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Nitrification & Denitrification
ENV 501
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Chapter 5
Biological Nitrogen Removal
Chapter Overview
Wastewater treatment
Nitrification
Denitrification
Emerging technologies
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LEARNING OBJECTIVES:
After studying this chapter, you should be able to:
1.
To understand the chemical and biological processes in nitrogen
removal
2.
To become familiar with the operation of nitrogen removal
processes and the impact of aerobic, anaerobic and anoxic
conditions on them
5.1
INTRODUCTION
Carbon and nitrogen are the major pollution sources that contribute to environmental
quality problems. All of the pollution sources : i.e., municipal, industrial and agricultural must
be managed in order to reduce the carbon and nitrogen concentration within a certain level
to improve the quality of the environment.Problem which are associated with carbon and
nitrogen are : (i) imbalance of natural ecological systems and increase of eutrophication, (ii)
depletion of dissolved oxygen in surface water which kill fish and create septic condition, (iii)
odor problems, (iv) contaminants that complicate water treatment, such as ammonia used
for water supplies that require an increase of chlorine dosage to achieve free chlorine
residual in the process of disinfection and (iv) increase risks to human health, such as NO3N concentration in the groundwater for potable use.
Nitrate is regarded as an undesirable substance in public water. Although it occurs naturally
in water, elevated levels of nitrate in groundwater usually result from human activities, such
as over use of chemical fertilizers in agriculture and improper disposal of human and animal
wastes (Nugent et al., 1988). Three basic types of biochemical transformations occur as
sewage is treated.
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The first is the removal of soluble organic matter. This is composed of dissolved carbon
compound such as detergent, greases and body wastes, which make up much of the BOD
content of the sewage. The second is the digestion and stabilization of insoluble organic
matter. These are the sewage solids, such as body wastes and food particles, which make
up the remainder of the BOD. The third is the transformation of soluble inorganic matter such
as nitrogen and phosphorus.
Nitrification and denitrification are processes that will biologically convert ammonia to
nitrogen gas. These processes are intentional in treatment facilities that have nitrogen
limitation as part of their effluent discharge permits. In other treatment plants these
processes will take place automatically when the environmental condition changes to select
for these organism.
The treatment plant operator needs to understand the biology of nitrification and
denitrification to anticipate and control these processes.Nitrogen is present in many forms in
the septic system. Most nitrogen excreted by humans is in the form of organic nitrogen (dead
cell material, protein, amino acids) and urea. After entering the septic tank, this organic
nitrogen is broken down fairly rapidly and completely to ammonia, NH3, by microorganism in
the septic tank. Ammonia is the primary form of nitrogen leaving the septic tank. In the
presence of oxygen, bacteria will break ammonia down to nitrate, NO3.
In a conventional septic system with a well aerated leaching facility, it is likely that most
ammonia is broken down to nitrate beneath the leaching field. Nitrification is an important
process in wastewater treatment. Ammonia can be quite harmful if discharged. Examples of
the impacts of ammonia in a receiving stream include, (i) creates dissolved oxygen sag in
receiving stream, (ii) toxic to fish and other aquatic life and (iii) nutrient input ( when oxidized)
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Nitrate can have serious health effects when it enters drinking water wells and is consumed.
Nitrate and other forms of nitrogen can also have deleterious effect on the environment,
especially in coastal areas where excess nitrogen stimulates the process known as
eutrophication.For this reason, many alternative technologies have been designed to remove
total nitrogen from wastewater. These technologies use bacteria to convert ammonia and
nitrate to gaseous nitrogen,N2. In this form nitrogen is inert and is released to the air.
Nitrogen sources in Municipal Wastewater
Total nitrogen reaching a municipal wastewater treatment plant averages 15 – 50 mg/l. 60%
is ammonia, 40% is organic nitrogen, < 1% is nitrite or nitrate.During the normal treatment of
domestic wastewater, secondary treatment may remove up to 30% of the total nitrogen
coming into the plant.
Nitrosomonas : Oxidation of ammonia to nitrite
Nitrobacter
5.2
: Oxidation of nitrite to nitrate
NITRIFICATION PROCESS
Nitrification is defined as a process in which bacteria ( nitrosomonas and nitrobacter)
convert ammonia in wastewater into nitrate. The process occurs in the biosphere, provided
that the environments as such that the nitrifying bacteria can exist. The nitrification process
is very important for the oxygen condition in soil, stream, lakes and biological wastewater
treatment plants.
