (rotary drum) in their anaerobic digester, and five without recuperative thickening, had specific methanogenic activity tested in all related streams, including dewatering feed, thickened return, final cake, and centrate. All plants had high speed centrifuges for final dewatering. The digester microbial community was also assessed through 16s pyrotag sequencing and subsequent principal component analysis (PCA). The specific methanogenic activity of all samples was in the expected range of 0.2-0.4 gCOD gVS(-1)d(-1). Plants with recuperative thickening did not have lower digester activity. Centrifuge based dewatering had a significant and variable impact on methanogenic activity in all samples, ranging between 20% and 90% decrease but averaging 54%. Rotary drum based recuperative thickening had a far smaller impact on activity, with a 0% per-pass drop in activity in one plant, and a 20% drop in another. However, the presence of recuperative thickening was a major predictor of overall microbial community (PC1, p = 0.0024). Microbial community PC3 (mainly driven by a shift in methanogens) was a strong predictor for sensitivity in activity to shear (p = 0.0005, p = 0.00001 without outlier). The one outlier was related to a plant producing the wettest cake (17% solids). This indicates that high solids is a potential driver of sensitivity to shear, but that a resilient microbial community can also bestow resilience. Sensitivity of methanogens to centrifuging does not rule out centrifuges for recuperative thickening (particularly where hydrolysis is rate-limiting), but may impose a maximum return rate to avoid digester failure.
Copyright © 2015 Elsevier Ltd. All rights reserved.
... Finally Recuperative Thickening is an alternative technique for AD intensification. The process aims at decoupling the SRT from the HRT (Vanyushina et al., 2012) and consists of thickening a portion of digested sludge and returning the high-solids concentrate back to the digester. However, depending on the thickening technology used, the methanogenic activity may be negatively impacted. ...
Sludge digesters are generally designed using empirical thresholds that were defined several decades ago, typically leading to large digesters displaying low organic loading rates (1-2.5 kgVS.m-3.d-1). However, the state of the art has significantly evolved since these rules were set, especially regarding bioprocess modelling and ammonia inhibition. This study demonstrates that digesters can be safely operated at high sludge concentration and total ammonia concentration up to 3.5 gN.L-1, without any sludge pretreatment. The possibility of operating sludge digesters at organic loading rates of 4 kgVS.m-3.d-1 by feeding concentrated sludge was identified through modelling and experimentally confirmed. Based on these results, the present work proposes a new mechanistic digester sizing strategy based on microbial growth and ammonia-related inhibition in lieu of historical empirical methods. Applying such method to sludge digester sizing could lead to very significant volume reduction (25-55%), which would result in reduced process footprint and more competitive building costs.
... Recuperative thickening (RT) is a relatively low cost technique for intensification of anaerobic digestion, with well understood fundamentals but with few formal references found on the topic. It increases effective solids retention time over hydraulic retention time by removing water from digestate and returning the high-solids concentrate back to the digester (Reynolds et al., 2001;Vanyushina et al., 2012). It has been shown that RT can increase capacity and performance (Cobbledick et al, 2016) while maintaining similar methanogenic activity when compared to conventional operation (Batstone et al, 2015). ...
... A very limited amount of work has been done on RT processes at lab-scale conditions. Vanyushina et al. [13] compared the performance of a single polymer flocculant to other dewatering techniques for lab-scale RT digesters (1 L capacity) operated at thermophilic conditions. However, they did not report any dewatering performance results involving the polymer flocculants. ...
There is considerable interest in recuperative thickening (RT), the process of recycling partially digested solids in the outlet stream of an anaerobic digester back into the incoming feed, as a 'high-performance' process to increase biogas production, increase system capacity, and improve biosolids stabilization. While polymer-induced flocculation is commonly used in full-scale RT operations, to date no studies have investigated the effect of flocculation conditions on RT process performance. Our goal was to investigate the effect of polymer type and dosage conditions on dewatering performance and biogas production in a lab-scale RT system. The type of polymer flocculant significantly affected dewatering performance. For example, the 440 LH polymer (low molecular weight (MW) polyacrylamide) demonstrated lower capillary suction time (CST) and filtrate total suspended solids (TSS) values at both dosage conditions than the C-6267 polymer (high MW polyacrylamide). The 4516 polymer (very high MW, copolymer of polyacrylamide and quaternized-N,N dimethylamino ethylacrylate) gave the best dewatering performance across test conditions. An examination of the dewatering performance for three RT digesters with different polymers found a strong correlation between the CST and filtrate TSS. The type of polymer flocculant had no significant effect on biogas productivity or composition; the average methane content was greater than 60% in good agreement with typical mesophilic anaerobic digestion results. Due to the highly variable nature of digestate from different wastewater treatment plants, the optimization of the polymer flocculation conditions is a critical task for which the lab-scale RT system used in this work is ideally suited.
