Title 19—department of health and senior services division 20—Division of Community and Public Health Chapter 3—General Sanitation

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Revised Statutes of Missouri and Revised Statutes of Missouri Supp. 2004.

19 CSR 20-3.060 Minimum Construction Standards for On-Site Sewage Disposal Systems

PURPOSE: This rule establishes minimum construction standards for on-site sewage disposal systems. In accordance with the authority granted in section 701.040, RSMo, this rule establishes the minimum standards and criteria for the design, location, installation and repair of individual on-site sewage disposal systems to promote the public health and general welfare and to protect the surface and ground waters of the state.

PUBLISHER’S NOTE: The secretary of state has determined that the publication of the entire text of the material which is incorporated by reference as a portion of this rule would be unduly cumbersome or expensive. Therefore, the material which is so incorporated is on file with the agency who filed this rule, and with the Office of the Secretary of State. Any interested person may view this material at either agency’s headquarters or the same will be made available at the Office of the Secretary of State at a cost not to exceed actual cost of copy reproduction. The entire text of the rule is printed here. This note refers only to the incorporated by reference material.

(1) General.

(A) Definitions. Definitions as set forth in Chapter 701, RSMo, On-Site Sewage Disposal Law shall apply to those terms when used in this rule unless the context clearly requires otherwise or as noted in this subsection. For the purposes of these standards, certain terms or words used here shall be interpreted as follows. The word shall is mandatory and the words should and may are permissive. All distances, unless otherwise specified, shall be measured horizontally:

1. Administrative authority—The governing body which may include, but is not limited to, county health departments, planning and zoning commissions, county building departments, county public works department, sewer districts, municipalities and the Missouri Department of Health which has, as authorized by statute, charter or other form of enabling authority, adopted these standards for individual on-site sewage disposal systems;

2. Aeration unit—Any sewage tank which utilizes the principle of oxidation in the decomposition of sewage by the introduction of air into the sewage;

3. Alluvium—Soil parent material which was transported and deposited in a running water setting;

4. Alternative—An individual sewage disposal system employing methods and devices as presented in section (6) of this rule;

5. Approved—Considered acceptable by the administrative authority;

6. Baffle—A device installed in a septic tank for proper operation of the tank and to provide maximum retention of solids. This includes vented sanitary tees and submerged pipes in addition to those devices normally called baffles;

7. Bedrock—That layer of geologic material which is consolidated;

8. Bedroom—Any room within a dwelling that might reasonably be used as a sleeping room. The number of bedrooms in a residence as given by an appraiser will be used in determining volumes in the sizing of on-site sewage disposal systems;

9. Black water—Liquid-carried waste from a dwelling or other establishment, which contains organic wastes, including excreta or other body wastes, blood or other body fluids, and garbage;

10. Building sewer—That part of the drainage system which extends from the end of the building drain and conveys its discharge to an on-site sewage disposal system;

11. Capacity—The liquid volume of a sewage tank using inside dimensions below the outlet;

12. Color—The moist color of the soil based on the Munsell soil color system;

13. Distribution pipes—Perforated rigid pipes that are used to distribute sewage tank effluent in a soil treatment system;

14. Dosing chamber (or pump pit or wet well)—A tank or separate compartment following the sewage tank which serves as a reservoir for the dosing device;

15. Dosing device—A pump, siphon or other device that discharges sewage tank effluent from the dosing chamber to the soil treatment system;

16. Dwelling—Any building or place used or intended to be used by human occupants as a residential unit(s);

17. Effluent—The liquid discharge of a septic tank or other sewage treatment device;

18. Gravelless system—An absorption system recognized by the administrative authority as an acceptable method of subsurface disposal of sewage without the required use of gravel. The following are examples:

A. Large diameter, eight inch (8") and ten inch (10") corrugated, perforated plastic pipe, wrapped in a sheath of spun-bonded filter wrap;

B. Chamber system; and

C. Drip irrigation;

19. Gray water—Liquid waste, specifically excluding toilet, hazardous, culinary and oily wastes, from a dwelling or other establishment which is produced by bathing, laundry or discharges from floor drains;

20. Grease trap—A device designed and installed so as to separate and retain oils and fats from normal wastes while permitting normal sewage or wastes to discharge into the drainage system by gravity;

