Lesson 14:

Biological Nutrient Removal

Objective

In this section we will answer the following question:

• How nitrogen and phosphorus can be removed through biological means.
• How nitrogen and phosphorus can be removed through chemical means.

Reading Assignment

Read the online lecture.

Lecture

Introduction

Biological nutrient removal (BNR) is a wastewater treatment process that aims to remove nutrients, such as nitrogen and phosphorus, from wastewater using biological processes. This is important because excessive levels of these nutrients in water bodies can lead to environmental problems, like eutrophication, where excessive plant and algae growth disrupts the balance of aquatic ecosystems. Excessive growth of algae can create taste and odor problems in drinking water supplies. Additionally, when the algae die off, it will create an oxygen demand in the water, resulting in reduced dissolved oxygen, which will result in death to the aquatic life present.

There are different methods used to removed these biological nutrients, with the most common approach being the use of an activated sludge system. In this type of system, the microbes, mainly bacteria, are used to biologically convert the nitrogen and phosphorus compounds present in the wastewater into less harmful forms before release.

Nutrients of Concern

The main nutrients of concern to wastewater treatment include carbon, nitrogen, and phosphorus. Nutrients may be removed chemically, physically through filtering, or biologically. Both phosphorus and nitrogen can be biologically removed, generally at a must more cost effective option.

Carbon

Carbon is the fourth most abundant element on earth and is very stable. Anything organic, but not a mineral, contains carbon. Typically the carbon concentration can be measured using the Biochemical Oxygen Demand (BOD) test. BOD represents the amount of oxygen consumed by the organic matter for biochemical oxidation to occur. The carbon content can also be determined by the volatile solids content of the sample.

Nitrogen

Nitrogen is found in all living things and is fundamental to amino acids, protein, DNA and RNA. Like carbon, nitrogen is needed to accomplish biological growth and removal. There are different forms of nitrogen present in wastewater that the NPDES (discharge permit) must address. These include ammonia nitrogen, nitrate nitrogen and Total Kjeldahl Nitrogen (TKN), which is the combination of ammonia nitrogen and organic nitrogen. Nitrogen may enter the wastewater through anything organic, such as human waste and urea as well as common agricultural fertilizers.

The presence of nitrogen compounds can cause problems that phosphorus can't including:

• Ammonia toxicity to fish and other aquatic life causing die-off
• Reduction in chlorine disinfection efficiency
• Adverse health effects if nitrates enter the drinking water supply
• Reduction in water's suitability for reuse

Phosphorus

Typically, wastewater influent has a total phosphorus concentration ranging from 5-9 mg/L and is needed for biological growth and treatment. Sources of phosphorus include human waste, runoff, fertilizers, detergents, food industry, water softeners and phosphoric acid, which is used in metal plating. Even though phosphorus is found in all living things, it is never found in elemental form and is very unstable. There are different types of phosphorus that are found in wastewater, including orthophosphate, polyphosphate and organically bound phosphates. Typically the NPDES (National Pollutant Discharge Elimination System) permit deals with Total Phosphorus, which is a combination of all types of phosphorus.

Phosphorus Type	Typical Concentration, mg/L
Orthophosphate	3.0 - 4.0
Polyphosphate	2.0 - 3.0
Organic phosphate	0.7 - 1.0
Total Phosphorus	5.7 - 8.0

 

Influent Nutrient Ratios

The concentration of nutrients entering the utility can help you determine whether you can successfully perform biological phosphorus removal. The conventional nutrient ratio of influent wastewater streams is 100: 5: 1 (carbon: nitrogen: phosphorus). Documentation indicates that a 20: 1 BOD: Phosphorus ratio must exist for the removal of phosphorus. Some studies show up to a 100: 1 ratio is needed. This is because the Biochemical Oxygen Demand (BOD) provides the the food and bugs needed for efficient phosphorus removal. If a sufficient BOD is not present, some utilities will add additives to the influent stream to aid in the biological removal process.

Bacteria Groups

There are different groups of bacteria that perform different functions within the biological treatment process. The operator is responsible for creating the best environment for the group of bacteria needed. This is known as microbial population selection.

The main groups of microorganisms in play for biological nutrient removal include:

• Obligate Aerobes: Bacteria that require either atmospheric or dissolved oxygen to survive and reproduce (oxic).
  
  
• Obligate Anaerobes: Bacteria that survive and reproduce in the absence of dissolved oxygen.
  
