Sand bioreactors can be designed and constructed in three ways: open, covered with a roof, or buried.
The surface of an open sand bioreactor is always exposed to outside elements. Open bioreactors offer the best accessibility for maintenance and observations of ponding, a precursor to clogging. They are also the least expensive type to construct. Open sand bioreactors are exposed to variations in hydraulic load, due to precipitation and variations in temperature. In cold climates, a layer of ice forms on the sand surface but the applied wastewater will easily flow underneath the ice. In addition, weeds tend to grow on open sand bioreactors which can become an eyesore, and provide a protected, moist place for animals to inhabit. Regular raking eliminates weeds.
By burying a sand bioreactor, it is protected from extreme fluctuations in temperature and hidden from view. The final depth of a sand bioreactor ranges from 4-5 feet, in order to accommodate for the 24 inches of sand depth, the supporting gravel and drainage system, the surface distribution system in a gravel cap, and a 1-foot layer of insulating soil. Therefore, it is common for a buried sand bioreactor to be constructed in a shallow excavation and be mounded above the ground surface (Figure 4).
Access and easy observation to the buried sand bioreactor has been eliminated. Observation ports become the only way to determine the onset of ponding. To assure a 20-year design life, buried sand bioreactors should be designed for extremely small loading rates to reduce the need for maintenance and the likelihood of ponding. If ponding does occur, a second buried sand bioreactor built on a site can be used to rest the primary bioreactor. The second sand bioreactor will only be used for a few months during the resting period, therefore it can be smaller than the primary.
A roof cover over a sand bioreactor may be an appropriate compromise. The roof protects the bioreactor from precipitation and extreme fluctuations in temperature. The shade provided by the roof also eliminates weed growth on the sand surface. However, roofs add to the construction cost, and must be constructed with easy access, wind and snow loads, durability, and appearance in mind. Figure 5 shows some photographs of roofs over sand bioreactors.
Roof designs vary greatly and must perform three functions: insulate the bioreactor, allow for easy access for maintenance, and be esthetically pleasing. The most common roofs are lifted off as a single piece, or in sections as necessary. Some roof designs used on sand bioreactors include a sloped, fixed roof built on posts about two feet above the sand surface. In this way an operator can reach under the roof edge to make observations and access the bioreactor to perform necessary maintenance. Hinges can be placed along one edge of flat or monosloped roofs so they can be propped up. Roofs are made of a variety of materials ranging from small roof trusses with sheathing and shingles to match nearby buildings, to 2 x 4 frames with exterior plywood and roofing paper. Corrugated metal and fiberglass have also been used.