Bioresource Technology 131 (2013) 6–12

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Bioresource Technology

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Kinetics of psychrophilic anaerobic sequencing batch reactor treatingflushed dairy manure

0960-8524/$ - see front matter � 2012 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.biortech.2012.11.147

⇑ Corresponding author. Tel.: +1 509 335 3743; fax: +1 509 335 2722.E-mail address: chens@wsu.edu (S. Chen).

Jingwei Ma a, Liang Yu a, Craig Frear a, Quanbao Zhao a, Xiujin Li b, Shulin Chen a,⇑a Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USAb Center for Resources and Environmental Research, Beijing University of Chemical Technology, Beijing 100029, China

h i g h l i g h t s

" A new biomass retention strategy for solids containing influent was presented." Influent solids were used as natural biofilm support media for high rate digestion." The technology showed a good performance despite short HRT and low temperature." There is free of clogging hazard in biofilm support media caused by manure fiber." Four microbial growth kinetic models were compared for biofilm kinetics study.

a r t i c l e i n f o

Article history:Received 2 October 2012Received in revised form 29 November 2012Accepted 30 November 2012Available online 12 December 2012

Keywords:PsychrophilicKineticsASBRAnaerobic digestionDairy manure

a b s t r a c t

In this study, a new strategy, improving biomass retention with fiber material present within the dairymanure as biofilm carriers, was evaluated for treating flushed dairy manure in a psychrophilic anaerobicsequencing batch reactor (ASBR). A kinetic study was carried out for process control and design bycomparing four microbial growth kinetic models, i.e. first order, Grau, Monod and Chen and Hashimotomodels. A volumetric methane production rate of 0.24 L/L/d of and a specific methane productivity of0.19 L/gVSloaded were achieved at 6 days HRT. It was proved that an ASBR using manure fiber as supportmedia not only improved methane production but also reduced the necessary HRT and temperature toachieve a similar treating efficiency compared with current technologies. The kinetic model can be usedfor design and optimization of the process.

� 2012 Published by Elsevier Ltd.

1. Introduction

Livestock farms in US produce a total of about two billion tonsof manure each year (Gillespie and Flanders, 2010), which accountsfor 8% of the total US anthropogenic bio-methane emissions (USEP-A, 2010). Anaerobic digestion (AD) is an alternative to livestockwaste management that offers economic and environmental bene-fits. Besides alleviating manure-associated greenhouse gas (GHG)emissions and farm-generated odors, AD of animal waste providesfertilizers rich in nutrient, and biogas as renewable energy.

Wider adoption of AD for animal manure management has beenlimited primarily by economics. This is especially true in someapplications where the wastewater is relatively dilute such as influshing dairies. Flushed manure handling systems are widely em-ployed within large-scale dairy farms due to their reduced laborand mechanical failures (Powers et al., 1997). However, flushing

systems produce a waste stream with total solids of 1–2%, nega-tively impacting conventional AD treatment processes due to thefact that diluted manure increases digester size and heatingrequirements. Anaerobic digestion at psychrophilic temperaturecan alleviate this concern, if corresponding reduction in biogas pro-duction rates due to the lower utilized temperature can be over-come through high microbial accumulation (Kashyap et al.,2003). By inference, assuming adequate psychrophilic operation,the main concerns with using an anaerobic digester for dilute man-ure treatment is the challenges in achieving higher solids retentiontime (SRT) required to retain microbial biomass and reducing re-quired size. Typical designs such as continuous stirred-tank reactor(CSTR) or plug flow (PF) digesters cannot accomplish such decou-pling of SRT and HRT (hydraulic retention time) (Zaher et al., 2008).

