• Septic tanks and soakaways

    A septic tank and soakaway still remains the simplest and most cost effective way of treating sewage from domestic properties where the site characteristics allow.

With some 300,000 septic tanks in England and Wales, a septic tank and soakaway still remains the simplest and most cost effective way of treating sewage from a property where the site characteristics allow.

WCI has over 30 years’ of expert knowledge on all aspects of septic tanks (commonly, but incorrectly referred to as cesspits). Services include problem identification, repair, maintenance, new septic tank installation and Environmental Health and Environment Agency issues.

Benefits at a glance

No electricity required

Completely silent

Simple maintenance

Compact and reliable

Low upfront cost

Environment Agency compliant

What is a septic tank?

A septic tank is a settlement tank in which raw incoming sewage is retained long enough for suspended solids to settle out as sludges and for liquid organic matter to undergo anaerobic decomposition. A scum forms on top of the tank consisting of fats and soaps, and sludge settles at the bottom of the tank. A stand-alone septic tank will reduce the strength of raw incoming sewage by 30-50% under ideal conditions. The relatively clear liquid, called septic tank effluent, which lies in between the scum and sludge, leaves the tank and then undergoes secondary treatment e.g. a soakaway, reed bed or sewage treatment plant.

Types of septic tank.

Types of Septic Tank

Historically septic tanks were built of engineering brick or stone and pointed, or built of concrete block and rendered. Often the inlet dip pipes were salt glazed clay 4” pipe work. The tank would consist of one, two or infrequently three chambers. The chambers were often connected using dip pipes or gaps in the partition wall, as shown in the diagram below. Wooden baffles might also be found within older tanks to hold back the scum and prevent carryover into the soakaway. Often a separate pipe would supply air to the tank via a nearby vent.

Modern septic tanks are either of the ‘onion’ type or, more frequently, of the ‘torpedo’ type. All septic tanks will generally incorporate a calmed inlet of some type as well as a dip pipe at the outlet to prevent the scum layer from flowing through the outlet. Some cylindrical septic tanks incorporate a baffle or a final effluent filter to further reduce the level of suspended solids within the discharged effluent.

What is a soakaway?

Soakaway image - watermark removed

Soakaways (also known as leach fields or drainage fields) are used to disperse effluent from a septic tank or a sewage treatment plant into the ground. Foul water soakaways are always linear in nature and must not be pits or crates which are used for surface water dispersal.

The size of the soakaway is generally defined by the linear length of the soakaway and will be dependent on the flows the soakaway will receive, the porosity ground into which the soakaway disperses and whether the effluent is treated (by a sewage treatment plant) or from a septic tank.

A percolation test will need to be carried out to determine the porosity of the soil (the ability of the ground to absorb water). This data is used to calculate the length of soakaway needed. A soakaway might not be the best secondary treatment solution, especially in places where the ground is made up of primarily clay soil that absorbs water very slowly, where the level of the natural water table is very high and/or the land area available is limited or too steep.

Soakaway design and installation is subject to building regulations (reference BS6297:2007 + A1: 2008 Code of Practice for the design and installation of drainage fields for use in wastewater treatment). A percolation test will be required to determine the porosity of the soil and design flows should be calculated according to British Water Flows and Loads 4. WCI can assist with all aspects of the design and installation of a soakaway.

Common soakaway problems

WCI receives many calls a year regarding failed soakaways. The correct design and installation of a soakaway will avoid these problems. However, here is a list of common faults.

  • Roots entering and choking the pipe work
    Water thirsty roots will often find their way into soakaways and, over time, roots can block the soakaway pipe work. This can be prevented by only installing soakaways away from shrubs and trees and ensuring the soakaway includes a geotextile covering during installation.
  • Insufficient porosity
    A soakaway is only as good as the ability of the surrounding ground to soak up the water it receives. As stated in Section H of the building regulations: ‘drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and trial hole tests have been favourable’. In other words, the soakaway must be designed according to the actual soil characteristics.
  • Incorrect installation
    Soakaways laid at incorrect gradients, incorrect depths and with incorrect layouts will not function correctly. For example, the deeper a soakaway is installed, the less oxygen is available for the aerobic bacteria in the soakaway to break down the septic tank effluent. Often the deeper the dig the more chance of hitting the water table and impermeable clay subsoils or parent rock which are all detrimental to soakaway performance. Further, if a soakaway is not laid at the correct gradient, water will pool at one end of the soakaway resulting in outcropping or septic effluent rising to the surface.
  • High water table
    WCI usually has more calls to our offices in winter regarding soakaway failure. This is because the water table is much higher in the winter months and can result in the backing up of wastewater towards the property as the septic tank effluent cannot be dispersed through the soakaway. The solids within the septic tank mix and when the water table drops, solids from the tank are distributed in the soakaway adding to its failure. If a watercourse or flowing ditch is available to you, a sewage treatment plant or reed bed could be installed to prevent such a problem.

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