The Type A filter is a buried, pressure-dosed, single-pass sand filter, meaning that effluent passes through the filter one time. The filter will be buried beneath the ground and lined on the bottom and sides with a heavy- duty plastic that will not allow water in or out. Six inches of gravel and a four-inch collection pipe that carries the treated effluent to the drainfield are placed on the liner at the filter bottom. On top of the gravel is a 24-inch layer of sand covered with more gravel. Small diameter distribution pipes running through the upper gravel layer bring effluent to the sand surface from the pump tank. The entire filter is covered (that is, it's buried) with a sandy soil that will allow air to reach the top of the underground filter (see Figure 1). Type A filters cover the largest surface area of the three systems and require less maintenance, but will require more sand and cost more to install than other systems. A small amount of sewage effluent is pumped over the filter surface about four to six times per day. After going through the filter one time, the treated effluent flows to the drainfield to be absorbed into the soil.

Type B is a free-access, pressure-dosed, single-pass filter, that allows access to the top layer of sand for maintenance. A removable cover may be used, or the sand surface can be left open if there are not any homes nearby. This type of filter requires less sand, a depth of only 18 inches, and a smaller surface area than Type A, but may require more frequent maintenance. Sand may have to be periodically replaced in some of these filters. These filters can be contained in a lined excavation or in the bottom halves of pre-cast tanks (see Figure 2). Again, the sewage effluent flows through this type of filter one time before it is discharged to the drainfield and soaks into the soil.

Type C, a free-access, pressure-dosed, recirculating filter, is similar to Type B, but requires 24 inches of sand depth and covers the smallest surface area of the three types. With this type of sand filter, a small amount of effluent is applied to the top of the filter surface every hour or one-half hour (24 to 48 times per day). After each pass through the Type C filter, about 75 percent of the effluent is returned to the dosing tank and reapplied to the filter, hence the name recirculating sand filter (see Figure 3). The other 25% of the treated wastewater flows to the drainfield. When the effluent finally reaches the drainfield, it will have been treated about four times. An added advantage of the Type C filter is that it converts the nitrate form of nitrogen (which can be a pollutant) into nitrogen gas that escapes into the atmosphere where it is harmless. This reduces the possibility of water pollution due to nitrogen. In nitrogen-sensitive water basins, this could serve as a helpful way of reducing nitrates from septic systems by up to 50 percent. Some of the system specifications for the three filters are included in Table 1.

All of these sand-filter pretreatment systems can provide a tremendous improvement in the quality of treatment we get from septic systems located in North Carolina's sensitive-receiving environments. These can include ground waters and surface waters used for public water supplies, shellfish harvesting waters, sole-source aquifers on the Outer Banks, and nutrient-sensitive rivers, lakes and ponds across the state. Sand filters can provide an extra measure of public health protection for on-site systems in challenging soil conditions. Many of the public health concerns about failing septic systems recently in the news can be significantly reduced through the use of pretreatment systems such as pressure-dosed sand filters.

One of the requirements in the state's pressure-dosed sand filter approval is that a licensed soil scientist must verify that the system will work on many of the more difficult sites. Also, the system design must be engineer-approved or the state must pre-approve standard designs. Therefore, manufacturers, engineers and/or health departments must submit these standard designs to the state for pre-approval before the systems can be used. One such approval was issued in 1999 to Orenco and a second is in process. For the 1999 approval see http://www.deh.enr.state.nc.us/oww/Innovati/iwws-97-1a.pdf. Also, note that a state- certified operator must monitor these systems twice a year (at a minimum) and the certified operator must check-out the system prior to the Health Department issuing an Operations Permit.

Please address any questions to Dr. David Lindbo.

This page created by
Roland O. Coburn, Research Technician I on 2/18/03.