When the stream discharge permit for an industrial wastewater is set, the required level of treatment can either be based on pre-set effluent limits (effluent based) or on effluent limits which will achieve a pre-set level of stream quality (water-quality based). To develop water-quality based limits, an understanding of the bio-chemical and physical dynamics of the receiving stream is necessary; that is, the cause-and-effect relationship between loading and stream quality response must be established. Traditionally, this is done by creating a water quality model, which is based on carefully designed field investigations.