Biochemistry of Nitrogen Reduction
in
Onsite Systems
22nd Annual Maryland Groundwater Symposium
© Orenco Systems, Inc
7/25/2013
#1
Primary Treatment Won’t Work If ...
• Tanks aren’t watertight
• Tanks aren’t large enough to ...
∼ Settle the sludge
∼ Float the scum
∼ Digest the wastewater
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7/25/2013
#2
Secondary Treatment Won’t Work If ...
• Treatment unit doesn’t provide
enough air and time to ...
∼ Decompose organics
∼ Transform ammonium
nitrogen to nitrate
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7/25/2013
#3
Two General Types of Treatment
Attached growth
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Suspended growth
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#4
Nitrogen
• Nitrogen (N2) in the environment exists primarily in the earth’s
atmosphere as a colorless, tasteless, and generally inert gas
• Air, by volume, is composed of about 78.08% nitrogen, with
oxygen (O2) following at about 20.95%
• Nitrogen is assimilated (absorbed/digested) and/or fixed from
the air in living things
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7/25/2013
#5
Nitrogen Family
• Organic nitrogen … Organic-N
• Ammonium nitrogen … NH4+
• Ammonia nitrogen … NH3-N
• Nitrite nitrogen … NO2-N
• Nitrate nitrogen … NO3-N
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7/25/2013
#6
Total Kjeldahl Nitrogen (TKN)
TKN = Organic-N + NH3-N
Therefore … Organic-N = TKN - NH3-N
And … TN = TKN + NO2-N + NO3-N
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7/25/2013
#8
Nitrogen in Urine and Feces
• Urine contributes about 75%+ of the nitrogen found in domestic
wastewater
• Feces contributes about 20% ± of the nitrogen.
• Soaps and cleaners, when unregulated, can contribute greatly to
the phosphorous and TN concentrations.
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7/25/2013
#9
Ammonification
• Ammonification is the transformation of organic nitrogen to
ammonium (NH4) and ammonia (NH3) inorganic forms,
depending on pH levels.
• The initial phase of ammonification occurs in the septic
environment of the primary tankage (70% ±) and about 3/4th of
the remaining organic nitrogen is converted to ammonia
through the secondary aerobic process.
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7/25/2013
#10
Nitrification
• Nitrification is the microbial conversion of ammonium to
nitrate.
• Nitrification is a two-phase process that occurs in the highly
aerobic environment of secondary and advanced treatment
processes
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7/25/2013
#11
Nitrification
In the first phase, ammonium oxidizing autotrophic bacteria
known as nitrosomonas convert ammonia to nitrite in the
following manner
2NH4+ + 302 nitrosomonas > 2NO2- + 4H+ + 2H2O.
In the second phase of nitrification, a nitrite oxidizing
bacteria known as nitrobactor converts nitrite to nitrate as
follows:
2NO2- + 02 nitrobactor > 2NO3© Orenco Systems, Inc
7/25/2013
#12
Nitrification Limiting Factors:
• High organic concentrations … BOD, COD, etc
• Lack of sufficient dissolved oxygen … DO greater
than 2 mg/L is preferred
• Insufficient alkalinity … filtrate CaCO3 should be
around 100 mg/L or greater
• Low pH … maintain greater than 6, with optimum
range between 7.2 and 9
• Low Temperature … should be 10°C or higher
• Toxic Inhibitors … e.g., high chloride levels (keep
below 180 mg/L)
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7/25/2013
#13
Nitrification … What’s Needed
• Adequate aeration volume for solids retention time (SRT > 8 10 days typical of suspended growth)
• Sufficient aeration capacity needed for dissolved oxygen (DO)
concentration (4.57 mg O2/mg NH3-N)
• Sufficient alkalinity/pH, (7.14 mg alkalinity as CaCO3 per mg
NH4-N oxidized – pH >6)
• No significant toxicity (e.g. trace organics, chlorides, …)
• Temperature control above 45°F
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7/25/2013
#14
Denitrification
• Denitrification is the microbial utilization of nitrate under
anoxic conditions. (DO < 0.3 mg/L is preferred)
• Denitrification is a multi-stage process where nitrate (NO3-) is
reduced to nitrogen gas (N2), which is liberated back into the
atmosphere after rising to the liquid surface in tiny bubbles.
