NYCDEP Reassesses Its Nitrogen Management Program

by Alfonso Lopez and Diane Hammerman

The New York City Department of Environmental Protection (NYCDEP) owns and operates 14 wastewater treatment plants in New York City. Five of these plants are in the process of being upgraded to provide for nitrogen removal. These plants include the four Upper East River (UER) water pollution control plants (WPCP), Wards Island, Hunts Point, Tallman Island, and Bowery Bay and the 26th Ward plant in Jamaica Bay. These five plants will undergo major reconstruction and be upgraded to step-feed biological nutrient removal (BNR) to achieve compliance with a Nitrogen Administrative Consent Order relative to nutrient removals pursuant to Long Island Sound Study goals.

The NYCDEP originally planned to upgrade all five plants according to "full" step-feed BNR designs. This program provided for extensive instrumentation and controls, and was based on very conservative parameters including: aeration design based on a peak-day load, not taking credit for denitrification, allowing for 100 percent return activated sludge (RAS), and no credit for primary settling tank removals.

In a re-examination of the basis of planning for the long range Nitrogen Management Program, the NYCDEP identified that the inherent conservatism used to develop the detailed full step-feed BNR resulted in a capital-intensive program which contributed to a cost escalation in excess of $500 million, raising the total cost of the upgrades to $3.9 billion with $1.3 billion for the BNR portion alone. A reassessment showed that a modified step-feed BNR process configuration could allow New York City to achieve the goals and limits of the consent order within the set time frames at a reduced cost, without materially changing the technological backbone on which the program was based.

The conservatism manifested itself in the design approach toward sizing and redundancy of process equipment for systems such as the return activated sludge, waste activated sludge, flow split, air supply, etc. This conservatism has now been reduced. And, in order to distinguish the more efficient design from the earlier approach, it has been identified in design guidance documents as the "advanced basic" step BNR process configuration.

Table 1 provides a comparison of full step BNR to the advanced basic step BNR process.

While reassessing the planning baseline, parallel research has developed increased confidence in separate centrate treatment both with respect to performance and to providing options for reliable operating modes.

The current nitrogen management program incorporates a revised planning criteria and the increased operating flexibility and reliability introduced by the separate centrate treatment process option. The operating modes at each facility will be determined on a plant specific basis, and seasonal process modifications will also be possible. Table 2 presents the current plan for plant process configurations for New York City's Upper East River (UER) WPCPs based on detailed analysis and performance projections.

Analysis of the updated plan as presented in Table 2 indicates that the projected performance capability of the process configuration is expected to achieve compliance with the Nitrogen Administrative Consent Order limits by the scheduled end dates. Like the original full step BNR plan, preliminary modeling results for the advanced step BNR configuration indicate that the NYCDEP will be able to achieve yet further removals with the addition of carbon.

The NYCDEP will also continue to examine emerging and innovative technologies in the industry and to test some technologies previously examined. By examining innovative technologies, the NYCDEP hopes to identify and implement additional strategies to provide the most cost-effective and environmentally sound solutions for further enhancement of water quality.

Step-feed BNR with biological treatment of centrate will continue as the backbone of the program, but the range of possible enhancement strategies has not as yet been fully evaluated. One example of the innovative technologies that the NYCDEP currently plans to evaluate on a demonstration scale is the SHARON Process at Wards Island. This is an innovative yet proven separate centrate treatment process utilized for a number of years in the Netherlands. This technology was originally brought to the NYCDEP's attention by the Nitrogen Technical Advisory Committee (NTAC), a group of preeminent scholars and researchers. The NYCDEP will continue to work with NTAC to identify a comprehensive range of innovative technologies or approaches to enhance nitrogen reductions beyond those required by the consent order.

Table 1. Summary of the components of full step BNR and advanced basic step BNR

Full Step-Feed BNR	Advanced Basic Step BNR
Install baffles and mixers	Install baffles and mixers
Flow control to each pass	Flow balancing
Automated aeration system with dissolved oxygen control	Aeration balancing with dissolved oxygen optimization
Process air blowers for 100% nitrification of maximum day influent total nitrogen load (N + 2)	Process air blowers based on existing footprint, credit for denitrification, assimilation, primary settling tank attenuation, and refractory total nitrogen
2 switch zones and 1 pre-anoxic zone per pass	1 or 2 switch zones and 1 pre-anoxic per pass
RAS system for 100% design dry weather flow (N + 2)	RAS system based on existing footprint and solids flux analysis/modeling results
Settled/unsettled centrate to pass "A" for WPCPs with dewatering facilities	Separate centrate treatment in existing aeration tanks (except Tallman Island)
Froth control systems (hoods, surface wasting, RAS chlorination)	Froth control systems (hoods, surface wasting, RAS chlorination)

Table 2. UER WPCP step-feed BNR with biological treatment of centrate configurations

Water Pollution Control Plants	Step BNR Upgrade Configuration	Centrate Treatment Options
		Selected Centrate Treatment Mode	Process Options
Hunts Point	Full step BNR	Centrate to pass "A"	Separate centrate treatment
Wards Island	Advanced basic step BNR	Separate centrate treatment in two existing aeration tanks	Centrate to pass "A"
Bowery Bay	Advanced basic step BNR	Separate centrate treatment in an existing aeration tank	Centrate to pass "A"
Tallman Island	Advanced basic step BNR	Centrate to pass "A"

—Alfonso Lopez and Diane Hammerman

Back to top

If you have experienced any difficulty with the website, please email the web master.