Automated handling of activated carbon in bulk bags

Two cities, one in the United States and the other in Canada, over 1100 km apart, were experiencing nearly identical problems, caused by manual addition of powdered activated carbon to their municipal water treatment systems. Both cities not only eliminated a health hazard caused by carbon powder dust, but also reduced the amount of labour required to add powdered activated carbon to the system, by installing a bulk bag unloading system.

The city of Swift Current draws its municipal water supply from a reservoir formed by a dam on the Swift Current Creek at an average rate of 6.8 million litres a day with a peak rate of 25 million litres a day. Potassium permanganate is added at a concentration of 1.5 parts per million (ppm), then powdered activated carbon is added in the form of a slurry consisting of 0.13% to 0.47% solids, and the water is gravity fed into a low well at a rate of approximately 114 litres per minute. Reaching a final concentration of 20 ppm, the carbon’s function is to adsorb organic materials, eliminating unwanted tastes and odours.

After addition of liquid alum, sludge conditioners and polymers in the clarifier, filtered water is pumped to a clear well located at the water treatment facility with a capacity of 820,000 litres and to two reservoirs within the city itself, each holding 6.8 million litres, for distribution to residents of the city of Swift Current. The city of Lafayette draws its water from two local reservoirs formed by dams on the Boulder Creek and from one reservoir on the South Boulder Creek. Consumption is approximately 7.5 million litres per day during the winter, but can reach as much as 38 million litres per day during the summer months. From the reservoirs, water flows 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.

Figure 1: City of Lafayette - bulk bag loading.

After final filtration, the water is pumped into three storage tanks, which together can hold up to 53 million litres of water, for distribution to residents of the city of Lafayette.

Powdered activated carbon was added manually

Prior to the installation of the bulk bag unloading systems, operators at both facilities were emptying 16-20 kg 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 20 microns and a bulk density of 34.4 kg/m³. “The slightest air movement causes carbon dust everywhere, and it cakes and smears on everything it touches, including your clothes and your skin,” says Rudy Holland, city of Swift Current Superintendent of Water Treatment.

“We tried using a dust collection system, but it never worked properly,” says Holland. Also, the capacity of the system was limited by the small screw feeder, which took 16 hours to transfer the powdered carbon from the 450 kg capacity hopper into the liquid slurry eductor. Since the water treatment facility operates only one shift a day - staffed for 8½ hours, then unmanned and controlled by computer for 15½ hours - the hopper, if not completely filled at the beginning of the unmanned shift, could potentially run out of carbon when personnel were not present to refill it.” City of Swift Current operators were opening and emptying as many as twenty-five 23-25 kg bags of powdered activated carbon a day into the hopper. City of Lafayette operators were carrying a 16-18 kg bag of powdered carbon up a set of metal stairs and emptying it directly into the aeration and flocculation facility. “This not only created hazardous carbon dust, but, since we used about two to four bags a day during the winter and as many as 10 to 17 bags a day during the summer, it also required extensive manual labour and placed operators working alone on the midnight shift in danger of falling and injuring themselves,” explains Ed Zimbleman, Lead Operator. “In addition, bags would occasionally break, requiring operators to hose down the entire area.”

Figure 2: City of Swift Current - bulk bag discharger.

Bulk bag unloading and feeding systems eliminate hazards

At both facilities, installation of an automated bulk bag unloading and feeding system eliminated dust caused by untying, discharging, collapsing and removing individual small bags, while increasing operator safety and reducing manual labour.

The city of Swift Current now purchases 500 kg bags and unloads 1-3½ bags per week. The city of Lafayette unloads between one 408 kg bulk bag per week during the winter and as many as 4½ bags per 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.

Mounted on the side of both unloading frames is a Bag-Vac dust collector that draws displaced air and dust from a hopper vent during discharge, and collapses empty bags dust free, ready for tie-off and removal.

Two systems have important differences

At the city of Swift Current, a hoist from a 7 m high ceiling-mounted monorail loads a 500 kg bag onto a 1.5 m high half-frame bulk bag discharger mounted on a 2 m high platform. The discharger frame is integrated with a 3 m high frame that holds a 900 kg capacity hopper (for the contents of two bags), a twin-screw volumetric feeder, washdown hopper and liquid slurry eductor. The powdered activated carbon flows from the bulk bag to the hopper and then to the feeder which meters it into the washdown hopper and eductor to blend with the 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 city of Lafayette an operator forklift loads a 114 kg bag in a lifting frame onto a full-height 3 m high discharger. The powdered carbon flows through the telescoping tube and a downspouting which extends through the floor to a receiving hopper contained in a 2.1 m high frame which also holds the twin-screw volumetric feeder, washdown 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 450 litres per minute, resulting in a final carbon concentration of 5 to 7 ppm. “We basically just set it and forget it,” says Zimbleman.