Assessing the Impacts of Membrane Residuals on Wastewater Treatment Facilities and Effluent Quality


Given the benefits derived from membrane treatment of water and wastewater, interest in using membranes is growing throughout the world, and at an ever increasing rate. As expected, the focus has been on the benefits of the product water (permeate) side of the system, but the system sidestreams (concentrates) bring their own special set of issues. As with any new technology or application, there are benefits and liabilities. Membrane separation is a non-destructive technology: the membranes merely concentrate the rejected pollutants in a sidestream. 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? This paper provides an overview of a completed WateReuse Foundation Research project to asses these impacts. The paper briefly reviews the literature search, data collection methods (user web-based survey), utility partner interviews, and a review of 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 research also identified many of the water balance impacts that can significantly increase the concentration of residuals entering a wastewater system: many times water utilities sell 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.

To account for all these variables, two models were developed. Black & Veatch’s research partner, the University of North Carolina at Chapel Hill (UNC) developed a model to predict the concentration of specific chemicals in concentrates reaching a WWTP. Black & Veatch developed an over arching system wide mass balance model. The B&V model comprises all of the elements of separation processes that account for generation of the concentrate at the source and all of the elements of separation processes that account for transport/fate through the WWTP. The result is a model to predict the concentration of components in the effluent at the WWTP and in the waste solid streams. Multiple concentrate sources can be included. Both models are needed for the guidance manual that has been prepared as part of this project. There are points of entry issues/ impacts from the membrane reject streams and there are more global quality issues/impacts into and out of the WWTP.

The Guidance Manual, to be made available by the WateReuse Foundation in 2006 provides an overview of this work and detailed guidance on the use of the models including tips on how to collect necessary input data and how to structure the model for a specific community’s collection and treatment system(s).

In short, this paper presents a summary of the just completed WateReuse Foundation (WRF) study titled Impacts of Membrane Residuals on WWTPs.

The demand for clean water is increasing worldwide, and available clean raw water supplies are dwindling. As a consequence, water treatment technology has had to evolve to meet the challenge of producing clean water for domestic and industrial uses from marginal to poor supplies. What are some of the characteristics one might normally associate with a poor raw water quality? Such a water supply would usually have an elevated total dissolved solids (TDS) concentration, among other things. Among the many constituents of a typical water supply, TDS has always been one of the more difficult and expensive to remove. In recent years, the growth of reverse osmosis as a treatment option has opened a new dimension in water treatment. Membrane treatment is gaining popularity in both private and public sectors and is being applied to both small and large flows. It is the growth of membrane treatment, and the subsequent increase of membrane concentrate disposal, that was the driver for this research project.

Continued research into membrane technology has led to the development of a number of different applications, which include reverse osmosis (RO), nanofiltration (NF, or low pressure RO), ultrafiltration (UF), and microfiltration (MF). While each of these systems targets different materials for removal, they are all alike in that they do not destroy any pollutants; they merely accumulate the pollutants into a concentrate or brine sidestream, with each membrane system
producing a sidestream with unique characteristics.

What happens to that sidestream of rejected material? Where does it go? Typically, the residuals are discharged to the utility’s wastewater collection system and ultimately end up at the local WWTP. The materials carried in the brine streams are typically not encountered in such high concentrations. At such elevated concentrations, do they have any adverse impact to the POTW, which includes the WWTP and the collection system? This research project, funded by the WateReuse Foundation, was established to ascertain the answers to just such questions, based on current data to be acquired from a variety of different utilities.

The project, five major components that included:

  • A literature review;
  • A structured survey of utilities;
  • Contacts with a variety of utility partners to gather data on their experiences with brine;
  • The development of models to assess point source and system-wide impacts of high TDS discharges to the system; and
  • The development of a guidance manual to help utilities utilize the developed models and set of data gathering mechanisms for themselves.

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