ch2mbr

Nitrogen Removal from Wastewater

The focus is on biological nitrogen removal, in which two biological processes are used: nitrification and de-nitrification.
Nitrification is a two step microbiological reaction in which ammonia nitrogen is converted into nitrite by Nitrosomonas and subsequently
into nitrate by Nitrobacter. In the de-nitrification process the produced nitrate is converted into harmless nitrogen gas.

1. Optimized Design of Nitrogen Removal

1-1 Determination of nitrogen removal capacity

The concepts of nitrification and de-nitrification capacity will be used to determine the extent of nitrogen removal possible
in an activated sludge system, given the influent nitrogen- and COD composition and the values of the kinetic parameters.
First a new parameter will be introduced, i.e. the amount of nitrate available for de-nitrification.
This parameter is defined as the fraction of the nitrification capacity that will be effectively available
for de-nitrification in the anoxic reactor for the selected values of the mixed liquor- and sludge recirculation factors.
Now, as a function of the applied sludge age, the values of nitrification capacity, nitrate available
for de-nitrification and de-nitrification capacity will be compared, taking into account the maximum anoxic sludge
fraction and minimum required aerobic sludge age. It will then be possible to predict the minimum effluent
nitrogen concentration- and composition, as a function of the applied sludge age.

1-2 Optimized design of nitrogen removal

A variable of crucial importance is introduced: the ratio between nitrogenous and organic (COD)
material in the influent. It will be demonstrated that for each value of the sludge age it can be established whether
it is possible achieve complete nitrogen removal and if not, what configuration will be
superior: pre-D or Bardenpho. Now, it will be easy to select the operational sludge age for which compliance
to the specified effluent limits is possible and to finalize the design of the activated sludge system.

2. Nitrification

Nitrification is the term used to describe the two-step biological process in which ammonia (NH4-N)
is oxidized to nitrite (NO2-N) and nitrite is oxidized to nitrate (NO3-N).

Since nitrate formation from nitrite is far faster than nitrite formation from ammonia,
only a negligible amount of nitrite exists at steady state in a typical reaction condition.

The need for nitrification in wastewater treatment arises from water quality concerns over

·  the effect of ammonia on receiving water with respect to DO concentrations and fish toxicity
·  the need to provide nitrogen removal to control eutrophication
·  the need to provide nitrogen control for water-reuse applications including groundwater recharge.

2-1 Process Description

The autotrophic bacteria responsible for nitrification not only grow slower than heterotrophic bacteria
(systems designed for nitrification generally have much longer hydraulic and solids retention times than those for
systems designed only for BOD removal), but also are more susceptible to the environmental changes such as temperature,
pH, and the toxic chemicals in wastewater. High ammonia concentration can inhibit nitrification,
but the threshold concentration varies depending on pH. In general high level ammonia is more toxic at low
pH because unionized dissolved NH3 can penetrate microbial cells easier to cause physiological problems.
High nitrite (NO2-) level in mixed liquor indicates a problem in nitratation, which is normally a fast reaction,
but it is not clear whether the nitrite itself is toxic to the nitrobacter that is responsible for nitratation.

Nitritation by Nitrosomonas (Slow)	NH4+ + 1.5 O2 → NO2- + 2H+ + H2O
Nitratation by Nitrobacter (Fast)	NO2- + 0.5O2 → NO3-
Overall reaction	NH4+ + 2 O2 → NO3- + 2H+ + H2O
Molecular weight(Dalton	NH4+ + 2 O2 → NO3- + 2H+ + H2O
Molecular weight(Dalton)	14 (N only) 2×32
Alkalinity (eqiv. CaCO3)	0   0   0   -2   0

According to above equations, two moles of acidity are produced, which also means two moles of alkalinity are consumed,
when one mole of NH4+ is oxidized. Here, NH4+ is considered neutral since it consists of one alkalinity (NH3)
and one acidity (H+). The specific oxygen requirement for ammonia nitrogen oxidation can be calculated based
on above equations. Assuming no new autotrophic microorganisms are produced during the reaction,
4.57 mg O2(=2×32/14) and 7.14 mg alkalinity as CaCO3 (= 2×50/14) are consumed to oxidize 1 mg NH4-N.

