Publication Summary

A post-upgrade Class II and receiving water study were performed at the Walla Walla WTP and Mill Creek in February 1986. Equipment and process malfunctions reduced the WTP performance. Despite these problems, water quality in Mill Creek below the WTP met Class A and B criteria.

A post-upgrade Class II and receiving water study were performed at the Walla Walla WTP and Mill Creek in February 1986. Equipment and process malfunctions reduced the WTP performance. Despite these problems, water quality in Mill Creek below the WTP met Class A and B criteria. Comparisons with a 1981 receiving water survey indicated the plant now prevents instream ammonia and chlorine toxicity problems below the plant. The dissolved oxygen (D.O.) profile below the plant also improved. The WTP effluent continues to elevate nitrogen and phosphorus levels in the creek which may cause far-field eutrophication problems. The 1981 total maximum daily load (TMDL) evaluation was reviewed. Changes in plant operation and NPDES permit limits suggested by the 1981 TMDL evaluation continue to adequately protect Mill Creek against D.O. and ammonia toxicity violations. However, instream ammonia toxicity may become a problem when the plant reaches its maximum discharge capacity.