UV Disinfection of Low Transmittance Pharmaceutical Wastewater

An UV disinfection system located at an activated sludge plant treating wastewater from a research pharmaceutical facility was performing unsatisfactorily. The plant had to rely on chlorine addition to meet its fecal coliform limit. Initially, interference from ferric chloride, which is used for phosphorus removal, was suspected. Ferric chloride was replaced with alum, but no improvement in UV effectiveness was noted. A review of the plant performance data indicated a direct correlation between effluent COD concentration, UV transmittance and UV effectiveness. A subsequent plant process evaluation revealed an inadequate operation of the activated sludge system. The plant was reseeded and the operation was optimized resulting in improved COD removal and UV performance. However, even optimized biological treatment did not improve effluent UV transmittance beyond the 40 to 50% range, considerably less than the 65% level, which is typical for treated municipal effluent. The poor UV transmittance and substandard UV effectiveness was attributed to UV-adsorbing organics which were incompletely degraded in the activated sludge process.