Conductivity measurements as measure and control methode for municipal wastewater treatment

Conductivity measurements can be used to control the function of sewage treatment systems. Conductivity is a dependent of the salininity and conductivity measurements gives indirect information of the composition of wastewater, registration of ongoing reaction and precipitation, process control and for alarm. An exempel is the projects deammonification and chemical denitrification, there conductivity measurements have been used to study biological nitrogen reduction from supernatant through reduction of ammonia to nitrogen.

The figure shows conductivity in a SBR (Sequencing Batch Reactor) for biological phosphorus removal. At biological phosphorus removal the activated sludge must be exposed to two alternating conditions; an anaerobic (neither oxygen nor nitrate) environment and an aerobic. In the anaerobic phase, the microorganisms in the sludge release phosphate and counterions and the concentration of dissolved phosphate increases. During the aerobic phase, the sludge takes up phosphate and the concentration of dissolved phosphorus decreases to a lower value than at the beginning. The changes of the phosphate concentration can be followed by measuring the conductivity. 

Conductivity values have been measured at different locations in the process at two WWTP (Waste Water Treatment Plant) in the Stockholm region; Käppalaverket, Lidingö and Himmerfjärdsverket, Botkyrka. At Duvbacken WWTP in Gävle conductivity have been measured in inflow and outflow January 23, 2005 to January 22, 2007 and at Lotsbroverket WWTP in Mariehamn conductivity have been measured at inlet, outlet and after presedimantation January to September 2006.

The three WWTP with biological nitrogen reduction Käppala, Himmerfjärdsverket and Lotsbroverket showed a decrease in conductivity in the process with 21 - 28 %, while Duvbacken WWTP which don´t have biological nitrogen reduction showed no decrease in conductivity. The contribution of phosphate ions to conductivity is based on calculations of analyses of sewage at Hammarby Sjöstad about 1 %. With uptake of counter ions biological phosphorus removal can reduce conductivity with about 3 %. Ammonia nitrogen and alkalinity which is reduced at biological nitrogen removal contributes to conductivity with 33 % and 14 % respectively. The conclusion is therefore that it is mainly biological nitrogen reduction which gives decrease in conductivity in the waste water treatment process.

Levlin E. (2009) Conductivity measurements for controlling municipal wastewater treatment, International scientific seminar, Research and application of new technologies in wastewater treatment and municipal solid waste disposal in Ukraine, Sweden and Poland October, Ustron, Poland, November 23-24, 2007 paper.