In practical situations, however, some nitrogen atoms are used to produce new microorganisms.
The following equations represent nitritation and nitratation reactions in typical nitrification condition (Tchobanoglous, 2002).
Due to the partial loss of nitrogen to the new cells, the O2 and alkalinity consumptions based on the treated NH4-N
are slightly lower than those values above. Although the exact values vary system by system due to the varying
autotrophic sludge yields, equivalent oxygen and alkalinity consumptions are often assumed at 4.3 mg O2/mg N and 6.8 mg CaCO3/mg N.

Nitritation	55NH4+ + 76 O2 + 109 HCO3- → C5H7O2N (cell) + 54NO2- + 57H2O + 104H2CO3
Nitratation	400NO2- + NH4+ + 4H2CO3 + HCO3- +1950O2 → C5H7O2N (cell) +3H2O +400NO3-

2-2 Environmental Factors

2-2-1 pH and alkalinity effect

The optimum pH for nitrification is known to be around 7.5, which is the middle ground of the two optimum pH
forNitrosomonas and Nitrobacter that grow fastest at pH 7.8-8.0 and at pH 7.3-7.5, respectively.

It is well know that nitrification rate slows down at a pH below 7.0 and it ceases at around pH 6.0. However,
it must be noted that the diminished nitrification with decreasing pH is only true when pH drops or fluctuates rapidly.
If mixed liquor is acclimated at a low pH for extended period of time, nitrification occurs even at a pH below 6.0 (Tarre, 2004).

Though full scale studies are rare, many lab-scale studies have shown the possibilities that some of the minor
species in Nitrosomonas and Nitrobacter genera grow fast at low pH and replaces major species.
It is also possible that the same species playing a role at high pH adapts to low pH.

Excessive pH fluctuation can occur at low alkalinity, which in turn decreases the activity of nitrifiers.
Therefore, high alkalinity is crucial to prevent pH from excessive fluctuations especially when TKN loading fluctuates.

Although the concentration of dissolved CO2, which is used as building blocks of autotrophs (or nitrifiers),
decreases as alkalinity decreases, there is no enough evidence to prove low CO2 concentration
in mixed liquor directly hinders the growth of autotrophs in field condition.

2-2-2 Temperature effect

It is well known that the growth of nitrifiers, which is already slow, slows down further at low temperature,
which possibly makes nitrifiers washed out and the maximum growth rate of nitrifiers
decreases less than a half when temperature decreases from 14 oC to 6 oC in long-term tests.

Maximum nitrification rate per cell mass also decreases with temperature, however the slower
growth rate at lower temperature does not directly mean nitrification rate decreases
proportionally in continuously running biological systems due to the following reasons.

·  First, there is a great deal of redundancy in nitrifier population due to the long SRT
    in biological nutrient removal (BNR) processes especially in MBR. In MBR, the high solids retention time (SRT)
    of 12-30 days enables the enrichment of autotrophs. Therefore, poor nitrification is not commonly observed
    in MBR treating municipal wastewater as long as there are no drastic changes in water pH, temperature, TKN loading, etc.
·  Second, if there is any residual ammonia left over as a result of a slower nitrification,
    nitrifier can grow faster due to the available food, NH4+-N. This self-correcting mechanism can keep
    nitrification efficiency high as long as there is no drastic changes in reaction condition.

