Bioresource Technology 101 (2010) 6361–6367

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Bioresource Technology

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Comparison of atmospheric pressure effects on the anaerobic digestionof municipal solid waste

Jianguo Jiang a,b,*, Xuejuan Du a, Siio Ng a, Chang Zhang a

a Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR Chinab Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, PR China

a r t i c l e i n f o

Article history:Received 17 December 2009Received in revised form 4 March 2010Accepted 15 March 2010Available online 1 April 2010

Keywords:Anaerobic digestionAtmospheric pressureHigh plateauMunicipal solid waste

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.03.068

* Correspondence to: Jianguo Jiang, Department oEngineering, Tsinghua University, Haidian District, Befax: +86 10 62783548.

E-mail address: jianguoj@tsinghua.edu.cn (J. Jiang)

a b s t r a c t

This study compared the performance of anaerobic digestion (AD) of municipal solid waste under 101 kPa(R system) and 65.8 kPa (RPC system) – the atmospheric pressure on the Qinghai-Tibetan Plateau, China.Gas production, gas composition, degradation of volatile solids, pH, alkalinity, volatile fatty acids, andammonia concentration were analyzed to examine how the two systems responded to change in theorganic loading rate. The RPC system had a stronger buffering capacity and lower gas production rateand could achieve a higher loading rate. The pH in RPC recovered more rapidly in the starting stageand remained higher than that in R during the whole experiment, with an average difference of 0.2. Dur-ing days 38–55, 56–70, and 71–125, the average methane production rates were 380.16, 318.67, and402.21 L kg�1 VS in R and 367.40, 299.04, and 275.06 L kg�1 VS in RPC, respectively.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Anaerobic digestion (AD) has been increasingly applied to treatsolid waste worldwide, including in high-altitude regions. A prop-erly functioning AD system can achieve highly efficient biogas pro-duction to reduce fossil fuel consumption and greenhouse gasemission and can transform organic waste into high-quality fertil-izer (Tafdrup, 1995; Edelmann et al., 2005).

The performance of AD of the organic fraction of municipal solidwaste (OFMSW) generally depends on operational parameters suchas temperature, pH, and organic loading. Previous studies haveconcluded that the rates of biogas production with thermophilicdigesters are higher than those with mesophilic digesters regard-less of hydraulic retention time (HRT), although thermophilicdigesters are lower in energy conservation and exhibit less stability(Gallert and Winter, 1997; Kim et al., 2006). As the temperaturedecreases, the AD digester performs lower degradation rate of vol-atile solids (VS) and less biogas production (Chen et al., 2009). Thevalue of pH is the pivotal factor that affects the methane produc-tion efficiency. Studies have suggested that a pH range of 7–8 issuitable for obtaining higher biogas production and degradationof VS (Dinamarca et al., 2003; Zhang et al., 2005a). By Liu et al.(2008), it was observed that the optimal value of pH is 7.10 under

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f Environmental Science andijing 100084, PR China. Tel./

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mesophilic temperature, and the cumulative methane productionwas increased about 35% in optimal pH. The starting stage of ADare significantly influenced by two most important factors – inoc-ula (Castillo et al., 2006) and total solid content (Li et al., 2009).Many other factors, including feedstock (Carneiro et al., 2008a,b),HRT (Salminen and Rintala, 2002; Rincón et al., 2008), stirring(Stroot et al., 2001), and ammonia inhibition (Sung and Liu,2003; Fricke et al., 2007) may also affect the performance of AD.Álvarez et al. (2010) have developed a linear programming optimi-sation method for determining the most adequate ratios of differ-ent co-substrates that provide an optimised biodegradationpotential or biokinetic methane potential.

Highlands cover one-fourth of China’s territory and occurmostly in the Qinghai-Tibetan Plateau, which has an average alti-tude of about 4000 m. Atmospheric pressure decreases with anincreasing altitude. According to Henry’s Law, at a given tempera-ture, a decrease in total pressure will result in a reduction of thepartial pressure and solubility of CO2. This then changes the equi-librium of CO2 in the liquid phase and consequently affects the pHand buffering capacity of a system (Rittmann and McCarty, 2002).Hayes et al. (1990) reported obtaining higher methane content un-der a higher pressure in a digester and less methane at a lowerpressure. Álvarez et al. (2006) set up reactors on a high plateau(3000–4000 m altitude) in Bolivia to study biogas production frommanure; their results showed little difference between high andlow altitudes. However, the experiment only considered gas pro-duction, and it is difficult to know whether the other parametersof AD or equipment restrictions limited the outcomes of their