21. Ground absorption sewage treatment and disposal system—A system that utilizes the soil for the subsurface disposal of partially treated or treated sewage effluent. The following are examples:

A. Chamber system—A system that uses an open bottom structure which forms an underground chamber over the soil's infiltrative surface. The wastewater is discharged into the chamber through a central weir, trough or splash plate and is allowed to flow over the infiltrative surface in any direction;

B. Conventional soil absorption system—A system that distributes effluent by gravity flow from the septic or other treatment tank and applies effluent to the soil through the use of a seepage trench or bed;

C. Dosing soil absorption system—A system that distributes effluent by a pump or automatic siphon to elevate or distribute effluent to the soil through the use of a seepage trench or bed;

D. Drip soil absorption system—An experimental system that distributes effluent through drip lines in a grid pattern (also known as trickle irrigation); and

E. Pressure distribution system—A soil absorption system that distributes effluent by a pump and smaller diameter distribution piping with small diameter perforations to distribute effluent;

22. Hazardous waste—Any waste or combination of wastes, as determined by the Hazardous Waste Commission by rules, which, because of its quantity, concentration, or physical, chemical or infectious characteristics, may cause or significantly contribute to an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness, or pose a present or potential threat to the health of humans or the environment;

23. High ground water—Zones of soil saturation which include: perched water tables, shallow regional groundwater tables or aquifers, or zones that are seasonally, periodically or permanently saturated;

24. High-water level—The highest known flood water elevation of any lake, stream, pond or flowage or the regional flood elevation established by a state or federal agency;

25. Holding tank—A watertight tank for temporary storage of sewage until it can be transported to a point of approved treatment and disposal;

26. Horizon—A layer of soil, approximately parallel to the surface, that has distinct characteristics relative to adjacent layers;

27. Individual sewage disposal system—A sewage disposal system, or part of a system, serving a dwelling(s) or other establishment(s), which utilizes subsurface soil treatment and disposal;

28. Intermittent sand filters—Intermit-tent sand filters are beds of granular materials twenty-four to thirty-six inches (24–36") thick underlain by graded gravel and collecting pipe. Waste water is applied intermittently to the surface of the bed through distribution pipes or troughs and the bed is underdrained to collect and discharge the final effluent. Uniform distribution is normally obtained by dosing so as to flood the entire surface of the bed. Filters may be designed to provide free access (open filters) or may be buried in the ground (buried filters or subsurface sand filters);

29. Matrix color—The dominant color of a soil material;

30. Mottling—Spots or splotches of color interspersed in the dominant (or matrix color) of a soil material. Mottles may be of a wide variety of colors;

31. Mound system—A system where the soil treatment area is built above the ground to overcome limits imposed by proximity to water table or bedrock or by rapidly or slowly permeable soils;

32. Non-ground absorption sewage disposal system—A facility for waste treatment designed not to discharge to the soil, land surface, or surface waters, including, but not limited to, incinerating toilets, mechanical toilets, composting toilets and recycling systems;

33. Other establishment—Any public or private structure other than a dwelling which generates sewage;

34. Pan—A soil horizon compacted, hard or very high in clay content. These horizons are usually very slowly permeable. Common pans in Missouri are claypans and fragipans;

35. Perched water table—A saturated zone above and separated from the water table by a horizon which is unsaturated;

36. Percolation rate—The time rate of drop of a water surface in a test hole as specified in subsection (2)(C) of this rule and expressed in minutes per inch;

37. Permeability—The ease with which liquids and gases move within the soil or rock;

38. Plastic limit—A soil moisture content below which the soil may be manipulated for purposes of installing a soil treatment system and above which manipulation will cause compaction, puddling and smearing, as determined by the administrative authority. This is not to be confused with plastic limit as used or defined in the Unified Soil Classification System;

39. Privy—An outhouse or structure used for receiving human excrement in a container or vault beneath the structure;

40. Registered geologist—A person who meets the requirements of Chapter 256, RSMo;

41. Restrictive horizon—A soil horizon that is capable of perching groundwater or sewage effluent and that is brittle and strongly compacted or strongly cemented with iron, aluminum, silica, organic matter or other compounds. Restrictive horizons may occur as fragipans, iron pans or organic pans and are recognized by their resistance in excavation or in use of a soil auger;

