|RECIRCULATING SAND FILTER SYSTEMS
RECIRCULATION VALVE (RSF) (RV)
RATIO BOX (RSF)(RB)
SEQUENCE OF OPERATION RECIRCULATING SAND FILTERS
Wastewater will flow from the house to a septic tank. For sites up to four (4) bedroom houses, a 1250 gallon septic tank is recommended. If a two compartment tank is used the first compartment is 2/3 of the over all capacity of the tank. This tank configuration is provided to enhance the initial treatment of the wastewater.
The septic effluent is discharged into a recirculating pump tank of the same size. . This tank is provided with a pump controlled by a timer to periodically dose a recirculating sand filter. The recirculation rate is adjustable to allow matching of the recirculation rate to the total effluent volume. This will allow the operator to achieve the best effluent quality at the lowest operating cost.
The low water safety switch is normally in the up position to assure enough water is in the chamber to dose the filter beds. The timer will initiate a single pump run time operation in increments (usually every 1 & 1/2 hr). The pump will run as set by the pump run time dial.
For the pump to be automatically actuated the pump switch must be on "auto". The pump may also be operated in the "manual" position.
The pump is sized to dose the recirculating sand filters as prescribed in the design criteria. The wastewater is then filtered through the sand, collected by underdrains and passed back through the pump chamber where it is either dumped into the chamber by the recirc-valve or allowed to pass through to discharge. In the event the pump chamber is full, the ball check in the Recirc-Valve prevents the wastewater from reentering the pump chamber. The bypass will continue until enough wastewater is discharged from the sand filter and pump lowers the water level in the chamber to allow the recirculating wastewater to reenter.
The effluent from the underdrains may be split by a ratio box to discharge a standard per cent of flow such as 25% going to discharge and the remainder going to discharge. This type of system will allow for a finite recirculation ratio.
In either system, valve or ratio-box, in the event of pump failure the water will continue to rise in the pump chamber and the alarm will sound and continue to run. In the event excessive wastewater enters the chamber between doses, the alarm will sound. The next pump cycle may eliminate the alarm, thus an intermittent alarm may indicate excessive water use, a peak flow condition, or a minor leak.
The frequency and amount of each filter dose should be adjustable to achieve the desired balance of operational cost and effluent water quality. The higher the frequency of the dose the more oxygen that will be drawn into the filter bed and the longer the filter runs between maintenance events. The higher the recirc ratio the higher the quality of effluent and the less odors from the filter.
Operational flexibility is helpful to fine tune the sand filter and to adjust for peak flows or infiltration from rainwater. However the system is very forgiving and can handle substantial peak flows and down time with little change in water quality.
RECIRCULATING SAND FILTER SYSTEM (RSF) with RECIRCULATION VALVE (RV)
The American Manufacturing Company, Inc. Recirculating Sand Filter Treatment package with Recirc-Valve is based on the EPA design Standard. The system will provide enhanced effluent quality with simple maintenance and very low operational costs. The Recir-Valve requires the final discharge to be determined by the elevation of the pump station. The filter is dosed periodically by a timer. The periodic dose is continuous so filter resting is less, and maintenance events may increase but the quality potential is enhanced due to the filter being continuously active.
TANKS ARE AVAILABLE WHEREVER SEPTIC SYSTEMS ARE SOLD. Most septic tank manufacturers should already have the necessary forms that can easily be adapted to configure this system. Recirculating filters differ from single pass filters in that a Recirculating filter is much smaller and a device is required to return filtrate to the dosing tank. Coarser media can also be used because of the recirculation to provide trouble-free operation.
RECIRCULATING SAND FILTER SYSTEM (RSF) with RATIO BOX (RB)
The American Manufacturing Company, Inc. Recirculating Sand Filter Package design with a Ratio Box is based on the EPA design Standard. The system is designed to provide enhanced effluent quality with simple maintenance and very low operational costs. The Ratio box allows the final discharge to be elevated with the sand filter therefore not determined by the elevation of the pump station. The filter rests during low flow so filter runs may be longer but the quality may be detracted from during restart after long rest periods.
TANKS ARE AVAILABLE WHEREVER SEPTIC SYSTEMS ARE SOLD. Most septic tank manufacturers should already have the necessary forms that can easily be adapted to configure this system. Recirculating filters differ from single pass (intermittent) filters in that a Recirculating filter is much smaller and a device is required to return filtrate to the dosing tank. Coarser media can also be used because of the recirculation to provide trouble-free operation.
HIGH EFFLUENT QUALITY are typically achieved from recirculating sand filters. Studies have shown that very high quality effluent can be produced with this system and the system is one of the easiest to maintain.
|BOD|| = ||99% Reduction
|SS|| = ||97% Reduction
|Total Nitrogen|| = ||45% Reduction
Recommended effective size of the media ranges from 0.8 to 1.5mm, the coarse material (1.2mm) being the preferred. Uniformity coefficients should be 2 to 4.0 (less than 3 preferred) (Teske, 1978; Hines and Favreau, 1975).
The media must be carefully placed in the filter to prevent stratification of the material. It should be homogeneous throughout its depth. To prevent the media from piping into the gravel below, two to three inches of 1/4 to 3/8 in. pea gravel may be placed on the bottom or on top of gravel before adding the media.
