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A problem that can be encountered with protein skimmers that have a drain line, is overflowing of the collection container. If for some reason the protein skimmer starts to produce a very wet skimate, then the collection container can be rapidly filled. And if the system is unsupervised, this can cause some major problems as the protein skimmer continually draws water from the system and it spills out of the collection container.

A very simple solution to this is the use of a waste collection container that shuts down the protein skimmer when it is full. The way to do this is by causing a significant amount of back pressure to the collection cup. The increased pressure then forces down the foam/water level within the foam riser, decreases the amount of air input into the skimmer and dramatically reduce skimate production (in some cases even shut it off).

A diagram below shows how this is achieved. In the diagram shown case, the container is fully sealed with the exception of the skimate/air inlet from the protein skimmer collection cup drain and the air outlet. As the container fills up with skimate it eventually reaches the base of the air outlet pipe. At this point the air no longer has a way of exiting from the container, trapping the air within the container and hence within the top of the protein skimmer. Air continues to be introduced into the protein skimmer, but it has nowhere to go. Therefore, the air pressure increases and starts to push the foam/water level down within the protein skimmer foam riser. A lower foam/water level produces a drier skimate and eventually a point is reached where skimate production is halted entirely.

Diagram showing the basic design and principle of an automatic shut off skimate collection container.

There are a large number of ways of construction this, from something as simple as a plastic drum with two pipes inserted and sealed through the lid, to construction of the entire unit out of acrylic. All you need is an air tight container with a sealed inlet to the protein skimmer collection cup drain, and an outlet pipe that is 10cm or so down into the collection container.

An even simplier way of achieving this shut off is to have the collection cup drain line go down some distance into an open top container. I have used 2-3 litre plastic milk bottles with the drain line going down to about 10-15 cm depth. This is sufficient to slow down the foam jet skimmer when skimate production goes wild and stop it from overflowing.

Note that some commercially available protein skimmers use this principle within their collection cups.

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