Current through Register Vol. 41, No. 3, September 23, 2024
A.
Biological nitrification is a process whereby autotrophic nitrifying bacteria
convert ammonia nitrogen to nitrate nitrogen. This process is capable of
removing most of the nitrogenous oxygen demand from domestic wastewater but
does not remove the nitrogen itself. Should nitrogen removal be required,
denitrification facilities must follow nitrification facilities. Although the
nitrification phenomenon has been observed for some time, unit process design
for optimum nitrification performances has only recently been employed.
If adequate performance data are not available, pilot
plant evaluation for a particular application shall be completed prior to a
full scale design proposal for upgrade of existing facilities. The recommended
minimum or maximum design capacities are provided as guidelines and should be
used if actual performance data or pilot plant evaluations do not provide
sufficient design information.
B. Single stage design. Single stage systems
should be considered for cases where nitrification must be provided only during
periods when wastewater temperatures are above 13°C (55°F). For cases
where nitrification must be provided for prolonged periods of temperatures less
than 13°C, two stage activated sludge, biological nutrient removal, or
combinations with fixed film growth systems should be considered.
1. The reactor design shall prevent
short-circuiting. Plug flow basins should be used, with consideration given to
dividing the reactor into a series of compartments by installing dividers
across the basin width with ports through the dividers.
2. The aeration capacity shall be sized for
the peak ammonia load. Where data are not available on ammonia variation, a
peak hourly ammonia load (lbs/day) of 2.5 times the average load (lbs/day)
should be assumed. The aeration supply should have a capacity determined by the
following formula where automated blower controls linked to D.O. probes are
provided:
| Aeration supply = |
800 cu. ft. per total pounds of
(BOD5 +NOD) |
| where: NOD = |
4.6 x total Kjeldahl nitrogen (TKN) |
| BOD5 = |
5 day BOD entering the aeration basin |
The peak BOD5 and NOD must be used
to ensure around-the-clock nitrification. The above air quantity should be
doubled if automated blower controls are not provided. The design should
maintain a D.O. concentration greater than 1.0 mg/l.
3. Aeration basin detention time should be
based upon pilot plant data on the specific wastewater involved. Proper control
of industrial discharges must be provided to minimize the possibility of
biological toxins upsetting the nitrification rates. The following minimum
criteria are suggested for municipal wastewaters free of significant industrial
wastes and which are subjected to primary settling prior to aeration.
a. Sludge age = 10 days or more and F/M =
0.25 or less
where: F/M = total daily lbs BOD5
to aeration basin divided by average lbs active biomass in aeration
tank.
b. Active biomass is
measured by the volatile portion of the suspended solids concentration within
the aeration basin (MLVSS).
4. Nitrification will destroy 7.2 lbs of
alkalinity per pound of NH3-N oxidized. If the wastewater is deficient in
alkalinity, alkaline feed and pH control must be provided. Sufficient
alkalinity should be provided to leave a residual of 30-50 mg/l after complete
nitrification.
5. The design of
final clarifiers will be similar to secondary clarifiers serving suspended
growth reactors. The basin shall be equipped with a surface skimming device. A
minimum biomass return rate of 25% and a maximum of 100% of the average daily
flow shall be provided.
C. Two-stage design. To assure year round
nitrification, a two-stage system is considered necessary. Superior performance
of the two-stage systems for both BOD and NOD removal is attributed to the
selection of an optimum biomass. The BOD5 entering the
second stage should be 50 mg/l or less to prevent a washout of the nitrifying
bacteria. Properly operated contactors or high rate activated sludge systems
should provide acceptable first stage systems. The second stage activated
sludge system should remove at least 50% of the remaining
BOD5 and provide oxidation of 85% to 100% of the ammonia
nitrogen.
1. The aeration basin should be of
the plug flow type with a minimum of three baffled chambers. The basin should
be sized to handle the "design peak" ammonia load at the lowest expected
operating temperature and optimum pH.
2. Available information indicates that the
optimum pH for nitrification of wastewater ammonia will be in the range of 8.2
to 8.6. Limited research results have indicated that the nitrifying bacteria
can acclimate to pH values less than 8.0. It is recommended that the following
information be used for guidance until additional operational information is
available concerning the effect of pH:
| pH |
Fraction of Optimum Nitrification Rate |
| 8.4 - 8.6 |
1.00 |
| 8.2 |
0.98 |
| 8.0 |
0.95 |
| 7.8 |
0.88 |
| 7.6 |
0.80 |
| 7.4 |
0.68 |
| 7.2 |
0.58 |
| 7.0 |
0.48 |
| 6.8 |
0.38 |
| 6.6 |
0.30 |
| 6.4 |
0.24 |
| 6.2 |
0.18 |
| 6.0 |
0.13 |
Lime feed capability should be provided to maintain the
pH in the aeration basin within optimum range. Quantities of lime needed should
be based on (i) pH adjustment of incoming wastewater, (ii) destruction of
natural alkalinity of 7.1 lb CaCo3/lb NH3 oxidized, and
(iii) maintaining residual alkalinity of 30-50 mg/l. When adequately buffered
wastewaters are treated, it may be more economical to add additional tank
capacity in lieu of operation at optimum pH.
