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

9VAC25-790-900. Nutrient reduction.

Article 10
Nutrient Control

A. The goal of nutrient reduction is to produce an effluent quality to meet effluent limitations for phosphorus, ammonia nitrogen and total Kjeldahl nitrogen (TKN). All designs should be based on pilot plant studies or full scale operating data obtained at design loadings.

The following nutrient control processes will be considered:

1. Natural Systems—Aquatic plant removal (APU) and proper plant management.

2. Suspended growth systems with adequate sludge treatment and management.

3. Attached growth system.

4. Covered anaerobic ponds.

5. Packed bed filters.

B. Aquatic plant systems. This natural treatment process involves three phases: aquatic plant growth, harvesting and management. Design should be based on seasonal climate and available sunlight in accordance with the provisions of this chapter. The basin or channel shall be based on achieving the required removal rate at the minimum encountered liquid temperature and shall include sufficient capacity to achieve permit requirements during periods of low temperatures and little or no sunlight.

1. It has been reported that for maximum nitrogen assimilation, theoretical detention times vary from days to weeks. The detention time is considered directly related to pond immersion temperatures (between 12°C and 25°C) and independent of temperature between 25°C and 33°C. Detention time can be shortened by biomass control.

2. Culture depths should be established to achieve optimum nitrogen assimilation. Adjustments in detention time should be considered for the variation in culture depth. Basin or trench depth should be as shallow as possible and be designed to prevent seasonal performance problems.

3. Facilities should be provided for the addition of nutrients, such as carbon dioxide, iron and phosphorus, as required.

4. Plant harvesting is the primary means of biomass control but can also serve to remove suspended solids and chemical precipitants. Harvesting of aquatic plant biomass is divided into three phases: concentration, dewatering and drying. Biomass concentrations of 1.0% to 2.0% by weight can be achieved by either coagulation, flocculation and sedimentation by various coagulants and by use of gravity filters (e.g., Sandborn Filter) with filtrate return. Further concentration to 10% to 20% solids is possible with dewatering by filtration or by self-cleaning centrifugation. Microscreens and upflow clarifiers are not recommended because of operation problems and design deficiencies.

5. Biomass control can be accomplished by use of fixed scrapers or floating harvesters with water surface barriers, or by providing settling areas in basins or other flow channels from which the plants are harvested.

6. Drying of harvested plants can be accomplished by air drying on asphalt pavement or other suitable pavement that will allow mechanical spreading and collections. Drainage should be returned to the treatment works.

7. Biomass management includes (i) disposal through incineration and landfill (may be subject to permit or certificate issuance); (ii) reuse through processing as a high protein animal food supplement and (iii) agricultural use as a soil conditioner or fertilizer.

C. Biological nutrient removal.

1. Phosphorus removal. Phosphorus control typically involves the use of activated sludge biomass exposed to varying levels of dissolved oxygen. Anaerobic conditions select organisms that release phosphorus and store carbonaceous substrate. Biomass is processed through anaerobic conditions to a combination of anoxic and aerobic conditions. The subsequent exposure to dissolved oxygen results in biological metabolism of stored organics with subsequent uptake and storage of phosphorus by the biomass.

a. Anaerobic conditions are defined as a reactor volume containing less than 0.2 mg/l of both dissolved oxygen and nitrate-nitrogen. This selection may be provided within a reactor or reactors (mainstream processes) utilizing controlled recycling of activated sludge. Processed flows from additional treatment operations (sidestream processes) may also be utilized.

b. The efficiency of biological phosphorus removal is highly dependent on the influent levels of phosphorus and biodegradable substrate (BOD or COD). The optimum ratio of process influent total (five-day) BOD to phosphorus appears to be approximately 20 to achieve final effluent levels of phosphorus of one mg/l or less.

c. It is necessary to reduce dissolved oxygen and nitrate levels within influent and recycled flows to the anaerobic reactor to levels that will not exceed a level of 0.2 mg/l within the anaerobic biomass. The anaerobic reactor should be subdivided into two or more compartments with a total hydraulic retention time of one hour or more. The anaerobic fraction of the process biomass should not be less than 25% of the total. An operating mean cell residence time of 10 days or more should be provided for optimum phosphorus removal.

d. For final effluent limitations requiring less than three mg/l of total phosphorus, the need for effluent filtration, or chemical addition, to remove suspended solids shall be evaluated.

