Resource Efficient Microbial Bioprocesses for Shortcut Nitrogen and Phosphorus Removal from Wastewater

Roots, Paul

doi:https://doi.org/10.21985/n2-18ss-k286

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Resource Efficient Microbial Bioprocesses for Shortcut Nitrogen and Phosphorus Removal from Wastewater

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Excess loading of reactive nitrogen and phosphorus into the environment from human activities has resulted in widespread eutrophication and the degradation of surface water quality and wildlife habitat. Wastewater is the dominant point source of nutrient loading into waterways, and thus represents a critical opportunity for treatment and prevention of downstream pollution. However, conventional biological processes for nutrient removal from wastewater are energy intensive; wastewater treatment currently consumes 3% of the total electrical energy demand in the United States. Emerging processes in biological wastewater treatment include energy saving methods for shortcut nitrogen removal such as anaerobic ammonia oxidation (anammox) via the partial nitritation/ anammox (PN/A) process, and the nitritation-denitritation process, both of which require less organic carbon and aeration than conventional processes. Application of these processes to mainstream wastewater, subject to low temperatures and fluctuations in flow due to wet weather events, remains a challenge due to the growth of undesirable nitrite oxidizing bacteria (NOB) at low temperatures and the low growth rate of anammox. My research focuses on the integration of shortcut nitrogen removal processes with biological phosphorus removal (which can facilitate phosphorus recovery via struvite precipitation) in the challenging environment of mainstream wastewater. I have investigated two treatment trains; a one-stage process (single reactor) and a two-stage process (separate reactors for nitrogen and phosphorus removal), both of which demonstrated robust combined nitrogen and phosphorus removal at lower energy demand than conventional processes. The first treatment train, a single sludge method for combined shortcut nitrogen, phosphorus and carbon removal, was the first to demonstrate the compatibility and consistent performance of nitritation-denitritation and biological phosphorus removal at the moderate mainstream wastewater temperature of 20 °C. Shortcut nitrogen removal likely improved biological phosphorus removal performance due the increased nitrogen removal (due to lower carbon requirements) and resulting lower concentrations of oxidized nitrogen recycled to the anaerobic zone. Another important finding of this study was the contribution of organic carbon to the suppression of NOB, namely by providing a nitrite sink through denitritation in anoxic zones. The second treatment train was a two-stage process, and the A-stage comprised a high rate biological phosphorus reactor for combined phosphorus and carbon removal. This reactor advanced understanding of low solids retention time (SRT) biological phosphorus removal systems by being the first to demonstrate a washout SRT for Candidatus Accumulibacter polyphosphate accumulating organisms (PAOs) in real mainstream wastewater. Moreover, it was the first to investigate their diversity in low SRT systems via the clade-level dynamics of Accumulibacter. Accumulibacter clades IIA, IIB and IID dominated the PAO community at the lowest SRT values, while clades IA and IC were washed out, suggesting that certain clades may have higher growth rates and thus be better adapted to low SRT operation. An integrated fixed-film activated sludge system comprised one of the B-stage reactors of the second treatment train and demonstrated the robust nature of anammox biofilms and their long-term compatibility with low temperature, low concentration environments such as mainstream wastewater. The challenge of NOB out-competition in mainstream deammonification was clearly illustrated in this process, which limited nitrogen removal due to excess nitrate production. Nitrogen removal dramatically improved by rerouting 10% of the influent flow around the A-stage reactor, thus increasing the influent sCOD-to-ammonia ratio by 35%. This provided an additional nitrite sink via denitrification (the other being anammox) that, like the one-stage nitritation-denitritation reactor discussed above, aided in NOB out-competition and contributed to total nitrogen removal. This reactor illustrated the critical role of organic carbon in mainstream deammonification, which lacks the usual selective pressures for AOB over NOB that exist in the sidestream (high temperatures and elevated free ammonia). A parallel B-stage reactor for the second treatment train also demonstrated the difficulty of NOB out-competition in mainstream deammonification, but also led to the first clear demonstration of Nitrospira comammox as the dominant ammonia oxidizers in a wastewater bioreactor. This discovery suggested that the very conditions of mainstream deammonification – low temperatures, long solids retention times, low dissolved oxygen and ammonia concentrations – may inadvertently select for comammox. Comammox seem to offer a relatively low-energy path for nitrification due to their success in this low dissolved oxygen reactor and may even be compatible with anammox given the increasing research into denitratation-anammox. In all, these results suggest that the application of shortcut nitrogen removal technologies coupled with biological phosphorus removal holds potential for decreasing the carbon and ecological footprint of wastewater treatment.

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