It is important to note that this process required and consumes oxygen. This contributes to
the BOD or biochemical oxygen demand of the sewage. The process is mediated by the
bacteria Nitrosomonas and nitrobacter which require an aerobic (presence of oxygen)
environment for growth and metabolism of nitrogen. Thus the nitrification process must
proceed under aerobic condition.
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
Chapter 5
Nitrification is a two step process
Step 1: The microorganism known as Nitrosomonas convert the ammonium into nitrite
oxidation of ammonia to nitrite
NH3
+
O2
+
Nitrosomonas sp bacteria
NO2
Step 2 : The microorganism known as nitrobacter convert nitrite into nitrate. Oxidation of
Nitrite to Nitrate
NO2
+
O2
+
NO3-
Nitrobacter sp bacteria
OR
The process for the ammonium oxidizing bacteria is
Nitrosomonas sp bacteria
NH4+
+
NO2-
3/2 O2
+
H20
+
2H+
The process for the nitrite oxidizing bacteria is
Nitrobacter sp bacteria
NO2-
+
½ O2
NO3-
5.2.1 Factors Influencing Rate of Nitrification
The rates of nitrification reaction are highly dependent on a number of environmental
factors. These include the substrate and oxygen concentration, temperature, pH and the
presence of toxic or inhibiting substances. The kinetics can be described with the Monod
expression, and in most cases the rate is limited by the first step, the oxidation of the
ammonia. At steady state, the amount of NO2- present is usually small and can be ignored in
initial design
 Temperature dependency
Nitrifying bacteria are slow growing and sensitive to temperature as well. The rate of
nitrification is greatly influenced by temperature. As the temperature increase, the
nitrification rate increase. Temperature of greater than 20°C up to about 35°C enhance
nitrification. Nitrification is inhibited at 10°C or less.
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Optimum temperature is about 25°C. Nitrification slow down dramatically or may stop
together at around 5°C. MCRT will need to increase when the temperature decrease to
maintain nitrification.
 Oxygen requirement for nitrification
Nitrifying bacteria are especially sensitive to low oxygen concentration, and providing
enough oxygen in wastewater treatment plant is often the most important design
consideration.The nitrifying bacteria are homogenous group of bacteria, characterized for
example by a low growth rate, this is due to the low energy yield, which is linked to the
oxidation of ammonium and nitrite, respectively.
The slow growth is a major problem for the nitrification in biological treatment plants. Most
nitrifying bacteria are autotrophic and thus use carbon dioxide as the carbon sources.
The carbon dioxide should be reduced before the carbon form part of the cell mass and
this reduction takes place through the oxidation of the nitrogen sources of the organism
concerned.The nitrification process is an autotrophic purely aerobic process that
consumes a large amount of oxygen. For every pound of ammonium oxidized, 4.6
pounds of oxygen is consumed. To put this in perspective, for every pound of BOD
removed, there is approximately 1 pound of oxygen consumed . It is highly recommended
for facilities that nitrify to maintain a DO reading at least 2.0 mg/l. In the nitrification zone,
so as to not inhibit the growth of the nitrifying bacteria. Biological oxidation of ammonia
can be expensive in term of energy costs.
 pH dependency
pH has strong effects on nitrification rates. The reactions occurs fastest when pH is from
7.5 – 8.5 although if bacteria exist in flocs or films the pH at the cell surface will be lower
than the bulk pH due to the production of H+ ions. The exact mechanism by which pH
effect the reaction rates are not fully understood, although it is believed that inhibitors
particularly from the neutral NH3 and HNO2 species, become important.
 Inhibiting substances – lack of toxins
Many substances can potentially inhibit the nitrification reactions. Metal are particularly
strong inhibitors of the reactions. When exposed to more than one inhibitor, the extent of
inhibition increase greatly.Nitrifying bacteria are more sensitive to inhibitory compound
such as heavy metals that are the BOD reducing bacteria. The nitrifying bacteria would
be the first ones to die off.
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 Alkalinity during Nitrification
The oxidation of ammonia to nitrate adds hydrogen ions produces acids to the water and
consumes alkalinity. The theoretical reaction shows that between 7.07 and 7.14 mg of
alkalinity (as CaCO3) is consumed for every mg of ammonia oxidized. Rule of thumb is 10
to 1. The result is an overall reduction in alkalinity and a possible drop in pH. Sufficient
alkalinity must be present to buffer the acids produced during nitrification. For every mole
of NH4+ - N which oxidized to NO2-, approx. 2 moles of HCO3- are consumed and this
correspond to 2 equivalent of alkalinity. This is essential for the nitrification of relatively
soft water where the pH in the water can be low that the nitrification process is limited or
stops completely.
