Digested Sewage Sludge as Seed for Batch Test of Anaerobic Biodegradability
Research was carried out to optimize the use of digested sewage sludge (DSS) as a seed for batch tests of food waste anaerobic biodegradability. Digested sewage sludge was used as the seed because of its abundant supply and previous use as a seed. A seed volume of 1:1 g VS basis to the substrate provided a stable anaerobic process, however it inundated the 20 g food waste sample by 25 times more volume. In an attempt to reduce the seed volume, thus increasing the sample size, the seed was thickened via gravity settling. Although the solids content doubled to 2.4% TS with the concentrate, the percentage of methane was similar. About 120 g of food waste was an appropriate sample loading for 1.5 L of seed with methane production decreasing when sample size increased to 160, 200 or 240 g. The larger ratio of seed to substrate allows for a successful digestion without pH adjustment. The methane production of food waste was more consistent when 7 days old seed (DSS was adapted to test temperature for a week) was used compared to a fresh (1 day old) one. The 7 day-old seed also produced less methane, thereby improving the estimation of methane production from the substrate. A reduced acid concentration and reduced initial acid production rate with stored seed is likely to have led to these results. In conclusion, the use of 7 day old DSS is recommended to ensure a consistent anaerobic degradability test result with minimal methane background production.