Decolourization of wood-ethanol stillage using a granular activated carbon packed anaerobic expanded-bed reactor : a thesis presented in partial fulfilment of requirements for the degree of Master of Technology in Biotechnology at Massey University /
The anaerobic treatment (including decolourization) of wood-ethanol stillage from the Forest Research Institute (FRI) wood-hydrolysis pilot plant at Rotorua has been investigated using granular activated carbon (GAC) packed expanded-bed reactors. Specifically, bioregeneration of the GAC in the reactors in terms of organic and colour removal has been considered. Two 7.2 1 anaerobic expanded bed (AEB) reactors were designed and built. Reactor One (R1) was used for the anaerobic digestion of raw wood-ethanol stillage and Reactor Two (R2) for the decolourization of anaerobic lagoon pretreated wood-ethanol stillage. For R1, a desulphated stillage feed (to 500 mg.1-1sulphate) was used. Depending on the organic loading rate (OLR), the additions of nitrogen (N), phosphorus (P) and alkalinity reagent ranged from 240-350 mg.l-1, 80-250 mg.l-1
and 2.5- 4.5 ml 20% w/v NaOH per litre feed respectively. Only N and P feed supplements were used for R2 at 240 and 80 mg.l-1
respectively. The reactors' performance and stability were closely monitored through analyses of volatile fatty acid's, pH, alkalinity, colour, chemical oxygen demand (COD), sulfide, biogas production rate and methane composition, solids concentrations, N and P. After operating R1 for 227 days, it was demonstrated that this system, is superior to the previous systems reported for the treatment of a similar stillage. A non-maximal OLR of 29.0 kg tCOD.m-3.d-1 at 0.85 d hydraulic retention time (HRT) with total and soluble COD (tCOD and sCOD) removals of 74.5 and 83.5% respectively were achieved. Digestion stability was excellent with acetate at 160 mg.l-1, propionate at 490 mg.l-1 and a gas methane composition of 61.0%. The colour loading rate was 4.7 kg chloroplatinate.m-3.d-1 with a 75% colour removal. Higher colour and COD removals may be obtained by operating at a longer HRT (e.g. the percentage colour and sCOD removals were 90.6 and 91.8% respectively at a 2 d HRT). Previously, no significant colour removal for the anaerobic digestion of wood-ethanol stillage has been reported. In this study, only approximately 9% w/v of the chromophoric materials present in the wood-ethanol stillage are particularly recalcitrant to anaerobic degradation. The methane gas yield was near to that predicted by theory (99.7% at 2 d HRT) with a very low sludge yield (2.8% based on 91.8% sCOD removal). Consequently, the AEB reactor had a very low nutrient requirement for effective treatment. In terms of reactor stability, it can accomodate very high hydraulic loading rates (less than 0.85 d) without problems of cell washout. The use of activated carbon medium also provides a toxicity sequesting potential against biological inhibitors present in the wood-ethanol stillage. Continuous bioregeneration of the GAC in R1 has also been demonstrated using sCOD and colour breakthrough curves for GAC adsorption with and without biological activity. Microbial degradation of the chromophoric species has been confirmed using UV-visible spectrophotometric scans. Little methanogenic activity was observed in R2 in its 191 days of operation due to the recalcitrant nature of the anaerobic lagoon pretreated stillage. Only approximately 20% bioregeneration of GAC in terms of colour removal was achieved at a colour loading rate of 1.2 kg chloroplatinate.m-3.d-1. This study has demonstrated that the GAC packed expanded-bed reactor (R1) provides a very effective treatment of wood-ethanol stillage (including decolourization) while recovering a very significant portion (89%) of the stillage energy. Considerable capital and operating cost savings are possible using the AEB system since effective treatment can be achieved in a single step utilizing a relatively small reactor with minimal nutrient, sludge disposal and GAC regeneration or replacement costs. The only disadvantages of the system are the carbon cost, a long start-up period of 5 months and a recycle energy cost to maintain an expanded-bed. It is believed that they can partly be reduced by using a GAC carrier with a smaller particle size. Anaerobic digestion, utilizing a GAC packed expanded-bed reactor, thus represents a cost effective and commercially attractive option for the utilization/disposal of wood-ethanol stillage.