By Craig Favill, Transvac Systems Ltd. and David Boger, Flexicon Corporation
When a water treatment plant faced a spike in pesticide concentration exceeding the allowable concentration limit for incoming water, it was forced to shut down. In order to provide clean drinking water to users, water had to be diverted from a regional water treatment plant until the problem could be solved.
The solution ultimately chosen was a mobile, trailer-mounted carbon dosing system, housed in a six-metre long steel shipping container. It was delivered and activated within one day, without costly and time-consuming site preparation, construction or complex components. The water treatment facility was restored to compliance, as the dosed carbon successfully removed pesticide traces from the main water stream.
Supplied by Transvac Systems, the TransPAC mobile powder handling and carbon dosing system includes a bulk bag discharger, two flexible screw conveyors, and a Transvac ejector system for mixing and injecting a slurry of powdered activated carbon (PAC) into the water stream.
It only requires connections to an electric power supply, the 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, and simple to control.
From the split-frame bulk bag discharger, PAC is automatically transferred from a half tonne bulk bag, through a flexible screw conveyor, to a surge hopper. From there, a second flexible screw conveyor meters the powder into the ejector.
A forklift loads the bag-loading frame and 500 kg bulk bag onto the stationary discharger frame inside the shipping container. Once the bag spout is untied, the powder flows into a 75 mm diameter flexible screw conveyor leading to the surge hopper. A second 60 mm diameter flexible screw conveyor moves the carbon powder from the hopper outlet to the intake of the ejector that accurately doses the PAC 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 water flow. Low and high level sensors in the surge hopper signal the controller to start or stop flow through the first flexible screw conveyor when the hopper contents reach the low or high level.
The carbon dosing portion of the TransPAC system includes a header tank for incoming water, a booster pump and the ejector. 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 can pose handling problems
Powdered activated carbon adsorbs the pesticide on its surface, and the carbon and adsorbed material are subsequently removed as sludge in the flocculation process. However, 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 Spout-Lock® clamp ring. This creates a secure, dust-tight connection between the clean side of the bag spout and clean side of the bag spout interface.
Each flexible screw conveyor consists of a stainless steel screw rotating inside a durable polymer tube that contains the fine powder as it is conveyed. The conveyor discharge is likewise dust-free, as powder exits through a transition adapter located forward of the drive at the discharge end, thereby preventing it from contacting bearings or seals.