This is part of an article I wrote a few years ago on sumps and overflows with current (comments) thrown in...
Overflows
Overflows are very useful when constructed correctly. The water on the surface of the aquarium tends to have the more nutrients and waste like household dust and oily films from cooking than any other area. Properly constructed and installed overflows will draw this water off of the surface and send it to the sump area where biological and mechanical filtrations are normally placed. If the power goes out or the return pump fails, the overflow should only allow the water above a certain level to drain into the sump therefore preventing the sump to overflow. A properly placed overflow with enough flow will skim all of the dust and oily films that collect on the water surface. There is a lot of debate on what the proper flow through an overflow should be. The flow range seems to be from the tank volume times 1 to tank volume times 10 or more. Flow volume, on tanks with built in overflows, is usually limited to the bulk head diameter the manufacture used.
(I would build the overflow box without teeth)
No Overflow is Fail Safe
Any overflow has the possibility of failing. A common cause of the failure is a snail checking out the standpipe, getting stuck and blocking the flow. Anything blocking the standpipe prevents flow from the display tank to the sump. The return pump however, will keep pumping water from the sump to the display tank until it is empty. This can cause flooding and in my case an irate wife. This is why we are now going to cover how to prevent this by sizing the sump to match the overflow.
Calculating Sump Size and Water Level
Calculating the sump size and configuration can be very critical. I like to have as large of sump as possible. However, in my opinion, compartmentalizing the sump is the key to success. You will need to calculate the amount of water that the display tank can hold if the overflow fails. This is done by measuring the distance from the display tank’s normal water level surface to the rim of the tank. Let’s call this measured value the height (H). All measurements will be in inches. Measure the length (L) and the width (W) of the tank. The calculation to determine the volume in gallons it can hold above the overflow is H x L x W / 231.
Let’s use my tank as an example for this calculation. I have 1.5 inches from the normal water level in my tank to the top rim of the tank. My tank is 60” long and 18” wide so the calculation is 1.5”x60”x18” equals approximately 7 gallons (I round down just to be safe). So when I made my sump, I made the volume of the last compartment from the pump intake to the rim of the compartment a little less than 7 gallons. Now if my overflow fails, I can not flood the living room. Using this method has saved me more than once.
The next thing that we need to do is determine the overall size of the sump. During a power outage water will drain from the display tank to the sump. This water will come from two sources. The first source is the overflow. The water in the tank will drain down to the top of the internal overflow box. The second source is from the return pump line. This is the easiest source to miss since many of us will forget that the water will siphon from the tank to the sump from this line though the pump. One trick I like to do is to drill a 1/4” hole in the return line just below the water surface. This hole will allow siphon to break when the water level drop below the hole. The calculation to determine the extra volume needed during a power outage is similar to the one we used before. Measure the distance in inches from the normal water level to the level the water will drop to when the return pump turns off. Let’s call this measurement height (H). We will use the length and width measurement we used before.
The calculation to determine the additional sump water volume needed above normal water levels is HxLxW/231. Using my tank as an example, the water level drops ¾” when the return pump turns off. So the minimum additional volume needed in the sump is ¾”x60”x18”=3.51 gallons. To be on the safe side always round up when doing this calculation. My current sump is a 23 gallon acrylic tank that measures 12”x28”x16”. The last compartment holds 6.5 gallons and the maximum water level in the sump is 7”. This leaves me with 9 gallons (5”x28”x16”/231 ) of volume remaining for any extra overflow.
Thanks,
Scott
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jpndave wrote:
