SITE INVESTIGATION 0F CHANGES IN CONVEYED WATER QUALITY AFTER LINING
MAINS WITH BLAST FURNACE SLAG CEMENT MORTAR GLASGOW FIELD TRIALS
Report No DWI0285
Jul l989
SUMMARY
I OBJECT
To investigate the effect on water quality of lining two dead end
mains with blast furnace slag cement mortar in low and moderately
low alkalinity waters.
II REASON
Cement mortar is widely used to internally renovate corroded iron
water mains throughout the UK. In some cases the pH levels of the
water passing through newly lined mains can be elevated above the EC
directive maximum advisable values. To alleviate this, WRc has
developed a modified mortar replacing 65% of the ordinary Portland
cement with blast furnace slag (BFS). The new BFS mortar, during
laboratory and field trials has been found to reduce the pH in the
conveyed water by up to one pH unit in comparison to OPC. The trials
in Glasgow have been conducted as a final part of a series of field
trials to investigate the effect on pH of using BFS mortar in waters
of varying alkalinity. Two sites were chosen, one at Millerston
where the alkalinity of the water was approximately 14mg/1 as CaCO3,
a level not previously examined and at Howwood where the alkalinity
was approximately 22mg/l as CaCO3.
III CONCLUSIONS
- At a water alkalinity of approximately 19-23mg/l as CaCO3
(Howwood trial) the pH of the conveyed water in contact with the
blast furnace slag cement mortar for 1 hour remained above the EC
directive MAV of 9.5 for a period of 9 weeks. For a contact time of
6 hours the conveyed water pH remained above 9.5 beyond the duration
of the trial 15 weeks after first returning the length to service.
- At a water alkalinity of approximately 12-16mg/l as CaCO3
(Millerston trial) the pH of the conveyed water in contact with the
blast furnace slag cement mortar for 1 hour remained above the EC
directive MAV of 9.5 for a period of 12 weeks. For a contact time of
6 hours the conveyed water pH reached a value of 9.5 units, 15 weeks
after returning the length to service.
- Initial pH values at Millerston may have been artificially low
compared to the Howwood trial as high flows were maintained over the
trial length due to consumer supply problems. However on rectifying
the flow, pH values stabilised rather than increased.
- The concentrations of aluminium, calcium, potassium and sodium
within the conveyed water was initially high, with aluminium above
its EC directive MAV at both sites and potassium above its
respective limit at Millerston. Potassium concentrations had fallen
sufficiently by the eighth day but aluminium remained above the EC
directive MAV, 9 weeks after lining at Millerston and 5 weeks after
lining at Howwood.
The high concentrations of aluminium measured at the Howwood site
was aggravated by the high source water levels of aluminium.
IV RECOMMENDATIONS
- Although the conveyed water pH values remained above the EC
directive MAV for a significant period of time after lining, the use
of blast furnace slag cement "Mainline" shows benefits in terms of
pH when compared to Ordinary Portland cement. Therefore, if no
alternative renovation or renewal technique is available for
critical condition mains the use of blast furnace slag mortar is
currently the best option and should be continued to be used.
- With the high levels of pH and metals occurring within the
conveyed water the first few days after lining, a flushing programme
could be initiated to alleviate water quality problems during this
period.
- Cement mortar lining of dead ends or long rural lengths where
little or no flow exist should be avoided. However, in certain
circumstances flows could be artificially created through the mains
by joining overland dead ends or by having controlled wastages.
- Source water to proposed relining areas should be monitored and
if feasible controlled to reduce potential water quality problems
over a short period, especially pH and aluminium.
- Further field measurements should be taken at both Howwood and
Millerston 4 months after the initial return to service. Values
after a 1 year period would also provide useful information.
Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.