Long term experiences of sequencing batch reactor-system and wetland treatment from a municipal wastewater treatment plant in Sweden, operated with low temperature wastewater

A biological wastewater treatment plant, Nynashamn treating municipal wastewater and septic sludge operated with a combination of sequencing batch reactor (SBR) units and constructed wetland is presented in this paper. The plant has to treat low temperature wastewater in winter time, still with demands for a biological nitrogen removal. Treatment results from a 13 year operation period are presented. Special attention was given to the nutrient removal during low temperature conditions. The combination of a SBR system along with classical chemical precipitation and a polishing step based on 'natural' extensive treatment has been a sustainable way to keep the discharge levels low. The combined treatment with SBR and the wetland at the Nynashamn plant has resulted in improved discharge levels typically as follows (annual mean values); BOD7 3 mg/l, to be compared with the formal consent value of <15 mg/l, total P < 0.1 mg/l, to be compared with the formal consent value of <0.5 mg/l and total N 7 mg/l, to be compared with the formal consent value of <15 mg/l. It is also important to underline that the change of process train has resulted in a substantial saving of the precipitant agent for phosphorus removal. The needed dosage is now 50% of the previous dose, before the implementation of the SBR-units.