HYDRAULIC LOADING RATE
Hydraulic loading rates are dependent on the media size. They should range from 3 to 5 gpd/sq ft based on forward flow. A recirculation rate of 3:1 to 5:1 (recycle: forward flow), is used (Hines & Favreau,1975).
Distribution over the sand surface should be provided primarily by low pressure distribution manifolds closely spaced. Timer operated pumps dose the wastewater onto the filters every 30 to 60 min. for a period of 5 to 10 minutes. Multi-home units may be dosed less frequently (20 min. every 2 to 3 hours).
Open recirculating filter surfaces are not recommended and must be protected from erosion, rain and algae growth. Spray screens or similar devices may be necessary.
The active dosing volume within the recirculation tank should be at least 5 times the raw influent volume entering the tank between dosing cycles. For small installations, 1/4 to 1/2 day volume of the septic tank is used. The volume can go as high as 1 full day volume.
Precast filters are usually made from the bottom half of a large septic tank or a whole smaller one. Make sure there is acceptable sand depth and air space. Other types of enclosures are poured concrete or masonry walls set on a poured slab. If any part of the filter wall is exposed, it should be well insulated against freezing temperatures. Covers are necessary in severe climates plus they may be used to shade the sun to prevent excess algae/plant growth and preventing unwanted traffic over the area by animals and children.
Modern recirculating filters should be checked regularly (annually). At the higher loading rates, clogging of the filter surface will occur, slowing infiltration. If the surface remains ponded between doses, insufficient aeration of the media will occur, lowering the effluent quality and life of the system. If the filter surface remains ponded between doses, the filter should be taken out of service for rejuvenation.
Resting, raking of the surface and sand equipment are common rejuvenation techniques (EPA,1980). Raking may be sufficient to break up the clogged surface to allow continued loading, but raking combined with 3 or 4 months resting seems to prolong filter runs (Univ. of Wis.,1978). Eventually, however, removal of some of the sand becomes necessary.
SAND FILTER CONSTRUCTION
The recirculating sand filter is easily constructed from a 1500 gallon septic tank. These tanks normally have 45 square feet of surface area. Therefore one filter, if designed as a high rate filter (10 gallons / day / per square foot), will service a 3 bedroom house. Two filters would service the 3 bedroom house at 5 gpd/sq.ft. The filter can easily be built on site per the following.
The distribution lateral provides an excellent method to disperse the effluent over the bed. This lateral curve will allow the designer to determine heads necessary to operate the system. Design the head at the lateral to operate at 1' to 2' over the invert of the orifice.
The Recirculating Sand Filter Package shall be a provided by American Manufacturing Company Inc. The Package shall include the items as indicated on the next page.
|ITEM # || ||COMPONENTS
|1|| ||RECIRC VALVE or RATIO BOX
|2|| ||CONTROL PANEL with FOUR FLOATS
|3|| ||SWITCH SET BAR
|4|| ||SAND FILTER COVERS
|5|| ||DISTRIBUTOR LATERAL SETS
|6|| ||PUMP DISCHARGE KIT
|7|| ||PUMP TANK RISER and LID
|8|| ||SEPTIC TANK|| ------ BY OTHERS
|9|| ||PUMP TANK|| ------ BY OTHERS
|10|| ||FILTER MODULES WITH AGGREGATE|| ------ BY OTHERS
|11|| ||MISCELLANEOUS CONNECTION PIPE|| ------ BY OTHERS
- The Ratio-Box shall be suitable for connection to 4" schedule 40 PVC or 4" S & D PVC pipe. The box shall be non-corrosive hard plastic suitable for use in this environment and be provided with an at grade riser. The return effluent shall enter the box and split the flow up to 6 ways. The Recirc Valve shall be by American.
- The controller shall be NEMA 4X and provided with separate breakers for the alarm/pilot and pump circuits. The pump run dial (ON TIME) which will be set for how long the pump will run (from 15 SEC TO 10 minutes). The adjustable pump frequency dial (OFF TIME or TIME OF DAY, depending on control model) provides the pump frequency time control for each time the pump activates (typically every 1-1/2 hours). The control shall be U/L listed with color coded wiring, and be manufactured by American Manufacturing Company Inc.
- The pump chamber shall be provided with a switch-set-bar complete with NEMA 3R junction box (by others) for mounting outside the pump station riser, NEMA 4X box with strain relief seals and vertical bar support. The switch set tree shall attach to the 1" vertical float support bar with a quick release compression coupling.
- The sand filter covers shall have pressure treated lumber suitable to support minimum walking loads on a corrugated non-corrosive, non - translucent and vented cover. The cover shall be double hinged by a single peripheral treated wood boarder which will set on the support and be provided with a locking hasp.
- The Distributor laterals shall be sized for the pump and provide suitable operation under a wide range of heads. The laterals shall be 1-1/2" dia. PVC pipe with equal sized orifices alternate sides of the pipe to match specification curves. The pipe shall be setup to receive a distal end stand pipe for setting head pressure and shall be wrapped in fiberglass spray screens attached with at least twice the pipe diameter. The laterals are to be supported on bricks on the sand surface. The laterals shall be pre-manufactured and as supplied by American Manufacturing Company Inc.
- The pump system shall be provided with a H-O-A switch and the alarm with an auto-reset T-N-S switch. The high level alarm shall be provided with an override selector switch by American to select whether or not to pump down the top portion of the tank in case of excess flow. The pump discharge shall have a gate valve, check valve, and cam-lock type corrosion resistant disconnect.