Nutrient Removal from Anaerobic Digester Side-Stream at the Blue Plains AWTP

Implementation of a novel sidestream treatment configuration will provide the Blue Plains AWTP sidestream treatment process the flexibility to operate in four different process modes including the MAUREEN, SHARON, STRASS and DEMON configurations. Each process confers specific benefits to the AWTP that will need to achieve stringent nutrient limits to support ongoing efforts within the Chesapeake Bay area. In order to maximize the operating cost savings, while also assuring that treatment goals are achieved, the MAUREEN process configuration can be deployed in two seasonal modes, providing differing levels of preferential nitritation and bioaugmentation. To maximize operating cost savings, when performance is not limited by temperature, configurations such as SHARON, STRASS or DEMON would be utilized.

Wastewater treatment plants needing to remove nitrogen are increasingly considering sidestream treatment for anaerobic digester dewatering flows as both, an economical means to reduce capital and operating costs, as well as a mechanism to fortify the performance of the main plant. The Blue Plains AWTP, is a two sludge plant, with chemically enhanced primary treatment, followed by a high rate BOD removal stage, and a separate nitrification/denitrification stage.

With a design flow of 370 mgd the Blue Plains AWTP will need to process 1.5 mgd of anaerobic digester dewatering flow. This flow will contain as much as 1500 mg/L NH4-N representing as much as one third of the biological process nitrogen load for nitrification and denitrification. Nitrogen removal is achieved by first nitrifying ammonia to nitrite or nitrate followed by denitrification of nitrite or nitrate to nitrogen gas. The air requirements for nitrification and the carbon requirements for denitrification are substantially reduced if the process is controlled to stop at nitrite. The HRT and SRT requirements are also reduced by operating the process at higher temperatures close to 35ºC, as is typically present in anaerobic digester dewatering recycles.

Sidestream treatment processes for nitrogen removal can be generally grouped into two
categories –

  1. Nitrogen elimination with bioaugmentation of the main plant process
  2. Nitrogen elimination with no bioaugmentation of the main plant process

The preferred process configuration would be dependent on the combination of loadings, treatment performance requirements and economics of treatment that will vary from plant to plant, and has been observed to vary within a given plant on a seasonal basis. Some of the modes described can be operated in parallel using multiple trains for treatment thereby maintaining overall treatment process robustness provided by the bioaugmentation processes, while also tapping into the enhanced treatment economics more readily obtained through the processes that do not provide bioaugmentation.

To be able to adjust to this range of operating requirements, the Blue Plains AWTP is actively considering four methods to reduce nitrogen processing costs by removing it from its side-stream before it reaches the mainstream process. These methods are:

  1. Bioaugmentation and enrichment of sidestream recycle (MAUREEN process)
  2. Nitritation/ denitritation of sidestream recycle using chemostat (SHARON process)
  3. Intermittent nitritation/ denitritation of sidestream recycle (STRASS process)
  4. Intermittent deammonification of sidestream recycle (DEMON process)

This paper will show how selection of the proper performance assumptions, particularly with respect to bioaugmentation and enrichment processes is key in the development of the economic model that will be used to select the appropriate process operating mode. A detailed description of each process option, energy, oxygen and carbon balance, SRT and HRT requirements, and pros and cons is discussed. The overall treatment strategy would allow DCWASA to operate two of these four process configurations in parallel using multiple treatment trains.

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