Anaerobic co-digestion of biosolids and organic fraction of municipal solid waste by sequencing batch process
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Keywords:Co-digestionBiosolidsOrganic fraction of municipal
because it not only reduces the volume of MSW and biosolids, through mixing each other . Paper materials in MSW,
F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 4 8 5 4 8 9
omstabilizes MSW and biosolids, but also recovers energy fromMSW and biosolids in the form of methane, and produces aresidue that can be used for soil conditioning . But theimplementation of anaerobic biological treatment of MSW at
suchas office paper andnewspaper, have a carbon-to-nitrogen(C/N) ratio ranging from 173:1 to greater than 1000:1, andsewage sludge has a C/N ratio varying from6:1 to 16:1 , whilethe suggested optimum C/N ratio for anaerobic digestion is inBecause the production of municipal solid waste (MSW) andbiosolids frommunicipal wastewater treatment plant (WWTP)continues to grow, the interest in alternative MSW and bio-solids management strategies intensifies, while the disposalcapacity via traditional landfilling and incineration is dimin-ishing [1,2]. Anaerobic biological treatment of organic fractionof MSW (OFMSW) and biosolids is an acceptable alternative
various constituent populations of the microbial consortium, anumber of factors can upset the anaerobic digestionprocess, forexample, the methanogenesis inhibition by excessive volatilefatty acids (VFA) accumulation and the inhibition of propionate-and butyrate-degrading acetogens by high hydrogen levels [4,5].
Anaerobic digestion of biosolids and anaerobic digestion ofMSWhave been both investigated by researchers . Digestionof nutrient-deficient MSW and biosolids can be improved1. Introductionthe full-scale level has been limited becauhigh cost and some technical problems.
The anaerobic digestion of organic maversion of dissolved and particulate organiane and carbon dioxide via a series of in
Corresponding author. Tel.: +86 731 8821228E-mail addresses: email@example.com, zhangp
0378-3820/$ see front matter 2007 Publisdoi:10.1016/j.fuproc.2007.11.013metabolisms. Because of many complex interactions betweenThe co-digestion of biosolids and organic fraction of municipal solid waste was comparedwith the direct digestion of biosolids. Addition of organic fraction of municipal solid wasteimproved carbon-to-nitrogen (C/N) ratio from 8.10 to 17.68 or 20.55 in the feedstock, and ledto a higher volatile fatty acid (VFA) concentration of about 1500 mg/L in the solution and alower solution pH under 6 in the beginning 20-day during digestion. Although the totalbiogas yield increased by the mixed feedstock, the biogas yield peak was postponed forabout 10 days and the maximum biogas yield rate was reduced. After digestion, over 30% oftotal solid (TS) and over 65% of volatile solid (VS) were removed. However, the CODconcentration in the effluent was still higher than 2300 mg/L and needed further treatmentbefore discharge.
2007 Published by Elsevier B.V.solid wasteVolatile fatty acidBiogas yieldA R T I C L E I N F O A B S T R A C TAnaerobic co-digestion of biosmunicipal solid waste by sequ
Panyue Zhanga,, Guangming Zenga, GuangmBibo Zhanga, Maohong Fanc
aCollege of Environmental Science and Engineering, Hunan UniversitbSchool of Municipal and Environmental Engineering, Harbin InstitutcInstitute of Physical Research and Technology, Iowa 50011-3020, U
www.e l sev i e r. cse of the relatively
tter entails the con-c matter into meth-terrelated microbial
; fax: +86 731 firstname.lastname@example.org (P. Zhan
hed by Elsevier B.V.ids and organic fraction ofcing batch process
ng Zhangb, Yin Lia,
hangsha 410082, Chinaf Technology, Harbin 150090, China
/ l oca te / fup rocthe range of 20:1 to 30:1 . Therefore, the mixing of biosolidsand MSW can provide suitable nutrients for the digestion.
In this study, biosolids and OFMSW fromkitchen, vegetablemarket and yard were used as the feedstock in the co-diges-tion system. Experiments were carried out in the sequencing
batch model. The VFA, COD, pH, biogas yield and organicsubstance removal were investigated.
2. Experimental work
Biosolids used were waste activated sludge taken from of a
were sealed. Characteristics of the feedstock components invarious reactors were listed in Table 2. During digestion, inter-mittent mixing for a few minutes each two hours was done toprovide uniformdistribution of substrates and solids.When the
Table 2 Components of characteristics of the feedstock invarious reactors
Reactor Biosolids (g) Rice (g) Mixed sample (g) C/N (g/g)
1 495.3 0 0 8.102 495.3 2.5 11.9 17.683 495.3 4.2 22.3 20.55
Table 1 Properties and characteristics of biosolids, riceand mixed sample
Parameter Units Biosolids Rice Mixed sample
TS % 1.56 42.16 60.31VS % of TS 54.14 87.32 89.45TOC % of TS 24.51 41.05 42.04TN % of TS 3.03 0.76 1.65pH 7.7Alkalinity (as CaCO3) mg/L 0.7
486 F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 4 8 5 4 8 9WWTP in Changsha, china. Before experiments the biosolidswere stored in a refrigerator under 4 C.