6362 J. Jiang et al. / Bioresource Technology 101 (2010) 6361–6367

experiment. In fact, by changing the total pressure in a digester,toxicity effects can be diminished or avoided. For example,decreasing the pH by raising the partial pressure of CO2 can reducethe non-ionized ammonia concentration and avoid ammonia inhi-bition. Conversely, increasing the pH can lower the level of non-ionized hydrogen sulfide (Vavilin et al., 1995). This suggests thatby appropriately adjusting the total pressure in a digester, severalinhibitory effects on AD may be eliminated. Zhang et al. (2005b)tried absorbing CO2 in the gas phase to reduce the CO2 partial pres-sure and found it to be helpful in preventing acidification.

This study examined the influence of atmospheric pressure onthe performance of AD of OFMSW, using a self-designed AD exper-imental system that simulates different pressures. The reactorswere operated semi-continuously under two pressures: 101 and65.8 kPa, with the latter representing the average atmosphericpressure of the Qinghai-Tibetan Plateau. Gas production, gas com-position, degradation of volatile solids (VS), pH, alkalinity, volatilefatty acids (VFAs), and ammonia concentration were analyzed andcompared.

3

11

2

1

5

8

10

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Gasexit

(a)

(b)

Outlet

4

Inlet

1

2

3

Fig. 1. Schematic diagram of the two anaerobic digestion systems: (a) RPC: (1) heater;controller; (7) electronic switch; (8) stirrer; (9) hand switch; (10) hot water pipe; (11) olagging cloth; (2) resistance wire; (3) stirrer; (4) valve; (5) temperature controller; (6) s

2. Methods

2.1. Apparatus

The experiments were conducted using two semi-continuousAD systems: one with pressure control (RPC) and the other without(R), as shown in Fig. 1a and b, respectively. Both systems had aworking volume of 5 L.

Fig. 1a illustrates the RPC system. The digester was cylindrical,with a diameter of 15 cm and a height–diameter ratio of 3. The in-let and outlet were placed at the top and bottom of the digester,respectively. A bivalve design was applied to the inlet to minimizethe pressure change caused by incoming air. Other ports on the topof the digester included a gas exit, pressure probe jack, tempera-ture probe jack, pH probe jack, and spare jacks. A stirring devicewas connected to the digester. Pressure, temperature, and stirringfrequency in the digester could be adjusted using the multi-func-tion controller. Hot water passed through the water pipe insidethe digester to maintain the temperature of the materials inside.

14

9

7

13

6

12

Connected tothe components

Connect to atmosphere

5

T

5

T

6

7

(2) water pump; (3) inlet bottle; (4) valve; (5) pressure gauge; (6) multi-functionutlet collector; (12) buffering bottle; (13) vacuum pump; (14) flow meter; (b) R: (1)tirrer controller; (7) flow meter.

J. Jiang et al. / Bioresource Technology 101 (2010) 6361–6367 6363

Pressure control was the synergic result of the controller, pres-sure probe, electronic switch, and vacuum pump. A predeterminedpressure could be set with the controller. When the pressure in thedigester (measured by the pressure probe) rose above the designedlevel, the electronic switch opened and repeatedly discharged theexcess gas until the pressure decreased. The pressure variationwas less than ±5 kPa.

Fig. 1b depicts the R system. The digester is the same as in theRPC system except that the inlet and outlet are not connected topressure control components but to the atmosphere. Heating isprovided by the resistance wire and the lagging cloth working to-gether, under the control of a temperature controller. A gas exit islocated at the top and connected to a flow meter.

2.2. Feedstock

AD performance can vary significantly for different kinds ofwaste (Mandal and Mandal, 1997; Krzystek et al., 2001; Bouallagui

Table 1Composition of artificial organic waste.

Constituent Percentage

Rice 34.65Vegetables 27.49Meat 13.55Beans 5.90Oil 0.83Salt 0.80Paper 1.88Garden waste 5.96Other 8.94

Table 2Properties of artificial organic waste.

Item Value

Total solids (TS, % wet) 23.73Volatile solids (% wet) 22.51C/N 24.83Density (g mL�1) 1.012

Table 3Operating parameters of each phase of the experiment.

