Bioresource Technology 127 (2013) 9–17

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Bioresource Technology

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Performance and energy economics of mesophilic and thermophilic digestionin anaerobic hybrid reactor treating coal wastewater

Anushuya Ramakrishnan a,⇑, Rao Y. Surampalli b

a UT-School of Public Health, Division of Epidemiology, Human Genetics and Environmental Sciences, Houston, TX, United Statesb US Environmental Protection Agency, Kansas City, KS, United States

h i g h l i g h t s

" Performance of mesophilic and thermophilic AHR was compared for phenolics removal." HRT’s ranging from 3 to 0.5 d with OLR of 1.12–6.72 g L�1 d�1 were employed." At each HRT, thermophilic AHR gave better phenolics/COD removal and biogas yield." Kinetic model showed higher substrate utilization in thermophilic reactor." 11,550 MJ d�1 more energy can be generated using thermophilic AHR than mesophilic.

a r t i c l e i n f o

Article history:Received 27 August 2012Received in revised form 18 September 2012Accepted 23 September 2012Available online 29 September 2012

Keywords:PhenolicsCoal wastewaterMesophilicThermophilicMethane

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.09.071

⇑ Corresponding author. Tel./fax: +1 713 500 9393.E-mail address: anushuyar@gmail.com (A. Ramakr

a b s t r a c t

Two anaerobic hybrid AHRs (AHR), mesophilic (35 �C) and thermophilic (55 �C) were operated with coalwastewater at different hydraulic retention times (HRT) ranging from 3–0.5 to 3.12–0.6 d with organicloading rates (OLR) of 1.12–6.72 g L�1 d�1. Synthetic coal wastewater with an average chemical oxygendemand (COD) of 2240 mg L�1 and phenolics concentration of 752 mg L�1 was used as substrate. At eachHRT, the thermophilic AHR gave a better performance, measured in terms of phenolics/COD removal andgas production. The specific methane yield was also higher for thermophilic AHR at each HRT comparedto mesophilic one. The volatile fatty acid concentration in the effluent increased with the lowering ofHRT. The Stover–Kincannon model was applicable at both temperatures and showed higher substrate uti-lization in thermophilic AHR. Energy economic study of the AHRs revealed that 11,938 MJ d�1 moreenergy can be generated using thermophilic AHR than mesophilic.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Coal conversion processes (coal gasification and coal liquefac-tion) are predicted to meet global energy demand in decades tocome, with coal set to fuel 44% of electricity (IEA, 2010). A typicalcoal gasification plant that gasified 42,475,269,888 liters of coalper day generated approximately 8495.05–22,653.4 million litersper day of wastewater that has to be processed. Wastewaters fromcoal gasification contain 60–80% of phenolic compounds (includingphenol, methyl phenols and C2-phenols) along with aromaticnitrogen and sulfur containing compounds and aliphatic acids(Singer et al., 1978). Biodegradation of phenolic constituents incoal wastewater is generally more cost-effective than the physico-chemical treatment processes (Loh et al., 2000). Anaerobic treat-ment of coal wastewaters was carried out initially in anaerobic

ll rights reserved.

ishnan).

filters employing activated carbon which served to adsorb the toxicpollutants and acted as a carrier for bacterial adhesion (Suidanet al., 1983; Nakhla et al., 1990). Further developments resulted re-search investigations employing continuous anaerobic treatmentof mixed phenolic compounds without activated carbon (Fanget al., 1996 and Tawfiki et al., 2000). Towards the successful treat-ment of coal conversion effluents, a prudent selection of treatmentprocess should be made that guarantees simultaneous degradationof major phenolic compounds (phenols, cresols, and dimethyl phe-nols (DMPs)).

Extensive research has been conducted and documented on theapplication of upflow anaerobic sludge blanket (UASB) process forone stage treatment of phenol containing wastewaters (Veereshet al., 2005). However, few constraints and problems have been re-ported to be major obstacles for the successful operation of UASBreactors including poor granulation and sudden washout of sludgein extreme cases (Lettinga et al., 1980). Hybrid reactor configura-tion, combining a UASB reactor in the lower zone with an