Many efforts have been made to increase microbial biomassretention with different digester configurations, such as fixed-bed and hybrid reactors (Borja et al., 1994; Demirer andChen, 2005; Umana et al., 2008; Wilkie et al., 2004; Zaher et al.,2008), and have been successfully applied at low temperature

Nomenclature

l specific microbial growth rate (/d)lm maximum specific microbial growth rate (/d)rm microbial growth rate (g/L/d)X0 influent biomass concentration (g/L)X effluent biomass concentration (g/L)V digester working volume (L)Q flow rate (L/d)S0 influent substrate concentration (g/L)S effluent substrate concentration (g/L)h hydraulic retention time (d)b endogenous decay constant (/d)k maximum specific substrate utilization rate (gVS/g/d)k’ first order rate constant (/d)Ks half-saturation constant (gVS/L)

K dimensionless kinetic parameter of Chen and Hashimotomodel

Y growth yield coefficientc contois coefficientB volume of methane produced under standard condition

per gram of substrate loaded (L CH4 STP/gVSloaded)B0 volume of methane produced under standard condition

per gram of substrate loaded at infinite retention time (LCH4 STP/gVSloaded)

M volumetric methane production rate (L CH4/L/d)P specific methane productivity (L CH4/gVSloaded)L organic loading rate (gVSloaded/L/d)

J. Ma et al. / Bioresource Technology 131 (2013) 6–12 7

(Siggins et al., 2011; Zhang et al., 2012). A variety of external arti-ficial biofilm support media, such as spherical plastic trickling filtermedia, floating support media, automobile tires, and zeolite havebeen employed in anaerobic biofilm digesters to enhance biomassretention. The addition of artificial support media occupies sub-stantial digester volume, which automatically lowers the digesterefficiency. Moreover, the artificial biofilm support media are vul-nerable to clogging caused by manure fiber, which impedescommercialization.

A concept of biofilm retention with influent solids was pre-sented in the authors’ previous studies (Frear et al., 2010; Wanget al., 2011). It was reported that anaerobic microorganisms havea strong affinity to manure fiber, which can serve as natural bio-mass support media in a high rate digester. Biomass retentionusing manure fiber as natural support media seems a promisingapproach for anaerobic treatment of flushed dairy manure. In vir-tue of no artificial biofilm support media, the concern regardingmechanical failure caused by media clogging is removed. Alongwith low maintenance, the required digester size and cost are re-duced. Anaerobic sequencing batch reactors (ASBRs) are knownto be capable of uncoupling HRT with SRT for biomass retentionwith a particular sequence of operation of ASBR exerting selectionpressures to microbes for immobilization (Liu et al., 2005; Wanget al., 2011). Wang et al. (2011) showed that an ASBR digester,which retained high concentration of biomass in the form of fi-ber-attached biofilm by selection pressure, exhibited compara-tively high methane yield and production rate. However,applications of this technology require technical information forprocess design and optimization.

Although simple, the ASBR operation involves complex pro-cesses whose design and optimization can be facilitated usingmathematical models. Kinetic modeling, being a useful tool in pro-cess analysis, design, and system control can be established by pre-cise determination of kinetic coefficients. Process kinetics alsodetails the effects of operational factors and reaction environmenton the substrate utilization rates. A variety of kinetic models havebeen developed to describe microbial growth kinetics. A first ordermodel is the simplest model for microbial growth with theassumption of first-order degradation, which has been used oftento describe hydrolysis limited digestion with respect to particulatesubstrate (Gavala et al., 2003). Monod model is the most widelyused kinetic model which was developed as a result of empiricalanalysis (Monod, 1949). Grau et al. (1975) and Chen and Hashim-oto (1978) improved the Monod model by predicting that effluentsubstrate concentration is proportional to influent substrate con-centration. However, it was assumed that microbial growth kinet-

ics in anaerobic biofilm reactors followed Monod or first-ordermodels in most literature (Batstone et al., 2002; Buffière et al.,1998; Huang and Jih, 1997; Rittmann and McCarty, 2001). The lackof appropriate models in the literature though shows thatimprovements can still be made. For example, it has been hypoth-esized that the Chen and Hashimoto model is capable of character-izing biofilm growth kinetics with an improved performance,compared to the Monod model, due to its dependence on influentsubstrate concentration.

