Nitrogen Recovery in WasteWater Treatment - Term Paper Example

Nitrogen Recovery in WasteWater Treatment Nitrogen is an essential nutrient for plants and animals. Approximately 80 percent of the Earth’s atmosphere is composed of nitrogen, and it is a key element of proteins and cells. Nitrogen is a constituent in human sewage. The major contributors of nitrogen to wastewater are human activities such as food preparation, showering, and waste excretion. The most common forms of nitrogen in wastewater are: Ammonia (NH3), Ammonium ion (NH4+), Nitrite (NO2‐), Nitrate (NO3‐), Organic nitrogen. Nitrogen in domestic wastewater consists of approximately 60 to 70 percent ammonia‐nitrogen and 30 to 40 percent organic nitrogen (Tchobanoglous et al. 2003; Crites and Tchobanoglous 1998). Environmental Effects Health Effects from Drinking Groundwater Contaminated with Nitrates- Human health concerns from nitrates in groundwater used as a drinking water source primarily focus on methemoglobinemia, however some studies suggest that nitrates may increase the risk of birth defects and development of certain cancers in adults. Surface Water Pollution with Nitrogen- The harmful effects of eutrophication due to excessive nitrogen concentrations in the aquatic environment have been well documented. Algae and phytoplankton growth can be accelerated by higher concentrations of nutrients, leading to harmful algal blooms, hypoxia, and loss of submerged aquatic vegetation (SAV). In addition to stimulating eutrophication, nitrogen in the form of ammonia can exert a direct demand on dissolved oxygen (DO) and can be toxic to aquatic life. Even if a wastewater treatment plant (WWTP) converts ammonia to nitrate by a biological nitrification process, the resultant nitrate can stimulate algae and phytoplankton growth. Removal of nitrogen from wastewater is a complex process, even for large wastewater treatment plants. Quality control of nitrogen removal processes from individual onsite wastewater systems is even more difficult to manage. Most of the nitrogen is released as nitrate (NO3-), which is highly mobile in the soil water. Wastewater treatment has generally been defined as containing one or more of the following four processes: (1) preliminary, (2) primary, (3) secondary, and (4) advanced (or tertiary) treatment. Preliminary treatment consists of grit removal, which removes dense inert particles, and screening to remove rags and other large debris. Primary treatment involves gravity settling tanks to remove settleable solids, including settleable organic solids. The performance of primary settling tanks can be enhanced by adding chemicals to capture and flocculate smaller solid particles for the precipitation and removal of phosphorus. Secondary treatment follows primary treatment in most plants and employs biological processes to remove colloidal and soluble organic matter. EPA classifies advanced treatment as “a level of treatment that is more stringent than secondary or produces a significant reduction in conventional, non‐conventional, or toxic pollutants present in the wastewater” (U.S. Public Health Service and USEPA 2008). Effluent filtration and nutrient removal are the most common advanced treatment processes. Nitrogen Removal Processes The biological removal of nitrogen is carried out through a three-step process: (1) the conversion of ammonia from organic nitrogen by hydrolysis and microbial activities, called ammonification; (2) the aerobic conversion of ammonia to nitrate by reacting the ammonia with oxygen in a process called nitrification; and (3) the conversion of nitrate to nitrogen gas by reacting the nitrate with organic carbon under anoxic conditions in a process called denitrification. The nitrification process is accompanied by the destruction of alkalinity (e.g., bicarbonate, HCO3-, is neutralized to carbonic acid, H2CO3). Alkalinity is recovered as part of the denitrification process with the generation of hydroxide. The chemical equations involved in the biological conversion of nitrogen are as follows: 1. Formation of ammonia from organic nitrogen by microorganisms (ammonification): Organic nitrogen NH4+ 2. Nitrification to nitrite by Nitrosomonas species and other autotrophic bacteria genera: NH4+ + 3/2 O2 + 2HCO3- NO2- + 2H2CO3 + H2O 3. Nitrification to nitrate by Nitrobacter species and other autotrophic bacteria genera: NO2- + ½ O2 NO3- 4. Denitrification by denitrifying microorganisms with no oxygen present. The most common and widely distributed denitrifying bacteria are Pseudomonas species, which can