10 A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17

anaerobic filter in the upper zone (Guiot and van den Berg, 1985) inorder to address the limitations of UASB reactors. AHRs belong tothe category of high rate anaerobic reactors and have uniqueadvantage that the hydraulic retention time (HRT) and solid reten-tion time (SRT) are uncoupled, thus enhancing the retention ofslow growing anaerobes promising a higher substrate reduction.AHRs also retain higher biomass at lower HRT and attain betterperformance at low temperatures due to the presence of filterzone. Filter zone not only ensured additional surface area for bac-terial attachment and growth above the sludge blanket (Tilche andVieira, 1991), but also guaranteed an improved stability and toler-ance to shock loading. The filter in addition to its physical role forbiomass retention also contributes to biological activity leading toadditional chemical oxygen demand (COD) removal in a zone thatis lacking biomass in classical UASB reactors (Oleszkiewicz et al.,1986). Since its inception, AHR has been successfully employedfor the treatment of industrial wastewaters, such as petrochemical(Augoustinos et al., 1989), pthalic (Tur and Huang, 1997) and phar-maceutical (Oktem et al., 2007). Anushuya and Gupta (2006) em-ployed AHR for the treatment of simulated coal wastewaters atmesophilic conditions (35 �C). They operated the AHR at variousoperating conditions (varying HRTs, C/N ratios, and recycling ra-tios) to guarantee a successful treatment of coal conversion efflu-ents and ensure a simultaneous degradation of major phenolicsubstrates (phenols, cresols, and dimethyl phenols (DMPs)) (Anus-huya and Gupta, 2008a,b).

Thermophilic UASB treatment of industrial wastewater has at-tracted much interest in the past two decades (Wiegant, 1985;van Lier et al., 1992). However, little information on the anaerobicdegradation of phenol under thermophilic condition is available sofar. Fang et al. (2006) reported the anaerobic treatment of phenolin wastewater under thermophilic condition. Over 99% of phenolwas effectively degraded in UASB reactor at 55 �C with 40 h HRTfor a wastewater containing 630 mg L�1 of phenol, correspondingto 1500 mg L�1 of COD and an organic loading rate (OLR) of0.9 g-COD L�1 d�1. Thermophilic process offers several merits, suchas removing pathogenic microorganisms and eliminating the needof cooling for effluent of high temperature. Thus, it is of rationalinterest to evaluate the performance of anaerobic treatment ofphenolic compounds in coal wastewater under thermophilicconditions.

For any specific wastewater, evaluating the potential of thermo-philic digestion process requires the assessment of whether it hasreal advantages over mesophilic digestion system. Examination ofliterature pertaining to the comparison of thermophilic and meso-philic AHR systems shows mixed results in favor of and againstthermophilic digestion. Kundu et al. (2012) studied the effect ofoperating temperature on the microbial community profiles in ahigh cell density hybrid anaerobic AHR. They operated the AHRat different temperatures (37, 45 and 55 �C). They reported thatthe AHR operated at 37 �C showed the best performance in termsof COD removal (2.04 kg COD m�3 d�1) and methane productionrate (0.56 m3 m�3 AHR day�1) compared to the AHR operated at55 �C with COD removal (1.93 kg COD m�3 d�1) and methane pro-duction rate (0.47 m3 m�3 AHR day�1). Reports by Harrison andDague towards the application of anaerobic filters for the treat-ment of non-fat dried milk revealed that there was little differencein the gas produced by both the AHRs at OLR up to 20 kg COD m�3 -d�1 at a longer HRT (48 h). However, at a lower HRT (12 h) themesophilic filter produced about 35% less gas than thermophilicfilter (56 �C) at OLR in the range 5.5–13.75 kg COD m�3 d�1. Themaximum OLR that could be attained by thermophilic UASB wasonly marginally higher than that for a mesophilic UASB, 11.4 com-pared to 10.0 kg COD m�3 d�1 for treating wastewaters from theproduction of instant coffee reported by Dinsdale et al. (1997). Thisstudy was conducted to examine the relative performance of mes-

ophilic and thermophilic AHRs at a series of HRTs pertaining to dif-ferent organic loadings.

2. Methods

2.1. Anaerobic hybrid reactors

Two identical AHRs were made of transparent acrylic plasticsheet with inner dimensions of 0.1 � 0.1 m, length of 1.5 m, wallthickness of 0.006 m and effective volume of 13.5 L respectively.The AHRs were provided with hopper bottom of 0.15 m lengthand a feed inlet pipe (u = 0.025 m) diameter to avoid choking dur-ing operation. The inlet end opens towards the bottom of the AHRwhich allows feed to first strike at the bottom and then gets evenlydistributed while rising upward in a hopper bottom. Gas liquid so-lid separator (GLSS) device (0.08 � 0.08 m) is placed at the top byassuming 20% of the AHR volume with inclined walls at 50�. Bafflesof sufficient overlap were provided below the GLSS in order toavoid the entry of biogas into settling compartment. An outlet pipe(u = 0.015 m) was provided at the top which is connected to theeffluent tank. The AHRs were provided with six equidistant sam-pling ports (u = 0.025 m) along its height to facilitate sampling. Afilter media of height 30.48 cm made of PVC rings (u = 0.0254 mand length 0.025 m) was provided at the middle of the AHR. The fil-ter media had a thickness of 0.064 m, dry weight of 102.9 g/L.About 215 rings were packed in all the four AHRs for consistency.The surface area of each ring was 0.0628 m2 and the total surfacearea occupied by the packing was 13.50 m2. The AHR was main-tained at a temperature of 55 �C for thermophilic condition andone of 35 �C for the mesophilic condition in a temperature con-trolled water bath. The performance of the AHR was monitoreddaily by measuring biogas production, phenolics and chemical oxy-gen demand (COD), alkalinity, volatile fatty acids (VFA), volatilesuspended solids (VSS) and pH.

