Successfully Reducing Effluent Total Nitrogen using Conventional Nutrient Removal Strategies

Authors:

  • Joe Rohrbacher, Katya Bilyk, Paul Pitt, Ronald J. Latimer, and Rosalyn Matthews - Hazen and Sawyer

Introduction
Municipal wastewater treatment facilities (WWTFs) are under increasing regulatory pressure to reduce effluent total nitrogen (TN) to very low levels. Many facilities are now required to remove TN to 3 mg/L, and an increasing number of facilities are faced with more stringent effluent limits. Conventional technologies (enhanced nutrient removal and/or denitrification filters) can typically reliably remove TN to 3 mg/L, and many facilities have documented performance achieving effluent TN concentrations down to 2 mg/L. In order to reliably achieve effluent TN concentrations below 2 mg/L, tertiary membrane processes including reverse osmosis (RO) have been proposed. The implementation of RO to remove nutrients results in an order-of-magnitude increase in the greenhouse gas emissions per mass of additional nutrient removal and a 50% increase in net present capital and operating costs in comparison to conventional nutrient removal technologies (WERF, 2011).

The purpose of this paper is to document the performance of several WWTFs that are achieving effluent TN concentrations of less than 2.5 mg/L with the use of conventional technologies. A minimum of six case studies will be presented representing nearly 30 years of historical operation. The operational characteristics of these facilities including influent wastewater composition, solids handling processes, and nitrogen removal performance will be evaluated and discussed in order to identify the drivers and barriers to achieving TN reduction towards 2 mg/L and below through the use of conventional technologies. This information will be beneficial to utilities facing effluent TN limits below 3 mg/L and provide a potential framework for sustainably and cost effectively reducing effluent TN to 2 mg/L or below.

Results and Discussion
Plant A is a 9.5 mgd biological nutrient removal (BNR) facility using a two-stage Modified Ludzak Ettinger process followed by denitrification filters. Effluent TN averages 2.1 mg/L at a current average flow of 3.3 mgd. Plant B is a five-stage enhanced nutrient removal (ENR) plant followed by denitrification filters rated at 60 mgd. The average effluent TN concentration from Plant B is 2.3 mg/L. Plant A has fairly consistent levels of effluent ammonia, organic nitrogen, and nitrate and nitrite (NOx-N). Plant B has variable effluent NOx-N and consistently low ammonia and organic nitrogen.

Plants C and D are five-stage ENR processes followed by denitrification filters. Each plant is rated at 6 mgd. Plant C has achieved monthly average effluent TN concentrations below 2 mg/L since June 2011, and the average reported effluent TN concentration from Plant D since October 2010 is 1.3 mg/L. Wastewater sampling performed at Plants C and D indicates a favorable carbon to nutrient ratio for nutrient removal. The sampling at Plant C also indicated that solids recycle streams may be a minor additional nutrient load to the BNR process.

Plant E is a 40 mgd five-stage ENR facility which currently achieves effluent TN concentrations of approximately 2.1 mg/L with no supplemental carbon addition. Plant F is an 18 mgd facility that utilizes a three-stage BNR process followed by denitrification filters to reduce average effluent TN concentrations to 2.1 mg/L. Plant E and F are both located in Florida.

One common characteristic of these plants is reliable and stable full nitrification, and organic nitrogen and NOx-N comprise the majority of the remaining effluent nitrogen. Percentile plots of effluent TN data for Plants A, B and C are very similar between plants, with a median effluent TN concentration between 1.9 and 2.2 mg/L. The 90th percent effluent values were between 3.0 and 3.8 mg/L.

In conclusion, many wastewater treatment facilities have the ability to optimize their conventional enhanced nutrient removal/denitrification filter process to reduce effluent TN well below 3 mg/L and in some cases, achieve effluent TN concentrations of less than 2 mg/L on a regular basis. Influent wastewater characteristics, solids handling operations, inert organic nitrogen, supplemental carbon feed and solids removal will impact level of TN removal achievable. Since the addition of tertiary membrane processes such as RO results in increased capital and operational costs and order-of-magnitude greenhouse gas emission increases for limited, if any, environmental benefit, it is incumbent on the regulatory agencies to work with the municipal wastewater sector in setting realistic and sustainable effluent nutrient limits.

References
Water Environment Research Foundation (2011) Striking the Balance between Nutrient Removal in Wastewater Treatment and Sustainability (NUTR1R06n); Alexandria, Virginia.

To request a copy of the full paper, contact the author at jrohrbacher@hazenandsawyer.com.

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