  
• Facultative Organisms: Bacteria that can perform oxic, anoxic, and anaerobic reactions.
  
  

To break that down even further into specific bacteria groups, let's look at some of the microbes involved in nitrogen and phosphorus removal.

• Nitrosomonas: An autotrophic bacteria that oxidizes ammonia to nitrite during nitrification.
  
  
• Nitrobacter: An autotrophic bacteria that oxidizes nitrite to nitrate during nitrification.
  
  
• Pseudomonas: Autotrophic and heterotrophic bacteria that reduce nitrate to gaseous form (nitric oxide, nitrous oxide and nitrogen gas) during denitrification.
  
  
• Phosphate-Accumulating Organisms (PAOs): Aerobic heterotroph that acts like a sponge that is capable of storing the phosphorus it accumulates in the form of orthophosphate. They have a unique metabolism that allows them to not only store the phosphorus accumulated, but release it in the anaerobic environment when the phosphorus concentration becomes limiting. This ability makes them invaluable in wastewater treatment. PAOs are typically added to the aeration basin of the activated sludge treatment process.

In biological nutrient removal (BNR) systems, nitrification is the controlling reaction because ammonia-oxidizing bacteria lack functional variation, have stringent growth requirements and are sensitive to environmental conditions. In other words, they are very picky microbes that need to have specific conditions available before they are prominent. Even though nitrification is the driving factor of BNR, it does not actually remove the nutrients from the waste stream. The further step of denitrification is needed to convert the oxidized form of nitrogen (nitrate) to nitrogen gas. Nitrification occurs in the presence of oxygen (aerobic conditions) and denitrification occurs in the absence of oxygen (anoxic conditions).

Nitrogen Removal

Biological Methods

Biological nitrogen removal is a two-step process involving nitrification and denitrification.

During nitrification, bacteria (Nitrosomonas) oxidize ammonia to nitrite in the presence of oxygen. Another type of bacteria (Nitrobacter) then oxidizes the nitrite (NO₂) to nitrate (NO₃).

Denitrification is a reducing process that occurs in the absence of oxygen, under anoxic conditions, using heterotrophic bacteria (Pseudomonas) to reduce the nitrate to nitrogen gas (N₂).

To implement biological nitrogen removal, utilities use a combination of aerobic and anoxic or anaerobic treatment zones. In the aerobic zone, nitrification occurs as the ammonia is converted to nitrate. In the anoxic or anaerobic zones, denitrification takes place, converting nitrate to nitrogen gas under oxygen-depleted conditions.

It is worth noting that the efficiency of biological nitrogen removal can be influenced by factors such as temperature, pH, dissolved oxygen levels, and the availability of carbon sources for denitrification.

Non-biodegradable organic nitrogen that is in particulate form cannot be removed through nitrification/denitrification. They require the physical separation of solids through the sedimentation/filtration processes.

Nitrification

As previously mentioned, during nitrification, bacteria, such as Nitrosomonas, oxidize ammonia to nitrite in the presence of oxygen. Another type of bacteria, Nitrobacter, then oxidizes the nitrite (NO₂) to nitrate (NO₃). This all occurs in the aeration basin and requires a large amount of oxygen for the conversions. This reaction takes about 4.6 mg of oxygen to oxidize 1 mg of nitrogen.

For optimum nitrification to occur the operator should strive for the following conditions:

• A solids retention time (SRT) long enough to allow a stable population of nitrifiers to develop in the process. The target SRT will vary with temperature, DO, pH, and ammonia concentrations:
  • Maintain a temperature greater than 45°F to provide a stable environment for nitrifiers to thrive.
  • Maintain a pH between 6.8 and 8.0 for the nitrifiers to perform properly.
  • Nitrification is usually not affected by dissolved oxygen above 2.0 mg/L.
  • The nitrification process consumes alkalinity at a rate of 7.14 mg CaCO₃ per 1 mg of nitrogen. For the pH to stabilize an effluent alkalinity measurement of 50 - 100 mg/L is recommended.
    
    
• A hydraulic retention time (HRT) long enough to allow the biomass enough time to react with the ammonia. Systems with longer HRTs are less likely to have an ammonia break-through due to temperature changes, or variations in flow and loading.
  
  
• Eliminate toxic substances (organic and inorganic) from the system because nitrifiers are very sensitive to many of them.

Nitrification can occur in either a single sludge system or a separate sludge system.