The main objective of this study was to evaluate the perfor-mance and kinetics of the new biomass retention strategy duringpsychrophilic ASBR digestion of flushed dairy manure. A kineticmodel with aim to find a more appropriate biofilm growth modelwas derived and assessed for substrate utilization and methaneproduction. Both HRT and OLR (organic loading rate) are consid-ered as the most important parameters for digester operation.Hence, the effects of HRT and OLR on methane production werethe primary output of the kinetic model.

2. Methods

The aforementioned biomass retention technology for treatingflushed dairy manure at psychrophilic temperature were evaluatedin five lab scale digesters operated in sequencing batch mode. Ki-netic properties of psychrophilic ASBR digesters were then ana-lyzed and a kinetic model was derived for system optimization.

2.1. Feedstock and seed

Fresh dairy manure was collected from the Washington StateUniversity Dairy Center in Pullman, WA, USA and stored at 4 �Cprior to use. Before addition to digesters, manure was diluted withtap water to mimic flushed dairy manure, which resulted in mixedliquor containing 9.1 g/L total solids (TS) and 7.6 g/L total volatilesolids (VS). Anaerobic sludge containing a microbial communityof hydrolyzing, acid producing, acetate producing and methaneproducing microbes was sampled from an anaerobic digester inthe Pullman Wastewater Treatment Facility with TS of 17.1 g/Land VS of 11.7 g/L.

2.2. Experimental setup and operation

Five digesters (64 cm in height and 10 cm in diameter), eachwith working volume of 6 L, were operated as ASBR at respectivecycle times of 2, 4, 6, 8 and 10 days while the other operation con-ditions remained constant (50% exchange ratio, 5 min settling time

Table 1Operating condition for ASBR.

Reactor HRT (d) OLR (gVSloaded/L/d)

Exchange ratio(%)

Total cycle time (d) Cycle time

Feeding(min)

Reaction(d)

Desludge(min)

Settling(min)

Decanting(min)

R1 4 2.0 50 2 5 2 5 5 5R2 8 1.0 50 4 5 4 5 5 5R3 12 0.7 50 6 5 6 5 5 5R4 16 0.5 50 8 5 8 5 5 5R5 20 0.4 50 10 5 10 5 5 5

8 J. Ma et al. / Bioresource Technology 131 (2013) 6–12

and 20 days SRT). This operation produced HRT of 4, 8, 12, 16 and20 days in each digester (Table 1). Settling time was determinedbased on the authors’ previous study to form biofilm which at-tached on the surface of manure fiber (Wang et al., 2011). Each di-gester was mixed with a separate impeller driven by a respectivemotor at 100 rpm. Intermittent mixing was carried out with10 min in every 2 h. Manure and anaerobic sludge were introducedto each digester at 1:1 volume ratio when experiments started.Digesters were then placed in a low temperature chamber (22 �C)and operated in sequencing batch mode which consists of fivestages: filling, reaction, desludge, settling and discharging in onecycle. At the end of the reaction stage, mixed liquor was dischargedunder complete mixed state to control SRT, and supernatant wasdischarged after settling stage to control HRT in each digester.Evaluation of system performance for each HRT was carried outduring pseudo steady state conditions, when biogas production,methane content and effluent COD variations were less than 10%(Karim et al., 2005).

2.3. Biochemical analytical methods

TS, VS and COD analyses were done according to StandardMethods (APHA, 1998). The volume of biogas from the digesterwas determined by water displacement method. Content of CH4

and CO2 were determined using a Varian gas chromatograph (PaloAlto, CA, USA) equipped with a thermal conductivity detector. ARestek (Bellefonte, PA, USA) shincarbon column (2 � 1/16 inch)with silcosteel packing material (100/120 mesh) was used, andnitrogen served as the carrier gas. Each type of gas was quantifiedbased on a calibration curve. All samples were measured at the endof each cycle.

