Conveyance of biosolids sludge cake from dewatering equipment to further processing or truck transport can be challenging, even under favorable design conditions.

The South Florida city of Port St. Lucie recently upgraded the cake conveyance system at its Glades Wastewater Treatment Facility, installing a progressive cavity pump fitted with an auger feeder to replace shaftless screw conveyors. 

Low-profile challenges

Port St. Lucie, population 181,000 and growing, operates the Glades treatment facility (12 mgd design) and the Westport Wastewater Treatment Facility (6 mgd design). Because the Glades plant lies close to a residential area, a low-profile design was required for all structures, including the new biosolids dewatering building.

The low-profile design meant that new centrifuges would be located at ground level instead of elevated on a second floor. An interconnected series of cascading shaftless screw conveyors transported dewatered biosolids cake from the centrifuges to hauling trucks located in an adjoining building.

The complete process consisted of nine conveyors, including one vertical conveyor with a conveyance height of 35 feet and an intermediate collection bin. The path from centrifuges to the trucks was tortuous. The system was not easy to maintain, and the vertical conveyor and elevated conveyors broke down frequently.

The biosolids cake at the Glades plant originates mainly from domestic wastewater and averages 17 percent solids. The centrifugal pumping equipment familiar to plant personnel would not be applicable. After considering various conveyance options, the plant team chose a progressive cavity cake pump (SEEPEX).

Reliable design

Progressive cavity pumps are used at numerous Florida wastewater plants in similar applications. Among its advantages at Glades, progressive cavity pumping eliminated the vertical conveyors, the elevated cake storage bin, and some elevated conveyance equipment. The pump was delivered with a control system that allows it to operate independently. The system uses a sophisticated laser level control, dry run protection, and overpressure protection for reliability.

With the new pump in place, the number of overhead electric motors and conveyance sensors in the dewatering building was greatly reduced. In addition, the discharge pipe and valve system are neater and cleaner in appearance.

The Glades plant presented numerous obstacles to a successful design. The dewatering building’s low-profile construction left little room for a suitable installation inside the building. The waste discharge and cleanup under the centrifuges made for a harsh and wet environment.

The ultralow headroom underneath the centrifuges further limited access for system maintenance and operation. Integration of the pump’s sensor and speed controls into the plant’s Device-Net-based control system was another concern during the project planning phase. 

Exterior installation

The team ultimately decided to place the pump outside the building near the truck hauling bay. This location allows nearly uninhibited access during startup and for maintenance. Another conveyor was required to deliver the cake to the pump, but it is no longer in a problematic vertical configuration.

Two overhead horizontal discharge conveyors inside the truck loading bays were retained for normal use since they typically need minimal maintenance units. Minor modifications enabled the truck loading conveyors to accept cake discharge from the pump.

The discharge piping consists of 12-inch pipe and two overhead, motor-operated, heavy-duty slide gate valves at each existing overhead truck loading conveyor. All pipe elbows have an ultralong radius at more than 2.5 times the discharge pipe’s inside diameter. The electric motor operators on each discharge valve enable operation from loading bay floor level.

An emergency pressure relief valve on the discharge piping on the downstream side of the pump provides additional overpressure protection. The pump’s own control panel and touch-screen operating system has little interaction with the Device-Net system, a significant advantage to the plant operations and maintenance staff.

Continuous duty

The project also included installation of a new piped drainage system under the centrifuges to conduct waste liquids directly to the centrate drain header. In total, the upgrades improved the dewatering operations’ appearance, freed workers from daily waste cleanup, and improved overall system reliability.

The dewatering building upgrades were completed in April 2017, and pumping of cake began in May. Daily pump operation is typically smooth and continuous. The laser level control has been robust in matching and maintaining the pump’s speed with cake input, even during the heaviest rainfalls. Average pump discharge pressures range from 56 to 67 psi, depending on the flow path and the hauling trailer location.

About the authors

Richard M. Schoenborn, P.E., (rschoenborn@cityofpsl.com) is senior civil-utility engineer with the city of Port St. Lucie (Florida) Utility Systems Department. Bill Martiniere (bmartiniere@seepex.com) is a territory manager with SEEPEX.