Single Sludge System:

The single sludge system removes BOD and ammonia in the same reactor, using only one set of clarifiers. This type of system is very common since denitrification can be incorporated into the design. This would require the sludge to be properly mixed or adding a carbon source to aid in the process.

Separate Sludge System:

The separate sludge system removes BOD in one set of reactor/clarifier and removes ammonia in subsequent sets of reactor/clarifiers. This system protects the slow growing nitrifiers from toxic materials within the influent waste stream, allowing nitrification to occur. Separate systems are less common due to higher construction costs and because denitrification is not incorporated into the process.

Denitrification

Denitrification is a reducing process that occurs in the absence of oxygen, under anoxic conditions, using heterotrophic bacteria, such as Pseudomonas, to reduce the nitrate to nitrogen gas (N₂), which is released into the atmosphere.

For optimum denitrification to occur, certain conditions are necessary, including:

• An anoxic zone that has dissolved oxygen levels less than 0.1 mg/L.
  • Denitrifying bacteria are facultative and prefer to use oxygen to metabolize the carbonaceous BOD (CBOD). This means that any oxygen present within the anoxic zone will be used before the bacteria start to reduce the nitrate in the waste stream.
• A sufficient amount of readily degradable CBOD in the anoxic zone.
  • Denitrification consumes 2.86 mg CBOD per mg of nitrate denitrified. Carbon augmentation may be necessary if the CBOD to nitrogen ratios is low.

There are benefits to a treatment process that combines nitrification with denitrification. Denitrification will reduce 2.86 mg of the CBOD present per mg of nitrate denitrified, without the expense of adding air to the process. The alkalinity demand is reduced because 3.57 mg of alkalinity is produced per mg of nitrate denitrified.

Nitrogen is required for cell growth and reproduction where bacteria assimilate (take in) nitrogen from the wastewater in a process known as assimilation. The new biomass contains about 12% nitrogen. During the aerobic treatment process, most of the organic nitrogen is changed to ammonia in a process known as ammonification. The ammonia is then available to the nitrifying organisms present in the wastewater. A small portion of the organic nitrogen will remain in organic form and is removed physically through sedimentation/filtration or will pass through to the effluent. The forms of nitrogen that must be removed include ammonia nitrogen, nitrite nitrogen and nitrate nitrogen. Nitrogen in the form of ammonia (NH₃) or ammonium (NH₄⁺) can be removed by air stripping, breakpoint chlorination or biological methods, such as nitrification/denitrification.

Chemical Methods

Air Stripping

Air stripping is the process of moving air through the waste stream to remove chemicals called Volatile Organic Compounds (VOCs). VOCs easily evaporate, which means they can change from a liquid to a vapor (gas) readily. The air that passes through the wastewater helps evaporate the VOCs faster. After the stream is treated, the air and chemical vapors are collected and removed or vented outside with low enough levels.

The pH of the wastewater is increased to the range of 10.8 to 11.5 through lime addition after the sedimentation tank. During air stripping ammonium (NH₄⁺) is converted to volatile gaseous ammonia (NH₃) where it is then removed from the stripping tower. The elevated pH water will be sent to the top of the stripper while air is pumped up through the bottom. The air and gas will cause the ammonia to be removed from the stripper through the top. It then goes through the recarbonation process to bring the pH back to a normal range. The lime recovery process is where the lime goes to the slaker to get ready for reuse.

Air stripping is a reliable process for removing ammonia from wastewater. Ammonia is present in wastewater in the form of NH₃ (dissolved ammonia gas) and NH₄ (ammonium ions). At the typical pH of wastewater, almost all of the ammonia is in the form of NH₄, which can not be stripped. As the pH is increased, the equilibrium shifts and more ammonia gas (NH₃) will be present, which can be stripped. When the pH is above 10.8, virtually all ammonia is the form of ammonia gas, which can be removed by the stripping process.

Breakpoint Chlorination

Breakpoint chlorination can be used to remove nitrogen if enough chlorine is added to oxidize the chloramine compounds to nitrogen (N₂) and nitrous oxide (N₂O), both gases. These nitrogen compounds will then leave the waste stream and enter the atmosphere.

If you need to review breakpoint chlorination, we covered that in the Disinfection lesson. Just remember that if enough chlorine is added, complete oxidation of ammonia nitrogen will happen at the breakpoint.

