R317-3-10. Lagoons

10.1. Lagoon Siting

A. Distance from Habitation. A lagoon should be sited as far as practicable, with a minimum of 1/4 mile (0.4 kilometer), from areas developed for residential or commercial or institutional purposes or may be developed for such purposes within a foreseeable future. Site characteristics such as topography, prevailing wind direction, forests, etc., must be considered in siting the lagoon.

B. Prevailing Winds. The lagoon should be sited where the direction of local prevailing winds is towards uninhabited areas.

C. Surface Runoff. The lagoon should not be sited in watersheds receiving significant amounts of storm-water runoff. Storm-water runoff should be diverted around the lagoon and protect lagoon embankments from erosion.

D. Hydrology and hydrogeology. Close proximity to water supplies and other facilities subject to wastewater contamination should be avoided in siting the lagoon. A minimum separation of four (4) feet (1.2 meters) between the bottom of the lagoon and the maximum ground water elevation should be maintained.

E. Geology

1. The lagoon shall not be located in areas which may be subjected to karstification, i.e., sink holes or underground streams generally occurring in area underlain by porous limestone or dolomite or volcanic soil.

2. A minimum separation of 10 feet (3.0 meters) between the lagoon bottom and any bedrock formation is recommended.

10.2. Small Facilities. The Director will review and approve the construction of a lagoon for a design rate of flow less than 25,000 gallons per day (95 cubic meters per day) only if:

A. there are no other alternatives for wastewater treatment and disposal available to the applicant;

B. there is no other appropriate technology for wastewater treatment and disposal except lagoon; and

C. the applicant has resources to satisfactorily operate and maintain the lagoon.

10.3. Basis of Design. Design variables such as lagoon depth, number of units, detention time, and additional treatment units must be based on effluent standards for BOD₅, total suspended solids (TSS), E. coli, dissolved oxygen (DO), and pH.

A. Design for Discharging and Total Containment Lagoons

1. The design shall be based on BOD₅ loading ranging from 15 to 35 pounds per acre per day (16.8-39.2 kilograms per hectare per day).

2. The design for total containment lagoons shall be based on conservative estimates of precipitation, evaporation, seepage or percolation and inflow relevant to the site. A mass diagram showing each of the foregoing factors on a month-by-month basis, shall be prepared and submitted with the design and plans for review.

B. Design Depth. The minimum operating depth should be such that growth of aquatic plants is suppressed to prevent damage to the dikes, bottom, control structures, aeration equipment and other appurtenances.

1. Discharging or Total Containment Lagoons. The maximum water depth shall be 6 feet (1.8 meters) in primary cells. Greater depth in subsequent cells may be deeper than 6 feet provided that supplemental aeration or mixing is incorporated in the design. Minimum operating depth shall be three feet.

2. Aerated Lagoons. The design water depth should range from 10 to 15 feet (three to 4.5 meters). The type of the aeration equipment, waste strength and climatic conditions affect the selection of the design water depth.

3. Sludge Accumulation. The minimum depth of 18 inches (45 centimeters) for sludge accumulation shall be provided in primary cells of facultative lagoons.

C. Freeboard. The minimum freeboard shall be three (3) feet (1.0 meter). For small systems - less than 50,000 gallons per day (190 cubic meters per day), the minimum freeboard can be reduced to two (2) feet (0.6 meter).

D. Slope

1. Maximum Dike Slope. The inner and outer dike slopes shall not be steeper than 3 horizontal to 1 vertical (3:1).

2. Minimum Dike Slope. Inner dike slope shall not be flatter than 4 horizontal to 1 vertical (4:1). A flatter slope can be specified for larger installations because of wave action, but have the disadvantages of added shallow areas, that are conducive to emergent vegetation.

E. Seepage

1. The bottom of lagoons treating domestic sewage shall be no less than 12-inch (30 centimeters) in thickness, constructed in two six-inch (15 centimeters) lifts. The selection of the type of seals using soils, bentonite, or synthetic liners for the lagoon bottom shall be based on the design hydraulic conductivity, durability, and integrity of the proposed material.

2. Hydraulic conductivity of the lagoon bottom as constructed or installed, shall be such that it meets the requirements of ground water discharge permit issued under R317-6, (Ground Water Quality Protection rules). It shall not exceed 1.0 x 10^-6 centimeters per second.

3. The seepage loss may vary with the thickness of the bottom seal and hydraulic head thereon. Detailed calculations on the determination of seepage loss shall be submitted with the design. It shall not exceed 6,500 gallons per acre per day (60.8 cubic meters per hectare per day).

4. Results of field and laboratory hydraulic conductivity tests, including a correlation between them, shall meet the design and ground water discharge permitting requirements, before the use of lagoon can be authorized.

5. Hydraulic conductivity for the lagoon where industrial waste is a significant component of sewage, shall be based on ground water protection criteria contained in R317-6 (Ground Water Quality Protection rules).

F. Detention time

1. Discharging Lagoons. Detention time in the lagoon shall be the greater, and exclusive of the capacity provided for sludge build-up, of:

a. 120 days based on winter flow and the maximum operating depth of the entire system; or

b. 60 days based on summer flow and peak monthly infiltration/inflow.

c. The detention time shall not be less than 150 days at the mean operating depth for effluent discharge without chlorination. In order to meet bacteriologic standards in such a case, at least 5 cells shall be provided. The detention time and organic loading rate shall depend on climatic or stream conditions.

2. Aerated Lagoons

a. The detention time shall be the greater of:

(1) 30 days minimum; or

(2) the value determined using the following formula: E = (1/(1 + (2.3 x K₁ x t))) where: t = detention time, days; E = fraction of BOD₅ remaining in an aerated lagoon; K_l = reaction coefficient, aerated lagoon, base 10. For normal domestic sewage, the K₁ value may be assumed to be 0.12 day^-1 at 20 degrees Centigrade, and 0.06 day^-1 at one degree Centigrade.

b. The reaction rate coefficient for domestic sewage which includes some industrial wastes must be determined experimentally for various conditions which might be encountered in the aerated lagoons. The reaction rate coefficient based on temperature used in the experimental data, shall be adjusted for the minimum sewage temperature.

G. Aeration Requirements for Aerated Lagoons

1. The design parameters for the aerated lagoon should be based on pilot testing or validated experimental data.

2. When pilot testing is not conducted, the design should be based on two pounds of oxygen input per pound of BOD₅ applied (two kilograms of oxygen input per kilogram of BOD₅ applied). However, it may vary with the degree of treatment, and the concentration of suspended solids to be maintained. A tapered mode of aeration is permitted based on applied BOD₅ to each cell.

3. Aeration equipment shall be capable of maintaining a minimum dissolved oxygen level of 2 milligrams per liter in the lagoon at all times such that their circles of influence meet.

a. Circle of Influence. It is that area in which return velocity is greater than 0.15 feet per second as indicated by the manufacturer's certified data. Table R317-3-10.3(G)(3)(a) may be used when the manufacturer's certified data is not available.

b. Freezing. Suitable protection from weather shall be provided for aerators and electrical controls.

H. Industrial Wastes. For industrial waste treatment using lagoon, the design parameters shall be based on the type and treatability of industrial wastes using biological processes. In some cases it may be necessary to pretreat industrial waste or combine with domestic sewage.

10.4. Lagoon Construction Details

A. Cell Shape. The shape of all cells should be such that there are no narrow or elongated portions. Round, square or rectangular lagoons with a length not exceeding three times the width are most desirable. No islands, peninsulas or coves are permitted. Dikes should be rounded at corners to minimize accumulations of floating materials. Common-wall dike construction, wherever possible, is strongly encouraged.

B. Multiple Units

1. At a minimum, the lagoon system shall consist of three cells of approximately equal capacity designed to facilitate both series and parallel operations.

2. The Director may approve less than three cells on the basis of review of factors such as, the rate of flow, the need, treatment reliability, etc.

3. All systems shall be designed with piping:

a. to permit isolation of any cell without affecting the transfer and discharge capabilities of the total system, and

b. to split the influent waste load to a minimum of two cells or all primary cells in the system.

C. Embankments and Dikes

1. Material. Dikes shall be constructed of relatively impervious material and compacted to no less than 90 percent Standard Proctor Density at 3 percent above the optimum moisture density to form a stable structure. The area where the embankment is to be placed shall be from vegetation and unstable organic material.

2. Top Width. The minimum dike width shall be 8 feet (2.4 meters) and shall permit access by maintenance vehicles.

D. Lagoon Bottom

1. Soil. Soil used in constructing the lagoon bottom (not including seal) and dike cores shall be incompressible and tight and compacted at a moisture content of 3 percent above the optimum water content to at least 90 percent Standard Proctor Density.

2. Uniformity. The lagoon bottom should be as level as possible at all points. Finished elevations shall not be more than three (3) inches (7.5 centimeters) from the average elevation of the bottom.

3. Prefilling. The lagoon should be prefilled to a level which protects the liner, prevents weed growth, reduces odor, and maintains moisture content of the seal. However, the dikes must be completely prepared before the introduction of any water.

E. Construction Quality Control and Assurance. A construction quality control and assurance plan showing frequency and type of testing for materials used in construction shall be submitted with the design for review and approval. Results of such testing, gradation, compaction, field permeability, etc., shall be submitted to the Director.

F. Erosion Control

1. The site shall be protected from erosion. The design of control measures shall be based on factors, such as lagoon location and size, seal material, topography, prevailing winds, cost breakdown, application procedures, etc.

2. For aerated lagoons, the slopes and bottom shall be protected from erosion resulting from turbulence.

3. Exterior face of the dike slope shall be protected from erosion due to severe flooding of a water course.

4. Seeding. The outside surface of dikes shall have a cover layer of at least 4 inches (10 centimeters), of fertile topsoil to promote establishment of an adequate vegetative cover wherever riprap is not utilized. Prior to prefilling, adequate vegetation shall be established on dikes from the outside toe to 2 feet (0.6 meter) above the lagoon bottom on the interior as measured on the slope. Perennial-type, low-growing, native, spreading grasses that minimize erosion and can be mowed are most satisfactory for seeding on dikes. Alfalfa and other deep-rooted crops must not be used for seeding since the roots of this type are apt to impair the water holding efficiency of the dikes.

5. Riprap or equivalent material shall be placed from 1 foot (0.3 meter) above the high water mark to two feet (0.6 meter) below the low water mark (measured on the vertical) for protection from severe wave action.

a. Riprap. The interior face of dikes must be protected from erosion by riprap or other equivalent methods of erosion control.

(1) Riprap layer shall be of durable, angular, sound and hard, field or quarry stones, and shall be free from seams, cracks and structural defects.

(2) The thickness of riprap layer shall be at least 8 inches (20 centimeters).

(3) Stones to be used in the riprap layer shall meet the following requirements:

(a) A minimum of 50 percent of stones by weight, shall be of sizes between two-thirds and one and one-half of the layer thickness;

(b) No more than ten percent of stones by weight, shall be of a size less than one-tenth of the layer thickness;

(c) The specific weight of stones must range between 2.5 and 2.82;

(d) Durability shall be tested in accordance with ASTM Standard C-535, as amended, and stones wearing in excess of 40 percent shall not be used.

(e) Stones shall be graded and manipulated in size so as to produce a regular surface of dense and stable mass. A stable foundation for the placed riprap shall be provided at the toe of the dike.

10.5. Influent Piping

A. Influent and Effluent Structures

1. All influent and effluent structures shall be located to minimize short-circuiting within lagoons, and to avoid blocking of lagoon circulation. Such structures must have protection against freezing or ice damage under winter conditions.

2. Inlets to the primary cells shall meet the following criteria:

a. Surcharging of upstream sewer from the inlet manhole is not permitted.

b. Multiple influent discharge points for primary cells of 20 acres (8 hectares) or larger should be provided to enhance the distribution of waste load in the cell.

c. Discharge shall be in the center of a round or a square cell, or at the third point farthest from the outlet structure in a rectangular cell, or at least 100 feet (30 meters) from the toe of the dike.

d. All aerated cells shall have an influent line which distributes the load within the mixing zone of the aeration equipment. Multiple inlets may be considered for a diffused aeration system.

e. Force mains shall be valved at the lagoon, and may terminate in a vertically or horizontally discharging section. The discharge end of the vertical pipe must be located no more than one foot above the lagoon bottom. Flow velocities in the discharge section entering the lagoon must not be in excess of two feet per second.

