A Salty Diet – How Much is Too Much for the Local WWTP?

Membrane treatment of water and wastewater is growing throughout the world, and at an ever increasing rate. Understandably, the focus for most utilities is on the benefits of the product water (permeate), but the system sidestreams (concentrates) bring their own special set of issues. Several types of membranes are used in water and wastewater treatment, and the sidestream from each type contains pollutants in different concentrations.What impacts do these pollutants have on the wastewater treatment plant (WWTP) and the collection system and how does a system operator assess these impacts? Recent work to identify and assess possible impacts conducted by the authors for the WateReuse Foundation revealed that indeed impacts can be quantified and that a "salty" diet possibly can be a problem for POTWs. The paper briefly reviews the supporting data from a literature search and discusses the field data collection methods (user web-based survey). A highlight of this work was a number of utility partner interviews where information was gathered on issues such as corrosion; recycling and concentration of pollutants in the wastewater; process inhibition; WWTP effluent quality; and impacts of the pollutants on the receiving streams.The work also identified many of the water balance impacts that can significantly increase the concentration of residuals entering a wastewater system creating the salty diet. For example, many times water utilities sell membrane treated water to outside customers who do not return it as wastewater to the utility's collection and treatment system, which creates an imbalance that, may cause additional stress on both the wastewater treatment plant and the receiving stream.Based on the data collected and the discussions with the utility partners, this paper summarizes the findings and discusses the POTW impacts of a salty diet. What are the ranges that were found that might impact the systems, how have operators coped with changing conditions, and what does the future look like as more and more membrane discharges find their way to POTWs? A systemwide pollutant mass balance modeling tool developed during the research work will also be highlighted as a means for predicting solids loadings.