42. Rock fragments—The percentage by volume of rock fragments in a soil that are greater than two millimeters (2 mm) in diameter or retained on a No. 10 sieve which may include, but is not restricted to, chert, sandstone, shale, limestone or dolomite;

43. Sanitarian—A person registered either as a sanitarian or environmental health professional by the National Environmental Health Association or the Missouri Board of Certification for Environmental Health Pro-fessionals or employed as a sanitarian or environmental health professional by the administrative authority;

44. Seepage bed—An excavated area larger than three feet (3') in width which contains a bedding of aggregate and has more than one (1) distribution line;

45. Seepage trench—An area excavated one to three feet (1–3') in width which contains a bedding of aggregate and a single distribution line;

46. Septage—Those solids and liquids removed during periodic maintenance of a septic or aeration unit tank or those solids and liquids removed from a holding tank;

47. Septic tank—Any watertight, covered receptacle designed and constructed to receive the discharge of sewage from a building sewer, separate solids from liquid, digest organic matter, store liquids through a period of detention and allow the clarified liquids to discharge to a soil treatment system;

48. Setback—A separation distance measured horizontally;

49. Severe geological limitations—Site-specific geologic conditions which are indicative of rapid recharge of an aquifer and likely groundwater contamination. Locations with significant groundwater contamination potential should be investigated by a registered geologist to determine if the site has severe geological limitations. Standardized criteria for determination of severe geological limitations are available in the form Assessment of Individual On-Site Waste Disposal Geological Limitations from the Department of Natural Resources, Division of Geology and Land Survey;

50. Sewage—Any water-carried domestic waste, exclusive of footings and roof drainage. Domestic waste includes, but is not limited to, liquid waste produced by bathing, laundry, culinary operations, liquid wastes from toilets and floor drains and specifically excludes animal waste and commercial process water. Also known as wastewater;

51. Sewage flow—Flow as determined by measurement of actual water use or, if actual measurements are unavailable, as estimated by the best available data provided by Table 2A in subsection (1)(E) of this rule;

52. Sewage tank—A watertight tank used in the treatment of sewage which includes, but is not limited to, septic tanks and aeration units;

53. Sewage tank effluent—That liquid which flows from a septic tank or aeration unit under normal operation;

54. Significant groundwater contamination potential—Any condition which would cause or indicate rapid recharge of an aquifer. This includes, but is not limited to, the following conditions or parameters: a water sample from an on-site well which exceeds drinking water standards with respect to fecal coliform; a hydrologic connection is established between the on-site waste disposal system and any well; a disposal field to be placed in Class V soils or soils with a percolation rate less than ten minutes per inch (10 min./in.); a disposal field within one hundred feet (100') of the topographic drainage of a sinkhole; or a sewage tank with fifty feet (50') of the topographic drainage of a sinkhole;

55. Sinkhole—A land surface depression that is hydraulically connected with a subterranean passage developed by a solution or collapse into the underlying bedrock, or both;

56. Site—The area bounded by the dimensions required for the proper location of the soil treatment system;

57. Slope—The ratio of vertical rise or fall to horizontal distance;

58. Soil—The naturally occurring, unconsolidated mineral or organic material of the land surface developed from rock or other parent material and consisting of sand, silt and clay-sized particles and variable amount of organic materials;

59. Soil characteristics, limiting—Those soil characteristics which preclude the installation of a standard system, including, but not limited to, evidence of water table or bedrock closer than three feet (3') to the ground surface and percolation rates slower than one hundred twenty minutes per inch (120 min./in.);

60. Soil saturation—The condition that occurs when all the pores in a soil are filled with water;

61. Soil scientist—An individual who has a minimum of fifteen (15) semester credit hours of soils course work including a minimum of three (3) hours in the area of soil morphology and interpretations, and has a minimum of two (2) years of field experience;

62. Soil textural classification—Soil particle sizes or textures specified in this rule refer to the soil textural classification in the Soil Survey Manual Handbook No. 18, United States Department of Agriculture, 1993;

63. Soil treatment area—That area of trench or bed bottom which is in direct contact with the trench rock of the soil treatment system;

64. Soil treatment system—A system where sewage tank effluent is treated and disposed of below ground surface by filtration and percolation through the soil. It includes those systems commonly known as seepage bed, trench, drainfield, disposal field and includes mound and low pressure pipe systems;

