For Developers

Water Advice Note [coming soon]

Aylesbury Vale District Council, in partnership with River Thame Conservation Trust and the Environment Agency, are producing an advice Note for those designing and developing sites that are near to, or contain, aquatic features ranging from main rivers to small ditches, floodplains to ponds.

The note will be available this coming spring (2020). Detailed below is supplementary information that should be used in partnership with the advice note.

Private sewage systems

It is almost always best to connect new developments to the main sewer network rather than to install private sewage systems. This ensures waste water is treated centrally at a water company run sewage treatment works. Here effluent can be treated to a higher standard before it is discharged. Systems are also regularly maintained and the process is well regulated to ensure environmental standards are met.

In cases where it is not possible to connect to the main sewer network then it is important to ensure an appropriate private sewage treatment system is selected and properly installed. This can vary depending on the space available, substrate and capacity needed.

Septic tanks

Septic tank and soak away

All septic tank systems require a drainage field which the effluent can permeate through the ground as the final part of the treatment process. Their effectiveness is very dependent on the substrate and some substrates will just not be suitable. Poor substrate conditions will cause effluent to either drain too quickly, not giving it long enough to permeate through the ground or it will drain too slowly, leaving effluent sitting on the surface. In the first instance this can pollute groundwater and drinking water. For the latter this can pollute surface water including rivers and streams. Both can have a severe impact on wildlife.

Clay is often a poor substrate as it is very impermeable and will not allow effluent to permeate into the ground. When it is dry it can crack, allowing water to drain too quickly.

Undertaking a percolation test will identify whether the substrate is suitable and whether it meets the legal requirements. Undertaking this test during both the summer and winter months will capture the ground at its driest and wettest, and will assess whether the site will be suitable all year round.

Small sewage treatment plants

Small sewage treatment plant

Small sewage treatment plants can discharge effluent into a soakaway or directly into a flowing waterbody. As with septic tanks, soakaways must also meet set percolation rates.

Usually it is best to plumb treatment plants into a soakaway rather than directly into a flowing watercourse. This will add in a further treatment stage and can reduce the potential damage a pollution incident could have on a watercourses if the system stops functioning or is poorly maintained.

Multiple housing developments

Consider installing a larger combined private sewage system for multiple housing developments. A combined system, rather than many singular systems, will remove the maintenance burden from homeowners and mean that the performance of the system is not determined by individual decisions.

Combined sewage systems also offers the potential for effluent to be treated to a higher standard, reducing its impact on water quality and wildlife. In many places constructed wetlands are being used as a natural means to treat foul water to meet high environmental standards. They are typically made up of multiple pools through which water slowly travels, allowing sediments to settle out, and plants and organisms to remove pollutants.


Withers, P. JA., Jordan, P., May, L., Jarvie, H. P., Deal N. E., (2013)., ‘Do septic tank systems pose a hidden threat to water quality?’. Front Ecol Environ. 12(2): 123–130, doi:10.1890/130131