Pilot-Scale Investigation to Achieve Very Low Nitrogen and Phosphorus Effluents by Retrofitting a University of Cape Town (UCT) Process

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There are technologies for achieving very low nutrient levels during wastewater treatment, but since few facilities are currently operated to simultaneously remove nitrogen and phosphorus to very low levels, information is scarce on what effluent concentrations can be reliably achieved. We performed a pilot-scale study to evaluate reactor configurations and the amount of chemical additives needed to simultaneously achieve very low total nitrogen and total phosphorus levels in discharged effluents. Using a UCT-type process as a starting point, we modified the plant configuration to evaluate alternatives such as aerated-anoxic operation with step feeding for maximizing denitrification, and optimization of simultaneous nitrogen and phosphorus removal with methanol and alum as chemical supplements. Filtered and unfiltered samples were collected and analyzed to simulate the effect of filtration as a tertiary treatment.

In the absence of an external carbon source, it was difficult to substantially lower total nitrogen in the treated water. With methanol as an external carbon source, we were able to reach a total nitrogen concentration as low as 2.1 mgN/L. The distribution of nitrogen species in filtered effluents showed that the two largest contributors to the remaining nitrogen were nitrate and residual organic nitrogen. Phosphorus removal by a combination of biological and chemical processes, followed by filtration allowed reaching total phosphorus levels in the filtered effluent consistently below 0.4 mgP/L, with the best performance achieving a total phosphorus of 0.06 ± 0.02 mgP/L when there were high levels of alum, methanol, and internal recycle. Measurements of orthophosphate and total phosphorus in filtered effluents revealed that the majority of the phosphorus remaining in the effluent was not orthophosphate, but a residual fraction of non-readily reactive phosphorus.
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