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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
8/11/2020

9VAC25-790-460. Standards.

A. The minimum degree of treatment to be provided shall be adequate in design to produce an effluent in accordance with this chapter, that will comply with the provisions of the State Water Control Law and federal law, and any water quality standards or effluent limitations adopted or orders issued by the State Water Control Board or Department of Environmental Quality. The expected performance levels of conventional treatment processes are described in subsection F of this section.

B. Industrial flows. Treatment works receiving industrial wastewater flows at a rate or volume exceeding 90% of the combined average daily influent flow can be designed and operated through the applicable requirements imposed by the State Water Control Board/Department of Environmental Quality, provided that public health and welfare protection issues are resolved. Otherwise, consideration shall be given to the character of industrial wastes in the design of the treatment works. In such cases, the treatability characteristics of the combined (sewage and industrial) wastewater shall be provided and addressed in the treatment process design. Pilot-scale testing as described in this chapter may be required to predict the full-scale treatment works operations.

C. Design loadings. Design loading refers to the established capacity of a unit operation or treatment process to reliably achieve a target performance level under projected operating conditions. Component parts and unit operations of the treatment works shall be arranged for greatest operating convenience, flexibility, economy, and to facilitate installation of future units.

1. Treatment works to serve existing sewerage systems shall be designed on the basis of established average sewage characteristics with sufficient capacity to process peak loadings. Excessive inflow/infiltration is an indication of deficiencies in the sewerage system and the design engineer shall provide an acceptable plan for eliminating or handling these excessive flows so that there will be no discharge of inadequately treated wastewaters or impairment of the treatment process.

2. A new treatment works must be designed in accordance with anticipated loadings. Table 3, found in this section, presents generally accepted minimum design flows and loadings. Deviations from Table 3 shall be based on sound engineering knowledge, experience and acceptable data substantiated in the design consultant's report. Numbers of persons per dwelling shall be based upon planning projections derived from an official source.

3. The design of treatment process unit operations or equipment shall be based on the average rate of sewage flow per 24 hours except where significant deviation from the normal daily or diurnal flow pattern is noted. The design flow for industrial wastewater flow contributions shall be determined from the observed rate of flow during periods of significant discharge or, in the case of proposed or new contributions, the industrial owner shall provide flow projections based on existing facilities of a similar nature. The following factors shall be included in determining design flows:

a. Peak rates of flow delivered through conduits as influent to the treatment process unit operations.

b. Data from similar municipalities, if applicable.

c. Wet weather flows.

4. The design organic loading should be based on the results of acceptable analytical testing of the wastewater or similar wastewater and shall be computed in the same manner used in determining design flow.

5. All piping and channels shall be designed to carry the maximum expected flow. If possible, the influent interceptor or sewer shall be designed for open channel flow at atmospheric pressure. If a force main is used to transmit the influent to the treatment works, a surge or equalization basin should be provided upstream of biological unit operations to provide a more uniform loading. Bottom corners of flow channels shall be filled and any recessed areas or corners where solids can accumulate shall be eliminated. Suitable gates and valves shall be placed in channels to seal off unused sections which might accumulate solids and to provide for maintenance.

D. Pilot plant studies. Pilot plants are defined as small scale performance models of full size equipment or unit operation design. The physical size of pilot plants varies from laboratory bench-scale reactors, with volumetric capacities of one or more liters up to several gallons, up to larger capacity arrangements of pumps, channels, pipes and tankage capable of processing thousands of gallons per day of wastewater.

Pilot scale studies are to include detailed monitoring of treatment performance under operating conditions similar to design sizes, including the proper loading factors. A sampling and analytical testing program is to be developed by the owner and evaluated by the department in order that the results of pilot plant studies can be utilized to verify full size designs.

E. Grease management. An interceptor basin or basins shall be provided to separate oil and grease from wastewater flows discharged to sewage collection systems whenever such contributions will detrimentally affect the capacity of the collection system or treatment works such that permit violations will actually or potentially occur, or such contributions will result in an actual or a potential threat to the safety of the operational staff. Interceptor basins shall be located in compliance with the Statewide Building Code as close to the source of oil and grease as practical. Interceptor basins shall be sized in accordance with the applicable building codes and local standards but shall be designed as a minimum to retain the volume of flow containing the oil or grease for each continuous discharge occurrence. But interceptor basins shall also provide a minimum volume in accordance with the following:

1. Provide two gallons of volume for each pound of grease received; or

2. Provide a minimum retention period of three hours for the average daily volume of flow received.

Interceptor basins shall be routinely maintained, including the periodic, scheduled removal of accumulations of oil and grease, within a portion of the basin volume as necessary, to prevent detrimental effects on system operation. The oil and grease shall be handled and managed in accordance with state and federal laws and regulations.