The ammonia in wastewater will react with hypochlorous acid to form monochloramine, dichloramine and trichloramine. If enough chlorine is added to react with the inorganic and nitrogenous compounds, then the chlorine will react with organic material to produce chloroorganic compounds. As more chlorine is added, the chloroorganic compounds are destroyed, producing chloramines. When the chloramines are oxidized, nitrogen gas (N₂) and nitrous oxide (N₂O) are produced. This means that if enough chlorine is added, complete oxidation of ammonia nitrogen will happen at the "breakpoint".

Phosphorus Removal

Biological Methods

Typically, influent wastewater has a total phosphorus concentration ranging from 5-9 mg/L and is needed for biological growth and treatment of the waste stream. Phosphorus removal techniques can also take advantage of microbes to remove phosphorus from the waste stream. This process is known as biological phosphorus removal and is possible because of the microbe's ability and need to utilize the phosphorus in energy transfer and for cell components.

Most phosphorus in wastewater occurs as orthophosphate, polyphosphate and organic phosphate. During biological treatment, most of the polyphosphate and organic phosphate is converted to orthophosphate, which is a form readily assimilated by the microbes.

Earlier in the course we discussed the activated sludge process, which is also a biological treatment process. The organisms within the activated sludge use phosphorus in their cellular structure. A process known as endogenous respiration occurs in which the organisms utilize oxygen and phosphorus freely. This is known as Luxury Uptake. The organisms, known as polyphosphate-accumulating organisms (PAOs), will absorb more phosphorus than is required for life functions, storing the excess for use at a later time. The biological uptake of phosphorus by these organisms is known as Enhanced Biological Phosphorus Removal (EBPR). The PAOs grow best on volatile fatty acids (VFAs), so it is imperative to ensure they have an adequate supply of their food source - VFAs. This will only happen if the conditions are desirable in their environment.

Volatile fatty acids (VFAs) are the smallest molecules into which organic material can be broken down, consisting primarily of acetic and propionic acids. These acids are formed through fermentation, which can occur in the collection system, primary clarifier or fermentation tank. The biological removal of phosphorus removes 7 to 10 mg of VFAs for each mg of Phosphorus that is removed. If an adequate supply of VFAs are not produced naturally, chemical addition may be necessary to ensure an adequate PAO population exists for optimum phosphorus removal.

Luxury uptake occurs in an aerobic environment with sufficient dissolved oxygen present. Under these conditions, the PAOs take up orthophosphate using energy from the oxidation of organic matter by nitrate or dissolved oxygen and convert it to polyphosphate, which is stored in their cell. PAOs take up more orthophosphate than they released under anaerobic conditions.

All of this occurs in the main aeration basin of the process. Phosphorus is then removed from the system through waste sludge (WAS).

Under anaerobic conditions, the PAOs use stored polyphosphate as a source of energy. During the process of storing food, the polyphosphate used for energy is split apart into molecules of orthophosphate. These molecules can't cross the cell membrane by themselves since they are negatively charged (anions). During the process they bond with magnesium and potassium, which are positively charged (cations). During bonding, the charges are neutralized, allowing them to cross the cell membrane and pass from the cell into the waste stream. During this process the phosphorus is released in a main stream process.

Different kinds of microbes remove different amounts of phosphorus. One difference between these microbes is their ability to live with or without oxygen. Aerobic microbes need oxygen. That oxygen can be in the form of dissolved oxygen or chemically combined oxygen, like nitrate (NO₃). Strict anaerobes are microbes that live entirely without oxygen. They cannot function if there is any dissolved oxygen or nitrate around. Combining the two groups is facultative microbes, which can live with or without oxygen. The facultative microbes have one way of living when there's oxygen and another way when there isn't.

The majority of the microbes in a standard treatment process tend to be the aerobic microbes. Research shows, however, that the facultative microbes can remove about three times as much phosphorus than the aerobic microbes can. If the plant can grow more facultative microbes and less aerobes in the activated sludge, we should remove a lot more phosphorus. The key is to look at what happens to both kinds of microbes in the absence of oxygen. All the aerobes don't necessarily die without air. Many of them are real survivors. Without air, the tolerant aerobes go into a hibernation-like survival mode until they get oxygen again. The facultative microbes change the way they live to anaerobic mode. Let's summarize what happens to each group of microbes when the return sludge is made anaerobic:

• The strict aerobes die.
• The tolerant aerobes go into survival mode.
• The facultative microbes change into an anaerobic mode. However, there is no food in the return sludge, so there isn't much for them to do until there is some food.