2.2. AHR seeding

The AHRs were seeded with a mixture of anaerobic digestersludge (3 L) and partially granulated sludge (1 L). Granular sludgefor mesophilic AHR was obtained from a hybrid AHR treating coalwastewater acclimated to phenol, cresols and dimethyl phenols(Anushuya and Surampalli, 2012). Granular sludge for thermo-philic AHR was obtained from thermophilic anaerobic sludge cul-ture, which was enriched on mixture of phenol (490 mg L�1) andcresols (228 mg L�1) and dimethyl phenols (38.3 mg L�1) for3 months in a 15 L batch-fed AHR at a temperature of 55 �C withan HRT of 30 days. This culture was taken from the lab scale AHRwhich had been operated at mesophilic temperature (35 �C) for5 years (Anushuya and Gupta, 2006). Digester sludge for both mes-ophilic and thermophilic systems was obtained from M/S. Maha-nanda Dairy Works, Goregoan, Mumbai. Three liters of digestersludge containing 72.6 g of total solids (with a sludge volume index(SVI) of 32 mL g�1 SS�1) and 1 L of granular sludge containing62.1 g of total solids (with a SVI of 27 mL g�1 SS�1) was mixedand stored at 4 �C prior to use. The mixed anaerobic culture was fil-tered through a screen of 0.05 inch (1.2 mm) mesh size and con-centrated by settling for 2 h before being used as inoculum. Thetotal suspended and volatile suspended solids added to the AHRwere 130 and 80 g L�1 respectively.

2.3. Wastewater characteristics

The composition of full strength synthetic coal wastewater em-ployed in the present study with their constituent concentrationexpressed in (mg L�1) is as follows: phenolics (7519), ammonium

A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17 11

chloride (280), calcium chloride (100), dipotassium hydrogenorthophosphate (250), magnesium sulfate (100), iron sulfate (20),sodium bicarbonate (1000), and yeast extract (100). The character-istics of the synthetic coal wastewater with their constituent con-centrations expressed in (mg L�1) is as follows: organic carbon(7519), COD (21,100), phenolics (7519), total Kjedahl nitrogen(267), nitrite nitrogen (0.2), nitrate nitrogen (0.8), ammonia nitro-gen (6.4), total cyanide (0.21), total phosphorus (5) and pH 7.4. Thefull strength synthetic wastewater contained the following pheno-lic compounds with their constituents expressed in (mg L�1): phe-nol (4900); o-cresol (586); m-cresol (1230); p-cresol (420); 2,4-dimethyl phenol (63); 2,5-dimethyl phenol (63); 3,4-dimethylphenol (44) and 3,5-dimethyl phenol (213) as major phenolic com-pounds (Anushuya and Gupta, 2008b).Volatile fatty acids in thesynthetic wastewater were represented in (mg L�1) as follows:acetic acid (28.0); propionic acid (16.0); butyric acid (5.0) and vale-ric acid (3.5), respectively. About 10% of the full strength syntheticcoal waste water was used in the experiment along with majornutrients and trace metal solution. Trace metal solution was pre-pared in distilled water containing (mg L�1), zinc chloride (0.05);cupric chloride (0.05); manganous sulfate (0.03); ammoniummolybdate (0.05); aluminium chloride (0.05); cobaltous chloride(0.05); nickel chloride (0.05) and resazurin (0.2).1 ml of this solu-tion was added per liter of the feed solution. By adding sodiumbi-carbonate an influent alkalinity of 1100 mg L�1 was maintainedin the AHRs. Influent COD was monitored prior to feeding to esti-mate the organic loading and suitable influent volume accordingto the experimental design. Feed was thoroughly mixed for15 min prior to each feeding using a high speed mixer (DriveI T,1.8 kW, 950, 145 rpm). Well mixed feed was stored in 15 L con-tainer and supplied to the AHRs using two peristaltic pumps Wat-son Marlow 601 S and FMI lab pump (RB 30 D) fitted with astandard gear (Nord 148 Gear; SK 01–80L/4; 0.75 kW; Gear ratio:14.75; rpm 95) that were well calibrated to determine the requiredflow rate that resulted in HRT of 3–0.5 d for mesophilic and 3.12–0.6 d for thermophilic AHR respectively.