F. Expected performance. Conventionally designed sewage treatment unit operations and processes should result in an expected performance level when processing design loadings in accordance with this chapter (see Table 4 of this section). A conventional arrangement of unit operations would include primary and secondary phases. The primary phase involves the use of suspended solids setting basins called primary clarifiers. The secondary phase typically includes a biological reactor and secondary clarifier to maintain a population of microorganisms (biomass) capable of achieving a significant reduction of organic matter (Biochemical Oxygen Demand) contained in the sewage. Advanced treatment processes will include primary, secondary and tertiary phases, typically involving filtration unit operations. Conventional processes can be modified to provide for reduced levels of nutrients in the treated effluent as described in Article 9 (9VAC25-790-870 et seq.) of this part. The use of nonconventional processes to achieve required performance levels shall be considered in accordance with the provisions of Article 2 (9VAC25-790-380 et seq.) of this part.

TABLE 2.
BUFFER ZONE REQUIREMENTS FOR PRIMARY AND SECONDARY SEWAGE TREATMENT UNIT OPERATIONS*.

A. Unit Operations That Are Totally Enclosed(1)

DESIGN FLOW, gpd

BUFFER ZONE(4)

1. <1,000

None

2. >1,000 to <500,000

50 feet

3. Greater than 500,000

100 feet

B. Unit Operations Using Low Intensity Mixing or Quiescent System(2)

DESIGN FLOW, gpd

BUFFER ZONE(4)

1. <40,000

200 feet

2. >40,000 to <500,000

300 feet

3. Greater than 500,000

400 feet

C. Unit Operations Using Turbulent High Intensity Aeration or Mixing(3)

DESIGN FLOW, gpd

BUFFER ZONE(4)

1. <40,000

300 feet

2. >40,000 to <500,000

400 feet

3. Greater than 500,000

600 feet

*Notes:

(1)For example, package plant with units totally enclosed as an integral part of its design and manufacture. A package plant treatment works is defined by these regulations as a preengineered and prefabricated structural arrangement of tankage and channels with all necessary components for onsite assembly and installation. The design flow of package plants should be less than 0.1 mgd. Also frequent agricultural use of Class I treated sludge.

(2)For example, covered basins, bottom tube aerated facultative lagoons or ponds, or surface flow application of treated effluent. Also, frequent agricultural use of Class II treated sludge.

(3)For example, uncovered surface mixed basins or trajectory spray irrigation for land application of treated effluent. Also frequent agricultural use of Class III treated sludge.

(4)Discharge locations shall be located no closer than 100 feet and up to 200 feet from any private or public water supply source.

TABLE 3.
CONTRIBUTING SEWAGE FLOW ESTIMATES TO BE USED AS A DESIGN BASIS FOR NEW SEWAGE WORKS.

Discharge facility(1)

Contributing Design Units

Flow gpd

BOD5 #day(3)

S.S. #day

Flow duration, hours

Dwellings

Per person

100(2)

0.2

0.2

24

Schools w/showers and cafeteria

Per person

16

0.04

0.04

8

Schools w/o showers w/cafeteria

Per person

10

0.025

0.025

8

Boarding Schools

Per person

75

0.2

0.2

16

Motels @ 65 gal. per person (rooms only)

Per room

130

0.26

0.26

24

Trailer courts @ 3 persons/trailer

Per trailer

300

0.6

0.6

24

Restaurants

Per seat

50

0.2

0.2

16

Interstate or through highway restaurants

Per seat

180

0.7

0.7

16

Interstate rest areas

Per person

5

0.01

0.01

24

Service Stations

Per vehicle serviced

10

0.01

0.01

16

Factories

Per person/per 8-hr. shift

15–35

0.03–0.07

0.03–0.07

Oper. Per.

