FINAL Nitrogen Control In Wastewater Treatment Plants V4

Forms of nitrogen found in wastewater: Nitrogenous gasses (N2O, N2) Organic nitrogen (urea, amino acids, fecal material) Ammonia (NH3 as a dissolved gas) Ammonium (NH4 ions in solution) Nitrate (NO3-) and nitrite (NO2-) ions in solution Total Kjeldahl Nitrogen (TKN) is the sum of the ammonia and organic nitrogen Total Nitrogen (TN) is the sum of TKN and nitrate and nitriteNitrate (NO3-) is a primary contaminant in drinking water and can cause a human healthcondition called methemoglobinemia (blue baby syndrome). This is due to the conversionof nitrate to nitrite (NO2-) by nitrate reducing bacteria in the human gastrointestinal tract.Oxygen bound in the nitrite can oxidize the iron in hemoglobin to form methemoglobin.Since methemoglobin is incapable of binding molecular oxygen, the result is a bluishcoloration of the skin and suffocation or death may occur if left untreated. As part of theSafe Drinking Water Act (SDWA), EPA has set the maximum contaminant level (MCL)for nitrate in drinking water at 10 mg/L.Typical effluent permit limits for nitrogen compounds in wastewater effluent vary, butmost all are based on the location of final effluent discharge. A wastewater plant thatdischarges to a spray field may not have a limit on nitrogen while a plant that dischargesto rapid infiltration basins (percolation ponds) may have an effluent nitrate limit of 12mg/L. A treatment plant discharging to a nearby stream, river or wetland may have atotal nitrogen limit of 3 mg/L, or an unionized ammonia (NH3) limit of 0.2 mg/L.In order to meet these limits, we must operate treatment plants to not only removecarbonaceous biochemical oxygen demand (CBOD) and total suspended solids (TSS),but to convert nitrogen compounds to less noxious forms. This requires operators to useprocesses that they may not be entirely familiar with. Many new package wastewatertreatment plants are built with anoxic tanks, which require different process controlmethods than a basic extended aeration treatment plant. If a wastewater treatment plantdoes not have an anoxic tank, cycling the aeration blowers on and off may be needed tocreate an anoxic zone or time to allow for denitrification in the aeration tank. Theseconcepts will be discussed further in the following sections. A side benefit of cycling theblowers is a reduction of electricity consumption, thus saving some money on the electricbill.9

Nitrogen ConversionDepending on environmental variables such as temperature and pH, nitrogen enters atreatment plant in various forms.AMMONIFICATIONStarting at the toilets in our homes to the main sewer line out in the street, nitrogen ismostly in the form of organic nitrogen (urea, amino acids, fecal material). Through aprocess called hydrolysis, the particulate organic nitrogen begins to convert to ammoniaor ammonium. Hydrolysis is the conversion of particulate organic material into formsthat are small enough to be taken up and consumed by bacteria. The amount of ammoniaand ammonium formed depends on the liquid pH and temperature. When the pH of thewastewater is acidic ( 6.9) or neutral (7.0), the majority of the nitrogen is ammonium(NH4 ). When the pH rises to 8.0 and is increasing, the ammonium begins to shift inform to dissolved ammonia gas (NH3). At pH 10 and higher, almost all the ammoniumhas converted to dissolved ammonia gas (see figure 3). Ammonification is alsoaccomplished by bacterial decomposition.Figure 3:Relationship between ammonium (NH4-N)and ammonia (NH3-N) at various pH levelsTypically by the time the sewage enters the domestic wastewater treatment plant, muchof the organic nitrogen has been converted to ammonium, roughly a 60/40 split. Asoperators of wastewater treatment plants, we do very little to control this, sinceammonification is a natural process that occurs through the collection system all the wayto the head works of our treatment plants.Ammonia stripping towers are used by some industries and wastewater treatment systemsto remove nitrogen from the waste stream by intentionally raising the pH to 11 (or10

