Sembcorp Bournemouth Water
Alderney works are positioned close to the head of the Bourne Stream in
Alderney, and there is an ongoing daily background discharge to the
stream of approx. 0.6 megalitres (ML) - that's 600,000 million litres/day - some 7
litres/second (l/s). As an indication of what that means to the
hydrology of the stream, average dry weather flow measured at the
Branksome Wood Road gauging weir is about 37 l/s.
water discharged is from the Stour &
Avon, which are chalk rivers. The water flows through a wet acid
heathland environment, designated SSSI. The resultant mix has had
an impact on the type of instream and bankside vegetation found in the
slow sand filters used in the water purification process at the Alderney
site need to be backwashed to remove the build-up of matter. The
water used in backwashing (perhaps for 4-5 days in a month) is drained
into two settling tanks and discharge through aeration baffles into the
surface water drainage system. The
discharge during this period tends to increase to between 1.2 - 1.6 ML/day (13-19
l/s). When filters run to waste the discharge is greatest -
perhaps 2.5 ML/day (29 l/s).
discharge from BWHW may well be the primary source of water for the
stream during periods of drought.