Temperature phased anaerobic digestion increases apparent hydrolysis rate for waste activated sludge

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<ul><li><p>ca</p><p>on</p><p>Article history:</p><p>Received 28 September 2010</p><p>used to treat, stabilise, and reduce the quantities of organic</p><p>wastes prior to disposal or beneficial re-use. Anaerobic</p><p>the effluent nitrogen concentration. Process adaptations</p><p>include the removal of primary settlings and primary sludge</p><p>streams; and increased retention times for biological nutrient</p><p>removal (BNR) processes, resulting in increased sludge age.</p><p>Increased sludge age results in waste activated sludge with</p><p>wastewater treatment processes have introduced new chal-</p><p>lenges for anaerobic digestion, as poor degradability of acti-</p><p>which is generally accepted as the rate-limiting step in</p><p>anaerobic digestion. Pre-treatment can enhance overall</p><p>digestion, and requires a minimal capital investment in</p><p>comparison with methods such as aerobic digestion (Ros and</p><p>Zupancic, 2003). Temperature phased anaerobic digestion</p><p>* Corresponding author. Tel.: 61 7 3346 9051; fax: 61 7 3365 4726.</p><p>Avai lab le a t www.sc iencedi rec t .com</p><p>els</p><p>wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 9 7e1 6 0 6E-mail address: damienb@awmc.uq.edu.au (D.J. Batstone).digestion has been used extensively in municipal wastewater</p><p>treatment to stabilise primary sludge and activated sludge.</p><p>Over the past decade, municipal wastewater treatment</p><p>processes have adapted to meet reduced discharge limits on</p><p>vated sludge requires long digester retention times, higher</p><p>mixing costs, and also results in poor gas production.</p><p>Incorporating a pre-treatment into anaerobic treatment</p><p>may enhance the sludge digestion by accelerating hydrolysis,1. Introduction</p><p>Anaerobic digestion is a biological decomposition process</p><p>inherently low degradability, as inertmaterials in the influent,</p><p>as well inert decay products accumulate in the activated</p><p>sludge (Gossett and Belser, 1982). Adaptations of modernReceived in revised form</p><p>26 November 2010</p><p>Accepted 28 November 2010</p><p>Available online 4 December 2010</p><p>Keywords:</p><p>Temperature phased anaerobic</p><p>digestion</p><p>Thermophilic pre-treatment</p><p>Mesophilic pre-treatment</p><p>Waste activated sludge</p><p>Hydrolysis rate0043-1354/$ e see front matter 2010 Elsevdoi:10.1016/j.watres.2010.11.042It is well established that waste activated sludge with an extended sludge age is inherently</p><p>slow to degrade with a low extent of degradation. Pre-treatment methods can be used prior</p><p>to anaerobic digestion to improve the efficiency of activated sludge digestion. Among these</p><p>pre-treatment methods, temperature phased anaerobic digestion (TPAD) is one promising</p><p>method with a relatively low energy input and capital cost. In this study, an experimental</p><p>thermophilic (50e70 C)emesophilic system was compared against a control meso-</p><p>philicemesophilic system. The thermophilicemesophilic system achieved 41% and 48%</p><p>volatile solids (VS) destruction during pre-treatment of 60 C and 65 C (or 70 C) respec-</p><p>tively, compared to 37% in the mesophilicemesophilic TPAD system. Solubilisation in the</p><p>first stage was enhanced during thermophilic pre-treatment (15% at 50 C and 27% at 60 C,</p><p>65 C and 70 C) over mesophilic pre-treatment (7%) according to a COD balance. This was</p><p>supported by ammoniaenitrogen measurements. Model based analysis indicated that the</p><p>mechanism for increased performance was due to an increase in hydrolysis coefficient</p><p>under thermophilic pre-treatment of 60 C (0.5 0.1 d1), 65 C (0.7 0.2 d1) and 70 C(0.8 0.2 d1) over mesophilic pre-treatment (0.2 0.1 d1), and thermophilic pre-treat-ment at 50 C (0.12 0.06 d1).