The Basic Process of Nutrient Removal

Successful wastewater treatment is dependent upon a healthy population of micro-organisms to consume organic materials. These micro-organisms can be found in the various treatment stages including the bioreactor and digester.

The Bioreactor

In 1996, the City of Saskatoon enhanced its wastewater treatment process with the addition of a Biological Nutrient Removal (BNR) system.   This BNR system uses micro-organisms to naturally remove unwanted nutrients from wastewater. It uses several different microbial processes to produce a nutrient-reduced final effluent.

The BNR process utilizes three conditions to remove phosphorous, organic compounds, and nitrogen. These conditions are aerobic (oxygen rich), anoxic (bound oxygen), or anaerobic (absence of oxygen). Each condition serves a specific purpose. There are approximately 4 main species of microorganisms working to break down the wastewater in the system. Moving from an aerobic to an anaerobic area of the treatment process the metabolic activity of the microorganism changes to remove phosphorous. The same microorganism in a different area of the treatment process with different oxygen conditions will remove BOD (biochemical oxygen demand) and break down solids.

The end result is a final effluent from which 95% of the BOD and TSS (total suspended solids) and over 90% of the phosphates have been removed. The introduction of the BNR process has resulted in a significant improvement in our effluent discharge and also to the environment of the South Saskatchewan River.

The Digesters

Throughout the entire treatment process, settled solids are removed and sent to large anaerobic (no oxygen) tanks called digesters. Here, microorganisms consume the organic compounds leaving behind high quality biosolids, rich in phosphorous and nitrogen. The biosolids are pumped north of the city and stored in large asphalt-lined cells until it can be used by local farmers to fertilize their fields. During the digestion process methane gas is produced in the digesters as a by-product. The gas is captured and used to heat the digesters and parts of the plant during the winter months.