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

Abstract

Anaerobic digestion provides an array of positive environmental benefits such as reducing greenhouse gas emissions, replacing mineral fertilizers, producing renewable energy and treating waste. However, pitfalls in anaerobic digestion such as poor methane yields, process instability, process failure and regional shortages of feedstock have limited the full exploitation of the anaerobic digestion process.



The research presented in this thesis deals with the assessment of the possible negative or positive impacts of feedstock characteristics 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.



It was found that ensiling crops results in insignificant losses in energy, total solid and wet weight. In addition, no significant difference was found in methane yields between the ensiled and fresh crop samples. The importance of correcting for losses of volatiles in total solids determination was pointed out and it was shown that failing to do so could be the main reason why many previous publications report increased total solid based methane yields after ensiling. Increased methane yield in silages may therefore be an effect of an analytical error rather than an effect of using ensiling as a pretreatment prior to anaerobic digestion.



Anaerobic digestion of crop biomass is known to be particularly limited by nutrient availability. Direct nutrient supplementation in crop mono-digestion in this research demonstrated an efficient biogas process at the shorter hydraulic retention times commonly applied in co-digestion of crop biomass and manure. The high degradation efficiency was evidenced by high methane yields, comparable to maximum expected yields generated under controlled conditions, and low volatile fatty acids accumulation. As a result of nutrient addition, the digestate could comply with certification standards for bio-fertilizer. Also, viscosity problems commonly reported for crop mono-digestion were not observed in this study, which could be another effect of nutrient addition.



Co-digesting of waste biomass and crop biomass led to significant improvement in methane yield per ton of feedstock and carbon to nitrogen ratio as compared to digestion of only the waste biomass. Biogas production from crops in combination with waste biomass also eliminated the need for addition of micronutrients normally required in crop mono-digestion. Co-digestion was also presented as a means of feedstock supplementation to curb feedstock shortages in waste-based anaerobic digestion processes. In addition, inhibitors in anaerobic digestion such as free ammonia and light metal ions were diluted, a condition which can lead to an overall viable biogas process



Anaerobic pre-treatment led to the solubilisation of particulate organic matter in sewage sludge. This solubilisation could have led to the improved methane yield, methane production rate and reduction in volatile solids.



Applying different feedstock improvement solutions to the various feedstocks investigated, i.e. nutrient addition, co-digestion and pretreatment, were demonstrated as effective means of enhancing the methane yield of the feedstock thereby improving the overall anaerobic digestion process.

Abstract (Swedish)

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.

• author: Ivo Achu, Nges
• supervisor: Lovisa Björnsson
• opponent: PhD Uellendahl, Hinrich, Aalborg University, Denmark.
• organization: Biotechnology
• publishing date: 2012
• type: Thesis
• publication status: published
• subject: Industrial Biotechnology
• keywords: Anaerobic digestion, Biofertilizer, Renewable energy, Sustainable operation
• pages: 198 pages
• defense location: Lecture Hall B, at the Center of Chemistry and Chemical Engineering, Sölvegatan 39, Lund University Faculty of Engineering
• defense date: 2012-06-14 10:30:00
• ISBN: 978-91-89627-83-3
• language: English
• LU publication?: yes
• id: 44dd2cf7-057a-4354-939e-4adcc2768ef5 (old id 2541422)
• date added to LUP: 2016-04-04 12:00:02
• date last changed: 2018-11-21 21:08:26