Anaerobic co-digestion of potato processing wastewater with pig slurry and abattoir wastewater
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Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 83:16581663 (2008)
Anaerobic co-digestion of potatoprocessing wastewater with pig slurryand abattoir wastewaterMaria Monou,1 Nicolas Pafitis,2 Nicoletta Kythreotou,1 Stephen R Smith,3
Dionissios Mantzavinos2 and Despo Kassinos11Department of Civil and Environmental Engineering, University of Cyprus, 75 Kallipoleos Str., 1678 Nicosia, Cyprus2Department of Environmental Engineering, Technical University of Crete, Polytechneioupolis, GR-73100 Chania, Greece3Centre for Environmental Control and Waste Management, Department of Civil and Environmental Engineering, Imperial College London,London SW7 2AZ, UK
BACKGROUND: Small-scale experimental investigations were carried out on the anaerobic digestion of potatoprocessing wastewater and its co-digestion with pig slurry and/or abattoir wastewater. A simple and rapidprocedure was used to determine the suitability of these wastes for digestion.
RESULTS: During the initial 5-day acclimation phase, the seed (digested brewery waste) was replaced by thetest waste before allowing the tests to incubate without further addition, where methanogenesis was measured.Although potato processing wastewater has low pH, with high fat content treatment via anaerobic digestionwas still feasible in spite of low methane production. Co-digestion with pig slurry and abattoir wastewater wastherefore investigated to enhance the process. Pig slurry improved the process, which, when co-digested withpotato processing wastewater in equal ratio achieved 72% volatile solids removal, 35mL average daily biogasproduction and 32% maximum methane content in 22days (following the acclimation period). Co-digestion withabattoir wastewater did not improve the digestion process due to poor buffering and low pH value.
CONCLUSION: Anaerobic co-digestion may be a feasible treatment option for industrial bio-wastes and livestockwastes produced in Cyprus and indeed in similar other countries of comparable market size and activities. 2008 Society of Chemical Industry
Keywords: anaerobic digestion; potato processing wastewater; pig slurry; abattoir wastewater
INTRODUCTIONWastewater from the potato processing industrycontains a high concentration of biodegradablecomponents such as starch and proteins.1 Thecomplex nature of this wastewater is due to ratherhigh concentration of suspended solids, high contentof insoluble fraction of COD, high content of BOD andsignificant quantities of potential foaming substances,such as proteins and fats. Furthermore, potentiallytoxic substances have been detected in the potatoprocessing wastewater. The wastewater may containproteins which are anaerobically difficult to degradeand phenols such as L-dopa which cause toxicityproblems if anaerobic sludge is exposed to L-dopain the presence of volatile fatty acids (VFAs).2 Potatoand its industrial by-products typically contain highquantities of soluble organics that would rapidly beconverted into VFAs. Insufficient buffer capacity insingle stage digesters may inhibit methanogenesis asa result of the pH decrease.3 However, there havebeen successful cases of potato digestion such as that
performed by Weiland4, where 300500 m3 of biogasper ton of dry matter was produced with a 5070%degradation from potato pulp (1821% dry solids)and potato thick stillage (1418% dry solids).5
Within the industrial sector in Cyprus, the wastesarising from potato and snack food can be difficultto treat, particularly during the high peak summermonths where large quantities are produced. There aretwo potato and snack food industries in Cyprus thathave constructed an onsite wastewater treatment plantfor the purpose of treating the wastes generated frompotato crisps. The wastes produced are potato skins,soil, oil and wash-water. The significant quantities ofproteins and fats, oil and grease (FOG) present inpotato processing wastewaters have been the causeof some major inefficiencies such as inhibition ofmicrobial activity, foaming, scaling and/or sludgeflotation that affect the performance of the anaerobicreactors. These problems have been moderated byinstalling a FOG removal system, where most of theremoved FOG is transported to a central wastewater
Correspondence to: Despo Kassinos, Department of Civil and Environmental Engineering, University of Cyprus, 75 Kallipoleos Str., 1678 Nicosia, CyprusE-mail: email@example.com(Received 27 February 2008; revised version received 8 April 2008; accepted 21 April 2008)Published online 23 May 2008; DOI: 10.1002/jctb.1979
2008 Society of Chemical Industry. J Chem Technol Biotechnol 02682575/2008/$30.00
Anaerobic co-digestion of mixed waste products
treatment plant. One of the most problematic island-wide characteristics is the densely packed livestock-breeding units within certain areas, the majority ofwhich are the pig farms. This is mainly due tointensive farming activities, increased production ofmanure and higher density of animals per area, allconcentrated in specific areas. Farms are often locatednear residential areas and water bodies. In addition tonuisance odours, unsuitable lagoons used for storageresult in leachate or runoff to neighbouring soils,agriculture and water bodies. Abattoirs on the islandhave previously been a major target of public scrutinydue to large quantities of the waste inappropriatelydisposed. Until recently, solid wastes from abattoirswere discarded in improperly managed landfill sites,while liquid wastes were placed in absorption pitswith no pre-treatment. The wastes have been found inwater bodies and discharged to neighbouring fields atunacceptably high loading rates.
