That's a load better for the facility operators
Carbon powder dust was causing a health hazard to water treatment workers in the US city of Lafayette and Swift Current, Canada. But bulk loading systems solved the problem.Lafayette in Colorado and Swift Current in Canada’s Saskatchewan are more than 1,100km apart but were experiencing nearly identical problems caused by manual addition of powdered activated carbon to their municipal water treatment systems. Both cities eliminated a health hazard caused by carbon powder dust and at the same time reduced the amount of labour required to add the activated carbon to the system − simply by installing bulk bag unloading systems.
Swift Current draws its municipal water supply from a reservoir formed by a dam on the Swift Current Creek at an average rate of 5,700m3/day and peaking at as much as 21,000m3/day. The water is treated by adding 1.5ppm of potassium permanganate, followed by the powdered activated carbon pre-mixed into the form of a slurry consisting of 0.13% to 0.47% solids.
The water is then gravity fed into a low well at a rate of about 95l/min. Reaching a final concentration of 20ppm, the carbon’s function is to adsorb organic materials, eliminating unwanted tastes and odours. After further additions of liquid alum, sludge conditioners and polymers in the clarifier, filtered water is pumped to a 680m3 clear well located at the water treatment facility and to two 5,700m3 reservoirs within the city itself, ready for distribution to residents of the city of Swift Current. Over in the US, Lafayette draws its water from two local reservoirs formed by dams on the Boulder Creek and one on the South Boulder Creek. Consumption is 7,600m3/day in winter and reaching a summer peak of as much as 38,000m3/day.
Water flows from the reservoirs into an onsite aeration and flocculation facility similar to that used by the city of Swift Current, where powdered activated carbon and other treatment chemicals are added. After final filtration, the water is pumped into three storage tanks, that together can hold up to 50,000m3 of water for distribution to residents of the city of Lafayette.
Prior to the installation of the bulk bag unloading systems, operators at both facilities were emptying 15-20kg bags of powdered activated carbon generating carbon dust and putting physical strain on workers.
Powdered activated carbon is an extremely fine powder with an average particle size of only 20µm and a bulk density of 345kg/m3. Its flow characteristics ensure that the slightest air movements cause the powder to get everywhere and become a health hazard to the operators.
Unfortunately, at the Swift Current facility a dust collection system failed to address this problem. Material transfer time was also an issue as the capacity of the system was limited by the small screw feeder, which took 16 hours to transfer the powdered carbon from the 450kg capacity hopper into the liquid slurry eductor.
Since the water treatment facility operates only one shift a day – staffed for eight hours then unmanned and controlled by computer for 16 hours – the hopper, could potentially run out of carbon, if not completely filled at the beginning of the unmanned shift, when personnel were not present to refill it.
Swift Current operators were opening and emptying as many as 25 25kg bags of powdered activated carbon daily into the hopper. Lafayette operators were carrying 18kg bags of powdered carbon up a set of metal stairs and emptying them directly into the aeration and flocculation facility. This not only created hazardous carbon dust, but since usage was as high as 17 bags a day during the summer season, it required extensive manual labour and placed operators working alone on the midnight shift in danger of falling and injuring themselves. Also, the bags would occasionally break, creating more work and requiring operators to hose down the entire area.
At both facilities, installation of an automated bulk bag unloading and feeding system eliminated dust caused by opening, discharging, collapsing and removing individual small bags. This not only increased operator safety, but also reduced manual labour.
Swift Current now buys 500kg bags and unloads one to 3.5 bags a week, while Lafayette uses 400kg bulk bags at a rate of one a week and up to 4.5 a week during the peak summer months.
The Flexicon bulk bag dischargers incorporate devices that contain dust and promote complete discharge. A manual Spout-Lock clamp ring is raised pneumatically by a Tele-Tube telescoping tube, allowing an operator to make a high-integrity, dust-tight connection with the bag spout.
The telescoping tube is then released to exert continuous downward tension on the clamp ring and, in turn the bag spout, which elongates the bag as it empties to promote complete discharge.
Flow is additionally promoted by Flow Flexer pneumatically actuated plates that raise and lower opposite bottom edges of the bag, causing the activated carbon to flow into and through the bag spout.
A Power Cincher flow control valve encircles the upper portion of the bag spout, allowing gradual discharge once the drawstring is untied, as well as retying of partially empty bags. A Bag-Vac dust collector system is mounted on the side of both unloading frames, drawing displaced air and dust from a hopper vent during discharge and collapses empty bags dust-free, ready for tie-off and removal.
At Swift Current, a hoist from a 7.5m high ceiling-mounted monorail loads a 500kg bag on to a Half-frame, bulk bag discharger mounted on a 2m high platform.
The outlet of the bulk bag discharger is connected to a frame-mounted hopper, holding up to 1,000kg of material (two bags), which is in turn connected to a twin-screw volumetric feeder; whose outlet is connected to a wash-down hopper and liquid slurry eductor. The powdered activated carbon flows from the bulk bag to the hopper and is fed to the wash-down hopper and eductor which blends it with the 95l/min water streaming toward the low well.
A control panel regulates the dust collector, the bag activators, feeder, and flow-promoting air pads on the side of the hopper.
At the Lafayette site, an operator, with the aid of a forklift truck, loads a 400kg bag attached to a lifting frame onto a full-height bulk bag discharger. The powdered carbon flows through the telescoping tube and into down-spouting which extends through the floor to a 140l capacity receiving hopper mounted in a frame which also supports the twin screw volumetric feeder, wash-down hopper and liquid slurry eductor. Controls govern the dust collector, feeder and air fluidisers for hopper flow and notify the operator to change empty bags. Continuous blending of powdered activated carbon from the volumetric feeder with fresh water creates a slurry, consisting of as little as 0.006% solids in the winter to as much as 0.05% solids in the summer.
It is added to the clarifier at a rate of 450l/min, resulting in a final carbon concentration of 5 to 7ppm.
Swift Current’s system was designed by a local consultant in Regina, SK, with the help of Mequipco, a local manufacturer’s representative, equipment supplier and systems integrator based in Calgary, Alberta. The Lafayette system, however, was designed and installed by Flexicon, supported by Process Control Equipment Co. Lafayette evaluated two other similar machines in addition to the Flexicon system. It found that Flexicon could tailor designs and scope.
Several commissioning modifications were made after the system was installed. Additional flow promotion was easily retro-fitted to stop the wash down hopper from clogging.
Jets with plastic nipples that circulated the water around the hopper instead of just spraying into it, eliminated the clogging.