This study assessed the feasibility of a novel vacuum-enhanced anaerobic digestion technology, referred to as IntensiCarbTM (IC), under mild vacuum pressure (110 mbar), compared to a control (conventional fermenter), and evaluated the impact of the vacuum on the activities of various microbial groups. Both fermenters (test and control) were operated with mixed (50% v/v) municipal sludge at solids concentrations of 2%-2.5%, pH of 7.8-8.1, 40-45˚C, a theoretical solids retention time (SRT) of 3 days with different hydraulic retention times (HRT). The intensification factor (IF) of the IC, defined as SRT/HRT, was controlled at 1.3 and 2.0. Simultaneous thickening and fermentation intensification were achieved. Compared with the control, the IC, despite the shorter HRTs, achieved 29.5% to 90.2% increase in the VFA yield (79 to 116 mg ΔVFA/ g VSS vs 61 mg ΔVFA/ g VSS), and 16.2% to 56.4% increase (280 to 377 mg ΔsCOD/ g VSS vs 241 mg ΔsCOD/ g VSS), in the hydrolysis yield. Fermentate from the IC exhibited comparable specific denitrification rates to acetate. Further, the solids-free condensate contained low nutrient concentrations, and thus was far superior to a typical centrates from dewatering as a carbon source. No adverse effects of vacuum on the activity of fermentative bacteria and methanogens were observed. This study demonstrated that the IC can be deployed as an intensification technology for both fermentation and anaerobic digestion of biosolids with the additional significant advantage, i.e. elimination of sidestream ammonia treatment requirements.
Recuperative thickening (RT) process was introduced to further upgrade anaerobic digestion of hydrothermal high-solid sludge. Continuous mesophilic (MD-R) and thermophilic (TD-R) digestion with RT (MD-R) were operated synchronously, with corresponding single digestion without RT as controls, namely MD and TD. The MD-R and TD-R increased biogas production rates by 22.8% and 11.0%, and achieved 16.6% and 9.7% higher volatile solids reductions, respectively. The improved performance was partly attributed to increased hydrolysis rate, with 11.2% and 7.4% higher for the MD-R and TD-R than the controls, respectively. The RT increased the numbers of total archaea in the mesophilic and thermophilic systems by 844% and 108%, and the numbers of dominant archaea by 50.4% and 38.1%, respectively, which promoted the degradation of organic matter and the production of biogas. Thus, RT is applicable to further upgrade digesting high-solid sludge.
The widespread existence of polychlorinated biphenyls (PCB) in wastewater results in the retention of abundant PCB in waste activated sludge (WAS), which has become a global concern. Till now, the effects of PCB on methane production during WAS anaerobic digestion was still limited. This study aimed to investigate whether and how PCB affects methane production. Results showed that the increased PCB concentration led to the less methane produced. At the highest concentrations of PCB (100 mg/kg DS) in this study, the methane production (112 ± 6 L CH4/kg VS) was significantly reduced by 26.6 ± 0.1% compared to the control (153 ± 7 L CH4/kg VS). Correspondingly, VS destruction at the studied concentrations showed the similar trend. In addition, the dewaterability evaluation indicated that the PCB resulted in the deteriorative sludge dewaterability. The mechanism studies suggested that the decreased methane production with the increased levels of PCB was attributed to the suppression on hydrolysis-acidification and methanation processes. This also was supported by the decreasing key enzymes activities associated with methane production (protease, cellulase, acetate kinase (AK) and coenzyme F420). The relative activity of F420 at 100 mg/kg DS of PCB even reduced to 78% ± 3%.
This review is on the research literature published in 2012 related to the physico-chemical processes for water and wastewater treatment. The review is divided into seven sections, including coagulation/flocculation, sorption, filtration, sedimentation, flotation, oxidation, and air
stripping.
