Projects
Treatment plant installation, Silsoe, Case Study
We were approached by our domestic customer in Silsoe, Bedfordshire, who had an old septic tank with various problems and which was not compliant. They wanted a new Treatment Plant fitting in its place.
Objective
Our objective was to install a new treatment plant and re-route the existing drainage to this, from the old septic tank. The septic tank was not particularly old, but was not compliant to discharge effluent into the ground under new EA and Building Regs guidance, and the land drainage or soakaway was not coping well in discharging the effluent, this is one of the most common reasons for septic tanks failing.
About the project
Prior to ordering the new treatment plant, we had to take some levels to site to ascertain the invert level of the existing drainage, this level is critical in determining the length of the ‘neck’ on the new plant, which are all made to order. We use Marsh Industries for all our Treatment Plants and Pump Stations, and have been doing so for over 15 years now. We gave our customer the choice of either an 8-person plant or a 10-person, and they chose to go with the 8-person.
We started by excavating for and installing the new treatment plant, leaving all of the existing drainage running into the old septic tank whilst the bulk of the work was being carried out on site. The plant was laid on concrete and completely surrounded in concrete. The plant has to be filled with water whilst the concrete is being packed around the outside, with the level of the concrete never coming higher than the level of the water inside. On the following day, with the concrete having ‘cured’, the water can then be pumped out. The strength of the plant is dependent on the concrete surround, they are not designed to take the weight and the pressure of the ground around the outside without the concrete.
The effluent from the new Treatment Plant was to disperse into a watercourse on the boundary of the property, which is by far the best way to legally discharge of the effluent, and it’s also the cheapest way, much cheaper than excavating for and laying land drainage. The levels worked for the effluent to discharge to the stream by gravity, which was great. This pipework was installed after the plant install, and the grass reinstated on top.
The plant was conveniently located adjacent to a double garage, with full electrics. This was ideal for connecting the plant up to. We excavated for and laid a 63mm black electric duct to the outside of the garage. The duct was surrounded in sand with electric warning tape to cover, the trench backfilled and the grass reinstated. Our customer appointed their own electrician, but we liaised with him as to what was required. As per building regs, a treatment plant needs its own isolator, which the electrician sited on the inside of the garage. Quite often they are sited outside, but either is acceptable.
We then had to divert the existing drainage from the old septic tank, to the new treatment plant. We excavated for all the pipework first before making the switchover. With everything dug out ready for the final piece of pipework, we ‘bunged’ the pipework from the house and switched the pipework over. This way, our customer was able to continue to flush their toilets and carry on using the system throughout.
The existing septic tank was quite extensive and the customer asked that we fill this huge void underneath their driveway, with some of the spoil that had been generated from the excavation for the new treatment plant. To enable us to do this, we asked the customer to have the septic tank emptied one last time after the switchover had been done. This enabled us to fill it with the excavated material. We compacted it best we could on top, and reinstated with grass, getting rid of all the old manhole covers on the top of the septic tank.
With the electrical connection done by the customer’s electrician we checked the new system over and everything was working great. We reinstated some tarmac on the edge of the driveway, but most of the reinstatement was in grass. We managed to salvage most of the turf that we had taken up and put this back, which just left a few gaps here and there, which we filled with fine-grade topsoil and grass seed.
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