Publication Summary

An announced Class II inspection was conducted September 16-19, 1996 at the city of Walla Walla Wastewater Treatment Plant, located in Walla Walla County. Prior to the inspection, a new procedure of adding alum to the influent was introduced and this produced substantially different treatment conditions than were typical.

An announced Class II inspection was conducted September 16-19, 1996 at the city of Walla Walla Wastewater Treatment Plant, located in Walla Walla County. Prior to the inspection, a new procedure of adding alum to the influent was introduced and this produced substantially different treatment conditions than were typical.

Influent characteristics were generally similar to typical weak domestic influent. Better than expected reduction in BOD5 across the primary clarifiers was attributed to the addition of alum. Good to moderate reductions in TSS, BOD5, and CBOD5 were achieved across the treatment plant. Nitrification was also observed. Based on results determined during the inspection (irrigation season discharge), it was projected that the ammonia nitrogen concentration may exceed water quality criteria at the chronic boundary during the Mill Creek discharge period (winter season discharge). The 24-hour composite BOD5 concentration exceeded the irrigation season′s NPDES permit monthly average limit, but was below the permitted weekly average limit. The percent reduction in BOD5 concentration across the plant may be less than the monthly average reduction required by the permit. The pH results for all Ecology grab samples were less than the permitted minimum value. Chlorine may exceed the permit narrative limit. All other inspection results were within applicable effluent concentration and load limits stipulated for the irrigation discharge period. Conditional comparisons of results to the Mill Creek discharge permit limits showed an exceedence of the ammonia nitrogen and total residual chlorine limits. CBOD5 exactly equaled the Mill Creek discharge permitted monthly average limits. Walla Walla should ensure that all permitted parameters remain within permit limits during all discharge periods.

Whole effluent priority pollutant organics and metals concentrations were generally within water quality criteria. Conditional comparisons of inspection results to the water quality criteria found that gamma-BHC and lead exceeded the chronic criteria. Dilution at the Mill Creek chronic boundary would not be expected to reduce these concentrations to within the criteria. Walla Walla should ensure that these concentrations do not exceed water quality criteria. Agronomic analysis of effluent concentrations found that pH and adjusted sodium adsorption ratio (Rna) indicate some deleterious effects to crops and soils might be expected. Area farmers have adjudicated water rights to the Walla Walla treatment plant effluent, and it is recommended that Walla Walla evaluate the effluent′s impact on irrigated cropland. The sludge selenium concentration exceeded the pollutant concentration limit specified in the EPA standards for land application of sewage sludge. Walla Walla should ensure concentrations meet EPA standards. Ecology analysis of TSS and BOD5 splits found differences between Ecology and Walla Walla samples. Walla Walla should review sampling techniques to ensure representative samples.