Wet Weather BNR Operations in New York City

Authors:

  • S. D. Galst, R. Sharp, P. Pitt - Hazen and Sawyer
  • K. Mahoney, A. Deur, K. Beckmann - NYCDEP
  • M. Motyl - CH2M Hill

The New York City Department of Environmental Protection (NYCDEP) is currently under Consent Judgment to upgrade to and operate eight of their 14 Wastewater Treatment Plants (WWTPs) in the Biological Nitrogen Removal (BNR) process. These upgrades are being conducted at four WWTPs discharging to the Upper East River and four WWTPs discharging to Jamaica Bay. The BNR process selected for these upgrades is a four-pass (Passes A through D), step-feed process with anoxic/aerobic operations in each pass.

Impact of Wet Weather on BNR Treatment Process:
The BNR treatment process is reliant on the biological population in the aeration tanks; without sufficient nitrifying biomass, the nitrification process will suffer and effluent nitrogen concentrations goals are compromised. During wet weather operations, the flow to the Aeration Tanks increases by as much as two to three times the typical dry weather influent flow. If not well managed, increased flows can result in a higher solids loading to the Final Settling Tanks (FSTs), which can lead to FST overload and solids washout. This problem is two-fold, in that effluent TSS limits may be exceeded by the high solids loading rates to the FSTs, and vital biomass for the nitrification process is lost. Strict limits on effluent Total Nitrogen discharges requires that a wet weather strategy be used to avoid process upsets that would otherwise lead to permit violations.

Wet Weather Treatment Implementation:
In order to avoid solids losses during wet weather events, a Wet Weather Operating Plan (WWOP) has been developed that targets the re-direction of PE flow to later passes in the aeration tank during wet weather events. By increasing more flow through Pass D, the nitrifying solids inventory can effectively be stored in Passes A and B, and the plant can still achieve a contact-stabilization measure of treatment during the storm. Additionally, the dilution effect of increased flows to Pass D reduces the Aerator Effluent Mixed Liquor Suspended Solids (AEMLSS) concentrations, thereby reducing the solids loading rates to the FSTs.

Testing of the Wet Weather Strategy:
Implementing the WWOP involved the determination of Primary Effluent (PE) gate settings to achieve the desired flow distribution that would enable the proper storage of solids in Passes A and B. In order to determine the optimal gate settings for wet weather operations, both modeling and in situ testing were conducted at the NYCDEP’s Battery E demonstration facility, located at the Wards Island WWTP. Battery E is a 25 MGD high-rate, 4-pass step-feed BNR aeration tank built in order to demonstrate BNR operational strategies. Battery E has flow measurement on the PE gates for each pass, and is also equipped with a dedicated wet weather “bypass gate” at the head of Pass D that can be opened during wet weather events to direct more flow to Pass D.

The in situ WWOP testing was conducted over the course of storm event whereby the Pass D bypass gate was utilized in two different configurations.
Strategy 1: Pass D Bypass Gate open 35%, Pass A Gate throttled to 33%
Strategy 2: Pass D Bypass Gate open 100%, Pass A Gate throttled to 33%

Application of Wet Weather Strategy to BNR WWTPs:
Unfortunately, not all NYCDEP BNR WWTPs are equipped with the level of PE gate control and flow measurement that Battery E has. As such, the gate settings must be calculated by hydraulic analysis to provide the plant with setting which will allow for optimal flow distribution during wet weather events. A full hydraulic analysis was conducted for the Bowery Bay WWTP, recently completed with BNR construction and currently operating in BNR mode, to provide gate settings for wet weather operation to achieve flow distributions similar to those observed during the Battery E testing. Also, CFD modeling of the hydraulics within the plant was conducted to ensure accurate flow measurement was available during wet weather. With these settings and measurement techniques, the plant will be able to program settings that, once triggered by an increased influent flow reading, will adjust the PE gates automatically to their optimal wet weather position. This will ensure that the solids inventory will be maintained and the FSTs will not be overloaded with solids.

For more information, please e-mail the author at sdailey@hazenandsawyer.com.

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