North Country Leisure is a long-established and successful family-owned business with three popular residential parks in Lancashire. Like many park owners, father-and-son team Geoff and Steven Barlow turned first to a firm of consulting civil engineers for advice on replacing their on-site wastewater treatment system at their Sea View residential park, only a couple of miles up the Ribble Estuary, a National Nature Reserve and therefore very environmentally sensitive and valuable, from Lytham St. Annes in Lancashire. On-site wastewater treatment system design isn’t something that’s taught in any depth, if at all, on university civil engineering courses, so the Barlows were referred on to specialist package treatment plant manufacturer WPL. All credit to the civil engineering company in this instance: many try to retain management of the project to make some money but simply contribute to cost and confusion for “their” client in what is a very niche area from both a technical and regulatory point-of-view.
Experienced though they are in the design and manufacture of package sewage treatment plants in their Hampshire factory, WPL only offer turnkey systems to wastewater treatment utilities companies like United Utilities or Thames Water. Even then they don’t get involved in ground-working. WPL therefore turned to Hutchinson, their long-standing partner in the north of England and Scotland, to take on the overall design, installation and ongoing maintenance of the sewage treatment system.
110 residential home pitches were served by an existing crude sewage package pump station in the centre of the park. From this pump station the raw sewage was pumped several hundred yards across the site to a balance tank linked to an older septic tank, from where the flow by gravity was to a rotating biological contactor sewage treatment plant just a few yards away. The RBC was showing signs of wear, tear, fatigue and imminent failure both audibly and visually, creaking and clunking and rotating very much out-of-balance.
Two private houses belonging to neighbouring BAe Systems drained directly to the septic tank, as did the Sea View’s laundry room: these needed to be taken account of in the design too.
Devising a new system for Sea View was relatively straightforward – there are few problems we haven’t solved already at least once in our 45 years. A regular one is if the new plant cannot be sited very close to the existing plant then the existing sewage collection pipework depths become a real issue, in which case a “lift” pump station often has to be added to the design. The biggest challenge at Sea View, however,was the lack of space in which to site a new plant anywhere, let alone near the existing one. Options were discussed with neighbouring landowners but after much thought, the least expensive solution was to sacrifice a pitch adjacent to the existing pump station in the centre of the site.
It might look like a big area, but you need some space to operate one of these safely...
...in order to "dig a hole" big enough for tanks like these:
And there were two of them, side-by-side, with a pump station downstream. And we still needed to work around them safely with a smaller machine.
Loss of revenue is an issue for any business owner so we always look for and cost out alternatives to try to avoid last-resort solutions. In this case the pill was not so bitter as it proved possible to decommission and backfill the old balance tank, septic tanks and failed treatment plant to make a new pitch to replace the one sacrificed.
In an ideal world, the existing crude sewage package pump station in the centre of the park would have been taken out of the design and sewage allowed to flow by gravity into the new treatment plant, since the fewer components there are in any system the better. Removing the pump station and having a gravity flow into the new treatment plant would have saved ongoing maintenance and power consumption costs, reduced potential odour problems and increased the robustness of the system by relying on gravity drainage rather than pumps. However, the existing foul drainage collection network ran too deep into the pump station, at 2m below ground, to allow it to be redirected straight to the new treatment plant.
The inlet pipework can be seen on the left hand side, just above the wastewater.
“Deep invert” treatment plants can be accommodated but their installation becomes more complicated and therefore more expensive from a civils point-of-view: deeper excavations mean more spoil to dig and remove from site, for which larger machines are needed, more time is taken, there's an increased likelihood of finding the water table, more excavation protection required for Health and Safety, etc. Can you imagine the treatment plant shown above being another metre and a half taller/deeper? Access for maintenance becomes an issue, too, turning a one-man job into a more expensive two-man job because of Health and Safety legislation as it relates to entering confined spaces and lone working.
The existing pump station was therefore retained in the design to pump the crude sewage into the new adjacent treatment plant. This was useful as it offered the possibility of a “half-in, half-out” treatment plant installation. We were pleased to have this option because the closeness of other pitches and the pump station enclosure wall to the excavation was a concern, as was the possibility of finding the water table so close to the Ribble Estuary. Of course, we discussed the options with the Barlows befrehand so that they were aware of these possibilities, not only from a cost point-of-view but also from an amenity point-of-view for their existing residents. We also discussed what the Barlows’ preferred solutions might be, and put them into a cost/benefit context, rather than just pushing on with what was easy for us. We appreciate that when you spend six figure sums on wastewater treatment systems you want to know why your money is being spent as it is and our customers agree that our extensive experience delivers best value.
The frequency of pumping from the existing pump station was changed to pump little and often into the treatment plant, and the primary settlement tank of the treatment plant enlarged to facilitate better solids separation of the macerated raw sewage. Such details are small but they are essential to the trouble-free running and effective treatment performance of the system and their consequent minimal impact on residents and the environment – remember that the Environment Agency, rightly, is never far away. Consideration of such detail only comes with years of experience of wastewater treatment system design and maintenance. The biochemistry and fluid flows of wastewater treatment process engineering isn’t an area into which straightforward ground-workers often stray and, when they do, lots of goodwillafter the event will never solve the problems of intrinsically poor system design. Once a poorly-designed system is in the ground there is a limited amount that can be done with it to correct its shortcomings.
We're aware of other details on caravan parks, too, like spiders' webs of electrical cables and drinking water supply pipes: we're at pains to avoid them for our own sake as much as yours!
A new secondary-treated-effluent pump station was to be installed immediately downstream of the new treatment plant to pump the treated wastewater back towards the original discharge point in the estuary. The existing “rising main” pipe that originally sent the crude sewage to the balance and septic tanks was “re-purposed” for this, which saved cost and avoided disruption on the established site. Of course, the treated effluent pump station had to be sized and fitted out with pumps and internal pipework to work with that existing rising main – an easily overlooked consideration.
The new package pump station pumps the secondary treated efluent to the outfall into the estuary.
New gravity discharge pipework was laid through the balance and septic tanks and then through the decommissioned carcass of the old treatment plant. A new, small, crude sewage pump station was installed to pick up the wastewater from the two private houses and the laundry room, and a short length of rising main laid to the nearest gravity sewer.
The new "feeder" pump station serving the private housing and laundry, all behind the wall.
It was certainly challenging to work in such a small area to install such a physically large treatment plant. Fortunately WPL’s modular system, with separate primary, treatment and final settlement tanks, could be designed to fit perfectly. Less expensive single tank systems are available but the additional costs and complicated logistics of having a crane on site eat up obvious capital cost differences. And in this case a single tank would not have fitted in the available space, so WPL’s modular system would have been the “go to” solution anyway.
As mentioned earlier, thought had been given to leaving the plant “half in, half out” but, when it came to it, the ground was stable and the weather was good so Keith, one of our machine drivers with, quite literally, a lifetime’s experience of ground-working, and Ian, our Contracts Manager, with almost as much “time in the saddle”, took the decision to work towards a flush finish to minimise the visual impact of the installed system, much to the great satisfaction of nearby residents and the Barlows themselves.
“The work from start to finish was exemplary,” said Geoff and Steven. So much so that they didn’t invite quotes from any other companies for the new sewage treatment plant and four feeder pumping stations at their Midwood park, between Lancaster and Preston, the design, supply and installation of which followed on only a matter of weeks from Sea View.