There is actually nothing to a sump. Sump are very basic, simply being a container into which water flows, then out again. It is as simple as that.
Why a sump?
A sump is not a requirement for a reef aquarium and isn't suitable for all situations. A perfectly successful system can be maintained without one. However, it can certainly make somethings simplier and easier. Some of the reasons that a sump are used are as follows.
Equipment out of view
Increased system water volume
Ease of access to equipment for tuning and maintenence
Bubble separation
Outlet location for filtration equipment
Example arrangement of sump within a system.
What equipment placed within the sump?
What is located within the sump really depends on what the sump is being for and the equipment requirements of the system. Pretty much any piece of equipment or filtration device can be located within the sump if required. The more common pieces of equipment or items that a sump will container are as follows.
Protein skimmer
Heaters
Calcium reator
Filter floss
Activated carbon
Phosphate sponge
Bioballs
Macro algae
Turf algae
Refugium
Deep sand bed
Float valve/switch
Probes to monitor water parameters
Live rock
Banished livestock
Acclimatising livestock
A float valve or switch is perfect for a sump, providing an easy way to control the salinity of the system. To do this, place a baffle in the sump between the inlet and outlet. When some of the system water volume disappears due to evaporation, then the water level will fall on the outlet side of the sump (or at the inlet of the pump). This is where a float valve or switch needs to be located. The narrower this compartment is, then the larger height change will occur for a given volume of water evaporated, which then means more precise control over the salinity.
Location
Typically a sump is located underneth the display tank within the stand, with the water coming into the sump via gravity feed from a weir overflow. Water is then pumped back to the main tank using a return pump. However, this isn't the only location or configuration that can be used, with the various options available listed below. Additionally, the sump does not even have to be located close to the tank, with on the floor below and outside being options.
Below the display tank within the stand
Below the display tank, beside or beind the stand
Within the display tank, as a separate section using a baffle
Above the display tank
Beside the display tank
Remote to the display tank, outside the house or on the floor below
Material
A sump can be made of any material / container, as long as it is inert and will hold water. The most commonly used container for a sump is a glass aquarium, with acrylic aquariums and plastic tubs being other options. Glass aquariums have the advantage that they are relatively cheap, square, warp very little and are transparent. Plastic tubs (such as those from a hardware store) are advantageous as they are cheap, come in a wide variety of sizes and are easy to drill through.
Moving water from sump to tank: return pump
The return pump used with the sump can either be located inside or outside, i.e. external or submerged. In the case of outside there needs to put a hole through the side of the sump and a bulkhead to seal it. Be careful what submergible pumps are uses, that it is powerful enough to pump the water back up to the display tank. Most powerheads are not designed to handle this kind of back pressure that is generated by having a pipe filled with water elevated to a couple of metres.
Never use two pumps, one in the display tank and one in the sump, to move water between each tank. It is impossible to get the flowrates exactly the same, so in a short period of time one of them will either overflow or drain. One needs to use an overflow, stand pipe or siphon box.
Moving water from tank to sump: overflow
The built in overflow or siphon box is the way to get water from the tank to the sump. Preference is for a built in one, but the tank has to be drilled in order to have this. An internal overflow consists of either a baffle separating a compartment form the rest of the tank, or a stand pipe. In the case of the baffle, it is set to a height such that it is below the top of the tank. When the water level reaches the top, it then flows over into the compartment formed by the baffle. Inside this compartment is located a pipe through the tank that then goes to the sump. A stand pipe is just a vertical piece of pipe that is just below the water level required. When the water goes above this level, water then flows down through the pipe. This method is a little more dangerous, because it is easier for the pipe to block and cause the tank to overflow.
Diagram of the principle of an internal overflow.
Diagram of the principle of a stand pipe.
A siphon box can be used on any tank, and is normally just hung over the side. It consists of two boxes, one on the inside of the tank and the other on the outside. Joining the two boxes is a U-tube which is filled with water, and both ends are submerged in water. As water flows out of the outside box water is then siphoned through the U-tube from the internal box to fill it back up. And then water flows into the internal box as it is at a lower level than the rest of the water in the tank. Make sure if a siphon box is used, it is large enough and rated to the flowrate running through it. Also be careful that the flow through the siphon tube is fast enough otherwise bubbles accumlate and the siphon will eventually stop. Then the floor ends up wet.
Diagram of the principle of a siphon box.
Water flowrate
The water flowrate into the tank, from the sump, is controlled by the water pumping rate of the return pump. The flowrate out of the tank, down to the sump, is controlled once again by the pumping rate of the return pump. Basically if the overflow system is designed correctly it will remove water from the tank as fast as it is returned (within reason). The return pump used has to be sized correctly to the capacity of the overflow or the overflow sized correctly for the return pump being used.