5.2.2
Testing for nitrification
Ammonia N (NH3-N) – Influent and effluent – If substantial nitrification is taking place
you would see a significant decrease.Nitrite (NO2) – Effluent generally < 5 mg/l, anything
greater would be considered high.Nitrate (NO3) – Effluent – Expect around zero if there is no
nitrification taking place. If nitrification is occurring, expect nitrates anywhere from 3 – 15
mg/l or greater. Alkalinity (measured as CaCO3) Influent and effluent – Expect a significant
decrease if nitrification is taking place. Effluent concentrations < 50 mg/l indicates potential
for pH problems.
5.2.3 Problem at nitrification plant
High BOD’s can results if nitrification is taking place within the lagoons and there is
enough residual ammonia remaining in the effluent. When you collect an effluent
sample and set up BOD’s you are adding any nitrifying bacteria that may be present in
the effluent to the BOD bottle. They may feed upon the remaining ammonia and thus
use up D.O in the process, which will show up as an elevated BOD. Effluent BOD’s
High BOD
have been reported as > 80 mg/l under these circumstances. If this suspected of
causing long terms problems, perform and inhibited BOD test by adding nitrification
inhibitor. This results of this test are referred to as the carbonaceouous BOD, or
CBOD should be considerably lower than the BOD if nitrification is taking place and
will be a more reliable indicator of how the plant is functioning. The CBOD cannot be
used for reporting purposes unless it is specified in your permit.
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Low effluent pH may result if there is insufficient alkalinity present to buffer the acids
produced during nitrification. Remember nitrification also used up alkalinity. There
Low pH
should be at least 50 mg/l of alkalinity remaining in the effluent to prevent low pH
problems. If this is a long lasting problem, alkalinity may need to be added to the
systems
Nitrite can sometimes accumulate within the process due to the slowing down of the
High Cl2
Demand
nitrite forming bacteria – due to temperature changes. Nitrate formers are more
temperature sensitive. Nitrite has a high chlorine demand. 5 parts of Cl2 are used up
for every part of NO2- oxidized to NO3-. Under normal condition, NO2- levels will be
minimal, from 0-5 mg/l
APPLICATION OF NITRIFICATION
Nitrification is the biological process by which ammonia is first converted to nitrite and then
nitrate. Nitrification can be achieved in any aerobic biological process at low organic loadings
and where suitable environmental conditions are provided. Nitrifying bacteria are slower
growing than the heterotrophic bacteria, which comprises the greater proportions of the
biomass in both fixed film and suspended growth systems. The key requirement for
nitrification to occur, therefore, is that the process should be so controlled that the net rate of
accumulation of biomass, and hence, the net rate of withdrawl of biomass from the system,
is less than the growth rate of nitrifying bacteria (Barners and Bliss, 1983). The processes
currently used in treatment of wastewater for nitrification are presented as follows.
Nitrification in Trickling Filters
The extent of nitrification in trickling filters depended on a variety of factors ; including
temperature, dissolved oxygen, pH, presence of inhibitors, filter depth and media type,
loading rate and wastewater BOD (Parker and Richard, 1986). Low rate trickling filters
allowed the development of a high nitrifying populations. For rock media filters, organic
loading should not exceed 0.16 kg BOD5/m3/day. (USEPA,1975). Higher loading rates (0.36
kg BOD5/m3/day) were allowable in plastic media trickling filter because of the higher surface
area of the plastic media (Stenquist, et al.,1974). If two trickling filter were used,
heterotrophic growth occurs in the first filter and nitrification in the second filter (Bitton,
1994). Boller and Gujer (1986) conducted a pilot plant study of tertiary trickling filters,
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recommending a media surface loading rate of 0.4 g NH3-N/m2/day for complete nitrification
(effluent NH3-N<2.0 mg/l) at a water temperature of 10°C.