65. Standard system—An individual sewage disposal system employing a building sewer, sewage tank and the soil treatment system commonly known as seepage bed or trenches, drainfield or leachfield;

66. Toilet waste—Fecal matter, urine, toilet paper and any water used for flushing;

67. Trench rock—Clean rock, washed creek gravel or similar insoluble, durable and decay-resistant material free from dust, sand, silt or clay. The size shall range from one inch to two and one-half inches (1"–2 1/2"). If limestone, dolomite or other crushed white rock is used, it shall be washed and be a minimum size of one and one-half inches (1 1/2");

68. Valve box—Any device which can stop sewage tank effluent from flowing to a portion of the soil treatment area. This includes, but is not limited to, caps or plugs on distribution or drop box outlets, divider boards, butterfly valves, gate valves or other mechanisms;

69. Very slowly permeable—Soils, bedrock and soil horizon or layer having a vertical permeability less than one inch (1") in twenty-four (24) hours;

70. Wastewater—same as sewage as defined in paragraph (1)(A)50. of this rule;

71. Wastewater stabilization pond—A sealed earthen basin which uses the natural unaided biological processes to stabilize wastewater (also known as a sewage lagoon);

72. Water table—The highest elevation in the soil or rock where all voids are filled with water, as evidenced by presence of water or soil mottling or other information. This includes perched water tables or perched zones of saturation; and

73. Watertight—Constructed so that no water can get in or out below the level of the outlet.

(B) Applicability. For this rule, on-site wastewater treatment and disposal system means all equipment and devices necessary for proper conduction, collection, storage, treatment and disposal of wastewater from a dwelling or other facility producing sewage of three thousand gallons (3000 gals.) or less per day. Included within the scope of this rule are building sewers, septic tanks, subsurface
absorption systems, mound systems, intermittent sand filters, gravelless systems, aeration unit wastewater treatment systems and single family wastewater stabilization ponds. Com-mercial or industrial facilities and developers of subdivisions must first contact the Depart-ment of Natural Resources concerning compliance with the Missouri Clean Water Law and Regulations before applying for any approvals or permits under this rule.

(C) Responsibilities.

1. The design, construction, operation and maintenance of sewage treatment and disposal systems, whether septic tank systems, privies or alternative systems, shall be the responsibility of the designer, owner, developer, installer or user of the system.

2. Actions of representatives of the administrative authority engaged in the evaluation and determination of measures required to effect compliance with the provisions of this rule shall in no way be taken as a guarantee or warranty that sewage treatment and disposal systems approved and permitted will function in a satisfactory manner for any given period of time. Due to the development of clogging mats, which adversely impact the life expectancy of normally functioning ground absorption sewage treatment and disposal systems and variables influencing system function which are beyond the scope of this rule, no guarantee or warranty is implied or given that a sewage treatment and disposal system will function in a satisfactory manner for any specific period of time.

3. Prior to the issuance of a permit to install or effect major repair of an on-site sewage disposal system as regulated by Chapter 701, RSMo, plans and specifications shall be required for review. Approval by the administrative authority shall be required for—

A. Plans for absorption field showing the following:

(I) Field locations with slope(s) indicated or with contour lines based on field measurement. If field areas are essentially flat or of uniform grade, spot elevations will be required for alternate systems;

(II) Field layout, length, spacing, connection, pipe sizes and cleanout details, invert elevations of flow distribution devices and laterals, valves and appurtenances;

(III) Trench plan and profile drawings and flow distribution device details;

(IV) Location and design of associated surface and ground water drainage systems;

(V) Name, address and telephone number of the person(s) drafting the plans; and

(VI) Any other information required by the administrative authority; and

B. Alternative systems whether or not specifically described in this rule.

4. The entire sanitary sewage system shall be on property owned or controlled by the person owning or controlling the system. Necessary easements shall be obtained permitting the use and unlimited access for inspection and maintenance of all portions of the system to which the owner and operator do not hold undisputed title. Easements shall remain valid as long as the system is required and shall be recorded with the county recorder of deeds.

(D) Minimum Set-Back Distances. All on-site wastewater treatment and disposal systems shall be located in accordance with the distances shown in Table 1.
(E) Sewage Flow Rates. Table 2A or 2B shall be used to determine the minimum design daily flow of sewage required in calculating the design volume of sanitary sewage systems to serve selected types of establishments. The minimum design volume of sewage from any establishment shall be one hundred gallons (100 gals.) per day. Design of sewage treatment and disposal systems for establishments not identified in this rule shall be determined using available flow data, water-using fixtures, occupancy or operation patterns and other measured data.

