Technologies Available to Meet Numeric Nutrient Criteria and their Associated Impacts
Authors:
- Katya Bilyk, Joyeeta Banerjee, Leah Teuber, Tony Greiner, Paul Pitt - Hazen and Sawyer
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Figure 1: Typical Treatment Train required for NNC requirements
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Figure 2: Comparison of Carbon Footprints for Various Unit Processes Used to Treat 20 mgd of Wastewater for 1 Year
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Table 2: A comparison of overall carbon footprints associated with treating to LOCT and LOAT with the carbon footprint of LOAT calculated to be almost 40 times higher than that associated with LOCT.
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Figure 1: Typical Treatment Train required for NNC requirements -
Figure 2: Comparison of Carbon Footprints for Various Unit Processes Used to Treat 20 mgd of Wastewater for 1 Year -
Table 2: A comparison of overall carbon footprints associated with treating to LOCT and LOAT with the carbon footprint of LOAT calculated to be almost 40 times higher than that associated with LOCT.
The purpose of this paper is to use two case studies to discuss (a) strategies for complying with ultra-low nutrient limits and Numeric Nutrient Criteria (NNC) and (b) the economic and environmental impacts associated with the treatment technologies needed to meet these nutrient limits.
The first case study is from the Falls Lake Watershed in central North Carolina, which is currently in the process of passing draft nutrient rules that will require plants to meet total nitrogen (TN) levels of 1.6 mg/L starting in 2016, and 1.1 mg/L in 2030, and total phosphorus (TP) levels of 0.075 mg/L. The second case study is from Plantation, Florida, where state-of-the-art technologies were piloted to meet state-wide NNC that will require plants to meet TN limits of 0.82-1.73 mg/L and TP limits of 0.069 – 0.415 mg/L. These limits are more stringent than what was implemented in the initial round of the Chesapeake Bay Program, which has thus far required point sources to meet the mass equivalent of limits of 3 mg/L TN and 0.18 mg/L TP.
Experts generally agree the limits of conventional wastewater treatment technology (LOCT) are somewhere around 2 mg/L TN and 0.1 mg/L TP, with influent characteristics, sidestream treatment, equalization, and operator skill level being key factors in achieving these levels. To treat to beyond these limits more expensive and complex treatment schemes are needed, such as high-pressure membranes (reverse osmosis (RO) and nanofiltration (NF)), oxidation (ozone, UV, peroxide), and/or ion exchange (IX). The two case studies involved piloting and/or evaluation of the treatment processes shown in Figure 1 above.
Results
The Plantation pilot documented TN levels less than 1.5 mg/L were reliably met. The lessons that can be universally applied are to consider the speciation of nitrogen, including the role of bioavailable and biodegradable organic nitrogen, how much supplemental carbon is required, operator experience, instrumentation, reuse opportunities, and optimization. The paper will integrate discussion of these key issues with a discussion of technologies used to meet ultra low limits, including: denitrification filters, advanced oxidation, ballasted flocculation, ion exchange, and NF/RO membranes.
The economic evaluation of these treatment technologies indicates that utility rates will be severely impacted because of the higher capital and O&M costs associated with these treatment systems with the burden being shifted to the rate payers. Order-of-magnitude costs developed using pilot-scale experience suggests that to meet at effluent quality of 1.2/0.075 mg/L TN/TP respectively, total present worth costs of conventional treatment with MF/RO treatment can be almost 11 times higher compared to LOCT. Further cost analysis of these alternatives will be presented.
While RO and IX are leading candidate technologies because of their high removal capabilities, brine disposal can make these technology even more expensive in certain regions where land availability is scarce, or ocean outfalls are impractical. To meet the effluent criteria in the Falls Lake Watershed, the only suitable brine treatment alternative is zero liquid discharge, an experimental evaporation process. The cost of drying brine concentrate is also prohibitively expensive on the order of $3.00 per 1000 gallons treated. Several treatment facilities in Florida currently utilize RO treatment for water reclamation but overall it is worth mentioning that less than 1 percent of RO facilities in the world are for wastewater applications and none treat the full forward flow with peaking factors. Therefore, as an industry, there are significant challenges that will need to be solved to meet NNC.
The purpose of this paper is also to present a greenhouse gas emissions perspective of evaluating the promising technologies that can meet ultra-low nutrient limits (TN < 2 mg/L, TP < 0.1 mg/L) and their carbon footprints. With the increasing regulatory focus on capping carbon emissions, carbon footprints are necessary to be considered because of the enormous power demands associated with technologies like RO. Figure 2 compares the carbon footprints of each unit process at a representative 20 mgd BNR treatment plant with a configuration similar to Figure 1. The carbon footprint added by microfiltration, brine disposal, and RO are one to two orders of magnitude higher than for the aggregate footprint of technologies needed to treat to LOCT.
A comparison of overall carbon footprints associated with treating to LOCT and LOAT is shown in Table 2 with the carbon footprint of LOAT calculated to be almost 40 times higher than that associated with LOCT.
While NNC assures better local water quality, the burden is being shifted to the air phase with the higher percent greenhouse gas emissions from these treatment technologies. Further evaluation of the holistic environmental benefit accrued by treating to LOAT instead of LOCT is recommended.
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