EXPERIMENTAL-STUDY ON THE CONTROL OF NUTRIENTS IN ACTIVATED-SLUDGE TREATMENT

Laboratory experiments were conducted to determine the need for additi on of phosphorus during the treatment of pulp and paper mill waste wat ers by the activated sludge method. The study also included the testin g of different modifications of the activated sludge method (a complet ely mixed, three completely mixed reactors in series, anaerobic/aerobi c) to see how different forms of phosphorus and nitrogen (total-N, NH4 - N, NO-2 - N, NO-3 - N) are present in the influent and effluent. T he tests were conducted using waste water from two newsprint/magazine paper mills and from a bleached sulphate pulp mill. Different loadings and levels of phosphorus addition were applied. When paper mill waste water was treated at normal loading (sludge load was c. 0.3 kgBOD/(kg MLVSSd)), a small phosphorus addition was needed to secure efficient operation. The optimum BOD:P ratio was about 100:0.4, in which case th e treated effluent had a total phosphorus content of c. 0.5 mg/l (abou t 70% reduction), a soluble phosphorus content of c. 0.3 mg/l and a ph osphate phosphorus content of well below 0.1 mg/l. Larger phosphorus a dditions produced no further improvement in treatment results (BOD red uction c. 90% and COD c. 75%). Doubling the loading gave poorer result s and the situation could not be rectified by adding phosphorus. Addit ion of phosphorus was not needed when treating pulp mill waste water, as has also been found when running activated sludge treatment plants at several mills. The BOD reduction (c. 95%) was excellent under all c onditions. The COD reduction was 30-55%, AOX 30-35% and chlorophenols 90-95%. The total phosphorus content of the treated effluent was 0.3-0 .7 mg/l when no phosphorus was added. This treatment also resulted in extremely low phosphate phosphorus levels. The biosludge contained 0.5 -1.9% phosphorus, 0.5-0.8% when pulp mill waste waters were treated an d occasionally around 2% for the paper mill. The experiments showed th at it might be possible to operate the pulp mill treatment plant with even less phosphorus in relation to BOD compared with the BOD level of waste waters to which no phosphorus has been added. The mill could co nsider removing the excess phosphorus originating from lime mud neutra lization before the waste water arrives at the treatment plant. In tre ating both these waste waters there is the risk of really high phospho rus discharges if care is not taken with the phosphorus addition. A ty pical situation of this type arises if the plant is run on the old ''t extbook rule'' of BOD:P = 100:1. The nitrogen was added as urea result ing in the BOD:N ratio of 100:(2.5-4.5). Total-N in the paper mill unt reated waste water was in the range of 8.5-13 mg/l and in the effluent 2.5 - 5.0 mg/l, i.e. the removal was 55-75%. NH+4 - N in the influent was in the range of 1.5-3.0 mg/l and was totally removed in most of t he runs. The concentration of (NO-2 - N + NO-3 - N) was only 40-50 mug /l, the removal was 0-85 % depending on the conditions. The activated sludge modification ''three completely mixed reactors in series'' yiel ded the best results when all parameters were taken into account.