Operational results
To compare the impact of IFAS on nitrification at the WWTP, monthly average TAN effluent concentrations were compared for time periods when the plant was operating with and without IFAS. Data indicates that full implementation of the IFAS system at Plants One and Two has had a significant impact on stabilization of the nitrification process.
As can be seen from Figure 1, after installation of IFAS, fluctuations in flows did not produce the TAN concentration exceedances observed prior to the retrofit. For example, high flow events in 2005, 2008 and 2011 prior to IFAS resulted in significant wash out of MLSS, including nitrifiers. This resulted in long recovery periods (about six months) before TAN effluent concentration dropped below the effluent objective. After IFAS implementation, similar high flow events in March 2012 and April 2013 did not cause significant effluent TAN spikes.
- Figure 1 – Total ammonia nitrogen concentrations.
It is noted that a minor spike in TAN occurred in the late summer of 2013, corresponding to the time period when one aeration tank had to be taken out of service.
Other effluent indicators also validated performance of the IFAS system. Cross referencing effluent un-ionized ammonia concentrations showed a parallel decrease and stabilization of toxic un-ionized ammonia. This is to be expected since un-ionized ammonia is a direct function of TAN concentration, pH and temperature. Further validation was completed by confirming the increase in effluent nitrate levels after the IFAS implementation. As per the nitrification chemistry described earlier, nitrate levels are expected to increase as nitrification improves.
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Seasonal aggregates of the average TAN concentrations shown in Figure 2 substantiate the fact
that IFAS provides considerable advantages over a conventional suspended biomass process during the coldest seasons. For the period of January through March, effluent total ammonia levels were 30% of the comparative period when the WWTP was operating without IFAS. The impact of IFAS was less during the warmer months, with effluent TAN levels being 70% of pre-IFAS conditions.
A review of the annual maximum peak daily TAN effluent concentrations gathered from daily samples shows a similar ability of the IFAS system to attenuate peak TAN levels. As shown in Table 2, the maximum recorded TAN levels dropped by 60% after implementation of the full-scale IFAS.
Summary
Upgrading the Peterborough WWTP with an IFAS system allowed maximum use of the existing infrastructure and increased efficiency by augmenting the total amount of biomass in the system. This approach eliminated the need to build new aeration tanks and secondary clarifiers, saving some $13 million in construction costs.
Converting the secondary treatment train to IFAS has lowered average TAN to 1.25 mg/l (37% of original levels) and resulted in a corresponding reduction in un-ionized ammonia. It achieves its greatest efficiency during the coldest winter months.
Operators have noted that the ammonia reduction in the “IFAS Zone” has been approximately 80%. The food to micro-organism ratio (F:M Ratio) has changed from 0.4 to 0.28-0.30, leading to a decrease in return activated sludge. Effluent total suspended solids and carbonaceous biochemical oxygen demand have continued to be below effluent limits. Long-term effluent results demonstrate the potential for IFAS to be successfully used in retrofits to increase plant capacity, meet more stringent effluent criteria, or to increase rated capacity to maximize existing tankage volume use within the secondary treatment train.
Valera Saknenko, P. Eng., Vincent Nazareth, P. Eng., and Roberson Gibb are with R.V. Anderson Associates Limited. Patrick Devlin, HBSc., and Krista Thomas are with the City of Peterborough. This article appeared in ES&E’s January/February 2015 issue.