2. Nitrogen removal. This process involves activated sludge biomass subject to anoxic conditions to promote the reduction of nitrate nitrogen to nitrogen gas that escapes to the ambient air.

a. Anoxic conditions are defined as a dissolved oxygen level of 0.2 mg/l or less and a nitrate nitrogen level exceeding 0.2 mg/l.

b. Complete denitrification can recover 15% or more of the dissolved oxygen utilized for complete nitrification. In addition, denitrification can recover approximately one-half of the alkalinity utilized for nitrification.

c. A sufficient level of carbonaceous energy in the form of a biodegradable organic substrate must be provided to the anoxic reactor to achieve the design denitrification potential. The degree of nitrogen removal will be a function of the ratio or the carbonaceous energy level available, to the level of TKN oxidized to nitrate nitrogen. The minimum ratio of influent total (five-day) BOD to TKN appears to be approximately 10 or more to achieve effluent levels of 10 mg/l or less of total nitrogen.

d. Complete denitrification may require at least two anoxic stages with a total hydraulic retention time of one hour or more. The anoxic mass fraction should be based on the specific growth rate of the nitrifying/denitrifying microorganisms and the operating mean cell residence time. However, the anoxic mass fraction should be approximately 25% or more of the system biomass.

e. A flexible operating mean cell residence time should be provided around a typical value of 10 days depending on the wastewater temperature. The capacity to recycle flow of nitrified activated sludge to the anoxic reactor should exceed three times the average daily raw sewage flow.

D. Denitrification. If pilot plant data cannot be obtained for the specific wastewater involved, denitrification reactors should be sized through acceptable kinetic models. The average wastewater characteristics for both raw influent and primary (settled) effluent, if applicable, shall be established as follows:

a. Both the total and soluble BOD and COD and the biodegradable fractions of each parameter.

b. The nitrogen fractions; ammonia, TKN, NO3-N.

c. Total and soluble phosphorus.

d. The specific growth rate of nitrifying bacteria.

e. The design wastewater temperature and pH.

1. A supplemental organic substrate feed to a denitrifying reactor may be utilized to achieve denitrification if the influent and recycled flows from the mainstream process do not provide a sufficient amount of substrate. Methanol is commonly used because of lesser cost and lower sludge yield. Methanol requirements should be computed as follows:

Methanol requirements (mg/l) = (2.47) (Influent Nitrate-Nitrogen (mg/l)) + (1.53) (Influent Nitrite-Nitrogen (mg/l)) + (0.87) (Influent Dissolved Oxygen Concentration).

Chemical feed pumps shall be provided in duplicate. Alternate organic substrate sources may be considered with chemical dosages determined stoichiometrically.

2. The amount of methanol or other organic substrate source feed must be closely controlled because excessive feed would result in a residual BOD in the treatment works effluent. A means of automatically pacing the feed to the incoming nitrate concentration shall be provided. Flow pacing shall not be acceptable because of varying nitrate concentrations.

3. The denitrifying reactor shall be followed with an aerated stabilization tank with sufficient detention time to remove any excess oxygen demand resulting from organic substrate source addition and to polish the treatment works effluent.

4. Clarifiers should be designed with a maximum settling overflow rate of about 1,200 gallons per square foot per hourly day at peak flow. A surface skimming device with provisions for returning scum to the denitrification tank shall be provided. The design should be similar to that of secondary clarifiers as provided in this chapter.

5. Dual return pumps shall be provided, each with the capacity to return a minimum of 100% of average flow upstream of the denitrification reactors. Provisions shall be made to transport sludge from the settling basin to the nitrification system in the event that nitrifying sludge is unavoidably discharged into the denitrification system.

6. Denitrification design should address the following parameters:

a. Sludge yield. 0.15 to 0.25 pounds of cells per pound of methanol; 0.5 pounds of cells per pound of glucose, 0.1 pounds of cells per pound of COD (based on methanol).

b. Sludge age. Minimum sludge age to allow mitosis is one-half day at 20°C to 30°C and two days at 10°C. With a safety factor of seven, a design sludge age of 3.5 to 14 days should be considered for temperatures of 10°C to 30°C, using the wastewater temperature dictating the design values.

c. pH. Satisfactory performance can be obtained at pH values of 5.6 to 9.0. The optimum pH range is 6.5 to 7.5. Facilities for pH adjustment should precede the denitrification reactor if necessary.

d. Mixed liquor volatile suspended solids—1,200 to 2,000 mg/l.

e. Detention time--two to four hours.

7. Ponds utilized for denitrification shall be considered on an experimental basis only. Ponds must be covered to prevent wind mixing and photosynthetic oxygen production. Unsuccessful operation has been reported for temperatures below 14°C.

