An Introduction to Septic Tank Sewage Treatment Systems

Septic Tank Sewage Treatment Systems

Septic Tank Sewage Treatment Systems ( or septic tank systems as they are called in some parts of the world,) treat sewage at its location, rather than transporting the sewage to a sewer system or larger treatment system nearby. Because many septic tank systems throughout the world are not functioning properly, various laws have been passed to help prevent the serious environmental and human health problems they can cause.

Several Acts or Regulations, in different parts of the world set requirements for minimum treatment standards, new construction and replacement of systems. Some local jurisdictions require disclosure of system status and location upon property transfer, and may have stricter requirements. Therefore if you're not sure your system is in compliance with the law, check with your local government office.

Types of Septic Tank Systems

There are various types of septic systems. The most common type in a lot of countries consists of an underground septic tank of differing types, which then connects to a soil treatment system, usually a land drainage system or drainfield, a mound, or an at-grade system.

Septic tanks

Untreated wastewater from a property flows into the septic tank, where the solids separate from the liquids. Some solids, such as soap scum or fat, will float to the top of the tank to form a scum layer. Heavier solids, such as human and kitchen wastes, settle to the bottom of the tank as sludge. Self forming bacteria in the tank help the system "digest" these solids or sludge. The remaining liquids flow out of the tank to a land drainage system or drainfield. Baffles built into the tank hold back the floating scum from moving past the outlet of the tank. It is generally recommended that septic tanks be pumped out annually, or the sludge and scum layers be measured at least every year so that solids don't wash out into the soil treatment system. Solids can clog the soil and limit its ability to properly treat the septic-tank effluent.

In the UK the older types of septic tanks comprised of two rectangular chambers, usually built in brick or stone and latterly concrete. The inlet pipe into the first chamber ends in a dip pipe which travels down the at least 450mm (18") below top water level (TWL), whilst the chamber is a minimum of 1500mm (5'-0") deep from TWL. The dip pipe is usually 100mm (4") dia. min. and has a vertical pipe for rodding. This first stage chamber is usually W width and 2W length. The outlet pipe from the first chamber also consists of a dip pipe but the bottom of the pipe is a min. of 300mm (12") below TWL. The inlet pipe into the second chamber also ends in a dip pipe which travels down the at least 450mm (18") below top water level (TWL), whilst the chamber is a minimum of 1500mm (5'-0") deep from TWL. The dip pipe is usually 100mm (4") dia. min. and has a vertical pipe for rodding. This second stage chamber is usually W width and W length. The outlet pipe from the second chamber also consists of a dip pipe but the bottom of the pipe is a min. of 300mm (12") below TWL.

There should also be a vent pipe from the first stage chamber and preferably the second stage as well to vent any gasses, that rise from the sludge, to atmosphere. Suitable covers should always be placed completely over a septic tank capable of withstanding an adults weight, and to avoid children / animals falling into the tank. (Sometimes it pays to have light duty covers over the tank and to place a lockable strap over the covers.)

There are these days, commercially manufactured septic tanks, which can be made in GRP or other suitable stable materials, which commonly are spherical in shape with a narrow shaft at the top to a manhole ground level. These tanks have built into them several baffles which perform the same function as the dip pipes and separate out the heavier solids to the bottom and let the greases / scum , and effluent rise to the top. These types of tanks are most commonly used in the developed world as they can readily and quickly be made, delivered rapidly to site, and placed in the ground easily.

Care should however be taken to ensure that these tanks will not rise out of the ground, when they are emptied, due to high water tables in the ground !

The effluent from a septic tank, still contains about 70% of the polluted matter in the sewage, and hence there is a need for further treatment of the liquid from the tank.

Land Drains or Drainfields

This part is perhaps the most important part of the Septic Tank sewage treatment process. The land drains or drainfield are an underground soil treatment system, and it receives partially treated sewage from the septic tank. The soil on a site must be suitable for a drainfield to work properly.

Local soil and water conservation districts can sometimes give a general idea of the ability of the soil on your property to handle a septic system. In some parts of the world it is necessary to use a professional who can give you an accurate assessment on your particular site. However more frequently it is better to carry out percolation tests on the ground strata to ascertain its suitability for land drains or drainfields.