2.4. Development of kinetic model

Kinetic models predicting methane production were derivedassuming digesters operated at steady state conditions. Four modeltypes were considered in this study in order to choose a modelwith appropriate fit for microbial growth kinetics during anaerobicdigestion of flushed dairy manure in ASBR digester.

First order Model (Gavala et al., 2003):

l ¼ kSS0 � S

� b ð1Þ

Grau Model (Grau et al., 1975):

l ¼ lmSS0� b ð2Þ

Monod Model (Monod, 1949):

l ¼ lmSKS þ S

� b ð3Þ

Chen and Hashimoto Model (Chen and Hashimoto, 1978):

l ¼ lmSKS0 þ ð1� KÞS� b ð4Þ

Microbial growth mass balance was considered as the basis formodel derivation, and rate of microbial growth can be written as:

rm ¼dXdt¼ lX ð5Þ

Thus, mass balance equation for microbial growth can be givenas follows:

VdXdt¼ QX0 � QX þ rmV ð6Þ

At steady state (dXdt ¼ 0), the above equation can be simplified as

l ¼ 1h

ð7Þ

By use of each of the above models, steady state effluent sub-strate concentrations were derived and are listed in Table 2. Takingthe Chen and Hashimoto model as an example, by substituting Eq.(4) in Eq. (7), effluent substrate concentration can be expressed as

S ¼ KS0ð1þ bhÞðK � 1Þð1þ bhÞ þ lmh

ð8Þ

If B denotes the volume of methane produced at standard con-dition per gram of substrate loaded to the digester, and B0 is thevolume of methane produced at standard condition per gram ofsubstrate loaded at infinite retention time, then the biodegradablesubstrate in the digester will be directly proportional to B0�B, andB0 will be directly proportional to the biodegradable substrateloading (Chen and Hashimoto, 1978). Therefore, the following rela-tionship can be derived:

SS0¼ B0 � B

B0ð9Þ

The above equation can be rearranged to give:

B ¼ B0ðS0 � S

S0Þ ð10Þ

The volumetric methane production rate equals B times OLR:

M ¼ BS0

hð11Þ

By use of Eqs. (10) and (11) can be expressed as:

M ¼ B0

hðS0 � SÞ ð12Þ

The volumetric methane production rate with respect to HRTcan be derived by substituting steady state substrate concentrationof effluent into Eq. (12). Using Chen and Hashimoto model as anexample, Eq. (12) can be written as follows:

M ¼ B0

hS0 �

KS0ð1þ bhÞðK � 1Þð1þ bhÞðlmhÞ

� �ð13Þ

J. Ma et al. / Bioresource Technology 131 (2013) 6–12 9

The volumetric methane production rate with respect to OLRcan be expressed as:

M ¼ B0 L� KLðLþ bS0ÞðK � 1ÞðLþ bS0ÞðlmS0Þ

� �ð14Þ

The specific methane productivity with respect to HRT can beexpressed as:

P ¼ B0 1� Kð1þ bhÞðK � 1Þð1þ bhÞðlmhÞ

� �ð15Þ

The specific methane productivity with respect to OLR can beexpressed as:

P ¼ B0 1� KðLþ bS0ÞðK � 1ÞðLþ bS0ÞðlmS0ÞÞ

� �ð16Þ

The VS removal efficiency with respect to HRT can be writtenas:

E ¼ 1� Kð1þ bhÞðK � 1Þð1þ bhÞðlmhÞ ð17Þ

The volumetric methane production rate, specific methane pro-ductivity and removal efficiency with respect to the other threemodels were also derived for model comparison and listed in Table2.

