Effluent produced by the wool scouring process is highly polluted with emulsified grease, dirt particles, salts, and detergent. The major problem in treating this waste stream is the wool grease which is resistance to biodegradation. The removal of grease from the effluent would lead to a more readily degradable waste stream, and therefore suitable for further biological treatment processes. This study aimed to investigate anaerobic destabilisation (flocculation), rather than degradation, of wool grease emulsion from wool scouring effluent (WSE). The process therefore can serve as a pretreatment step, prior to a conventional biological process.
The results from this study show that emulsified wool grease in WSE could be removed by bioflocculation under anaerobic conditions. After 110 days of continuous operation, a two-stage anaerobic process treating a high grease (> 10 g/L) effluent removed 70 to 90% grease and approximately 60 to 86% COD at a combined hydraulic residence time (HRT) of 4 to 10 days. With low grease (<10 g/L) effluent grease removal was reduced. At a HRT of 3 days a single stage anaerobic process removed 40 and 44% grease (37 and 43% COD) at 20 °C and 37 °C respectively. Since the supernatant of the treated effluent still contained residual grease of over 1.5 g/L, further purification was necessary. The supernatant was readily treated by an aerobic activated sludge process, reducing the grease concentration from about 1.5 g/L to less than 0.1 g/L, in the final effluent, with an HRT of 3 days.
Methane production and volatile fatty acids consumption of both the above anaerobic systems were negligible. The majority of the grease was removed by flocculation as a result of anaerobic bacterial activity. The mechanisms of this process were investigated by a series of batch experiments. It was found that: (1) appropriate gentle mixing between wool scouring effluent (WSE) and anaerobic sludge resulted in the absorption of wool grease from the liquid phase to the sludge phase, (2) further estabilisation of the wool grease emulsion was obtained when the mixed liquor is left undisturbed. The process thus required a short gentle mixing period of approximately 15 minutes to enable complete contact between the sludge and WSE, and a longer settling period of 2 to 4 days to provide appropriate time for the microbes to destabilise wool grease emulsion and transfer it to the sludge phase.
The process of destabilising the wool grease from wool scouring liquor was found to result from the activities of suspended microbes in the anaerobic sludge, which could successfully grow in WSE, rather than the bulk biomass as required in a conventional anaerobic digestion process. General microscopic observation indicated that during the process of bioflocculation a large number of mixed bacterial cells (> 1Q8 cells/nil) were present in the supernatant and only a small number appeared within the flocculated grease. No evidence of bacterial cell aggregation was observed in the process. It was hypothesised that the mechanism involved the partial degradation of detergent.
Detergent analysis revealed that anaerobic microbes' (taken from the sludge of a municipal wastewater treatment plant) had an ability to partially degrade non-ionic surfactants (nonylphenol polyethoxylates - NPEO) by shortening the hydrophilic ethoxylate chain, resulting in the reduction of surfactant properties. This is likely to be one factor causing coagulation and subsequent flocculation of wool grease in the liquor. Other factors such as production of biopolymers and enzymes by microbes may also play a role, and should be further investigated as they beyond the scope of this thesis.
Ten different bacteria strains were isolated from the supernatant of successfully flocculated WSE samples. Six strains were found to grow in raw WSE as a pure culture. Only three strains caused some flocculation of wool grease, although the reduction of grease from the supernatant was not as effective (20-30%) as that using the mixed culture (60-80%). However, the results were not reproducible when different WSE samples were used, thus no definite conclusions could be obtained from this experiment.
The efficiency of anaerobic bioflocculation was found to vary greatly (30% to 80% grease removal) depending on the source of wool scouring effluent The concentration of bacterial substrate, grease and free detergent (rather than total detergent) were all found to effect the efficiency of the process. At a constant loading rate, the efficiency of the process was found to increase with increased grease concentration in WSE. A rationalisation of the scouring process to minimise detergent use and produce higher concentration grease and suint WSE is a likely benefit of bioflocculation process. These findings lead to the recommendation of a proposed treatment scheme.
The main conclusion drawn from this study is that the anaerobic biological removal of wool grease in WSE is due to the destabilisation of the wool grease emulsion resulting in grease flocculation. Since the process does not require further additives, such as chemical flocculant or oxygen, the removal of the bulk of the grease by simple anaerobic bioflocculation appears to be a useful part of an economic treatment system
