Project Focus: Reservoir system tastes sweet for Welsh Water
The installation of a £1M source management system at Pontsticill Reservoir helped Welsh Water tackle high MIB levels resulting from algae growth
Geraint Long, Welsh Water:
“What we did here was to treat the root cause of the issue. If we had treated the symptom, then we probably would have been looking at a GAC plant which we would have had to build offsite, and £5-10 million of investment, probably more. By treating the root cause we’ve dramatically reduced that.”
John Gillett, Gurney Environmental:
“Rather than the water quality peaking and troughing, with different qualities and different contaminants throughout the year, you will now see much more even quality, colour and condition throughout the year. It makes treating it in the water treatment works much simpler.”
Roy Samuel, Welsh Water’s Capital Delivery Manager for South East Wales:
“We are very pleased to be using this innovative management system in the reservoir at Pontsticill. This investment will improve the water quality for customers in the area and ensure they continue to receive a top quality supply of drinking water long into the future.”
Installing a system which de-stratifies Pontsticill Reservoir has allowed DÅµr Cymru Welsh Water to tackle water quality issues and allowed proactive management of its water resource.
This summer the company invested £1.5M in improvements at Pontsticill, which is located in the Brecon Beacons, north of Merthyr Tydfil. Around £1M of this was spent on the installation of a system - the ResMix 5000cc – which it is hoped will prove the key to ensuring the drinking water originating from the reservoir is more consistently pleasant tasting in the years to come.
In recent times the reservoir has had periodic problems with methylisoborneol (MIB) a compound resulting from algae growth, which is particularly prevalent in the summer months. While not a health risk, MIB can give water a musty smell and taste, and requires additional treatment processes to eliminate.
“In 2011/12 we started experiencing taste and odour issues coming from the Pontsticill reservoir,” says Geraint Long, solutions manager at DÅµr Cymru Welsh Water. “We looked at a few different solutions: the normal solution to that would have been a GAC [granular activated carbon] type installation, which is high opex cost and high capital cost to put in place.
“But on-site at Pontsticill there was limited space and we really had limited options.
“So we started looking at the root cause of where we were finding issues, the MIB which was in the reservoir, and whether we could do anything to manage our reservoir source more effectively to reduce the taste and odour issues further down the line.”
Dating from 1927, Pontsticill reservoir holds over 15 billion litres of water – equivalent to 6,000 Olympic sized swimming pools. Part of the reason algae was flourishing in the resource was stratification – the process by which water becomes divided into layers with warm, highly oxygenated water on the top and cold, low-oxygen water beneath. Welsh Water commissioned a report from consultants Black & Veatch into systems that could reduce the effects of this stratification.
The two main options were a system using compressed air which works by oxygenating the water from the bottom up, or the ResMix, which is installed on the surface and uses two ‘stirrers’ with a diameter of 5m each to circulate the water from the top, pushing warmer, oxygenated air into the depths of the reservoir.
“The systems all work on the same principle of de-stratifying the reservoir, but from the report that we had it was deemed that the ResMix would be more effective at doing that than the compressed air systems, and more energy efficient as well,” says Long.
ResMix has been used in Australia for 20 years, but is relatively novel in the UK: its first application on these shores was by Scottish Water in 2009. Its energy efficiency advantages stem from the fact that it works on a different principle to the alternatives, explains John Gillett, Managing Director at Gurney Environmental, the firm which supplies the system in the UK.
“ResMix really is the first system that allows the water utility companies to start to proactively manage their water resources,” says Gillett. “Up until now the only options they’ve had have been compressed air systems, which are high-energy systems and so are only typically switched on once a problem occurs to try and get it to go away. ResMix is different, it’s a low energy system using about one-twentieth of the energy requirement that a compressed air system does. It can be run 24 hours a day, 365 days a year; so for the first time, the water companies can economically, cost-effectively and sustainably manage the water quality in their reservoirs.”
Because of the size and depth of Pontsticill, Gurney recommended the use of the largest available system, the 5000cc, which has not been deployed in the UK before.
“From our evaluation, we concluded that the installation of a ResMix 5000cc - which is a two-unit system with a 5kw motor on each unit - would be suitable to create uniform conditions in this reservoir, both in temperature and in dissolved oxygen,” says Gillett. “The Resmix 5000 is a big unit – 5 metres in diameter, and it moves 10 tonnes of water a second from the surface to the bottom of the reservoir with only 5 kilowatts of energy. It’s a very energy efficient way of moving a lot of water around.”
Installation from the air
Welsh Water employed Gurney Environmental to supply and install the system, which is constructed by WEARS Australia. It needed to be shipped 10,333 miles from Toogoolawah in Queensland, Australia, before being assembled on the banks of Pontsticill reservoir. Due to its size and weight, it had to be installed into the reservoir using a helicopter which positioned it in the water.
“We found quite early on that quite a lot of these old reservoirs are quite remote and the roads up to them are fairly narrow,” Gillett continues. “Getting cranes to them is sometimes not easy to do, and even if you can get a crane in, typically you can’t get the reach to the water depth that we need. So helicopters present a way of doing this which is a lot safer - we don’t have to bring very heavy machinery up on site and position it on the dam wall, for instance, and it just works very well and very efficiently.
“We were on site for about a week and a half assembling the units, and then on the day of installation, the helicopter came in. Within about half an hour both units were in the water, and the helicopter could disappear - it was all very quick.”
The positioning of the system was a matter for consultation between Gurney and Dwr Cymru’s process design and reservoir safety teams.
The deepest point of the reservoir is the most efficient for it to operate successfully, but it needed to be positioned where it would not impact on users of the reservoir, which is used for recreation and water sports.
Despite the installation taking place in August, Welsh Water have opted not to make it fully operational until late October, when MIB levels naturally decline.
“We’re due to switch the system on within the next couple of weeks,” explains Long, who was speaking to WWT in early October. “We’ve decided on a commissioning period of a fortnight to get it up and running properly. That’s so we can monitor and manage the resource to monitor the effect that it is having on MIB. We’ll switch it on at 10%, and review it over a three or four day period, then push it up to 20%, again review over a three or four day period, and so on.
“We’ve had to wait some time because the MIB is currently in there; ideally what we’d like is for the MIB to come back down to zero, and then turn it on in order to reduce the levels for next year.
“MIB builds up in April and May, and the hot months of July and August are the worst period for algae growth. In the later stages of the summer, mid-September, levels start coming down again, and by November they will be gone. That’s why we’ll be looking at the end of October to switch it on.”
Overall, Long feels that it was the right solution for Pontsticill, and the 106,000 customers served by the reservoir in the Merthyr area should soon be feeling the benefits. The results will be monitored closely to see if other Welsh Water reservoirs might benefit from similar investment.
“What we did here was to treat the root cause of the issue,” he says. “If we had treated the symptom, which was the bad tasting water, then we probably would have been looking at a GAC plant which we would have had to build offsite. With land purchasing costs, site design and build, and the actual filter you would have been looking at £5-10 million of investment, probably more. By treating the root cause we’ve dramatically reduced that.”
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