The overland flow process is the application of wastewater along the upper portion of uniformly sloped and grass-covered land and allowing it to flow in a thin sheet over the vegetated surface to runoff collection ditches. The primary objective of this process is treatment of wastewater. Utilization of this process does result in a discharge and therefore a waste discharge permit from the Texas Water Commission is required. This process is best utilized on soils with low permeability. The performance of the overland flow process is dependent on the detention time of the wastewater on the vegetated sloped area. Therefore, in order to meet a specified effluent criteria, the hydraulic loading rate, the application rate, and the effectiveness of the distribution system are essential design considerations. For detailed process design guidance, the latest edition of the Environmental Protection Agency Technology Transfer Process Design Manual for Land Treatment of Municipal Wastewater may be used.     
       (1) Hydraulic loading rate. The hydraulic loading rate and application rate can vary depending on levels of pretreatment, quality of effluent, temperature, and other climatic conditions. A hydraulic loading rate of 1.5 to 2.0 inches per day and an application rate of six to eight gallons per hour per foot of slope width are suggested as general guides. The design rates selected and their justification shall be submitted in the design report.      
       (2) Wastewater storage. Storage capacity for inclement weather conditions shall be provided. To minimize the impact of algae on the treatment performance, this storage shall be designed as an off-line basin, used only as needed and emptied as soon as possible by blending with other pretreated wastewater prior to application. To control odors, provisions for aeration in the storage basin should be considered.      
       (3) Soil testing. For the overland flow process, the soil profile evaluation should extend to a depth of at least three feet. The soil sampling and testing specified in subsection (b) of this section shall be representative of the soil to this depth.      
       (4) Other design considerations.                (A) The overland flow process treatment area shall be subject to the same buffer zone requirement as a treatment plant.                 (B) The minimum slope length for the applied wastewater shall be 100 feet.                 (C) The sloped areas to receive wastewater shall be uniformly graded to eliminate wastewater ponding and short circuiting for the length of the flow. Site grading procedures and tolerances shall be included in the specifications. Minimum slopes shall equal or exceed 2.0%; maximum slope shall not exceed 8.0%. The application site shall be protected from flooding.                 (D) The application cycle should provide a maximum of 10 hours for dosing followed by a minimum period of 14 hours of resting.                 (E) The method of application shall provide uniform coverage of the area.                 (F) A vegetative cover shall be provided on the application site. The plant types selected shall be suitable for overland flow conditions and shall provide uniform coverage of soil to prevent short circuiting and channelization of the area.                 (G) Wastewater quality and disinfection requirements for overland flow process discharges will be established by the discharge permit.                 (H) An effluent sampling station shall be provided prior to discharge to surface waters. The sampling and reporting requirements will be established by the discharge permit.        
     Source Note:        The provisions of this §317.10 adopted to be effective November 26, 2015, 40 TexReg 8341