The OFMSW was collected from kitchen, vegetable marketand yard, including rice, vegetable, grass etc. Rice was isolatedseparately. The mixed sample consisted of vegetable leaves(52.7%), grass (31.2%) and of carrot (16.1%), which was inso-lated and then minced to small pieces (about 1 cm1 cm).
Properties and characteristics of biosolids, rice and mixedsample are shown in Table 1.
2.2. Anaerobic digestion procedure
Anaerobic digestionwas carried out in 3 sets of digestion reactorwith a volume of 1 L in sequencing batch model (see Fig. 1).Reactors were installed in a water bath with a temperature of38 C to keep the material in reactors at a temperature of 37 C.After the feedstock was added into the reactors, the reactorsFig. 1 Sketch of the anaerobic digester.3.1. VFA and pH variation during anaerobic digestion
Anaerobic digestion includes normally three steps, hydrolysis,fermentation (or acidogenesis) and methanogenesis. Throughhydrolysis and fermentation the particulate materials areconverted to soluble compounds, and further degraded toacetate, hydrogen, carbon dioxide, propionate and butyrate.The production of a large amount of VFA leads to the decreaseof solution pH. Non-methanogenic microorganisms respon-sible for hydrolysis and fermentation can be adapted to low pHwhile methanegons will lose activity at low pH. Thus me-thanogenesis can be inhibited significantly at low pH. pH is animportant control parameter during digestion.
Fig. 2 shows VFA variation during digestion period. In thebeginning of digestion VFA concentration increased rapidly topHwas lower than 5, Na2HCO3 solution with a concentration of1 mol/L was added from point 3 to adjust the pH in reactors;liquid samples in reactorswere taken frompoint 6, and their pH,VFA, COD were measured each other day; biogas produced wascollected in the U-pipe 7 and was recorded every day. Afterdigestion, total solid (TS) and volatile solid (VS) of thematerial inreactors were analyzed.
In all experiments the following data were determined: biogasyield, pH, TS and VS, elemental content (C and N) of biosolidsand OFMSW, soluble COD, total alkalinity and VFA content.
All analytical procedures were performed in accordancewith Standard Methods .
3. Results and discussionFig. 2 VFA variation during digestion period.
OFMSW provided more substrates for methanogenesis and
Fig. 3 pH variation during digestion period. Fig. 5 Biogas yield per day during digestion period.
487F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 4 8 5 4 8 9a peak at about 3000-4000 mg/L within 10 days; in succession,the VFA concentration kept at a relatively stable level of about500, 1500 and 1500 mg/L for the reactor 1, 2 and 3; after 30-day digestion, the VFA concentration began to decreasegradually. The OFMSW added is degradable organic matters,which are easily converted into VFA in the solution.
Fig. 3 displays pH variation during digestion period. Whenthe solution pH is lower than 5, 10 ml Na2HCO3 solution wasadded into the reactors to avoid the inhibition of methano-genesis by low pH. This occurred mainly in the fermentationstep. The solution pH in reactor 1 was higher than that in theother two reactors; it was always higher than 6.5 after seven-day digestion, while the solution pH in reactor 2 and 3 washigher than 6.5 only after 27-day digestion. The addition ofOFMSW results in formation of more VFA and thus low pH inthe initial period.
3.2. Soluble COD variation during digestion period
Soluble COD represents the soluble organic matters inreactors, which are produced by hydrolysis and fermentationduring digestion. The particulate material is converted intosoluble compounds, which leads to the increase of the solubleCOD concentration in solution. These soluble organic mattersare the substrates of the methanogenesis, and are furtherconverted into methane and carbon dioxide gradually. Thesoluble COD in the solution changed during digestion periodas shown in Fig. 4. After 6-day digestion, the soluble COD of thesolution reached the highest value-14728, 16832 and21040mg/L for reactor 1, 2 and 3, respectively. In succession,Fig. 4 Soluble COD concentration of the solution duringdigestion.the solubleCODof the solutiondecreasedandkeptata relativelystable valueof about 5000, 10000 and 15000mg/L for reactor 1, 2and 3, respectively. At the end of digestion, the soluble COD ofthe solution was still very high, and the effluent needed to befurther treated before discharge.
3.3. Biogas yield and solid removal
Anadvantage of anaerobic digestion is the recovery ofmethane,whichcanbeconverted intoenergy. Fig. 5 showsbiogasyieldperday during digestion. In the beginning, hydrolysis and fermen-tation were dominant, and biogas yield was very low. After 8-day reaction the biogas yield in the reactor 1 increased, andreached the highest during the period from the 13th day to the17th day; in succession, the biogas yield decreased, and after 29-day reaction nomore biogas was produced. For reactor 2 and 3the biogas yield peaks were postponed to the period from the20th day to the end (when the digestion in reactor 1 ended, thereaction in reactor 2 and 3 still went on).