use hydrogen, methanol, carbohydrates, organic acids, alcohols, benzoates, and other aromatic compounds for denitrification: NO3- + organic carbon N2 (g) + CO2 (g) + H2O + OH- In Biological Nutrient Removal (BNR) systems, nitrification is the controlling reaction because ammonia oxidizing bacteria lack functional diversity, have stringent growth requirements, and are sensitive to environmental conditions (Jeyanayagam, 2005). Nitrification by itself does not actually remove nitrogen from wastewater. Rather, denitrification is needed to convert the oxidized form of nitrogen (nitrate) to nitrogen gas. Nitrification occurs in the presence of oxygen under aerobic conditions, and denitrification occurs in the absence of oxygen under anoxic conditions. There are a number of BNR process configurations available. Some BNR systems are designed to remove only Total nitrogen (TN) or Total phosphorus (TP), while others remove both. The configuration most appropriate for any particular system depends on the target effluent quality, operator experience, influent quality, and existing treatment processes, if retrofitting an existing facility. BNR configurations vary based on the sequencing of environmental conditions (i.e., aerobic, anaerobic, and anoxic) and timing (Jeyanayagam, 2005). Although the exact configurations of each system differ, BNR systems designed to remove TN must have an aerobic zone for nitrification and an anoxic zone for denitrification, and BNR systems designed to remove TP must have an anaerobic zone free of dissolved oxygen and nitrate. Often, sand or other media filtration is used as a polishing step to remove particulate matter when low TN and TP effluent concentrations are required. Sand filtration can also be combined with attached growth denitrification filters to further reduce soluble nitrates and effluent TN levels (WEF and ASCE/EWRI, 2006). Common BNR system configurations include: Modified Ludzack-Ettinger (MLE) Process – Continuous-flow suspended-growth process with an initial anoxic stage followed by an aerobic stage; used to remove TN. A/O Process – MLE process preceded by an initial anaerobic stage; used to remove both TN and TP. Step Feed Process – Alternating anoxic and aerobic stages; however, influent flow is split to several feed locations and the recycle sludge stream is sent to the beginning of the process; used to remove TN. Bardenpho Process (Four-Stage) – Continuous-flow suspended-growth process with alternating anoxic/aerobic/anoxic/aerobic stages; used to remove TN. Modified Bardenpho Process – Bardenpho process with addition of an initial anaerobic zone; used to remove both TN and TP. Sequencing Batch Reactor (SBR) Process – Suspended-growth batch process sequenced to simulate the four-stage process; used to remove TN (TP removal is inconsistent). Modified University of Cape Town (UCT) Process – A/O Process with a second anoxic stage where the internal nitrate recycle is returned; used to remove both TN and TP. Rotating Biological Contactor (RBC) Process – Continuous-flow process using RBCs with sequential anoxic/aerobic stages; used to remove TN. Oxidation Ditch – Continuous-flow process using looped channels to create time sequenced anoxic, aerobic, and anaerobic zones; used to remove both TN and TP. Denitrification Filters- Denitrification filter is usually placed after the secondary treatment process. One of their advantages is that in addition to providing nitrogen removal (complete nitrate removal) they act as an effluent filter. Used to remove TN. Cyclically Aerated Activated Sludge- In a cyclically aerated activated-sludge system, the aeration system is programmed to turn off periodically, allowing denitrification and nitrification to occur in the same tank. Used to remove TN. Moving-Bed Biofilm Reactor- It consists of small plastic media (carrier elements) in an anoxic or aerobic zone that allow attached growth to occur. Such high-rate biofilm processes are highly efficient in removing organic and nitrogen loads. Used to remove TN. Membrane Bioreactor- It consists of anoxic and aerobic zones followed by a membrane that filters the solids from the mixed liquor. Used to remove TN. Biodenitro Process- It is a variation of the oxidation ditch, consists of two oxidation ditches side by side. Influent is fed alternately to the ditches, allowing anoxic and aerobic zones to form for nitrification and denitrification. The ditches are alternately aerated and not aerated, and mixing is maintained by the flow in the ditches. The ditches periodically switch modes, and the overall