FINAL Nitrogen Control In Wastewater Treatment Plants V4

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NITROGEN CONTROL IN WASTEWATER TREATMENTPLANTSSecond EditionCopyright 2009 by Ron Trygar, CET and the University of Florida Centerfor Training, Research and Education for Environmental Occupations(TREEO)All rights reserved. No part of this publication may be reproduced by anymeans, electronic, mechanical, photocopying, recording or otherwise,without the prior written permission of the copyright owner (s).i

Preface to the Second EditionThe original edition of this manual was written in November of 2000 as a study guide forwastewater operators preparing for their Florida class C state exam. It seemed at the timethat operators needed a little extra reference material about nitrification anddenitrification that they did not get from the required coursework for the state exam. TheState of Florida Department of Environmental Protection (FDEP) Operator CertificationProgram asked for assistance in preparing a short technical manual that could be given toexam applicants as supplemental study information. I wrote the first edition of thismanual while working for the Florida Rural Water Association as a Wastewater CircuitRider.Once the manual was completed, many copies were distributed by the FDEP to operatorsupon receiving their exam applications. Over the years the exams have changed, therequired coursework has been revised and the need to send operators a nitrogen controlsupplement manual has ebbed.I realized that the first edition was still being used by operators for studying, butmoreover to help them in operation of their wastewater treatment facilities. Since stateand federal regulations have become more stringent, the need for updated informationwritten for operators about this subject became apparent. Technology has progressed,methods of monitoring have improved and specific plant operational information hasbecome available. The first edition did not appear to adequately cover all the recentchanges.It is my hope that operators of wastewater treatment plants will find this informationuseful in their quest to remove nitrogen from the waste stream, meet regulatory limits andcontinue to enhance the environment.Ron Trygar, CETMarch, 2009ii

AcknowledgementsI wish to thank the Florida Department of Environmental Protection OperatorCertification Program and the Florida Rural Water Association for the opportunity towrite the first edition, and the University of Florida TREEO Center for its assistance withthis second edition.I also wish to personally thank the following people for their review and edits of thetechnical information contained within: Dr. Bill Engel, PhD, CET, Director, UF TREEO CenterMr. Edward M. Toby III, CETMr. Michael Cherniak, CET, Senior VP, Woodard CurranMr. James Clifton, Superintendent, Simsbury, Connecticutiii

About the AuthorRon Trygar is the Senior Training Specialist for Water and Wastewater programs at theUniversity of Florida TREEO Center, and is responsible for most all water andwastewater related training. Ron instructs courses related to energy conservation attreatment facilities, activated sludge process control and troubleshooting, biologicalnutrient removal, microscopy, and a variety of exam review courses that help operatorsprepare for the state exams. Ron teaches sections of the Water Facility Security andWater Distribution System courses, and also is the instructor of the UF Dept. ofContinuing Education correspondence and on-line Class C Drinking Water Operatorcourse.Ron has worked in the water and wastewater treatment industry for more than twenty fiveyears in a variety of locations and jobs. He also worked for several years as Training andEducation Coordinator in the US Virgin Islands for a contract O&M company based onSt. Thomas and St. John. While there, he helped many operators become licensedthrough the Association of Boards of Certification (ABC) certification program. Ronreturned to Florida in 2004 and started Trygar Consulting Services, a corporation aimedat training and troubleshooting for water and wastewater system operators before hejoined TREEO full-time in 2008. Ron has been a Certified Environmental Trainer (CET)since 1998 and has provided training for associations and regulatory agencies such as theFDEP, at FW&PCOA Short Schools, the USABlueBook Company, FWEA trainingevents and at local school environmental programs. Along with the FDEP NortheastDistrict in Jacksonville, Ron helped begin the FRWA/FDEP joint wastewater operatorcertification review classes that are still given around the state today. Ron holds a Floridaclass A wastewater license and a Florida class B drinking water operator’s license. Hehas experience in many types of treatment processes such as trickling filters, RBC’sSBR’s, oxygen activated sludge; from small package plant operation to advanced wastetreatment including nitrogen and phosphorus removal. The majority of his experience hasbeen in Florida, but Ron began his wastewater treatment career in Virginia Beach,Virginia and his home state of New Jersey.Ron is a member of the FDEP Operator Certification Program Exam Review Committee,a member of WEF, FWEA and FW&PCOA, and on the CET Board of Certification forNESHTA.iv

Table of ContentsSection One: IntroductionPageWhat is Nitrogen?8Why do we Remove Nitrogen from Wastewater?8Forms of Nitrogen in Wastewater9Section Two: Nitrogen ConversionAmmonification10Nitrification11Summary of Conditions Required for trification Process Description15Ammonification, Summary15Nitrification, an Oxidative Process16Denitrification, a Reduction Process16Section Three: Common Problems That Nitrogen Can CauseRising Sludge18Ashing19Foam/scum20Nitrite Lock21Section Four: Methods of Converting NitrogenThe Wurhmann Process22The Modified Ludzack-Ettinger Process23The Four Stage Bardenpho Process24v