1. Volume determination. In determining the volume of sewage from single family dwellings, the minimum flow rate shall be one hundred twenty gallons (120 gals.) per day per bedroom. The minimum volume of sewage from each single family dwelling shall be two hundred forty gallons (240 gals.) per day. When the occupancy of a single family dwelling exceeds two (2) persons per bedroom, the volume of sewage shall be determined by the maximum occupancy at a rate of sixty gallons (60 gals.) per person per day.

2. Other establishments. For establishments or housing developments other than a single family residence, either Table 2A shall be used to estimate the sewage flow rate or actual measured flow rate for existing systems may be used. Values for estimated sewage flow for establishments having food service operations shall be increased by a factor of one and one-half (1.5) to compensate for the high organic strength. Grease traps shall be required at food service facilities, meat markets and other places of business where the accumulation of grease or oils can cause premature failure of a soil absorption system. The following design criteria shall be met:

A. The grease trap shall conform to Plumbing & Drainage Institute Standard PDI-G101 or equivalent;

B. The grease trap shall be plumbed to receive all wastes associated with food handling and no toilet wastes;

C. The grease trap liquid capacity shall be sufficient to provide for at least five gallons (5 gals.) of storage per meal served per day, at least two-thirds (2/3) of the required septic tank liquid capacity, or a capacity as determined in accordance with the following:

LC = D × GL × ST × HR/2 × LF

where LC = grease trap liquid capacity


D = number of seats in dining area

GL = gallons of wastewater per


(1.5 single-service; 2.5 full-


ST = storage capacity factor = 2.5

HR = number of hours open

LF = loading factor (1.25 interstate


= 1.0 other highways and recreational areas

= 0.8 secondary roads);

D. Two (2) or more chambers must be provided, with total length-to-width ratio at least two to one (2:1). Chamber opening and outlet sanitary tee must extend down at least fifty percent (50%) of the liquid depth;

E. Access manholes, with a minimum diameter of twenty-four inches (24”), shall be provided over each chamber and sanitary tee. The access manholes shall extend at least to finished grade and be designed and maintained to prevent surface water infiltration. The manholes shall also have readily removable covers to facilitate inspection and grease removal; and

F. Where it has been demonstrated that specially designed grease interceptors will provide improved performance, the grease trap liquid capacity may be reduced by up to fifty percent (50%).

3. Population to be served. Unless satisfactory justification can be given for using lower per-unit occupancies, the figures in Table 2B shall be used in determining the population for which to design the sewage works.

4. Reduction in sewage flow. Reductions in design sewage flow rates may be allowed by the administrative authority on a case-by-case basis depending upon water conservation plans. Sewage flow rates may be reduced up to forty percent (40%) for gray water systems where the toilet wastes are discharged to a holding tank and disposed of off-site or where waterless toilets are utilized.

(2) Site Evaluation.

(A) All proposed sites for on-site sewage treatment and disposal systems shall be evaluated for the following:

1. Either percolation tests or soil conditions, properties and permeability as determined by a soil morphology examination; a profile pit shall be required for all new installations in order to conduct soil morphology examination;

2. Slope;

3. The existence of lowlands, local surface depressions, rock outcrops and sinkholes;

4. All required setback distances as required in subsection (1)(D) of this rule;

5. Surface water flooding probability and depth to water table;

6. Location of easements and underground utilities;

7. Amount of available area for the installation of the system and an area for replacement;

8. Location of homesite or dwelling as well as management of surface runoff water from those buildings;

9. Any other cultural feature, such as roads, streets and the like in the surrounding areas which influences surface and subterrainal flow of water on or near the proposed site; and

10. Any significant groundwater contamination potential.

(B) Preliminary Soils Information. During a site evaluation reference may be made of county soil survey reports which are available from the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) for a majority of the counties in Missouri. NRCS soil survey reports should not be used as sole final determination for a specific site, but only as a guide to which soils are expected in a given area.

(C) Soil Permeability and Soil Percolation. Soil permeability and soil percolation are two (2) different soil features with no direct correlation.