higher) with calcium oxide (lime), pumping the wastewater/lime solution into a towerpacked with plastic or wood media and allowing the ammonia gas to be released from thesolution as the treated wastewater splashes over the media and flows downward throughthe tower. This type of physical/chemical method of nitrogen removal is not common indomestic wastewater treatment plants, but may be found at industrial waste treatmentplants.NITRIFICATIONNitrification is the biological conversion of the ammonium to nitrate nitrogen, and is atwo-step process. In the first step, aerobic bacteria known as Nitrosomonas convertammonium to nitrite. Another group of aerobic bacteria called Nitrobacter finish theconversion of nitrite to nitrate. The reactions are generally coupled and proceed rapidly tothe nitrate form; therefore nitrite levels at any given time are usually low.These bacteria known as ‘nitrifiers’ are strict aerobes, which means they must have freedissolved oxygen (O2) to perform their work, and are active only under aerobicconditions. Complete nitrification requires approximately 4.6 pounds of oxygen forevery pound of ammonium converted to nitrate. In comparison, CBOD can be consumedwith only about 1.5 pounds of oxygen. The growth rate of nitrifiers is affected by theconcentration of dissolved oxygen (DO), and at DO less than 0.5 mg/L the growth rate isminimal. Typical operational guidelines call for a minimum DO concentration of 1.0mg/L at peak flow and an average daily DO concentration of 2.0 mg/L. For nitrificationto proceed the oxygen should be well distributed throughout the aeration tank and itslevel should not be below 1.0 mg/L.Similar to humans, activated sludge organisms need nutrients to survive and reproduce.Nitrifying bacteria are no different, and need calcium in their diet. Luckily, there isusually enough calcium in the raw wastewater in the form of calcium carbonate (CaCO3)to allow nitrifiers to survive nicely. Later in this manual, we will discuss theconsequences of insufficient calcium (alkalinity) in the wastewater. As the nitrifiers usethe ammonium as an energy source, they consume the calcium carbonate as a carbonsource.The process of nitrification produces acids. This acid formation, along with the calciumcarbonate reduction, can lower the pH of the MLSS and cause a decline in the growthrate of nitrifying bacteria. The optimum pH for Nitrosomonas and Nitrobacter organismsis between 7.5 and 8.5 and nitrification stops at pH levels at or below 6.0. Approximately7.14 pounds of alkalinity (as CaCO3) are consumed per pound of ammonia oxidized tonitrate.Water temperature also affects the rate of nitrification. Nitrification reaches a maximumrate at liquid temperatures between 30 and 35 degrees C (86oF and 95oF). Attemperatures of 40oC (104oF) and higher, nitrification rates fall to near zero. Conversely,cold water temperatures can also affect the rate of nitrification. As water temperaturedecreases, the nitrifiers slow down. Many wastewater treatment plants increase the11

MLVSS amounts during the winter months to increase the amount of aerobic autotrophicbacteria available to perform nitrification efficiently.Nitrifying bacteria are sensitive organisms, and react quickly to environmental changes.Rapid changes in liquid temperature can shock nitrifiers and other organisms we relyupon to treat our wastewater. As seasons change, cold fronts can bring swift changes inair temperatures. Water temperatures change fast as well, and treatment plants withmechanical aeration equipment where the MLSS is thrown into the air (figure 4) can seetemperature decreases much faster than facilities with submerged, compressed airdiffusers. We can see the effects of a MLSS temperature drop of one degree or more perday as cloudiness in secondary clarifiers, and an increase in effluent ammonium levels.Figure 4: Mechanical aeration unitSludge Age and Mixed Liquor amounts are also integral components in the nitrificationprocess. When performing sludge age calculations (MCRT or SRT) to determine thedetention time required for nitrification, the capacity of the oxic (aerated) portion of theplant should be used. Since anoxic or fermentation basins are not aerated and nitrifyingorganisms are strict aerobes, the capacity of these basins should not be included incalculations for oxic SRT. Extended aeration (package type) wastewater plants are morecapable of nitrification than contact-stabilization and some other activated sludgemodifications due to the high sludge age and long periods of aeration.Toxicity and sources of inhibition to microorganisms present problems to operators andnitrifying organisms. Some of the most toxic compounds to nitrifiers include cyanide,thiourea, phenol and heavy metals such as silver, mercury, nickel, chromium, copper andzinc. Some of these compounds can enter a wastewater treatment plant from landfillleachate and septage. Nitrifying bacteria can also be inhibited by high concentrations offree forms of their own substrate. Nitrite oxidizing bacteria are sensitive to free nitrousacid, and ammonium oxidizing bacteria are sensitive to free ammonia (NH3). Increasedlevels of free ammonia can decrease nitrifier growth rates. Some treatment plants thatmay have increased influent organic nitrogen and ammonia levels include plants thatserve highway rest areas and wastewater plants serving schools.12