Table of Contents, continuedSection Five: Measuring and Monitoring NitrogenSludge Settleability25Nitrogen Series Testing26Dissolved Oxygen (DO)27Oxidation Reduction Potential (ORP)28Alkalinity, as Calcium Carbonate (CaCO3)32Section Six: A Few Suggestions on Overall Plant OperationPlants with Aerobic Digestion34Plants with Anaerobic Digestion34vi

ReferencesThe following resources were used in producing this manual:EPA: Package Treatment Plants MO-12, EPA 430/9-77-005, April 1977EPA: Summary Report: The Causes and Control of Activated Sludge Bulking andFoaming, EPA 625/8-87/012, July 1987EPA: Manual: Nitrogen Control, EPA 625/R-93/010, September 1993EPA: Handbook: Retrofitting POTWs, EPA 625/6-89/020, July 1989EPA: Nitrification and Denitrification Facilities/Wastewater Treatment, EPA 625/4-73004a, August 1973EPA: Aerobic Biological Wastewater Treatment Facilities, EPA 430-9-77-006, March1977Bitton, Gabriel; Wastewater Microbiology, 1994University of Florida, TREEO Center, Troubleshooting Activated Sludge Problems inWastewater Treatment, June 1997University of Florida, TREEO Center, How to Improve Wastewater Package PlantOperations, 1996University of Florida, TREEO Center, Troubleshooting and Optimizing WastewaterTreatment Systems for Small Communities, Nutrient RemovalDiagger, G., CH2M Hill. Paper: Nutrient Addition and Nutrient Removal in BiologicalSystems, presented at WEFTEC, Anaheim, CA, October 2000Muirhead, W., Appleton, R. Keys to Better Nitrification, WE&T 12/2007Barnard, J., Black & Veatch, Paper: Biological Nutrient Removal: Where We Have Been,Where We Are Going?, 2007Water Environment Federation Press, MOP 29, Biological Nutrient Removal Operationin Wastewater Treatment Plants, 2005Jenkins, D; Richards, M; Daigger, G. Manual on the Causes and Control of ActivatedSludge Bulking, Foaming and Other Solids Separation Problems, 3rd Edition, (2004)CRC PressDabkowski, R. Applying Oxidation Reduction Potential Sensors in Biological NutrientRemoval Systems, Hach Company, 2007vii

List of IllustrationsFigure 1Figure 2Figure 3Figure 4Figure 5Figure 6Figure 7Figure 8Figure 9Figure 10Figure 11Figure 12Figure 13Figure 14Figure 15Figure 16Figure 17Figure 18Figure 19Cover photo; oxidation ditch wwtpAlgae growing in a small creekRelationship between ammonia and ammonium at variouspH levelsMechanical aeration unitElectric timer for aeration blower controlSludge pop-ups in a small plant clarifierRising sludge pop-ups on small plant clarifierAshing on clarifier surfaceLight scum on clarifier surfaceWuhrmann processModified Ludzack-Ettinger processFour stage Bardenpho processSettleometer with mixed liquor that has risenColorimeter showing influent ammonia nitrogen resultDissolved oxygen meterORP meter and DO meterORP tableORP meter and DO meterAlkalinity testing in a labviii781012151718192022232425262729303133

IntroductionPurpose of this manualThis manual is designed to help wastewater treatment plant operators understand theconcepts of nitrification and denitrification, how various forms of nitrogen in wastewaterare measured, problems nitrogen can cause at an activated sludge treatment facility andmethods to correct these problems. The manual was also produced as a study guide foroperators preparing for wastewater treatment plant exams.It is my hope that the wording and the format of the material contained within is easy tofollow for all wastewater treatment plant personnel, from new trainees to seasoned,experienced operators. It is written for operators, by an operator.Cover Photo:Oxidation ditch WWTPwith anoxic area highlighted7

What exactly is nitrogen? Nitrogen is an element, and listed on the periodic table of elements at period 2 as agas Nitrogen was discovered in 1772 by Daniel Rutherford in Scotland An atom of nitrogen has 7 electrons, 7 protons and 7 neutrons A molecule of nitrogen (N2) is made up of two atoms of nitrogen An atom of nitrogen has an atomic weight of 14.0067 The molecular weight of N2 is 28.0134 Nitrogen gas makes up about 78% of the air we breathe Nitrogen gas is not toxic, however Nitrogen compounds such as ammonia gas (NH3) are toxic in high concentration,and Nitrogen containing compounds like cyanide (CN-) are lethal in very small amounts When dissolved nitrogen gas comes out of a solution, such as human blood, thebubbles can cause ‘the bends’, a scuba diving hazardWhy should we remove nitrogen from wastewater?In its various forms, nitrogen can deplete dissolved oxygen in receiving waters, stimulateaquatic plant growth (figure 2), exhibit toxicity toward aquatic life, present a publichealth hazard, and affect the suitability of wastewater for reuse purposes. Wastewatereffluents containing nutrients such as nitrogen and phosphorus can cause eutrophication,the excessive growth of aquatic plants and/or algae in lakes, streams, rivers, wetlands orany surface water subject to runoff.Figure 2: Algae growing in a small creek8

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

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