Septic Tanks

Septic Tanks - 1...

Description...

Wastewater is collected in the building's plumbing system and conveyed outside by gravity to the buried septic tank. Within the septic tank, gravity causes the solids to sink to the bottom of the tank, and the oils/grease and scum float on the surface of the liquid in the tank. This is called primary treatment. Effluent from the septic tank is then conveyed either by gravity flow or pumping to either further treatment/pretreatment processes, or final treatment and disposal in a subsurface soil disposal system. Some anaerobic digestion of organic solids typically occurs in septic tanks, with the rate dependent on the wastestream and conditions in the tank. Septic tanks typically have outlet piping structures ("tee" outlets) which allow wastewater to exit by gravity at a liquid elevation which should be several inches below the liquid surface, and well above the solids/sludge level in the tank so as to prevent solids and grease/scum from leaving the tank. Occasional inspection of the sludge levels in tanks, or regularly scheduled pumping intervals, are thus important for maintaining the proper functioning of septic tanks.

Common Modifications...

Septic tanks are most commonly constructed of precast concrete, but may also be constructed of cast-in-place concrete, fiberglass, or polyethylene. If tanks are constructed of lightweight materials, such as fiberglass, it is important to "anchor" the tank in place with concrete or by other means if there is any indication of high groundwater which might cause flotation of the tank. During the past five to ten years at least two different types of proprietary filters have been developed for use in septic tanks. These have become increasingly used to filter the effluent prior to discharging either to the disposal field, or further treatment processes. One type of filter uses a series of plastic trays, and another filters the effluent through a series of long tubes and screens as the effluent flows upward through the outlet pipe of the septic tank. These filters can provide for enhanced solids removal, with associated BOD reduction. Manufacturers recommend that the filters be cleaned once every two to three years.

Technology Status...

Septic tanks are used by 25 to 30 percent of the residences in the United States. It is a well-developed technology.

Applications...

Septic tanks can be used by individual residences and establishments for providing primary treatment of wastewater prior to (1) further treatment/pretreatment, (2) final onsite subsurface treatment and disposal, or (3) conveyance to a centralized collection and treatment system for final treatment and disposal.

Limitations...

If site conditions are such that primary treatment is not sufficient prior to final onsite subsurface disposal, then additional treatment will be necessary if the effluent is to be disposed of onsite.

Typical Equipment / Number of Manufacturers...

Septic tanks are supplied locally from several sources.

Performance...

Performance of septic tanks is a function of proper sizing, design, and installation, as well as use which is in keeping with the design assumptions. For typical residential wastewater, primary treatment by septic tanks can provide for approximately 40 to 60% removal of both BOD₅ and TSS if the tank is sized for a detention time of about 3 to 4 days. Some organic nitrogen removal occurs through the solids removal process.

Residuals Generation...

Numerous studies have been conducted to determine sludge accumulation rates for septic tanks under various conditions, and to develop recommendations for pumping (sludge/septage removal) intervals. Results and recommendations from these studies tend to vary widely. US EPA literature generally recommends that septic tanks be pumped at least once every 2 to 5 years.

Overall Reliability...

Operating without mechanical equipment, and if properly constructed and installed, septic tanks can have service lives in excess of 20 years and be considered highly reliable.

Operation and Maintenance Requirements...

As discussed above under "Residuals Generation", septic tanks should be pumped at an average frequency of 2 to 5 years, in order to prevent too much sludge and solids build-up.

Potential Environmental Impacts...

Tables given below, from the "EPA Design Manual for Onsite Wastewater Treatment and Disposal Systems" are provided on the following page which shows typical concentrations of key parameters for effluent from residential septic tanks. A thorough site evaluation must be conducted in order to determine the appropriate level of pretreatment prior to final onsite subsurface disposal. Septic tanks can only provide for primary levels of treatment. Thus, some type of further treatment will need to be provided for if the soil and subsurface conditions at the site are such than ground or surface water may be adversely impacted by applying typical septic tank effluent.