result is that the water passes through multiple aerobic and anoxic zones before discharge. Used to remove TN. Schreiber Process- The patented Schreiber countercurrent aeration process can provide nitrification and denitrification in one basin. The wastewater enters a circular basin equipped with a rotating bridge that provides mixing. Aeration is provided by fine-bubble diffusers attached to the bridge. Used to remove TN. InNitri Process (Nitrification) - The ammonia-laden water is treated in a separate nitrification reactor before recycling to the plant headwork’s. This therefore reduces the ammonia-nitrogen load in the recycle stream. The recycle stream then provides a seed of nitrifying bacteria to the main reactor. Used to remove TN. Bio-Augmentation Batch Enhanced (BABE) (Nitrification) - The BABE process is a variation on the InNitri process. In BABE, the reactor is a batch system that is fed batches of return sludge from the main activated sludge system along with the sidestream. This batch reactor is operated both aerobically and anoxically and therefore both nitrifies and denitrifies the sidestream. Used to remove TN. Nitritation–Denitritation- A recently developed alternative to conventional nitrification/denitrfication is the process of nitritation, where only nitrite is produced aerobically. Used to remove TN. Benefits of nutrient removal Biological nutrient removal can lead to several operational improvements. In most cases, the addition of an anaerobic zone for biological phosphorus removal will increase the sludge density because of phosphorus accumulation, and reduce the growth of filamentous organisms because of the absence of DO, thereby improving settleability. A pre‐anoxic zone for denitrification can also lead to a more stable, better settling activated sludge process as the anoxic‐aerobic processes favor good settling floc‐forming bacteria over filamentous growth. Additional benefits of a pre‐anoxic zone include less aeration energy required in the aerobic zone as the nitrate produced can be used for BOD removal, and less sludge production compared to post‐anoxic treatment with supplemental carbon. Anaerobic and anoxic zones also provide better control of foaming if backmixing is eliminated and the recycle of NOx and DO to the zones is minimized. Further, good removal of nitrogen reduces concern over denitrification and floating sludge in the secondary clarifiers, and provides the option of over-sizing the clarifiers to better handle wet weather flows. Also, nutrient recovery and reuse is gaining national and international attention as a key aspect in sludge management plans. Rather than being disposed as a “waste,” sludge is now being harvested for valuable resources and used as an alternative source of energy. Works Cited United States Environmental Protection Agency. “Biological Nutrient Removal Processes and Costs”. Washington, DC: United States Environmental Protection Agency , June 2007. Web. 9 April 2011. Office of Wastewater Management. “Primer for Municipal Wastewater Treatment Systems”. Washington DC: United States Environment Protection Agency, September 2004. Web. 9 April 2011. Division of Environmental Health. "Nitrogen Reducing Technologies for Onsite Wastewater Treatment Systems." Olympia, WA: Washington State Department of Health, June 2005. Web. 9 April 2011. Shin Joh Kang, Kevin Olmstead, Krista Takacs, James Collins. "Municipal Nutrient Removal Technologies Refrence Document." Vol 1-Technical Report. Washington, DC: U.S. Environmental Protection Agency, 2008. Web. 9 April 2011. The Cadmus Group, Inc. “Nutrient Control Design Manual”. Ohio: United states Environment Protection Agency, August 2010. Web. 9 April 2011. http://www.epa.gov/nrmrl/pubs/600r09012/600r09012.pdf. WEF and ASCE. “Biological Nutrient Removal (BNR) Operation in Wastewater Treatment Plants ‐ MOP 29”. Water Environment Federation and the American Society of Civil Engineers. Alexandria, VA: WEF Press, 2006. Web. 9 April 2011. Tchobanoglous, G., F. L. Burton, and H.D. Stensel. “Wastewater Engineering: Treatment and Reuse”. New York, NY: McGraw‐Hill, 2003. Print. Crites R. and G. Tchobanoglous. “Small and Decentralized Wastewater Management Systems”. New York, NY: McGraw Hill, 1998. Print. U.S. Public Health Service and USEPA. “Clean Watersheds Needs Surveys 2004 Report to Congress”. 2008. Web. 9 April 2011. http://www.epa.gov/cwns/2004rtc/cwns2004rtc.pdf. Jeyanayagam, Sam. “True Confessions of the Biological Nutrient Removal Process”. Florida: Water Resources Journal, January 2005. Print. Read More