Mosinee Paper Effluent Toxicity and Dilution Studies, 1990 Environmental Conference Proceedings

New state regulations aimed at controlling the discharge of toxic substances to surface waters are now being promulgated throughout the United States. This paper presents a brief description of some of the general concepts pertinent to these new water toxics rules. The emphasis is on mixing zone dilution concepts applicable to both chemical-specific and whole effluent acute and chronic toxicity limitations. Measures which can be taken to enhance effluent mixing in receiving waters, and hence dilution credit in permit calculations, through outfall modification are discussed.

These concepts are illustrated by relating the recent experiences of the Mosinee Paper Corporation, located in North-Central Wisconsin. The Mosinee pulp and paper mill discharges about 45 ML/day (12 mgd) to the Wisconsin River, where the design low river flow for chronic permit limits (7Q1O) is about 2,300 ML/day (950 cfs), ostensibly a dilution factor of more than 50:l.

However, because of peculiarities of the outfall location and the new Wisconsin rules, initial agency determinations regarding chronic dilution credit were extremely stringent (i.e.,zero dilution credit). Following dye tracer studies, the agency substantially increased the chronic dilution allowance, but not to the point where Mosinee Paper could readily comply with permit limits. Subsequent demonstration studies, including extensive mixing zone and hydraulic modeling, along with a commitment by Mosinee Paper to relocate and modify their outfall will further increase chronic dilution credit to about 40:1, and through the use of a diffuser, provide an acute dilution factor of 20:l.

While the costs associated with outfall relocation are substantial (in excess of $1 million), the improved dilution credit will eliminate chemical-specific limits from the Mosinee permit and greatly enhance compliance reliability for whole effluent toxicity requirements. The costs associated with the alternative action, tertiary treatment for metals removal, would be an order-of-magnitude higher and compliance with permit conditions still would be questionable.