The RSF/Wetland treatment system uses two "fixed film" processes in series to achieve a very high degree of wastewater purification. The first process is a recirculating sand filter, which would more accurately be termed a recirculating gravel filter due to fact that the active part of the filter is a fine gravel rather than sand. This process consists of primary tanks for solids removal, a fine screen to further remove particles, recirculation tanks and finally the recirculating sand (gravel) filter. The settled and screened wastewater is then dosed intermittently onto a four foot deep bed of various grades of gravel with an underdrain to maintain air within the void spaces in the gravel bed. The void spaces provide contact of the wastewater with air for the oxygen required for efficient treatment. A bacterial film develops on the fine gravel media in the center of the bed, which reduces the organic strength of the wastewater - normally referred to as CBOD. The bacteria deeper into the bed convert the ammonia-nitrogen in the wastewater to nitrate (nitrification). The wastewater is "recirculated" through the gravel bed an average of 5 to 10 times prior to discharge to the wetland phase of treatment. The recirculation ratio is determined primarily by the organic strength of the wastewater. Because the RSF effluent and the primary tank effluent are both introduced to the recirculation tanks at the same point, anoxic zones develop in the recirculation tank, which serve to partially denitrify the wastewater prior to discharge to the wetland.
A maximum of 20% of the RSF effluent is diverted by gravity to the wetland where it undergoes further treatment. The wetland also typically has four to five feet of slightly coarser (1/2" diameter) gravel, which provides the surface for the bacteria grow. The gravel media in the wetland is always submerged as opposed to the RSF, which is always drained. This "flooded" condition maintains anoxic to anaerobic conditions within the wetland forcing the bacteria to use the oxygen in the nitrate ion (NO3) during respiration. Once the oxygen is stripped from the nitrate ion, it becomes nitrogen gas (N2) and the wastewater has been denitrified.
The treated wastewater is then suitable for final disposal via surface discharge or land disposal depending on the specific needs of the system. This treatment system is particularly well suited for commercial and small community applications with flow rates from 5,000 to 150,000 gallons per day.
If you have questions concerning this technique or would like to discuss your needs, please contact us at your convenience.