Shopping centers

Per 1,000 square foot of ultimate floor space

200–300

0.1

0.1

12

Hospitals

Per bed

300

0.6

0.6

24

Nursing Homes

Per bed

200

0.3

0.3

24

Doctor's offices in medical centers

Per 1000 square foot

500

0.1

0.1

12

Laundromats, 9–12 machines

Per machine

500

0.3

0.3

16

Community colleges

Per student & faculty

15

0.03

0.03

12

Swimming pools

Per swimmer

10

0.001

0.001

12

Theaters (drive-in type)

Per car

5

0.01

0.01

4

Theaters (auditorium type)

Per seat

5

0.01

0.01

12

Picnic areas

Per person

5

0.01

0.01

12

Camps, resort day & night w/limited plumbing

Per camp site

50

0.05

0.05

24

Luxury camps w/flush toilets

Per camp site

100

0.1

0.1

24

Notes:

(1)Colleges, universities and boarding institutions of special nature to be determined in accordance with subdivision B 2 of this section.

(2)Includes minimal infiltrations/inflow (I/I) allowance and minor contributions from small commercial/industrial establishments.

(3)#/Day - Denotes pounds per day.

TABLE 4.
EXPECTED PERFORMANCE FOR VARIOUS CONVENTIONAL TREATMENT PROCESSES.

Effluent Value Range(1) (mg/l)

A. Primary/secondary treatment process.

BOD5(2)

TSS(2)

1. Primary

100–180

100–150

2. Facultative Aerated Lagoon

24–45

24–30

a. With Clarification

b. Without Clarification

3. Biological contactors

24–50

24–50

4. Activated Sludge

24–30

24–30

5. Biological Plus Filtration(3)

10–20

5–15

6. Primary plus constructed wetlands(4)

24–40

24–40

7. Primary plus Aquatic Ponds(5)

20–30

20–30

B. Advanced treatment process.

BOD5

TSS

PO4-P

NH3-N

1. Physical chemical(6) and

45–95

20–70

1–10

20–30

a. F

20–70

1–20

1–10

20–30

b. F & AC

5–10

0.1–10

1–10

20–30

2. Biological(7) and

a. C & S

12–20

12–24

0.5–10

5–30

b. C, S, & F

6–11

0.5–15

0.5–10

5–30

c. C, S, F & AC

1–5

0.1–5

0.1–10

5–30

d. Microscreening

(1) 21 microns @ 5 GPM/sq. ft.

2–14

1–14

20–30

5–30

(2) 35 microns @ 8 GPM/sq. ft.

5–20

3–17

20–30

5–30

3. BNR(8)

20–30

20–30

2–4

1–3

4. Other biological and natural treatment processes evaluated on a case-by-case basis.

NOTES:

(1)Ranges reflect normal expected upper and lower values for process, performance, considering design and operations variability. Upper range value reflects performance expected for conventional loadings.

(2)Effluent values for soluble phosphorus and ammonia nitrogen are not given for conventional primary and biological processes since these are not designed as nutrient removal processes. However, phosphorus is removed in biological sludge and ammonia is oxidized to nitrate in biological effluents. Typical effluent values range from 4 to 5 mg/l of total phosphorus and from nearly 0 to more than 30 mg/l ammonia, for fully nitrified to unnitrified effluent.

(3)Coagulant and polymer addition prior to filter to be provided.

(4)Subsurface flow microbial-plant filter system with a minimum detention of three days, or surface flow system with a minimum retention of six days.

(5)Aquatic pond providing one acre of surface area (5-foot depth) per 200 population equivalent or less.

(6)Physical - Chemical: means coagulation by aluminum, iron or other metal salts or, precipitation by lime, followed by clarification and may include filtration. Unit processes include, as a minimum, flash mix, flocculation, and sedimentation. Filtration operations will be necessary to achieve effluent TSS levels of 15 mg/l or less.

(7)Biological: means any of the biological treatment processes including activated sludge and its process variations, attached growth systems including various filters, and facultative and fully aerated lagoons which are capable of producing a secondary effluent containing 30 mg/l BOD5 and TSS or less.

(8)Biological Nutrient Removal performance will be a function of influent levels of nutrients with typical influent values of 4 to 6 mg/l of PO4-P and 20 to 40 mg/l of NH3-N. Additional nitrification operations would be necessary to achieve TKN levels of less than 10 mg/l. Denitrification may produce effluent total nitrogen levels of 5 to 10 mg/l.

LEGEND:

C = Coagulation S = Sedimentation F = Filtration and AC = Activated Carbon

BNR = Biological Nutrient Removal

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-520 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-460, Virginia Register Volume 20, Issue 09, eff. February 12, 2004; Errata 20:12 VA.R. 1526 February 23, 2004.

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