Costly, time-consuming and persistent maintenance problems,
coupled with unacceptable odor levels, prompted a major southwest wastewater
treatment plant to scrap its sludge handling system.
The facility had been using a conveyor to move the sludge
cake from belt filter presses to holding hoppers. Continual spillage of sludge
cake off the conveyor proved an insurmountable problem, especially as the plant
With a rated capacity of 17.5 mgd, the Northwest Wastewater
Treatment Plant (NWTP), owned and operated by El Paso Water Utilities Public
Service Board, El Paso, Texas, currently processes about 9 mgd, serving
primarily a residential user base.
NWTP has invested heavily to ensure that odorous treatment
units are fully enclosed and exhaust air is scrubbed before being discharged
into the atmosphere. The plant's effluent is filtered to reduce turbidity and
is disinfected before discharge into the Rio Grande River, with a portion being
used as reclaimed water in industrial and irrigation applications.
The original 5 mgd NWTP, built in 1986, operated with two
belt filter presses and two short conveyors used to transport the final 18% to
22% biosolids to waiting semi-trailers.
When the NWTP tripled its capacity in 1995, a third filter
press was added, with the existing 20-in. wide conveyor belt extended several
yards to accommodate the additional sludge output.
During the pre-expansion operation, the 15-fpm chain and
sprocket conveyor system, with rectangular scoop-type collecting pans, moved
sludge cake directly to trucks for disposal. When the plant expanded, two 20 cu
ft capacity holding hoppers were added. The extended conveyor then collected
dewatered sludge from all three presses and carried it at a 45° incline to
a central trough, where two new screw conveyors would split flow into the
holding hoppers. The three presses are located 60 ft, 40 ft and 20 ft from the
When the plant operated all three presses, the finished
sludge from each would be deposited onto the main conveyor. But pans became overloaded,
sludge spilling off the conveyor and onto the floor.
This sludge overflow prevented the required amount of sludge
from reaching the holding hoppers. Additionally, noxious odors rose from the spillage,
which often was not cleaned up for eight to ten hours. This could be a serious
problem since the plant is located in a highly odor-sensitive part of the city,
six miles west of downtown El Paso.
Maintaining the conveyor system with its chain-belt mounted
rubber pans proved expensive and labor-intensive. When the conveyor reached the
end of the line, ripped pans would stretch open and allow sludge to fall
through. Operators, washing down the conveyor at day's end, would have to
change ripped pans, a time-consuming task as each pan was attached with eight
nuts and bolts.
The conveyor was literally ripping itself apart and
occasionally jumped the track, because of excess accumulation of spilled sludge
in the sprockets. Operators could take two to three days to repair the
Looking for a permanent solution, NWTP turned to TP Pump
& Pipe Company in Albuquerque, N.M., a regional fluids handling firm. They
supplied three type BTE 17-12 PC pumps manufactured by seepex Inc. in Enon,
Ohio. The 25-gpm capacity sludge pumps have gray cast iron housings, NBR
stators, and Duktil-coated hardened tool steel rotors.
The BTE design can pump up to 35% solids sludge, has an
enlarged rectangular feed hopper with removable compression zone, and a high
volume ribbon-screw auger with enlarged diameter and pitch. The hopper length
was matched to the discharge of the dewatering device.
During operation, the positive displacement pump's single
external helix rotor turns within a double internal helix stator to form progressively
moving cavities, creating the pumping action. The pump's output is directly
proportional to its speed, and its customized stator ensures an identical
compression ratio along the entire length of the rotor/stator interface.
A seepex pump was installed at the discharge of each filter
press. A specially designed tapered stainless steel chute was fabricated to
allow dewatered sludge to gravity feed into each BTE unit's open hopper, with a
20-in. clearance to minimize odor. The three pumps transfer the sludge via a
6-in. pipeline to the central trough feeding the two screw augers.
The pumps were installed one at a time, the farthest (60 ft)
first, to assure each unit's performance.
After the units had been in operation for several months,
the farthest pump began to show signs of wear. To correct this, the plant
installed a 2-in. chemical injection ring to meter polymer into the annulus of
the sludge pipe, using a separate seepex PC metering pump. While the polymer
injection system lowered the system pressure, the wear problem on the most
distant pump persisted.
NWTP resolved this pump abrasion issue in August 2000, by
upsizing the capacity of the farthest two pumps by 25%—a simple matter of
switching out the pumps' rotors, stators, and some fittings. The original PVC
6-in. pipeline was replaced with stainless steel, and the new dewatered sludge
transfer system has operated smoothly and efficiently ever since.
As a result of installing the new seepex PC pumps, the NWTP
has significantly reduced maintenance costs and improved operator morale:
pumps rarely require repair; when they do, two operators can change a unit's
rotor and stator in just two hours, and
new pumps have eliminated the unpleasant, odorous task of spillage cleanup,
very important since El Paso heat can make odors unbearable.
Products In Action: Pumps