The Recirculating Sand Filter: (or multiple pass sand filter) is typically used in a commercial/communal applications (usually with larger flows) and has an exposed bed of gravel . The Ontario Building Code lists the recirculating sand filter as a tertiary treatment system and is suitable for use with conventional gravity drainfields or a Shallow Buried Trench type drainfield or other forms of final disposal. The recirculating sand filter is hydraulically loaded at 200 l/m2/d.
Recirculating Sand Filters are an excellent method for bringing wastewater that varies in volume and strength up to tertiary treatment levels: BOD & TSS levels below 10 mg/L and total nitrogen reductions of 40-50%. Complete RSF kits from Sand Filtration Inc can handle every onsite treatment need, from a 3,785 liters per day installation to 3,785,412 liters per day effluent sewer system. Typically installed flush-to-ground, (exposed bed of gravel) they're the ideal solution for treating commercial waste from restaurants, schools, or RV parks. Sand Filtration Inc RSF's require little power to operate, and there's no need for a full-time operator. A Sand Filtration Inc. Recirculating Sand Filter makes sense when treatment quality is important and cost matters.
A finished Recirculating Sand Filter - Note river stone landscaping and exposed bed of gravel.
Construction:
The plywood frame to hold the liner in place is constructed first. A 50mm layer of fine sand is placed in the bottom of plywood frame. The 30mm PVC liner is installed then the 200 mm slotted underdrain and pump vaults (if applicable) along with a 200mm layer of large clear stone. A 600 mm layer of engineered gravel is placed in the Sand Filter. This layer is topped with a 50 mm layer of pea gravel on which a grid of effluent distribution laterals, with cold weather orifice shields, is placed then a final layer of pea gravel is applied over the distribution laterals. Note: The plywood frame is backfilled on the outside (which keeps the plywood frame rigid) at the same time the layers of gravel are applied.
Treatment Train:
In these on-sight sewage systems, there is a primary tank containing an effluent filter and a flow modulating orifice. Effluent from the relatively clear zone of the primary septic tank, between the scum and the sludge layer, enters the Biotube Effluent Filter through its inlet holes. Effluent then enters the space between the housing and Biotubes, utilizing the entire screen surface for filtering. Particles larger than 3 mm are detained in the interior spaces where continued decomposition of organic material occurs.Once the effluent has been screened through the Biotubes, it flows through the modulating orifices at the outlet of the filter. If tank inflow becomes temporarily excessive, the fluid level in the tank will rise as the modulating orifices slow the flow through the tank, allowing maximum settling of solids. In this manner instantaneous peaks during the day are buffered. A flow balancing tank is therefore not required in these types of systems.
The size of the primary treatment tankage determines hydraulic retention time and the frequency of solids removal. Based on the annual average flow, the primary tank volume is designed to provide for sludge storage and removal at approximately a 5 year frequency and a minimum hydraulic retention time of 24 hours in the clear zone of the primary tank.The primary treatment tank is buried with access risers to ground surface. Access covers are fiberglass and coloured green to blend with grass surface.
Recirculation Tank: Effluent from the primary tank flows by gravity, or is pumped to the recirculation tank. As the effluent is discharged to the recirculation tank it mixes with filtered effluent which has already passed through the sand filter. The mixed effluent flows through the tank where it is pumped at timed intervals to the Recirculating Sand Filter. An underdrain in the sand filter returns the effluent back to the tank where it once again mixes with fresh effluent from the primary Tank.When the recirculation tank is full, a simple floating ball valve in the recirculating tank splitter valve (RSV) diverts approximately 20% of the filtered effluent to disposal and the remaining 80% is discharge to the recirculation tank where the cycle repeats again. The design objective is that each litre of effluent is passed through the sand filter at least 4 to 5 times prior to discharge. If the recirculation tank water level is low, then 100% of the filtered effluent is returned to the recirculation tank and once again pumped through the sand filter. The recirculation tank is a precast concrete tank. The recirculation tank is equipped with pumps housed in duplex screened pump vaults and a recirculating tank splitter valve. Each pump vault is equipped with a 600 mm access riser to allow for pump inspection and maintenance.
The recirculation tank is raised with access risers to ground level. Access covers are fiberglass and coloured green to blend with grass surface
.Recirculating Sand Filter
The sand filter consists simply of an excavation in the ground, a liner frame of 7/16" waferboard which is lined with a 30 Mil PVC liner. The liner is filled with filter sand which meets a specific grading specification. The wood frame is designed to hold the shape of the filter liner stable until the surrounding soil becomes stable after construction. The wood frame will eventually bio-degrade and the sand filter liner will remain stable in place.
An underdrain is installed in the filter prior to backfilling with filter media. The 200mm slotted underdrain returns the filtered effluent back to the recirculation tank. A pressure effluent distribution grid (sand filter laterals) is installed on the surface of the filter sand . 100 to 150mm of pea gravel is then placed over the top of the laterals.
The effluent distribution grid (sand filter laterals) is designed to operate in distinct separate zones. Each zone is fed by a high head pump which is dedicated to that zone.
The sand filter is sized according to the formula A= Total Flow per day/Hydraulic Loading Rate.
Drainfield Dosing Tank
Effluent which has been diverted to disposal from the recirculation tank flows by gravity to a precast concrete drainfield Dosing Tank and from there to final disposal.