(a) If a sludge thickening system is used, the following
criteria are required:
(1) Capacity. The maximum monthly sludge production
rate must be used as the basis for sludge thickening system sizing
and design.
(2) Flexibility.
(A) A sludge thickening system must have a bypass to
the digester.
(B) A wastewater treatment facility with a design flow
greater than 1.0 million gallons per day must have:
(i) at least two sludge thickening units;
(ii) an alternate means of sludge thickening; or
(iii) an alternate sludge disposal method.
(b) Specific Requirements for a Mechanical Gravity
Thickener.
(1) Equipment Features.
(A) A mechanical gravity thickener must have:
(i) a low-speed stirring mechanism for continuous mixing
and flocculation;
(ii) sufficient sludge storage, or sufficient additional
storage in other external tanks; and
(iii) a means of controlling the rate of sludge withdrawal.
(B) A mechanical gravity thickener must use either
a chemical addition or dilution water feed system.
(C) A scraper mechanical train must be capable of withstanding
any expected torque load. The working torque load must not exceed
10% of the manufacturer's recommended torque load.
(2) Design Basis.
(A) The design of a mechanical thickener must be justified
in the engineering report.
(B) The executive director may require data from a
pilot study or similar mechanical gravity thickener operating under
similar conditions.
(C) The mechanical gravity thickener surface loading
rate must be at least 400 gallons per day per square foot, but not
more than 800 gallons per day per square foot.
(D) The minimum side water depth for a mechanical gravity
thickener is 10 feet.
(E) A circular mechanical gravity thickener must have
a minimum bottom slope of 1.5 inches per foot.
(F) The peripheral velocity of a scraper must be at
least 15 feet per minute but no more than 20 feet per minute.
(G) A mechanical gravity thickener must prevent short-circuiting.
(c) Specific Requirements for a Dissolved Air Flotation
(DAF) Basin.
(1) Equipment Features.
(A) A DAF basin must have a bottom scraper that functions
independently of the surface skimmer.
(B) A recycle pressurization system for a DAF basin
must use effluent instead of potable water.
(C) A DAF basin must have a polymer feed system that
meets the requirements of §217.247(r) of this title (relating
to Chemical Pretreatment of Sludge).
(D) A DAF basin must be located in a covered building
with positive air ventilation.
(2) Design Basis.
(A) The design of a DAF basin must be justified in
the engineering report.
(B) The executive director may require data from a
pilot study or similar DAF basin operating under similar conditions.
(C) The hydraulic loading rate must not exceed 2.0
gallons per minute per square foot.
(D) The solids loading rate must be at least 1.0 pound
per hour per square foot, but not more than 4.0 pounds per hour per
square foot.
(E) The air to solids weight ratio must be at least
0.02, but not more than 0.04.
(F) A retention tank system must have a minimum pressure
of 40 pounds per square inch, gauge.
(G) A skimmer must have multiple or variable speeds
that allow an operational range of at least 1.0 foot per minute, but
not more than 25.0 feet per minute.
(d) Specific Requirements for a Centrifugal Thickener.
(1) The design of a centrifugal thickener must be justified
in the engineering report.
(2) The executive director may require data from a
pilot study or data from a similar centrifugal thickener operating
under similar conditions.
(3) A centrifugal thickener must be preceded by pretreatment
to prevent plugging of a nozzle or excessive wear in the bowl.
(4) The centrate is subject to the requirements of §217.242
of this title (relating to Control of Sludge and Supernatant Volumes).
(e) Specific Requirements for a Gravity Belt Thickener.
(1) Equipment Features.
(A) A gravity belt thickener must include a wash water
system capable of providing 60 gallons per minute of flow per meter
of belt width at a pressure of at least 60 pounds per square inch.
Booster pumps may be used.
(B) A gravity belt thickener must include a polymer
feed system that meets the requirements of §217.247(r) of this
title.
(C) A filtrate drainage system must be sized to remove
the full hydraulic capacity of a gravity belt thickener without accumulation
or ponding of filtrate.
(2) Design Basis. Unless otherwise justified in the
engineering report, gravity belt thickener sizing must be based upon:
(A) a maximum solids loading of 1,250 pounds per meter
of belt width; or
(B) a maximum hydraulic loading of 250 gallons per
minute per meter of belt width.
(3) Gravity belt thickener filtrate is subject to the
requirements of §217.242 of this title.
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