1. Soil permeability is that quality that enables soil to transmit water or air. It can be measured quantitatively in terms of rate of flow of water through a unit cross section of saturated soil in unit time under specified temperature and hydraulic conditions.

2. Soil percolation rate is based on a standard method which includes a twenty-four (24)-hour presoak in a six to eight inch (6–8") diameter hole to the depth of the proposed absorption field. After the presoak, water is poured into the hole to a level of eight inches (8") above the bottom. The drop in water level is then measured at thirty (30)-minute intervals until a stabilized rate is obtained. Results are recorded as minutes per inch (min./in.) for the water level to drop.

(D) Procedures for Percolation Tests and Profile Holes. Two (2) types of site evaluations are acceptable. Each type depends upon the technical expertise of the individual conducting the evaluations. When percolation tests are slower than sixty minutes per inch (60 min./in.), the design must be drafted and signed by a registered engineer unless site suitability and system sizing has been determined by soil evaluation in accordance with section (7) of this rule. This would apply to all systems except for lagoons or other systems that do not use the soil for treatment. When percolation tests are slower than one hundred and twenty minutes per inch (120 min./in.), on-site sewage disposal systems shall not be permitted, except for lagoons or other systems designed in accordance with sections (6) and (7) of this rule. The administrative authority will determine which method(s) is to be used. The types of site evaluations are described as follows:

1. Percolation tests only. This type of site evaluation is where site suitability and sizing of the soil absorption system is made by percolation tests and there is no other evaluation of soil characteristics. This type of site evaluation can be used only for siting and sizing standard systems in areas which are not classified as having significant groundwater contamination potential. When using this type of evaluation, only percolation rates between ten minutes and sixty minutes per inch (10–60 min./in.) will be acceptable. Percolation tests shall be conducted by an engineer, sanitarian, registered geologist, soil scientist or a person who has been trained and certified by the Department of Health in accordance with section 701.040(2), RSMo. These tests shall be performed in accordance with the following procedure:

A. A minimum of four (4) percolation test holes are required with three (3) of the holes around the periphery within the proposed soil absorption site and one (1) in the middle of the proposed soil absorption site;

B. Each test hole shall be six to eight inches (6–8") in diameter, have vertical side walls and be bored or dug to a depth of the bottom of the proposed soil absorption system;

C. The bottom and sides of the hole shall be carefully scratched to remove any smearing and to provide a natural soil surface into which water may penetrate. All loose material shall be removed from the bottom of the test hole and two inches (2") of one-fourth to three-fourths inch (1/4–3/4") washed gravel shall be added to protect the bottom from scouring;

D. The hole shall be carefully filled with clear water to a minimum of twelve inches (12") over the soil bottom of the test hole and maintained for no less than four (4) hours. The hole shall then be allowed to swell for at least twenty-four (24) hours. In sandy soils, the saturation and swelling procedure shall not be required and the test may proceed if one (1) filling of the hole has seeped away in less than ten (10) minutes;

E. In sandy soils, the water depth shall be adjusted to eight inches (8") over the soil bottom of the test hole. From a fixed reference point, the drop in water level shall be measured in inches to the nearest one-eighth inch (1/8") at approximately ten (10)-minute intervals. A measurement can also be made by determining the time it takes for the water level to drop one inch (1") from an eight-inch (8") reference point. If eight inches (8") of water seeps away in less than ten (10) minutes, a shorter interval between measurements shall be used but in no case shall the water depth exceed eight inches (8"). The test shall continue until three (3) consecutive percolation rate measurements vary by a range of no more than ten percent (10%);

F. In other soils, the water depth shall be adjusted to eight inches (8") over the soil at the bottom of the test hole. From a fixed reference point, the drop in water level shall be measured in inches to the nearest one-eighth inch (1/8") at approximately thirty (30)-minute intervals, refilling between measurements to maintain an eight-inch (8") starting head. The test shall continue until three (3) consecutive percolation rate measurements vary by a range of no more than ten percent (10%). The percolation rate can also be made by observing the time it takes the water level to drop one inch (1") from an eight-inch (8") reference point if a constant water depth of at least eight inches (8") has been maintained for at least four (4) hours prior to the measurement;

G. Percolation rate shall be calculated as follows:

(I) The time interval shall be divided by the drop in water level to obtain the percolation rate in minutes per inch;