2.4. Analytical methods

The analytical procedures for all tests were as outlined in theStandard Methods for the Examination of Water and Wastewater(APHA, 2005). Phenolic compounds were determined by gas chro-matogram (GC) (Agilent Model: 6890 No. G1530, USA) according tothe procedure of Fang et al. (1996). Biogas generation was mea-sured by using water displacement method for a collection periodof 3 h, twice in a day and average value for the particular day hasbeen estimated. For measuring the biogas, the vent pipes of thereactors are connected with 5 L aspirator bottle filled with water.As the biogas enters the bottle filled with water, it displaces samevolume of biogas through the stop cock which was collected andmeasured to find out total amount of biogas produced during thegiven period. The composition of biogas was determined by GC.The system used a 16.3 m glass column (3 mm ID) with a porapakQ (mesh size 80–100) support which was maintained at 90 �C. Thecarrier gas was helium (35 ml min�1) and the sample size was1.0 ml. The content of methane in the biogas was determined asfollows. A known volume of the head space gas (V1) produced ina serum bottle was syringed out and injected into another serumbottle that contained 20 g potassium hydroxide (KOH) per liter.This serum bottle was shaken manually for 3–4 min so that allthe CO2 and H2S were absorbed in the concentrated KOH solution.The volume of the remaining gas (V2), which was 99.9% methane,in the serum bottle was determined by means of syringe. The ratioof V2/V1 provided the content of methane in the head space (Ergu-der et al., 2000). Volatile fatty acids (VFA) in the effluent were mea-

sured in conformity with the procedure outlined by Anushuya andGupta (2006).

2.5. Operational strategy

Operation of the AHRs began with the synthetic phenolic waste-water with a mean COD of 2240 mg L�1 and phenolics concentra-tion of 752 mg L�1. The AHRs were operated at 6 HRTs between3.0–0.5 d during Phase I–VI for mesophilic and 3.12–0.6 d for ther-mophilic conditions respectively. The subsequent changes in OLRare given in Table 1. In phase VII and VIII, the concentration ofphenolics used in the feed was increased to 980 and 1128 mg L�1

respectively. The AHRs were operated for a period of 300 days todetermine the effect of temperature on phenolics removal in coalgasification wastewater.

3. Results and discussion

3.1. Performance of the AHRs at different HRTs and organic loadings

Performance of the AHRs at different operating conditions isshown in Figs. 1a and 1b, Figs. 2a and 2b and Figs. 3a and 3b.

During phase I, the effluent COD and phenolics levels in themesophilic AHR remained at 381 and 112.8 mg L�1respectively(Figs. 1a and 2a). COD removal efficiency remained at 83% andthe phenolics removal efficiency during this period was 85%(Figs. 1a and 2a). However, thermophilic AHR gave a better perfor-mance with lower effluent COD and phenolics levels at 234 and67.7 mg L�1 respectively (Fig. 1b and Fig. 2b). COD and phenolicsremovals were 6.0% higher in the thermophilic AHR in this phase(Figs. 1a and 2a). Alkalinity and VFA levels in the thermophilicAHR were 1.02% and 28.2% higher than that of the mesophilicAHR (Table 2). Methane yield was 15% higher in the thermophilicAHR compared to the mesophilic one (Figs.3a and 3b).

During phase II, the effluent COD and phenolics levels in themesophilic AHR remained at 336 and 97.7 mg L�1respectively(Figs. 1a and 2a). COD removal efficiency remained at 85% andthe phenolics removal efficiency during this period was 87%(Figs. 1a and 2a). However, thermophilic AHR recorded a betterperformance with lower effluent COD and phenolics levels at 219and 56.4 mg L�1 respectively (Figs. 1b and 2b). COD and phenolicsremovals were 5.2% and 5.5% higher in the thermophilic AHR inthis phase (Figs. 1a and 1b). Alkalinity and VFA levels in the ther-mophilic AHR were 0.8% and 17% higher than that of the meso-philic AHR (Table 2). Methane yield was 14.28% higher in thethermophilic AHR compared to the mesophilic one (Figs. 3a and3b).