Summary of conditions required for effective nitrificationOxic SRT, days:There must be enough time in the aeration tanks tocomplete nitrification. Typically SRT higher than 10 dayswill allow for nitrification.MLSS, mg/L:Since nitrifying bacteria make up a small percentage (about5%) of the total organisms in the MLSS, there must beenough MLSS to allow nitrification. Most extendedaeration facilities have no problem providing sufficientMLSS.Alkalinity, mg/L as CaCO3: There needs to be enough alkalinity to allow nitrifiers tocomplete their work. Influent alkalinity should be 200 –250 mg/L, and aeration tank effluent shouldhave at least 50 mg/L as CaCO3 remaining.Influent Ammonia, mg/L:Ammonia present as a dissolved gas caninhibit nitrifying bacteria if present in highlevels. Ammonia concentrations of 60 mg/Las NH3 and higher can harm these sensitivebacteria.Toxic or inhibitory compounds:Cyanide, heavy metals and other organicand inorganic chemicals can inhibitnitrification. Be careful when using chlorinefor filament control!Dissolved Oxygen, mg/L:DO should be in the range of 1.0-2.0 mg/LOxidation Reduction Potential, mV:ORP should be 100 mV or higherpH., su:Optimum pH for nitrifiers is 7.5 to 9.0,nitrification can stop at pH less than 6.013

DENITRIFICATIONDenitrification is an anaerobic respiration process in which nitrate serves as the electronacceptor. In simpler terms, denitrification occurs when free dissolved oxygen levels aredepleted and nitrate becomes the primary bound oxygen source for facultativeheterotrophic microorganisms. Heterotrophic organisms can obtain their carbon andenergy from the same source – by biodegrading influent wastewater containing carbon.We measure the influent carbon by sampling for and running a carbonaceous biochemicaloxygen demand (CBOD) test.When bacteria break apart nitrate (NO3-) to gain the oxygen (O2), the nitrate is reduced tonitrite (NO2-) then quickly to nitrous oxide (N2O), and finally nitrogen gas (N2) as theoxygen is stripped away by the microorganisms. Since nitrogen gas has low watersolubility, it tends to escape as gas bubbles once the surrounding liquid is saturated withnitrogen. These gas bubbles can become bound in the settled sludge floc in clarifiers andcause the sludge to rise to the surface.Conditions Required for Effective DenitrificationConditions that affect the efficiency of denitrification include nitrate concentration,anoxic conditions, presence of organic matter, pH, temperature, alkalinity and the effectsof trace metals.Since denitrifying bacteria are facultative organisms, they can use either dissolvedoxygen or nitrate or sulfate as an oxygen source for metabolism and oxidation of organicmatter. If dissolved oxygen and nitrate are present, facultative bacteria prefer to use thedissolved oxygen. This will occur since dissolved oxygen is readily available and yieldsmore energy to the organisms. Therefore it is imperative to keep dissolved oxygen levelsas close to zero as possible in anoxic basins or timed anoxic cycles. Excessive dissolvedoxygen in basins designed as anoxic is possible through aerated return sludge (air lift typeRAS), excessive splashing of liquid streams into anoxic basins, and air diffusion beingused for tank mixing instead of using mixing pumps or mixing devices.Another important aspect of denitrification is the presence of organic matter to drive thedenitrification reaction. Organic matter may be in the form of raw wastewater, foodprocessing wastes, or chemical sources such as methanol, ethanol, acetic or citric acid.When these sources are not present, bacteria may depend on internal (endogenous)carbon reserves as the source of organic matter. This material is released during the deathphase of organisms, and may not be a consistent enough source of carbon to drivedenitrification to completion. Whatever organic source is used to drive the denitrificationreaction, it should be fed consistently and at a rate to keep denitrification levelsmaximized. Conversely, it is important to avoid raising effluent CBOD values and avoidspending excessive money on organic sources such as methanol.An advantage of denitrification is the production of alkalinity and an increase of pH.Approximately 3.0 to 3.6 mg of alkalinity (as CaCO3) is produced per milligram ofnitrate reduced to nitrogen gas. Optimum pH values for denitrification are 7.0 to 8.5.14

