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Gravity Fed Filters - Bottom Drains

21st Nov 2019

A bottom drain is used on a pond which has a gravity fed filter system. Water and pond waste leave the pond via the bottom drain, travel along a length of pipe, and into the pond’s filter. Bottom drains are located on the pond floor, and should be positioned in the deeper areas of the pond.Bottom drains consist of; a sump, which is set into the pond’s base and is attached to the pipe-work, and a dome, or cap, which covers the sump.The sump is supplied with a flange ring, which can be screwed to its top lip. If the pond is liner-based, the liner should be sandwiched between the flange ring and the sump lip, and adhered with a suitable sealant, creating a water-tight seal. If the pond is concrete the flange ring is not required. The top of the sump should be set level with the pond’s final rendered layer.A gap of usually around 2 to 5cm is set between the top of the sump and the dome, and should stop smaller pond fish from entering the filter system.

As the pipe-work used in the construction of a gravity fed filter runs under the pond and is usually set in concrete, bottom drains should be considered at the outset of pond planning, and installed as the pond is constructed.Bottom drain set ups are a much more efficient than having a pump situated in the pond, pushing water to a filter. Bottom drains can handle larger solids than most types of pond pumps, and are far less likely to block or become hampered by debris.A pond can be built with multiple bottom drains along the floor, leaving no areas in the pond for sludge to accumulate. Also, a submerged pond pump requires hose connecting to the filter, and a power cable, both of which have to be concealed to improve the pond’s aesthetics.To obtain a maximum flow-rate and reduce the build up of debris in the pipe-work connecting the bottom drain to the filter, the pipes should run as directly as possible, and any bends should have a large curved radius, as apposed to a sharp 90° elbow. It is recommended that the pipe-work between the bottom drain and the filter should be Solvent Weld Pressure pipe.

The number of bottom drains needed on a pond will depend on the size and shape of the pond. A single bottom drain will normally draw debris from an area of 4 to 8m2 of pond floor. The steepness of the base’s benching, and pond’s flow-rate will also be a factor in the number of bottom drains which are adequate a specific pond design.

Aerated Bottom Drains

There are some bottom drain models which have an air diffuser fitted into the dome, which can be connected to a separate air pump. This set-up will obviously increase the amount of dissolved oxygen in the pond, but will also improve the pond’s flow. The ascending column of bubbles produced by from the diffuser drags the pond water, creating a circulation system as demonstrated in the diagram below.