Are we creating widespread greenhouse gas production during biological wastewater treatment?

Kartik Chandran (Associate Professor, EEE)

Supria Ranade (EEE)

Joon Ho Ahn (EEE)

The removal of nitrogenous compounds from wastewater streams is one of the most challenging engineering and biochemical problems facing environmental engineers and water quality practitioners.Of the different options available for nitrogen removal from wastewater streams, biological nitrogen removal (BNR) is most attractive given its low cost and environmentally benign nature.Conventional BNR is achieved by sequential nitrification and denitrification.Nitrification involves predominantly autotrophic ammonia and nitrite oxidation, by two distinct classes of ammonia and nitrite oxidizing bacteria.Denitrification involves reduction of nitrate to dinitrogen gas via several aqueous and gaseous intermediates including nitrite, nitric oxide, nitrous oxide and dinitrogen gas.As such conventional BNR strategies are operationally intensive with respect to both aeration power (for nitrification ) and electron donor (for denitrification) requirements.However, if nitrification could be controlled to achieve just ammonia to nitrite oxidation, significant reductions in operating costs could be achieved (up to 25% in aeration power and 40% in electron donor).

The Chandran group in the Department of Earth and Environmental Engineering is investigating the molecular biology and microbial and chemical ecology of wastewater treatment bioreactors engaged in BNR via partial nitrification.We have successfully been able to “engineer” microbial communities to perform selective partial nitrification, approximately 90% or higher conversion of ammonia predominantly to nitrite (90% or higher).In our system, partial nitrification is achieved by operating the bioreactor at low DO concentrations (< 1.0 mg O2/L).Additionally, the system is completely automated in terms of operation and monitoring and requires extremely low maintenance and operator attention.

While we have optimally developed a solution for controlling water pollution, we find that partial nitrification produces significant levels of nitric oxide (NO), a green house gas, which can aggravate air pollution.In our reactor, effluent NO concentrations are as high as 2 parts per million, which is lower than the permissible exposure limit of 25 ppm but considerably high than general ambient levels of 20 parts per billion. Such high production of NO full-scale partial nitrification bioreactors is therefore alarming from both a public health and environmental health perspective.Indeed, several utilities around the nation are looking to implement partial nitrification as a cost-effective BNR strategy.Our results are therefore intended to alert regulators, practitioners and health administrators against the wide-spread application of partial nitrification strategies before additional measures are adopted to counter the significant gaseous emissions from these reactors.

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