Phase A B C

B1 B2 B3

Duration (d) 0–37 38–55 56–70

71–125 126–146

Feeding method 25 g/d when pH > 7 25 g/2 d

25 g/d

50 g/2 d

100 g/d

OLR (kg m�3 d�1) 0.78 1.76 1.76 6.82

4.5

5.5

6.5

7.5

0 25 50 7Tim

pH R

Phase A Phase

Fig. 2. pH profile of anaerobic digestion of artific

et al., 2005; Carneiro et al., 2008a,b). To avoid this influence in theexperiment, artificial organic waste was applied as described inTable 1, based on an investigation of the composition of municipalsolid waste in Tibet. Materials were crushed into small pieces(<10 mm) before being blended and stored at 4 �C until use. Tables1 and 2 show the compositions and properties of the artificial or-ganic waste.

2.3. Experimental design

The experiments were conducted in Beijing, where the pressureis about 101 kPa. The atmospheric pressures of R and RPC were setat 101 and 65.8 kPa, respectively. Except for pressure, the two sys-tems were operated under the same reacting conditions, includingtemperature (35 �C), stirring (10 min every 4 h at a speed of120 rpm), and amounts of input and output.

The whole experiment lasted 146 d which was divided intothree phases, and Table 3 lists the duration and operating param-eters of each phase. Phase A (0–37 d) was the starting stage, whereabout 70% feedstock and 30% inoculation sludge, obtained from thedigester of the Beijing Gaobeidian Wastewater Treatment Plant,were mixed and placed in the reactors. The pH of the digester con-tents was monitored daily and adjusted with 1 M NaHCO3. Whenthe pH rose above 7, the digester contents were regularly with-drawn from each reactor and replaced with a fresh feed of thesame weight via the inlet, following the feeding method inTable 3. Phase B started at day 38, when the pH stabilized above7, and was divided into three stages according to the organic load-ing rate (OLR) and feeding method. The OLR was low in the earlystage of the experiment, to achieve a more stable state of perfor-mance for comparison, and was then gradually raised to investi-gate the influence of different OLRs on reactor performance. TheOLR was raised to 6.82 kg m�3 d�1 in phase C on day 126 and main-tained at this level until the end of the experiment to examine theabilities of the systems to accommodate a high OLR.

2.4. Analyses

The cumulative gas production was measured by a gas flow me-ter (LML-2, Changchun Automotive Filters Co., Ltd.) at the relevantpressure and designed temperature. Methane and CO2 concentra-tions in the biogas were tested using a gas chromatograph – SHI-MADZU GC2010. The pH of the digester contents was determinedwith a pH-meter (Delta 320A, Mettler-Toledo International Inc.)at a specified time every day.

Total solids (TS) and VS were determined according to StandardMethods (Jiang, 2008). The TS content was determined after heat-ing (105 �C) for 24 h. VS was determined by igniting the residueproduced in the TS analysis in a muffle furnace (SX2-5-12, TianjingZhonghuan Experiment Electric Stove Co., Ltd.) at a temperature of600 �C for 2 h.

5 100 125 150e (d)

RPC

B Phase C

ial waste at 101 kPa (R) and 65.8 kPa (RPC).

6364 J. Jiang et al. / Bioresource Technology 101 (2010) 6361–6367

Ammonia was measured by an UV spectrophotometer (UV-2000, UNICO Instruments Co., Ltd.) with salicylic acid spectropho-tometry from the Standard Methods (MEP, 2002). The C/N ratiowas determined with an elemental analyzer (CE-440, Exeter Ana-lytical Inc.). Alkalinity analyses were carried out using a pH-meter(Delta 320A, Mettler-Toledo International Inc.) with potentiomet-ric titration method (MEP, 2002). The digester contents in theamount of 1 lL at a time were used for VFA determination usinga gas chromatograph (GC3486, Beijing Beifen Huapu AnalysisEquipment Technology Co., Ltd.) with a polar chromatographic col-umn (GDX-103, Beijing ZXYD Technology Co., Ltd.).

3. Results and discussion

3.1. pH

Fig. 2 shows the variation in pH of the two systems. Significantt-test results of phases A, B1, B2, and B3 were all 0.000 (a = 5%),indicating that the pH of the two systems differed significantly.

In the early stage of phase A, acidification was noted in the tworeactors, as the pH decreased to 4.7–4.9 on days 7–8. During thisperiod of acidification, the pH was adjusted with 1 mol L�1 NaH-CO3, whereupon the pH recovered to above 7.