Parameter	Concentration ( mg / L )
Total solids	680 - 1,000
Volatile solids	380 - 500
Suspended solids (TSS)	200 - 290
Volatile suspended solids (VSS)	150 - 240
BOD₅	200 - 290
Chemical oxygen demand (COD)	680 - 730
Total nitrogen	35 - 100
Ammonia (NH₃ - N)	6 - 18
Nitrites and nitrates (NO₂ and NO₃)	< 1
Total phosphorus	18 - 29
Phosphate	6 - 24
Total coliform	10¹⁰ - 10¹²
Fecal coliform	10⁸ - 10¹⁰

The above concentrations are for typical residential dwellings equipped with standard water-using fixtures and appliances (excluding garbage disposals), generating approximately 45 gpd. All values are for raw residential wastewater, and not effluent from septic tanks. Coliform "concentrations" are presented in organisms per liter.

Parameter	7 sites	10 tanks	19 sites	4 sites	1 tank
BOD₅
Mean (mg / L)	138	138	140	240	120
No of samples	150	44	51	21	50
COD
Mean (mg / L)	327	-	-	-	200
No of samples	152	-	-	-	50
Suspended solids
Mean (mg / L)	49	155	101	95	39
No of samples	148	55	51	18	47
Total nitrogen
Mean (mg / L)	45	-	36	-	-
No of samples	99	-	51	-	-

Energy Consumption...

Septic tanks do not require power consumption. Wastewater drains by gravity from the house to the tank.

Costs...

Costs for septic tanks depend upon a variety of factors including subsurface site conditions, location of and access to the site, and the type of tank. Costs of tanks, including installation, typically range from about $1.00 to $4.00 per gallon of tankage. Current rates for pumping septic tanks are from about $150 to $200 for tanks less than 2,000 gallons. If a tank is pumped once every 3-1/2 years, the cost for maintaining the tank will be about $50 per year, with a pump and haul cost of $175.

Capital : Tank (1,000 gallons) installed	$ 2,000
O & M : Pumped once every 3-1/2 yrs.	$ 4.17 / month
20 - year NPW (not including design / permitting costs)	$ 2,498.54

Aesthetic Considerations...

Septic tanks are buried below grade, and usually do not have significant visual impacts on the site.

State and Local Acceptance...

Septic tanks are the most frequently used pretreatment unit for the final onsite treatment and disposal of residential wastewater.

Septic Tank to Intermittent Sand Filter...

Description...

Sand filters have been used for many years by communities for treating both water and wastewater, and have for several decades been used for treating wastewater from individual residences and smaller systems. Intermittent sand filtration, which doses the wastewater onto a sand bed (usually 24" to 36" deep) "intermittently" during the day, can be used to treat septic tank effluent, or effluent from other types of pretreatment processes (such as aerobic tanks) . Properly designed and functioning sand filters can provide for enhanced BOD5 and TSS removal, pathogen reduction, and nitrification of wastewater effluent. Effluent from the intermittent sand filter is conveyed either by gravity flow or pumping to final treatment and disposal in either a subsurface soil disposal system, or a surface disposal system following disinfection.

Common Modifications...

Two types of intermittent sand filters have been used extensively for physical and biological wastewater treatment. These are buried and open intermittent sand filters. The surface of the filter may either be open to the atmosphere (though enclosed beneath a cover on the box), or the sand filter surface may be covered with several inches of coarse media, usually pea gravel, and backfilled with soil on top. The specific site conditions should govern the type of system used.

Technology Status...

Sand filtration is a relatively old technology. It has been used extensively for onsite and small community applications, with numerous installations in Texas. Substantial data has been collected from studies conducted in several states in the US on the performance of these systems.

Applications...