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7/25/2013
#15
Denitrification
Anoxic denitrification conversion in wastewater can be expressed by the
following reaction
C10H19O3N + 10NO3-
pseudomonas
> 5N2- + 3H2O + 10CO2 + 10OH- + NH3
Microbial denitrification consumes 4 mg carbon for each mg nitrate utilized
The rule-of-thumb in wastewater applications though, is to use a BOD/NO3–
range between 4:1 and 8:1.
Also, denitrification recovers 3.57 mg alkalinity (CaCO3).
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7/25/2013
#16
Typical Residential-Strength Screened
Septic Tank Wastewater Characteristics*
Average*
mg/L
Weekly Peak
mg/L
Rarely Exceed
mg/L
cBOD5
150
250
500
TSS
40
75
150
TKN
65
75
150
G&O
20
25
30
* From structurally sound and watertight tanks … Crites & Tchobanoglous 1998
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7/25/2013
#17
Oxygen Availability in Air
• 275 mg/L oxygen in free air, which directly interfaces with the
biomass ... in contrast to 9 mg/L DO holding capacity of water
for oxygen transfer
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7/25/2013
#18
Two General Types of Treatment
Attached growth
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Suspended growth
7/25/2013
#19
Effect of DO on Nitrification
DO greater than 2 mg/L is preferred
DO Effect on Nitrification
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
1
2
3
4
5
6
7
8
9
10
11
DO, mg/L
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7/25/2013
#20
Nitrifier Population vs BOD5/TKN ratio
Ratio of
Carbon to Nitrogen
0.5
2.5
5.5
10
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Percentage of
Total Population
35%
10%
5%
2.5%
7/25/2013
#21
Nitrification Reaction Rate vs BOD/TKN
1.2
Nitrifier Fraction fn;
Nitrification Rate Rn
1
Rn
0.8
0.6
0.4
fn
0.2
0
0
1
2
3
4
5
6
7
8
9
10
C/N Ratio
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7/25/2013
#22
Effect of pH on Nitrifiers
Filtrate alkalinity (CaCO3 ) should be maintained at 80-100 mg/L or greater
for optimum nitrification
pH effect on Nitrification Rate Rn
Maintain pH above 6, with optimum between 7.2 and 9 for nitrification
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
6
7
8
9
10
11
pH
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7/25/2013
#24
Temperature Effect on Nitrifiers
Winter effluent temperature should be 10°C or higher
Temperature Effect on Nitrification
Growth Rate µn
2.50
2.00
1.50
1.00
0.50
0.00
0
5
10
15
20
25
30
35
40
45
T, °C
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7/25/2013
#25
Effect of DO on Denitrification
DO Effect on Denitrification
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
DO, mg/L
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7/25/2013
#26
Why Recirculate Treated Effluent?
• To optimize wastewater treatment
• To blend and dilute influent concentrations, which promotes
more efficient hydrolysis
• To introduce nitrified effluent where there is a free carbon
concentration and potential for supporting denitrification.
• To accomplish greater ammonia and total nitrogen removal
• To accomplish greater carbon reduction through
denitrification utilization of 4 or more mg BOD5 per mg
nitrate removed.
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7/25/2013
#27
Example
80 mg Ammonium - N / L
∼ 95% transformed to NO3
So 95% of 80 mg/L is 76 mg/L
76 mg ammonium - N/L X 7.14 mg alkalinity/L ammonium - N
=542 mg Alk (CaCO3) / L is consumed
© Orenco Systems, Inc
7/25/2013
#28
In Closing:
• Must be aware of BOD/TKN ratio …
• Must maintain proper levels of DO
• pH needs to be maintained within normal operating ranges
• Temperatures should remain above 10C
• Water Softeners discharge should not be put in onsite systems
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7/25/2013
#29