Civil Engineering Reference
In-Depth Information
prior to its being fed to the belt press. Free water drains from the conditioned sludge
in the free drainage zone of the press.
The sludge then enters a two-belt contact zone, where a second upper belt is gently
set on the forming sludge cake. The belts with the captured cake between them pass
through rollers of decreasing diameter. This stage subjects the sludge to continuously
increasing pressures and shear forces. Pressure can vary widely by design, with the
sludge in most presses moving from a low-pressure section to a medium-pressure
section. Some presses include a high-pressure section that provides additional dewa-
tering. Progressively, more and more water is expelled throughout the roller section to
the end where the cake is discharged. A scraper blade is often employed for each belt
at the discharge point to remove the cake from the belts. Two spray-wash belt cleaning
stations using high-pressure water are generally provided to keep the belts clean.
Belt press performance is measured by the percent solids of the sludge cake, the
percent solids capture, the solids and hydraulic loading rates, and the required polymer
dosage. Several machine variables, including belt speed, belt tension, and belt type,
influence belt press performance.
Belt speed is an important operational parameter that affects cake solids, polymer
dosage, solids recovery, and hydraulic capacity. Low belt speeds result in drier sludge
cakes. At a given belt speed, increased polymer dosages result in higher cake solids.
With an adequate polymer dose, solids recoveries are improved by lowering belt
speeds. Hydraulic capacity increases at higher belt speeds, but the solids capture is
reduced.
Belt tension has an effect on cake solids, maximum solids loading, and solids
capture. In general, a higher belt tension produces a drier cake but causes a lower
solids capture, at a fixed flow rate and polymer dose. A drawback of using higher
tension is increased belt wear. For sludges with a large quantity of alum sludge, the
belt tension must be reduced to contain the sludge between the belts. The maximum
tension that will not cause sludge losses from the sides of the belts should be used.
Belt type is an important factor in determining overall performance. Most belts are
woven of polyester filaments, and they are available with weaves of varying coarseness
and strength. A belt with one of the coarser and stronger weaves may require high
polymer dosages, to obtain adequate solids capture.
Most manufacturers' belt presses can be equipped with sensing devices that may
be set to automatically shut off the sludge feed flow in case of underconditioning.
Both underconditioned and overconditioned sludges can blind the filter media. In ad-
dition, overconditioned sludge tends to drain so rapidly that solids cannot be evenly
distributed across the belt. Vanes and distribution weirs included in the gravity drainage
section help alleviate the problem of distribution of overconditioned sludge across the
belt, and inclusion of a sludge blending tank before the belt press can also reduce this
problem.
A belt press installation should be designed for daily washdown by hosing around
the press; therefore, drainage and safe walking area around the press are important.
The required flow rate for belt washing water is usually 50 to 100 percent of the
flow rate of sludge to the machine, and the pressure is typically 100 psi (690 kPa) or
more. Some belt presses recirculate washwater from the filtrate collection system, but
usually potable water is used. The combined flow of washwater and filtrate typically
contains between 500 to 2,000 mg / L of suspended solids. To minimize the effect on
finished drinking water quality by recycle streams, the filtrate should be either dis-
charged to a sanitary sewer or treated before recycle.