The application of anaerobic submerged membrane bioreactors was studied for the treatment of wastewaters containing suspended solids. A mesophilic and a thermophilic reactors were operated with a synthetic wastewater. The thermophilic reactor achieved higher volumetric loading rates than the mesophilic reactor, reaching 14 g COD/L · d (0.47 g COD/g VSS · d). The mesophilic reactor showed signs of overload, when reaching a volumetric loading rate of 10 g COD/L · d (0.32 g COD/g VSS · d). Cake formation was identified as the main factor governing applicable flux. Low levels of irreversible fouling were observed in both reactors. Low fluxes were attained and gas sparging was ineffective in increasing the critical flux.
A variety of test procedures for determination of anaerobic biodegradability have been reported. This paper reviews the methods developed for determination of anaerobic biodegradability of macro-pollutants. Main focus is paid to the final mineralization of organic compounds and the methane potential of compounds. Hydrolysis of complex substrates is also discussed. Furthermore, factors important for anaerobic biodegradation are shortly discussed.
Membranes themselves represent a significant cost for the full scale application of anaerobic membrane bioreactors (AnMBR). The possibility of operating an AnMBR with a self-forming dynamic membrane generated by the substances present in the reactor liquor would translate into an important saving. A self-forming dynamic membrane only requires a support material over which a cake layer is formed, which determines the rejection properties of the system. The present research studies the application of self-forming dynamic membranes in AnMBRs. An AnMBR was operated under thermophilic and mesophilic conditions, using woven and non woven materials as support for the dynamic membranes. Results showed that the formation of a cake layer over the support materials enables the retention of more than 99% of the solids present in the reactor. However, only low levels of flux were achieved, up to 3 L/m2 x h, and reactor operation was unstable, with sudden increases in filtration resistance, due to excessive cake layer formation. Further fine-tuning of the proposed technology involves looking for conditions that can control effectively cake layer formation
An existing anaerobic process, upgraded to an anaerobic membrane bioreactor (AnMBR) system, was commissioned in July 2008 to treat wastewater produced from salad dressings and barbeque sauce. Upgrading the existing anaerobic treatment process to an AnMBR was considered the best choice
as it reduced operating and maintenance costs, provided stable operation, consistently produced high quality anaerobic effluent and removes the strain on an existing aerobic process.The two years of AnMBR operation has shown that the system is very suitable for treating this industrial
wastewater as it provides COD, BOD, and TSS removals in excess of 99.4 percent. The AnMBR effluent BOD and TSS concentrations were consistently less than 25 mg/l and 2 mg/l, respectively, while influent BOD and TSS concentrations averaged 18,000 and 11,000 mg/l, respectively. The AnMBR system
provides superior performance and a very low rate of membrane fouling, with the aid of biogas scour across the membrane surface. The change in transmembrane pressure was negligible over the first twenty four months of operation and no citric acid cleaning events were required while operating
at a MLSS concentration up to 48,000 mg/l.
Anaerobic digesters are widely employed at wastewater treatment works for the destruction of volatile solids present in settled primary and waste activated sludge. Membrane separation techniques afford an effective method to separate solids from the digester suspension and recycle them to the digester. A commercially available polymer ultrafiltration and ceramic microfiltration membrane were tested for filtration of anaerobic sludge. For the filtration of suspensions with similar solids concentrations, the short term fluxes reached in ceramic microfiltration (200 tot 250 l m−2 h−1) were on the average tenfold those for polymer ultrafiltration (25 l m−2 h−1). The permeate quality of the ceramic microfiltration and polymer ultrafiltration was similar. Discontinuous coupling of the ceramic microfiltration to an anaerobic primary sludge digester increased the total solids concentration of the reactor from 22 g l−1 to 35 g l−1, but the reactor efficiency was not improved in terms of volumetric loading rates. The high frequency of displacement of the anaerobic sludge through the filtration unit disrupted the interaction between the different species in the anaerobic consortia due to shear stress. Ceramic microfiltration of digested sludge was estimated to represent an extra cost of 1.53 ECU (57.2 BEF) m−3 permeate compared to polymer ultrafiltration. In line implementation of membrane filtration in sludge digestion is not recommended in terms of improved microbial process performance. However, coupling membrane filtration to the final treatment of the digested liquor by polymer ultrafiltration or ceramic microfiltration opens perspectives because high fluxes can be achieved and a high quality permeate is produced.
