Avery Carlson, PhD Student

Background:

The Traverse City Regional Wastewater Treatment Plant (TCRWWTP) was built in 2004 and is one of the first continuous-flow membrane bioreactor (MBR) plants in North America. MBRs are designed to treat wastewater, combining biological treatment with natural bacteria and physical separation of liquids and solids by membrane filtration to produce a high-quality liquid effluent that flows back to the environment. Over time, the membrane’s ability to filter degrades as material builds on the surface or inside its pores. This occurrence is called “membrane fouling”—a damaging and detrimental phenomenon that occurs with membranes that is not well understood.

 The plant at Traverse City serves both residential customers and industry, and plays an essential part in maintaining compliance with regulations that allow for the discharge of treated water to the Grand Traverse Bay in northern Michigan. In early 2011, after over seven years of successful operation, the plant experienced a sudden membrane fouling event that nearly forced the plant to shutdown, which would have created a public health crisis. Since 2011, membrane fouling events have occurred at random intervals. Plant staff have been able to manage these emergencies so far, but at great monetary cost to the plant and the city. Any failure at TCRWWTP would have harmful environmental and public health consequences, as untreated wastewater would bypass the plant and flow into the Grand Traverse Bay. 

Relevance:

Our research looked at a variety of both novel and well understood factors that could be driving the MBR fouling events. Wastewater samples were collected over many months from the treatment plant to compare periods of normal operation with periods of membrane fouling. The results showed two mechanisms contributed to the fouling. First, high concentrations of salts, including sodium and calcium, were present in levels that contributed to making the biomass in the reactor susceptible to fouling. Second, high concentrations of organic (carbohydrate-like) material were forming a dense cake on the surface of the membrane. Together, these two mechanisms caused fouling on the membranes and the ability to move water through the membranes was diminished. We suspected that certain permitted industrial wastewater sources, when discharging to the sewer at the same time, created the conditions for the fouling to occur.   The utility can use this information to create a sustainable, long-term plan involving the treatment plant, industries, and city  to prevent future emergency fouling events. Furthermore, this study provides insight that can guide the maintenance of other MBR plants in operation. 

The outcomes of this work have been published and can be accessed at DOI: https://doi.org/10.2166/wst.2020.354. The citation is: Carlson, A. L., G. T. Daigger, N. G. Love and E. Hart. 2020. Multi-year diagnosis of unpredictable fouling occurrences in a full-scale membrane bioreactor. Water Science and Technology. 82(3):524-536.