During days 38–55, the pH in R and RPC fluctuated between7.09–7.24 and 7.19–7.46, with average pH values of 7.13 and7.31, respectively. It was found that the pH remained higher inRPC compared with the system under normal pressure and thatthis trend continued during the whole experiment.

During days 56–70, pH values in both systems decreased as aresponse to the OLR increase. In R, pH declined from 7.14 to 6.84,with a larger difference of 0.3 than that in RPC, where the pH de-creased from 7.4 to 7.26 for a difference of only 0.14. Feedingwas stopped on day 70 to prevent the systems from acidifying,and after 1 d, the pH in R and RPC increased dramatically to 7.25and 7.4, respectively, because a majority of the VFAs were con-sumed. Thereafter, the feed mode was changed to 50 g/2 d, andpH remained steady in both systems until the end of phase B. Inaddition, inspection of the variation in pH at the time at whichOLR changed showed that the waves in the RPC curve were smallerand recovered more quickly than those in the curves for the otherreactor.

The adjustment of the OLR in phase C led to significant changesin the AD systems. At the beginning of phase C, the pH in R de-creased dramatically, from 7.29 to 6.07. These results indicate thatthe acidification of R was a consequence of the decrease in the gasproduction rate. A different result was seen in RPC, where the pHvariation was considerably smaller. The pH decreased, reaching avalue of 7.14 at day 131, and then rose to stabilize at about 7.38for 6 d, followed by another decrease until the end of the experi-ment. The pH in RPC at day 146 was 7.07, still in the optimal rangefor the AD system (6.8–7.2).

The pH is determined according to the following equation (Ritt-mann and McCarty, 2002):

pH ¼ pKa;1 þ log½Alkalinity�=50; 000½CO2 ðgÞ�=½KH�

; ð1Þ

where

½Hþ�½HCO�3 �½H2CO�3�

¼ Ka;1 ¼ 5� 10�7 ð35 �CÞ;

½Hþ� þ ½Alkalinity� ¼ ½A�� þ ½HCO�3 � þ 2½CO2�3 � þ ½OH��;

½CO2 ðgÞ�½H2CO�3�

¼ KH ¼ 38 atm L mol�1 ð35 �CÞ;

where [A�] represents the summation of the molar concentrationsof all weak acid salts present, except bicarbonate and carbonate.The units of alkalinity are mg/L as CaCO3, and [CO2 (g)] is the partialpressure of CO2.

Eq. (1) indicates that the pH is controlled by the concentrationsof alkalinity and VFAs in the liquid phase and CO2 in the gas phaseof the reactor, assuming that CO2 equilibrium exists between thegas and liquid phases. According to Eq. (1), pH increases when[CO2 (g)] decreases. Therefore, a low-pressure environment wouldenable AD to proceed at a higher pH, such that the AD systemwould be better able to resist acidification and achieve a higherOLR. Zhang et al. (2005b) succeeded in reducing acidification dur-ing AD by absorbing CO2 to reduce the partial pressure of CO2.Their result implies that a higher OLR can be applied to the AD ofOFMSW in a high-altitude area.

3.2. Biogas production

Fig. 3 shows cumulative gas production, gas production rates,and pH in the three phases of the AD systems. Gas production ratesin B1 and B3 were calculated as the averages of every 2 d becauseof the feed mode adjustment. Gas production rates in phases A, B1,B2, and B3 were assessed by t-tests (a = 5%), and the Sig. resultswere 0.017, 0.330, 0.202, and 0.000, respectively.

3.2.1. Phase AFig. 3a presents the cumulative gas production in phase A. The

curves were similar at the beginning but started to separate atday 23. The cumulative gas production on day 37 was 26.36 L inR and 32.61 L in RPC. As shown in Fig. 3b, the gas production ratecurves of R and RPC were close in phase A, with no clear distinc-tions from each other (t-test, a = 5%, Sig. = 0.017). Obvious acidifi-cation occurred in both systems in the early stage, restrainingthe activity of methanogenic bacteria and thus leading to a lowgas production rate. This phenomenon can be explained by theimbalance of hydrolysis/acidogenesis and methanogenesis. Theoptimal pH of acidogenic bacteria is 5.8, while methanogenic bac-teria need a condition of pH above 6.2. The pH was adjusted usingNaHCO3, and as the pH was increased, bacteria groups developedand became acclimated, increasing the gas production rate.

The pH curves in Fig. 3b show a large increase starting on day 9,when the NaHCO3 was added. The line representing RPC recoveredmore rapidly, increasing above 7 2 d earlier than the line of R.Thereafter, the pH in RPC was higher than that in R (t-test,a = 5%, Sig. = 0.000), with an average difference of 0.23 betweenthe two curves.

