In order to select the proper operating conditions and assess the performance of an anaerobic digester, it is important to analyze the characteristics of an organic feedstock. In this study, food and green wastes were characterized daily and weekly, and digestibility was tested. The green wastes were collected from the City of Vacaville, California, and surrounding areas over a period of four months (July to October) and passed through a mesh screen to remove large particles. Based on the analysis of bi-weekly samples, the green wastes had an average moisture content (MC) of 65% and an average volatile solids to total solids ratio (VS/TS) of 78%. The food wastes were collected from the City of San Francisco over a two-month period (July and August) and they were ground and passed through a mesh screen screening. The daily average MC and VS/TS determined from a week-long sampling were 70% and 83%, respectively, while the weekly average MC and VS/TS determined from sampling over an eight-week period were 74% and 87%, respectively. The results showed that the composition of both food and green wastes varied minimally on a daily and weekly basis. The nutrient content analysis showed that both green and food waste contained well balanced nutrients for anaerobic microorganisms. The biodegradability and biogas and methane yields of food and green wastes were determined using batch anaerobic digestion tests performed at 50°C. After 10 and 28 days of digestion, food waste showed a methane yield of 338 and 435 mL/gVS respectively, and green waste showed 202 and 226 mL/gVS. The average VS destruction was 80.57% for food waste and 79.30% for green waste at the end of the 28-day digestion test. Moreover, biogas yields were 5078 and 2862.5 ft3/ton of food and green wastes with average methane contents of 73% and 55 %, respectively. Based on the sources of food and green wastes, it is expected that the biodegradability of food waste will be relatively consistent throughout the year but the biodegradability of green waste could have seasonable variation. The results of this study on the green waste would well represent the characteristics of green waste in the summer and early fall. Further study is needed for charactering the green waste generated in other seasons. These results support the UC Davis pilot digester project funded by California Energy Commission.
Biogas Yield Chart source: David House Biogas Manual: http://www.completebiogas.com/B_ARTI.html.