Anaerobic Co-Digestion Of Food Waste And Kraft Pulp Fibre To Enhance Digestate Dewaterability

Abstract: Digestate produced during anaerobic digestion of food waste is recognised as a good alternative to mineral fertilizer which could also be used to amend soil properties. This has conventionally been applied directly and unprocessed to nearby farms or processed and transported elsewhere. The latter option has gained recognition due to environmental restrictions coupled with soil nutrient management objectives but is an expensive venture. With increasing biogas production and AD plants across Europe, production of digestate has however exceeded its demand. Improving the dewaterability of the digestate has the benefit of reducing the cost and time of processing and handling. The principal aim of this experiment was to enhance the dewaterability of food waste digestate by the addition of pulp fibre to the AD process. In doing so, the study also investigated the effect of co-digestion of food waste and pulp fibre on the performance and stability of the digestion. Source separated food waste was digested at OLR of 3.5 ±0.1 g VS/L'd-1 for 163 days in 3 CSTRs with a working volume of 6L at HRT OF 23-26 days. Soft- and hardwood pulp fibres were added to 2 designated digesters for 104 days and increased stepwise at OLR of 0.5 ±0.1 g VS/L'd-1 PF until 1.5 ±0.1 g VS/L'd-1 PF with the 3rd digester serving as a control. 3 other post-digesters, each with a working volume of 1.41L were operated for 104 days with sludge from the 3 main digesters serving as inoculum and substrate. This was run at HRT of 7 days.  Pulp fibre addition of 1.5 ±0.1 g VS/L'd-1 OLR to 3.5 ±0.1 g VS/L'd-1 food waste increased the total biogas and methane production to 35-40% and 21-32% respectively. Though recording a higher biogas production, the corresponding specific methane production from the fibre addition was 12-8% lower than food waste digestion. Analysis of the digestate from post digestion showed that CST increased linearly from 595 ±13s for food waste digestate to 962 ±19s for pulp fibre addition. The experiment established a positive correlation between CST and organic matter content. Suspended solids increased from 128 ±10 mg/l for FW digestate to 177 ±12 – 237 ±10 mg/l for fibre addition. Addition of kraft pulp fibre types did not enhance the dewaterability of the digestate. However, the total methane production was enhanced by the addition of pulp fibre.