Our Ref: DWI/2/10 Event 30/10 Date: 9 June ... - Derry City Council

ADVERSE WEATHER INCIDENT AFFECTING SUPPLIES ACROSS NORTHERN IRELAND, 27 DECEMBER 2010 – 6 JANUARY 2011
SUMMARY TABLE OF EVENTS AT WATER TREATMENT WORKS (WTWS) DURING THE ADVERSE WEATHER INCIDENT
Annex 3
WTWs
Cabragh /
Gortlenaghan @
Cabragh
Dates
28/12/10 – 05/1/2011
(8 days)
Parameter
Failures
Manganese and
Iron in final water
Probable Cause
Requirement to bring Cabragh Borehole back into service as an emergency supply
due to sufficiency issues. There are naturally high levels of iron and manganese
present in this emergency water source.
Carmoney 28/12/10
(1 day)
Dorisland 29/12/10 – 04/01/11
(6 days)
Caugh Hill 29/12/10 – 03/01/11
(5 days)
Lough Macrory 29 /12/10 – 03/01/11
(5 days)
Ballinrees 29/12/10
(1 day)
Dunore Point 02/01/11 – 05/01/11
(3 days)
Lenamore Springs 04/01/11 – 07/01/11
(3 days)
Killylane 06/01/11 – 10/01/11
(4 days)
Turbidity in final
water
Aluminium in final
water & customer
tap samples
Turbidity & Iron in
final water
Aluminium in final
water
Aluminium in final
water
Aluminium in final
water & customer
tap samples
Turbidity in final
water
Turbidity and
Aluminium in final
water and in the
distribution system.
The very low level in the clear water tank led to an air lock which caused disturbance
in the sample line resulting in an unrepresentative sample.
Instrument malfunction caused problems with pH control which affected the
coagulation process. Critical process monitoring point failure resulted in a delay in
the problem being reported and investigated. This was compounded by the works
being unmanned over the holiday period, so manual tests were not carried out. The
breakdown in treatment went undetected and therefore corrective action to restore
water quality was delayed.
Instrument malfunction caused problems with pH control which affected the
coagulation and filtration processes.
The low temperatures affected the coagulation process, and this was compounded
by a number of temporary shutdowns at the works caused by problems with the
power supply.
Lime dosing pump failure caused problems with pH control which affected the
coagulation and filtration process.
Breakdown of the coagulation process due to Freeze/Thaw damage to the ‘‘dirty
wash-water” system which caused increased aluminium levels being introduced at
the head of the works.
Tankering operations into the holding tank, which were necessary because of
sufficiency issues caused particle disturbance.
Breakdown of the coagulation process after the primary coagulant had been
changed as part of ongoing operational work at this works.