3. Where performance data or pilot plant data
are not available, the following nitrification rates may be employed in the
design of the aeration basin. These rates are established for optimum pH. If
the design is based on a pH range other than the optimum range, the
nitrification rates should be reduced.
| Temperature (°C) |
Nitrification rate-lbs NH3 N nitrified/day/lb MLVSS
|
| 5°C |
.04 |
| 10°C |
.08 |
| 15°C |
.13 |
| 20°C |
.18 |
| 25°C |
.24 |
| 30°C |
.31 |
A MLVSS concentration of 1,500-2,000 mg/l is
recommended.
4. Either
diffused air or mechanical aeration may be used. The dissolved oxygen
concentration in the aeration basin should be based on obtaining 3.0 mg/l
during average conditions but should never fall below 1.0 mg/l during peak flow
conditions.
a. The design of the aeration
system should incorporate:
(i) critical
wastewater temperature,
(ii)
minimum dissolved oxygen concentration,
(iii) wastewater oxygen uptake rate,
(iv) wastewater dissolved oxygen
saturation,
(v) altitude elevation
of the treatment works,
(vi)
aerator efficiency.
b.
The stoichiometric oxygen requirement of the wastewater can be computed and
expressed as daily pounds using the following formula:
(O2 required) = BOD5 from 1st
stage + 4.6 (TKN)
5.
This oxygen requirement is somewhat conservative since neither all of the BOD
or NOD will be completely satisfied. In order to balance the summer oxygen
requirement, provisions for one or more of the following reactor adjustments
shall be included:
a. Reduce the MLVSS
concentration;
b. Adjust the pH;
or
c. Reduce the volume in service
and increase the oxygen supply in remaining volume.
6. Design information for optimum settling
rates is limited. However, it is recommended that the final clarifier design be
similar to secondary clarifiers when operating data or pilot plant information
is not available. A sludge return capacity of 100% to 150% of the average flow
is recommended. Continuous and intermittent sludge removal capability should be
provided. The waste sludge quantities typically will be small in comparison to
first stage activated sludge quantities and may be combined with first stage
activated sludges for further processing.
D. Fixed film design. Various types of
attached growth or fixed film unit operations have been studied to determine
their ammonia removal capabilities. Conventional standard rate contactors can
provide a significant amount of nitrification during warm months but, in
general, do not provide consistent year round nitrification. As in the
suspended growth systems, a separate fixed film unit operation for
nitrification is also deemed necessary to maintain consistent year round
performance. However, the use of fixed film biomass support surfaces within
aeration basins have demonstrated effective nitrification. Biomass support
surfaces would typically be located in the downstream end of aeration basins,
occupying the last one-third of the basin length. One of the major advantages
that fixed film nitrification seems to have over suspended growth nitrification
appears to be stability. Contactor type reactors used for nitrification
typically include synthetic media for enhancing the surface area to volume
ratio, which generally exceeds 25 square feet of total surface area per cubic
feet of media volume. These fixed film contactors generally may be classified
into one of the following types based on media construction:
a. Column or tower (top loaded).
b. Submerged surface (plates or
strands).
c. Rotating disc
(partially submerged).
1. Numerous variations
in features and arrangements of fixed film contactors have been investigated.
Significant nitrification should occur through a fixed film reactor, provided
that the biomass surface area is properly sized and uniformly loaded with
respect to influent levels of soluble BOD and ammonia nitrogen. No specific
design loading criteria or guidelines are proposed at this time. A hydraulic
loading of one gpm or less per square foot of specific media surface has
resulted in efficient nitrification of secondary effluent in previous studies.
Results of such studies also indicate that the organic loading should be
maintained at or below 10 pounds BOD5 per day per 1,000
cubic foot of media surface. The results of pilot plant studies for specific
applications should provide design loading values. Review of fixed film
nitrification design will be approached on a case-by-case basis. Influent
wastewater characteristics affecting nitrification performance include:
a. Soluble BOD.
b. Ammonia Nitrogen.
c. Temperature.
d. pH.
e. Alkalinity.
f. Toxicity (nitrifier inhibitors).
2. The values of nitrification
performance are valid for wastewater temperatures greater than 16°C
(60°F). At a given loading rate, ammonia removal efficiency decreases
nonlinearly with decreasing wastewater temperature.
| Loading Rate (gpm/square foot) |
Nitrification Performance % Removal of Ammonia |
| .50 |
90 |
| .75 |
85 |
| 1.00 |
80 |
| 1.50 |
75 |
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
