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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations

9VAC25-790-910. Biological nitrification.

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:


Fraction of Optimum Nitrification Rate

8.4 - 8.6


























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













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









Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-970 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-910, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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