(II) The slowest percolation rate of the four (4) tests shall be used to determine the final soil treatment system design. Where the slowest percolation rate varies by more than twenty minutes per inch (20 min./in.) from the other tests, a detailed soils morphology evaluation must be conducted to justify a design based upon the average percolation rate; and

(III) For reporting the percolation rate, worksheets showing all calculations and measurements shall be submitted; and

H. Depth to bedrock or other restrictive layer shall be determined in areas where it is known that bedrock may exist at depths less than ten feet (10'); and

2. Soil morphology. This evaluation shall be conducted by a soil scientist unless an engineer, registered geologist or sanitarian has had special training and field experience to determine the required soil characteristics. This type of evaluation is recommended for sites that are classified as having significant groundwater contamination potential, severe geological limitations or severe limitations relating to restrictive layers. Section (7) of this rule contains criteria for this type of site evaluation. Since this type of soil analysis pertains to the factors that relate directly to permeability, no percolation test is required, however the administrative authority may retain the option of requiring percolation tests for additional information in determining site suitability.

(3) Building Sewers. Building sewers used to conduct wastewater from a building to an on-site wastewater treatment and disposal system shall be constructed of material meeting the minimum requirements of American Society for Testing and Materials (ASTM) Standards and listed by that agency for such use. Suitable materials meeting ASTM standards include: Acrylonitrile, butadiene styrene (ABS), cast iron pipe, concrete pipe, copper or copper-alloy tubing, polyvinyl chloride (PVC) or vitrified clay pipe. Although listed by ASTM, asbestos cement pipe will not be accepted due to potential health hazards to installers. Building sewer specifications are as follows:

(A) Size. Building sewers shall not be less than four inches (4") in diameter;

(B) Slope. Building sewers shall be laid to the following minimum slope:

1. Four-inch (4") sewer—twelve inches (12") per one hundred feet (100'); and

2. Six-inch (6") sewer—eight inches (8") per one hundred feet (100');

(C) Cleanouts. A cleanout shall be provided at least every one hundred feet (100') and at every change in direction or slope if the change exceeds forty-five degrees (45°). A cleanout should be provided between house and tank; and

(D) Connection to Sewage Tank. The pipe going into and out of the sewage tank shall be schedule 40 PVC or cast iron and shall extend a minimum of two feet (2') beyond the hole of excavation for the sewage tank.

(4) Sewage Tanks.

(A) General. All liquid waste and washwater with the following exceptions shall discharge into the sewage tank. Roof, garage, footing, surface water, drainage, cooling water discharges and hazardous wastes shall be excluded from the sewage tank. Backwash from water softeners and swimming pool filtration systems may be excluded from the sewage tank. In such event of excluding swimming pool filter backwash, the Depart-ment of Natural Resources shall be contacted for applicability of a discharge permit. All sewage tank effluent shall be discharged to a soil absorption system that is designed to retain the effluent upon the property from which it originated. All tanks regardless of material or method of construction shall—

1. Be watertight and designed and constructed to withstand all lateral earth pressures under saturated soil conditions with the tank empty;

2. Be designed and constructed to withstand a minimum of two feet (2') of saturated earth cover above the tank top; and

3. Not be subject to excessive corrosion or decay. Metal sewage tanks shall not be used unless specifically allowed by the administrative authority on a case-by-case basis. The tank shall be thoroughly coated inside and out with a bituminous or other suitable coating. Any damage to the bituminous coating shall be repaired by recoating. Additionally, plastic sanitary tees shall be used for the inlet and outlet for the sewage tank. The administrative authority shall use Table 3 regarding minimum gauge thickness for metal sewage tanks.

(B) Septic Tanks. Septic tanks, regardless of material or method of construction, shall conform to the following criteria:

1. The liquid depth of any septic tank or its compartment shall be not less than thirty-six inches (36"). A liquid depth greater than six and one-half feet (6 1/2') shall not be considered in determining tank capacity;

2. No tank or compartment shall have an inside horizontal dimension less than twenty-four inches (24");

3. Inlet and outlet connections of the tank shall be protected by baffles or sanitary tees as defined in paragraph (4)(B)6. of this rule;

4. The space in the tank between the liquid surface and the top of the inlet and outlet baffles shall not be less than twenty percent (20%) of the total required capacity, except that in horizontal cylindrical tanks, this space shall be not less than fifteen percent (15%) of the total required liquid capacity;

