One of the biggest problems faced by water treatment plants is bacterial growth, or biofouling, on reverse osmosis membranes, causing them to clog up. Traditionally, this is combated by adding environmentally harmful biocides to the water to be treated. NWTR opted for a different solution, applying its very own invention: a BODAC filter (Biological Oxygen-Dosed Active Carbon).
After ultrafiltration, the WWTP effluent passes through a BODAC filter. Adding oxygen optimises the conditions for bacterial growth, to the point that they will use up all the nutrients present in the water. Now the water has been depleted of all nutrients, bacteria will no longer be able to grow, thus preventing the accumulation of biofouling in the rest of the process. The clean water continues its way to the RO membranes. The filters are rinsed periodically to remove any excess bacteria, which are then returned to the WWTP. This process requires no chemicals at all, apart from oxygen.
The integration of an active carbon filter in a pure water plant was an experiment, but it has proven to work very well in practice. In fact, we have even learned that BODAC filters also remove pharmaceuticals from water. Since 2018, NWTR, the Vechtstromen water board and WLN have worked with Wetsus and STOWA to study the (biological) processes in BODAC filters and map them in greater detail.