Administrative Code

Virginia Administrative Code
11/28/2022

Article 10. Nutrient Control

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)

20°C

10 minutes

15°C

22 minutes

10°C

45 minutes

5°C

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.

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:

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.

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.

9VAC25-790-920. Ammonia stripping.

Ammonia stripping is the chemical-physical process by which dissolved ammonias are converted to gaseous ammonia and removed from the wastewater by changes in the surface tension of the air-water interface. The removal of ammonia nitrogen in treated effluent is the objective of this treatment unit operation.

1. Ammonia stripping typically involves the addition of lime to treated effluent (secondary or advanced treatment), followed by agitation in the presence of air. Wastewater effluent with an adjusted pH of 10 or more is usually allowed to flow downward through special media. The ammonia gas which develops is stripped out by the passing contact with outside air. These ammonia stripping towers become inoperable at temperatures below freezing (32°F or 0°C wet bulb). Therefore, before consideration can be given to ammonia stripping the minimum air temperature must be determined and provisions made to prevent freezing.

2. For effective conversion of ammonium to ammonia gas the system pH must be maintained at a minimum of 10.5 on a continuous basis. The elevation of the pH of the wastewater for conversion of the ammonium to ammonia should be selected from the ammonia solubility curve versus pH.

Ammonia stripping units may be of countercurrent operation or utilize cross flow air movements. Minimization of scale formation may be obtained by countercurrent operation.

3. The loading applied to the stripping reactor should not exceed 1,250 pounds of wastewater ammonia per hour per square foot of cross-sectional media area. The gas-liquid ratio shall normally be in the range of two to four expressed in terms of pounds per square foot per hour of air, divided by pounds per square foot per hour of wastewater.

4. The reactor media shall be (i) resistant to continuous loading of high pH liquid; (ii) consist of material which can be readily cleaned of scale deposits; and (iii) structurally sound. Facilities shall be provided for media cleaning, consisting of either manual means, or high pressure jets of water, or other approved means. Provisions shall be made for treatment of washdown waters and scale removed from the packing media.

5. In areas where reliability is questionable due to physical restraints of the system, a back-up system for nitrogen removal shall be required. Duplicate pumping units are required where pumping is employed to apply or remove the liquid.

6. Facilities shall be provided for post-pH adjustment.

7. Considerations shall be given to remote locations for stripping reactors in relation to bodies of water. Each proposal shall include sufficient information to substantiate expected plume dispersion areas and, if necessary, removal of ammonia gas from the plume.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-980 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-920, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-930. Ion exchange.

Ion exchange may be utilized as a unit operation in which ions are exchanged between two different materials, usually a solid-liquid, but may involve a liquid-liquid exchange. In wastewaters, the exchange usually involves a solid resin material consisting of readily ionized compounds. Treated effluent (secondary or advanced treatment) passes at a controlled rate through a certain volume of resin within a contactor. The removal of 90-95% of the ammonia nitrogen can be achieved by such treatment. Ion exchange may also be utilized for removing heavy metals, nitrates, phosphates, sulfides, phenol, and chlorophenols from wastewaters.

1. The process specifically designed for ammonia nitrogen removal uses a clinoptilolite resin. Many of the design considerations are applicable to other types of ion exchange treatment, including:

a. Flow, total dissolved solids, suspended solids, ion specific concentrations, alkalinity, pH, and resin structure.

b. The rate of exchange based upon selectivity of the resin, the exchange capacity of the resin, waste strength, and the effluent requirements.

c. The exchange capacity and break through point.

d. Certain contaminants which create treatment problems in the operation of ion exchange. Where these contaminants exist, their removal shall be provided for if necessary through the methods of pretreatment listed in Table 10.

TABLE 10. METHODS OF PRETREATMENT.

Contaminant

Effect

Removal

Suspended Solids

Blinds or seals resin media with particles

Coagulation and filtration

Organics

Large molecules (e.g. humic acids) will foul strong base

Carbon absorption or use of weak base resins only resins (high pH)

Oxidants

Slowly oxidizes resins. Functional groups become liable (unstable)

Avoid prechlorination or neutralize the chlorine.

Iron, Manganese, and Dissolved Solids

Coats resin with charged particles.

Chemical clarification or aeration depending on nature of solids.

2. Clinoptilolite mineral should be crushed and screened resulting in particle sizes in the range of 20 X 50 mesh. Ion exchange capacities and selectivity shall be determined in pilot plant studies for the particular wastewater in question. The pH of the influent to the exchange resin contactor should be in the range of 4-8.

3. The following parameters shall be considered for design of the ion-exchange contactor:

a. Flow rates in the range of five to 15 resin volumes per hour are normal but the specific design loading shall be confirmed by pilot studies or performance data.

b. The contactors may be gravity or pressure type units.

c. A minimum of two units is required. The number of contactors required is governed by the length of cycle which can be achieved while still meeting effluent quality goals. This shall be determined by pilot tests on the specific wastewater involved.

d. The number of contactors shall be adequate to treat the maximum flow rate in compliance with appropriate permit or certificate requirements, with one contactor out of service for maintenance and an appropriate number out of service for regeneration.

e. Means must be provided to uniformly distribute the influent flow and regenerant flow over the entire area of the contactor.

f. Make-up clinoptilolite storage shall be provided, as well as a water slurry transfer system to move the clinoptilolite from storage to the contactor.

g. Facilities to wash the clinoptilolite prior to transfer to the contactor shall be provided. Means to transfer clinoptilolite from a contactor to the storage system for washing should also be provided.

h. The process shall be controlled by a control system which will automatically initiate and program the regeneration cycle and return the contactor to normal service.

i. Each contactor shall have a flow totalizer. Also, each contactor shall have a flow rate controller to maintain equal flows to all contactors.

j. Each contactor shall be equipped with an efficient surface wash device.

4. With a neutral regenerant, provisions shall be made for a contactor backwash supply with minimum capacity equivalent to 10 gpm/sq ft of contactor area. If wastewater temperatures exceed 25°C (72°F) for prolonged periods, a greater capacity may be required. If a high pH regenerant is used, a minimum backwash capacity of 15 gpm/sq ft should be provided.

5. Regeneration facilities shall be provided for the ion exchange resin. Regeneration may be by high pH regenerant or neutral pH regenerant. Supportive data from fully operational units or from a pilot plant shall be provided to demonstrate acceptability of the proposed regeneration method.

6. Treatment or recovery of regenerant shall be provided. The design should provide for removal of ammonia with recovery of the regenerant through either (i) electrolytic treatment at neutral pH, or (ii) air stripping, or (iii) steam stripping, at elevated pH. Supportive data from a fully operational unit or pilot plant shall be provided to demonstrate acceptability of electrolytic treatment at neutral pH and steam stripping at elevated pH.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-990 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-930, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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