(a) Design requirements. (ii) Wastewater loading rates. - (i) Location. Wastewater lagoons shall be located more than 500 feet (152 m) from existing habitations.
- (A) Facultative. The primary cells of a facultative (non-aerated) pond system shall be limited to a maximum BOD application of 40 lb/acre/day (44.8 kg/ha/d) at average design loading conditions.
- (B) Aerated. Aerated lagoons shall be designed for an organic loading of less than 10 lb BOD5/day/1,000 cu ft (160 kg/1,000 m3/d) for completely mixed systems, and less than two lb BOD5/day/1,000 cu ft (32 kg/1,000 m3/d) for aerated non-completely mixed systems. Aeration equipment shall be sized to maintain a minimum dissolved oxygen of two mg/L. Completely mixed systems are mixed to provide 1/4 hp/1000 cu ft mechanical mixing or 10 cfm/1000 cu ft of air mixing.
- (C) Nonsurface water discharging ponds. Nonsurface water discharging ponds shall be designed on the basis of a water balance that considers evaporation and seepage. Water balance calculations shall be submitted with the plans and specifications. The BOD5 loading for non discharging ponds shall not exceed 14 lb/acre/day (15.7 kg/ha/d) based on the average annual BOD5.
- (iii) Storage. Nonsurface water discharging lagoons shall be designed to provide sufficient storage to retain all wastewater and rainfall during the wettest year of record during a ten (10) year period of record. Seepage shall be controlled to maintain a minimum water depth of two (2) feet (0.6 m) in the primary cell during the driest occurring year of a ten (10) year period.
- (iv) Inlet.
- (A) Location. The inlet pipe to the primary cell of a facultative lagoon shall be at least thirty (30) feet (9.2 m) from any bank. It shall terminate at a point away from the outlet by a distance of at least equal to or greater than 2/3 of the longest lagoon dimension. In aerated systems, the influent line shall be located in the mixing zone of the aeration equipment.
- (B) Apron. Provide a concrete apron at the inlet pipe termination with minimum dimensions of four feet by four feet (1.2 m by 1.2 m).
- (C) Influent manhole. An influent man-hole shall be provided prior to the lagoons. The influent pipe in the influent manhole shall be at least six (6) inches (0.15 m) above the normal operating water level of the primary lagoons.
- (D) Flow distribution. Flow distribution for multiple primary cells shall be provided to effectively split hydraulic and solids proportionately.
- (v) Inlet and outlet structures.
- (A) Location. Inlet and outlet structures shall be easily accessible by plant operators and located to minimize short circuiting within the cell. A level control structure shall be provided at the outlet of each cell.
- (B) Level control. Provide controls to permit varying water levels between two (2) feet and six (6) feet (0.6 m to 1.8 m). Provide baffling at the outlet to prevent scum overflow. Multiple draw offs in the final cell shall be provided. At least one (1) shall be located at the two (2) foot (0.6 m) level.
- (vi) Interconnecting piping.
- (A) Location. Piping between lagoon cells shall connect to the preceding cell outlet control structure and discharge into the subsequent cell. The pipe shall discharge at least ten (10) feet (3.05 m) from the toe of the slope on the lagoon bottom and shall terminate on the concrete apron that is at least four (4) feet by four (4) feet (1.2 m by 1.2 m).
- (B) Elevation. The piping shall discharge at the floor of the lagoon.
- (C) Material. Interconnecting piping shall be any acceptable pipe designed to resist low pressures and adequately protected from corrosion.
(b) Number of lagoons cells. A lagoon system with a total area greater than one (1) acre (0.4 ha) shall have at least three (3) cells in series. Smaller systems and nondischarge pond systems shall have at least two (2) cells. The maximum size cell shall be twenty (20) acres (8 ha).
(c) Lagoon configuration. - (i) Shape. Rectangular cells shall have a maximum length to width ratio of 5:1. No sharp corners nor dead-end coves are permitted.
- (ii) Water depth. Facultative ponds shall be designed to have water depths of not less than two (2) feet, nor more than six (6) feet (0.61 m to 1.8 m). Aerated lagoons shall be designed to have water depths of not less than four (4) feet nor more than fifteen (15) feet (1.2 m to 4.6 m).