</p><p> 2010 Elsevier Ltd. All rights reserved.a r t i c l e i n f o a b s t r a c tAdvanced Water Management Centre (AWMC), Environmental Biotechnology CRC, The University of Queensland, St Lucia,</p><p>QLD 4072, AustraliaTemperature phased anaerobihydrolysis rate for waste activ</p><p>Huoqing Ge, Paul D. Jensen, Damien J. Batst</p><p>journa l homepage : www.ier Ltd. All rights reserveddigestion increases apparentted sludge</p><p>e*</p><p>ev ier . com/ loca te /wat res.</p></li><li><p>Thermophilicemesophilic TPAD has been shown to be an</p><p>and water temperature of approximately 20 C in the Elanorawastewater treatment plant, located at Gold Coast, Australia.</p><p>The feed was prepared monthly by centrifuging the sludge to</p><p>a total solids (TS) concentration of 2e3%, and subsequently</p><p>stored below 4 C. Regular analysis was performed to deter-mine the characteristics and consistency of the feed material.</p><p>Table 1 shows the average characteristics of the activated</p><p>sludge feed based on 14 feed collections over 15 months.</p><p>2.2. Start-up and operation</p><p>Two identical two-stage systems were used throughout.</p><p>These consisted of thermophilic pre-treatment (TP) and</p><p>wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 9 7e1 6 0 61598effective treatment for increasing methane production and</p><p>volatile solids (VS) destruction, compared with a single-stage</p><p>mesophilic digestion. Han and Dague (1997) reported 39% VS</p><p>destruction of primary sludge achieved in a TPAD system</p><p>(55 C, 3 days hydraulic retention time (HRT) and 35 C), whichwas higher than 32% in a single-stage control system (35 C).A correspondingmethane production in the TPAD systemwas</p><p>also 16% higher over the control system. An improvement of</p><p>methane production with a TPAD system treating activated</p><p>sludge was also observed by Bolzonella et al. (2007). They</p><p>found the highest specificmethane production from the TPAD</p><p>(70 C, 2e3 days HRT and 37 C) was 370 ml gVSadded1 , 30e50%</p><p>higher than that from a single-stage control system (37 C).Extending pre-treatment HRT to 5 days did not improve</p><p>methane production further.</p><p>Nges and Liu (2009) evaluated the effect of pre-treatment</p><p>temperature, and reported that the digestion performance of</p><p>mixed primary and activated sludge was not influenced by</p><p>thermophilic temperature, as the same VS destruction of 42%</p><p>was achieved at pre-treatment temperatures of both 50 and</p><p>70 C (2 days HRT). However, this result was still greater thanthe VS destruction achieved in the single-stage mesophilic</p><p>control (39%). Watts et al. (2006) reported a substantial</p><p>improvement of activated sludge digestion with increased</p><p>thermophilic temperature. They found that a TPAD system</p><p>(47 C, 2 days HRT and 37 C) achieved the similar VSdestruction of 24% as a single-stage mesophilic digester</p><p>(37 C). VS destruction was not improved with the thermo-philic temperature increased to 54 C, but was significantlyenhanced to 34% at 60 C.</p><p>The majority of research has focused on achieving</p><p>improved performance by varying pre-treatment conditions</p><p>during TPAD, with performance comparisons against single-</p><p>stage thermophilic or mesophilic anaerobic digestion.</p><p>However, there is little analysis to determine the nature of the</p><p>pre-treatment process; whether it improves the rate or extent</p><p>of subsequent sludge degradation, or both properties. As</p><p>a result, optimal pre-treatment conditions (temperature, pH</p><p>and HRT) have not been established. Additionally, thermo-</p><p>philicemesophilic TPAD rarely has been evaluated in</p><p>a parallel comparison of mesophilicemesophilic with the</p><p>same retention times. This study is based on our previous</p><p>investigation of primary sludge (Ge et al., 2010), and further</p><p>investigates pre-treatment mechanisms of TPAD on waste</p><p>activated sludge, with a direct comparison against a control</p><p>mesophilicemesophilic process.