Bottom Drains & Skimmers

Bottom Drains
Our ponds will produce heavier than water solids, mostly fish waste, that being heavier than water will settle on the pond bottom. We use what we call “Bottom Drains” (BDs) to sweep the bottom clean of this heavier than water solids. Bottom Drains are designed to pull from the circumference or sides of the drain, instead of straight down through an open hole, thus sweeping the bottom clean around the Bottom Drain. Bottom Drains usually flow into either a settlement filter, allowing the heavier than water solids to settle out, or into a Sieve, which separates the heavier than water solids from the main water stream. Different size BDs will handle different size ponds. For the BD to be effective, the flow rate has to match the BD pipe size. If the flow rate is to slow, sediment will settle out in the BD piping. The more you flow, the larger your sweep radius will be. And the size of the settlement chamber has to match the flow rate of the BD to give enough “Dwell Time” to allow the heavier than water solids to settle out. In general terms the settlement chamber should be 10% of the flow rate.
A three inch BD will effectively sweep a 4 foot radius, a three inch bottom drain needs a minimum flow of 1500 gallons per hour, (gph) to prevent sediment from settling out in the line. This 1500 gph needs a 150 gallon (10%) settlement tank to effectively settle out the heavier than water solids. 2500 gph/250 gallon tank is better and preferred for a three inch BD.
A four inch BD will effectively sweep a 6 foot radius, a four inch bottom drain needs a minimum flow of 2500 gallons per hour, (gph) to prevent sediment from settling out in the line. This 2500 gph needs a 250 gallon (10%) settlement tank to effectively settle out the heavier than water solids. 3500 gph/350 gallon tank is better and preferred for a 4 inch BD.
The line from the bottom drain flowing into the settlement chamber is what we call gravity flow. The pump does not take suction off this line, but takes suction off the settlement tank, thus dropping the level of the settling tank, As this level becomes lower than the pond, water from the pond will flow through the BD into the settling tank to fill the void. Pumps should never be hooked directly to a BD, as the heavier than water solids will get minced up going through the pump impeller and become almost impossible to separate out. This is why we use settling tanks before the pump. As you can see, the settlement tank has to be buried in ground, or in some kind of filter pit as water seeks its own level, the BD will always try and keep the settlement tank filled to the pond level.
Bottom Drain effectiveness can be enhanced by adding an air diffuser to the top of it. Most air diffusers for this application are 9 inches in diameter. The column or air rising to the surface will carry a column of water with it. This rising water column in the center of the pond will create a counter current of water flowing down along the outer pond walls, then across the bottom to the BD, thus sweeping the bottom clean even better.
Bottom Drain performance can also be enhanced with under water returns commonly called TPRs or GPRs. These are under water returns, usually about 16 inches off the bottom of the pond that will direct the bottom currents in a circular flow around the BD, thus freeing settled waste so it can be carried away down the BD.
Settling chamber size can be reduced with the aid of a Pre-Filter or rotating Micro Screen on the outlet of the settling chamber. The one problem with settling chambers is they do not remove the fish waste from the water column, but just collect it in wait for us humans to either dump it to waste, or pump it out to waste. We humans have to do something to physically remove the solids the settlement chamber has collected. Until we do this, the fish waste we have collected is still in the water column polluting the pond.
A Sieve in place of a settling tank will actually physically separate and remove the solid waste from the water column and hold it in a separate compartment awaiting removal by us humans. Sieves are smaller than settling chambers, but harder to construct our selves (DIY) and more costly to buy.

Skimmers
Skimmers remove floating stuff like leaves, pine needles and so forth. They also remove suspended and floating “Fines” that tend to cloud out water. Also and most important, the skimmer will remove the layer of Dissolved Organic Compounds (DOCs) that form on the pond surface. With out a skimmer the pond surface will collect a layer of DOCs, it will look like an oily film floating on the surface, effectively sealing the pond surface and suffocating the pond. This would be just like laying a piece of Visqueen over the pond surface.
The opening of the skimmer will have a weir, a floating device designed to float up and down with water level, thus skimming only the surface. This weir will also aid in keeping fish out of the skimmer. Always go with the widest weir available. A 16 inch weir will skim twice as much water as an 8 inch weir at the same flow rate. Any good skimmer will have some kind of removable “Leaf Basket”, that will trap lg stuff like leaves. You can remove this basket and dump and hose it out. The rigid solid poly leaf baskets are easier to use than the ones made with netting. Some skimmers will provide additional mechanical filtration usually in the form of matts or brushes. The kind of skimmer you need will depend on your pond’s environment. A dirty pond, one that has lots of leaves and other floating debris will need a larger weir and leaf basket than a clean pond, such as one that is inside a building.

Steve Joneli
High Desert Koi