Septic tank and soakaways failures

Posted on 27 Mar 2015 by Fabian Belin

Most septic tanks and soak-aways have problems, usually involving drain pipes backing up or the septic tank overflowing, resulting in the effluent backing seeping into ditches and polluting the environment. Most of the time problems with the septic tank are to do with the soak-away and not the septic tank itself.

Failure of soak-aways depends on several factors, including the maintenance of the septic tank, the soil type, the soil quality, the height of the ground water table but also the specification of the septic system itself.

Common signs of a soakaway failure are effluent pooling on the ground surface or foul odours (bad smells) coming from the septic tank or drains, but also visible pollution of nearby ditches and streams, slow flushing toilets or gurgling sounds from the drains, overflowing toilet, shower, or bath and dips in the ground surface near to the septic tank and drain runs.

Any of these signs: lift the drain inspection chambers to check if there is any standing effluent in them. The drain inspection chambers should be clear flowing with no material buildup.

  • The main reasons for septic tank and sewage system failure

Reason 1: Poor septic Tank Maintenance

Most septic tanks are unfortunately poorly maintained as owners are often under the misguided belief that they are saving money. For instance onion shaped fiberglass septic tanks have a lower quality effluent than the traditional brick concrete two chamber septic tanks.

Septic tanks have a capacity for storing 12 months of sludge and thus require emptying at least once every 12 months. The soils porosity is very reduced if the sludge gets washed into the soak away system. This will result in preventing the effluent from soaking away.

Reason 2: Wrong soakaway Depth after septic system

Section H of the Building Regulations 1.39 which states: ‘Drainage fields should be designed and constructed to ensure aerobic contact between the liquid effluent and the subsoil’. Unfortunately most modern ‘onion’ shaped, fiberglass septic tanks have outlet levels that are set at greater than a metre below ground, resulting in a soakaway using anaerobic conditions after the septic system.

Septic tank soakaways should be constructed in soil containing oxygen. This “aerobic” soil is only found in the first top meter of soil. The ground is acting as a medium to disperse the effluent as well as used as part of the system to break up and digest the effluent to be treated. This sewage water treatment  is carried out by aerobic bacteria in the soil.

This is the reason why the soakaway depth should not be greater than a meter below ground level, including the 300mm gravel bed normally present beneath the pipe to avoid the soakaway to be trying to operate in anaerobic conditions.

A different type of bacteria exists and will produce a slime which will reduce the length of life of the soakaway rather than breaking down the effluent, leading to the soils porosity reduction. This will occur under “non-aerobic” conditions and because of the lack of oxygen.

Reason 3: Soil Type and / or ground  water table

Clay and clay based soils have poor porosity: it is impossible to disperse the septic system effluent satisfactorily in such non-porous grounds. Soakaways may often fail in the first 5 years of their life in such bad porosity soils.

Excess sodium (salt coming from detergents such as washing powders or sometimes from cooking vegetables) in clay soils and / or soils with fine particles of silt can cause a reaction called sodium binding: the clay particles bind together resulting in a waterproof layer forming along the soakaway trench.

High levels of suspended solids - greater than 1200mg per litre, similar than the levels can be found in badly maintained sewage treatment units – can be found in such bad porosity soils, resulting in the reduced life of the soakaway. The porosity will equally be reduced in sand soils by such high level of suspended solids, and the black slime created as a result of the anaerobic process.

When the ground water table is higher than the septic tank outlet level, which happens especially in winter, then the soakaway will back fill the septic tank. This process can cause the settlement chambers of the septic tank to mix with the clarified effluent. The soils porosity will then be reduced by a resulting effluent full of solids.

Reason 4: Septic Tank construction and septic system capacity

Soakaways are designed to cope with a certain volume of daily wastewater used. An increase in the volume of wastewater produced implies the soakaway  to be redesigned to match to the new treatment capacity needed.

The septic tank corrosive environment may deteriorate metal parts but also mortar joints over time. This septic system corrosion includes the internal parts such as fins, rods and metal struts and bolts in the “onion” type septic tanks as well as the joints in the brick built septic tanks. This corrosive action may lead to an internal collapse of the septic system, leading to a dramatic rise of suspended solids in the final effluent, causing the soakaway to fail shortly after. The septic tank is therefore unable to clarify the sewage.