In order to determine B0, mass balance equation of substrateconsumption can be written as:

VdSdt¼ QS0 � QSþ rsV ð18Þ

Rate of substrate consumption

rs ¼dXdt¼ �k0S ð19Þ

At steady state (dSdt ¼ 0), effluent substrate concentration can be

expressed as:

S ¼ S0

ðhk0 þ 1Þð20Þ

Substituting Eq. (20) into Eq. (12) yields:

S0

M¼ h

B0þ 1

B0k0ð21Þ

B0 of manure depends on the species, ration, age of the manure,collection, and storage and bedding material. Values of B0 for dairymanure range from 0.21 to 0.27 (Chen and Hashimoto, 1978; Hus-ain, 1998). B0 of flushed dairy manure used in this present studywas determined by plotting HRT versus S0/M according to Eq.(21). The slope of the curve was used to calculate B0 as 0.24 LCH4/gVSloaded.

An endogenous decay constant b of 0.03/d was used for themodel simulations (Husain, 1998). Values of kinetic parametersfor each model were estimated using the Curve Fitting functionin SigmaPlot 11 (Systat Software, Inc.) using the Marquardt–Leven-berg algorithm with 200 iterations.

3. Results and discussion

3.1. Effect of HRT on biogas production

Biogas generation in the ASBR digesters with different HRT isrepresented in Fig. 1. It can be seen that the time needed to reachsteady state condition is associated with HRT. The longer the HRT,the more time required for start-up. After operating periods of 34,62, 72 and 80 days, digesters R1, R2, R3, R4 and R5 corresponding

to HRT of 4, 8, 12, 16 and 20 days reached steady state condition,respectively.

The volumetric biogas productivity during manure bio-methan-ization was also related to HRT in each digester. The highest volu-metric biogas production was observed at the shortest HRT of4 days (digester R1) with rate of 0.37 L/L/d, while digester R5, run-ning at the longest HRT of 20 days, showed the lowest rate of0.14 L/L/d. Zaher et al. (2008) reported similar results (volumetricmethane production rate of 0.20 L/L/d at 5 day HRT and 0.10 L/L/d at 17 day HRT) from a tire supported fixed-bed digester treatingflushed dairy manure but under mesophilic temperature. A muchlower rate using a zeolite supported CSTR digester even at meso-philic conditions was obtained by Borja et al. (1994). Powerset al. (1997) reported a fixed-bed digester with a low methane pro-duction at 2.3 day HRT. The manure fiber supported psychrophilicASBR digester showed good performance as compared againstthem.

Biofilm was expected to be formed in all digesters due to selec-tion pressure in terms of settling time (Wang et al., 2011). How-ever, washout of biomass was present and digester failureoccurred when the HRT was further shortened to 2 days, which isbecause too short of an HRT at start-up period exceeded the micro-organism growth limits (Rittmann and McCarty, 2001). A practicallimit with a minimum HRT of 4 days was required at the start-upperiod for biomass retention within the digester.

It should be noted that, compared with our previous studies, thepresent study was conducted under psychrophilic temperature(22 �C) instead of mesophilic condition (35 �C). However, the di-gester in this study showed a comparable performance with thatof a mesophilic digester (volumetric biogas production rate of0.25 L/L/d and specific methane productivity of 0.20 L/gVSloaded),indicating that biomass retention provides a cost-effective methodfor uncompromised anaerobic digestion rate at lower temperaturewith less energy consumption due to the reduction of heat re-quired (Connaughton et al., 2006; Lettinga et al., 2001).

3.2. Effect of OLR on biogas production

OLR in each digester varied according to different HRT since theinfluent substrate concentration is fixed. The effects of OLR on vol-umetric methane production rate and specific methane productiv-ity were plotted in Fig. 2A. Specific methane productivity showedan inverse relationship with respect to OLR, reducing slightly untilOLR reached 1 gVSloaded/L/d at which point further OLR extensionled to steep drops in specific methane productivity.

The volumetric methane production rate and specific methaneproductivity are often two competing performance parameters.An increasing OLR favors volumetric methane production ratebut leads to impaired specific methane productivity. It seems acompromise in OLR should be employed with an OLR of around1 gVSloaded/L/d yielding high values for both parameters. The samesituation is applied HRT with a HRT around 4–6 days being favoredfor both parameters (Fig. 2B).

