Anaerobic bioflocculation of wool scouring effluent: the influence of non-ionic surfactant on efficiency

Wool scouring effluent (WSE) contains high concentrations of wool grease emulsified by non-ionic surfactants (nonylphenol polyethoxylates – NPEO). The short-term treatment (1-7 days) of this effluent with anaerobic bacteria resulted in partial grease flocculation. However the efficiency of this process varied largely (30% to 80%) with the source of wool scouring effluent used. The concentration of free surfactant, rather than total surfactant, was found to be the likely reason for the variation in efficiency. In order to elucidate the mechanisms of anaerobic biological flocculation a detailed surfactant analysis was performed. This revealed that anaerobic microbes (taken from sludge of a municipal wastewater treatment plant) had an ability to partially degrade NPEO by shortening the hydrophilic ethoxylate chain causing coagulation and subsequent flocculation of wool grease from the liquor.

Treatment of wool scouring effluent by anaerobic bioflocculation

Biological destabilisation of the wool grease/water emulsion in wool scouring effluent using anaerobic bacterial activity (biological flocculation) was investigated. The aim of biological flocculation is to remove the bulk of wool grease which is the major source of COD, therefore serving as a pretreatment step, prior to classical biological processes either aerobic or anaerobic. In a semi-continuous system, a two-stage anaerobic bioflocculation process was employed to treat a high grease (> 15 g l−1) wool scouring effluent (WSE). After 110 days of operation, the process showed removal of 70 to 90% grease at a combined hydraulic residence time (HRT) of 4 to 10 days. With low grease (< 10 g l−1) WSE grease removal was lower. At an HRT of 3 days a single stage bioflocculation process removed 40% grease. The supernatant from the process was easily treated by activated sludge process reducing grease concentration from about 1.5 g l−1 to less than 0.1 g l−1 in the final effluent (HRT 3 days). Methane production of the process was negligible. Most of the grease was removed by flocculation as a result of anaerobic bacterial activity. The mechanisms of the process were investigated by a series of batch experiments and found to be; (1) appropriate gentle mixing between WSE and anaerobic sludge results in the absorption of wool grease from the liquid to the sludge phase, (2) further destablisation of the wool grease emulsion is obtained when the mixed liquor is left undisturbed. The latter was due to bacterial activity and growth on organics contained in WSE.