Long term trials with membrane bioreactor for enhanced wastewater treatment
Within the project Stockholm’s Framtida Avloppsrening (SFA, Stockholm’s future wastewater treatment), Henriksdal wastewater treatment plant (WWTP) in Stockholm, Sweden, is being extended and rebuilt for increased capacity and enhanced treatment efficiency. The new process configuration at the Henriksdal WWTP has been designed for a capacity of 1.6 million population equivalents (PE) which is about twice as much as today. The design maximum flow of the biological treatment is 10 m3/s which is equivalent to 850 MLD. In addition, the treatment process has been designed to reach low nutrient concentrations in the effluent (5 mg BOD7/L, 6 mg TN/L and 0.2 mg TP/L). The extension of the plant will include new primary treatment, new primary settlers and a new treatment step for thickening of primary and waste activated sludge. The reconstruction will include retrofitting of the existing conventional activated sludge (CAS) tanks with a new membrane bioreactor (MBR) process containing 1.6 million m2 of membrane area. Digestion of thick sludge will be done at thermophilic conditions instead of mesophilic digestion of thin sludge.
To increase the knowledge on membrane technology for wastewater treatment in Nordic conditions, Stockholm Vatten och Avfall (SVOA) decided in 2013 to conduct long-term pilot scale studies at the R&D facility Hammarby Sjöstadsverk, located on the premises of the Henriksdal WWTP. In 2017 it was decided to supplement the MBR pilot with a sludge treatment line in order to study the future digestion process. The pilot scale studies are carried out in cooperation with IVL Swedish Environmental Research Institute. This report presents the results from year 2017 (project year 4) of the pilot scale studies.
During 2017, a large focus was put on optimising the phosphorus removal, to verify the process design of the future Henriksdal WWTP in order to comply with the future effluent requirement of TP < 0.2 mg/L. The control for dosing of the precipitation chemicals was fine-tuned and the set-point for effluent phosphorus was lowered during the year. It was shown that the future effluent requirement can be accomplished, even though more chemical addition was needed compared to when operating at current effluent requirement.
Optimisation of resource consumption related to the membrane operation has also been in the spotlight during 2017. Trials to reduce the amount of scouring air used in the membrane tanks and the amount of chemicals used for membrane cleaning have been performed as well as a trial to increase the time period between recovery cleaning events. Even though these trials were not finished by the end of 2017, and will continue in 2018, indications show that there are large potential savings in both chemical and energy use when operating the membrane tanks, without risking any decrease in membrane capacity.
In order to study any possible differences in cleaning effect and membrane performance, the acid used for cleaning one of the membrane tanks (MT1), was changed, in early 2017, from citric to oxalic acid, whereas the other membrane tank (MT2) was continuously cleaned with citric acid. The results showed that the effect of cleaning with oxalic acid was at least as good as when cleaning with citric acid. Since oxalic acid is cheaper than citric acid, there is a large economic saving potential in switching to oxalic acid. Also, high phosphorus concentration peaks detected in the effluent in connection to citric acid cleaning events, was not detected in connection to oxalic acid cleaning events.
The installation of the sludge treatment line (including sludge thickening, anaerobic digestion and sludge dewatering) continued throughout the year and in September the process was started up through seeding of sludge from the Henriksdal WWTP to the anaerobic digester. By the end of 2017, the process was still in a start-up phase and a more detailed follow-up will be carried out in 2018.
A two-year long study on mapping of micro pollutants through the treatment process, such as pharmaceutical residues, micro plastics, bacteria, PFAS and chloro-organic halogens was started during autumn 2017 and the results of the first sampling campaign (out of a total of four planned campaigns, study is ending in 2019) is presented in this report. It shows that several substances are reduced in the process, but some have higher concentrations in the effluent compared to the influent, which might indicate that some substances are re-formed in the process. Further conclusions will be drawn once results from the following sampling campaigns are retrieved (in 2019).