- (iii) Removal of lagoon cells from operation. Bypass piping for primary lagoon cells and aerated lagoon cells shall be provided.
- (iv) Lagoon freeboard. A minimum freeboard of two (2) feet (0.6 m) shall be provided. Greater freeboard shall be provided for wave runup, where required.
(d) Construction requirements. - (i) Dike.
- (A) Material. Dikes and embankments shall be of relatively impervious and stable material, and compacted to at least 95 percent of maximum density (ASTM D698-78). Embankment fill shall be free from organic material, rock larger than six (6) inches (15.2 cm) and construction debris. The area where the embankment is to be constructed shall be stripped of vegetation and roots.
- (B) Top width. Dikes and embankments shall be constructed with minimum top width of eight (8) feet (2.4 m).
- (C) Slopes. Interior slopes shall be from three (3) to four (4) horizontal to one vertical, and shall be stable under varying water level conditions. Interior slopes that are surfaced with concrete paving or riprap may be constructed at slopes of two (2) or more horizontal to one (1) vertical. Exterior slopes shall be three (3) or more horizontal to one (1) vertical and shall prevent the entrance of surface water to the lagoon.
- (ii) Seeding. Exterior slopes and interior slopes that are not riprapped shall be seeded with dryland grasses, unless another equivalent method for soil erosion control is provided.
- (iii) Erosion control. Interior embankments except cells smaller than one (1) acre shall be protected from wave action with riprap, paving, or other erosion resistant material, unless it is demonstrated that the ponds are sheltered from wind or where wind velocity is low and erosion will not occur.
(e) Lagoon sealing. - (i) Lagoon sealing. The seepage through the pond bottom and side walls shall not cause a violation of the groundwater standards as described in Chapter 8 (Quality Standards for Wyoming Groundwaters) of the Wyoming Department of Environmental Quality, Water Quality Rules and Regulations. Liners shall be required if the wastewater characteristics or site conditions will not insure the protection of the groundwater for which it is classified.
If the applicant cannot document that the facility poses no threat to groundwater and elects not to perform a subsurface study in accordance with Chapter 3, Section 17(a) and (b), then the groundwater shall be protected from contamination by the wastewater with a liner equivalent to three (3) feet (1 m) of soil having a permeability of 10-7cm/sec or less. When an applicant performs a subsurface study, the requirements for the liner shall be determined based on the results of the study and the groundwater protection required. In no instance shall the maximum seepage rate exceed 1/8 inch per day (3.2 mm/day) in the primary pond(s).
Following construction of the lagoons, but prior to startup, a testing program shall be conducted to demonstrate the effectiveness of the sealing program. Should the testing program show the lagoon seal to be less effective than the above requirements, the seal shall be modified and retested until it succeeds.
- (ii) Synthetic liners.
- (A) Material. Synthetic liners shall be essentially impervious. The minimum lining thickness shall be 30 mils. The liner material shall be resistant to organic materials typical of sewage. The liner shall be resistant to sunlight or shall be covered with 12 inches (30.5 cm) or more of soil at all locations including the lagoon bottom and side slopes.
- (B) Liner stabilization. Where the seasonal high groundwater is above the bottom of the lagoon, the liner shall be stabilized to prevent it from rising.
- (C) Appurtenances. A leak detection system and/or air release mechanism may be required.
(f) Aerated systems. - (i) Air requirements. Aerated ponds shall be designed to maintain 2 mg/L of dissolved oxygen or more throughout the pond contents.
- (ii) Equipment requirements.
- (A) Number. Surface aerators shall be provided at intervals of 200 feet (61 m) or less. The lagoon shall be protected from erosion from the aeration equipment. At least two surface aerators or brush aerators shall be provided. With the largest unit out, the remaining units shall be capable of transferring the average day oxygen demand. Each diffused aeration system shall be provided with at least two blowers. With the largest blower out of service, the remainder shall be capable of supplying the design air flow rate.
- (B) Removal. All equipment shall be accessible and removable from the edge of the lagoons. Provisions for dewatering shall be made for removal or repair of diffusers.
020-11 Wyo. Code R. § 11-18