A TECHNOLOGY REVIEW FOR CO-DIGESTION OF FOOD WASTE WITH BIOSOLIDS AND/OR MANURE

Co-digestion is a fairly new concept in which more than two or more substrates are digested together. Previous studies have indicated that co-digestion offers several advantages including additional gas production, improved nutrient balance and potential tipping fees for waste treatment. On the other hand, it requires additional facilities for gas handling and treatment, increases solids loading to dewatering facilities and may have impacts on recycle stream, mixing and heating requirement and quality of biosolids produce. Therefore, the merits and limits need to be thoroughly investigated. This paper reviews the technologies suitable for co-digestion and co-digestion experiences in terms of feed mixture, organic loading rates and other operational parameters. A variety of foodwaste is suitable for co-digestion as long as mixtures of foodwaste and biosolids or manure are correctly determined. The studies agree that high organic loading rates (OLRs) cause metabolite inhibition (ammonia or hydrogen sulfide) or fatty acid accumulation which reduces the digester performance. OLR values of no higher than 0.10 1bVS/cf-d are typically recommended to avoid such inhibition especially during the start up periods. It is also shown that the process failure is sometimes linked to in balanced carbon and nitrogen ratio in feed mixture. In order to reduce ammonia inhibition or volatile fatty acid accumulation, C/N ratio should be maintained between 16/1 and 25/1. A digester feed TS concentration of 3.5–10% have been typically used to avoid mixing problems and overloading of the digesters. The percentage of food waste and wastewater biosolids in the reviewed full-scale applications ranged between 5–30%. When full-scale studies were considered, the number of mesophilic digesters in operation substantially exceeds thermophilic digesters. The main reason is perhaps the additional gas production in thermophilic operation may not offer a reasonable payback period to compensate the necessary capital investment for heat and gas system upgrades and for digester heating.