Wastewater treatment with nanofiltration in pilot-scale

Sammanfattning: During the spring semester 2018, an investigation about supplementary cleaning of the waste water at Dow Chemical site in Landskrona was done. Different water-based binders from acrylic monomers are produced by Dow and large amounts of water is used for such processes. The wastewater, that is treated currently by ultrafiltration (UF), contains different chemical residues such as surfactants, residual monomers and polymers. The UF content also varies from batch to batch because of different products being produced at Dow Landskrona. From UF, permeate and concentrate are produced. The concentrate is collected and sold as a product known as (TN-1), while the permeate should be approved before sending it to the municipal wastewater plant (WWTP). From both economic and environmental perspectives, the UF permeate should be filtered further by nanofiltration (NF) to produce sufficient clean water to be reused. In this study, a pilot plant with NF90-4040 membrane from Dow Filmtec was used to investigate the nanofiltration. The UF permeate from Dow was filtered further to produce NF permeate and concentrate. The operating pressure was 5 and 10 bars and the temperature varied from 33 to 38˚C, while pH was in the range of 8 and 10. The conductivity and pH was measured for UF permeate, NF permeate and concentrate at Dow. The flux with different volume reduction (VR) was also measured. Bacterial growth analysis was performed in Dow, while total organic carbon (TOC) and molecular mass analysis for the NF permeate and concentrate were performed in Lund university. After 12 batches performed at Dow, the conductivity and TOC values in the NF permeate were still higher than in the city water. Therefore, RO membrane is recommended to be used as a second stage after NF to reduce the conductivity and TOC. A screening of flat sheet membranes was performed at Lund university with three different RO membranes. XLE and SW30-HR from Dow Filmtec and RO98 pHt from Alfa Laval. The XLE membrane was chosen further because of good retention. The NF permeate had also a smell issue which also had been solved by trying different methods. Active carbon column was the suitable one which already exists in Dow. For the full scale, six NF and three XLE membranes were determined to handle the wastewater. The full-scale should be able to handle even the worst-case scenario at Dow. NF membranes are effective to produce clean water and the pure water flux (PWF) was obtained after each chemical cleaning. Yet another RO membrane stage is required to meet the water specifications used in the process, especially when the UF permeate is considered as a worst-case. A cost estimation was also conducted for full scale in Dow Landskrona. The investment cost was estimated to be 1.9 million SEK and the operating cost at 200 000 SEK per year. During the payback period, the cost of treated wastewater costs 28 SEK/m3 and after the payback period the cost of the treated wastewater is reduced to 11 SEK/m3. Chemical precipitation was also tested as an alternative method for the NF concentrate treatment. Jar tests were performed to conclude which chemicals to be used. The test has showed that it is possible to precipitate the surfactants and to be removed from the NF concentrate. Further studies need to be done for larger amounts to decide whether this method is suitable or not. Keywords: wastewater; nanofiltration; ultrafiltration; chemical precipitation; pilot-scale.