Sand bioreactors are fixed-film biological treatment units. They are in the same category as trickling filters and rotating biological contactors, and are sometimes referred to as sand filters. As wastewater is applied to the top of a bed of sand, gravity draws it down between the sand particles. The surface of the sand grains is soon colonized by a film of microorganisms, as shown in Figure 1. The microorganisms draw nutrients, organic matter, and oxygen from the wastewater and air that passes through the sand bed. The sand also traps suspended solids. Sand bioreactor effluent is typically very clear with low biochemical oxygen demand (BOD5) and ammonia levels. Small sand bioreactors can serve individual dwellings, or larger ones can be used to serve an entire community.
Figure 1. Microrganism growth on sand grains in sand bioreator system.
Sand bioreactors differ from trickling filters and rotating biological contactors in that the small grain size of the sand media does not allow for sloughing of excess biomass. If continually overloaded, the sand tends to clog. In this way they are fail-safe, requiring the owner or operator to take corrective action. A malfuctioing sand bioreactor backs up rather than release poorly treated effluent to a receiving stream. These sand bioreactors also differ from those which receive secondary treated effluents. Sometimes a filter of sand is used after an aerobic treatment plant to physically filter the wastewater in order to remove excess suspended solids and "polish" the effluent. A filter of sand used for effluent polishing serves a different function than a sand bioreactor, has different design requirements, and is not covered in this document.
Sand bioreactors respond well to gradual increases in wastewater loading. Therefore, they are very appropriate for new developments with a gradual build out rate. Sand bioreactors also tolerate fluctuations in flow, especially changes from a negligible flow to very high flows. In this way, sand bioreactors are appropriate for seasonal use and recreational areas.
This bulletin is intended for designers and regulators of wastewater treatment systems. The types, design criteria, construction, operation, and maintenance practices for sand bioreactors are described along with regulatory and permit requirements.