Environmental Engineering Reference
In-Depth Information
the drum from 1/8 to 1 rpm. Normally, solids pickup is indirectly related
to the drum speed. The drum is partially submerged in a vat containing
the conditioned sludge. Submergence is usually limited to 1/5 or less of
the filter surface at a time.
10.7.2.1.3 Chemical Conditioning
Sludge dewatered using vacuum filtration is normally chemically
conditioned just prior to filtration. Sludge conditioning increases the
percentage of solids captured by the filter and improves the dewater-
ing characteristics of the sludge; however, conditional sludge must be
filtered as quickly as possible after chemical addition to obtain these
desirable results.
10.7.2.2 Operational Considerations
The rotating drum picks up chemically treated sludge. A vacuum is
applied to the inside of the drum to draw the sludge onto the outside of
the drum cover. This porous outside cover or filter medium allows the
filtrate or liquid to pass through into the drum and the filter cake (dewa-
tered sludge) to stay on the medium. In the cake release/discharge mode,
slight air pressure is applied to the drum interior. Dewatered solids are
lifted from the medium and scraped off by a scraper blade. Solids drop
onto a conveyor for transport for further treatment or disposal. The fil-
trate water is returned to the plant for treatment. The operator observes
drum speed, sludge pickup, filter cake thickness and appearance, chem-
ical feed rates, sludge depth in vat, and overall equipment operation.
Sampling and testing are routinely performed on influent sludge solids
concentration, filtrate BOD and solids, and sludge cake solids concen-
tration. Several vacuum filter operational problems and causal factors,
along with recommended corrective actions are listed below:
•
symptom 1.
Filtrate has a high level of solids.
Causal factors:
Improper coagulant dosage; filter media binding
Corrective actions:
Adjust coagulant dosage; recalibrate the
coagulant feeder; clean synthetic cloth with steam and deter-
gent; clean steel coil with acid bath; clean cloth with water or
replace cloth.
•
symptom 2.
Filter cake is thin with poor dewatering.
Causal factors:
Filter media binding; improper chemical dosage;
inadequate vacuum; drum speed too high or drum submergence
too low
Corrective actions:
Clean synthetic cloth with steam and deter-
gent; clean steel cloth with acid bath; clean cloth with water
or replace cloth; adjust coagulant dosage; recalibrate coagulant
feeder; repair vacuum system; reduce drum speed; increase
drum submergence.
