Fosse Septics, Filter Beds and Micro-stations

What are my options if I need to replace my system?

There are several options

  • a conventional “fosse toutes eaux” with filter bed (sand, natural fibre or reed)
  • a compact filter system or
  • a micro-stations.

What is a traditional fosse system?

The conventional “fosse toutes eaux” with filter bed works by collecting all waste water in the fosse itself where it is separated and the lighter suspended solids are digested by aerobic bacteria and the heavier sludge waste digested by anaerobic bacteria. As this only results in a reduction of pollutants, some germs and pathogens still remain which could pose a real threat of contamination or disease if released directly into waterways. As such the effluent then needs to pass through a filtration process where the remaining waste and toxic particles are removed before the resulting liquid is either allowed to soak away or be siphoned off .

What are the pros and cons of traditional fosse systems?

Proven to be effective in purifying waste water without requiring any electrical input, these systems are ideal for properties that are not inhabited all year round or where the environmental impact is important. However, they do require a sizeable amount of land and can be costly and timely to install. They are also prone to flooding and often require a pumping system to overcome land lie difficulties. As such these systems should not be used where land is limited, where there is a high water table, where the soil is of a water retaining nature or where there is a problematic terrain.

What is a compact filter system?

A compact filter system is where the filtering process takes place in an enclosed compact unit. Usually, water enters a “fosse toutes eaux” where it undergoes pre-treatment through separation and anaerobic and aerobic digestion. The water is then passed using a gravity led system over a series of filtering layers where the water is clarified to an acceptable level.

What are the pros and cons of a compact filter system?

As these filter systems are compact and enclosed, both the space required and the chances of flooding are significantly reduced. However, as they still rely on a downward movement of water they remain unsuitable for very problematic terrains and will often require a pumping system to ensure the effective movement of water into, or out of, the system.

What is a micro-station?

Micro-stations compact units that collect and treat used water without the need for a large filter bed. These units are made up of numerous chambers, basically a settlement chamber for waste separation and bacterial pre-treatment, an aeration chamber where the remaining liquid effluent undergoes enhanced biologically treatment and a clarification chamber which removes any remaining fine solids before the effluent can be discharged.

What are the pros and cons of micro-stations?

Micro-stations tend to be quick, easy and cheap to install and the minimal space required makes them ideal for properties with little land or where disruption to existing gardens needs to be kept to a minimum. However, as they rely on an electrical input to run the compressor, they are unsuitable for holiday or secondary homes and, as they vary in size and depth, the presence of bedrock may make their installation financially non-viable.

What is a “fosse toutes eaux”?

A “fosse toutes eaux” is simply a fosse that collects all waste water from a property. This includes bathroom, kitchen and utility room waste water. It does not include rainwater.

Do micro-stations need to be emptied?

De-Sludging needs to be undertaken on all systems with any sort of “fosse toutes eaux” and/or settlement chamber. The frequency of de-sludge depends on the system and on the number of people using the system. Official recommendations are to de-sludge conventional fosses when 50% and for microstations when the settlement chamber is 30% full. Only a licensed waste collector should carry out the de-sludging process.

What is anaerobic digestion?

Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is through this process that solid matter which falls to the bottom of the fosse is broken down.

What is aerobic digestion?

Aerobic digestion is the process of oxidizing and decomposing the organic part of the sludge by micro-organisms in the presence of oxygen. In aerobic wastewater treatment systems, micro-organisms feed on organic materials to stabilize them, and reduce biological oxygen demand and suspended solids in the wastewater. Since oxygen needs to be present the process can be accelerated by introducing further air movement. In this way the compressor used in micro-stations can speed up significantly the breakdown of effluent in water.

Where does a micro station discharge to?

The clarifeid water from the micro-station may be discharged into a road drain, a ditch, and (with the correct permission) watercourses such as a river, stream or lake. Because the effluent is so highly oxygenated, it can actually improve the oxygen quality of the receiving watercourse.