1. LOADS OF FAECAL INDICATOR BACTERIA TO
SHELLFISH WATERS BEFORE AND AFTER
IMPROVEMENTS IN SEWERAGE INFRASTRUCTURE
Crowther, J.,1 Kay, D.,2 Campos, C.J.A.3
1 University of Wales, Trinity Saint David, Lampeter
2 IGES, University of Wales, Aberystwyth
3 Cefas, Food Safety Group, Weymouth Laboratory
METHODS
Numbers of faecal coliforms (FC) and enterococci
(EN) discharged per unit of time were estimated for
seven shellfish water catchments covering different
levels of sewerage infrastructure improvement.
These improvements involved the installation of ultra-
violet disinfection (tertiary treatment) in one or more
sewage treatment works (STWs) and in five (the
exceptions being the Conwy and Chichester
Channel), an increase in storage capacity and
installation of screens in one or more intermittent
discharges (IDs) (combined sewer overflows, storm
overflows, etc.) during the period 2000–2010.
Generic data from previous CREH studies were used
to quantify loads of bacteria from sewerage sources.
Densities of human residences and livestock were
used to predict bacterial concentrations in rivers and
to make source apportionment estimates. Bacterial
concentration data were combined with river flow
data to quantify loads. Pollution sources were
categorised as:
 Sewerage sources that have been improved: key
STWs and IDs associated with these;
 Other catchment sources: other STWs, IDs and
agricultural sources.
RESULTS AND RECOMMENDATIONS TO IMPROVE SHELLFISH WATER QUALITYAIM OF THE STUDY
To assess the effectiveness of water company
investment in reducing loads of faecal indicator
bacteria to shellfish waters in England and Wales.
 Results showed substantial reductions in FIO loads after sewerage improvements.
 In the six catchments where improvements were made to key STWs, treated sewage effluents from these
works now contribute ≤0.6% to the total bacterial loads impacting on the shellfish waters.
 In the five catchments where IDs were improved
and on the assumption of a 90% reduction in the
estimated volume of ID flow following
improvement, the IDs post-improvement contribute
only ≤7% of the total bacterial loads.
 Under a ‘worst-case scenario’ in which the
estimated ID flow volumes are greater pre-
improvement and are reduced by 50% following
improvement, ID contributions increase to >50%.
 Preliminary source apportionment estimates
indicate that both sewage- and agriculture-related
sources contribute significantly to the present
bacterial loads from most catchments investigated.
Reduction (%) in total
loads of faecal coliforms
and enterococci during
the summer following
STW improvements. No
improvements in the
Conwy.
Estimated source apportionment of total faecal coliform
loads to seven shellfish water catchments during the summer.
More data on STW treated effluent and ID loads and agricultural best management practices are
required to allow accurate characterisation of bacterial loads at catchment level.
The most likely area of future intervention would be the extension of UV to other STWs in the
shellfish water catchments.
-100 -80 -60 -40 -20 0
Chichester Channel
Poole Harbour West
Yealm
Fal/Ruan
Taw/Torridge
Ribble
Faecal coliforms
-100 -80 -60 -40 -20 0
Chichester Channel
Poole Harbour West
Yealm
Fal/Ruan
Taw/Torridge
Ribble
Enterococci
© Crown Copyright (2012)
0 20 40 60 80 100
Poole Harbour West
Yealm
Fal/Ruan
Taw/Torridge
Conwy
Ribble
%
IDs associated with STWs at which UV-disinfection has
been installed
Other sewerage-related sources
Agriculture-related sources