3.3. Kinetic modeling

3.3.1. Evaluation of kinetic modelsFirst-order, Grau, Monod and Chen and Hashimoto models were

chosen to determine the most appropriate model for the kinetics ofmethane production from flushed dairy manure in an ASBR diges-ter. Results are presented in Fig. 3 and fitting accuracy is listed inTable 3. On the account of poor correlation with data sets, theMonod model was not recommended for kinetic analysis. Further-more, the Monod and first-order model failed to predict the volu-metric methane production rate decreases at extremely shortHRT, which led to reduced accuracy in the simulation. The limita-

Table 2Kinetics models used in this study.

Kinetics coefficients First-order Grau Monod Chen&Hashimoto

Specific growth rate l ¼ kSS0�S� b l ¼ lm S

S0� b l ¼ lm S

KSþS� b l ¼ lm SKS0þð1�KÞS� b

Effluent substrate concentration S ¼ S0ð1þbhÞhðkþbÞþ1 S ¼ S0ð1þbhÞ

lmh S ¼ Ksð1þbhÞhðlm�bÞ�1 S ¼ KS0ð1þbhÞ

ðK�1Þð1þbhÞþlmh

Volumetric methane production rate M ¼ B0h S0 � S0ð1þbhÞ

hðkþbÞþ1

� �M ¼ B0

h S0 � S0ð1þbhÞlmh

� �M ¼ B0

h S0 � Ksð1þbhÞhðlm�bÞ�1

� �M ¼ B0

h S0 � KS0ð1þbhÞðK�1Þð1þbhÞþlmh

� �Specific methane productivity P ¼ B0 1� 1þbh

hðkþbÞþ1

� �P ¼ B0 1� 1þbh

lmh

� �P ¼ B0 1� Ks ð1þbhÞ

S0hðlm�bÞ�S0

� �P ¼ B0 1� Kð1þbhÞ

ðK�1Þð1þbhÞþlmh

� �Removal efficiency E ¼ 1� 1þbh

hðkþbÞþ1 E ¼ 1� 1þbhlmh E ¼ 1� Ksð1þbhÞ

S0hðlm�bÞ�S0E ¼ 1� Kð1þbhÞ

ðK�1Þð1þbhÞþlmh

10 J. Ma et al. / Bioresource Technology 131 (2013) 6–12

tion of the Monod model resides in the effluent substrate concen-tration (S) being independent of the influent substrate concentra-tion (S0), with organic loading notably having been found toaffect digester performance. Saravanan and Sreekrishnan (2006)pointed out that the assumption of substrate degradation de-scribed within the Monod model is questionable in biofilm reac-tors. The Chen and Hashimoto model and Grau model explicitlyaccount for the influent substrate concentration, so they are ableto overcome this constraint and predict S as a function of S0, witheffluent substrate concentration being directly proportional to theinfluent substrate concentration. The Chen and Hashimoto modelsuccessfully fit the peak volumetric methane production rate at4 days HRT and the microorganisms’ washout at 2 days HRT. TheChen and Hashimoto model included the influence of S0 in the ki-netic expression in order to express mass transfer limitations(Chen and Hashimoto, 1978). Mass transfer limitation can lead lto vary with initial substrate concentration. Therefore, the Chenand Hashimoto model was selected for development of a derivedmodel to conduct the kinetic analysis for an ASBR digester treatingflushed dairy manure.

3.3.2. Model simulationIn the derived model, values of lm and K were the variables

identified to characterize the digester performance and for fitting.lm is the maximum specific growth rate of microorganisms ex-pressed as per day. K is a dimensionless kinetic parameter indicat-ing digester performance. K is equal to Yc, where Y is growth yieldcoefficient and c is the Contois coefficient. From the data presentedin Fig. 2, it can be seen that the experimental data and derivedmodel predictions are in good agreement (R2 > 95%), showing thevalidity of the model. The lm and K values calculated from the de-rived model are 0.36 d�1 and 0.23, respectively.

