Anaerobic digestion of dairy manure wastewater, food and fruit waste, a sustainable source of bio-energy and waste management

Sakeus, Kafula (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The potential for anaerobic co-digestion (AD) organic waste streams such as dairy manure waste water, fruit and food waste is a well-established process. A pilot study, utilizing a new constructed 40.248 m3 plug-flow, three chamber anaerobic reactor was conducted to determine the effects of temperature, electrical conductivity (EC), pH, organic loading rate (OLR) over a 20 day hydraulic retention time (HRT) on biogas yield, waste volume reduction, water soluble minerals, reduction of pathogenic microorganisms (Escherichia coli O157:H7), phytotoxcity and anaerobic microbial population dynamics. The reactor was equipped with a build-in temperature sensor which allowed for the internal temperature to be recorded from the circulation of water via an 8L biogas geyser. The reactor was also fitted with three ports connected to the three individual chambers, from which samples for EC and pH were lifted for analysis. Dairy manure wastewater had a total soluble (TSS) concentration (2.59 g/l) which was considerably lower than the anticipated 10% TSS. This meant the reactor had an OLR of 2.59 kg/m3reactor/day (Total suspended solids) and 1.97 kg/m3reactor/day volatile suspended solids (VSS). To maintain an organic retention time (ORT) equivalent to HRT, co-digestion of fruit and food waste was than calculated based on the TSS and VSS content of dairy manure wastewater. The results validated that the reactor’s temperature plays an important role in the production of biogas and reduction of pathogenic microorganisms of the digestate. Due to the low temperatures during the trial period, no viable biogas yields were detected using the current water displacement system and thus no biogas quality analysis were conducted. Temperature data confirmed that at temperatures below 20 °C, there was no linear relationship between the salinity indicator, EC and temperature. In addition, these low psychrophilic temperatures at a HRT of 20 days had no comparable impact on the reduction of pathogenic microorganisms. However, a 97% reduction in TSS entering the reactor was observed in all digestates. Water soluble minerals entering the reactor through feedstock were comparable to the soluble mineral concentrations the digestate. Although, the digestates contained high concentrations of mineral nutrients such as ammonium which were above the legal limit of waste water to be discharged in the environment. At concentrations below 75% none of the digestates showed significant phytotoxicity effects quantified with tomato seed germination. While, the data also seem to suggest that OLR had a noticeable influence on microbial population dynamics. Overfeeding the reactor induced an instant decreased on pH and also increased the microbial population species that colonize the different chambers. Under current sub-operating conditions compounded by low OLR, the AD reactor was not competitive enough to replace the current composting operation.

AFRIKAANSE OPSOMMING: Die potensiaal vir anaërobiese mee-vertering (AD) van organiese afvalstrome soos suiwelmis afvalwater, vrugte- en voedselafval is 'n goed gevestigde proses. Vir ‘n loodsstudie, is gebruik gemaak van 'n nuut geboude 40. 248 m3 propvloei, drie-kamer anaërobe reaktor om die uitwerking van temperatuur, elektriese geleiding (EG), pH en organiese laai tempo (OLR) oor 'n 20 dag hidrouliese retensietyd (HVT) op biogas opbrengs, afvalstroom volume vermindering, water oplosbare minerale konsentraises, die vermindering van patogene mikro-organismes soos (Escherichia coli O157: H7), fitotoksisiteit en mikrobiese anaërobiese bevolking dinamika te bestudeer. Die reaktor is toegerus met 'n inhuis temperatuur sensor wat dit moontlik gemaak het om die interne reaktor se temperatuur te monitorword sirkulasie van water deur 'n 8 L biogas verhitter. Die reaktor is ook toegerus met drie poorte wat gekoppel is aan die drie individuele kamers, waaruit monsters vir EG en pH ingesamel kon wordvir ontleding. Suiwelmis afvalwater het 'n totale oplosbare wastetof (TOVS) konsentrasie getoon wat aansienlik laer is as die verwagte 10% TOVS. Dus het die reaktor het 'n OLR van 2. 59 kg / m3reactor / dag (TOVS) en 1. 97 kg / m3reactor / dag vlugtig gesuspendeerde vastestowwe (VSS) getoon. Om 'n organiese behoud tyd (ORT) gelykstaande aan HVT te bereid, is mee-vertering van vrugte- en voedselafval geïmplimenteer gebaseer op die TSS en VSS inhoud van suiwelprodukte mis afvalwater. Die resultate bevestig dat die temperatuur van die reaktor 'n belangrike rol in die produksie van biogas en die vermindering van patogene mikro-organismes uit die afvloeiwater speel. As gevolg van die lae temperature, is geen lewensvatbare biogas opbrengs waargeneem in die die huidige water-verplasing-stelsel nie en dus is geen biogas kwantifisering gedoen nie. Temperatuur data het wel bevestig dat by temperature onder 20 ° Cr geen lineêre verhouding tussen die soutgehalte aanwyser EG en temperatuur bestaan nie. Verder het hierdie lae temperature op 'n HVT van 20 dae geen vergelykbare impak op die vermindering van patogene mikro-organismes gemaak nie. 'n Vermindering van 97% in TSS in die reaktor is wel waargeneem in alle afvloeiwater. Water oplosbare minerale in die invoer van die reaktor was vergelykbaar met die in die afvloeiwaer. Alhoewel, die afvloeiwater hoër konsentrasies van minerale soos ammonium bevat het wat die wettige perk oorskry het fir die storting van afvalwater in die omgewing. Konsentrasies onder 75 % van al die afvloeiwater het geen beduidende fitotoksisiteit effekte op tamatie saad ontkieming getoon nie. Die data daarop dat OLR 'n merkbare invloed op die mikrobiese bevolkingsdinamika het. Oorvoeding die reaktor het 'n direkte effek getoon op pH en ook die toename in mikrobiese spesies bevolking wat die verskillende kamers koloniseer. Onder die huidige sub-optimale omstandighede, was die AD reaktor was nie mededingend genoeg om huidige stelsel van kompostering te vervang nie.

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