Click here to enlarge imageThe dual-filter system began to experience clogged screens in a matter of minutes after start-up. The filters automatically signal a cleaning cycle when either a 7 psi pressure differential is sensed across the filters or an adjustable preset timer has expired. As long as the differential pressure switch remains closed, the filter will repeat cleaning cycles until a preset number of cycles has been reached and then the controller will shut down the cleaning process to prevent dumping too much water to drain and send a fault signal. This signal was being activated repeatedly.
When the filters were opened for inspection the screens were found to be completely encased in a black, stringy, tarry sludge that even plugged the large 3/8" holes in the coarse protective screen. Further investigation showed a layer of sludge 12-15 inches thick had built up on the floor of the basin from which the pumps supplying the filters drew effluent. The vertical turbine pump inlets were located about 6" off the floor of the basin.
Screen filters are not sludge filters. A vacuum truck was called to remove the sludge layer. After pumping resumed from the cleaned basin, the filters worked as expected, with cleaning cycles triggered by the timer every two hours.
Had the original engineers investigated a little further in the preliminary design stage they would have realized that an old coarse-screen continuous cleaning filter had been operating in this area for many years. But instead of flushing the debris it captured back to the head-works of the treatment plant, it dumped the debris back into the basin from which it received its effluent. Decades of accumulated solids had built up inside the basin forming the dense sludge layer encountered by the new filtration system.
It was soon noted that two or more consecutive cleaning cycles caused air pressure to build up in the drain line. At one point, the vice president of the filter manufacturing company was completely soaked with treated but unchlorinated effluent when a clamped rubber joint in the drain line blew apart while he was training plant personnel on the filtration system.
The situation occurred because the 6" drain line from the filters traveled underground about 150 feet to a wet well about 5 feet in diameter. Inside the wet well were two, 2" residential basement-type sump pumps. Both 2" discharge lines were tied together into a single 2" line leaving the wet well. This greatly decreased the pump-down capacity of the wet well system. One flush of the filtration system would raise the water level in the wet well above the drain inlet elevation, completely submerging the 6" inlet drain pipe.