Vortex System Proves Effective Mixing Biosolids In Storage Tanks

The old work maxim — “If it ain’t broke,
don’t fix it” doesn’t always apply in the real world.

Take the case of the Springfield Metro Sanitary District
(SMSD), in Springfield, Illinois. Beginning in 1973, the SMSD sprayed
accumulated biosolids onto crops in an ongoing land application program. It
worked well until 1993 when the U.S. Environmental Protection Agency created
new regulations regarding nitrate levels. If the SMSD wanted to continue its
program, something would have to change.

To get into compliance, the SMSD chose a design that
actually improved the process. The major modifications — Aquastore tanks
coupled with the JetMix Vortex Mixing System — proved more energy
efficient, nearly maintenance-free and, above all, effective.

In moving from problem to solution, overlapping concerns had
to be resolved. The compliance issues led to upgrade issues and ultimately to
mixing issues.

Specifically, the SMSD, a wastewater treatment facility that serves Springfield, Illinois and surrounding communities, needed to meet the pathogen and vector-attraction reduction requirements and prevent nitrate levels from contaminating groundwater. After careful study, the SMSD selected a process that would allow it to store digested biosolids that accumulated when the sludge could not be applied.

What is the best way to mix biosolids that have settled over
long periods of time? The district found a workable solution in the JetMix
Vortex Mixing System.

“Basically, the tanks were installed to store the sludge
during periods when it couldn’t be applied, winter time and harvest time,
for instance,” explains William H. Peffley, an environmental engineer
with Crawford, Murphy & Tilly, Inc., the consulting engineers contracted by
the SMSD to design the upgrades. “Then, the mixing system would resuspend
the stored sludge for a relatively short period just prior to
application.”

Not only were these modifications effective, they kept the
SMDS within its project budget. Originally, the district estimated that
compliance would cost about $3.5 million. The eventual solution cut that figure
in half. Also, the storage-and-mixing solution allowed the SMSD to meet
regulations with little impact on its disposal program.

“The alternatives would have entailed a lot of
expensive construction, but the district chose a method that saved a
substantial amount of money,” said Peffley.

SMSD operates two domestic wastewater treatment plants
— Spring Creek and Sugar Creek. Upgrades at the Sugar Creek aerobic
facility included two aboveground storage tanks, three new mixing pumps and two
relocated biosolids farm pumps. At the Spring Creek facility, the new equipment
included two storage tanks, two new mixing pumps and two new biosolids farm
pumps.

The choice of storage tanks seemed the easy part. Choosing
an appropriate mixing system was a more complex issue.

“The sludge settles when you store it for a long
time,” said Peffley. “You have to get it stirred up to be able to
pump it out and apply it to the ground.”

Settled sludge results in accumulation of solids,
“dead areas” in the center of tanks, or chunks of sludge that rise
to the surface. These problems make pumping the sludge for land application
more difficult. Adequate mixing would be crucial.

The trouble with most mixing systems is that they vary in
effectiveness. Continuous mixing — the traditional approach — is
expensive, requiring a lot of energy and maintenance. Options such as fixed
liquid jets and pivoting propeller mixers are only partially effective in mixing the stored contents.

The other, more intriguing option was infrequent mixing. The
process uses far less energy as the biosolids need only be mixed several days
before application. However, while intermittent mixing has definite advantages,
it poses a major problem: obtaining a homogenous mixture often is
difficult—and a homogenous mix is necessary for the best land
application.

Still, the district was attracted to the energy savings.
After careful evaluation, SMSD opted for the JetMix system as a solution to the
mixing problem.

The system, which includes rotatable nozzles, was developed
especially to handle the kinds of problems associated with long-term sludge
storage and sludge disposal. It sweeps all areas of a storage tank by creating
a unique torroidal flow pattern. Essentially, its floor-mounted nozzles combine
with the action of chopper pumps to keep solids suspended or to resuspend
solids that have settled, resulting in the desired homogenous mix.

The 
system’s floor nozzles and top nozzles can be adjusted to tackle
unique mixing problems. The nozzles are normally aimed in the same circular
pattern, generally perpendicular to the radius of the tank on which the nozzle
is located. If unusual conditions develop in one sector of the tank, the
nozzles can easily be redirected by the operator to clear up the situation and
then returned to the original position. The top nozzle is a unique item that
can be rotated around its horizontal axis as well as its vertical axis. Thus an
operator can use it to wash down walls, if desired.

The system is relatively easy to operate. The nozzle
arrangement creates a good rotation of the tank contents to resuspend and keep
solids in suspension. The need for nozzle adjustment is not generally required
on a normal basis for most applications. Even after the tank sits idle for
several months, the solids can be resuspended without involved operator
attention. Maintenance is simplified because only the external pump requires
periodic service.

“Let’s say that the tank contents have settled
for quite a while and you’ve got the solids on the bottom and the
supernatant on top,” says Peffley. “The way the system is designed,
you can draw the liquid down and then pump it back. You can rotate the nozzles
to different angles and directions, which gives you the ability to resuspend
all of those solids in a very short amount of time.”

By using two chopper pumps, the SMSD can resuspend solids at
each of its plants in 36 to 48 hours.

“A chopping impeller shears and breaks up the big
chunks of solids and sends them back as smaller chunks,” Peffley
explained. “So, it is a combination of the flow and the chopper
pumps.”

The energy savings using a JetMix system are substantial.
The SMSD estimates that it saves $170,000, or approximately 90 percent of the
power costs otherwise required during a three-month storage period at the two
facilities. Annually, that adds up to $680,000.

The new equipment at both plants allowed SDMS to store
biosolids in the digesters or in drying beds during times when land application
is prohibited. More expensive options, such as buying additional land, would
have cost as much as $7 million.

Also, the district realizes significant savings on
maintenance.

“The JetMix system is a very low-maintenance
option,” said Peffley. “Essentially, it is self-cleaning.”

When normal care is taken, the SMSD tanks can be pumped
empty of biosolids, and the storage-tank nozzles can also be used to clean the
tank through a combination of spraying patterns.

Mark J. Crump is chief engineer at Liquid Dynamics Corp. For further information, phone Liquid Dynamics Corp. at 630-513-8366.

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