3.2.2. Phase BFig. 3c presents the cumulative gas production in phase B. As

shown in Fig. 3d, during days 38–55, the OLR was 0.78 kg m�3 d�1,and the gas production rates of R and RPC were 2.10 and 2.12 L/d,respectively. The difference between the two curves was not signif-icant (t-test, a = 5%, Sig. = 0.330). During this period, pH rose from7.09 to 7.24 in R and from 7.19 to 7.46 in RPC. Methanogenic bac-teria are more active than acidogenic bacteria at this range of pH,and consumed a great deal of VFAs. Meanwhile, VS degradationin the two systems did not differ significantly (t-test, a = 5%,Sig. = 0.028), suggesting similar activity of hydrolysis/acidogenesisbacteria in both systems. The average methane production rates(AMPR) during days 38–55 were 380.16 L kg�1 VS, in R and367.40 L kg�1 VS in RPC.

During days 56–70, the OLR was twice that in the previous per-iod, reaching 1.76 kg m�3 d�1. The gas production rates of R andRPC in this stage stabilized at around 3.42 and 3.31 L/d, represent-ing increases of 62.9% and 56.1%, respectively, due to the increasein substrate. However, the VFAs produced could not be consumed

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R-biogasRPC-biogasR-pHRPC-pH

(a)

(b)

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(d)

(e)

(f)

Fig. 3. Comparison of gas production rates and cumulative gas production of the two systems: (a) cumulative gas production in phase A; (b) gas production rates in phase A;(c) cumulative gas production in phase B; (d) gas production rates in phase B; (e) cumulative gas production in phase C; (f) gas production rates in phase C.

J. Jiang et al. / Bioresource Technology 101 (2010) 6361–6367 6365

immediately and pH decreased from 7.14 to 6.84 in R and from 7.4to 7.26 in RPC. The average methane production rates during days56–70 were 318.67 L kg�1 VS for R and 299.04 L kg�1 VS for RPC,indicating that there was no obvious difference in the activity ofmethanogenic bacteria in the systems.

During days 71–125, the feed mode was changed from 25 g/d to50 g/2 d to study the influence on the performance of the AD sys-tems, and the gas production rate was calculated as the averageof every 2 d. As shown in Fig. 3d, the gas production rates in thetwo systems fell at the beginning of the increase in OLR, possiblybecause the bacteria systems were unable to immediately acclima-tize to the new conditions. The subsequent increase in R from days74 to 82, and in RPC from days 72 to 80, suggests that the bacteriacommunity had gradually adapted to the new conditions. The aver-age gas production rate in R was around 4.31 L/d, a 26.0% increasecompared to the previous period, whereas that in RPC was around3.01 L/d, a decrease of 9.1%. The pH in R fluctuated between 7.08and 7.54, 0.2–0.3 lower than that in RPC, which fluctuated between7.22 and 7.56. The average methane production rate of R duringdays 71–125 increased to 402.21 L kg�1 VS, whereas the AMPR ofRPC remained at about 275.06 L kg�1 VS.

In phase B, the gas production of the AD system under lowerpressure was remarkably less than that under normal pressure.The results of VFA tests showed no accumulation, and the pH curve

of RPC was continuously higher than that of R. Together, this evi-dence indicates that the difference in gas production betweenthe two systems resulted from acidogenic bacteria being restrainedby the high pH in RPC. Further proof of this suggestion was pro-vided by the VS degradation results for this phase, which were2–4% lower in RPC than in R.

3.2.3. Phase CCumulative gas production in phase C is shown in Fig. 3e. To

investigate the response of the two systems to a high loading rate,the OLR was further increased to 6.82 kg m�3 d�1 during thisphase. The gas production rate of the R system rapidly increasedto 9.74 L/d at day 127, about 235% of the prior level, and then grad-ually decreased to 7.43 L/d at day 132. The pH of this system de-creased continuously, from 7.29 to 6.37, inhibiting the activity ofmethanogenic bacteria.

Unlike the R system, the gas production rate of RPC increasedsteadily until day 143, reaching 10.99 L/d, approximately 373% ofthe previous production, and the pH fluctuated between 7.14 and7.4. After this, the RPC gas production rate gradually decreased to4.79 L/d, while pH decreased to 6.85. Clearly, in this phase, the sys-tem under lower pressure performed better at a high loading rate.In this phase, average methane production rate in RPC was177.21 L kg�1 VS.