Mesophilic anaerobic digestion (AD) of biodegrad-able wastes has the potential to achieve efficientpollution reduction and the additional advantage ofconserving energy, thus providing opportunities forecological and economical benefits from these wastestreams.6 Furthermore, AD would eliminate off-gasair pollution (for example, from emissions of volatileorganic compounds and ammonia from open lagoons),reduce water and groundwater pollution and requireconsiderably less land area in comparison with uncon-trolled lagooning disposal methods currently in use.Several studies show that, generally, the sensitivityof the AD process may be improved by combiningseveral waste streams.710 For example, co-digestionof manures with agro-residues would overcome theproblem in the digestion of agro-residues alone main-taining a stable pH within the methanogenesis range,11
which is associated with low pH of the substrate itself,poor buffering capacity and the possibility of highVFAs accumulation during digestion.10,12 Further-more, co-digestion would overcome ammonia inhi-bition observed in pure manure digestion as potatoprocessing wastewater is typically low in terms of TotalKjeldahl Nitrogen. Generally, manure is consideredto be an excellent co-substrate due to its natural highbuffering capacity, high ammonia content and a widerange of nutrients needed by the methanogens.13,14
Thus, the successful combination of different wastescan increase biogas production and the nutrient con-tent of the digestate may be improved enhancing its
value as a fertiliser. Furthermore, potentially negativeeffects of toxic compounds on the co-digestion processmay be reduced due to dilution effects.15
This work identified two of the main organic wastestreams produced in Cyprus and investigated theirsuitability for co-digestion with potato processingwastewater. Co-digestion was investigated as anapproach to overcome the inhibition caused by highFOG content observed for the digestion of potatoprocessing wastes. The aim was to combine thedifferent types of wastes to increase buffer capacityto reduce any potential inhibition of methanogenicactivity. The focus was on achieving the highestmethane production rate and treatment of the testwastes. As part of the study, a small-scale laboratorytest procedure was developed to rapidly screen wastemixtures for co-digestion.
MATERIALS AND METHODSCharacteristics of substratesLiquid organic wastes were collected from livestockbreeding units and industrial units as follows:
Brewery wastewater (used as seed inoculum):samples were collected from the influent and effluentof a full-scale UASB reactor treating brewerywastewater.
Potato processing wastewater: samples were col-lected from the influent of a full-scale UASB reactortreating potato processing wastewater.
Pig slurry from a farm with a capacity of about5000 pigs: samples were collected from a temporarycement-lined storage lagoon with a submersiblemixer, prior to disposal in soil lagoons.
Abattoir wastewater: centrate samples were col-lected from the centrifuged liquid wastes prior totreatment in the biological unit at the largest abattoiron the island.
Waste materials were used immediately in theanaerobic digestion tests or maintained in a refrigeratorset at 4 C prior to use; their main properties are shownin Table 1.
