PILOT-PLANT EXPERIENCE WITH AN AEROBIC MOVING-BED BIOFILM REACTOR FOR TREATMENT OF NSSC WASTE-WATER

Norske Skogindustrier A.S operates Sande Paper Mill A/S, a neutral sulphite semi-chemical (NSSC) corrugating mill. The mill currently discharges 1500 m3/day of wastewater containing 36 tonnes of COD and 15 tonnes of BOD7. A pilot plant test programme to demonstrate the application of a moving bed biofilm process was initiated in 1991. The objective of the pilot plant study was to obtain high COD removal efficiency at high organic loads, i.e. 25 kg COD/m3.day. If this was possible, two unused oil tanks (2 x 1000 m3) at the mill could provide sufficient reactor volume for biological treatment of the mill wastewater. The process tested was the KMT Moving Bed Process. This process was developed quite recently and had not been tested for the treatment of pulp and paper wastewater. The process is based on using floating biofilm carrier elements with a large inside surface area. The pilot plant at Sande Paper Mill included two reactors in series and a settling tank. Each reactor bad a volume of 0.523 m3 and both were filled with elements to about 70%. The pilot plant was operated continuously for seven months. The organic load was increased in increments from about 10 kg COD/m3.d to about 65 kg COD/m3.d on the first reactor. Results show that it is possible to achieve high removal efficiency at high organic loads. At an organic load of about 25 kg COD/m3.d on the plant (i.e. 50 kg COD/m3.d on the first reactor), COD and BOD7 reductions through the pilot plant were 70% and 96% respectively. Toxicity removal was about 98% (Microtox). Sludge production was 0.2 kg TS per kg COD removed or 0.35 kg TS/kg BOD removed. The main part of the soluble COD removal took place in the first reactor where oxygen consumption was about 3 times higher than in the second reactor. Organic load of about 60 kg COD/m3.d on the first reactor is the upper limit for efficient operation of the KMT process when treating the NSSC wastewater. No clogging was observed during the test period and the process seems to be both stable and robust.