6366 J. Jiang et al. / Bioresource Technology 101 (2010) 6361–6367

Álvarez et al. (2006) found that the pressure effect (at pressuresof 65.8 and 101 kPa) was not significant in any case. In the presentstudy, the same result was obtained under a low loading rate.However, the situation was quite different at a high loading rate,where the performance of the two systems under different pres-sures varied significantly.

3.3. Degradation of VS

Fig. 4 shows the variation in the VS degradation rate and theVFAs during the experiment. On days 38–55, the average degrada-tion rates of VS in R and RPC were 93.3% and 94.3%, respectively,which were both at high levels due to the low OLR and long HRT.On days 56–125, the OLR of both systems remained unchanged,but the VS degradation rates in R and RPC gradually decreasedfrom 94.02% and 91.85% (at 57 d) to 85.06% and 84.80% (at122 d), respectively. The degradation rate of VS in R was alwayshigher than that in RPC, especially during days 71–125, possiblybecause high pH inhibited the activity of acid-producing bacteriain RPC. In phase C, the OLR was increased to 6.82 kg m�3 d�1, short-ening the HRT to 30 d, and the degradation rate of VS in RPC furtherdecreased to 78%.

As expected, when the OLR was increased, the degradation rateof VS decreased, which may be explained by the shorter HRT of or-ganic matter in the reactors. Similar results were reported by Bu-joczek et al. (2000). Resch et al. (2006) suggested that HRT > 30 dwould be beneficial for obtaining a high level of degradation andgas conversion of organic waste, which contains large amounts oflong-chain fatty acids and proteins. It was observed by Rincón

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VS

(%)

R-VSRPC-VSR-VFARPC-VFA

Phase A Phase

Fig. 4. Variation in VS degradation and VF

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N-N

H3

(mg·

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Fig. 5. Variation in alkalinity and ammon

et al. (2008) an appropriate buffering capacity and high stabilityof the experimental system for HRT in the range of 17–108 d anda destabilization and deterioration of the process and an inhibitionof anaerobic system occurred at an HRT of 15 d.

As an intermediate of AD from organic matter degradation, VFAconcentration is an important index of the metabolism of anaero-bic microbes. High VFAs lead to acid inhibition of the digestion sys-tem. The concentration of VFAs in an AD reactor can reflect thebioactivity of methanogenic bacteria and the degree of systemdeterioration. In phase B, the VFA concentrations in both systemswere less than 500 mg/L and did not accumulate. In phase C, theVFA concentrations increased rapidly, resulting in a pH decreasein both systems.

3.4. Gas composition

The average methane contents of R and RPC biogas during days56–125 were 59.25% and 60.01%, respectively. The results of t-testsrevealed no significant differences in the methane contents of Rand RPC biogas (Sig. = 0.021). Theoretically, a decrease in pressurereduces the solubility of gas, leading to its discharge from the sol-vent. As the solubility of CO2 is 40 times that of CH4 (pH 7.0 and35 �C), more CO2 will be discharged when the pressure decreases.Hayes et al. (1990) found that the pH and gas pressure of the sys-tem were the main factors that influenced gas composition, andthe methane content of two systems with a pressure differenceof 70 kPa deviated by 5%. In the present study, there was no obvi-ous difference in the methane content of R (101 kPa) and RPC(65.8 kPa).

92 102 112 122 132 1420

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VFA

(mg/

L)

B Phase C

As at 101 kPa (R) and 65.8 kPa (RPC).

2 92 102 112 122 132 142d)

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Alk

alin

ity (

mg·

L-1

)

RPC N-NH3

RPC alkalinity

B Phase C

ia at 101 kPa (R) and 65.8 kPa (RPC).

J. Jiang et al. / Bioresource Technology 101 (2010) 6361–6367 6367

3.5. Other parameters

Two additional parameters, alkalinity and ammonia concentra-tion, were monitored in the experiment (Fig. 5).

Alkalinities of the two systems, which was affected by bicarbon-ate and ammonia produced from the degradation of organic waste,tended to rise during the experiment. The highest alkalinities in Rand RPC were 6976, and 6498 mg/L, respectively, and were higherin R than in RPC. Rittmann and McCarty (2002) pointed out that asalkalinity increases, the pH in the reactor becomes higher. How-ever, pH is not sensitive to increases in alkalinity after the pHand alkalinity reach about 7.4 and 5000 mg/L (as CaCO3), respec-tively. Therefore, the variation in pH in the later stages of theexperiment did not have a strong association with alkalinity.

