October 2013, Vol. 25, No.10

Problem solvers

Automated PAC system improves safety and efficiency  

Problem : Manual powdered activated carbon (PAC) distribution proved difficult, time-consuming, and hazardous.   

Solution : Installation of automated system reduces work and improves performance. 

Consolidated Mutual Water Co. (Lakewood, Colo.) serves a sprawling suburban area west of Denver. The company’s Maple Grove Water Treatment Plant was upgraded in 2009 to produce between 56,775 and 68,130 m3/d (15 and 18 mgd).

The facility’s treatment process requires the addition of powdered activated carbon (PAC) to adsorb and remove dissolved organic contaminants that could cause unwanted tastes and odors. The upgraded facility included a PAC system that required manual dumping. 

With the manual system, operators had to carry 20-kg (45-lb) bags up a flight of stairs and manually dump the contents into a storage hopper above the feeder. “It was messy, potentially dangerous, and prone to generate dust,” said Chuck Conway, Maple Grove Water Treatment Plant operator. “The problem became especially severe during periods of peak consumption, when we might have to add as many as 20 bags of PAC per day.” 

Because the facility is located in a dry area that receives an average of only 381 mm (15 in.) of precipitation per year, irrigation increases dramatically during the summer. “The same amount of water is treated and consumed during June through August as during the other 9 months of the year combined,” Conway said. “During periods of low demand, one bulk bag may last several days. During peak periods, however, two or three bulk bags per day may be needed.” 

The amount of PAC added also varies according to the types of organisms producing the unwanted tastes and odors. Some require only a 4- to 5-mg/L dose for effective control, while others may require 30 to 40 mg/L.  

With the extreme variations in PAC application and difficulties of manual application, the company decided to look for a new PAC-distribution system. A different bulk-bag unloading and feeder system was considered but rejected because it loosened material from bags with an air-injection system. Another water company in the state recommended its system, which uses mechanical plates that raise and lower the bag to promote flow while the material remains completely enclosed during transfer, said Chris Jones, water facility manager at Consolidated Mutual Water. 

The company decided to take the recommendation and installed the Flexicon Corp. (Bethlehem, Pa.) Bulk-Out® BFC-model bulk-bag discharger. The system consists of a rigid frame with a cantilevered I-beam and an electric hoist and trolley. The operators now place the bulk bag on the floor from a pallet truck and activate the electric hoist to insert the bag onto the discharger frame. With the bag secured on the discharger, the operator pulls the bag spout — positioned directly above a twin-screw volumetric feeder — through a Power-Cincher® flow-control valve that cinches the bag spout. 

Below the flow-control valve is a manual Spout-Lock® clamp ring that is raised by a Tele-Tube® telescoping tube for a dust-tight connection. The operator then unties the drawstring and opens the flow-control valve to initiate flow. The telescoping tube then lowers, applying continual downward tension to keep the spout taut as the bag empties and elongates. At the same time, Flow-Flexer® bag-activators raise and lower the bottom edges of the bag, directing material into the outlet spout and raising the bag into a steep “V” shape to promote total discharge of contents into the volumetric feeder, Conway explained.  

“The flow-control valve remains open the entire time the bag is emptying, with no other valves or controls modulating the flow into the feeder,” Conway said. “We have an in-line scale between the bag and the hoist with a remote readout that allows the operator to ascertain how much PAC remains in the bag to anticipate bag changes. Eventually, we plan to run a signal-transmission line into the plant so we can take this reading in the operations control center.” 

The feeder meters PAC into a two-stage tank, where it is mixed with water to form the slurry that feeds the treatment process. “We are currently pumping the slurry at a rate of 15 gal [56.8 L] per minute, but this rate is being evaluated and may change,” Conway said. 

Because PAC is a fine powder, the slightest breeze can spread carbon dust everywhere. By automating the bag-discharge process, the company has been able to eliminate dust and operator safety problems while increasing efficiency by freeing operators to perform more-productive tasks, Conway said. 

“We now receive PAC in [408-kg] 900-lb bulk bags that we unload [onto] the bulk-bag discharger so our operators no longer need to carry them,” Conway said. “A secure seal between the bulk bag and the feeder almost completely eliminates dust.”  

At a treatment rate of 30 mg/L (30 ppm), the facility requires up to 1700 kg (3750 lb) of PAC daily, while the higher volume will require about 2041 kg (4500 lb). “As the population of our service area continues to increase, we are projecting as many as five bulk bags per day eventually, but our bulk-transfer system will be able to handle that easily,” Conway said. 

“By discontinuing manual dumping of bags and instead unloading [408-kg] 900-lb bulk bags, we cut manual effort by at least 2 to 3 hours per day,” Jones said. “In labor savings alone, we are saving more than $200 to $300 per day — plus additional savings in receiving PAC in bulk bags, rather than small bags.”