The key to making a difference in the effectiveness of this selection process is to give the microbes food when they have no oxygen. The tolerant aerobes will just stay in survival mode. The facultative microbes will eat and reproduce because they have food to eat with no competition from the dormant aerobic microbes. Under these conditions the facultative microbes are given a large advantage. If there is only so much food to eat the facultative microbes have already taken a large portion of it. By the time all the microbes get oxygen again, there is less food available and many more facultative microbes to compete with the surviving aerobes for the food that is left over.

This process can be accomplished in at least two different ways. In one way, part of the return sludge can be made anaerobic. The difference is that now some of the settled sewage is put together with the return sludge to provide food to the facultative microbes. Another way is to make all of the return sludge with all of the settled sewage anaerobic before starting the aeration process.

When all of the return sludge with all of the settled sewage is made anaerobic, the process is referred to as a main stream process.

When just part of the return sludge is made anaerobic, the process is referred to as a side stream process.

There are advantages and disadvantages to each type.

Phosphorus can be removed from the wastewater through chemical processes as well.

Chemical Methods

Lime Precipitation

Lime can be added to the rapid mix tank to raise the pH of the wastewater. When phosphorus combines with calcium hydroxide causing flocculation, the phosphorus/lime compounds will settle out and be removed as sludge. The calcium carbonate (lime) can be reclaimed from the sludge and reused.

In the lime precipitation process, three reactions take place to remove phosphorus from the waste stream: coagulation, flocculation and sedimentation. These principles are the same as the water treatment processes. Neutralization of the charged particles, which allow particle agglomeration, forming clumps that will settle out more easily through gravitational forces.

Alum Flocculation/Filtration

When aluminum sulfate (alum) comes into contact with phosphorus, it reacts to form aluminum phosphate, which is capable of flocculation and will settle out in a clarifier. Aluminum phosphate does not settle as well as the lime compound, therefore a pressure or mixed media gravity filter usually follows the sedimentation process.

There are a few differences between using lime and alum. Alum is more expensive than lime and can not be reclaimed and reused as lime can be. The chemical reaction that occurs itself is different since alum is an acid and lime is a base. The alkalinity (bicarbonate) that is already present in the wastewater will react with the alum first to form an aluminum hydroxide floc. Once the alkalinity is reduced, the alum will react with the phosphate present. When using alum, the optimum pH for phosphorus removal is 6.0, which can be maintained through the alum feed system. As more alum is added, the pH will be reduced because liquid alum is acidic.

Modified BNR Processes

The Bardenpho Process

Nitrogen Removal

A process known as the Bardenpho process is able to remove biological nutrients, such as nitrogen and phosphorus, in four stages. This is accomplished through complete nitrification in the aeration basin and recycling the nitrate-rich mixed liquor back to the anoxic zone at the head of the aeration basin. In this initial anoxic zone (reduced oxygen supply), the bacteria remove the oxygen from the nitrate for their life processes, leaving nitrogen gas, which is released into the atmosphere. A second anoxic zone and aeration chamber is for further denitrification to achieve maximum nitrogen removal.

The influent wastewater is anaerobic and mixed with organics which are subject to anaerobic fermentation for a time period of 2 to 4 days at a temperature of 30 - 40°C. This fermentation "broth" is then sent through alternating aerobic-anoxic cycles in batch reactors.

When under anaerobic conditions, phosphorus is secreted from the microbes that accumulate phosphorus (PAOs). When the fermentation broth is introduced to the aerobic zone, the phosphorus is taken up by the accumulating microbes. Nitrifying bacteria oxidize the ammonia nitrogen in this stage. When the final anoxic tank is filled with this "broth", the oxidized nitrogen is converted to nitrogen gas by the bacteria.

Nitrogen within the waste stream is mainly in the form of ammonia, which passes through the first two zones of the process without any change. It is only in the second pass through the system that the sludge has aged enough to perform complete nitrification, converting the ammonia nitrogen to nitrates and nitrites. This is accomplished by recycling the mixed liquor from the main aeration basin back to the anoxic zone at the beginning of the process. This internal recirculation allows nitrification to occur in the aeration basin. Flow that goes back to the anoxic zone creates an environment that encourages denitrification.

Phosphorus Removal

Phosphorus can also be removed by this process by adding a fifth chamber to the beginning of the Bardenpho Process, which is anaerobic in nature. This will facilitate phosphorus removal as well as nitrogen. In the anaerobic chamber the bacteria will release stored phosphorus because there is no dissolved oxygen present. The phosphorus will be taken up within the aeration basin, just as described above. The ultimate removal of phosphorus is through sludge wasting.