Trials at a wastewater treatment works indicate that cross-flow microfiltration, using flexible woven fibre tubes, could significantly enhance the performance of anaerobic digesters. The cross-flow microfiltration unit decouples the solids residence time from the liquid residence time and thus enables the volumetric throughput of the digester to be significantly increased while maintaining a constant solids residence time and volatile solids destruction. hi trials on a pilot-plant digester, the volumetric throughput was almost doubled, from 70 to 130 l d-1, while the solids residence time was maintained at 26 days. The economic feasibility of the coupled system was evaluated, using permeate flux data obtained from a cross-flow microfiltration unit coupled to a full-scale digester. The evaluation indicates that the coupled process is economically favourable when compared to conventional digester systems. The coupled system is currently being developed into a full-scale process. A full-scale CFMF unit is being constructed and will be coupled to a full-scale digester. Over a three year project the long-term reliability and operability of the process will be investigated, together with its effect on digester fluid dynamics and biomass viability.
This paper will report the observed flux rates and cleaning requirements of two membrane systems coupled to an anaerobic sludge digestion pilot. This study was the second of two pilot projects designed to increase the processing capacity of municipal anaerobic sludge digesters.
The first project analyzed the impact of shear on biomass activity in anaerobic sludge digesters. It was found that biomass activity did increase at increasing shear rates. However, it was also found that certain thickening polymers could limit the achievable increase in biomass activity. In order to circumvent this process limitation, alternate thickening methods not depending on thickening polymers were being considered. Membranes seemed to fit the bill, but it wasn't clear how well sludge might thicken in the absence of polymers. It was decided to analyze the membrane thickening concept in a second pilot study.
The aim of the second study was to investigate the application of membranes for recuperative thickening during anaerobic sludge digestion rather than for thickening of raw sludge prior to anaerobic sludge digestion. This pilot project was carried out at our facility in Camden, New Jersey using non-thickened mixed primary and secondary sludge from the Gloucester County Utilities Authority (GCUA) in Paulsboro, New Jersey. The performances of two membrane systems were evaluated during the course of this study. One of the membrane systems was an oscillating cross flow system with Teflon sheet membranes and the other a tubular cross flow system with a membrane of titanium oxide sintered onto stainless steel. By operating the digester and one of the membrane systems for an extended period of time we hoped to gain insight into the longevity of the membrane system when treating a dirty substrate such as municipal sewage sludge.
The AGF process is the anaerobic activated sludge process using biogas flotation for liquid solids separation (Burke 1991, Burke 1992). The AGF pasteurization process (Burke 2000) includes the additional step of pasteurizing and re-digesting the thickened waste solids to convert an even greater portion of the biosolids to gas.
The AGF pasteurization process was installed at Southwest Suburban Sewer District's 3.0-mgd (1136 m³/d) Salmon Creek plant in 1998. Installation of the process involved converting an unused 18-foot (5.5 m³) diameter thickener to a gas flotation (AGF) unit and installing a 220-gallon (0.83 m³) pasteurization tank. This paper presents the results of the AGF operation over the first 60 weeks of operation. This paper also presents the pasteurization pilot results using waste solids from the AGF float concentrate. Full-scale pasteurization results will be reported at the Biosolids 2001 Conference. Table 1 compares the solids handling process at Salmon Creek before and after AGF installation.
The process stabilized the digester and eliminated digester foaming. Volatile solids conversion to gas increased from 57 to 73% while the total solids removed from the plant decreased 26%. Operation of the belt press decreased from an average of 4 days per week to 1 day per week. The process increase digester capacity by a factor of 3 times its existing capacity (SRT/HRT = 3.04). The process also consumed less energy than that required to mix and heat an existing mesophillic digester.
A six month pilot investigation confirmed that concentrated float recycle could be pasteurized and redigested to achieve an additional 10 to 12% volatile solids destruction. The final biosolid product was essentially odorless, pathogen free, and fully stabilized.
Anaerobic membrane bioreactor and online ultrasonic equipment used to enhance membrane filtration were coupled to form a hybrid system (US-AnMBR) designed for long-term digestion of waste activated sludge. The US-AnMBR was operated under volatile solids loading rates of 1.1-3.7 gVS/L·d. After comprehensive studies on digestion performance and membrane fouling control in the US-AnMBR, the final loading rate was determined to be 2.7 gVS/L·d with 51.3% volatile solids destruction. In the US-AnMBR, the improved digestion was due to enhanced sludge disintegration, as indicated by soluble matter comparison in the supernatant and particle size distribution in the digested sludge. Maximum specific methanogenic activity revealed that ultrasound application had no negative effect on anaerobic microorganisms. Furthermore, implementing ultrasound effectively controlled membrane fouling and successfully facilitated membrane bioreactor operation. This lab-scale study demonstrates the potential feasibility and effectiveness of setting up a US-AnMBR system for sludge digestion.