Figure 3. Recirculating sand bioreactor system.
Intermittent (single pass) sand bioreactors receive wastewater from a primary treatment unit through periodic surface applications totaling 0.5 to 1 gal/ft2/day. The wastewater is allowed to flow through the sand by the force of gravity between applications. Air penetrates the surface of the reactor between each wastewater application, and newly applied wastewater pushes trapped air down through the sand. Through intermittent applications of small amounts of wastewater, the filter remains aerobic.
Intermittent sand bioreactors are constructed of a 24-inch deep bed of medium to coarse sand. A supporting layer of gravel and a collection drain are placed beneath the sand to collect treated wastewater for ultimate disposal. Figure 2 shows a cross section of an intermittent sand bioreactor system.
Cattaragus, New York, a community of 200 homes, treats all of its wastewater through 4 intermittent sand bioreactors. The wastewater is pretreated in one of 4 parallel, 2,500 gallon septic tanks and is applied in 3 to 4 doses per day to a 36-inch deep bed of sand at a dry weather application rate of 1.5 gal/ft2/day and a wet weather application rate of 6.25 gal/ft2/day. After UV disinfection, effluent with an average BOD5 of 7 mg/l is discharged to a stream.
Recirculating sand bioreactors resemble intermittent sand bioreactors in that they receive periodic surface applications of wastewater, but are smaller. About 3 to 5 gal/ft2/day of primary treated wastewater is mixed with 9 to 25 gal/ft2/day of treated effluent. The mixture is applied in small doses, throughout the day, to the surface of a sand bioreactor. Therefore, in addition to the sand bioreactor, this system requires a recirculation tank to mix treated and untreated wastewater, the appropriate pump, and controls. Another important difference between a recirculating and an intermittent sand bioreactor is the sand specifications. Coarse sand is used to construct recirculating sand bioreactors, whereas intermittent sand bioreactors utilize medium to coarse sand. Figure 3 shows a cross section of a recirculating sand bioreactor system.
Figure 3. Recirculating sand bioreactor system.
Oriskany Falls, New York, a community of 900 homes, treats all of its wastewater through 4 recirculating sand bioreactors. The wastewater is pretreated in a septic tank. The septic tank effluent and 33% of the sand filter effluent are mixed in a recirculation tank and applied in 12 doses per day at 10 minutes per dose. The sand beds are 36 inches deep with an application rate of 4 gal/ft2/day. After UV disinfection, effluent with average BOD5 of 2 mg/l and ammonia ranging from 1.1 to 2.0 mg/l is discharged to a stream.
Recirculating the effluent several times has many advantages. The partially treated wastewater applied to the filter surface has practically no odor. Because the applied wastewater has a lower BOD5, more can be applied with each dose, resulting in a smaller bioreactor.
By mixing partially treated wastewater containing nitrate with full strength, anaerobic septic tank effluent, some denitrification can occur to achieve nitrogen removal. Denitrification rates of up to 50% have been achieved with recirculating sand bioreactors. Denitrification rates increase at higher temperatures.