Volatile fatty acid production during anaerobic mesophilic digestion of solid potato waste

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<ul><li><p>Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 79:673677 (online: 2004)DOI: 10.1002/jctb.1012</p><p>Volatile fatty acid production during anaerobicmesophilic digestion of solid potato wasteWilson Parawira,1,2 Marika Murto,1 John S Read3 and Bo Mattiasson11Department of Biotechnology, Lund University, PO Box 124, SE-22100 Lund, Sweden2Department of Biochemistry, University of Zimbabwe, PO Box MP167, Mt Pleasant, Harare, Zimbabwe3Department of Applied Biology and Biochemistry, NUST, PO Box 219 Ascot, Bulawayo, Zimbabwe</p><p>Abstract: The production of volatile fatty acids by anaerobic digestion of solid potatowaste was investigatedusing a batch solid waste reactor with a working capacity of 2 dm3 at 37 C. Solid potato waste was packedinto the digester and the organic content of the waste was released by microbial activity by circulatingwater over the bed, using batch loads of 500 g or 1000 g potato waste. The sequence of appearance ofthe volatile fatty acids was (acetic, propionic); (n-butyric); (n-valeric, iso-valeric, caproic); (iso-butyric).After 300h digestion of potato waste on a small scale, the fermentation products were chiefly (mgg1 totalVFAs): acetic acid (420), butyric acid (310), propionic acid (140) and caproic acid (90), with insignificantamounts of iso-butyric acid, n-valeric and iso-valeric acids. When the load of potato solids was increased,the volatile fatty acid content was similar, but butyric acid constituted 110mgg1 and lactic acid 400mgg1</p><p>of the total volatile fatty acids. The maximum soluble chemical oxygen demand (COD) achieved underthe experimental conditions used was 27 and 37 g COD dm3 at low and high loadings of potato solids,respectively. The total volatile fatty acids reached 19gdm3 of leachate at both loads of potato solid waste.Gas production was negligible, indicating that methanogenic activity was effectively inhibited. 2004 Society of Chemical Industry</p><p>Keywords: acidogenesis; anaerobic digestion; hydrolysis; leachate; potato; solid waste; volatile fatty acids</p><p>NOTATIONCA Caproic acidHAc Acetic acidHPr Propionic acidi-BA Iso-butyric acidi-VA Iso-valeric acidLA Lactic acidn-BA Butyric acidn-VA Valeric acidTVFAs Total volatile fatty acidsVFAs Volatile fatty acids</p><p>1 INTRODUCTIONIn recent years, research on the acidogenic phaseof anaerobic digestion has increased considerably.Hydrolysis and acidogenesis are the first steps in theanaerobic digestion of complex organic materials whenthey are degraded into methane and carbon dioxide.These basic steps involve conversion of the polymerspresent in organic matter into soluble monomers,which are quickly fermented into VFAs (HAc, HPr,n-BA, i-BA, n-VA and i-VA), hydrogen (H2) andcarbon dioxide (CO2) by the rapidly growing andpH-insensitive acidogenic bacteria.1,2 The HAc, H2</p><p>and CO2 are converted to methane by methanogens.2</p><p>Many kinds of bacteria are involved in hydrolysis andacidogenesis and, therefore, several kinds of organicacids and alcohols are usually produced.3</p><p>Hydrolysis is reported to be the rate-limiting stepduring anaerobic digestion of complex organic waste.4</p><p>The rate of hydrolysis depends on the extracellu-lar enzymes produced by fermentative acidogens,the biomass concentration, the substrate concentra-tion and the specific surface area of the particulatesubstrate.5 Accumulation of organic acids and low-ering of pH are known to lead to suppression ofmethanogenic activity and process failure in single-stage methanogenic, reactors.6 The concept of atwo-stage system was thus proposed to improve theprocess5 stability and efficiency.2 Using a two-stageanaerobic process, hydrolysis and acidification canbe conducted in the first digester at a pH, temper-ature and hydraulic retention time optimal for thefermentative, acidogenic bacteria. Acetogenesis andmethanogenesis can then be done in the second reactorat conditions optimal for those processes.</p><p>Detailed knowledge of the acid phase of anaerobicdigestion is useful in a number of situations. Microbialanaerobic acidogenesis could be a useful means of</p><p> Correspondence to: Bo Mattiasson, Department of Biotechnology, Lund University, PO Box 124, SE-221 00 Lund, SwedenE-mail: bo.mattiasson@biotek.lu.seContract/grant sponsor: Swedish Agency for Research Cooperation (SAREC)(Received 19 September 2003; accepted 22 January 2004)Published online 6 May 2004</p><p> 2004 Society of Chemical Industry. J Chem Technol Biotechnol 02682575/2004/$30.00 673</p></li><li><p>W Parawira et al</p><p>producing organic acids from organic waste thatcould be used in denitrification, dephosphatationor methanisation.7,8 VFAs are essential as energyand carbon sources for the microorganisms involvedin the biological removal of nutrients (nitrogenand phosphorus) in wastewater treatment.9 Theorganic acids produced by the degradation of largequantities of residues from organic waste can alsobe used to make biodegradable plastic such