Digestion test procedureA simple process was used, adapted from Fernandeset al.,16 to rapidly screen the suitability of differentwaste types for AD treatment. Experiments were
Table 1. Properties of various wastes used in this study. Numbers in brackets show standard deviation
Untreated potatoprocessingwastewater Raw pig slurry
Centrifuged liquidabattoir wastewater
before biological treatment
pH 9.8 4.6 7.4 6.7Conductivity (mS cm1) 2.3 3.1 31.4 2.7TS % (w/w) 0.3 1.2 (0.3) 2.9 (0.1) 0.6 (0.1)VS (% TS) 25.9 18.3 (5.6) 38.5 (5.1) 25.4 (4.5)
Fresh and digested brewery wastewater in equal volume ratios
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conducted in 500 mL plastic bottles providing aneffective volume of 400 mL. Each digester was startedup by adding the test sample and seed inoculum ofdigested brewery waste in equal volume (200 mL)ratios. The seed was a composition (50:50% involume) of raw and digested brewery wastes, usedas the inoculation source to establish the anaerobicmicrobiology. Air in the headspace was evacuatedwith nitrogen gas before sealing with plastic screw caps(covered with grease) to provide an airtight seal andmaintain anaerobic conditions in the digesters. Thedigesters were incubated in a temperature-controlledwater bath at 35 C and mixed by manual shakingat least three times a day. Gas production in eachdigester was measured after shaking using a waterdisplacement technique. The baseline gas productionfrom the brewery wastes was also measured using adigester filled with the inoculation mixture.
Digestion tests were performed in duplicate. Potatoprocessing wastewater was assessed individually andin combination with pig slurry or abattoir wastewater,in equal volume (200 mL) ratios. The anaerobicdigestion test was split into two phases. The digesterwas equilibrated initially by stepwise replacement ofthe inoculant source with the test sample. Thus,after mixing, about 100 mL of digestate was removedand an equivalent volume of the test sample wasadded on a daily basis over a 5-day period. Followingacclimatisation, the digesters contained only the testwastes and were monitored for a further period ofseveral days to allow maximum biogas production.The digesters were sealed and no further wasteadditions occurred during this period. The pH wasmonitored daily and adjusted by adding 10 N NaOHsolution if the digester pH dropped below 6. Similarprocedures were followed for pig slurry and abattoirwastewater, which were also tested individually toassess their digestibility.
ANALYTICAL METHODSSamples of the wastes removed from the digestersduring the acclimatisation period, and at intervals of35 days during the incubation stage, were analysedfor the following parameters:
pH was monitored using a pH electrode meter(Inolab, Multilevel 3 Inolab, WTW, USA).
Total solids (TS) and total volatile solids (VS). TSwere determined at 104 C to constant weight andVS were measured by the loss on ignition of thedried sample at 550 C.
Biogas production and composition. Biogas pro-duction was determined by a water displacementmethod. The composition of the biogas was mea-sured using a portable gas analyser instrument ofthe type used for monitoring landfill gas (GA 2000,Geotechnical Instruments, UK). The instrumentis able to analyse methane, oxygen, carbon diox-ide and the balance gas (principally nitrogen) in
percentages and carbon monoxide and hydrogensulphide in ppm. Gas was recovered directly frominverted 100 mL measuring cylinders used for waterdisplacement through a tube connected to the topof the apparatus.
Statistical analysisData from the duplicate and repeated experimentswere compared using single factor analysis of variance(ANOVA). The F critical values were calculated usingan alpha value equal to 0.05.
RESULTS AND DISCUSSIONAcclimatisation phaseDuring the initial acclimation phase, low biogasproduction, minimal or zero methane quantities, pHdecrease and TS and VS increase were observedfor the digestion and co-digestion processes. Thedecline in pH value and minimal or zero methaneproduced in the low biogas yields can probablybe explained by the accumulation of VFAs duringthis phase.17 Given that the working volume was400 mL, a retention time (hydraulic and cellular)of 5 days during the acclimatisation phase may nothave permitted full development of the slow-growingmethanogen community within the completely mixedreactor. The conditions created probably resulted inthe biochemical thermodynamics within the microbialpopulations favouring the fermentative acidogenicphase of AD.2 The expected increase in TS and VSwas a result of the simple physical mixing processgiven the TS and VS loading in the test wastesduring the equilibration phase. The TS content ofthe inoculum was much lower than the test wastesand the VS content lower than the pig slurry andabattoir wastewater. Therefore, progressive additionof test wastes raised both TS and VS and the rateof addition exceeded the rate of removal by digestionduring the equilibrium phase.
