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Can Anaerobic Digestion Coexist with Limit of Technology Nutrient Removal? A US Perspective

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The challenge of reliably achieving Limit of Technology (LOT) nutrient limits has necessitated a reevaluation of the generally accepted norms in plant design approaches and the metrics of nutrient removal process reliability. This has been reflected in the industry's continued support for research on the novel issues confronting wastewater utilities called on to achieve LOT nutrient standards. One of the areas where design and operational norms are being questioned lies in the envelope of applicability of anaerobic biosolids treatment processes. Whereas anaerobic sludge processing has been accepted as the norm for facilities treating flows greater than a few MGD, preliminary indications from facilities in the Chesapeake Bay area are that the need to achieve LOT treatment has reduced the application envelope of anaerobic digestion for biosolids processing.

Dewatering of anaerobically digested biosolids results in the generation of a significant soluble nitrogen and phosphorus dewatering liquor that needs to be handled through biological processing. With a loading that can represent 30–50% of the influent nitrogen loading to the biological unit process, the dewatering liquor can significantly reduce the processing capacity of the biological nutrient removal process and introduce additional operational complexities.

This may include the use of supplemental carbon to enhance denitrification, as the dewatering liquor will reduce the BOD to TKN ratio for the load applied to the biological reactor. The use of supplemental alkalinity may also be required as the dewatering liquor typically contains only 40–50% of the net alkalinity requirement for nitrification. The net result is a significant increase in capital improvements, operating complexity and costs associated with the liquid treatment processes.

However, at the same time an ever-morphing energy, environmental and regulatory climate is applying significant pressures towards the selection of treatment approaches that assure a sustainable long term operating vision. Anaerobic digestion of biosolids represents one vehicle towards development of a sustainable operating strategy for wastewater treatment facilities.

Coupled with an unstable landscape with regard to energy security, and biosolids reuse and disposal opportunities, drivers that are not purely economic can significantly sway the decision-making process.

Furthermore, the economics and reliability of nutrient removal in facilities practicing anaerobic digestion of biosolids is being actively modified at this time with the adoption of sidestream treatment processes for significant reduction of the nitrogen loads from anaerobic biosolids dewatering processes. As the process and economic benefits imparted by sidestream treatment become better quantified through ongoing research efforts and the analysis of the first full scale integrated sidestream treatment facilities in Europe, the economics of deployment of anaerobic digestion appear increasingly positive. The recent experiences with anaerobic de-ammonification processes such as ANAMMOX and DEMON, in particular, have served to reduce the operating burden on liquid side nutrient removal that has traditionally been ascribed to anaerobic digestion process.

This paper will use a case study from a large North American wastewater treatment plant to assess the economic and non-economic factors that drove the decision making process relating to incorporation of anaerobic digestion into the process scheme. The Blue Plains Advanced Wastewater Treatment Plant, a 370 MGD, two sludge nutrient removal facility serving the greater DC Metro area will serve as the template for the case study. The facility is undergoing a programmatic expansion that may include the deployment of a new anaerobic digestion facility with innovative sludge minimization technology, to replace the currently operated lime stabilization process. The case study will highlight the balancing of economic and non-economic factors in the decision making process, and the economic and process reliability enhancements provided by the application of advanced sidestream treatment technology for treatment of the high nitrogen recycles produced by anaerobic digestion processes. In the case of Blue Plains, provision of anaerobic digestion, when coupled with advanced sidestream treatment for nitrifier and methanol degrading denitrifier bioaugmentation, was shown to enhance nutrient removal process reliability. Coupling anaerobic digestion with a side stream process that targeted operating cost reductions (such as ANAMMOX) but would not provide significant bioaugmentation capacity did not material enhance treatment process reliability, although it reduced operating costs significantly.

This poster presentation will provide a comprehensive analysis of the factors influencing the decision-making process for the deployment of anaerobic digestion facilities, and a presentation of the re-valuation of the economics of nutrient removal that can be obtained through the integration of sidestream treatment for nitrogen removal.
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Document Type: Research Article

Publication date: 2007-01-01

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