Florida’s Numeric Nutrient Criteria & the Potential Importance of Dissolved Organic Nitrogen (DON)
Authors:
- Rosalyn Matthews, Robert Sharp, Paul Pitt - Hazen and Sawyer
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Table 1: Summary of Florida’s Pending Numerical Nutrient Standards
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Table 2: Effluent Nutrient Levels and Significance of DON at Various Florida Plants
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Table 1: Summary of Florida’s Pending Numerical Nutrient Standards -
Table 2: Effluent Nutrient Levels and Significance of DON at Various Florida Plants
In January 2009, the EPA declared that Florida’s existing, and long-standing, narrative nutrient discharge criteria were insufficient to protect water quality. EPA entered into a legal agreement in August 2009 with environmental non-governmental organizations to propose numeric nutrient standards. Subsequently, the EPA proposed numeric nutrient criteria for lakes, streams and springs in January 2010 that are to be finalized in Oct 2010. In addition, the EPA will release numeric nutrient criteria for estuaries, coastal waters, and South Florida flowing waters (including canals) by November 2011, and finalize by August 2012. A numeric nutrient standard is defined the maximum nitrogen and/or phosphorus concentration in a water body that will permit that water body to maintain its designated use. The EPA claims that “Numeric criteria provide a target for restoring waters to a healthy condition and a target to limit nutrient sources before problems start so that currently healthy waters do not become impaired.” Table 1 summarizes the EPA proposed numerical nutrient criteria for different zones in Florida.
The EPA has proposed numeric total nitrogen and total phosphorus criteria for streams and lakes in four of the zones (Panhandle, N. Central, Peninsula and Bone Valley), and a nitrate/nitrite nutrient criterion for all springs. The EPA established stream criteria based on a “reference site” approach where 25% of reference waters would fail to meet proposed nutrient criteria and would be classified as impaired. The criteria are not based on causal relationships between nutrient concentrations and biological responses in streams and may lead to impairment designations to ‘recover’ water bodies to nutrient levels they would not naturally meet. The Florida Department of Environmental Protection (FDEP) estimates that 35% of Florida’s most pristine surface waters will fail the EPA’s proposed in-stream criteria. In addition, the EPA has pending nitrogen criteria for estuaries which may lead to downstream protection values that are even more restrictive than the proposed in-stream criteria. The EPA proposed baseline nutrient criteria for lakes vary based on maximum Chlorophyll A levels, lake color level and alkalinity (alkaline or acidic lakes).
The EPA proposed criteria are expressed as an annual geometric mean not to be exceeded more than once in a 3 year period. The long term average of annual average geometric mean values also must not exceed the criteria. The inherent variability of lake and stream nutrient levels may lead to exceedances of the standard due to natural conditions or from variability of sampling procedures. In Florida an estimated 93 wastewater treatment plants have direct or indirect surface water discharges to oceans, while approximately 42 have discharges to fresh water bodies. A majority of these facilities will be impacted by the EPA proposed nutrient criteria.
Significance of Dissolved Organic Nitrogen
The profound and significant changes in Florida’s nutrient regulations will impact many municipalities, requiring significant treatment plant upgrades to either meet the new regulations or to provide high quality effluent for reuse applications. BNR treatment plants have traditionally focused on the removal of inorganic nitrogen species (NH3, NO2, NO3) to meet their effluent TN standards. However, as TN limits for many Florida plants become more stringent (<2 mg/l), the importance of dissolved organic nitrogen (DON) will become more significant. The typical effluent DON from a BNR plant ranges from 0.5 to 2.5 mg/l, which alone could exceed the TN limit for a lake or stream discharge in Florida. The technical and fiscal demands required to meet these new limits will be difficult for many plants to meet. Under this type of strict regulation, there is a growing need to properly measure and characterize effluent nutrients, and determine their bioavailability and biodegradability.
Researchers have been developing methods to determine the bioavailability of effluent DON and phosphorus and their true impact on receiving waters. Additional research has been ongoing to characterize effluent DON according to size fractionation and biodegradability to determine the sources and fates of DON in a treatment plant, and better understand the best technologies for removing different types of DON (ie. particulate vs. colloidal vs. truly dissolved). The concentration of DON in a plant effluent can be influenced by influent characteristics, plant design and operation, and environmental factors. However, there is no consensus on how to properly characterize effluent DON to allow for either the selection/design of better treatment processes to remove DON or a mechanism for regulatory relief for fractions of DON that will not impact water quality due their recalcitrance or lack of bioavailability. Similar concerns are associated with effluent phosphorous and its treatability and bioavailability.
Impact of Numerical Nutrient Regulations on Effluent DON
This paper analyzes the current effluent organic nitrogen levels from BNR plants across Florida to determine the potential impact of the new EPA numerical nutrient regulations. A selection of results are summarized in Table 2, which shows the treatment technology and receiving water for four plants, as well as their recent average effluent nutrient concentrations. The data shows that under current discharge criteria the effluent DON levels for the OCU NWRF and Altamonte Springs RWRF plants are not the most significant fraction of effluent TN. These plants could trim a significant amount of nitrogen from their effluent with relatively modest upgrades and process optimization. However, if the proposed numerical criteria are promulgated, the current effluent DON levels would become the dominant form of nitrogen in three of the plants. For the NWRF plant, the current effluent DON would actually exceed the new criteria even if they were able to completely remove all inorganic forms of nitrogen. These results demonstrate how DON can be a key factor in meeting strict nitrogen standards.
The need to remove residual effluent DON poses both technical and financial challenges since effluent DON is the most difficult nutrient to remove at low levels. Especially strict standards push the limits of BNR technologies, and create new technical and regulatory challenges for the industry. These criteria and how they will be implemented will be compared to other nutrient criteria from other parts of the United States. Additionally, characterization of effluent DON with respect to bioavailability and the potential role these fractions of DON may play in forming new or revised discharge criteria will be presented. Finally, environmental sustainability issues associated with trying to meet increasingly strict nutrient standards, including GHG production and carbon footprint will be discussed.
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