Growing without pain
The population of the Dorset town of Gillingham is expanding fast. So when its existing wastewater system was struggling to cope with peak demands, action had to be taken. Installing four of Nordic Water's Dynasand moving sand bed filters proved a fine solution.
The installation in its present configuration is designed to meet anticipated needs till 2013. But with additional media and designed capacity for a potential fifth unit, it is expected to meet Gillingham's needs until the year 2020.
Gillingham is the most northerly town in Thomas Hardy's county of Dorset. But far from the novelist's isolated rural idyll, it is a rapidly growing, bustling small town, well serviced, and well connected by road and rail.
The town has seen faster growth than most around the UK - one house in five has been built since the millennium, and at 10,500 population, Gillingham has tripled in size since the 1950s, and is expected to continue to grow, expanding by a further 15-20% over the next 20 years.
It's a geographic and demographic profile that brings economic success - but riding alongside that there are external and situational challenges that require resolution, if the town is not to be defeated by an infrastructure that fails to keep up with existing and projected growth.
Take wastewater. There is an ever-growing need for water to be reused as we become increasingly aware that it is not a limitless resource. Hand in hand with that, legislation requires any water returned to rivers after treatment to be cleaner and purer than ever before and it is likely treatment standards will continue to rise.
As a result more treatment is required, particularly in towns like Gillingham, where there is a high proportion of residents who have retired and tend to be at home all day.
A reasonable answer would be simply to increase the capacity of the existing treatment plant, but there are problems there. First, land is expensive, and residents would object to ever larger treatment sites. But second, working to the limits of design capacity, the existing system was anyway struggling to cope with peak demands and more of a problem than this was the need to meet rising consent standards into the nearby River Stour.
Jill Smith was Wessex Water's commissioning engineer for the scheme, and claims the Gillingham project was relatively quick from inception to completion, and in its way quite straightforward - though not without the unique challenges every project faces.
"It had been clear for some time that something had to be done about Gillingham, as it was one of the fastest growing towns around," she says. "The only question really was to work out the best way to do it. The existing system could cope with the flow volume, but with the increasing flows the quality of the output into the River Stour needed work to maintain consent standards. And the standards are becoming ever more stringent, so we had to look at not just what we needed at the time, but within a reasonable future horizon as well.
"Our challenge was the way the consents on outfall water are measured. It depends on the finite amount of organic solids dispersed to the river, not the actual state of purification of the water itself. This means that if you double the flow of water through the system, you have to halve the solids held within it after treatment, so you don't increase the total amount of solids put into the river."
She continues: "Then there was the question of cost. Because we were working with the growing population and the increased flows that come with that, we couldn't apply for funding through Ofwat in the way you can if you're investing and upgrading simply to meet higher consent standards. So ultimately our decisions were guided by the need to remove more solids to reach consent standards, to get rid of the additional ammonia content that comes with the increased flow, and to do this in the most cost effective manner."
The original installation follows a classic and traditional trickling filter design, time tested and used in many hundreds of similar treatment plants across the UK. A site on the outskirts of town (though the town is now creeping ever closer), close to the river - and adjacent to the railway line where in the 1950s it was thought residential development would never take place - features initial screening for solids before passing through a primary settlement tank.
Secondary treatment occurs in a series of eight bio-filter beds before passing the water through humus tanks then gravity feeding to a final sump from which outflow to the river is controlled. Storm tanks introduce temporary holding storage for exceptional flows, and feedback through the system to reduce the risk of pollution in such cases.
Wessex Water determined that the flow volume was less of a problem than the need to clean the final effluent - so the hydraulic capacity of the system did not need to be increased, but its purification effectiveness needed work. A study of the alternatives indicated the most efficient, compact, and cost effective way to achieve this was to install a tertiary treatment system following from the existing primary settlement tanks and bio-filter beds. The company decided that aerated sand filters were probably the best way to remove both suspended solids and ammonia.
Four DynaOxy units were installed in a compact group at the lowest point of the site, allowing them to receive their input flows from the existing process units. Each filter is 6m high, and contains a filtration medium of basalt sand. The system operates all units from a central single input to maximise efficiency, and filters solid particles as the flow is pumped to the bottom and rises up to the top of the unit.
The introduction of pumped air to the bottom of the filter helps to lift the dirty sand to the top where the media is washed for continuous reuse. Process air is also added within the media bed to provide air for biological treatment to remove ammonia from the effluent.
A ten-day post-installation testing programme with the plant working in fully automatic mode confirmed the plant could meet compliance standards at actual and anticipated flow and ammonia load rates, and sampling was taken at regular and frequent intervals for evaluation. After the ten days, further time was added to look at performance under more widely varying flow conditions.
Additional ammonia was dosed into the system to ensure the system outputs would remain within the required consent criteria even at the expected future flow levels, and all tests produced results within - sometimes well within - acceptable consent ranges and design limits.
The treated effluent from the units is used for more than just the internal automatic media washing inside the filtration units, and is clean enough to be used to wash down within the site itself, including cleaning purposes and washing the screens at the initial input to the plant. This reduces the cost of running the plant and its impact on the environment.
After protracted use the system works well and in day-to-day running is virtually automatic, requiring a weekly visual check on the different equipment elements and a monthly monitoring of generated reports, mostly concerning the results of sand sinking speed and of media washwater flow within the units.
The continuing growth of Gillingham is taken into account as well. The effectiveness of the Dynasand units can be augmented with a marginal top up of filtration media - each unit can take approximately a further metre of basalt sand that will increase effectiveness. And, predicting the need, when the four units were installed, the pipework and other infrastructure was put in place to allow for a fifth unit to be brought in and commissioned with minimal disruption.