Incubation phaseA decline in pH was observed for the digester con-taining potato processing wastewater and potato andabattoir wastewater during the acclimation phase. Asolution of 10 N NaOH was therefore added whennecessary to increase the pH of these treatments towithin the optimum range for methanogenic bac-teria (6.57.5). Once in the incubation phase, nofurther addition of 10 N NaOH was required, as thepH stabilised at approximately pH 7. The releaseof ammonia from blending with the more alkalinesubstrate pig slurry provided the necessary bufferto counterbalance the potential for rapid accumula-tion of VFAs produced from the highly degradablepotato waste.14,18 Therefore, no pH adjustment wasneeded for this treatment. Likewise, no adjustmentwas needed for the digesters containing pig slurryand abattoir wastewater as pH remained between 6.5and 7.3 throughout. As seen in Fig. 1, the digesters
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0 50 100 150 200 250 300 350 400 450 500 550 600 650 700Elapsed digestion time (hours)
potato wastewater and pig slurry
potato and abattoir wastewater
Figure 1. Change of pH during the anaerobic digestion or co-digestion of various wastes.
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700Elapsed digestion time (hours)
End of waste equilibrium phase
potato wastewater and pig slurry
potato and abbatoir wastewater
Figure 2. Cumulative biogas production during the anaerobic digestion or co-digestion of various wastes.
were maintained within the optimum pH range formethanogenesis, i.e. 6.3 to 7.8 indicating adequatebuffer capacities to compensate for acidogenesis.1921
The eventual accumulation of NH3-N during anaero-bic co-digestion with pig slurry was responsible for thehighest subsequent increase in pH towards the end ofthe process.
Increased biogas production, indicating the onsetof complete AD, commenced after 144 h at the endof the waste equilibrium phase of the test. Themean cumulative biogas production for the digestiontest with potato processing wastewater and its co-digestion tests is shown in Fig. 2. Figure 3 displaysthe average methane concentrations measured duringthe incubation phase. Carbon dioxide concentrationsdetected were low and remained fairly constant; onaverage, 0.68% for potato wastewater, 4.6% for pigslurry, 0.23% for abattoir wastewater, 0.88% forpotato wastewater and pig slurry and 0.33% for potatoand abattoir wastewater.
The potato processing wastewater and pig slurryco-digestion gave the maximum overall biogas yieldcompared with co-digestion with abattoir waste orindividual potato wastewater digestion, most probablybecause the wastes had the highest TS content
and high biodegradability by AD. The biogas yieldsfor pig slurry AD and abattoir wastewater AD arealso shown in Fig. 2. The net biogas yield andmaximum methane composition of the experimentsare summarised in Table 2. Potato wastewater andpig slurry produced high biogas volumes during co-digestion, reflecting their higher digestibility. Thisis a function of the relative TS contents of thewastes as on an equivalent volume basis, the pigslurry contained the highest TS. The pig co-substrateproduced a higher net gas yield (0.41 m3 kg1 VSloaded) compared with the abattoir co-substrate andthe potato substrate alone (0.25 and 0.37 m3 kg1
VS loaded respectively). Similarly, the maximummethane concentration was highest for the potatoand pig slurry co-digestion experiment. Co-digestionwith abattoir wastewater decreased biogas yield asmay be expected due to the TS dilution whenthe potato and abattoir wastes are mixed. Minimalquantities of methane were detected suggesting thatmethanogenesis was not the dominant microbialreaction for types of waste. The lower pH andfats in these wastes probably contributed to thisfact.
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150 200 250 300 350 400 450 500 550 600Elapsed digested time (hours)
Potato wastewater + Pig slurry
Potato + Abattoir wastewater
Figure 3. Methane composition during the anaerobic digestion or co-digestion of various wastes.
Table 2. Biogas production and VS destruction (standard deviation denoted in brackets) during anaerobic digestion of various wastes
Waste typeMaximum net gas yield(m3 kg1 VS loaded)
Maximum methaneconcentration (%) VS destruction (%)
Potato proc. wastewater 0.37 20.5 60.1 (1.5)Potato proc. wastewater + pig slurry 0.41 31.8 72.1 (6.4)Potato proc. + abattoir wastewater 0.25 4.1 60.3 (7.5)Pig slurry 0.25 52.9 42.9 (4.9)Abattoir wastewater 0.11 5 ND
ND: not determined
However, as also seen in Table 2, AD of pigslurry resulted in a net biogas yield of 0.25 m3 kg1VS loaded and a maximum methane concentrationaverage of 52.9%. Therefore, the improvement inpotato processing wastewater and pig slurry co-digestion occurs at the expense of the pig slurrydigestion. It is uncertain why the mean net biogasyield, per unit VS basis, was smaller for the pig slurry.A possible explanation is that CO2 was absorbedthrough an alkalinity reaction with the waste, whichcould also explain the larger CH4 concentrationin biogas produced from this compared with thepotato wastewater. It was found that the microbialactivity in the digestion of abattoir wastewater favoursfermentation. The biogas yield produced from the ADof abattoir wastewater was less than that producedfrom the co-digestion with potato wastewater in thisexperiment. Furthermore, a negligible amount ofmethane was detected. The process was inhibitedat the acetogenesis phase, possibly due to theaccumulation of long-chain fatty acids from thehydrolysis of the high lipid content, which severelyinhibit methanogenic activity.22 Hence, co-digestionwith potato wastewater encourages methanogenicactivity. However, this occurs at the expense of thepotato wastewater AD.
