Abstract
Dairy and potato are two important agricultural commodities in Idaho. Both the dairy and potato processing industries produce a huge amount of waste which could cause environmental pollution. To minimize the impact of potential pollution associated with dairy manure (DM) and potato waste (PW), anaerobic co-digestion has been considered as one of the best treatment process. The purpose of this research is to evaluate the anaerobic co-digestion of dairy manure and potato waste in terms of process stability, biogas generation, construction and operating costs, and potential revenue. For this purpose, I conducted 1) a literature review, 2) a lab study on anaerobic co-digestion of dairy manure and potato waste at three different temperature ranges (ambient (20-25˚C), mesophilic (35-37˚C) and thermophilic (55-57˚C) with five mixing ratios (DM:PW-100:0, 90:10, 80:20, 60:40, 40:60), and 3) a financial analysis for anaerobic digesters based on assumed different capital costs and the results from the lab co-digestion study. The literature review indicates that several types of organic waste were co-digested with DM. Dairy manure is a suitable base matter for the co-digestion process in terms of digestion process stability and methane (CH4) production (Chapter 2). The lab tests showed that co-digestion of DM with PW was better than digestion of DM alone in terms of biogas and CH4 productions (Chapter 3). The financial analysis reveals DM and PW can be used as substrate for full size anaerobic digesters to generate positive cash flow within a ten year time period. Based on this research, the following conclusions and recommendations were made:
► The ratio of DM:PW-80:20 is recommended at thermophilic temperatures and the ratio of DM:PW-90:10 was recommended at mesophilic temperatures for optimum biogas and CH4 productions.
► In cases of anaerobic digesters operated with electricity generation equipment (generators), low cost plug flow digesters (capital cost of $600/cow) operating at thermophilic temperatures are recommended.
* The ratio of DM:PW-90:10 or 80:20 is recommended while operating low cost plug flow digesters at thermophilic temperatures.
► In cases of anaerobic digesters operated without electricity generation equipment (generators), completely mixed or high or low cost plug flow digesters can be used.
* The ratio of DM:PW-80:20 is recommended for completely mixed digesters operated at thermophilic temperatures;
* The ratio of DM:PW-90:10 or 80:20 is recommended for high cost plug flow digesters (capital cost of $1,000/cow) operated at thermophilic temperatures;
* All of the four co-digested mixing ratios (i.e. DM:PW-90:10 or 80:20 or 60:40 or 40:60) are good for low cost plug flow digesters (capital cost of $600/cow) operated at thermophilic temperatures. The ratio of DM:PW-90:10 is recommended for positive cash flow within the ten year period if the low cost plug flow digesters are operated at mesophilic temperatures.