Effect of co-digestion with water hyacinth, inoculum concentration and dilution on biogas production of fish waste

Abstract

A report from the FAO (Food and Agriculture Organization) estimates that 9.1 million tons of Fish wastes (FW) are thrown away each year. Water hyacinth (WH) is the most invasive water weed that thrives in freshwater water and has impacts on the environment, ecology, and society. The waste generated is either discarded onto the open ground or disposed into pits which result in environmental pollution and health hazards. FW and WH can be used for biogas production. However, FW digests quickly and produces a lot of ammonia and Volatile fatty acids (VFAs) which inhibit the digestion when digested alone, it has also a low carbon to nitrogen (C/N) ratio; consequently, it is not directly suitable for anaerobic digestion (AD). In this regard, co-digestion with WH (high C/N ratio) prior to AD is necessary. To examine the effects of the operating factors: Substrate ratio (WH:FW), Inoculum concentration (IC) and Dilution, Central composite design (CCD) in Design Expert 13 software was used to design experiments of three factors at five levels which resulted in 17 runs. Laboratory scale experiments were carried out under mesophilic temperature (37 °C). The maximum biogas with the highest methane content of 68.15% was produced at the substrate ratio of 25:75 g/ 250 mL (WH:FW), 15 g/ 250 mL of IC and dilution of 95 mL. The biogas yield increased by 16.1% and 32.4% compared to pure WH and FW respectively. This implied that the operating conditions were efficient in maximising the biogas yield. FW had a lower C/N ratio; further study needs to consider co-digestion with other higher C/N ratio substrates.