The accumulate biogas yield are shown in Fig. 6. For reactor1 the maximum biogas yield rate was about 170.36 ml/(m3d)from the 13th day to the 17th day; while with the addition ofOFMSW the high biogas yield period was postponed to theperiod from the 16th day to the end for the reactor 2 and theperiod from the 20th day to the end for reactor 3, and themaximum biogas yield rate for reactor 2 and 3 was 88.96 and125.72 ml/(m3d), respectively. After 35-day digestion, thereaction in reactor 2 and 3 still went on. The addition ofFig. 6 Accumulative biogas yield during digestion period.
 M.E.Williams, Integrated solidwastemanagement, in: F. Kreith(Ed.), Handbook of SolidWasteManagement, McGraw-Hill Inc.,
Retention d 35 35 35
N Oimproved the total biogas yield. In this process the biosolids andOFMSW were treated simultaneously. Though the addition ofOFMSW improved the C/N ratio from about 8:1 to 20:1 (thesuitable C/N ratio for digestion), the biogas yield rate decreased,the biogas yield peakperiodwas postponed, and the specific gasyield also reduced (shown in Table 3), whichmight be caused bythe accumulation of VFA to a high concentration. The optimalreaction conditions should be further studied.
As shown in Table 3, the experiments lasted for 35 days.During the experiment period, the methane content was atabout 55-56% of the total biogas produced and there were no
% 56.1 55.3 55.1
L biogas/g VS removed 0.31 0.15 0.20
ml biogas/(m3d) 170.36 88.96 125.72
g 7.71 16.01 23.06
g 4.02 8.30 15.09
TS removal % 47.86 48.16 34.56VS infeedstock
g 4.17 11.51 17.75
g 1.53 2.61 6.04
VS removal % 63.31 77.32 65.97Table 3 Performance data for experiments duringoperations
Parameter Units Reactor Reactor Reactor
1 2 3
488 F U E L P R O C E S S I N G T E C Hsignificant differences amongdifferent reactors. According thebiogas yield in the Figs. 5 and 6, the digestion reaction inreactor 1 completed fully, and 47.86%of TS and 63.31%of VS inthe feedstock was converted, indicating the existing of somenon-degradable matters in the feedstock, which agrees withthe observation of Speece . Although the digestion reactionin reactor 2 and 3 did not complete fully, over 65% of VS andover 30% of TS was converted in reactor 2 and 3. After thedigestion and further dewatering, the volume of biosolids andOFMSW can be reduced significantly. TS and VS removalincreased in the reactor 2 because of the addition of OFMSW,comparedwith the situation in the reactor 1. But in the reactor3, TS and VS removal reduced. TS in the reactor 3 was aboutthree times higher than that in the reactor 1, correspondingly,the inorganic matters and non-degradable organic mattersincreased. Thesemight influence the TS andVS removal in thereactor 3.
In this study, the co-digestion of biosolids and OFMSW wasinvestigated, compared with sole digestion of biosolids. C/Nratio of the feedstockwith sole digestion of biosolids in reactor1was 8.10, and C/N ration in reactor 2 and 3was adjusted by
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 I. Angelidaki, L. Ellegaard, B. Ahring, A comprehensive modelof anaerobic bioconversion of complex substrates to biogas,Biotechnol. Bioeng. 63 (1999) 363372.mixing different amount of OFMSW into biosolids to 17.68 and20.55, respectively. Addition of OFMSW in the feedstock led toa higher VFA concentration of about 1500 mg/L in reactor 2and 3, while the VFA concentration in reactor 1 kept at about500mg/L. The accumulation of VFA resulted in decrease of thesolution pH and could inhibit the methanogenesis. Althoughthe total biogas yield increased with the feedstock of biosolidsand OFMSW, the biogas yield peak period was postponedfor about 10 days and the maximum biogas yield rate wasreduced form 170.36 ml/(m3d) in reactor 1 to 88.96 and125.72 ml/(m3d) in reactor 2 and 3, respectively. After di-gestion over 30% of TS and over 65% of VS were removed, andthe volume of biosolids and OFMSW could be reduced sig-nificantly. COD concentration in the effluent was still high,and the effluent needed to be treated further.
This study was financially supported by the National ScienceFund of China for Distinguished Young Scholars (50225926,50425927), the China Postdoctoral Science foundation(2004036127) and Program for New Century Excellent Talentsin University (531105050030).
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489F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 4 8 5 4 8 9
Anaerobic co-digestion of biosolids and organic fraction of municipal solid waste by sequencing.....IntroductionExperimental workMaterialAnaerobic digestion procedureAnalytical
Results and discussionVFA and pH variation during anaerobic digestionSoluble COD variation during digestion periodBiogas yield and solid removal