Anaerobic bioconversion of the organic fraction from the fruit processing industry

[摘要] ENGLISH ABSTRACT:South Africa is a developing country that relies heavily on its agricultural sector foreconomical welfare especially in the Western Cape Province. However,development gives rise to new technologies, new products, economical stabilityand unfortunately also to the production of larger volumes of liquid and solidwaste.Anaerobic composting is becoming a very attractive treatment option forsolid waste disposal because of its unique operational advantages and two valueaddedby-products, compost and biogas. Over the last decade progress has beenmade in anaerobic digestion of solid wastes, but no literature could be found onthe anaerobic composting of apple and peach pomace.The main objective of this study was to develop a method to anaerobicallycompost apple and peach pomace. In the first phase important operationalparameters were identified and a method was developed to optimise theparameters. In the second phase of the study, the scaling-up and optimisation ofthe process were the major objectives.During the first phase of this research 2 L modified glass containers wereused as composting units. The most important operational parameters (leachatepH, inoculum source and size, and initial moisture levels) were identified.Anaerobic compost from previous tests, brewery granules and anaerobic sludgewere also used as inocula and evaluated for the best source of microbes. Afteroptimising all the identified parameters, good results were obtained, whichincluded higher biogas production, good volume reductions, less bad aromas anda compost product with a neutral pH.After developing the 2 L laboratory-scale method to compost the applepomace anaerobically, the next step was to ascertain if the method would work iflarger volumes of solid fruit waste were composted. A special 20 L compostingunit made of PVC was designed to suit the operational requirements of theanaerobic composting process. It was also decided to mix apple pomace andpeach pulp together and to use this solid waste source as part of the compostingsubstrate.Different inocula, including cattle manure, anaerobic sludge, brewerygranules and anaerobic compost produced in the previous tests, were used. Although good results were obtained with the anaerobic compost and cattlemanure as inoculum, the aim was also to decrease the composting period byshortening the pH stabilisation period. To achieve this, it was decided to addNaHC03 to the substrate to be composted to facilitate a faster pH stabilisation.The composting period was subsequently shortened to 25 days with satisfactoryresults, which included a volume reduction, biogas production and faster pHstabilisation.An upflow anaerobic sludge blanket (UASB) bioreactor was also used toassist the composting process by facilitating the removal of the VFA's present inthe composting leachate. This proved to be a valuable addition to the compostingprocess as the UASB bioreactor also provided the composting units with a'moisturising liquid', which was 'enriched' with a consortium of active anaerobicbacteria when the effluent from the bioreactor was re-added to the compostingunits.With all the operational parameters in place, good results were obtainedand these included a volume reduction of 60% (m/m), a good biogas production, acomposting period of only 25 days, a compost that was free of bad aromas, a finalcompost pH of > 6.5, final leachate COD values of less than 3 000 rnq.l, and afinal leachate VFA's concentration of between 0 and 250 rnq.l.If in future research further scaling-up is to be considered, it isrecommended that the composting unit be coupled directly to the UASBbioreactor, thus making the process continuous and more practical to operate. Ifthe operational period of the anaerobic composting set-up could be furthershortened and the inoculum adapted so that the process could be used for thetreatment of other difficult types of solid wastes, it would probably beadvantageous for the fruit processing industry to use this method as anenvironmental control technology.

[发布机构] Stellenbosch University