EDTA-REDUKTION I EN MULTIBIOANLÄGGNING / EDTA-Reduction in a Multibio Plant

A pilot plant study with biological treatment has been carried out at Mörrum Bruk. The pilot plant was built as the future external treatment plant, a multistage biology or a so-called Multibio. The total retention time in the biological stages was slightly more than 7 hours. The volume load was about 5,5 kg COD/m3 d. The sludge content in the active sludge stage varied between 3,5 and 4,0 kg MLSS/ m3. The sludge age usually did not exceed 5 days. The plant has been loaded with bleaching filtrate and white water from a drying machine. One of- the two parallel bleaching lines is used for ECF- and TCF-bleaching, which is why incoming water is continuously shitting character.

A clear connection between accessible phosphorous and the reduction from EDTA was established. With optimum conditions the EDTA is reduced with more than 90 %. When the phosphorous is low in the untreated effluent, the EDTA-reduction is deteriorated and during some time it almost ceased completely. When the doses of phosphorous was increased the EDTA-reduction returned immediately. Even with the EDTA-reducing organisms in the sludge during the periods with low phosphorous, there was no EDTA-reduction. The tests show that current understanding concerning the connection between pH, sludge age, retention time and the reduction of the EDTA is not correct, at least not if the plant is designed as a Multibio. pH in untreated effluent was during some periods lower than 5 and the reduction has nut been influenced by short retention time and low sludge age. The COD-reduction had an average of 73% during the test runs. The sludge production has been approximately 0,1 kg MLSS/kg COD reduction.