Connected to the septic tank by an underground pipe, the drainfield may consist of trenches or a seepage bed. The bottom of the trench's of the drainfield should generally be 900mm (3'-0") above the seasonally high water table, or bedrock.

In some areas of the world, the seasonally high water table is within a 600mm (2'-0") of the ground surface. Therefore land drains or drainfields may be typically installed close to the surface of the ground, but care must be exercised as it is possible that the polluted water may break through the ground surface. In other areas there may be clay ground present close to the ground level, and where this occurs then it is very unlikely that the use of land drains or a drainfield will prove satisfactory.

In a land drain or drainfield, wastewater moves from the septic tank into perforated pipes, bedded on and surrounded in, gravel-filled trenches. These pipes should be laid fairly flat, at a gradient of about 1 in 200, so that the effluent is dispersed along the whole length of the pipe run. The perforations in the pipe should preferably be larger than 6mm (0.25") to avoid the effluent forming a biomass, which will soon result in blocking the holes. Sometimes this pipeline can be in a specially manufactured, large-diameter corrugated plastic tubing with a geotextile wrap around the pipe.

From the pipeline the pathogens, nutrients, and organic material are dispersed into the gravel where they are removed or neutralized as the liquid moves through into the soil. In a trench system, the effluent may flow into a series of drop boxes or manholes with another outlet to the next pipeline set at a higher level. As each set of trench lines reaches capacity, the drop boxes release liquids to the next set of pipes in the drainfield. Seepage beds are gravel-filled excavations similar to a trench, but are greater than three feet in width.

Seepage beds should be used only in flat areas where space is extremely limited.

Mounds

A sewage treatment mound is an elevated soil treatment system. Clean sand or small gravel is used to elevate the system so that it is at least 900mm (3'-0") above the seasonal high water table or bedrock. The mound's size, shape and height must be carefully designed to avoid failure of the system and to ensure even distribution of the septic-tank effluent throughout the mound. The design must take into account property plot dimensions, local topography, the amount of sewage to be treated, and the rate at which the water flows through the soil.

Liquid from the septic tank is collected in a small pumping station or tank, and pumped under pressure into the mound in "doses" for treatment. Mounds lend themselves well to landscaping and provide effective treatment when carefully designed and built.

An at-grade system is also an elevated sewage treatment system, but instead of clean sand, it uses drainfield rock as the distribution medium. The sizing of an at-grade system depends on the amount of sewage to be treated and the rate at which water flows through the soils. At-grade systems also require a small pumping station or tank to pressurize the system for adequate sewage distribution across the soil for effective treatment.

Seepage pits and Soakaways

Other sewage systems that have been used in the past are seepage pits or large soakaways. These systems typically involve discharging septic tank treated sewage into a deep, cylindrical pit that is open on the sides and bottom. Sometimes these pits can be constructed using honeycombed brickwork, or concrete manhole sections with perforations in the walls. The holes is frequently filled with large stones or gravel and a cover (probably in concrete ) placed over the hole. If the ground strata for the whole depth is good and will absorb the effluent these can be satisfactory, but if not then these can cause problems as the end result will be a large hole filled with septic tank effluent.

Site Conditions and Installation

Not all sites are suitable for septic tank systems. Of primary concern is the type and porosity of the soil at the site. Soils that are too coarse or too fine can limit the effectiveness of the treatment system. Also the depth of the seasonally high water table or bedrock can also cause problems. Some of these problems may possibly be overcome by altering the design of the septic system.

However, septic systems generally cannot be placed in areas where the depth to bedrock or the water table is less than 900mm (3'-0") from the ground surface. Generally you cannot improve an unsuitable site to the point where a soil treatment system will work. It is always better to contact your Local Government office for help in finding out if your property would be suitable for installation of a septic system.

The size of a septic system is determined by the number of bedrooms in a home and the rate at which water flows through the soil on the property. When planning a septic system, be sure to consider future needs for expansion. A septic system cannot function properly if it is overloaded.

Finally, never plan a septic system without checking with your Local Government office (county, city, township) first. You will probably need a permit to design and install one, and permits could be refused for sites that are deemed unsuitable.

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For further impartial advice on any problems associated with all types of sewage and effluent treatment contact Johnston Smith Consulting - The Independent Specialists.