Nitrification in Rotating Biological Contactor
RBC biofilm has an initial adsorption of microorganisms to the disk surface to form 1-4 mm
thick biofilm that is responsible for BOD removal in rotating biological contactors. The
rotating disk provided a large surface area for the attached biomass. The first stages of an
RBC mostly removed organic matrials, whereas subsequent stage removed NH3-N as a
result of nitrification, when the BOD5 was low enough. Ammonia oxidizer could not
effectively complete with the faster growing heterotrophy that oxidize organic matter.
Nitrification occurs only when the BOD was reduced to approximately 14 mg/l and increase
with rotation speed (Weng and Molof, 1974). RBC performance was negatively affected by
low dissolved oxygen in the first stages and by low pH in the later stages where nitrification
occurred (Hitdlebaugh and Miller, 1981).
An innovative operational process using recirculation in RBC was developed and used to
improve nitrification (Klees and Silverstein, 1992). They found that it could improve
nitrification at all hydraulic loading rates; the positive effect of recirculation on nitrification
was due to the dilution of influent organic carbon. Degradable organic carbon inhibits
nitrification at concentration greater than 15 -20 mg/l BOD5; extremely low concentration of
influent BOD5 (less than 10 mg/l) did not improve nitrification.
Nitrification in Fixed bed reactor
Jimenez, et al (1987), conducted a pilot scale research by using a fixed bed reactor for
nitrification of the effluent from an extended aeration sewage treatment plant. The nonsettleable sewage influent of COD of 373 mg/l, NH4-N of 45 mg/l, SS of 297 mg/l and pH 8.1
were used in this study. With and HRT of 4 to 6 hours and recycle ratio of 3.5 removal
efficiencies of 70% of COD, 67% of SS and 95% of NH4-N were obtained.
Less than 10 mg/l effluent BOD and ammonia removal efficiency of 65-85% were achieved
in a four stage aerobic submerged biofilm reactor containing ball-ring (Gonzalez-Martinez
and Duqu-Luciano,1992). Nitrification was achieved in all 4 reactor operated at the loading
rate of 6-21g/m2/d and the HRT of 130 – 290 minutes.
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Nitrification in a conventional activated sludge process at low loading
Weismann (1994) studied the nitrification in a conventional activated sludge system and
found that it was relatively low for carbon removal and nitrification of sewage because
carbon removal and nitrification occurred in the same reactor with an activated sludge
system. This resulted in a population mixture of mainly heterotrophs and few autotroph. In
this kind of treatment system, it was not possible to enrich the autotrophic bacteria because
the slower growing autotroph were removed with the surplus sludge. It was necessary to
separate the autotrophic from the heterotrophic biomass in order to increase the specific
nitrification rate.
Suwa, et al (1989), conducted a research on simultaneous organic carbon removal
nitrification by an activated sludge process with cross flow filtration. Because of the recycle
of sludges with cross flow filtration, this process made the sludge retention time very long;
simultaneous carbon removal-nitrification was achieved quite well under the loading rate of
about 0.10 g BOD/g VSS/d. The efficiency of dissolved organic carbon removal was more
than 95% and nitrification was sufficient (NH3-N was not detected in the effluent)
Two stage activated sludge systems with separate carbonaceous oxidation and
nitrification systems.
The nitrification process requires a slow growing nitrifying bacteria with sludge that has been
aged for a long time and high dissolved oxygen concentration. In addition, they were
susceptible to inhibition by a wide range of compound at concentration so low as not to
affect the heterotrophic bacteria. For these reasons, it would seem sensible to separate the
process of carboneous removal and nitrogen removal into separate reactors (Horan, 1989).
Rimer and Woodward (1972) investigated separate carboneous oxidation and nitrification
systems and found that this process minimized sludge washout with the nitrification stage,
and the process could be operated successfully at a shorter detention time, lower MLSS and
solid retention time.
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5.3
Chapter 5
DENITRIFICATION
Denitirification is the biological process by which nitrate is converted to nitrogen and
other gaseous end products. This is typically done through an anoxic zone that precedes the
aerobic zone. This usually happens in the secondary clarifier or in a dedicated anoxic tank.
In the aerobic zone, nitrification takes place and produces nitrate. A portion of the mixed
liquor is returned to the head end of the anoxic zone ( for a source of nitrate). In the anoxic
zone, the lack of elemental oxygen causes the bacteria to derive their oxygen chemically
and they therefore convert the nitrate to nitrite and ultimately nitrogen gas.