Temperature affects the growth rate of denitrifying organisms, with increased growth rateat higher temperatures. Denitrification can occur between 5 to 30oC (41oF to 86oF), andthese rates increase with temperature and type of organic source present. The highestgrowth rate can be found when using methanol or acetic acid. A slightly lower rate usingraw wastewater will occur, and the lowest growth rates are found when relying onendogenous carbon sources at low water temperatures.Denitrifying organisms are generally less sensitive to toxic chemicals than nitrifiers, andrecover from toxic shock loads faster than nitrifiers.So how does it all work together?Nitrification/Denitrification Process DescriptionThere are several ways to encourage bacteria to perform the work of nitrogen removal.Some processes may be designed specifically for nitrification/denitrification usingseparate aeration and anoxic tanks (selectors) and others may use timers (Figure 5)controlling aeration blowers by turning them on and off.Figure 5: Electric timer for aeration blower controlAmmonification summaryStarting from the house service connection, organic nitrogen begins its transformation toammonia and ammonium, based on pH and temperature. As the wastewater reaches thetreatment plant for processing, some of the organic nitrogen has been converted toammonia or ammonium through the collection system. Roughly 30 – 40% may remain asorganic nitrogen, with about 60 to 70% coming in as ammonia/ammonium. We measurethe amount of organic nitrogen and ammonia as TKN of the influent wastewater. Most ofthe remaining organic nitrogen is finally converted to ammonia/ammonium in the15

treatment plant, but a very small amount of organic nitrogen may make it all the waythrough the processes and give us some problems in the chlorine contact tank, which wewill learn more about later.Nitrification, an oxidation processOnce the ammonium reaches the aeration tank, the aerobic autotrophic bacteria(nitrifiers) get to work. As long as there is enough DO left over after CBOD removal byaerobic and facultative bacteria, the nitrifiers will consume calcium carbonate whileoxidizing the ammonium as an energy source. Nitrosomonas bacteria take the lead byconverting the ammonium (NH4 ) to nitrite (NO2-), then the Nitrobacter step in to finishoff the conversion of nitrite to a final fully oxidized form called nitrate (NO3-). While thecalcium carbonate is being consumed, nitrous acid is being formed and the total alkalinityof the liquid begins to decline.By the time the wastewater has reached the end of the aeration tank system, The CBOD has been oxidized to low levels by aerobic and facultative bacteria asa carbon food source The incoming ammonia/ammonium has been oxidized to nitrite, then to nitrate Ammonia (NH3) and ammonium (NH4 ) levels have decreased, nitrate (NO3-)levels have increased Dissolved oxygen demand has decreased, residual DO has increased Total alkalinity (as calcium carbonate) has decreased pH may have decreased A total of 6.1 pounds of oxygen has been consumed by aerobic and facultativebacteria to oxidize CBOD and ammonia Significant bacterial floc has been formed, trapping suspended and colloidalsolids, including particulate organic nitrogenDenitrification, a reduction processHere’s where the process gets a little tricky. After the aeration tank, the MLSS normallyenters a secondary clarifier for settling and separation of the solids from the treated water.If there is a carbon source available for bacteria to consume, little or no dissolved oxygenand nitrate is present, denitrification can take place.In the settled activated sludge that becomes the sludge blanket in the clarifier, the freeavailable dissolved oxygen is rapidly used up by aerobic and facultative bacteria. Oncefree DO is gone, nitrification basically stops. If there is sufficient carbon present in theform of CBOD or carbon from within the bacteria itself, facultative organisms willcontinue to thrive. Since there is little to no free DO present, they use a different internalenzyme to allow the use of a combined oxygen source instead of free oxygen. The mostreadily available and easily reduced oxygen is found within the nitrate compound, NO3-.16

Facultative bacteria are able to consume carbon in a oxygen depleted environment, usingnitrate as an alternative oxygen supply. A bacterium reduces the nitrate molecule bytaking one oxygen atom off the nitrate (NO3-), reducing it to nitrite (NO2-). Anotherbacterium takes one more oxygen atom from the nitrite, reducing it to nitrous oxide(N2O). Finally, another bacterium takes the last oxygen reducing it to nitrogen gas (N2).Where does the nitrogen gas go? Some of it dissolves into the liquid solution, howeveronce the water is saturated with nitrogen gas, no more nitrogen can be dissolved in thewater. Microscopic bubbles of nitrogen form, becoming larger bubbles as they bump intoeach other. Some of the bubbles rise directly to the tank surface, some of the bubblesremain trapped in the MLSS floc they formed inside of. Once enough of these bubblesaccumulate inside the floc particle, the bubbles ma

vii References The following resources were used in producing this manual: EPA: Package Treatment Plants MO-12, EPA 430/9-77-005, April 1977 EPA: Summary Report: The Causes and Control of Activated Sludge Bulking and Foaming, EPA 625/8-87/012, July 1987 EPA: Manual: Nitrogen Control, EPA 625/R-93/010, September 1993 EPA: Handbook: Retrofitting POTWs, EPA 625/6-89/020, July 1989