CONDUCTIVITY FOR NUTRIENT CONTROL IN CTMP WASTE-WATER TREATMENT

The control and monitoring of nutrients is imperative for the efficient biological removal of organic matter. Adding too much chemicals will not only affect operating costs, but will introduce a significant secondary pollution to the receiving waters. If the influent organic load from a mill is relatively constant, nutrients can be dosed in proportion to the flow, which is the most common control method for nutrient addition. If, on the other hand, the organic load on the treatment is varying, a more sophisticated control is necessary for optimal operation. At the 60 000 t/a CTMP-mill at Vaggeryd in southern Sweden, being taken into operation in May 1989, a two-stage activated sludge plant with pre- and post-precipitation, has been installed for reducing the incoming BOD from 12 tons per day to less than 500 kilo per day. The results normally show a BOD- and COD- reduction of 96-98 % and 75-85 % respectively, or less than 250 or 1800 kg/d in the effluent. The possible use of conductivity as the measurement of the incoming organic content has been studied. For the actual wastewater, the correlation between conductivity and the COD-content in the influent has been good. As the COD in the influent can be predicted with less than 20% deviation by the use of conductivity measurement, the dosage of nutrient can be easily and reliably controlled.