Denitrification reaction
The conversion of nitrate to nitrogen gas, is reffered to as denitrification. This process can be
summarized as:
NO3- + 5/6 CH3OH
½ N2 + 5/6 CO2 + 7/6 H2O + OH-
The requirement for the denitrification process are : a) nitrogen in the form of nitrates, b) an
organic carbon source and ,c) an anaerobic environment.
This process is also mediated by bacteria. For the reduction of nitrate to nitrogen gas to
occur, the dissolved oxygen level must be at or near zero; the denitrification process must
proceed under anaerobic conditions. The bacteria also require a carbon food source for
energy and conversion of nitrogen. The bacteria metabolize the carbonaceous material for
BOD in the wastewater as this food source, metabolizing it to carbon dioxide. This in turn
reduces the BOD of the sewage, which is desirable. However, if the sewage is already low in
BOD, the carbon food source will be insufficient for bacterial growth and denitrification will
not proceed efficiently.
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Carbon and Nitrogen Removal Process
Currently, the process used for carbon and nitrogen removal can be divided into two major
groups : separated stage and single stage processes. For multiple stages of carbon and
nitrogen removal, there is a disadvantages for denitrification which occurs either in the
addition of external carbon or the recycle part of the effluent of nitrifying bacteria. Carbon
and nitrogen removal occurring in a single unit is a possibility to overcome these
disadvantages.
In general, the nitrification and denitrification processes can be applied for the removal of
nitrogen contained in the wastewater. The concentration of nitrogen in the wastewater can
be from low to high contents, which require an appropriate treatment process for the
removal. The application of a certain biological treatment method or process will vary,
depending on the location, land availability, social and economic concerns. Thus, both
traditional and innovative technologies for the nitrification and denitrification processes for
nitrogen removal from the wastewater need to be evaluated and applied properly. Thus the
protection of our environment and conservation of our resources can be warranted.
5.3.1 Factor affecting rate of denitrification reaction
pH and Alkalinity during denitrification : The optimum pH for denitrification is 7.0 to 7.5.
The denitirification process produces bicarbonate. Theory: 3.57 mg CaCO3 / mg N2 gas.
Alkalinity gain is about half the loss of nitrification.
Carbon source : The denitrifying bacteria require carbon for growth. In the clarifier or
another anoxic reactor, the carbon available due to endogenous respiration may be enough.
If outside sources of carbon are required raw wastewater, methanol or other sources are
sometimes used.
Clarifier observation during denitrification : If this is an older sludge condition, we could
expect clarifier conditions to fit this category: pin floc, ashing. If the sludge is allowed to sit in
the clarifier too long we would begin to see floating clumps come apart, the particles will
contribute to the ashing on the surface of the clarifier.
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Other Anoxic Processes : Some treatment facilities will design a process step or tank for
denitrification and gas stripping. Additional sections of the aeration tank with internal recycle
flows are used for this purpose. Gas stripping requires aeration of high turbulence before the
flow heads to the clarifier. The oxidation ditch is designed to provide for nitrification and
denitrification.
Any
wastewater
treatment
unit
that
is
going
to
remove
nitrogen
by
the
nitrification/denitrification process must be designed to provide both aerobic and anaerobic
areas so that both nitrification and denitrification can proceed.
Separated and combined nitrification and denitrification processes – Application of various
treatment technologies, either additional or innovative, depend on the location, social and
economic considerations. Also, it may vary for small and large wastewater treatment plants
for the application of nitrification and denitrification.
5.3.2 Problem of denitrification plant
While denitirification can be beneficial, if it occurs in the wrong part of the treatment
plant ( such as the clarifier), it can be a problem. Establishing the anoxic zone causes the
denitrification to take place in the anoxic zone when properly operated and prevents
denitrification from taking place in parts of the treatment system where we do not want it
such as the clarifier.
Denitrification is accomplished with many facultative organism, such as Achromobacter,
Bacillus and Pseidomonas. When elemental oxygen is depleted, facultative organism search
for another electron receptor such as nitrate, sulfate or carbonate ( in the order). The
denitrifying enzyeme is always present but is inhibited by dissolved oxygen. As soon as
elemental oxygen is depleted the facultative organism immediately begin using nitrate for
respiration. It is important to maintain proper nitrate recycle. ( 1 x to 2x influent flow) and a
DO near zero in the anoxic zone. Partial denitrification will produce nitrite ( that is toxic) and
cause the chlorine demand to jump way up.