As may be expected, the TS and VS concentrationsdecreased during the incubation phase of the test.The destruction of VS was computed from thedifference in VS content measured on day 5 of the test
(corresponding to the start of methanogenesis) and atthe end of the process. It was found that VS destructionwas dependent on initial TS concentrations, as alsosupported by Sung and Santha.23 Digestion of potatowastewater yielded 60.1% VS destruction and thisincreased to 72.1% for the potato and pig slurryco-digestion experiment. The co-digestion of potatoand abattoir wastewater was not considered to haveimproved the digestibility of the potato waste on itsown as a VS destruction of 60.3% was achieved.The high values found may be due to a synergisticresponse between the properties of the waste materialsincreasing digestibility. For example, the high VSconcentration (25.4%) in the abattoir wastewater co-substrate contributed to the increased VS degradation.However, the high fat, oil and grease content mayinhibit methanogenic activity24 thus accounting for theminor amount of methane produced. In addition, highVS destruction associated with low maximum methaneconcentration reflects imbalance between acidogenesisand methanogenesis. This may happen not only dueto low pH, but may be related to nutrient deficiency.
In general, the co-digestion of potato wastewaterand pig slurry was the most efficient process. Theseresults are in line with Kaparaju and Rintala5 whodemonstrated the feasibility of anaerobic co-digestionof potatoes and their industrial by-products with pigslurry. They calculated that 1 ton of potatoes when co-digested with 1215 m3 of pig slurry would produce328 m3 of methane. Thus, 5250 m3 of methane would
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be generated from 1 ha of potatoes (assuming ayield rate of 16 ton ha1). The power that could begenerated from 1 ha potato harvest would therefore beequivalent to 51 MW of heat or 6.3 MW of electricity.Anaerobic co-digestion of potato wastewater withpig slurries could therefore potentially provide cost-effective solutions to both farms and potato processingindustries required to undertake compulsory wastemanagement measures.
CONCLUSIONSA simple procedure is described to allow rapid screen-ing of different waste types and combinations foranaerobic digestion and co-digestion. The method-ology is straightforward to set up and monitor, rapidand cost-effective. Anaerobic digestion of wastes withlow pH and/or high fat is potentially problematic; inhi-bition caused by the high lipid content in these wastescould potentially account for low biogas production.In the case of anaerobic digestion of potato wastewaterwith abattoir wastewater, possible inhibition by longchain fatty acids present in these wastes preventedmethanogenesis from being the dominant microbialreaction due to the high oxygen content of substratefats. However, co-digestion of potato wastewater withpig slurry generally improved the process. Within thecontext of these experiments, co-digestion was con-sidered as a process for simultaneous treatment of twodifferent waste streams. Furthermore, co-digestionwas also seen as a solution to the potential ammo-nia inhibition occurring during pig slurry digestionand the inhibition caused by the readily acidifyingpotato waste, naturally low in pH.
The results presented here indicate that anaerobicco-digestion is a feasible treatment option for industrialbio-wastes and livestock wastes produced in Cyprus.As most of the livestock units and industrialareas producing these types of waste are oftenlocated together in Cyprus, co-digestion represents afeasible option and opportunities exist for developingcooperative ventures to establish industrial codigestionfacilities between the agricultural and industry sectors.
ACKNOWLEDGEMENTSThe authors thank the following industries andfarms for providing waste samples: Photos PhotiadesBreweries Ltd (The Cyprus Carlsberg Brewery),Peoples Coffee Grinding Co. Ltd (Potato and SnackFood Factory), Vroudas Farm (Dairy Cattle), LagosFarm (Pig) and Kofinou Abattoir.
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