5. Inlet and outlet baffles shall be constructed of acid-resistant concrete, acid-resistant fiberglass or plastic;

6. Sanitary tees shall be affixed to the inlet or outlet pipes with a permanent waterproof adhesive. Baffles shall be integrally cast with the tank, affixed with a permanent waterproof adhesive or with stainless steel connectors top and bottom;

7. The inlet baffle shall extend at least six inches (6") but no more than twenty percent (20%) of the total liquid depth below the liquid surface and at least one inch (1") above the crown of the inlet sewer;

8. The outlet baffle and the baffles between compartments shall extend below the liquid surface a distance equal to forty percent (40%) of the liquid depth, except that the penetration of the indicated baffles or sanitary tees for horizontal cylindrical tanks shall be thirty-five percent (35%) of the total liquid depth. They also shall extend above the liquid surface as required in paragraph (4)(B)4. of this rule. In no case shall they extend less than six inches (6") above the liquid surface;

9. There shall be at least one inch (1") between the underside of the top of the tank and the highest point of the inlet and outlet devices;

10. The inlet shall be not less than three inches (3") above the outlet;

11. The inlet and outlet shall be located opposite each other along the axis of maximum dimension. The horizontal distance between the nearest points of the inlet and outlet devices shall be at least four feet (4');

12. Sanitary tees shall be at least four inches (4") in diameter. Inlet baffles shall be no less than six inches (6") or no more than twelve inches (12") measured from the end of the inlet pipe to the nearest point on the baffle. Outlet baffles shall be six inches (6") measured from beginning of the outlet pipe to the nearest point on the baffle;

13. Access to the septic tank shall be as follows:

A. Manholes. Access shall be provided over both the inlet and outlet devices and to each tank compartment by means of either a removable cover or a manhole. Where the top of the tank is located more than eighteen inches (18") below the finished grade, manholes and inspection holes shall extend to approximately eight inches (8") below the finished grade. The extension can be made using riser of approved material and fitted with tight covers of heavy metal or concrete. Proper attention must be given to the accident hazard involved when manholes are extended close to the ground surface. Manhole risers are not required when the top of the tank is within eighteen inches (18") of final grade. All manhole openings must be provided with a substantial, fitted, water-tight cover of concrete, cast iron or other approved material. All manhole covers which terminate below grade shall be covered with at least six inches (6") of earth. Manhole covers which terminate above grade shall have either an effective locking device or otherwise be adequately sealed in a manner to prevent accidental access; and

B. A six-inch (6") inspection port shall be provided over the inlet and outlet baffles of each tank and terminate at or above grade. An inspection port shall not be used as a pumpout access. A manhole cover at or above grade may also serve in place of inspection ports;

14. Compartmentation of single tanks shall be in accordance with the following:

A. Septic tanks larger than fifteen hundred gallons (1500 gals.) and fabricated as a single unit shall be divided into two (2) or more compartments;

B. When a septic tank is divided into two (2) compartments, not less than one- half (1/2), nor more than two-thirds (2/3), of the total volume shall be in the first compartment;

C. When a septic tank is divided into three (3) or more compartments, one-half (1/2) of the total volume shall be in the first compartment and the other half equally divided in the other compartments;

D. Connections between compartments shall be baffled so as to obtain effective retention of scum and sludge. The submergence of the inlet and outlet baffles of each compartment shall be as specified in paragraphs (4)(B)7. and 8. of this rule;

E. Adequate venting shall be provided between compartments by baffles or by an opening of at least fifty (50) square inches near the top of the compartment wall; and

F. Adequate access to each compartment shall be provided by one (1) or more manholes with a minimum opening twenty inches (20") square or in diameter and located within six feet (6') of all walls of the tank;

15. The use of multiple tanks shall conform with the following:

A. Where more than one (1) tank is used to obtain the required liquid volume, the tanks shall be connected in series;

B. Each tank shall comply with all other provisions of this section;

C. No more than three (3) tanks in series can be used to obtain the required liquid volume; and

D. The first tank shall be no smaller than any subsequent tanks in series;

16. The liquid capacity of a septic tank serving a dwelling shall be based upon the number of bedrooms contemplated in the dwelling served and shall be at least as large as the capacities given in Table 4.

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