The value of lm is at the lower side of the wide range (0.041–0.912 d�1) reported for mixed and pure cultures of methanogens

Fig. 1. Volumetric biogas production rate at various HRT.

at temperature between 35–37 �C (Pavlostathis and Giraldogomez,1991). This may be due to the lower temperature and mixed cul-ture used in the kinetic coefficient determination. K is an indicatorof the overall performance of the digester (Chen, 1983). An increas-ing K indicates inhibition of fermentation while low K indicates ra-pid substrate degradation. K value for typical digesters range from0.6 to 2.0 with a mean of 1.06 (Hashimoto, 1982). The K value ob-tained from this study is lower compared with other studies,implying only short HRT is needed for anaerobic digestion offlushed dairy manure in an ASBR digester. Furthermore, K and lm

were reported to be independent of the influent substrate concen-tration for diluted organic waste stream (Chen and Hashimoto,1978). Hence, the lower K value could be attributed to high amountof biomass retained within ASBR digester.

From simulated profiles, digestion performance, as indicated byvolumetric methane production rate, was highest at near 4 days orat an organic loading rate of 2 gVSloaded/L/d (Fig. 2). Digester perfor-mance can also be represented by specific methane productivitywith this indicator pointing to maximums at 5–20 days and OLRof between 0.5–1.3 gVSloaded/L/day (Fig. 2). Comparison of thetwo performance indicators shows that an ideal HRT for ASBR feed-ing with flushed dairy manure would be around 4–6 days whileOLR can either be limited for enhanced specific methane produc-tivity or slightly increased for improved volumetric performance.

3.3.3. Model predictionThe derived model was then used to predict digester perfor-

mance, namely effluent VS concentration and VS removal effi-ciency. As Fig. 4 shows, the model successfully predicted effluentVS concentration with R2 of 0.91. However, the calculated resultsof VS removal efficiency do not fit well with the measurements.This is also because influent substrate concentration was not in-cluded in the VS removal efficiency calculation equation. AlthoughChen and Hashimoto (1980) states that the treatment efficiencydoes not depend on influent substrate concentration, it was notvalidated by experimental data of the present research.

3.4. Process performance comparison

Comparisons of the operation parameters and the methane pro-ductivities obtained in this study with the performance data fromother anaerobic biofilm reactors treating dairy manure are pre-sented in Table 4. As can be seen, except for this study, a varietyof different types of external artificial biofilm support media wereemployed in those anaerobic biofilm digesters to enhance biomassretention. It is clear from Table 4 that ASBR using manure fiber assupport media not only improved methane production but also re-duced the necessary HRT and temperature to achieve a comparabletreating efficiency. This digester expanded the capacity of anaero-bic digestion to dilute solid wastes treatment with no requirementof prior solids separation or the risk of biofilm support media clog-ging. The specific methane productivity obtained in this study washigher than most of others as shown in Table 4. It should be notedthat this high performance was actually attained at relatively low

Fig. 2. Changes of volumetric methane production rate and specific methaneproductivity against HRT and OLR, (d) volumetric methane production rate, (s)specific methane productivity, and (—) simulation profile.

Fig. 3. Comparison of simulation with different kinetics models.

Table 3Summary of model comparison with kinetic coefficients and fitting accuracy.

Models Kinetic coefficients R2 SRSEa

First-order k = 0.43 0.92 0.073Grau lm = 0.67 day�1 0.96 0.032Monod lm = 0.07 day�1 0.76 0.102

Ks = 0.24 g VSChen & Hashimoto lm = 0.36 day�1 0.99 0.011

K = 0.23

a The sum of residual squared error.

Fig. 4. Changes of effluent substrate concentration and treatment efficiency withderived model, (d) effluent VS data, (h) VS removal efficiency data, and (—)predicted profile.

