Rectangular Square Cube heat recovery systems from DDI Heat Exchangers are designed to handle sludge with more than 26 percent solids without plugging. Heat from sludge warmed for anaerobic digestion (95 degrees F Class B, 135 degrees F Class A) is recovered and used for direct heating of incoming cold raw material. By using direct sludge-to-sludge heat recovery, cold raw sludge can be heated to 75 degrees F before entering the digester, saving on boiler or electrical output, says Erwin Schwartz, company president.To avoid plugging, the DDI system uses parallel 3-inch-high rectangular channels (3- to 6-inch sludge side gap and