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5.3.3 Benefit of denitrification
Energy saving : Through nitrate uptake in the anoxic zones 60% of the oxygen
consumed during nitrification is returned to the process. Approximately 4.6 Lbs of oxygen is
consumed during nitirification while denitrification returns 2.86 Lbs of oxygen to the system.
That amounts to 60% of the oxygen required for nitrification. Aeration is one of the significant
costs for wastewater facility. Nitrification can almost double you aeration costs. The cost
saving could be significant.
Chemical saving : In the anoxic zone, approximately 3.6 Lbs of the alkalinity consumed in
the nitrification process ( approximately 7.2 Lbs) is returned to the process. That means we
get about 50% of the alkalinity we consumed back. This may reduce or eliminate the need to
add supplemental alkalinity.
Better Process Control : Significant cBOD uptake occurs in the anoxic zone , thus starving
troublesome filamentous organism of their food source. This results is a better settling
sludge and reduces or eliminates the need to chlorinate the return activated sludge.
Lower Sludge Production : Studies indicates that facilities that utilize denitrification reduce
overall sludge production by 5% or more. One of the rising costs today in many budget is
sludge disposal. A reduction in the quantity produced equates to a direct savings.
Better Quality Effluent : By reducing total nitrogen in our effluent, we reduces the amount
of fertilizer that is ultimately added to the receiving stream. Excess nitrate often times causes
excessive algae blooms. Also, nitrate can be a real problem for downstream water supply
intakes.
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5.4
Chapter 5
EMERGING TECHNOLOGIES
There are a number of alternative technologies available for ammonia removal. Both
breakpoint chlorination and ion exchange involve complex technologies that are high cost
and require skilled operators.
Air stripping of ammonia involves the passage of large quantities of air over the exposed
surface of the leachate, thus causing the partial pressure of the ammonia gas within the
water to drive the ammonia from the liquid phase to the gas phase. A commonly
encountered difficulty was that of temperature (Damhaug and Jahren, 1981). The use of
ammonia strippers would be too costly because of the volumes of air required (Knox, 1983).
The wind cooling effect of air passing up a column was also considered to be a source of
difficulty as the ambient temperature approached zero (Reynolds, 1982). A temperature drop
of around 5 to 6⁰C was found to be unexceptional. The presence of landfill gas in a landfill
sites does, however, add another dimension to the realistic application of such technology.
Landfill gas can be used to raise the temperature of the leachate, thus making the removal
of ammonia easier and less prone to the problems encountered by earlier researchers. As
the temperature rises, not only does the risk of freezing decline but also the amount of
ammonia gas available for removal increases.
There are two processes for removal from groundwater – stripping and GAC adsorption. The
only efficient method of aeration is air stripping in a countercurrent packed tower, where the
water trickles downward through the packing while air is forced up through the voids of
packing. However, because of low allowable maximum contaminant levels, aeration may not
be able to reduce the concentration to drinking water standards. Furthermore, it cold
climates, aeration is not nor feasible because of poor removal at low temperature and the
possibility of ice formation on the tower packing. The more costly process of adsorption by a
GAC filter may be used, instead of air stripping or be applied as a second stage following
partial removal by aeration.
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Advantages
 Process can be controlled to achieved required removal rates
 Not sensitive to toxic substances
 Process may be able to meet total nitrogen standards
Disadvantages
 Process temperature sensitive requiring either temperature contol or having varied
performance
 Subject to ambient temperatures
 Process often requires pH adjustment
 Carbonate scaling of packing
Ammonia stripping has the commonly perceived significant disadvantage of requiring pH
adjustment. In practice the conversion of ammonium ions to ammonia gas is a function of
both temperature and pH, raising the possibility that temperature alone may be adequate to
carry through the conversion necessary for air stripping to be effective. Ammonia nitrogen is
present in water in two forms. The first is as disassociated ammonia, NH4+, also referred to
as a ammonium ion. The second is as undissociated ammonia NH3, known as ammonia
gas. The equation governing the relationship between ammonia gas and the ammonium ion
is as follows:
NH4+ + OH-
NH3 + H2O
Dissociated ammonia is converted to undissociated ammonia is converted to undissociated
ammonia by the addition of a base, such as sodium hydroxide. It has also been noted that
as the temperature of the water increase so the amount of free ammonia gas also increase
(Srinath and loehr, 1974). The ratio of ammonia in the gas phase to the total ammoniacal
nitrogen referred to as “f” may be expressed as follows :
f = [NH3] / [NH3] + [NH4]
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5.4.1 Soul water strippers
Stripping processes are used to removed selected constituents from liquid streams.
