Intermittent Sand Filter

Intermittent Sand Filter Systems...

Description...

Filters are used when the soil is too shallow or too permeable and the effluent requires treatment prior to surface or subsurface discharge. Typically effluent is pre-treated in a septic tank and then flows, or is pumped over a bed of sand under which is a drainpipe collection system. Intermittent sand filters are those in which the effluent passes over the media only once and is then discharged to the disposal system.

Performance...

BOD of 30 mg / L and SS of 30 mg / L (intermittent - for a filter of 1 foot sand depth).

Advantages...

Effluent is low in suspended solids and BOD₅, and does not cause a clogging mat to form at the infiltrative surface in soil absorption systems

Natural permeability of the soils will be maintained over time and may increase

The lower organic content of effluent may allow reduction of area requirements for subsurface infiltration

Disadvantages...

Restricting activities or access to the portion of the property occupied by the sand filter may be objectionable to a home owner

Maintenance contracts with a professional operator may be necessary to ensure acceptable performance

Semiannual raking of the sand filter is necessary to prevent formation of a dense biomat

Electrical power is required for pump operation

Costs...

A recent cost estimate of the control panels, float assemblies, pumps, and hose and valve assemblies, alone for a sand filter system was approximately $1,000 US indicates that the total cost of the installation of such a system in Nova Scotia is more likely to be on the order of $5,000 to 10,000.

Recirculating Sand Filter Systems...

Description...

Filters are used when the soil is too shallow or too permeable and the effluent requires treatment prior to surface or subsurface discharge. Typically effluent is pre-treated in a septic tank and then flows, or is pumped over a bed of sand under which is a drainpipe collection system. Recirculating systems are those in which the septic tank effluent is discharged to a recirculation tank, in which raw septic tank effluent mixes with the filtrate. A portion of the return filtrate discharges to the disposal system, while the majority is pumped back up to run through the system. The filter may be open to the atmosphere or covered with a layer of pea gravel.

Performance...

Reported removal - N : 40 to 70 %

Quality : BOD 10 mg / L, TSS 10 mg / L

Below 10 degrees C nitrification can drop to 25 %

Advantages...

Higher nitrogen removal rates than those achieved with the intermittent sand filter

Effluent is low in suspended solids and BOD₅, and does not cause a clogging mat to form at the infiltrative surface in soil absorption systems

Natural permeability of the soils will be maintained over time and may increase

The lower organic content of effluent may allow reduction of area requirements for subsurface infiltration

Disadvantages...

Restricting activities or access to the portion of the property occupied by the sand filter may be objectionable to a home owner

Maintenance contracts with a professional operator may be necessary to ensure acceptable performance

Semiannual raking of the sand filter is necessary to prevent formation of a dense biomat

Electrical power is required for pump operation

Costs...

Costs for pumps control panels and piping recirculation tank, sand filter, and disposal field $5,500 US. However, a recent cost estimate of the control panels, float assemblies, pumps, and hose and valve assemblies, alone for a recirculating sand filter system was approximately $2,500 US indicates that the total cost of such a system is more likely to be on the order of $10,000.
"Septic Tanks & Biofilters"...

Sloping Sand Filters...

Description...

The sloping sand filter (SSF) is a variation on the the more commonly used vertical flow, under drained sand filters. In Nova Scotia it is used as a remedial system. The septic tank location, field layout, and a cross-section of a typical SSF field installation is shown in the Figure. Although the original design shows a collection trench and roadside discharge, as in shown in the Figure, recent designs allow flow through the ditch wall along the entire foot of the filter. The filter fill is a specified sand of 12 to 18 inches in depth. The SSF is typically used on lots with inadequate room for the installation of a replacement system due to either lot size restrictions or slowly permeable soils. On lots with slowly permeable soils the filter provides a large basal contact area over which effluent may be absorbed into the subsoil. If the host soil will not allow vertical downward movement of the effluent it receives adequate treatment during lateral flow through the length of the filter. Discharge is commonly out of the bottom of the filter, although at high water table times of the year some discharge may occur along the foot of the trench.

Performance...

There are currently hundreds of these systems installed and operating in Nova Scotia. They have proved to relatively trouble free. There is currently some consideration to use a modified design for the installation of new systems. Laboratory results showed excellent treatment over 5 m of lateral flow ;

Effluent Quality : Ammonia - N << 0.05 mg / L, Nitrate - N << 70 mg / L, BOD << 2mg/L, SS << 10 mg / L, Fecal coliforms 0 / 100 mL

Advantages...

Is a viable remedial option for sites with lot size restrictions that would preclude the installation of a replacement system

Provides very good treatment

Large systems have been used in Nova Scotia to replace malfunctioning package plants

Can be designed as a gravity feed low maintenance system

Disadvantages...

On low permeability sites there may be discharge at the foot of the filter during high water table times of the year, some homeowners may find this objerctionable

Costs...

Depending on lot topography a sloping sand filter system will cost $2,000 to $5,000.

Septic Tank and Gravel Absorption Trench...

Description...

This is the most common system that is used on sites with adequate soil depth above the water table. Heavy solids and greases are removed in the septic tank. The liquid flows out through a perforated pipe into the soil. Bacteria and oxygen purify the liquid as it moves down through the soil toward the water table. Shown here is a multiple trench design. Older systems are most likely to be area bed designs in which a large area was dug up, covered with gravel, on top of which pipes were laid. The trench layout is now preferred it allows better oxygen transfer to the sub-soil.

Performance...

The performance of a properly designed, operated, and maintained system depends on the local site and soil conditions. Even in very favorable conditions conventional trench disposal systems can result in nitrogen levels entering the groundwater that will exceed the drinking water guideline of 10 mg / L.

Advantages...

Low cost and low maintenance, particularly if installed as a gravity feed system

Serves to recharge groundwater

Disadvantages...

Even properly operating systems will contribute nitrogen to the subsurface

Systems do require some maintenace; this is often ignored as the system is buried and out of mind

Malfunctioning sytems can result in surface and groundwater contamination

Costs...

Trench systems for single family homes range from a low of $2,500 to upwards of $10,000 depending on site and soil conditions.

Contour Trench...

Arrangement of the disposal trench in one long thin field along a contour to maximize distribution of effluent perpendicular to the groundwater flow. A thin layer of sand is placed in the bottom of the trench to support biomat growth. This field layout also minimizes disruption to the site during installation and often leaves room for a replacement field.

C - 2 Contour Trench...

As above, the field is laid out along a contour, however, the trench is shallow, installed in the upper permeable soil horizon in order to maintain the separation between the bottom of the disposal bed and a limiting surface (water table, bedrock). The top of the trench is covered with a layer of imported fill which treats flows which exceed the hydraulic capacity of the trench.