Risk-Equivalent Seasonal Discharge Programs for Ice-Covered Rivers

Abstract

Seasonal waste discharge (SWD) programs allow different waste-discharge rates during different seasons of the year and reduce the cost of waste treatment. The efficiency of such programs for managing biochemical oxygen demand (BOD) and dissolved oxygen (DO) may be affected by reduced reaeration and slowed BOD decomposition on river systems that have ice-cover conditions. This paper presents a modified SWD program for managing BOD and DO in river systems that have ice cover during certain periods of the year. The program designs a set of seasonal uniform treatment removal levels such that the average percent removal over the year is minimized and the risk of water-quality violation is equal to that which would occur under a nonseasonal waste-discharge program. The uniform treatment levels during the ice-covered period are evaluated by simulating water quality based on reaeration coefficients that are nearly zero. Application of this program to a section of the St. John River indicates that the treatment levels for the ice-covered season are not the highest treatment levels required during the year. This suggests that the water-quality responses resulting during the ice-cover period are not the most critical conditions for the St. John River. A sensitivity analysis shows that such results occur even when the BOD decay rate is increased to a high value during the ice-covered period. Evaluation of two- and four-season SWD programs for the St. John River indicate that in the two-season case there is no advantage to separating the ice-covered period from other times of the year and that only marginal savings can be achieved by using a four-season program instead of a two-season program.