The 4-stage Cleaning Process: Stage 1 - The rainwater is first cleaned by a patented filter system positioned in the tank dome. As the water runs over the filter, the dirt and solids are caught in the filter basket and the clean water runs into the tank; Stage 2 - To allow the very fine particles to settle on the bottom of the tank, the filtered water is fed into the tank using a calmed inlet. This prevents permanent swirling of the water and also introduces oxygen to keep it clear; Stage 3 - Dirt particles, such as pollen, that are lighter than water settle on the surface of the water forming a floating layer. This floating layer is removed when the tank overflows and is washed away via the overflow siphon in the tank; Stage 4 - The collected rainwater is pumped out using a floating intake in the upper part of the tank. This takes the water from approximately 100mm below the surface - the site of the best quality water.

The Graf Carat S is unlike any other underground tank. It is the largest tank of its kind to be injection-moulded, resulting in a high quality engineered product that is consistent, strong and reliable. This process enables the Carat S to be manufactured as two easy to handle half shells, which can be stacked on top of each other for shipping, as well as saving space on site. The patented quick connection system, which has been laboratory tested to last more than 25 years, allows the tanks to be easily assembled (without any screws or bolts) in a matter of minutes, and uses a combination of a unique EPDM seal and sophisticated location pins. The ribbed design makes it light yet very strong and stable, enabling it to be installed in groundwater up to the middle of the tank.

The Carat S tank comes with a dome, which is designed to rotate independently of the tank, making it much easier to align the connections and pipes during installation. A dome extension is available along with a range of telescopic tank shafts with lids that are suitable for pedestrian and vehicle loading. These tank shafts slide up and down inside the dome offering adjustment for height as well as up to 5° of tilt, making it easy to get the lid flush with ground level.

These filter options fit neatly inside the dome of the Carat S tanks and come with 'Spannfix sleeves', which use Velcro for quick connection to the pipework. Filter package 1, which includes an overflow siphon with rodent guard and a calmed inlet, is suitable for rainwater that has been pre-filtered before it reaches the tank. The addition of a Graf Universal-Filter makes filter package 2 the recommended option for most underground systems. This has a removable filter basket and can manage roof areas up to a massive 350m². Filter package 3, which has a transparent lid for visibility and uses the self-cleaning Optimax filter, offers the best solution for low maintenance.

The tank connection kit uses special seals, which remain watertight even with slight tank misalignment

These easy to fit 110mm drainage pipes and fittings can be used to divert harvested rainwater to an underground tank, to connect tanks together and to run the overflow to a soakaway.