There are strict guidelines as to what type of septic tank system or packaged treatment plant you can install and where you can install it.

Up until April 2010 a consent to discharge was required by the Environment Agency but this has now been replaced by permit to discharge as part of the environmental permitting programme 2nd edition (EPP2)

Exemption forms are available for sewage treatment plants discharging 5m3 per day or less into surface water (the equivalent of a population of 27) or for septic tanks and packaged treatment plants discharging 2m3 per day or less into groundwater (the equivalent of a population of 11).

i have given some general guidelines in a bite size format, this is not intended to be a definitive guide and should not be treated as such, every site and situation is different and a specialist contractor should visit site before site specific recommendations are made, site visits may also be required by the relevant agencies prior to permission being granted for certain installations.

Other guidelines include;

• Septic tanks should have a capacity below the level of the inlet of at least 2,700 litres (2.7m 3 ) for up to 4 users. The size should be increased by 180 litres for each additional user.
• All sewage treatment plants now for sale must have been tested and hold the EN 12566-3 2005 Certificate
• Septic tanks should be sited at least 7m from any buildings, and preferably down slope and downwind.
• The soakaway must be a minimum of 10 metres from a watercourse, 15 metres from a building and 50 metres from a borehole or spring.
• Where they are to be emptied using a tanker, they should be sited within 15m of a vehicle access and at such levels that they can be emptied and cleaned without hazard to the building occupants and without the contents being taken through a dwelling or place of work .
• The soakaway must be designed to BS6297: 2007 and all percolation test results must be submitted. 
• Soakaway drains must be constructed in the aerobic soil layer, i.e. within 700mm. of ground level.
• If the discharge is to a soak away a sampling chamber must be provided before the soak away.
• Septic tanks may also be constructed in brickwork, concrete, or reinforced concrete, roofed with heavy concrete slabs. Brickwork should be of engineering bricks and be at least 220 mm thick. The mortar should be a mix of 1: 3 cement sand ratio. In-situ concrete should be at least 150 mm thick of C/25/P mix
• The inlet and outlet of a septic tank should be designed to prevent disturbance to the surface scum or settled sludge and should incorporate at least two chambers or compartments operating in series. Where the width of the tank does not exceed 1200 mm the inlet should be via a dip pipe.
• To minimise turbulence, provision should be made to limit the flow rate of the incoming foul water. For steeply laid drains up to 150 mm the velocity may be limited by laying the last 12m of the incoming drain at a gradient of 1 on 50 or flatter.
• The inlet and outlet pipes of a septic tank should be provided with access for sampling and inspection
• Septic tanks should be provided with access for emptying and cleaning. Access covers should be of durable quality having regard to the corrosive nature of the tank contents. The access should be lockable or otherwise engineered to prevent personnel entry.

I would also recommend that you study the building and drainage regulations set out here and visit the excellent Environment Agency website for further information if required.