Establishing nitrite shunt at full scale

Annual Conference

With an energy neutrality goal for both the Mangere and Rosedale Wastewater Treatment Plants (WWTPs) by 2025, Watercare has started the journey of implementing innovative techniques to existing processes in order to drive down energy consumption. A central focus of this work is on how to improve energy consumption in aeration systems, which can account for more than 40% of total site power consumption.

Mainstream nitrite shunt is an advanced concept that provides shortcut nitrogen removal in activated sludge nutrient removal processes. The objective of nitrite shunt is to minimise the production of nitrate by stopping the nitrification process at nitrite which then is converted to nitrogen gas. This provides a direct energy ‘payback’ of up to 25% and a 40% carbon requirement reduction for denitrification. It enables the opportunity to focus more heavily on carbon harvest from primary treatment processes.

At Mangere WWTP nitrite shunt process has been implemented at full scale on one of nine MLE step feed reactors (approximately 120,000 EP) followed by a roll out to three other reactors through an approach of reducing reactor sludge retention time and adjusting dissolved oxygen set points through a cascade control loop using online ammonia feedback. After trials with several variations of the control strategy, a successful method was achieved. Ammonia based aeration control was implemented in all aeration zones. Several months of results show that the nitrite shunt reactors out-perform the conventional reactors, in terms of effluent nitrogen parameters. Ammonia was on average less than 1mg/L and total nitrogen was lowered by 2.4 mg/L. Nitrite accumulation was also observed at approximately 0.4 mg/L compared to typical values of under 0.1 mg/L.

Activated sludge wasting is expected to increase by up to 28% which can be a challenge to the downstream thickening processes and needs to be investigated. No detrimental impact on sludge settlability could be observed except higher peak SVI values which could be influenced by several other factors. Using qPCR, there is evidence that the test reactor is accumulating more aerobic ammonia-oxidising bacteria (AOB) than nitrite-oxidising bacteria (NOB) with a ratio of NOB/AOB of 0.33 in the test reactor compared to a ratio of 0.8 in the control reactor. However, this was not evident in the three other reactors where the control strategy was later implemented.

The main challenges for the trial have been how to best adapt a control strategy that works with a plant designed in the 1990s for a conventional process. This paper describes the initial steps to set up the full scale trial, extension of the trial to other reactors, results and provides guidance on how to implement nitrite shunt. This application at the Mangere WWTP is gaining interest with utilities and professionals, who are keen to explore their own nitrite shunt adaptation.

This paper describes the process and challenges of implementing a full scale nitrite shunt process in one of nine step-feed reactor-clarifiers (RCs) and subsequently introducing it to three other RCs.

Conference Papers

ESTABLISHING NITRITE SHUNT AT FULL SCALE.pdf

pdf
1 MB
28 Sep 2018