E. Selector systems. These processes are designed to provide a competitive advantage to maintain a desired group of microorganisms within the process. Systems of this type, which will be accepted on an experimental basis (unless sufficient operating data are made available), are as follows:

1. Activated sludge biomass may be subject to extended aeration conditions to accomplish carbonaceous organic oxidation (oxic) and nitrification without settling. Denitrification may be accomplished by introducing the nitrified effluent from the reactor (the mixed liquor) to established anoxic conditions. The anoxic effluent mixed liquor is settled in a clarifier from which return sludge is recycled to the aeration phase for BOD removal. In this process a supplementary organic carbon source is not used, as endogenous respiration of the mixed liquor suspended solids will satisfy the carbon requirement for biological metabolism.

2. Activated sludge reactors may be utilized in series, followed by a clarifier, with nitrified activated sludge biomass returned to a combination of selectors or anaerobic or anoxic conditions established in separate basins. The anaerobic and anoxic reactors should be mixed at a level sufficient to keep the solids in suspension.

a. Nitrification is achieved under aerobic or oxic conditions and mixed liquor from the aerated basin, or basins, is returned to the anoxic basin, or basins, at rates up to and exceeding three times the average flow rate of the influent.

b. Denitrification is obtained under anoxic conditions. The nitrate contained in the aerobic mixed liquor is reduced by the facultative anaerobic bacteria in the anoxic basins using the influent organic carbon compounds as hydrogen donors. Influent ammonia is not nitrified in the anoxic phase.

F. Attached growth systems. Flooded and submerged fixed film contact reactors or biomass support surfaces can be considered for nitrification and denitrification applications in accordance with the provisions of this chapter and standards contained in this chapter. Such designs shall be verified through submission and evaluation of satisfactory operating data. Possible alternatives include (i) the use of biomass support surfaces located within the downstream sections of suspended growth reactors to provide ammonia oxidation; or (ii) the use of contact reactors for nitrification and denitrification.

1. Packed bed contact reactors should be designed in a manner similar to gravity deep bed filters or pressure filters. Provisions shall be provided for backwashing the reactor. Media may consist of silica, activated carbon, volcanic cinders, and acceptable synthetic materials. The smaller media will result in the retention of bacterial floc in the filter, resulting in increased head, and shortcircuiting of flow may develop through the filter, unless frequent backwashing is provided. Larger media permits operation without frequent backwashing, although contact times are reduced, resulting in an increase in effluent suspended solids. High density media larger than 1/2 inches in effective size could produce backwashing problems and may require additional backwashing capability.

2. Nitrate reduction of greater than 90% can be achieved with fixed film contact times of one hour for one inch aggregate and two hours for two-inch aggregate at temperatures above 12°C. The actual detention time necessary for the chosen media shall be based on pilot studies and should be varied in accordance with the specific surface area of the media and temperatures expected. For well rounded sand of two to three mm diameter, the following guidelines for reactor sizing are suggested when pilot plant data cannot be obtained

Wastewater Temperature

Fixed Film Reactor Time
(Based on Media Composed of Sand)


10 minutes


22 minutes


45 minutes


90 minutes

3. Provisions shall be made for feed of a biodegradable carbon source, if necessary, based on the guidelines specified for suspended growth reactors.

4. Additional clarification is not required following the packed bed filter, unless the permit specifies an effluent suspended solids concentration of five mg/l or less.

5. Limited experimental data are available for upflow contactors with fluidized media beds and any design must be supported by operating data obtained from existing installations or from a thorough pilot scale study, including requirements for chemical feed additions.

6. One-inch diameter stone media may be specified for upflow contactor media to allow upflow operation without exceeding the scouring flow rates that could result in backwashing or stripping of attached growth. The disadvantages of large media sizes include a reduction in contact time and increased effluent suspended solids as compared to smaller media such as sand. For one-inch diameter or less media, size should be specified to achieve nitrate nitrogen removals up to 90% with methanol feed at temperatures as low as 12°C. Provisions should be made to remedy any head loss build up during operation.

7. Actual upflow contact time should be provided in the range of one to four hours at flow rates of 0.2 to 0.4 gallons per minute per square foot respectively, for white silica sand media with an effective size of 0.6 mm and a uniformity coefficient of 1.5.

8. A source of carbon, if necessary, shall be applied to upflow contact reactor influent based on guidelines outlined for suspended growth reactors. Design should be based on minimum wastewater temperature and maximum influent nitrogen concentration. Provisions shall be made for conveying nitrogen gas from the system.

9. If the upflow contactor is operated properly, additional clarification should not be required, unless the permit or certificate issued specifies an effluent suspended solids level of 10 mg/l or less.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-960 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-900, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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