</p><p>2. Materials and methods</p><p>2.1. Substrate(TPAD), combines a short (1e3 days) thermophilic pre-treat-</p><p>ment stage (50e70 C) applied prior to a conventional meso-philic anaerobic digestion (35 C, 10e20 days). TPAD is highlyscalable as the process incorporates standard digestion</p><p>vessels and low quality heat is the main energy input.Substrate was waste activated sludge, collected from a bio-</p><p>logical nutrient removal (BNR) process with 10 days sludge agemesophilic pre-treatment (MP) pre-treatment stages (0.6 L, 2</p><p>days HRT), andmesophilicmethanogenic stages (4.2 L, 14 days</p><p>HRT), as shown in Fig. 1. The basic set-up and operation of</p><p>thermophilic (TP1)emesophilic (TP2) TPAD and mesophilic</p><p>(MP1)emesophilic (MP2) TPAD systems were described in</p><p>Ge et al. (2010). Approximately 0.3 L per day of substrate was</p><p>fed simultaneously by pumping 0.05 L through the pre-treat-</p><p>ment stage andmethanogenic stage at intervals of 4 h per day</p><p>(6 times daily, also weighed daily). Gas production was</p><p>measured daily from each reactor using tipping bucket gas</p><p>meters, and continuously logged. Reactor pH was also recor-</p><p>ded online from each reactor continuously. Each reactor was</p><p>inoculated using methanogenic inoculum from the meth-</p><p>anogenic second stage (35 1 C, 14 days HRT) of a lab-scalethermophilicemesophilic TPAD system (Ge et al., 2010). This</p><p>provided a diverse microbial community and a common</p><p>starting point for each reactor.</p><p>The systems were operated in parallel for over 15 months.</p><p>During this time the temperature of TP1 was altered to create</p><p>different operating periods:</p><p> Period 1: 50 C (186 days), including two periods of pH 5 bydosing 1 M HCL (Day 39e48, and Day 55e77)</p><p> Period 2: 60 C (100 days) Period 3: 65 C (67 days), including a period of HRT reducedto 12 days in TP2 (Day 330e356)</p><p> Period 4: 70 C (68 days).</p><p>The temperature of TP2, MP1 andMP2was held constant at</p><p>35 C during all periods. During periods where the pH of TP1was reduced, the pH of MP1 was also reduced, and periods</p><p>Table 1 e Characteristics of the waste activated sludgeused in this study.</p><p>Measure Activated sludge</p><p>TS (g L1) 25.4 0.1VS (g L1) 17.5 0.1pH 6.5e7.5</p><p>COD (g L1) 27.4 3.5VFA (g COD L1) 0.2 0.1TKN (g N L1) 1.9 0.5NH4</p><p>eN (g L1) 0.06 0.04</p><p>Error margins indicate standard deviation across 14 different feedcollections used in the study over 15 months.</p></li><li><p>er </p><p>TP</p><p>wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 9 7e1 6 0 6 1599where the loading of TP2 was increased, the loading of MP2</p><p>was also increased, as a result of HRT shortened to 12 days.</p><p>This was done in steps of 20% around the average in an</p><p>attempt to provide better model-parameter identifiability.</p><p>After each acid dosing period, the pH of TP1 and MP1 returned</p><p>to their natural levels of 6.6 and 6.8, respectively.</p><p>2.3. Chemical analysis</p><p>Pretreatment 0.6LTP1 = 50-70C</p><p>MP1 = 35C</p><p>FGas meter</p><p>Feed reservior</p><p>Feedpump</p><p>Digestpump</p><p>Water jacket temperature </p><p>control</p><p>Gas to exhaust</p><p>Fig. 1 e Schematic diagram of thermophilic pre-treatmentGas production and composition (H2, CH4, CO2) were analysed</p><p>by GCeTCD as described previously (Tait et al., 2009). Liquid</p><p>samples were collected from each reactor three times per</p><p>week. Analysis was performed for TS, VS, volatile fatty acid</p><p>(VFA), chemical oxygen demand (COD), total Kjeldahl nitrogen</p><p>(TKN) and ammoniumenitrogen (NH4eN). Analytical</p><p>methods were based on Standard Methods (APHA, 1998). The</p><p>preparation and measurement of VFA, soluble COD (COD(S))</p><p>and NH4eN were as described previously (Ge et al., 2010).</p><p>2.4. Calculation</p><p>2.4.1. VS destructionThe two calculation methods used to determine VS destruc-</p><p>tion were the Van Kleeck equation and the mass balance</p><p>equation. The Van Kleeck equation (1) assumes the amount of</p><p>mineral solids is conserved during digestion (Switzenbaum</p><p>et al., 2003), and uses the volatile fractions (VS/TS VSfrac)in the inlet and outlet as references.</p><p>VS destruction% VSfrac;in VSfrac;outVSfrac;in </p><p>VSfrac;in VSfrac;out</p><p> (1)</p><p>where VSfrac,in volatile fraction (VS/TS) in the inlet solids;VSfrac,out volatile fraction (VS/TS) in the outlet solids.