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External Carbon Sources for Nitrogen Removal

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External Carbon Sources for Nitrogen Removal United States Environmental Protection Agency August 2013 Wastewater Treatment Fact Sheet: External Carbon Sources for Nitrogen Removal DESCRIPTION Discharge permits for publicly owned treatment There are two major sources of organic carbon works (POTWs) and industries often include utilized in wastewater treatment operations. effluent limits for nutrients, including nitrogen. The sources are defined with respect to Total maximum daily loads (TMDLs) for whether they originate within the influent nutrients have and are being developed for many wastewater entering the treatment facility or water bodies throughout the United States. The are provided as an external supplemental TMDLs and resultant waste load allocations to carbon source added to the treatment system. protect impaired water bodies have resulted in Carbon sources are termed external when the more stringent effluent limits for total nitrogen. carbon substrate is sourced from outside the wastewater treatment process i.e., it is not In order to achieve very low total nitrogen limits derived from the influent wastewater or any of less than 6 mg/l through biological onsite treatment processes at the treatment denitrification, a readily biodegradable carbon facility. External supplemental carbon sources source must be available for the denitrifying are brought into the wastewater treatment organisms to use. A supplemental external process usually as pure compounds or high carbon source is often required when organic strength waste materials where concentrations material in the wastewater has been oxidized. can be as high as 1.5 g/L chemical oxygen This is especially true in denitrification processes demand (COD) to facilitate nutrient removal. that are located after the aeration process such Internal carbon sources refer to organic as post or second anoxic zone and denitrifying carbon substrates obtained either within filters. the influent wastewater (as an organic wastewater load entering into the plant from This fact sheet will provide information the influent) or from accumulated materials on external supplemental carbon sources to utility stored within the cells also referred to as managers and operators of wastewater treatment endogenous carbon sources. facilities that have existing nitrogen permit limits or will be required to remove nitrogen in CARBON AUGMENTATION FOR the future. NITROGEN REMOVAL Nitrogen removal involves the initial OVERVIEW transformation of ammonia and organic Bacteria utilize carbon as an energy source to nitrogen to nitrates via nitrification, and the drive metabolism as well as for the synthesis subsequent elimination of nitrogen through of new cellular material. Microorganisms obtain denitrification. Because nitrification typically their carbon needs from organic compounds or only occurs following carbonaceous biological from carbon dioxide. Heterotrophic oxygen demand (BOD) removal, the limiting microorganisms are able to utilize organic carbon factor for effective ddenitrification is often sources while autotrophic organisms utilize the absence of a readily biodegradable carbon carbon dioxide as a carbon source. When source that can be used as an effective microorganisms utilize organic carbon as a substrate by denitrifying bacteria during the substrate, energy is produced by the biochemical denitrification process. Without the oxidation of organic carbon to carbon dioxide. availability of a ready source of biodegradable carbon, denitrif ication will not occur, or will occur too slowly for sufficient nitrogen removal to occur. Figure 1: Illustration of Wastewater Treatment Process and Supplemental Carbon Feed Points supplemental carbon source to be added, as By using recycle schemes and step-feed almost all of the internal carbon sources have processes, nitrates are brought into contact with been utilized in the aeration process and only sources of readily biodegradable carbon in the a very limited amount endogenous carbon is plant. Processes such as the Modified available for denitrification. Ludzak-Ettinger (MLE) process enable contact between the nitrates formed at the back end of A wide range of carbon sources can be the wastewater treatment process and soluble used to meet the soluble COD needs for COD generally found in the influent wastewater denitrification. Commonly used sources streams by recycling nitrate laden process flows of external carbon include methanol, to the head of the treatment system. However, in ethanol, acetate, acetic acid, glycerol, the MLE, step-feed and sequencing batch reactor molasses sugar water and proprietary (SBR) processes, a supplemental external carbon formulations like MicroC™, a suite of carbon source might still be required to facilitate nitrate sources manufactured by Environmental removal when internal carbon is not available in Operating Solutions, Inc. high enough concentrations (see Figure 1 for MLE and post anoxic zone processes common The choice of carbon source typically will supplemental carbon feed points). depend on the evaluation of a number of product attributes, including: safety, cost, Denitrification processes that are located after handling requirements, ease of use, materials the aeration process such as post or second compatibility, as well as kinetics and yield anoxic zones and denitrifying filters will dynamics. The choice of a carbon source can generally always need an external have profound implications not just on the efficacy of nutrient removal, but also on plant huge impact on the prices of these carbon and personnel safety, sludge yields, aeration sources. adequacy, environmental sustainability, overall effluent quality and other factors. Agriculturally derived carbon sources such as Tables 1 and 2 provide an overview of molasses, glycerol, corn syrup, sucrose and different carbon sources as well as a MicroC™, tend to have more predictable and qualitative assessment of their ratings on a less volatile price profiles. In order to ensure variety of attributes and product that lifecycle costs associated with the external characteristics. carbon system reflect the potential for significant price movements, especially with IMPORTANT FACTORS TO CONSIDER regards to fossil fuel based carbon sources, WHEN SELECTING AN EXTERNAL evaluations for carbon source selection should CARBON SOURCE incorporate an analysis that incorporates historical price changes into the evaluative Safety framework. Safety is a very important consideration for carbon source selection. The use of external DOSAGE AND KINETIC carbon sources such as methanol, acetic acid CONSIDERATIONS and ethanol for enhanced nutrient removal has The dosage requirement refers to the amount expanded the scope of safety concerns in of COD that is required to remove each unit of wastewater treatment facilities. The most nitrate (i.e., the COD:N ratio, which is usually significant concerns are that of flammability expressed as lbs COD/lbs NO3-N removed). and explosion hazards associated with This ratio is affected by factors such as the methanol and ethanol. nature of the carbon source, the species of biomass supported, the electron donor The levels of flammability associated with the capacity of the carbon source, the solids carbon source selected will have an impact on retention time (SRT) of the treatment system the cost of the systems that will need to be put and the sludge yields associated with bacterial in place to ensure that the risk of fires and species supported by the carbon source. explosions are mitigated through compliance with applicable National Fire Protection Kinetic considerations typically focus on the Association (NFPA) fire and safety codes. specific denitrification rates and the Also, the hazards associated with handling biomass growth rates associated with the these products will require Occupational carbon source. This is generally a function of Safety and Health Administration (OSHA) the species of biomass that are selected for specific initial and annual recertification use in the treatment process when a carbon training of the plant operations staff. In 2006, source is utilized. When methanol is used as a a serious incident involving fatalities was carbon source, methylotrophic denitrifying recorded at a wastewater treatment plant using bacteria are selected for, resulting in a slower methanol as an external carbon source. This overall growth. Non methanol carbon sources tragic incident brought to the forefront the such as glycerol, acetic acid, and the need for safety considerations for flammable MicroC™ suite of products can be carbon sources, which include foam metabolized by the general heterotrophic suppression systems, explosion proof storage, bacterial populations found in the wastewater and spill containment. treatment process. Price Fluctuations Methylotrophic biomasses are known to have Many external carbon sources are derived very low growth rates. In colder from fossil fuel based raw materials. temperatures, the growth rate is even lower, Significant price fluctuations in the methanol, potentially leading to a reduced capacity for ethanol, and acetic acid markets can have a denitrification in the winter period Table 1: Product Characterization for Some External Carbon Sources QUALITY CONTROL Carbon sources are generally pure products When external carbon sources are applied for (e.g., methanol, ethanol, MicroC™), unrefined nutrient removal, especially in tertiary deep wastes, or purified waste materials derived bed biological filters, it is important for the from a variety of industrial
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