Sand filters which have been properly designed, constructed and maintained have demonstrated a capability of producing a high quality of secondary wastewater effluent. The lower organic and suspended solids content of the effluent may allow a reduction of land area requirements for subsurface disposal systems. Depending upon the particular conditions at a site, and the type and extent of treatment needed, sand filters may be well-suited for individual residences or businesses, small clusters of homes, or rural communities. Operation of sand filter systems does not usually require highly skilled personnel, and the systems themselves are moderately inexpensive to build. Some routine maintenance is necessary for sand filter systems, with the type and amount of maintenance dependent upon the particular sand filter design. Those requiring higer levels of ongoing maintenance may be best suited for clustered homes or small community systems where economies of scale tend to reduce maintenance and/or repair costs per user.

Limitations...

Land availability on the site may limit the applicability of an intermittent sand filter. Some odors may result from these systems, depending upon the particular design. In order for sand filters to continue functioning properly, they must be adequately maintained. Electrical power is usually required for intermittent sand filter units. This system must be designed by a licensed professional engineer, and requires special permitting review and consideration.

Typical Equipment / Number of Manufacturers...

All of the materials used for constructing sand filter units are available from several local sources. These include washed graded sand and gravel, geotextile fabric, piping, concrete, etc.

Performance...

For properly functioning intermittent sand filters, effluent with the quality ranges given below can be expected, with open sand filters (as contrasted with buried filters) tending to have the lower solids and BOD₅ concentrations in these ranges. Performance will depending upon the loading rate and the media size(s) of the filter.

Parameter	Average concentration range (mg / L)
BOD₅	3 - 30
TSS	10 - 40
NH₃	70 % - 98 % nitrification
Fecal coliform	2 to 3 log reduction

Residuals Generation...

Clogging of the surface of sand filters may occur as the pore space between the media begins to fill with inert and biological materials. For open sand filters with higher loading rates, it is necessary to periodically rake and / or remove and replace the upper few inches of media. The sand media removed must either be disposed of at a sanitary landfill, or regenerated if such operations are available and cost-effective locally. Septic tanks, or other sludge/solids accumulating pretreatment units must be periodically pumped.

Overall Reliability...

As stated above, properly designed, constructed and maintained sand filter systems can reliably produce a secondary quality of effluent. The process may accommodate significant variations in hydraulic and organic loading with little effect on effluent quality. However, studies of sand filters used for individual residences have shown that home owner neglect and / or their lack of knowledge about the system and its maintenance requirements, can result in problems with these systems.

Operation and Maintenance...

Pretreatment Units : If a septic tank is used to pretreat the wastewater prior to applying the effluent to the sand filter unit, the tank should be pumped periodically to remove sludge / solids. Other types of pretreatment units and processes will require maintenance of some type.

Sand Filter Units : For open sand filters with higher loading rates, in order to prevent clogging of the surface of the sand media, the upper layer of sand can be occasionally raked (assuming it is accessible). The upper few inches of sand may need to be removed and replaced periodically with clean sand (again, assuming it is accessible from the surface). It would most likely require two persons for some of these activities since heavy lifting may be involved. The spent sand must be transported to and disposed of at a permitted landfill, or regenerated for reuse. For buried intermittent sand filters occasional flushing of the distribution piping across the sand filter is needed to maintain relatively uniform distribution and treatment across the filter.

Potential Environmental Impacts...

Intermittent sand filters are capable of producing an effluent with secondary quality. If enhanced total nitrogen removal prior to final onsite disposal is necessary for a particular site's conditions, adverse impacts may result from the use of a septic tank followed only by an intermittent sand filter. As described above, sand filters must also be properly maintained in order to function properly. Some odors may result from sand filters, particularly if they improperly installed or maintained, depending upon the particular design. As long as aerobic conditions are maintained in the sand filter, significant odors should not occur. Sand removed from the systems must be transported to and disposed of at a permitted landfill, or regenerated offsite if this is feasible.

Energy Consumption...

It is estimated that the average pump operating in an onsite system (average of 20' of head and about 30 gpm, at an overall efficiency of about 30%), operating approximately 8 minutes per day, will use approximately 0.38 KW of power. This would be about 0.051 Kwh of energy per day.