2-2-3 Dissolved oxygen effect

Nitrification rate is also affected by dissolved oxygen (DO) level. DO of 1 mg/L is considered as a minimum
requirement to prevent any inhibition caused by the insufficient oxygen level (Tchobanoglous, 2002).
At lower DO than 1 mg/L, nitrification slows down in general. At below 0.5 mg/L DO,
nitrification nearly stops and de-nitrification starts to occur. However, if low DO condition sustains stably,
biological systems can eventually adapt to the low DO and nitrification occurs at below 0.5 mg/L. In one full scale
dairy wastewater treatment plant, author observed nitrification was accomplished nearly 100%
at a DO below detection limit while oxidation reduction potential (ORP) was at persistently around 0 mV.
Therefore, it appears that the most crucial factor affecting nitrification efficiency
in biological wastewater treatment is the stability of operational condition.

2-2-4 Toxicity effect

Nitrifying organisms are sensitive to a wide range of organic and inorganic compounds and at concentrations
well below those concentrations that would affect aerobic heterotrophioc organisms.
In many cases, nitrification rates are inhibited even though bacteria continue to grow
and oxidize ammonia and nitrite, but at significantly reduced rates.

In some cases, toxicity may be sufficient to kill the nitrifying bacteria. Nitrifiers have been
shown to be good indicators of the presence of organic toxic compounds at low concentration (Blum and Speec, 1991).

Compounds that are toxic include solvent organic chemicals, amines, proteins, tannins,
phenolic compounds, alcohols, cyanates, ethers, carbamates, and benzene. Because of the numerous compounds that
can inhibit nitrification, it is difficult to pinpoint the source of nitrification toxicity
for wastewater plants with inhibition, and extensive sampling of the collection system is normally needed to find the source.

2-2-5 Metals

Metals are also of concern for nitrifiers, and Skinner and Walker (1961) have shown complete
inhibition of ammonia oxidation at 0.25 mg/L nickel, 0.25 mg/L chromium, and 0.10 mg/L copper.

2-2-6 Un-ionized Ammonia

Nitrification is also inhibited by un-ionized ammonia (NH3) or free ammonia, and un-ionized acid (HNO2).
The inhibition effects are dependent on the total nitrogen species concentration, temperature, and pH.

3. De-Nitrification

The biological reduction of nitrate to nitric oxide, nitrous oxide, and nitrogen gas is termed de-nitrification.
Biological de-nitrification is an integral part of biological nitrogen removal, which involves both nitrification and de-nitrification.

Compared to alternatives of ammonia stripping, breakpoint chlorination, and ion exchange,
biological nitrogen removal is used in wastewater treatment where there are concerns for eutrofication,
and where groundwater must be protected against elevated NO3-N concentrations
where wastewater treatment plant effluent is used for groundwater recharge and other reclaimed water applications.

3-1 Process Description

Two modes of nitrate removal can occur in biological processes, and these are termed assimilating
and dissimilating nitrate reduction. Assimilating nitrate reduction involves the reduction of nitrate to ammonia for use in cell synthesis.

·  Assimilation occurs when NH4-N is not available and is independent of DO concentration. On the other hand,
·  Dissimilating nitrate reduction or biological de-nitrification is coupled
    to the respiratory electron transport train, and nitrate or nitrite is used as an
    electron acceptor for the oxidation of a variety of organic or inorganic electron donors.

3-2 De-nitrification Prerequisites

The necessary conditions for the de-nitrification process to develop in an activated sludge process can be summarized as:

·  Presence of a facultative bacterial mass, capable of using oxygen and nitrate or nitrite
·  Presence of nitrate and absence of dissolved oxygen in the mixed liquor (i.e. an anoxic environment)
·  Suitable environmental conditions for bacterial growth
·  Presence of an electron donor (nitrate redactor), i.e. organic material

3-3 De-nitrification Factors

3-3-1 Effect of ORP

Biological denitrification occurs when molecular oxygen (O2) is not sufficient for microbial respiration.

Under this condition, the combined oxygen in nitrate (NO3-N) can be used as an oxygen source
for hetrotrophs and the reduced nitrogen escapes as molecular nitrogen (N2) from mixed liquor.

The condition where denitrification occurs is called “anoxic” in order to distinguish it from “anaerobic” condition,
where no oxygen sources exist whether it is molecular oxygen or combined oxygen.

