PPS 21 Sustainable Development in the Countryside·Page 46·5.89
On-Site Sewage Treatment in Countryside Development
While connection to mains sewerage is preferred, countryside buildings often cannot connect to public systems and must rely on on-site treatment such as septic tanks or package treatment plants, with effluent dispersed through field drains to watercourses.
Accordingly it is desirable for new development to connect to mains services wherever possible. However, it is acknowledged that it will not be feasible for many buildings in the countryside to connect to a public water borne sewerage system and they will rely instead on some means of 'on-site sewage treatment', such as a septic tank or package treatment plant. Effluent from such installations is usually dispersed through a system of channels or field drains before percolating to the nearest watercourse.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/PPS 21 - Sustainable Development in the Countryside.pdf