The European Bathing Water Directive imposes strict limits on the bacterial quality of waters used by the public for bathing.

At Thurlestone the site chosen for the sewage treatment works needed to improve bathing water quality is adjacent to a site of special scientific interest (SSSI), into which the treated effluent discharges. The effluent is required to be of a high bacteriological quality to protect the bathing water and high chemical quality to protect the SSSI.

South West Water Services (SWWS) in conjunction with their consultants, WS Atkins, designed and installed a horizontal flow Phragmites australis reed bed to give tertiary treatment to the effluent from a trickling filter plant. The media for the reed bed consists of crushed basalt and limestone chippings to try to maximise the phosphate removal from the effluent. The reed bed has been in operation for two years and produced an effluent of excellent quality soon after it was commissioned. Particular attention has been paid to bacteria removal. Total and faecal coliform removal have averaged two logs. Suspended solids and biochemical oxygen demand have averaged less than 5 mg/l in the effluent, with removals greater than 75%. Ammoniacal nitrogen has averaged less than 2 mg/l. The bathing water into which the effluent eventually discharges has not failed the Directive criteria since the works was commissioned.