J. Ma et al. / Bioresource Technology 131 (2013) 6–12 11

HRT and low temperature so that this digester may be more cost-effectiveness than others.

3.5. Implications for dairy AD process design

A high rate AD process driven by high biomass retention insteadof mesophilic temperature (35 �C) appears to be an economical ap-

proach for methane recovery from flushed dairy manure (Frearet al., 2010). This study demonstrated a new biomass retentionstrategy with biofilm supported by manure fiber. Fibrous solidscontent in flushed dairy manure act as a natural biofilm supportmedium for high biomass retention as opposed to using externalmedia that might clog and add cost. As attributed to high concen-tration of biomass, the performance of this technology at psychro-philic temperature is comparable to that of other technology undermesophilic conditions. Taking advantage of successful in vitro bio-mass immobilization on dairy manure fibrous solids, ASBR mightbe employed as an optimum means to achieve high rate AD influshed dairy manure.

This novel biomass immobilization process expands the capac-ity of anaerobic digestion to dilute solids wastes with no need forprior solids separation and no hazard for media clogging. As a re-sult of requiring no artificial biofilm support media, the digesterstructure will be very simple and of lower cost. Sequencing batchmode operation procedures of ASBR are tailored to cater to non-continuous manure production and collection practices. The mul-ti-feeding procedures are well adapted to infrequent barn flushing.Owing to simple digester configuration and low maintenancerequirement, this technology is suitable for application to bothsmall farms and large CAFOs (confined animal feeding operations).This technology is also able to handle a wide range of TS from 1% to5% caused by varying flushing intensity and water usage. The ki-netic model and kinetic parameters obtained from this study canbe used for design and optimization of the process. A six-dayHRT and an OLR of 1.0–1.5 gVSloaded/L/day are recommended bythe kinetic model prediction. At optimized conditions, a volumetricmethane production rate of 0.24 L/L/d of and specific methane pro-ductivity of 0.19 L/gVSloaded are expected.

Table 4Performance data for different anaerobic biofilm reactors treating dairy manure.

Digester type Influent(g VS/L)

OLR(g VS/L/d)

Temperature(�C)

HRT(d)

Methanecontent (%)

Specificmethaneproductivity(L/g VSloaded)

Literature

ASBR with manure fiber as support media 7.60 1.26 22 6.0 73.4 0.19 This studyFixed-film reactor with spherical plastic

trickling filter media1.30a 4.07 23–24 2.3 65.0 0.10 Powers et al. (1997)

Anaerobic hybrid reactor with floating support media 9.87b 7.30 36 15.0 63.5 0.19 Demirer and Chen (2005)Fixed bed reactor with automobile tires 13.83c 2.77d 35 5.0 NA e 0.19 f Zaher et al. (2008)CSTR with zeolite support 47.10c 9.42d 35 5.0 NA 0.12 f Borja et al. (1994)Fixed bed reactor with tire rubber and zeolite 75.00 4.40 22–26 5.5 NA 0.18 f Umana et al. (2008)

a %TS.b %VS.c g COD/L.d g COD/L/d.e Not available.f L/g COD loaded.

12 J. Ma et al. / Bioresource Technology 131 (2013) 6–12

4. Conclusion

A successful biomass retention technology for treating flusheddairy manure at psychrophilic temperature was presented in thisstudy. A Chen and Hashimoto based model gave the best simula-tion with R2 of 0.99. The simulation of kinetic modeling indicatedthe best HRT and OLR were 4–6 days and 0.5–1.3 gVSloaded/L/day,respectively. Extended SRT was important to retain high concen-tration of biomass at low temperature, and to enhance the digesterperformance. When compared with other research, this technologyexhibited a better performance in terms of specific methane pro-ductivity while at shorter HRT and lower temperature.

Acknowledgements

The authors would like to thank the Washington StateUniversity Agricultural Research Center and China-US interna-tional collaborative project (No. 2011DFA90800) for funding thisstudy.

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