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Septic Tank Sewage Treatment Systems

Septic Tank Sewage Treatment Systems

Septic Tank Sewage Treatment Systems (or septic tank systems) 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 tank systems. The most common type in a lot of countries consists of differing types of underground septic tank, which then connect to a soil treatment system, usually a land drainage system or drainfield, a mound, or an at-grade system (the At-Grade Soil Absorption system is one of several soil absorption systems that can be used to treat and dispose of on-site wastewater through the soil).

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 adult’s 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.)

These days there are commercially manufactured septic tanks that are made in GRP or other suitable stable materials; these are commonly spherical in shape with a narrow shaft at the top to a manhole at ground level. These tanks have built into them several baffles that 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 be readily made, delivered rapidly to site and installed 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!

The effluent from a septic tank still contains about 70% of the polluted matter in the sewage, 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 that 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 trenches 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 neutralised 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.


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 mounds 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.

At-grade System

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 hole 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.

Maintaining Your Septic Tank System

The three most important things to remember about your septic system are:

  1. Do not overload it. Exceeding the capacity of the septic tank system to treat sewage can cause surface discharges. These discharges can create a health hazard on your property and pollute nearby water sources. Depending on the design of your septic tank system, you may be able to expand its capacity if necessary.
  2. Measure sludge and scum depths or routinely pump out the septic tank at least once every year. All septic tanks need pumping to reduce sludge and scum build-up. If you do not pump out your tank periodically, solids may wash out of the tank and into the soil treatment system, limiting its effectiveness.
  3. Do not put strong or hazardous chemicals into the system. Septic tank systems are designed to treat normal, household domestic sewage. Substances such as paints, paint thinners, solvents, pesticides and flammable liquids can kill the bacteria that provide the treatment in the septic tank and the drainfield. Furthermore, these chemicals can flow through the soil and contaminate surrounding ground water, making it unsafe to drink.

In addition, following these tips will help keep your septic tank system from becoming a problem:

  • Keep grease out of the system. It can easily block up the works and inhibit the bacteria that are necessary for proper functioning of the septic tank system.
  • Toilet paper is fine, but don't put other paper products such as cigarette butts, sanitary napkins, paper towels and disposable incontinence pads or nappies (diapers) into the system.
  • Don't use products advertised as septic system cleaners or starters. They are not needed and can harm the system. The bacteria that are naturally present in human wastes are all that are needed to "start" a system.
  • If you use a garbage or waste disposal system, be aware that it will require a bigger septic tank, with compartmented walls, and it will require more frequent emptying. It is better to compost kitchen wastes.
  • Don't drive any vehicles, or establish walkways, over the land drainage or soil treatment system. This can compress the soil in the drainfield, making it less effective.
  • Don't connect roof drains or surface water drains (drain tiles) to the foul system.  It can overload the septic tank system with extra water that does not need to be treated and will probably flush out all the solids you have settled in the tank!
  • Laundry wastewater contains soaps, dirt and grease, and should go into the septic tank not directly into any soil treatment system (and never as a surface discharge).
  • If your total septic tank system approaches capacity, find ways to conserve water. The most effective ways are to reduce the amount of water used in each toilet flush and install low-flow shower heads.

Symptoms of system overload

Warning signals of septic system problems include:

  • Sluggish toilet flushing
  • Plumbing backups
  • Gurgling sounds in the plumbing
  • Ground is boggy or wet over soil treatment area (system not working)
  • Odours inside or outside (tank / drains not vented or tank not emptied regularly)
  • Low spots in the area over soil treatment system
  • Tank rapidly fills up after emptying (water running back into tank from effluent disposal system)

Remember, every litre or gallon of water going into the septic system must go through the soil treatment system, which has a limited capacity to remove pollutants and bacteria. With proper installation and maintenance, your septic system can function well for many years.

SPE Limited
10 Kingfisher Business Park, Lakeside
Redditch, Worcestershire, B98 8LG

T 01527 529809
M 07850 399732
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An Introduction to Septic Tanks - Sewage Pumps and Sewage Treatment Plants An Introduction to Septic Tanks - Sewage Pumps and Sewage Treatment Plants