Costs...

Estimated initial intermittent sand filter unit costs, installed, and including septic tank for pretreatment, pump, pump tank, control and alarm	$6,500
Equipment (pump) repair / replacement costs, estimated at $ 60 / year)	$5/month
O & M (assumes buried intermittent sand filter), with a maintenance contract of $ 240 / year (est. two 180-day filter runs, with system checks and maintenance twice annually) 8 hrs. @ $ 15 / h * 2.0, including taxes, overhead, profit, and including pump / controls servicing	$20/month
Septage and sludge pumping once every 2 to 5 years	$4.17/month
Energy costs (using 0.051 Kwh / day energy use)	$0.124/month
20 - year NPW (not including design & permitting costs)	$10,002.22

Aesthetic Considerations...

Residential intermittent sand filters are either buried, or are contained in covered boxes approximately 3 to 4 feet high. These may be located within a reasonable distance of the septic tank (or other pretreatment unit), and placed so as to be the least visible (assuming that is desirable) . Some odors may be emitted from an open sand filter unit, although a properly designed, constructed and functioning unit should produce very little if any odor.

State and Local Acceptance...

Intermittent sand filters have been permitted and installed in Central and other portions of Texas, and in many other areas of the US. They have been used for several decades in the US.

Septic Tank to Recirculating Intermittent Sand Filter...

Description...

Sand filters have been used for many years by communities for treating both water and wastewater, and have for several decades been used for treating wastewater from individual residences and smaller systems. Recirculating sand filters dose the wastewater onto a sand bed (usually 24" to 36" deep) intermittently during the day, and recirculate the filtered effluent back to a recirculation (pump) tank where it is either dosed again onto the sand filter, or is discharged. These systems can be used to treat septic tank effluent, or effluent from other types of pretreatment processes (such as aerobic tanks). Properly designed and functioning recirculating sand filters can provide for enhanced BOD₅ and TSS removal, pathogen reduction, and both nitrification and denitrification of wastewater effluent to some extent, depending upon the design and operation of the unit. Recirculation ratios of 3:1 to 5:1 are typical for these units. Effluent from the recirculating intermittent sand filter is conveyed either by gravity flow or pumping to final treatment and disposal in either a subsurface soil disposal system, or a surface disposal system following disinfection.

Common Modifications...

The effluent from the intermittent sand filter may be routed to either the septic tank or to the recirculation (pump) tank. Some researchers have reported better denitrification when the sand filter effluent is recirculated to the septic tank.

Technology Status...

Sand filtration is a relatively old technology. It has been used extensively for onsite and small community applications, with numerous installations in Texas. Intermittent sand filtration has been used more widely than have recirculating sand filters. However, considerable data is available from recirculating units.

Applications...

Recirculating intermittent sand filters which have been properly designed, constructed and maintained have demonstrated a capability of producing a high quality of secondary wastewater effluent. At least some degree of denitrification (total nitrogen removal) may be accomplished with these systems. The lower organic and suspended solids content of the effluent may allow a reduction of land area requirements for subsurface disposal systems. A higher loading rate may be used for recirculating intermittent sand filters as compared with single-pass intermittent sand filters due to the larger media size used. Depending upon the particular conditions at a site, and the type and extent of treatment needed, sand filters may be well-suited for individual residences or businesses, small clusters of homes, or rural communities. Operation of sand filter systems does not usually require highly skilled personnel, and the systems themselves are moderately inexpensive to build. Some routine maintenance is necessary for sand filter systems, with the type and amount of maintenance dependent upon the particular sand filter design. Those requiring higer levels of ongoing maintenance may be best suited for clustered homes or small community systems where economies of scale tend to reduce maintenance and / or repair costs per user.

Limitations...

