Many people have asked about my sand/gravel filters so here's my construction thread. But 1st, all credit has to go to Kent Wallace. He is the one that originally showed me this type of filter. I build mine different from Kent, but the concept, and operation are the same.
This filter while providing some bio filtration is designed as a pump fed polishing filter. it is the last filter in line before going back to the pond. The water is pump fed in the bottom and filters up through progressively finer layers of gravel then sand. There are two outlets, one to your pond, and one to waste. When you want to flush it, you simply pull the plug from the waste line, put it in the pond line, and turn on your blower. The blower will blow up and dislodge all the trapped waste and flush it up and out the waste line. The progressively finer layers of gravel and sand will stay in place because of their weight and the air flushing going up ward. I am currently using 6 of these filters on my ponds and am very pleased with the results.
OK, Lets build a filter.
Start by cutting the top out of your barrel. Try and make a nice round accurate cut as we will be using this as a template later. Next drill a 3 1/4 inch hole in the center/bottom of your barrel. Use a hole saw for this to get a nice round hole. picture 1 This is for the 2 inch shower drain fitting for the water inlet. Now on one side of the barrel, measure down from the top 4 inches and drill a 3 1/2 inch hole there, using the 4 inch mark as the center of the hole. This will be the outlet to the pond. picture 2 On the opposite side of the barrel drill another 3 1/2 inch hole centered 9 inches down for the outlet to waste. picture 2
Now discard the gasket from your 2" shower drain and using 3M-5200 marine adhesive, install the shower drain in the bottom of the barrel. For the two outlets I use 3 inch electrical male and female adapters. Elec. conduit is different from PVC as the threads are straight cut and they will screw together tightly. Once again using 3M-5200 screw together a male and female adapter in each of the two outlet holes. picture 3 This completes the modifications on the barrel.
We also need a plug for the outlets. Just cut 4 inches of 3 inch PVC and glue a cap on one end.