What is a Filter press?
Filter press: Plate-and-frame and belt filter presses for sludge dewatering
A filter press is a dewatering machine that removes water from sludge by pressure. In a plate-and-frame (recessed-chamber) press, conditioned sludge is pumped into chambers lined with filter cloth and squeezed until a dry cake forms; in a belt filter press, sludge is sandwiched between two moving porous belts that pass over rollers of increasing pressure. Presses turn sludge of 2 to 5 percent solids into a cake of 15 to 35 percent, cutting the volume a plant has to haul by more than 80 percent. Klarifi’s facility data records mechanical dewatering as a unit process at US plants, and 341 biosolids and dewatering line items appear in the city capital plans it tracks.
Plate-and-frame versus belt filter press
A plate-and-frame press works in batches and produces the driest cake, often above 30 percent solids, which matters when biosolids go to landfill or incineration. It needs lime or ferric conditioning, an operator to open and clean the plates, and more floor space.
A belt filter press runs continuously, uses polymer rather than lime, and is cheaper and simpler, but the cake is wetter, typically 15 to 22 percent. It was the default at small and medium plants for decades and many belt presses installed in the 1990s are now due for replacement, which is why dewatering appears so often in capital plans.
Press or centrifuge?
Centrifuges produce a comparable cake to a belt press in a smaller, enclosed footprint with less odor and labour, at the cost of higher power use and polymer dose. Screw presses have taken share at small plants because they run slowly and unattended. The choice usually turns on the disposal route: the drier the cake needs to be, the more a plate-and-frame press or a dryer makes sense.
The water that comes off
The liquid squeezed out, called filtrate (or centrate from a centrifuge), carries high ammonia and phosphorus back to the head of the plant. Sidestream treatment of that return flow is its own project category at plants with nutrient limits.