Re-evaluating Surface Application Rates for Texas OSSF Systems
Clifford B. Fedler, John Borrelli
Approximately 25% of the nation’s housing units utilize on-site treatment and disposal systems. Mostly, on-site treatment consists of a septic tank-soil adsorption configuration, though surface disposal systems are used in areas where the soil is not suitable for an adsorption field. One of the concerns with the use of on-site sewage treatment systems is the potential for nitrate pollution of the groundwater resources. Current procedures for designing surface application systems for on-site sewage facilities (OSSFs), with an emphasis on aerobic systems, in Texas have been reviewed. Concerns with the current procedures for designing sprinkler systems include the sizing of the spray field area, the volume of effluent storage required, and the absence of the uniformity of sprinkler distribution patterns. Currently the spray field area is determined by the estimated daily volume of water applied divided irrigation water requirement (evaporation minus precipitation). A proper design needs to be adaptable to the many climates and soils that exist within the state, while maintaining the integrity of the environment. To meet this goal, an alternative, easy to follow, design procedure is proposed. The proposed design method incorporates the concept of water application rate, soil infiltration rate, crop water use, crop nutrient uptake rate, water application efficiency, and irrigation layout design and nozzle selection.
With any surface application system for wastewater effluent, control of the nitrogen applied is essential to minimize the impact on regional water resources, whether surface water or ground water. If an OSSF is designed with a typical type of sprinkler and no overlap of the spray pattern is provided, the potential mass of nitrogen that can move below the crops root zone can be substantial. The quantity of nitrogen that could potentially move below the crops root zone ranges from 16 percent of the nitrogen applied in East Texas to 59 percent of the nitrogen applied in West Texas. Poor distribution of the effluent applied on portions of the spray field may cause the nutrients (e.g. nitrate-nitrogen) to be applied at levels exceeding the plants assimilation capacity. If a sprinkler design provides an overlapping spray pattern and the wastewater application rate is limited based on the crops ability to utilize the applied nitrogen, the nitrogen that could potentially move below the crops root zone can be limited to 7 percent of the total nitrogen applied or less. This limited nitrogen movement is realized when the wastewater distribution uniformity coefficient is 80 percent or greater.
Another advantage of the proposed design procedure is the smaller land area requirement in some parts of the state. In east Texas for example, the land area reduction is about 27 percent while the land area required in West Texas increases by two times. The additional system requirements under the proposed design procedure are an increase in the number of sprinkler heads and a zone sprinkler controller. This latter device is required to cycle the application of the wastewater effluent to the various quadrants of the spray field.
It is recommended that the following changes be made to the current Texas Administrative Code 285 rules in order to provide for the least amount of negative impact our the states environment.
- All surface application systems designed for an on-site sewage facility should consider both a water balance and a nutrient balance for the final design.
- The layout of the site for effluent application should be in a block pattern such that the sprinklers can be arranged to have a head-to-head overlap. If this is not available, then the system should be designed such that the proper overlap can be provided in order to achieve a uniformity coefficient of 80 percent or greater.
- Spray head type of sprinklers should not be used in an OSSF system while the gear head type should be used.
- All sprinklers are designed to operate at an optimum pressure range to obtain the specified pattern of water distribution and the OSSF design pressure should be in the middle of the specified range. Sprinklers operating at pressures lower or higher than designed will produce unreliable patterns that will result in very low water application efficiencies and low application uniformity.
- The time used to apply the effluent should not exceed 1 hour and the average design should be 0.5 hours.
- The base water intake rate of the soil should follow that described by Saxton et al. (1986) provided more precise information on the soil is not available.
- The base soil infiltration rate should be set equal to the saturated hydraulic conductivity of the top 18 inches of soil.
- A check-off list of design considerations should be developed and used on all new and renovated designs of OSSF where surface application of the effluent is utilized.