Anaerobic digestion of crop and waste biomass: Impact of feedstock characteristics on process performance

Abstract: Popular Abstract in English In an ever energy hungry world and given the concerns about global warming, depleting reserves of fossil fuels and growing fuel prices, there is an urgent need for alternative renewable energy sources. Biogas production through anaerobic digestion in sync with other bio energy production technologies could replace or partially replace fossil fuels and hence curb greenhouse gas emissions. The digestate from the process can also replace mineral fertilizer leading to an overall sustainable operation. However, the biogas process has suffered a great many setbacks due to problems such as shortage and unavailability of feedstock, poor methane production, process imbalances and even process failure. Most successfully operated processes are thus usually maintained at very long hydraulic retention times (long digestion times) and low organic loading rates. There is therefore a need for improvement and expansion of the anaerobic digestion process. This thesis is a summary of six papers (I-VI) and represents my research in the field of biomethanation (biogas production) aiming at improving the efficiency of the biogas process through feedstock optimization. Feedstock is food for the microorganisms in the biogas process. The type, composition as well as the presence or absence of vital macro and micronutrients will influence the outcome of the process. The research presented in this thesis deals with the assessment of the possible negative or positive impacts of the characteristics of the feedstock on the efficiency of anaerobic digestion. In addition, it investigates ways of enhancing the methane yield of the feedstock by improving the feedstock characteristics. The feedstocks investigated were various energy crops, food industrial waste and sewage sludge. The improvement methods investigated were ensiling, nutrient supplementation, co-digestion and anaerobic pretreatment. Contrary to reports in previously published literature, ensiling as a pre-treatment did not improve the methane yield of crop biomass (Paper I). The reported increased methane yields in literature were suggested to be due to the presence of volatile compounds in silage which could have given analytical error. However, our findings indicated that ensiling could enhance the stability of an anaerobic digestion process, as was evidenced by little or no foaming in a silage fed process (Paper III) as opposed to fresh crop fed process, where foaming was rampant (Paper II). In a related pre-treatment study, i.e. anaerobic pre-treatment of sewage sludge prior to anaerobic digestion (Paper VI), solubilisation of particulate organic matter was observed. This could have led to an increase in methane yield and reduction in volatile solids. However, the solubilisation reported here was partly as a result of fermentation, a condition that often leads to the production of volatile compounds. The presence of these volatile compounds can lead to the same analytical error as was observed in the study presented in Paper I. It is therefore very important to thoroughly characterise ensiled (or ‘pre-fermented’) biomass so as to achieve better quantification of methane yields and of other total solids weighted parameters such as organic loading rate and reduction in volatile solids. For energy crops, we were able to demonstrate high methane yields comparable to maximum expected yields and process stability as evidenced by low VFAs accumulation in mono-digestion when both macro and micronutrients were supplemented in the process (Papers II and III). In the nutrient supplemented processes, it was possible to apply high organic loading rates at short hydraulic retention times, thereby increasing the treatment capacity through efficient feedstock utilization (Papers II and III). The nutrient addition was also balanced to make the digestate comply with certification limits for heavy metals content in bio-fertilizer for farmland application (Paper III). For the processes based on food industrial waste, co-digestion especially led to an improvement in methane yield per weight of feedstock and to a stable process through balancing the carbon to nitrogen ratio and diluting toxicants (Papers IV and V). It was also suggested that co-digestion of waste and crop biomass may eliminate the need for the micronutrient supplementation (Paper IV) that was applied in the studies presented in Papers II and III. In conclusion, this thesis shows that the performance and conversion efficiency of the biogas process can be improved by improving the characteristics of the feedstock. This is relevant for utilizing the limited available biomass in the most efficient manner.