When a water treatment plant in northern England faced a spike in pesticide concentration exceeding the allowable concentration limit for incoming water, the facility was forced to shut down. Plant managers then used water from a regional facility until the problem could be solved.

Their temporary purification solution arrived in a 20-foot steel shipping container. In it were a bulk bag discharger and two flexible screw conveyors made by Flexicon Corp. of Bethlehem, Pa., and Transvac Systems Ltd.’s ejector system for mixing and injecting a slurry of powdered activated carbon (PAC) into the water stream.

PAC is automatically transferred from a half-ton bag through a flexible screw conveyor to a surge hopper from which a second flexible screw conveyor meters the powder into the ejector. Supplied by Transvac, an English company that’s made ejectors since 1973, TransPAC can be activated in a day without costly and time-consuming site preparation, construction, or complex components.

The northern England facility was restored to compliance as the dosed carbon successfully removed pesticide traces from the main water stream.

Split-frame discharger fits inside container

TransPAC requires connections to an electric power supply, municipal water stream, and an external water supply. Environmental impact and site preparation are minimized, as well as the need for maintenance and planning permission. The system is safe to operate, simple to control, and doses accurately and steadily.

A forklift loads a bag-loading frame and 1,100-pound bag of carbon powder onto a discharger frame inside the shipping container. Once the bag’s spout is untied, the powder flows into a flexible screw conveyor leading to a surge hopper. A second flexible screw conveyor moves the powder from the hopper outlet to the intake of the ejector, which releases the powder into the municipal water stream.

The conveyors are curved to fit the tight space within the shipping container.

From the control panel, the operator sets the speeds of the conveyor drives to automatically dose the proper amount of PAC according to the site’s water flow. Sensors in the surge hopper signal the controller to start or stop flow through the first flexible screw conveyor when hopper contents reach low or high levels.

The dosing portion of the system includes a header tank for incoming water, a booster pump, and the ejector. The velocity of the water flowing through a venturi creates a low pressure zone in the ejector that entrains the carbon powder into the treated water stream at a rate set at the control panel. The unit operates with no moving parts.

PAC safety issues resolved

PAC adsorbs pesticide on its surface, and the carbon and adsorbed material are subsequently removed as sludge in the flocculation process. With an average particle size of only 20 microns, the extremely fine powder is prone to dusting.

Both the bulk bag discharger and flexible screw conveyors prevent dusting. The bag outlet spout is connected to the feeder by a clamp ring, creating a secure connection between the clean side of the bag spout and clean side of the bag spout interface.

Each screw conveyor consists of a stainless steel screw rotating inside a durable polymer tube that contains the fine powder as it’s conveyed. Conveyor discharge is likewise dust-free, as powder exits through a transition adapter located forward of the drive at the discharge end, thereby preventing powder from contacting bearings or seals.

In addition to emergencies related to pesticides, water utilities in the United Kingdom have used TransPAC to address taste and odor problems and as an alternative to traditional PAC batch dosing systems, which are large, complex and costly, and require long lead times by comparison.