Oxidation Ditch

An oxidation ditch can also perform biological nitrogen removal by creating an anoxic zone in parts of the ditch. By changing the location of the aerators and mixers, the oxidation ditch can operation like the Bardenpho process, creating a high recycle rate without the need for pumping the return.

Sequencing Batch Reactor (SBR)

A Sequencing Batch Reactor (SBR) performs all the necessary functions of nutrient removal in a single tank with variable water levels and timed aeration periods. The wastewater is added to a single "batch" reactor, treated to remove the undesirables, and then discharged. Equalization, aeration, and clarification can all be achieved using a single batch reactor. This type of system is uniquely suited for low or intermittent flow conditions of 5 MGD or less. Primary clarifiers are usually not required prior to an SBR unless the total suspended solids (TSS) or biochemical oxygen demand (BOD) are greater than 400-500 mg/L.

Ion Exchange Process

We have covered various ways of removing nitrogen and phosphorus from the waste stream. Now let's talk about one more: the ion exchange process. Think of this treatment method as using a nutrient magnet. At the heart of this process is the ion exchange resin, which is a specialized polymer matrix that has charged sites which can attract and release ions. This resin acts as a sponge.

During this process the nutrients are removed through adsorption, desorption and then regeneration. During adsorption, the wastewater is passed through a bed of ion exchange resin beads. The charged surfaces of the resin attract the unwanted nutrient ions. Once the resin beads are saturated with the targeted nutrient ions, they need to be regenerated. This is achieved through desorption, which releases the captured ions from the resion. During regeneration, a regenerant solution, often a strong salt solution like sodium chlorine (NaCl), is used to displace the captured ions from the resin. The nutrient ions are then exchanged for the chloride ions.

After desorption and regeneration, the concentrated nutrient-rich solution is separated from the resin. This solution can then be further treated to recover the captured nutrients, closing the nutrient loop. This removal process does require operational costs associated with regenerant chemicals and resin replacement.

Review

• Following the luxury uptake of phosphorus in an aerobic environment, the microbes are transferred to an anaerobic environment where the phosphorus is released.
• When using chemical precipitation for phosphorus removal the pH should be above 11 pH units.
• Nitrosomonas and Nitrobacter bacteria are autotrophs.
• The step-feed aeration process is not able to produce complete nitrification due to its short contact time.
• Monitoring alkalinity during the nitrification process is a good way to monitor process performance.
• If alkaline chemicals are over fed, the pH will increase, which will likely cause excess ammonia to form, which will inhibit the nitrifying organisms.
• If the Chemical Oxygen Demand (COD) increases during denitrification, it is most likely a result of an overdose of methanol.
• If nitrate levels suddenly increase in the denitrification unit it is an indication that the pH has drifted outside the 7.0 - 7.5 pH range.
• The most likely solution to a sudden nitrate increase in the effluent of a denitrification unit is to raise the pH by the addition of lime.
• If an ammonia stripping tower loses it ammonia removal efficiency, the most likely cause is a low water pH.
• The most likely solution when a stripping tower loses its ammonia removal efficiency is to add lime to increase the pH of the water.
• The primary process control parameter used to help minimize sludge bulking during enhanced biological nutrient removal is dissolved oxygen (DO).
• Phosphorus and nitrogen can be removed by the activated sludge process in a sequence of anaerobic, anoxic, aerobic tanks.
• To convert ammonium to gaseous ammonia for stripping removal, the pH must be raised above 10.5.
• The resin used for nitrogen removal in the ion exchange process is regenerated with sodium chloride (NaCl).
• In the 4-stage Bardenpho nitrogen removal process, ammonia is converted to nitrate in the aerobic zone.
• It takes 4.6 mg of oxygen to convert 1 mg of ammonia and nitrogen to nitrate nitrogen.
• The mean cell residence time (MCRT) of the 4-stage Bardenpho process ranges between 20 - 30 days.
• When using ferric for enhanced chemical phosphorus removal, the optimum pH range is 4 - 6 pH units.
• The DO level in the aerobic selector should be between 1.0 - 2.0 mg/L.
• The DO level in the anoxic selector should be less than 0.3 mg/L.
• The DO level in the anaerobic selector should be less than 0.1 mg/L.
• The most difficult foam to control is the foam produced by filamentous or Nocardia organisms.

Assignment

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Quiz

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