Land availability on the site may limit the applicability of recirculating intermittent sand filters. Some odors may result from these systems if they are not properly installed and maintained, depending upon the particular design. In order for sand filters to continue functioning properly, they must be adequately maintained. Electrical power is required for recirculating sand filter units. This system must be designed by a licensed professional engineer, and requires special permitting review and consideration.

Typical Equipment / Number of Manufacturers...

All of the materials and equipment used for constructing these sand filter units are available from several local sources. These include washed graded sand and gravel, geotextile fabric, piping, concrete, pumps and tanks, etc.

Performance...

For properly functioning recirculating sand filters, effluent with the following quality ranges can be expected, (with the effluent quality highly dependent upon the particular system's design, the local climate, and other factors).

Parameter	Average concentration range ( mg / L )
BOD₅	5 - 10
TSS	5 - 10
NH₃	70 % - 98 % nitrification
Fecal coliform	2 to 3 log reduction
Total nitrogen	30 % - 80 % removal

Residuals Generation...

Clogging of the surface of sand filters may occur as the pore space between the media begins to fill with inert and biological materials. It may be necessary to periodically rake and/or remove and replace the upper few inches of media. Sand media removed must either disposed of at a sanitary landfill, or regenerated if such operations are available and cost-effective locally. Septic tanks, or other sludge/solids accumulating pretreatment units must be periodically pumped.

Overall Reliability...

Operation and Maintenance Requirements...

Pretreatment Units: If a septic tank is used to pretreat the wastewater prior to applying the effluent to the sand filter unit, the tank should be pumped periodically to remove sludge / solids. Other types of pretreatment units and processes will require maintenance of some type.

Sand Filter Units: In order to prevent clogging of the surface of the sand media, the upper layer of sand can be occasionally raked (assuming it is accessible). The upper few inches of sand may need to be removed and replaced periodically with clean sand (again, assuming it is accessible from the surface). Two persons would likely be needed at the site for some of the maintenance activities since at least some heavy lifting may be involved. The spent sand must be transported to and disposed of at a permitted landfill, or regenerated for reuse. Occasional flushing of the distribution piping through the filter may be needed.

Potential Environmental Impacts...

Recirculating sand filters are capable of producing a very good effluent quality. Some degree of total nitrogen removal (denitrification) is possible from these systems, if that is important for the particular site conditions. As described above, sand filters must also be properly maintained in order to function properly. Some odors may result from sand filters, particularly if they are improperly installed or maintained, depending upon the particular design. As long as aerobic conditions are maintained in the sand filter significant odors should not occur. Sand removed from the systems must be transported to and disposed of at a permitted landfill, or regenerated offsite if this is feasible.

Energy Consumption...

Costs...

Estimated initial intermittent sand filter unit costs, installed, and including septic tank for pretreatment, pump, pump tank, control and alarm, and valves	$7,000
Equipment (pump) repair / replacement costs, estimated at $ 60 / year)	$5/month
O & M (does not include periodic sand removal / replacement and disposal if / as needed), with a maintenance contract of $ 240 / year (est. two 180 - day filter runs with system checks and maintenance performed twice annually) 8 hrs. @ $15/hour * 2.0, including taxes, overhead, profit, and including pump / controls servicing	$20/month
Septage and sludge pumping once every 2 to 5 years	$4.17/month
Energy costs (using 0.051 Kwh / day energy use)	$0.124/month
20 - year NPW (not including design & permitting costs)	$10,502.22

Aesthetic Considerations...

Residential recirculating sand filters are contained in covered boxes or lined excavations approximately 3 to 4 feet high, with the pump (or recirculation) tank usually below the ground surface. These may be located within a reasonable distance of the septic tank (or other pretreatment unit) , and placed so as to be the least visible (assuming that is desirable). Some odors may be emitted from the sand filter unit, although a properly designed, constructed, maintained and well functioning unit should produce very little if any odor.

State and Local Acceptance...

Recirculating sand filters have been permitted and installed in Central and other portions of Texas, and in other areas of the US.