Causes and Cures of Elevated BOD5 in the Secondary Effluent from a Kraft/NSSC Mill, 1997 Environmental Conference Proceedings

Boise Cascade Corporation operates a 1,200-ton-per-day bleached kraft/NSSC mill in Wallula, Washington. The mill uses a two-stage aerated stabilization basin (ASB) to treat its effluent. The ASB was originally built in 1977 and was upgraded in 1984 to meet increased mill production. Until April 1995, the ASB had run reasonably smoothly and has achieved design expectations.

In April 1995, the performance of the ASB system suddenly deteriorated, resulting in higher effluent biochemical oxygen demand (BOD5 ) loadings. The cause was initially presumed to be a sulfuric acid spill; however, instead of recovering, the ASB continued to experience low BOD5 removal efficiency.

Numerous measures were undertaken to correct the problem, including dredging the deposited solids from the both the first-and second-stage ASB, increasing addition of supple-mental nitrogen, increasing hydraulic retention time, reha-bilitating the diffused aeration system, and installing additional surface aerators in both the first and second stages.

These measures, by themselves, did not restore ASB per-formance to historical levels; thus unusually high effluent BOD5 continued. An evaluation of the fate of nitrogen forms within the ASB and a comparison of inhibited and uninhibited BOD5 test results pinpointed the primary cause of elevated secondary effluent BOD5 . It was found that nitrification was partially occurring within the ASB, creating a population of nitrifiers that were present in the secondary effluent. Nitrification con-tinued in the BODs bottle, oxidizing reduced nitrogen present both in the sample and in the dilution water.

The nitrification problem was reduced by dredging the ASB and by restricting the supplemental nitrogen feed to the AS B, which reduced nitrogen present in the ASB and resulted in fewer nitrifying bacteria and less nitrification in the BOD bottle. In this instance, a lower nutrient dose resulted in lower secondary effluent BOD5 , as measured by the uninhibited 5-day BOD test.