| (a) Design Requirements. The design requirements for
the stabilization processes in this section are based on only one
stabilization process being used at the wastewater treatment facility.
(b) Variance. An owner must request a variance in accordance
with §217.4 of this title (relating to Variances) if a design
includes a series of two or more stabilization processes. Any deviations
from the requirements of this section must be documented in the variance
request.
(c) Anaerobic Digestion.
(1) A wastewater treatment facility with a design flow
exceeding 0.4 million gallons per day must have at least two anaerobic
digesters.
(2) Anaerobic digesters may be operated in series or
in parallel. Each anaerobic digester may be used for treating sludge
from both primary and secondary clarifiers.
(3) Each anaerobic digester must have a means for transferring
a portion of its contents to another anaerobic digester.
(4) A wastewater treatment facility that has been granted
a variance to operate without multiple anaerobic digesters must have
an emergency storage basin for storing sludge during times when the
anaerobic digester needs to be taken out of service.
(d) Depth. An anaerobic digester must provide a minimum
of 6.0 feet of storage depth for supernatant liquor.
(e) Maintenance Provisions. A digester design must
allow access to each unit for maintenance.
(f) Digester Configuration.
(1) The bottom of a digester must slope towards a drainpipe.
(2) A flat-bottomed digestion chamber is prohibited.
(g) Access Manholes.
(1) The top of a digester must have at least two access
manholes and a gas dome.
(2) One manhole must have a sufficient diameter to
permit the use of mechanical equipment to remove grit and sand.
(3) A digester system must have a separate sidewall
manhole at ground level.
(h) Safety.
(1) The wastewater treatment facility operation and
maintenance manual must require the use of non-sparking tools, rubber
soled shoes, a safety harness, and gas detectors for flammable and
toxic gases when working in a digester.
(2) At least one self-contained breathing apparatus
must be maintained in operational condition and kept on site.
(i) Sludge Inlets and Outlets. To facilitate effective
mixing of the digester contents a digester must have:
(1) multiple sludge inlets located to prevent short-circuiting
and at least one inlet located in the center of the digester above
the liquid level at design flow;
(2) at least three recirculation sections; and
(3) at least three outlets.
(j) Digester Capacity.
(1) The digester capacity must be calculated using
the expected volume and character of the sludge. The engineering report
must include the calculations used to justify the design.
(2) The total digester volume must be based upon:
(A) the volume of sludge added;
(B) the percent solids and character of the sludge;
(C) the temperature to be maintained in the digester;
(D) the degree or extent of mixing to be obtained;
and
(E) the size of the installation with appropriate allowance
for sludge and supernatant storage.
(3) A digester must be able to maintain a minimum sludge
digestion temperature of 35 degrees Celsius, plus or minus 4 degrees.
(4) Sludge that will be disposed of in a landfill must
undergo at least 15 days of digestion for stabilization in the primary
digester. Sludge that will be land-applied must undergo at least 60
days of digestion for stabilization, or the period required to achieve
the necessary level of pathogen control and vector attraction reduction
as required by Chapter 312, Subchapter D of this title (relating to
Pathogen and Vector Attraction Reduction), whichever is less.
(5) A Completely Mixed System.
(A) A digester must have an average feed loading rate
of less than 200 pounds of volatile solids per 1,000 cubic feet of
volume per day in the active digestion volume.
(B) Complete mixing in 30 minutes or less is required
for:
(i) a confined mixing system, if gas or sludge flow
is directed through a vertical channel;
(ii) a mechanical stirring or pumping system; and
(iii) an unconfined continuously discharging gas mixing
system.
(C) A digester tank over 60 feet in diameter must have
multiple mixing devices.
(D) The minimum gas flow supplied for complete mixing
must be 15 cubic feet per minute per 1,000 cubic feet of digestion
volume.
(E) A complete mixing system must have a flow-measuring
device and a throttling valve.
(F) The minimum power supply for a mixing system is
0.5 horsepower per 1,000 cubic feet of digestion volume.
(6) Moderately Mixed Systems.
(A) A digestion system where mixing is accomplished
only by circulating sludge through an external heat exchanger must
be loaded at less than 40 pounds of volatile solids per 1,000 cubic
feet of volume per day in the active digestion volume. A design must
be based on the volatile solids loading in accordance with the degree
of mixing.
(B) The engineering report must include a justification
for the loading rates, if mixing is accomplished by another method.
(k) Gas Collection, Pipes, Storage, and Appurtenances.
(1) General Requirements. Each portion of a gas system
must maintain positive gas pressure under all normal operating conditions,
including sludge withdrawal.
(2) Safety Equipment.
(A) A gas system must include a pressure valve, vacuum
relief valve, a flame trap, and an automatic safety shut-off valve.
(B) Installation of water seal equipment on a gas pipe
is prohibited.
(3) Gas Pipes and Condensate.
(A) A gas pipe system must be designed for the volume
of gas expected.
(B) A gas pipe must be pressure tested for leakage
at 1.5 times the design pressure before a digester is placed into
service.
(C) A gas pipe must slope at least 1/8 inch per foot
to drain condensate.
(D) The main gas pipe from a digester must have a sediment
trap and a drip trap.
(E) Float controlled condensate traps are prohibited.
(F) A condensation trap must be accessible for daily
servicing and draining.
(G) A drip trap must be located at each low point in
the pipes.
(H) A gas pipe to each gas outlet must have a flame
check or a flame trap.
(I) A burner pilot must use natural or bottled gas.
(J) Each main gas pipe must have a flame trap with
a fusible shut-off.
(K) A gas pipe to a waste gas burner must have a pressure
valve and a vacuum relief valve.
(4) Electrical Fixtures and Equipment. The electrical
equipment near sludge digester pipes containing gas must be designed
to prevent potentially explosive conditions.
(l) Waste gas.
(1) A waste gas burner must be accessible for inspection
and maintenance and must be located at least 50 feet away from any
structure, if placed at ground level.
(2) A waste gas burner may be located on the roof of
the control building.
(3) A waste gas burner must not be located on top of
a digester.
(4) A discharge of less than 100 cubic feet per hour
of digester gas through a return bend screened vent with a flame trap
terminating at least 10 feet above a walking surface is allowed.
(m) Ventilation.
(1) An underground enclosure connected to an anaerobic
digester tank, gas pipe, or sludge equipment must have forced ventilation
in accordance §217.246 of this title (relating to Ventilation
and Odor Control).
(2) An underground enclosure must have a tight-fitting,
self-closing door to minimize the spread of gas.
(n) Gas Meter.
(1) An anaerobic digester system must have a gas meter
to measure total gas production.
(2) A meter must have a bypass.
(o) Manometer.
(1) A gas manometer must have a tight shut-off vent
and vent cock.
(2) A vent pipe must be extended from a manometer to
the outside of the building.
(3) A vent pipe opening must have a screen and be designed
to prevent the entrance of rainwater.
Cont'd... |