During phase III, the effluent COD and phenolics levels in themesophilic AHR remained at 290 and 82.7 mg L�1respectively (Figs1a and 2a). COD removal efficiency remained at 87% and the phen-olics removal efficiency during this period was 89% (Figs. 1a and2a). However, thermophilic AHR gave better efficiency leading tolower effluent COD and phenolics levels at 212 and 45.12 mg L�1

respectively (Figs. 1b and 2b). COD and phenolics removals were3.5% and 6% higher in the thermophilic AHR in this phase. Alkalin-ity and VFA levels in the thermophilic AHR were 2.4% and 18%higher than that of the mesophilic AHR (Table 2). Methane yieldwas 13.33% higher in the thermophilic AHR compared to the mes-ophilic one (Figs. 3a and 3b).

During phase IV, the effluent COD and phenolics levels in themesophilic AHR remained at 240 and 67.6 mg L�1 respectively(Figs. 1a and 2a). COD removal efficiency remained at 89.3% andthe phenolics removal efficiency during this period was 92%(Figs. 1a and 2a). However, thermophilic AHR gave better efficiencyleading to lower effluent COD and phenolics levels at 202 and

Table 1Operational sequence of AHR.

Phase Phenolics concentration (mg L�1) Mesophilic AHR Thermophilic AHR

HRT (d) OLR (g COD m�3 d�1) HRT (d) OLR (g COD m�3 d�1)

I 752 3.0 1.13 3.12 1.13II 752 2.5 2.62 2.66 2.62III 752 2.0 3.28 2.25 3.28IV 752 1.5 4.45 1.68 4.45V 752 1.0 5.76 1.2 5.76VI 752 0.5 6.72 0.6 6.72VII 980 1.0 7.26 1.2 7.26VIII 1128 1.0 8.22 1.2 8.22

Fig. 1a. COD removal in the mesophilic AHR.

Fig. 1b. COD removal in the thermophilic AHR.

12 A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17

Fig. 2a. Phenolics removal in the mesophilic AHR.

Fig. 2b. Phenolics removal in the thermophilic AHR.

A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17 13

30.08 mg L�1 respectively (Figs. 1b and 2b). COD and phenolicsremovals were 2% and 5% higher in the thermophilic AHR in thisphase. Alkalinity and VFA levels in the thermophilic AHR were2.06% and 15.4% higher than that of the mesophilic AHR (Table 2).Methane yield was 18.75% higher in the thermophilic AHR com-pared to the mesophilic one (Figs. 3a and 3b).

During phase V, the effluent COD and phenolics levels increasedin the mesophilic AHR remained at 200.5 and 82.7 mg L�1 respec-tively (Figs. 1a and 2a). COD removal efficiency remained at 91%and the phenolics removal efficiency during this period was 89%.(Figs. 1a and 2a). However, thermophilic AHR gave better efficiencyleading to lower effluent COD and phenolics levels at 160 and60.16 mg L�1 respectively (Figs. 1b and 2b). COD and phenolicsremovals were 1% and 3% higher in the thermophilic AHR in thisphase. Alkalinity and VFA levels in the thermophilic AHR were1.02% and 19.5% higher than that of the mesophilic AHR (Table 3).Methane yield was 11% higher in the thermophilic AHR comparedto the mesophilic one (Figs. 3a and 3b).

During phase VI, the effluent COD and phenolics levels increasedin the mesophilic AHR and stabilized at 268.8 and 105.3 mg L�1

respectively (Figs. 1a and 2a). COD removal efficiency declinedand stabilized at 88% and the phenolics removal efficiency also low-ered by 3% to 86% (Figs. 1a and 2a). Decrease in COD and phenolicsremoval efficiency can be attributed to the lower HRT and increasedphenolic loading into the AHR. Decline in the performance of thereactors could be attributed to the inhibition of bioactivity due tothe lower HRT and increased phenolic loading into the AHR. How-ever, thermophilic AHR gave better efficiency leading to lower efflu-ent COD and phenolics levels at 185 and 75.2 mg L�1 respectively(Figs. 1b and 2b). COD and phenolics removals were 4% each higherin the thermophilic AHR in this phase. Alkalinity and VFA levels inthe thermophilic AHR were 0.8% and 33% higher than that of themesophilic AHR (Table 3). Methane yield was 14.28% higher in thethermophilic AHR compared to the mesophilic one (Figs. 3a and 3b).

During phase VII, the effluent COD and phenolics levels in themesophilic AHR and stabilized at 379 and 107.8 mg L�1 respec-

Fig. 3a. OLR and methane yield in the mesophilic AHR.

Fig. 3b. OLR and methane yield in the thermophilic AHR.