Oxidation reduction potential (ORP) is used to monitor and control anoxic and anaerobic conditions.
Once oxygen supply stops, dissolved molecular oxygen serves as an electron acceptor until ORP decreases to around
+50mV according to Fig. 1. At +50mV, nitrate starts to serve as an electron donor until it depletes at the ORP
of around -50mV. If there is no additional nitrate or oxygen supply, sulfate becomes an electron donor and ORP decreases below -50mV.

The exact ORP that triggers denitrification varies quite a bit depending on water chemistry especially pH.
In addition, due to the low DO inside biological floc, denitrification can occur at relatively high ORP in bulk.
Therefore, the ORP values must be considered as guidelines with uncertainties of ± 50-100 mV.

For example, the triggering ORP for denitrification can varies between -100mV and 100 mV depending on situation.

3-3-2 Requirement of readily biodegradable COD/BOD

Denitrification requires electron donors to reduce the combined oxygen in nitrate. If methanol is used,
the reaction can be written as follow, where 1.905 mg methanol is required to reduce 1 mg NO3-N (=5×32/6/14).
If the methanol requirement is converted to COD requirement, it becomes 2.86 mg COD/mg NO3-N
considering the conversion factor of 1.5 mg COD/mg methanol.

6 NO3- + 5 CH3OH → 3N2 + 5CO2 + 7H2O + 6OH-
Molecular weight :
6×14(N only) 5 x 32

The HRT of anoxic tank cannot be prolonged too much due to the adverse effect on microorganisms at low DO/ORP condition.
HRT of anoxic tank is typically 0.5-2 hours in municipal wastewater treatment. Due to the short duration of wastewater
in anoxic tank, only readily biodegradable portion of COD (or BOD) can be utilized for denitrification.

The approximate specific denitrification rates (SNDR) of some of the popular carbon sources are summarized in Table 1.

Since denitrification is a fast reaction and the majority of hetrotrophs participate it,
the partial activity loss due to low temperature and other non-ideal conditions is not a major concern.

Overall, denitrification rate is mainly controlled by the amount and the quality of readily biodegradable COD supplied
to the anoxic tank. In general, BOD/TKN ratio in wastewater needs to be higher than 3, more preferably higher than 4,
to obtain a good nitrification in biological nutrient removal (BNR) processes.

Table. 1. Typical de-nitrification rates for various carbon sources (Tchobanoglous, 2002)

Carbon source	SNDR, (g NO3-N/g VSS/d)	Temperature (oC)
Methanol	0.21-0.32	25
Ethanol	0.12-0.90	20
Wastewater	0.03-0.11	15-27
Endogenous metabolism	0.017-0.048	12-20

Following graph (Fig. 1) shows the specific de-nitrification rate (SNDR) as functions of F/M ratio in anoxic
tank and the content of readily biodegradable BOD in feed wastewater to anoxic tank.

If F/M ratio is 0.5 g BOD/g MLSS/d in anoxic tank and readily biodegradable BOD takes 30% of the total BOD,
SNDR becomes ~0.125 g NO3-N/g MLSS/d. Following procedure can be used to determine the required anoxic tank volume.

·  Assume HRT of anoxic tank and calculate the tank volume.
·  Calculate SNDR required to denitrify using the nitrogen load to the tank,
    MLSS, and the anoxic tank volume. ·  Calculate F/M ratio in anoxic tank and find out SNDR from Fig. 2.
·  If the calculated SNDR is less than the SNDR from the graph,
    the initial HRT assumption is validated. Otherwise, increase HRT and repeat the sequence.

Fig. 1 Specific denitrification rate as function of F/M ratio and the ratio of readily biodegradable BOD to total BOD (Metcalf & Eddy, 2003).

