And they are costly to clean and maintain. After years of observation of these traps, and of the way water flows in stormwater drains, Mr Crompton and Mr Blanche came up with a new approach. They built a separator which sits to one side of the stormwater system. A channel deflects the flow from the stormwater into a concrete basin, inside of which sits a cylindrical metal basket with screen sides perforated like a cheese grater. The holes are set back obliquely so that the screen presents a smooth opaque surface to the oncoming flow.
Water can make a U-turn, however, and pass through the holes.
As stormwater is diverted to enter the basket, it swirls around the screen in a circular fashion. Litter is prevented from sticking to the screen by being continually scoured and moved on by the flow behind. The velocity of the water flow lessens towards the centre of the basin. This tends to push floating solids to the middle while the heavier solids sink as they are kept in continuous circular motion. Whatever floats just remains whirling in the centre of the trap.