The two most prevalent pollutant found in refinery wastewater which are susceptible to
stripping are hydrogen sulfide and ammonia resulting from the destruction of organic
nitrogen and sulfur compound during desulphurization, denitrification and hydro treating. The
use of steam within the process is the primary source of conveyance, as the condensation
occurs simultaneously with the condensation of hydrocarbon and in the presence of a
hydrocarbon vapor phase which contain H2S and NH3. Phenol also present in these phase “
sour water” condensates and can be stripped from solutions, although the efficiency of
removal is less than for sulfide and ammonia. Other aromatics also can be stripped from
solution at various level of efficiency.
AIR STRIPPING
Air and Vaporized
Contaminants
Off Gas
Treatment
Treated Oil
Gas
Contaminated
Groundwater or
wastewater
Packing or trays
Sump (Collect wastewater
Clean Air
Compressor
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Air stripping involves the mass transfer of volatile contaminants from water to air. For
groundwater remediation, this process is typically conducted in a packed tower or a low
profile aeration system. The typical packed tower air stripper includes a spray nozzle at the
top of the tower to distribute contaminated water over the packing in the column, a fan to
force air countercurrent to the water flow, and a sump at the bottom of the tower to collect
decontaminated water. Low profile air strippers are available in horizontal tray or vertical box
designs. Baffles are used to route contaminated water two or more times along the length of
the tray or height of the box. Air sparged through the bottom of the tray or through a vent
pipe in the bottom of the box passes up through the water to strip out volatile compounds.
Several trays or boxes can be used to increase flow capacity or stripping efficiency.
Auxiliary equipment that can be added to the basic air stripper includes an air heater to
improve removal efficiencies, automated control systems with sump level switches and
explosion proof components, and air emission control and treatment systems such as
activated carbon units, catalytic oxidizers, or thermal oxidizer. Packed tower air strippers are
installed either as permanent installation on concrete pads or on a skid or a trailer. Low
profile air stripper are small package units suitable for skid or floor mounting.
Aeration tanks strip volatile compounds by bubbling air into a tank through which
contaminated water flows. A forced air blower and a distribution manifold are designed to
ensure air water contact without the need for any packing materials. The baffles multiple
units ensure adequate residence time for stripping to occur. Aeration tanks are typically sold
as continuously operated skid mounted units.
The advantages offered by aeration tanks are considerably lower profiles ( less than 2
meters or 6 feet high) than packed column towers ( 5 to 12 meters of 15 to 40 feet high)
where height may be a problem, and the ability to modify performance or adapt to changing
feed composition by adding or removing trays or chambers. The discharge air from aeration
tanks can be treated using the same technology as for packed tower air discharge treatment.
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Nitrification & Denitrification
ENV 501
Chapter 5
Air strippers can be operated continuously or in a batch mode where the air stripper is
intermittently fed from a collection tank. The batch mode ensures consistent air stripper
performance and greater energy efficiency than continuously operated units, because mixing
in the storage tanks eliminates any consistencies in feed water composition.
Applicability
Air stripping is used to separate halogenated and nonhalogenated volatile organic
compound (VOCs) from water, but is ineffective for contaminants with low vapor pressure or
high solubility such as inorganic salts. The Henry’s law constant is used to determine
whether air stripping will be effective. Generally, organic compound with constant greater
than 0.01 atmosphere (m3/mol) are considered amenable to stripping. Some compounds
that have been successfully separated from water using air stripping include benzene,
toluene, ethylbenzene, and xylenes (BTEX), chloroethane, trichloroethylene, dicloroethlene
and perchloroethylene.
Limitation
The following factors may limit applicability and effectiveness of the process : (i) Suspended
solids or solids formed by precipitation of reduced iron or manganese species can foul
packing or tray, requiring pre treatment to remove the fouling agents.(ii) High concentration
of dissolved calcium or magnesium can react with carbon dioxide in the air, forming
carbonate scale on the packing or trays.(iii) Biological growth can foul the trays or packing,
requiring periodic column cleaning. (iv) Compound with low volatility at ambient temperature
may require preheating of the groundwater. (v) Off-gases will require treatment if the mass
emission rate exceeds locally allowed limits.
