Sand Filter for Swimming Pool

No problems. Simply enable an inlet port near the top of the drum and an outlet port near the bottom. The oultet port should have some form of flow distributor in it and arrange it so that the port size is smaller than the sand grains.

Fill it with a single-size sand. "Greensand" works well. Allow some head-space above the sand for what comes next.

Arrange a backwash connection. For this, valves are needed to isolate the flow to service and allow a washing connection to pass flow upwards through the bed and out the top to waste. The velocity inside the drum needs to be enough to expand the bed, and not enough to fully fluidise it so that no sand comes out the waste port. So some form of flowrate restiction has to apply during backwash.

Oh yes. And the drum will need to withstand, without bursting, the maximum pressure available either from the supply source or the backwash connection.

Instrumentation? Differential poressure measurement would be useful, as would forward and backwash flowrates. Frequency of backwash depends upon the flowrate and the dirt loading of the forward flow stream, which differential pressure will indicate given operational experience.

For information, suitable vessels are availabe for a few tens of £GBP upwards, depending upon size. GRP is the most common material-of-construction, though steel (usually stainless canisters of some sort) can sometimes be found. Sand is readily available at little cost virtually anywhere.

You have some good answers here, but I want to weigh in on this one a little if you will bear with me.

The design I would recommend would be as follows:

1) - In the bottom of the drum, insert a 2" pipe (here I would use schedule 40 PVC). This pipe should have ½ inch diameter holes drilled in it, 1 inch apart, two rows looking up, about 1½ inches apart. Seal the pipe where it goes through the drum wall. Cap the other end of the pipe. This will be your collection pipe. Put it on or near the bottom of the drum.

2) - Pour clean gravel that is about 3/8 inch to 1/2 inch in diameter over the pipe to about 3 inches above it.

3) - Pour about 4 inches of "pea gravel" about 1/8" to 1/4" over the larger gravel then

4) - Pour about 12 to 15 inches of silica sand over the pea gravel. This sand should be about .45 to .55 mm with a uniformity coefficient of 1.7 or less. This sand is commercially available from a sand supplier. It is available in bulk or in 100 pound bags. I would guess that a couple bags would suffice.

5) - Introduce your inlet near the top of the drum. The size of the pipe is dependent upon the pump you are using. I would guesstimate a 1¼ to 1½ inch pipe would be adequate. Extend that pipe to the center of the drum and turn an elbow downward. Then hang or weld something under the elbow to deflect the flow so it doesn't erode the sand under it.

This will provide you with a system that will remove the debris and impurities in the filtered water. If you find that the pool water you are considering has more larger solids in it than might regularly be found in that type of flow, you can put a layer of crushed anthracite coal about 6" deep on top of the filter sand. This should be 0.9 mm to 1.0 mm with the uniformity coefficient of 3.0 or less. This will be larger than the sand and remove the larger solids before the flow reaches the sand which will remove the finer particulates.

Since the interstices between the sand particles is much smaller than that in the anthracite, the filtering will have a two stage effect. However, the larger anthracite is lighter in weight than the samller sized sand. When the filter is backwashed as described by an earlier respondent, the weight of the filtering media will keep all the layers in place. The anthracite will remain on top, followed by the sand and then the layers of support gravel.

Controlling the backwash can be automated, or you can simply backwash the filter as needed. You will want to install a pressure gage in the inlet line to the filter. You will read the pressure with the filter clean, and then watch it during operation. Observe the pressure when the filter has operated a full day (always being mindful of the limitations of the drum. An oil drum is NOT a pressure vessel). If the pressure recorded after a day of use is significantly higher than the clean pressure, you may have to clean it more often. This will come by observation.

Of course, this can all be automated with pressure switches, etc. If that is your preference, I would be happy to describe a method to accomplish that, too.

I can't stress enough the pressure limitations of the oil drum. I have stated that it is not a pressure vessel, and you must be very careful when operating one in this manner. The exploding drum can cause physical harm as well as soaking you and the surroundings. Good luck!! Let us know how this works out.