Table 2Summary of results obtained over operational period at different HRTs.a

Parameters I II III IV

Mesophilic Thermophilic Mesophilic Thermophilic Mesophilic Thermophilic Mesophilic Thermophilic

Alkalinityi (mg L�1) (as CaCO3) 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10Alkalinitye (mg L�1) (as CaCO3) 1175 ± 32 1185 ± 38 1195 ± 33 1205 ± 35 1225 ± 66 1255 ± 60 1235 ± 6 1261 ± 8VFAi (mg L�1) 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10VFAe (mg L�1) 28 ± 12 39 ± 10 35 ± 11 42 ± 10 46 ± 10 56 ± 13 55 ± 11 65 ± 15VSS (g L�1) 34.5 ± 1.5 35.5 ± 2.8 33.5 ± 3.5 34.2 ± 2.8 32.5 ± 1.5 33.3 ± 2.8 31.5 ± 3.5 32.2 ± 2.8pH 7.8 ± 0.2 7.7 ± 0.2 7.7 ± 0.2 7.6 ± 0.2 7.6 ± 0.2 7.5 ± 0.2 7.5 ± 0.2 7.4 ± 0.2

Subscripts i and e refer to influent and effluent.a Values are averages of 50 determinations.

14 A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17

tively (Figs. 1a and 2a). COD removal efficiency stabilized at 87%and the phenolics removal efficiency also increased by 3–89%

(Figs. 1a and 2a). Increase in phenolics removal efficiency can beattributed to the 50% increase in HRT at the higher phenolics load-

Table 3Summary of results obtained over operational period at different HRTs.a

Parameters V VI VII VIII

Mesophilic Thermophilic Mesophilic Thermophilic Mesophilic Thermophilic Mesophilic Thermophilic

Alkalinityi (mg L�1) (as CaCO3) 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10 1100 ± 10Alkalinitye (mg L�1) (as CaCO3) 1255 ± 32 1268 ± 38 1275 ± 33 1285 ± 35 1355 ± 66 1475 ± 60 1655 ± 66 1750 ± 85VFAi (mg L�1) 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10 52.5 ± 10VFAe (mg L�1) 128 ± 14 159 ± 13 229 ± 15 342 ± 18 232 ± 16 376 ± 15 445 ± 10 585 ± 12VSS (g L�1) 30.5 ± 1.5 31.5 ± 2.8 29.5 ± 3.5 30.2 ± 2.8 28.5 ± 1.5 29.2 ± 2.8 27.0 ± 3.5 28.5 ± 2.8pH 7.2 ± 0.2 7.1 ± 0.2 7.0 ± 0.2 6.9 ± 0.2 6.9 ± 0.3 6.8 ± 0.2 6.8 ± 0.2 6.7 ± 0.2

Subscripts i and e refer to influent and effluent.a Values are averages of 50 determinations.

A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17 15

ing into the AHR. However, thermophilic AHR gave better effi-ciency leading to lower effluent COD and phenolics levels at 321and 88.2 mg L�1 respectively (Figs. 1b and 2b). COD and phenolicsremovals were 2% each higher in the thermophilic AHR in thisphase. Alkalinity and VFA levels in the thermophilic AHR were2.24% and 8.1% higher than that of the mesophilic AHR (Table 3).Methane yield was 6.25% higher in the thermophilic AHR com-pared to the mesophilic one (Figs. 3a and 3b).

During phase VIII, the effluent COD and phenolics levels in-creased in the mesophilic AHR and stabilized at 503 and146.6 mg L�1 respectively (Figs. 1a and 2a). COD removal efficiencydeclined and stabilized at 85% and the phenolics removal efficiencyalso declined by 2–87% (Figs. 1a and 2a). Decrease in COD andphenolics removal efficiency can be attributed to the 12.5% in-crease in phenolics loading into the AHR that was a shock to theAHR. The AHR took almost 10 days to reach normal phenolicsand COD levels. However, thermophilic AHR gave better efficiencyleading to lower effluent COD and phenolics levels at 436 and124.08 mg L�1 respectively (Figs. 1b and 2b). COD and phenolicsremovals were 2.2% and 2% higher in the thermophilic AHR in thisphase. Alkalinity and VFA levels in the thermophilic AHR were9.45% and 5.4% higher than that of the mesophilic AHR (Table 3).Methane yield was 5.9% higher in the thermophilic AHR comparedto the mesophilic one (Figs. 3a and 3b).

