AbstractAn instant coffee factory's physical waste treatment systems and their resulting waste streams were identified and samples of wastewater were analysed to determine the physical and chemical characteristics relevant to anaerobic digestion. Two wastewater types were chosen for treatment by anaerobic digestion, a wastewater with a high level of suspended solids and a wastewater with the majority of the suspended solids removed.
The waste with a high level of coffee grounds was studied in mesophilic and thermophilic batch studies and mesophilic and thermophilic CSTR reactors fed daily. Samples were analysed to determine the degree of waste removal and changes in proximate composition before and after digestion. Continuous mesophilic digestion was achieved at a OLR of 1.3 kgCOD m'May' 1 (25 day HRT) with the addition of Ca(OH) v nitrogen, phosphorus and trace metals. A COD and VS removal of 60% was achieved. Continuous thermophilic digestion could not be established beyond 50 days without a increase in total VFA occurring.
The coffee waste low in suspended solids was treated in single stage mesophilic and thermophilic UASBs. Using the same UASBs the effect of thermophilic preacidification was also studied.
Stable anaerobic digestion of settled instant coffee wastewater was achieved for over100 days in mesophilic and thermophilic UASB reactors. Thermophilic UASBs were seeded with mesophilic granules and converted to thermophilic operation by raising the temperature to 55"C in one step. Both the mesophilic and thermophilic UASBs achieved stable anaerobic digestion at OLRs of up to 10 kgCODm'd' and down to a HRT of 24 hours. The mesophilic UASB has superior performance than the thermophilic UASB but the thermophilic reactor achieved an higher OLR.
The thermophilic and mesophilic UASB reactors used in single stage operation were operated with thermophilic pre-acidification and studied over a period of more than 120 days. The thermophilic pre-acidification stage was operated with pH control or with l.Sgl' NaHCO, added to the feed with HRTs of 24, 18, 15 and 12 hours. Up to 38% of the total influent COD was converted to TVFA, the principal VFAs were nbutyric, acetic and propionic respectively. The thermophilic/mesophilic two stage system gave a consistent improvement in performance over the thermophilic/thermophilic two stage system especially at higher organic loading rates. The thermophilic/mesophilic system achieved greater COD removal than achieved by the thermophilic/thermophilic two stage system. Thermophilic preacidification gave an increase of 60% in the loading rate achievable by the mesophilic methanogenic stage and a 100% reduction in HRT compared with the single stage system.
|Date of Award||1998|
|Supervisor||Freda Hawkes (Supervisor) & Dennis Hawkes (Supervisor)|