Nutrient-limited biofilm pretreatment: an efficient way to upgrade activated sludge plants.

Application: By applying the principles described in the paper, mills may improve their effluent treatment considerably.

This study evaluated biological treatment of bleached kraft mill effluent in a nutrient (phosphorus) limited moving bed biofilm process followed by an activated sludge process on a pilot plant scale. Operating the process under phosphorus limitation with an influent BOD7:N:P-ratio of 400:10:l did not compromise organic removal. It resulted in excellent sludge settleability and reduced sludge production. At hydraulic retention times of 2.4-3 h in the biofilm process and 6.88 h in the activated sludge plant, the nutrient limited biofilm/activated sludge system removed 98% of BOD7 and 70%-80% of COD. The sludge volume index was in the range of 40-120 mL/g and the effluent TSS was around 20 mg/L. The biosludge yield was as low as 0.09 kg TSS/kg COD removed (0.17 kg TSS/kg BOD removed). Due to nutrient limitation, low discharge levels of nutrients could be achieved, around 0.3 mg/L of total P and 4 mg/L of total N. EDTA was degraded by 95% in the process.

As a consequence of nutrient (phosphorus) limitation, the biomass produced in the biofilm process became slimy. However, the excess sludge from the biofilm process was rapidly degraded in the subsequent activated sludge plant, explaining the low overall sludge yield in the process. The activated sludge was dominated by large compact flocs without any slime content. The results of the study indicate nutrient-limited biofilm pretreatment is an efficient way of decreasing the sludge yield and securing good sludge settling characteristics in activated sludge plants.

Welander and Olsson are with ANOX AB, Klosterangsvagen 11A, S-226 47 Lund, Sweden; Fasth is with S6dra Cell AB, Morrums Bruk, S-375 86 Morrum, Sweden. Email Welander at [e-mail].