Assessing the treatability of textile effluents in an activated sludge system.

Abstract

Assessing the treatability of a textile effluent through the activated sludge process required the development of analytical protocols and evaluating their suitability in providing receiving municipal wastewater treatment plants with systematic methodologies for predicting: (i) soluble dye effluent decolourisation through the activated sludge process (ii) impact of surfactants on oxygen transfer in the activated sludge system (iii) subsequent biodegradability of these surfactant effluents. Decolourisation was assessed through spectrophotometric computations of the mass of dye remaining in the activated sludge supernatant. Oxygen transfer was quantified from estimates of volumetric oxygen transfer coefficients which were computed from the modified form of the Lewis-Whitman interfacial mass transfer model which took into account the oxygen uptake rate from the respiring microbial species Biodegradability of the surfactant effluent was computed from the mass of soluble biodegradable substrates assimilated by the active sludge system during exogenous respiration The mass of the dye particles removed from solution attained an asymptotic value after 1 h and this implied adsorption equilibrium. A comparison between the adsorption equilibrium attained after 1 h and the municipal activated sludge system hydraulic residence time of 6 h led to the conclusion that soluble colour removal in receiving municipal activated sludge systems is not rate limited and it was therefore not necessary to accurately predict the adsorption kinetics. Instead, the adsorptive capacity of the activated sludge and extent of dye effluent decolourisation is of greater significance. Instantaneously after dosing the activated sludge system with the surfactant effluent, computed estimates of the volumetric oxygen transfer coefficient exhibited sudden and pronounced increments which simultaneously coincided with pronounced increments in the non-linear regression confidence level error bounds associated with each mass transfer coefficient computation. It was theorised that the surfactant effluent imparted some form of interference to the Clark dissolved oxygen sensor’s dissolved oxygen measurement mechanism and this resulted in erratic data points that did not fit onto the model. Comparative computations of volumetric oxygen transfer coefficients in the presence of a non-surfactant substrate such as CH3COOH should be conducted for purposes of elucidating increments in the mass transfer coefficients as a result of reaction-enhanced mass transfer from increments resulting from the impact of the surfactant effluent on either the liquid film mass transfer coefficient or the interfacial area or both. Further refinements are required in automating the methodology for computing volumetric oxygen transfer coefficients and generating scatter plots of the mass transfer coefficients as a function of time from automated real-time feeds of dissolved oxygen time series data logged by dissolved oxygen online instrumentation. Biodegradability numerical estimates were all far less than the estimates reported in literature by surfactant manufacturers and it was postulated that the erratic dissolved oxygen time series data points resulting from the dosing of the surfactant effluent were also extended to the biodegradability computations. It is also highly probable that the pronounced dissimilarities in biodegradability estimates were a result of either the presence of toxic components in the surfactant effluent which resulted in the gradual inhibition of microbial activity or a significant presence of slowly biodegradable and inert soluble substrates in the surfactant effluent which were not depleted through aerobic utilisation by heterotrophic microbial populations.