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Chapter 5
Nitrification & Denitrification
ENV 501
5.4.2 ION EXCHANGE
Contaminated Groundwater
or Wastewater
Ion Exchange
Resin
Regenerant
waste to
disposal
Regenerant
collection
Backwash
water
Regeneration
chemicals
Treated
water
Ion exchange treatment involves passing contaminated water through an ion exchange resin
so that contaminants exchange onto sites on the media, exhausting its capacity. After resin
capacity has been exhausted, resins can be regenerated for reuse. Ion exchange material
usually consists of synthetic organic polymers that contain ionic functional groups to which
exchangeable ions are attached. Inorganic or naturalpolymeric materials such as zeolites
may also exhibit useful ion exchange properties.
The most effective exchanger for nitrate removal is strongly basic anion resin that uses
sodium chloride fro regeneration. All anion exchange resins preferentially remove divalent
anions therefore both sulfate and nitrate ions are removed and replaced by chloride ions.
This constituents a major difficulty in processing water for nitrate removal since many of the
groundwater contaminated with nitrate also have as significant sulfate concentration. When
the capacity for extracting nitrate ions is depleted. A regeneration solution with a higher
content is passed through the bed to displace the sulfate and nitrate ions.
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Nitrification & Denitrification
ENV 501
RCl + SO42-
Nitrate removal
Chapter 5
RSO4 + Cl-
The quantity of waste brine from regeneration is about 5 percent of the water processed.
The amount varies with the sulfate content of the water being treated and the exchange
capacity of the resin, which decrease with usage. The major disadvantages of anion
exchange treatment for nitrate removal are high operating costs and the problem of brine
disposal.
Applicability
Ion exchange is used primarily to treat inorganic by removing dissolved ionic contaminants
such as metals (e.g., lead and copper) or anions (e.g., nitrate and ammonia) from aqueous
solutions.)
Limitation
The following factors may limit the applicability and effectiveness of the process:

Different resins are needed to remove cations and anions

Oil and grease can foul the exchange resin

Suspended solids or solids formed by precipitation of reduced iron or
manganese species in solution can foul the exchange resin.

The pH of the influent water may affect the ion exchange resin selection

Oxidants in groundwater may damage the ion exchange resin.

Wastewater is generated during the regeneration step and will require additional
treatment and disposal

Concentration of dissolved solids > 4,000 will rapidly exhaust bed capacity. The
technology typically is used to treat concentration up to about 200 to 500 mg/l
Duration
Ion exchange is used tot reat pumped groundwater or wastewater residual from a treatment
process. The operation and maintenance duration depends on the duration of the primary
process operation.
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Chapter 5
Nitrification & Denitrification
ENV 501
Raw
water
Drain
Mixed
Resin
Treated
Air
Out
Alkali
Backwash
Simultaneous
Regeneration
Mixing
Anion
Anion
Anion
Cation
Cation
Cation
Raw
water
Acid
Principle of mixed bed ion exchange: (a) service period; (b) Backwash period (c)
Simultaneos Regeneration
Advantages
 Produces a reclaimable aqueous ammonia product
 Process may be able to meet total nitrogen standard
 Ease of product quality control
Disadvantages
 Organic matter can cause resin binding
 High concentration of other cations will reduce ammonia removal capability.
 High capital and operating costs and requires highly skilled operator.
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Air
Nitrification & Denitrification
ENV 501
Chapter 5
At this point you should be able to:

To understand the chemical and biological processes in nitrogen removal

To become familiar with the operation of nitrogen removal processes and the impact
of aerobic, anaerobic and anoxic conditions on them
PART A: DEFINITION
Please define the following terms:
Nitrifying bacteria are chemoautotrophic. TRUE (
) FALSE (
)
The conversion of dinitrogen (N2) to ammonia and subsequently to organic nitrogen is
termed nitrogen fixation. TRUE (
) FALSE (
)
At the critical point in an oxygen sag curve, the rate of reaeration is _____________ the rate
of respiration.
In nitrification, ammonia is oxidized first to nitrite and then to _______________.
PART B: SHORT ANSWER
Answer the following questions:
In advanced wastewater treatment, it often involves removal of nitrogen and phosphorus
compound as well as suspended solids, dissolved inorganic salts and refractory organic.
Suggest the suitable method(s) to remove nitrogen compound
121 |N i t r i f i c a t i o n & D e n i t r i f i c a t i o n
Chapter 5
Nitrification & Denitrification
ENV 501
STUDY NOTES:
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