A Survey of Rotary Press Dewatering Technology

Rotary press sludge dewatering technology was developed in Canada in the 1980s. Within this technological category there are currently two major designs, the “rotary press” and “rotary fan press”. In order to update the WEF Manual of Practice and USEPA Solids Processing Design and Management Manual with information on this technology, Carollo Engineers investigated the major design types and conducted a survey of installations at municipal wastewater treatment plants across the United States.Rotary press dewatering relies on gravity, friction, and pressure differential to dewater sludge. Sludge is dosed with polymer and fed into a channel bound by screens on each side. The channel curves with the circumference of the unit, making a 180° turn from inlet to outlet. Free water passes through the screens, which move in continuous but slow, concentric motion. The motion of the screens creates a “gripping” effect towards the end of the channel, where cake accumulates against the outlet gate, and the motion of the screens squeezes out additional water. The cake is continuously released through the pressure-controlled outlet. The major elements of a rotary press are the polymer feed and mixing system, parallel filtering screens, a circular channel between the screens, the rotation shaft, and a pressure-controlled outlet.Areas of distinction between the rotary press and rotary fan press were found to be the screens, drive mechanism, and pressure differential. In the rotary press, the screens consist of two layers of perforated stainless steel, with each layer having different sieve size. The rotary fan press screens consist of fabricated wedgewire with small openings and linear gaps. The entry zones of rotary presses and rotary fan presses function much like the gravity phase of belt press dewatering. Free water “falls” through the filtering screen pores and is collected in a filtrate channel. Pressure builds gradually as the solids travel towards the machine outlet. Because the outlet controls the pressure at which cake can be released, cake solids accumulate against the outlet and are further dewatered via friction from the continuous motion of the screens. In the rotary press, the friction generated between the screens and the cake plug translates into mechanical pressure that deflects the cake away from the center and forces it sideways against the restricted outlet. In the rotary fan press, frictional force is also imparted in the outlet zone, but mechanical pressure is not generated to the same magnitude. In both designs, water that is released through friction is collected in the filtrate channel along with water released by gravity in the entry zone.The survey addressed performance, operational factors, and power usage. The size of facilities surveyed ranged from 0.5 to 20 million gallons per day (1900 to 76,000 cubic meters per day). The hydraulic loading rate of single-drive units ranged from 7 to 250 gallons per minute (26 to 950 liters per minute). The cake solids content ranged from 12 to 28 percent. Operators were able to control the performance of the rotary press by changing polymer type and dosage, feed rate, feed pressure, wheel speed, and outlet pressure. Accurate chemical conditioning was found to be mandatory for attaining design performance. Both design types were found to require minimal supervision between start-up and shutdown.