An increasing trend was also found in ammonia concentration.The concentration curves of the two reactors were almost the sameon days 0–70, with both tending to increase slowly. The ammoniaconcentrations of R and RPC rose to 616.73 and 601.86 mg/L at day67, respectively. Although the C/N of the artificial organic wastewas about 25, i.e., within the appropriate range for AD, a certainproportion of C was locked in the lignin of vegetables, paper, andgrass, making decomposition difficult and thus resulted in ammo-nia accumulation in both systems. With the change of feedstock,the ammonia concentrations of R and RPC mushroomed after day71, with averages on days 71–125 of 1214.63 mg/L for R and1101.55 mg/L for RPC. In phase C, the ammonia concentration ofR rose to 1544.59 mg/L, and that of RPC was maintained at1175.78 mg/L. At the higher loading rate, ammonia accumulationwas not apparent in the RPC system but was in the R system. Kapp(1992) reported that ammonia accumulation at high temperaturecan be avoided by increasing pressure in an AD system. In the pres-ent study, ammonia accumulation in AD at a medium temperatureoccurred more readily at atmospheric pressure than at lowerpressure.

4. Conclusions

The experimental results demonstrated the feasibility of apply-ing AD to treat OFMSW in high-altitude areas. Gas production wascomparatively lower in the system under 65.8 kPa than that under101 kPa. At a low loading rate and long HRT, pressure had no obvi-ous effect on the gas production of the AD system. However, at ahigh loading rate, the lower pressure enhanced the pH bufferingcapacity and had a positive effect on the stability of the AD system.Therefore, an AD system constructed in the low atmospheric pres-sure environment of the Qinghai-Tibetan Plateau can sustain ahigher loading rate. To clarify the mechanism of pressure effectson AD, further studies of two-phase AD systems under differentpressures are recommended.

Acknowledgements

We thank the Ministry of Science and Technology, China, forfunding this study (2006DFB93580), and the Tibet Energy Re-sources Research and Demonstration Center for support duringthe course of the study.

References

Álvarez, R., Villca, S., Liden, G., 2006. Biogas production from llama and cow manureat high altitude. Biomass and Bioenergy 30, 66–75.

Álvarez, J.A., Otero, L., Lema, J.M., 2010. A methodology for optimising feedcomposition for anaerobic co-digestion of agro-industrial wastes. BioresourceTechnology 101, 1153–1158.

Bouallagui, H., Touhami, Y., Cheikh, R.B., Hamdi, M., 2005. Bioreactor performance inanaerobic digestion of fruit and vegetable wastes. Process Biochemistry 40,989–995.

Bujoczek, G., Oleszkiewicz, J., Sparling, R., Cenkowski, S., 2000. High solid anaerobicdigestion of chicken manure. Journal of Agricultural Engineering Research 76,51–60.

Carneiro, T.F., Pérez, M., Romero, L.I., 2008a. Thermophilic anaerobic digestion ofsource-sorted organic fraction of municipal solid waste. BioresourceTechnology 99, 6763–6770.

Carneiro, T.F., Pérez, M., Romero, L.I., 2008b. Anaerobic digestion of municipal solidwastes: dry thermophilic performance. Bioresource Technology 99, 8180–8184.

Castillo, E.F., Cristancho, D.E., Arellano, V., 2006. Study of the operational conditionsfor anaerobic digestion of urban solid wastes. Waste Management 26, 546–556.

Chen, Z.Y., Cai, C.D., Shi, D.W., 2009. Effect of different temperature on anaerobicdigestion of livestock manures. Guizhou Agricultural Sciences 37, 148–151.

Dinamarca, S., Aroca, G., Chamy, R., Guerrero, L., 2003. The influence of pH in thehydrolytic stage of anaerobic digestion of the organic fraction of urban solidwaste. Water Science and Technology 48, 249–254.

Edelmann, W., Baier, U., Engeli, H., 2005. Environmental aspects of the anaerobicdigestion of the organic fraction of municipal solid wastes and of solidagricultural wastes. Water Science and Technology 52, 203–208.

Fricke, K., Santen, H., Wallmann, R., Huttner, A., Dichtl, N., 2007. Operating problemsin anaerobic digestion plants resulting from nitrogen in MSW. WasteManagement 27, 30–43.