B. Influent Discharge Apron

1. The influent line shall discharge horizontally into a shallow, saucer-shaped, depression extending below the lagoon bottom not more than the diameter of the influent pipe plus 1 foot.

2. The end of the discharge line shall rest on a suitable concrete apron large enough to prevent the terminal influent velocity at the end of the apron from causing soil erosion. A 2-foot (0.6 meter) square apron shall be provided at the minimum.

C. Flow Measurement. Influent flow to the lagoon shall be continuously indicated and recorded. Flow measurement and recording equipment shall be weatherproof.

D. Level Gauges. Level gauges with clear markings shall be provided in:

1. each cell to measure and manually record the depth; and

2. the primary flow measurement device structure to indicate the depth or the rate of flow.

E. Manhole

1. A manhole or vented cleanout wye shall be installed prior to entrance of the influent line into the primary cell and shall be located close to the dike as topography permits. Its invert shall be at least 6 inches (15 centimeters) above the maximum operating level of the lagoon and provide sufficient hydraulic head without surcharging the manhole.

2. A manhole is required for small systems to house flow measurement device. For larger systems, flow measurement device and related instrumentation must be housed in a headworks type structure.

F. Flow Distribution. Flow distribution structures shall be designed to effectively split hydraulic and organic loads equally to primary cells.

G. Material. The material for influent line to the lagoon should meet the requirements of material for underground sewer construction described in this rule. Unlined corrugated metal pipe is not permitted due to corrosion problems. The material selection shall be based on factors such as, wastewater characteristics, heavy external loadings, abrasion, soft foundations, etc.

10.6. Control Structures and Interconnecting Piping

A. Structure

1. As a minimum, control structures shall:

a. be accessible for maintenance and adjustment of controls;

b. be adequately ventilated for safety and to minimize corrosion;

c. be locked to discourage vandalism;

d. contain controls to permit water level and flow rate control, and complete shutoff;

e. be constructed of non-corrodible materials (metal-on-metal); and

f. be located to minimize short-circuiting within the cell and avoid freezing and ice damage.

2. Recommended devices to regulate water level are valves, slide tubes or dual slide gates. Regulators should be designed so that they can be preset to stop flows at any lagoon elevation.

B. Piping. All piping shall be of cast iron or other material for installation of underground piping. The piping shall be located along the bottom of the lagoon with the top of the pipe just below average elevation of the lagoon bottom. Pipes should be anchored and protected from erosion.

10.7. Effluent Discharge Piping

A. Submerged Takeoffs. For lagoons designed for shallow or variable depth operations, submerged takeoffs are required. Intakes shall be located a minimum of 10 feet (3.0 meters) from the toe of the dike and 2 feet (0.6 meter) from the seal, and shall employ vertical withdrawal.

B. Multi-level Takeoffs. For lagoons that are designed deeper than 10 feet (3 meters), enough to permit stratification of lagoon content, multiple takeoffs are required. There shall be a minimum of three withdrawal pipes at different elevations. Adequate structural support for takeoffs shall be provided.

C. Emergency Overflow. An emergency overflow should be provided to prevent overtopping of dikes. The hydraulic capacity for continuous discharge structures and piping shall allow for a minimum of 250 percent of the design flow of the system. The hydraulic capacity for controlled-discharge systems shall permit transfer of water at a minimum rate of six (6) inches (15 centimeters) of lagoon water depth per day at the available head.

10.8. Miscellaneous

A. Fencing. The lagoon area shall be enclosed with not less than 6 feet high chain link fence to prevent entering of livestock and to discourage trespassing. Fencing must not obstruct vehicle traffic on top of the dikes. A vehicle access gate of sufficient width to accommodate all maintenance equipment shall be provided. All access gates shall be provided with locks.

B. Access. An all-weather access road shall be provided to the lagoon site to allow year-round maintenance of the facility.

C. Warning Signs. Permanent signs shall be provided along the fence around the lagoon to designate the nature of the facility and advise against trespassing. At least one sign shall be provided on each side of the site and one for every 500 feet (150 meters) of its perimeter.

D. Service Building A service building for laboratory and maintenance equipment should be considered.

10.9. Industrial Waste Lagoons. The Director will review the design of lagoons for treatment of industrial wastes on the basis of such factors as treatability, operability, reliability, ground water protection levels, water quality objectives, etc.