By Valera Saknenko, Vincent Nazareth, Roberson Gibb, Patrick Devlin and Krista Thomas
The Peterborough Wastewater Treatment Plant (WWTP) in southern Ontario is a Class IV plant discharging to the Otonabee River. As a result of municipal growth, average capacity had to be increased from 60,000 m³/day to 68,200 m³/day, with a simultaneous improvement in effluent quality.
This plant re-rating was achieved by full conversion of the existing aeration tanks to a hybrid integrated fixed film activated sludge (IFAS) media system. Concurrent upgrades included improvements to the inlet works, construction of four new primary clarifiers, a sludge dewatering facility and a new septage receiving station.
Since September 2011, it has been operating as an IFAS plant, resulting in a number of process benefits, including lower average total ammonia nitrogen (TAN), non-toxic effluent, and greater resiliency of the nitrification process to both organic and hydraulic shock loads.
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To effectively evaluate the impacts of the IFAS system, it is important to first revisit some chemistry theory to appreciate the transformation process that occurs inside one.
The majority of nitrogen enters wastewater in the form of urea and fecal matter, and is then converted through hydrolysis to TAN. TAN (or more correctly TAN-N, which measures only the mass of nitrogen) is the sum of two molecules: NH3 (ammonia or un-ionized ammonia) and NH4 (ammonium or ionized ammonia). In essentially all solutions, including wastewater, both the ionized (NH4) and un-ionized (NH3) forms are present due to the principles of acid dissociation.
The un-ionized form is the compound that is most toxic to fish. Fortunately, for aquatic species, at pH levels in typical wastewater, NH4 is present in much greater concentrations than NH3 (100:1 ratio at pH of 7.4). However, the NH³ percentage rises with increasing pH and/or sewage temperature. This means that, for identical TAN measurements, warmer effluent results in greater toxicity due to the higher NH3 levels.
To limit un-ionized ammonia effluent concentration, TAN must be reduced through a two-step bacteriological conversion process (known as nitrification) during secondary treatment. Ammonia is first converted to nitrite (N02) using Nitrosomonas bacteria. Subsequently, the nitrite is converted to nitrate using Nitrobacter bacteria. The first process (conversion to nitrite) is the rate limiting process, meaning that nitrate is usually present in much greater concentrations. For aquatic species, the rate limiting reaction is critical, since nitrite is toxic at significantly lower concentrations.