3-3-3 Oxygen credit from de-nitrification

The oxygen demand in aeration tank decreases as much as the amount of COD treated in anoxic tank.
More precisely, the amount of oxygen delivered from nitrate in anoxic tank, which is equal to the amount of COD
consumed for de-nitrification, can be deducted from the total oxygen demand of wastewater.
Therefore, if anoxic tank exists, aeration costs tend to decrease as opposed to the system without anoxic tank.

This oxygen credit is theoretically estimated at 2.86 g O2/g NO3-N removed. Since 4.57 mg O2 is consumed
when 1 mg NH4-N is oxidized in theory, net oxygen consumption during the removal of 1 mg NH4-N is 1.71 mg O2/mg NH4-N
removed assuming no cell production. In practice, however, oxygen demand for nitrification and oxygen credit
from de-nitrification are assumed at 4.3 mg O2/mg NH4-N and 2.4 mg O2/mg NO3-N, respectively,
considering the nitrogen loss to cell mass. Overall, net oxygen demand for nitrogen removal becomes around 1.9 mg O2/g NH4-N removed.

3-3-4 Alkalinity Production from De-nitrification

De-nitrification produces one mole of alkalinity (OH-) as one mole of nitrate is removed.
Since two moles of alkalinity is consumed to produce one mole of nitrate, net alkalinity consumption during
the course of nitrification and de-nitrification is one mole per one mole of NH4-N (or TKN),
where the net one mole alkalinity consumption is in fact the disappearance of NH4+ itself.
The alkalinity production during the denitrification can help prevent pH from dropping
excessively especially when influent TKN is high and wastewater alkalinity is low.

Table 2 summarizes the oxygen and alkalinity consumption and production during nitrification and de-nitrification.

Process	O2 consumption			Alkalinity consumption
	Without cell production		With cell production	Without cell production		With cell production
	mol O2 /mol N	g O2 /g N	g O2 /g N<	eqv. Alk /mol N	g CaCO3 /g N	g CaCO3 /g N
Nitrification	2.0	4.57	4.3	2.0	7.14	6.8
Denitrification	-1.25	-2.86	-2.4	-1.0	-3.57	-2.9
Overall	0.75	1.71	1.9	1.0	3.57	3.9

Table 2. Oxygen and alkalinity consumption/production during nitrification and de-nitrification

4. Simultaneous Nitrification and De-nitrification (SNDN)

As a wastewater treatment process, microbial simultaneous nitrification-denitrification
is the conversion of the ammonium ion to nitrogen gas in a single bioreactor.

The process is dependent on floc characteristics, reaction kinetics, mass loading
of readily biodegradable chemical oxygen demand (rbCOD), and the DO concentration.

De-nitrification can occur at oxygen rich environment due to the low DO in the center of floc or in the bottom of biofilm.
Since oxygen must diffuse through microbial floc or biofilm while it is consumed by microorganisms,
the steady state oxygen concentration near the center of the floc can be low enough to trigger de-nitrification (Kaempfer, 2000; Daigger, 2007).

5. Anaerobic Ammonia Oxidation (Anammox)

The ANAMMOX® process is a cost-effective and sustainable way of removing ammonium from effluents and ammonia from waste gas.
Compared to conventional nitrification/denitrification savings on operational costs can reach up to 60%, while CO2 emission is reduced.

The ANAMMOX® conversion is an elegant shortcut in the natural nitrogen cycle.
Anammox bacteria convert ammonium (NH4+) and nitrite (NO2-) into nitrogen gas.
Paques developed the process for commercial purposes in cooperation with Delft University of Technology and the University of Nijmegen.
Since the first full-scale plant started up in 2002, many other ANAMMOX® plants were implemented worldwide.

The ANAMMOX® process can be used for the removal of ammonium from nitrogen rich effluents, which for example can be found in:

·  Municipal waste water treatment (sludge rejection water)
·  Organic solid waste treatment (landfills, composting, digestion)
·  Food industries
·  Manure processing industry
·  Fertiliser industry
·  (Petro) chemical industry
·  Metallurgical industry
·  Semi-conductor industry