Reclaiming A Tradition
Alabama’s rich mining tradition—a history that dates back nearly two centuries—and the economic lift it provides (a direct and indirect value of about $5 billion, according to a recent economic census) have not been without their downsides. Scars left by once highly productive strip mines dot the countryside, providing a harsh contrast to the natural beauty for which the state is known. Efforts to reclaim these sites have been ongoing for some time, but they are getting a boost lately thanks to the efforts of Jefferson County’s Village Creek Wastewater Treatment Plant (WWTP).
The Birmingham plant, which once simply landfilled biosolids from its operation, now converts them to a Class A material used in area strip mine reclamations. Aided by a pair of KSP 50 sludge pumps from Schwing Bioset, Somerset, Wis., that effectively move sludge during one of the final steps in the treatment process, the result has been a win-win for all involved. Land that was once an eyesore is again vibrant and pristine, and a material which, in the past, was viewed as nothing more than a waste product is helping make that rebirth happen.
First built in the early 1900s, Village Creek WWTP has undergone a series of modifications and expansions over the years, the last of which took place in 2001 and essentially reshaped the face of wastewater treatment in the area, according to Plant Manager Terry Lane.
“To bring the plant into compliance with the federal Clean Water Act, we added another full plant to operate alongside the existing one. In addition to that, we constructed a series of 20 surge basins to aid in times of peak volume. Doing all this took the plant from a rated capacity of 60 million gal per day (mgd) to about 420 mgd.”
While the basins have a static capacity of 90 million gal collectively, Lane said, during those peak times (torrential rain during hurricane season is not uncommon) they can process a huge volume through the newly configured plant.
“It only has a rated capacity of 60 mgd, but we’ve pushed 100 mgd through it with no problem. That’s allowed us to stay ahead of downpours that were associated with hurricanes Ivan, Katrina and Rita, and that’s saying something.”
Processing of wastewater at Village Creek is fairly straightforward: Sewage is first run past bar screens to remove coarse debris, then pumped into a grit removal system, then on to a primary settling tank. At that point, primary sludge is sent to anaerobic digesters, then on to dewatering and biosolids treatment. The balance of the liquid stream heads for secondary aeration, additional filtering and either chlorine or ultraviolet (UV) disinfection.
“It’s important to remember that we actually have two separate plants here, both working at the same time, so when the volume of water comes in, it is split,” said Rob Brown, the facility’s maintenance supervisor. “Not surprisingly, we also have two different types of disinfection at this facility: chlorine on the old side of the plant and UV on the new side. Because both plants are active, if the chlorine treatment process should go down for some reason, the effluent can be sent here to be treated using the UV.”
Sludge from the digesters is sent to a series of centrifuges for dewatering, then, at a dewatered rate of between 27% and 29% solids, sent via short collection screw conveyors to a Schwing Bioset 10-ft- diameter, 940-cu-ft sliding frame silo.
“The silo serves as a metering device of sorts,” Brown said. “Using it in that way allows sludge to be more evenly fed, again via screw feeders, into the Schwing sludge pumps.”
The unit at work at Village Creek, Schwing’s KSP 50VHD model, is a piston-style pump that originally was designed for use in concrete pumping projects. It is that demanding application that initially drew plant management to select the pumps to move the dewatered sludge.
“My boss at the time was involved in that decision,” Lane said. “His rationale was simple: He said we had some very dry solids that we had to push a fairly long way—material that would more than likely dry out even more in the lines as it moved. If a pump like that could move concrete, they felt it could move the dewatered sludge.”
To alleviate the issue of dryness, Village Creek staff chose to include Schwing’s pipeline lubrication system. Unlike other systems that inject polymer as the lubricating medium at great expense, this system lubricates the pipeline by injecting a thin film of water 360 degrees around an annular groove. This system imparts pressure reductions of greater than 50% in the pipeline.
“But it was the power of the pumps, more than anything, that won them over,” Lane said.
If laid in a straight line, the distance from the pumps to a pug mill, in which lime is added to the cake before transport, is relatively short—about 50 ft or so. That path, however, ran straight through a heavily trafficked area that would put workers at risk. To avoid that, the line was made to follow the contours of the building instead.
“By doing that, we had to take it up some 20 to 30 ft, have it make a 90-degree turn, snake around and over some storage bins, then route it to the lime machine,” Lane said. “So now we are dealing with about 150 ft of pipe rather than 50 [ft], and the power of those pumps becomes even more important.”
The final step in the process—adding lime to the biosolids—raises pH levels in order to decrease biological activity and reduce pathogen levels. Initially, Village Creek had not planned on needing alkaline stabilization. But, as Lane put it, nature had other plans.
“The original design was intended to be completely thermophilic, which we felt would kill all the bugs, giving us the Class A sludge we needed,” Lane said. “At that point, we hoped to store it in live-bottom hoppers, drop it directly into the trucks and haul it out for land application at the mines. Unfortunately, we were one of several plants nationwide that began seeing fecal regrowth. When our sludge went into the thermofluid digesters, the heat was causing the bugs to encapsulate themselves.”
“Generally, once they build that shell, they never come out again,” Lane continued. “However, after being impacted by a 2,700-rpm centrifuge, many of the bugs were coming alive—and doing so in a food-rich environment, which obviously caused them to multiply. So we now add lime, which kills any remaining bugs and makes the product ready for transport.”
New Lease on Life
When the biosolids are stabilized, they are loaded into dump trucks and hauled to one of several local mines with which the county has contracted to do reclamation. The biosolids are dumped and applied using specialized spreaders. The program has been in place for about 10 years, and in that time Village Creek has been sending a steady stream of about 0.5 million lb of biosolids per week. The results have been impressive, according to Lane.
“These locations have become revegetated to such a degree that we are able to grow hay at one of them and use the hay throughout our county departments,” Lane said. “This is a recycling program in every sense of the word. We are taking a product that was previously being sent to a landfill, finding a new use for it, growing a new product from it and using that new product—the hay—for road projects, erosion control, fill and so on. In the process, we have taken a strip mine that was once a blight on the landscape and made it a beautiful, vegetated, forested area; it is a superb application for it. And, given the number of mills and mines in this area, we can probably continue to do this for a long, long time. I’d say that’s a pretty good deal for everyone.”