The data revealed that thermophilic AHR gave superior perfor-mance than that of the mesophilic AHR at each of the HRTs tested.These results are similar to the observations of Ahn and Forster(2002) who reported a higher soluble COD removal significantlygreater at the thermophilic temperatures than at the mesophilictemperature. However, it is contrary to the reports of Duran andSpeece (1997) on the performance of two completely mixed tankAHRs operating at a HRT of 15 days. They reported that effluentCOD from thermophilic digester was higher (about 3500 mg L�1)compared to an identical AHR operating in the mesophilic range(about 2900 mg L�1). The difference in the results could be attrib-uted to the positive effect of increasing temperature on maximumspecific growth rate and substrate utilization rate, thereby result-ing in higher phenolics removal and enhanced biogas productionrate for thermophilic AHR. Performance of the AHRs showed thatthe optimal HRT that could result in higher phenolics and COD re-moval in mesophilic and thermophilic hybrid AHRs was 1 and1.2 d at an organic loading rate of 5.32 g L�1 d�1. Increase inphenolics loading resulted in a significant decrease in phenolicsremoval in mesophilic AHR compared to thermophilic one.Lowering of phenolics removal at lower HRT could be attributedto the inhibition of bioactivity by the higher phenolics concentra-tion. Similar trend was reported by Fang et al. (2006) uponlowering of HRT during the thermophilic anaerobic treatment ofphenol.

Daily biogas production data revealed that digestion in thermo-philic range was effective than mesophilic one at each HRToperated. The difference was significant at higher loading rates.

These results are comparable to reports of Borja et al. (1995)who reported that mesophilic AHRs showed a substantial decreasein methane production at lower retention times when comparedwith thermophilic AHRs. Specific methane yields (moles methaneg�1 COD) were also higher in thermophilic AHR at all the phases(Figs. 3a and 3b).

VFA concentrations produced by the AHRs demonstrate thatthermophilic process was more effective than the mesophilic onein the degradation of VFAs. The effluent from thermophilic AHRcontained only acetate and propionate all the applied HRTs. How-ever, acetate was dominant in both systems, but mesophilic AHRproduced appreciably higher concentration of acids in Phases VIIand VIII. The accumulation of these acids is compatible with thedecrease in pH during these phases (Table 3). Thermophilic AHRsproduced an effluent with VFA concentrations 144 and 120 mg L�1

greater than in the effluent from mesophilic AHR in phases VII andVIII. These results are compared to that reported by Harris andDague (1993) who reported the VFA concentration in thermophilicfilters to be greater than that of mesophilic filters by 150 mg L�1.

Alkalinity in the thermophilic AHRs was always higher thanthat of the mesophilic AHR. Volatile suspended solids (VSS) levelswere also higher in the thermophilic AHR compared to mesophilicone. Decrease in HRT resulted in a decrease in VSS in both thermo-philic and mesophilic AHR (Table 2 and Table 3). However thermo-philic AHR could effectively retain biomass due to the positiveeffect of increase in temperature on the maximum specific growthrate of microflora retained in it. The pH of the thermophilic processwas generally lower than that of the mesophilic process. This wasthe result of the higher VFA generation in the AHR. However, thepH was always maintained within the favorable range for anaero-bic bacteria. The pH balance could be attributed to increased alka-linity resulting from the degradation of phenolics in thethermophilic AHR. Similar results were observed from the degra-dation of phenolics in our previous studies (Anushuya and Gupta,2008a).

3.2. Modified Stover Kincannon model

Since the early 1970s, Stover and Kincannon have proposed adesign concept of total organic loading rate and established a ki-netic model for biofilm AHRs (Kincannon and Stover, 1982). In thismodel the substrate utilization rate is expressed as functions of theorganic loading rate by monomolecular kinetics for biofilm AHRssuch as rotating biological contactors and biological filters. A spe-cial feature of modified Stover–Kincannon model is the utilizationof the concept of total organic loading rate as the major parameterto describe the kinetics of an AF in terms of organic matter removaland methane production (Yu et al., 1998). The removal of organicsubstrate in the AF process can be determined on the basis of thesubstrate removal rate as a function of the substrate concentration.At steady state, the Stover–Kincannon model would define the rateof substrate removal (ds/dt) as:

Table 4Comparison of kinetic constants in the Modified Stover–Kincannon model.

S. No. Feed and AHR used S (g COD L�1) HRT Umax

(g L�1 d�1)KB

(g L�1 d�1)R2 Authors

1 Coal wastewater; mesophilic AHR 1.13–6.72 0.5–3.0 40.64 118.6 0.9846 Present study2 Coal wastewater; thermophilic AHR 1.13–6.72 0.5–3.0 186.23 248.30 0.9868 Present study3 Pharmaceutical wastewater; AHR 4.0–4.5 0.12–

1.25108.69 115.66 0.9997 Pandian et al. (2011)

4 Simulated wastewater; AHR 1–10 0.5–10 83.3 186.23 0.987 Buyukkamaci and Filibeli (2002)5 Simulated wastewater; UASB 4.214 0.25–

4.167.5 8.2 0.995 Isik and Sponza (2005)

6 Simulated paper mill wastewater; mesophilic AF 1.7–3.87 11.7–26.2

49.8 50.6 0.9711 Ahn and Forster (2002)

7 Simulated paper mill wastewater; thermophilic AF 1.7–3.87 11.7–26.2

667 702 0.9711 Ahn and Forster (2002)

8 Soybean wastewater, AF 7520–11450 1–1.45 83.3 85.5 – Yu et al. (1998)

Table 5Economic evaluation for mesophilic and thermophilic AHR.