</p><p>The mass balance equation (2) uses VS concentrations</p><p>(VSconc) in the inlet and outlet, expressed asthe results of the Van Kleeck calculation are influenced by</p><p>accumulation ofmineral inerts within the reactor (under non-VS destruction% VSconc;in VSconc;outVSconc;in</p><p> 100 (2)</p><p>where VSconc,in VS concentration of inlet; VSconc,out VSconcentrations of outlet.</p><p>Results of the mass balance calculation are sensitive to</p><p>systematic sampling issues, which may cause dilution, while</p><p>Main Digester 4LTP2=35CMP2=35C</p><p>PLC</p><p>FGas meter</p><p>Effluent drum</p><p>Effluentpump</p><p>Heatingcoil</p><p>AD system and mesophilic pre-treatment TPAD system.steady state conditions).</p><p>2.4.2. Extent of solubilisationExtent of sludge solubilisation in each pre-treatment stage</p><p>was calculated using the ratio of total solubilised products</p><p>(methane production and COD(S)) and particulate COD</p><p>concentration in the inlet feed (Song et al., 2005). Hydrogen</p><p>was not detected in either pre-treatment stage. Extent of</p><p>solubilisation can be expressed as</p><p>Extent of solubilisation% CODCH4 CODSoCODSiCODTiCODSi</p><p> 100</p><p>(3)</p><p>where CODCH4 methane production as mg COD during pre-treatment; COD(S)i COD(S) concentration of inlet; COD(S)o COD(S) concentration of outlet; COD(T)i total CODconcentration of inlet.</p><p>2.5. Mathematical analysis</p><p>Mathematical analysis was based on the IWA Anaerobic</p><p>Digestion Model No. 1 (ADM1) (Batstone et al., 2002). Imple-</p><p>mentation of ADM1 for a TPAD process is described by Ge et al.</p><p>(2010), with the input model of Nopens et al. (2009). Initial</p><p>conditions were adjusted based on measurements of organic</p><p>solids, organicacids, ammonia,TKN,etc.Therewereapprox420</p><p>input changes over 450 days used in the model. Degradability</p></li><li><p>60 C (Period 2) improved VS destruction in the TP system from</p><p>wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 9 7e1 6 0 6160034 1% to 41 1%. A further increase of VS destruction to48 2% was observed when thermophilic pre-treatmenttemperature was increased to 65 C (Period 3), but no furtherenhancement at 70 C (Period 4). Statistical analysis (studentst-test, a 0.05) confirmed that VS destruction in the TP system(65 C) was significantly greater than that achieved at 60 C,which was also a significant improvement over that achieved</p><p>at 50 C. VS destruction in the TP system during Periods 2e4was also significantly better as compared to the MP system</p><p>(students t-test, a 0.05).Additionally, pre-treatment pHwas temporarily lowered to</p><p>pH 5 twice during 50 C pre-treatment (Day 39e48 and Day55e77), which did not influence VS destruction in either</p><p>system compared to previously. Similarly, VS destruction in</p><p>each system was maintained as previously described when</p><p>HRT ofmethanogenic stages was shortened from 14 to 12 days</p><p>during Period 3 (Day 330e356).</p><p>Total methane produced from TP system was consistently</p><p>higher than that fromMP system except Period 1, as shown in</p><p>Fig. 3 and Table 2. This was confirmed by students t-testextent (fd) and apparent first order hydrolysis rate coefficient</p><p>(khyd) were the main parameters used to assess and compare</p><p>two TPAD systems. In each system, khyd and fd were simulta-</p><p>neously estimated to achieve the average optimal values for the</p><p>wholeTPADprocess,whichwere thenset todetermine the two-</p><p>parameter uncertainty surface for khyd and fd based on the</p><p>method of Batstone et al. (2003, 2009). In the TP system, confi-</p><p>dence regions were estimated based on system performance at</p><p>each pre-treatment temperature. For the MP system, operating</p><p>conditionswere constant throughout theexperiment; therefore</p><p>only one confidence region was estimated for comparison.</p><p>A 95% confidence limit was used, with appropriate F-value</p><p>(2.996) for 2 parameters and the number of degrees of freedom.</p><p>Van Kleeck VS destruction was used as a...</p></li></ul>

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