Gallert, C., Winter, J., 1997. Mesophilic and thermophilic anaerobic digestion ofsource-sorted organic wastes: effect of ammonia on glucose degradation andmethane production. Applied Microbiology and Biotechnology 48, 405–410.

Hayes, T.D., Isaacson, H.R., Pfeffer, J.T., Liu, Y.M., 1990. In situ methane enrichmentin anaerobic digestion. Biotechnology and Bioengineering 35, 73–86.

Jiang, J.G., 2008. The physical and chemical characteristics of solid wastes. In: SolidWaste Disposal and Resource, first ed. Chemical Industry Press, Beijing, pp. 26–37.

Kapp, H., 1992. The effect of higher pressures on the mesophilic and thethermophilic anaerobic sludge digestion. In: Proceedings of InternationalSymposium on Anaerobic Digestion of Solid Waste, Venice, 14–17 April 1992.pp. 85–94.

Kim, J.K., Oh, B.R., Chun, Y.M., Kim, S.W., 2006. Effects of temperature and hydraulicretention time on anaerobic digestion of food waste. Journal of Bioscience andBioengineering 102, 328–332.

Krzystek, L., Ledakowicz, S., Kahle, H.J., Kaczorek, K., 2001. Degradation of householdbiowaste in reactors. Journal of Biotechnology 92, 103–112.

Li, D., Sun, Y.M., Yuan, Z.H., 2009. Effect of solid content on start-up of mesophilicmiddle solid anaerobic digestion for water-sorted organic fraction of municipalsolid waste. The Chinese Journal of Process Engineering 9, 987–992.

Liu, C.F., Yuan, X.Z., Zeng, G.M., Li, W.W., Li, J., 2008. Prediction of methane yield atoptimum pH for anaerobic digestion of organic fraction of municipal solidwaste. Bioresource Technology 99, 882–888.

Mandal, T., Mandal, N.K., 1997. Comparative study of biogas production fromdifferent waste materials. Energy Conversion and Management 38, 679–683.

Ministry of Environmental Protection of the People’s Republic of China, 2002.Standard Methods for Water and Wasterwater Monitoring and Analysis, forthed. China Environmental Science Press, Beijing.

Resch, C., Grasmug, M., Smeets, W., Braun, R., Kirchmayr, R., 2006. Optimisedanaerobic treatment of house-sorted biodegradable waste and slaughterhousewaste in a high loaded half technical scale digester. Water Science andTechnology 53, 213–221.

Rincón, B., Borja, R., González, J.M., Portillo, M.C., Jiménez, C.S., 2008. Influence oforganic loading rate and hydraulic retention time on the performance, stabilityand microbial communities of one-stage anaerobic digestion of two-phase olivemill solid residue. Biochemical Engineering Journal 40, 253–261.

Rittmann, B.E., McCarty, P.L., 2002. Environmental Biotechnology: Principles andApplications. McGraw-Hill, New York, USA.

Salminen, E.A., Rintala, J.A., 2002. Semi-continuous anaerobic digestion of solidpoultry slaughterhouse waste: effect of hydraulic retention time and loading.Water Resources 36, 3175–3182.

Stroot, P.G., McMahon, K.D., Mackie, R.I., Raskin, L., 2001. Anaerobic codigestion ofmunicipal solid waste and biosolids under various mixing conditions—I.Digester performance. Water Resources 35, 1804–1816.

Sung, S., Liu, T., 2003. Ammonia inhibition on thermophilic anaerobic digestion.Chemosphere 53, 43–52.

Tafdrup, S., 1995. Viable energy production and waste recycling from anaerobicdigestion of manure and other biomass materials. Biomass and Bioenergy 9,303–314.

Vavilin, V.A., Vasiliev, V.B., Rytov, S.V., 1995. Modelling of gas pressure effects onanaerobic digestion. Bioresource Technology 52, 25–32.

Zhang, B., Zhang, L.L., Zhang, S.C., Shi, H.Z., Cai, W.M., 2005a. The influence of pH onhydrolysis and acidogenesis of kitchen wastes in two-phase anaerobicdigestion. Environmental Technology 26, 329–339.

Zhang, B., He, Z.G., Zhang, L.L., Xu, J.B., Shi, H.Z., Cai, W.M., 2005b. Anaerobicdigestion of kitchen wastes in a single-phased anaerobic sequencing batchreactor (ASBR) with gas-phased absorb of CO2. Journal of EnvironmentalSciences 17, 249–255.