aspolylactate polymers, an environmentally friendlyalternative to non-biodegradable plastics derived frompetrochemicals.10,11 Since the composition of organicacids in the medium influences the subsequentmethanogenesis reactions, it is not only interesting,but important, to study the product spectrum duringanaerobic acidogenesis. Initially, the liquefaction stageof each substrate could be performed in separatedigesters in order to obtain the best liquefactionyield and VFA conversion using a specialisedmicrobial ecosystem. The digestion pattern of eachindividual acid varies from one acid to anotherdepending on the substrate and physico-chemicalconditions.1214 Optimum operational conditionsfor the acetogenic/methanogenic phase have beenextensively studied6,8 but little information is availableon the acidogenic phase. The characteristics of theleachate in terms of specific VFA concentrationshave important effects on the performance of theacidogenic digester as an excess of one type of acidfavours the predominance of specific kinds of microbialconsortia.2,12</p><p>This study focuses on investigating the VFAproduction profile and changes in the characteristicsof the leachate during the acid-phase digestion processof potato waste at two concentrations of solids. Theraw material can be seen as representative of manyother kinds of cheap starch-rich biomasses suitablefor conversion to VFAs and other biochemicalssuch as lactic acid. Throughout the world hundredsof millions of tonnes of potatoes are producedannually and when they are processed into consumableproducts, enormous amounts of starch-rich wastes,that can be detrimental to environmental quality, arealso produced.</p><p>2 MATERIALS AND METHODS2.1 Experimental set-upCylindro-conical anaerobic reactors with a 2 dm3</p><p>working capacity were used. Solid potato waste wascut into small pieces using a kitchen blender and 500 gor 1000 g portions were loaded into two separatereactors. The potato waste had a total solids (TS)content of 190 g kg1 and a volatile solids (VS) contentof 950 g kg1 TS. To both reactors 800 cm3 waterwas added, and 200 cm3 of anaerobic sludge fromthe Ellinge municipal wastewater treatment plant insouthern Sweden was used as inoculum. This plantreceives domestic waste and industrial effluent, thelatter mainly from a potato-processing plant. The</p><p>reactors, operated at 37 C, were closed at the topwith butyl rubber stoppers to maintain anaerobicconditions. A wire sieve (3 mm gauge) was installed5 cm above the bottom of the cone in each reactor tosupport the solid waste substrate while still allowingthe liquid to pass through it. The leachate from thereactors containing microbes from the inoculum wasrecirculated at 9 cm3 min1 and sprinkled over thepacked bed of potato waste. The leachate percolateddown through the bed and out of the reactor througha part at the bottom. Samples were collected everyhour for the first 24 h and thereafter every 12 h untilthe experiments were stopped after 300 h.</p><p>2.2 Analytical methodsThe samples from the reactors were centrifugedat 3000 g for 3 min (WIFUG Lab CentrifugesSTUDIE-M, UK) and the supernatant was usedfor pH, COD and VFA analysis. Samples for VFAanalysis were acidified with concentrated sulfuricacid to stop microbial activity and stored at 20 Cuntil analysis. Before analysis, the samples werethawed and filtered through a 0.45 m filter (Minisart,Sartorius AG, Gottingen, Germany); the filtrates werecollected in HPLC sample vials. The individual VFAconcentrations were determined using HPLC, VarianStar 9000 (Varian, Walnut Creek, CA, USA), with aBiorad column, Cat 125-0115 (Hercules, CA, USA)for fermentation monitoring. The column temperaturewas 65 C, sulfuric acid (1 mM) was used as mobilephase and the liquid flow was 0.8 cm3 min1. Peakdetection was achieved with UV absorption at 208 nmand chromatograms were integrated and plotted usingStar chromatographic software. The TS, VS and CODwere determined according to standard methods.15</p><p>The COD equivalents of the five VFAs (derived fromthe complete oxidation reactions of the individualVFAs to CO2 and H2O), used to calculate the CODin the form of VFAs were: 1.066 (HAc), 1.512 (HPr),1.816 (BA), 2.036 (VA) and 2.204 mg dm3 (CA).16The following formula was used:</p><p>conversion of COD gg1 = COD of VFA(mg dm3)</p><p>Soluble COD(mg dm3)</p><p>3 RESULTS AND DISCUSSIONVFAs are among the main products of the acidogenesisof organic matter. The appearance of VFAs in theleachate indicated the initiation of acidogenesis within1 h of digestion, as can be seen in Table 1. HAcand HPr were the first VFAs to appear at bothpotato loads. n-BA was detected after 6 h at bothloadings of digestion. i-BA was detected after 7 h and15 h of digestion of 500 g and 1000 g solid potatowaste, respectively. n-VA and i-VA appeared after7 h of digestion at both loading. CA was detectedafter 9 h and 7 h of digestion of 500 g and 1000 gpotato solids, respectively. The presence of VFAsin the reactors was indicative of bacterial activity.The appearance of HAc and HPr has also been</p><p>674 J Chem Technol Biotechnol 79:673677 (online: 2004)</p></li><li><p>Mesophilic digestion of solid potato waste</p><p>Table 1. Appearance of volatile fatty acids and lactic acid duringhydrolysis and acidification of solid potato waste at batch loads of</p><p>500 g and 1000g</p><p>Time of appearance (h)</p><p>VFA 500 g 1000 g</p><p>HAc 1 1HPr 1 1n-BA 6 6n-VA 7 7i-VA 7 8CA 9 7LA 9 10i-BA 7 15</p><p>reported within 1 h in a study on anaerobic mesophilicdigestion of vegetable market waste.12 The anaerobicconditions and frequent contact of waste with leachatein the solid bed reactors provided conditions forenrichment, growth and development of acidogenicand hydrolytic bacteria.12 Their combined action onthe potato biomass resulted in the breakdown ofcomplex substrates to simpler materials, thus resultingin solubilisation of the organic content.</p><p>The time course for the production of VFAs andLA is shown in Fig 1. The fermentation productsafter 300 h digestion of potato were chiefly (mg g1total VFAs): HAc (420), n-BA (310), HPr (140)</p><p>Figure 1. Time course for the production of VFAs and lactic acidduring hydrolysis/acidification of potato solids at loads of 500 g (a)and 1000 g (b).</p><p>and CA (90) as well as low amounts of i-BA, n-VA and i-VA using 500 g potato solids. With 1000 gpotato solids, the fermentation products were mainly(mg g1 total VFAs): HAc (410), LA (400), n-BA(110) and CA (40), low amounts of HPr and no n-VA or i-VA. The production of HAc, n-BA and i-BAincreased continuously with 500 g potato solids duringthe period of the study, while the production of CAand HPr was almost constant after 50 h and 150 h ofdigestion, respectively. The same pattern was observedfor HAc with 1000 g potato solids, but the productionof n-BA, i-BA and CA remained constant after 50 hof digestion. Identification of the individual VFAsformed is important, since it may provide valuableinformation on the metabolic pathways involved inthe process. HAc, HPr, n-BA and i-BA are formeddirectly from the fermentation of carbohydrates andproteins, as well as during the anaerobic oxidation oflipids.3,17 These results are similar to those found byHoriuchi et al3 when they studied the production oforganic acids from a complex medium in an anaerobicreactor with pH control. The high production of n-BAin this study is mainly attributed to the larger amountof carbohydrate (starch) present in the substrate.9,18</p><p>HAc is considered to be the major precursor ofmethane.12 The conversion of HAc and n-BA ishigher than that of HPr, n-VA and i-VA. The straight-chain (normal form) fatty acids were found to havea higher decomposition rate than their branched-chain (iso form) acids by Wang et al.1 In this study,straight-chain forms were the dominant products,therefore it would be expected that the amountof methanogenesis would be good when used as asecond step in the process of biogas production frompotato solid waste. The formation and consumptionof some VFAs, such as HPr, n-BA, i-BA, n-VAand i-VA, demonstrate the conversion between theseacids via -oxidation followed by isomerisation andalso their conversion to acetic acid by acetogenicbacteria.18 The concentrations of VFAs increasedwith increasing concentrations of potato solids. Theseresults show that the concentration of the substratehad a considerable effect on the distribution of theacidification products.</p><p>The results also indicate that the occurrenceof the individual VFAs decreased as the chainlength increased. The higher molecular weightVFAs, including n-VA and i-VA, were presentat low concentrations at both loadings of potatosolids. These VFAs are mainly associated with thefermentation of proteins and they could be formedvia reductive deamination of single amino acids orby oxidationreduction between pairs of amino acidsvia the Stickland reaction.12,1820 The total Kjeldahlnitrogen in potato is very low (15 g kg1 of the TS)hence the production of these acids was not assignificant as those of acetate and butyrate. It has beenreported that the production of the four acids, n-BA,i-BA, n-VA and i-VA, during the digestion of non-proteinaceous substrates is minimal. Neither of these</p><p>J Chem Technol Biotechnol 79:673677 (online: 2004) 675</p></li><li><p>W Parawira et al</p><p>VFAs is normally produced under the conditions oflow partial pressure of H2 encountered in acidogenicdigestion systems.18</p><p>The relationship between TVFAs concentrationand soluble COD at the different total solidsconcentrations is shown in Fig 2. The amount ofTVFAs reached approximately 19 g dm3 at bothloadings after 300 h. However, the total organic acidcontent with 1000 g potato solids was 31 g dm3 whenthe TVFA was combined with lactic acid. SolubleCOD is a parameter that represents the extent ofhydrolysis and solubilisation. Solubilisation of organicmatter, in terms of soluble COD, increased rapidlywithin the first 100 h and then gradually to reach amaximum of 27 g COD dm3 with 500 g potato solids,and 37 g COD dm3 with 1000 g potato solids after300 h of digestion. The recirculation of the percolatingculture through the solid bed promoted hydrolysisand acidifica...</p></li></ul>