Packed Bed Filters
Packed bed filters can be broken into two groups: single pass units and multi pass units. As the titles suggest, wastewater is either passed though the filter once or multiple times. These filters pre-treat effluent from the septic tank before infiltration into the soil where the unsaturated depth above groundwater or bedrock is not great enough to provide sufficient treatment. They may also be used around sensitive areas such as lakes and areas where aquifers recharge. Packed bed filters often treat domestic wastewater, but they can also treat industrial wastewater.
Single and Recirculating sand filters
The purpose of a sand filter is not only to remove sediment and suspended solids, but mainly to provide biological aerobic treatment of the wastewater. This is considered secondary treatment. The first six inches of the filter is where most biological treatment occurs. Here is also where suspended solids and BOD are removed. Sand filters are very effective at lowering suspended solids as the media in the filter holds onto the solids well. Fecal coliform bacteria removal ranges from 2 to 4 logs (99- 99.99 percent)
This table shows % BOD removal from single pass sand filters.
The above picture shows a single pass sand filter. The construction of a sand filter starts with the excavation of the area the filter will be in. A hole is dug large and deep enough for the size filter needed for the project. A sand filter for a single family home is typically built 1.5 times larger than the calculated peak daily flow. Soft material such as sand is then filled in on the bottom if the area is rocky to prevent puncturing of the liner. Most commonly for households a prefabricated concrete liner is placed in the hole, but liners are sometimes PVC or polypropylene. Protection of groundwater is the main reason that sand filters are lined. On top of the linere there is an underdrain outlet with a minimum of 0.1 percent slope to prevent backwash. The pipes transport the clean water to the soil absorption system for a single pass unit or to the recirculation tank for a multi-pass unit. The pipes are covered with washed gravel which provides a porous medium for the wastewater to flow through.
Smaller gravel is placed over this to prevent the filter media from mixing with the larger gravel of the underdrain outlet. The filter is filled with media which could include, sand, gravel, peat, crushed glass, bottom ash, foam chips, and synthetic textile materials. Media choice depends on filter area, dose volumes, and dosing frequency. Then the dosing pipes which are connected to a pump are added. These pipes apply the wastewater to the sand filter.
Traditionally, hydraulic loading rates have been used as the basis of design to determine the dosing volume of the system (how much wastewater is released at a time to be treated.) However, organic loading is more appropriate because packed bed filters are primarily aerobic treatment units. The current standards state that BOD5 loadings should not exceed 5 lb/1,000 cubic feet per day. Smaller, more frequent dosing volumes are desirable because the soil is unsaturated meaning better contact with the media and a longer retention time. Therefore there is more opportunity for the wastewater to be treated. There are various distribution methods including pressurized pipe networks with spray nozzles, drip distribution, and surface flooding. The dosing volume can be timed or controlled by the amount of water in the sand filter.
A sand filter being constructed
From Waukesha County Health Department
Many filters are covered to prevent the presence of algae from the sunlight which could clog the filter. Other reasons include odor, cold weather, snow, and leaf accumulation. The filter must be aerated to allow the bacteria to live so the cover must let air in but still be water tight so not to collect storm water.
A sketch of a single pass sand filter design
Differences in Recirculating Filters
Recirculating filters, like single pass units pre-treat effluent from septic tanks before it is released into the environment. Instead of all the water in the underdrain flowing to the soil absorption system, the pipes return some of it to the recirculation/dosing tank. Here the water is combined with the effluent from the septic tank. Recirculating filters can be smaller and have less odor than single pass units. Recirculating filters use a more coarse material and have a higher hydraulic loading than single pass units therefore reducing the removal of fecal coliform bacteria. However, almost all ammonia is removed through nitrification.
A table of BOD removal in a Recirculating sand filter
AdvanTex is another kind of aeration treatment unit. The filter itself is a textile material contained in a fiberglass unit. According to the EPA, the AX10 model have 20,000 square feet of surface area in a relatively small space. The unit is only 10 square feet. Operating cost is relatively low. The components of the system include a septic tank, the AdvanTex unit itself, and a drain field. Typically, the Advantex unit is placed on top of the septic tank. Like sand filters, AdvanTex are biological reactors which rely on bacteria for treatment of the wastewater. The textile of the filter is where the colonies of bacteria grow and treat the water.
Typical effluent from AdvanTex systems contains BOD, suspended solids, and total nitrogen levels of less than 10 mg/L. For proper operation AdvanTex systems should be maintained annualy.
Photo by Eli Gottfried
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