Parameters Mesophilic Thermophilic

Maximum percent COD reduction at lowestHRT

88 92

Reduction in COD for a 50 KLD plant, kg d�1 3.68 � 104 3.85 � 104

Maximum methane yield, (m3 kg�1 CODrem) 0.325 0.340Methane production (Nm3 d�1) 12374 12945Energy generation (MJ d�1) 2,58,709 2,70,647Coal equivalent of methane, metric tonne d�1 11.99 12.54Oil equivalent per metric tonne of hard coal 59.9

barrels62.7 barrels

Annual fuel savings in million ($) 2.17 2.275

16 A. Ramakrishnan, R.Y. Surampalli / Bioresource Technology 127 (2013) 9–17

ds=dt ¼ Q=VðSi � SeÞ ð3:1Þ

where dS/dt is defined in two ways as follows:

ds=dt ¼ UmaxðQsi=VÞ=KB þ ðQsi=VÞ ð3:2Þ

ds=dt ¼ kXSe=Ks þ Se ð3:3Þ

This can be linearized as follows:

ðds=dtÞ�1 ¼ V=QðSi � SeÞ ¼ KB=Umax � V=QSi þ 1=Umax ð3:4Þ

where, dS/dt, substrate removal rate (g L�1 d�1); Umax, maximumutilization rate constant (g L�1 d�1); V, clean-bed volume of the AF(l); Q, inflow rate (L d�1); Si, substrate concentration in the influent,g COD L�1 d�1; Se, substrate concentration in the effluent, gCOD L�1 d�1; KB, saturation value constant (g L�1 d�1); k, maximumrate of substrate removal (L d�1); X, microorganism concentration(VSS) in the AF (g L�1); KS, half-velocity constant (g L�1).

If (dS/dt)�1 is taken as V/[Q(Si � Se)], which is the inverse of theloading removal rate and this is plotted against the inverse of thetotal loading rate V/(QSi), a straight line portion of intercept 1/Umax

and a slope of KB/Umax results. Since ds/dt approaches Umax as QSi/V,the organic loading rate, approaches infinity in Eq. 3.4., Umax can bebelieved to be the maximum utilization rate constant. Umax and KB

values obtained for thermophilic AHR (186.23 and 248.30) werehigher than that of the mesophilic AHR (4.064 and 118.61) (Ta-ble 4). The Umax and KB values obtained in this study were higherthan values found by Pandian et al. (2011); Buyukkamaci and Fili-beli (2002); Isik and Sponza (2005); Yu et al. (1998) (Table 4).However, the thermophilic substrate utilization (248.3 g L�1 d�1)was lesser than that reported by Ahn and Forster (2002)(702 g L�1 d�1) reported for simulated paper mill wastewater.These results reveal that the thermophilic AHR has a much highermaximum utilization rate constant than the mesophilic AHR. Thevalues of correlation constant (R2) confirm that modified stover-

kincannon model can be used to describe the performances of ther-mophilic AHRs as well as mesophilic ones.

3.3. Energy economics of thermophilic and mesophilic AHR

Table 5 shows the energy potential and economics of biogasgeneration from mesophilic and thermophilic AHR. The estimateshows that for a coal gasification plant of 50 KLD capacity, thermo-philic AHR can result in additional COD reduction of 17649 kg andproduce 571 m3 of more methane per day. This results in addi-tional energy saving of 11,938 MJ d�1 using thermophilic AHRtechnology than mesophilic one.

4. Conclusions

The results of the study reveal that thermophilic anaerobicdigestion in terms of the specific methane yield, effluent qualityand process stability was superior to the mesophilic. ThermophilicAHR was stable and well functioned with a higher retention of ac-tive microorganisms that showed higher substrate affinity. VFAlevels in the effluents of both mesophilic and thermophilic AHR in-creased with decrease in HRT. The Stover–Kincannon model wasapplicable to both AHRs and thermophilic AHRs had the maximumsubstrate utilization rate compared to mesophilic. Energy eco-nomic study of the AHRs revealed that 11,938 MJ d�1 more energycan be generated using thermophilic AHR.

Acknowledgement

The views and opinions expressed in this paper are those of theauthors.

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