Current through Register Vol. 41, No. 3, September 23, 2024
A.
Sludge thickening to decrease the liquid fraction should be considered for
volume reduction and conditioning of sludges prior to treatment and management.
Biological sludges returned to reactors should be thickened to provide for
effective control of biomass. Prior to conventional treatment of biosolids,
thickening should be provided to reduce volume and to condition the raw sludge
flow.
B. General design. Thickener
design shall provide adequate capacity to meet peak demands. Thickeners should
be designed to prevent septicity during the thickening process.
1. A sludge handling bypass around the
thickening process is required. Dual units or alternate storage is required for
all treatment works of greater than 1 mgd capacity.
2. Thickeners shall be provided with a means
of continuous return of supernatant for treatment. Provisions for side-stream
treatment of supernatant should be considered.
3. Consideration should be given to any
potential treatment advantages obtained from the blending of sludges from
various treatment processes.
4.
Odor control shall be addressed with consideration being given to flexibility
of operations and changes of influent sludge characteristics.
C. Gravity systems. Clarifiers or
gravity thickeners sufficiently sized for clarification will provide for
thickening. However, the use of mechanical stirring devices will significantly
improve the performance of gravity thickeners. Mechanical thickeners employ low
speed stirring mechanisms for continuous mixing and flocculation within the
zone of sludge concentration. In this manner, liquid separation is enhanced.
1. Conventional overflow rates for gravity
thickeners should be in the 400-800 gpd per square foot range. The engineer
shall provide the basis and calculations for the nonconventional surface
loading rates. The side water depth of conventional gravity thickeners shall be
a minimum of 10 feet. Circular thickeners shall have a minimum bottom slope of
1-1/2 inches per radial foot.
2. A
gravity sludge thickener shall be so designed as to provide for sludge storage,
if sufficient storage is unavailable within other external tankage. Sludge
withdrawal from gravity thickeners should be controlled and adjusted, and
variable speed pumps should be provided.
3. Gravity thickeners should be provided with
bottom scraping equipment to enhance sludge removal. The scraper mechanism
peripheral velocity should be in the 15 to 20 feet per minute range.
a. The scraper mechanical train shall be
capable of withstanding extra heavy torque loads. The normal working torque
load shall not exceed 10% of the rated torque load.
b. A method to correct blockage of the
scraper mechanism and restore operation from a stalled position should be
provided in accordance with the Operation and Maintenance Manual.
4. Alternative designs should be
based on data obtained from a pilot plant (relatively small scale test
equipment) program. Chemical addition and dilution water feed systems should be
evaluated for use to optimize performance.
D. Dissolved air flotation. Dissolved air
flotation (DAF) basins shall be equipped with bottom scrapers to remove settled
solids and surface skimmers to remove the float established through release of
pressurized air into the sludge inflow. The bottom scraper should function
independently of the surface skimmer mechanism. Dissolved air flotation units
should be enclosed in a building. A positive air ventilation system and odor
control shall be provided.
1. Conventional
design parameters include:
a. Maximum
hydraulic loading rates of 2.0 gallons per minute per square foot of surface
area (gal/min/sq. ft.).
b. A solids
loading rate in the range of 0.4 to 1.0 pounds per hour per square foot of
surface area (lb/hr/sq. ft.) without chemical addition. A solids loading rate
of up to 2.5 lbs./hr./sq. ft. may be used if appropriate chemical addition is
provided (9VAC25-790-860) .
c. An air supply to sludge solids weight
ratio in the range of 0.02 to 0.04.
2. The recycle ratio should be in the 30% to
150% range. The recycle pressurization system should utilize DAF effluent or
secondary effluent if use of potable water is not available. The retention tank
system shall provide a minimum pressure of 40 psig.
3. A polymer feed system shall be provided.
The feed system shall meet the requirements of this chapter.
4. Alternative design should be based on data
obtained from a pilot plant test program if sufficient operational performance
data is not available.
5. Skimmer
design shall be multiple or variable speed such as to allow normal operation in
the less than one fpm range, with the capability of a speed increase to 25
fpm.
E. Mechanical
separation. Filters or centrifuge can be used to thicken sludges. The process
shall be preceded by pretreatment to remove material that can plug the media,
nozzles or cause excessive wear.
1. Provisions
for the addition of appropriate coagulants to the sludge inflow to the filter
or centrifuge shall be considered.
2. The design basis and calculations for
nonconventional loading rates shall be submitted for evaluation.
3. Filtrate or centrate shall be returned to
the head of the primary units, aeration basins, or a separate side-stream
treatment system.
Statutory Authority
§ 62.1-44.15 of the Code of Virginia.
