Current through Register Vol. 41, No. 3, September 23, 2024
A. The
design of anaerobic digesters should provide an optimum environment for
effective microbial degradation of the organic matter in sewage sludge. The
digester system design shall address separation and removal of liquid or
supernatant. The production of methane gas (CH4) should
be optimized. Digester gas should be utilized as a fuel whenever
practical.
B. Design. A minimum of
two anaerobic digesters, or enclosed reactors, shall be provided, so that each
digester may be used as a first stage or primary reactor for treating primary
and secondary sludge flows generated at a treatment works with a design flow
exceeding 0.5 mgd. Additional digesters are provided to treat the total flow of
primary and secondary sludge generated at treatment works with sewage design
flows exceeding one MGD.
1. Where multiple
digesters are not provided, a storage facility or adequate available sludge
processing system shall be provided for emergency use so that the digester may
be taken out of service without unduly interrupting treatment works
operation.
2. Each digester should
have the means for transferring a portion of its contents to other digesters.
Multiple digester facilities should have means of returning supernatant from
the settling digester unit to appropriate points for treatment.
3. Provisions for side-stream treatment of
supernatant shall be included when the supernatant load is not included in the
treatment works design.
4. Multiple
sludge inlets and draw-offs and multiple recirculation section and discharge
points (minimum of three) to facilitate flexible operation and effective mixing
of the digester contents to optimize treatment for pathogen control and vector
attraction reduction shall be provided. One inlet shall discharge above the
liquid level and be located at approximately the center of the digester to
assist in scum breakup. Raw sludge inlets should be so located as to minimize
short circuiting between the inlets and either the supernatant draw-off, or
sludge withdrawal points.
5. The
proportion of depth to diameter should provide for a minimum of six feet
storage depth for supernatant liquor, or the proportion of total volume
allocated for supernatant should be 10% or more.
6. The digester bottom shall slope to drain
toward the withdrawal pipe. At least one access manholes shall be provided in
the top of the digester in addition to the gas dome. One opening shall be large
enough to permit the use of mechanical equipment to remove grit and sand. A
separate side wall manhole shall be provided at the basin floor level.
a. To facilitate emptying, cleaning, and
maintenance, the digester design shall provide for access and safety
features.
b. In accordance with
VOSH requirements and these regulations, the operation and maintenance manual
should specify: nonsparking tools, rubber soled shoes, safety harness, gas
detectors for inflammable and toxic gases, and at least one self contained
breathing apparatus.
C. Loadings. Where the composition of the
sewage has been established, digester capacity shall be computed from the
volume and character of sludge to be digested. The total digestion volume shall
be determined by rational calculations based upon such factors as volume of
sludge added, its percent solids and character, the temperature to be
maintained in the digesters, the degree or extent of mixing to be obtained,
expected formation of inactive deposits, and the size of the installation with
appropriate allowance for sludge and supernatant storage. These detailed
calculations shall be submitted to justify the basis of design.
1. The design average detention time for
sludge undergoing digestion for stabilization shall be a minimum of 15 days
within the primary digester, but longer periods may be required to achieve the
levels of pathogen control and vector attraction reduction necessary for the
method used for sludge management.
2. The digester shall be capable of
maintaining a minimum average sludge digestion temperature of 35°C
(95°F) with the capability of maintaining temperature control within a
4°(+/-)C range.
3. If unheated
digesters are utilized, they shall have the capacity to provide a minimum
detention time of 60 days within the digestion volume in which sludge is
maintained at a temperature of at least 20°C (68°F).
4. For digestion systems where mixing is
accomplished only by circulating sludge through an external heat exchanger, the
system shall be loaded at less than 40 pounds of volatile solids per 1,000
cubic feet of volume per day or at a volumetric rate that provides not less
than a 30 day detention time in the active digestion volume. The design
volatile solids loading should be established in accordance with the degree of
mixing provided.
5. Where mixing is
accomplished by other methods, loading rates shall be determined on the basis
of information furnished by the design consultant.
D. Completely mixed systems. For digesters
providing for intimate and effective mixing of the digestion volume contents,
the systems shall be designed for an average feed loading rate of less than 200
pounds of volatile solids per 1,000 cubic feet of volume per day or at a
volumetric loading that provides 15 days or more detention time in the active
digestion volume.
1. Confined mixing systems
include (i) arrangements where gas or sludge flows are directed through
vertical channels; and (ii) mechanical stirring, or pumping systems. Both
confined mixing and unconfined continuously discharging gas mixing systems
shall be designed to ensure complete turnover of digestion volume every 30
minutes. For tanks over 60 feet in diameter, multiple mixing devices shall be
used.
2. Unconfined, sequentially
discharging gas mixing systems shall be designed using the number of discharge
points and gas flow rates shown for the various tank diameters as listed in
this section, unless sufficient operating data is submitted and approved to
verify the performance reliability of a alternative designs.
3. Gas discharge lines (lances) mounted on a
floating cover or top designed to accumulate gas emissions shall extend to the
base of the vertical side wall while the cover is resting on its landing
brackets. For floor mounted diffuser boxes or lances mounted to a fixed cover,
gas discharge shall extend to the base of the vertical side wall.
| DESIGN CRITERIA FOR MULTIPLE DISCHARGE
MIXING SYSTEMS, SEQUENTIAL DISCHARGE |
| Maximum Diameter (Ft.) |
| Tank Diameter |
20-30 |
31-40 |
41-50 |
51-60 |
61-70 |
71-80 |
81-90 |
91-100 |
101-110 |
| (Minimum Number of Points) |
| Discharge Points |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
| Minimum Gas Flow |
| Gas Flow (CFM) |
95 |
95 |
95 |
150 |
150 |
150 |
200 |
250 |
300 |
4. The minimum gas flow supplied for complete
mixing shall be 15 cubic feet/min./1000 cubic feet of digestion volume. Flow
measuring devices and throttling valves shall be used to provide the minimum
gas flow.
5. The design power
supplied for mechanical stirring or pumping type complete mixing systems shall
be capable of achieving a minimum of 0.5 horsepower per 1,000 cubic feet of
digestion volume.
6. Where low
speed mechanical mixing devices are specified, more than one device shall be
provided unless other mixing devices are also provided.
E. Gas collection. All portions of the gas
system, including the space above the liquid surface in the digester, storage
facilities and piping shall be so designed that under all normal operating
conditions, including sludge withdrawal, the gas will be maintained under
positive pressure.
1. All enclosed areas where
any gas leakage might occur shall be adequately ventilated.
2. All necessary safety facilities should be
included where gas is produced in accordance with VOSH requirements.
3. Pressure and vacuum relief valves and
flame traps, together with automatic safety shut-off valves, may be
provided.
4. Water seal equipment
shall not be installed on gas piping.
5. Gas piping shall be of adequate diameter
to provide a velocity less than 12 feet per second at a flow of two times the
average rate and shall slope to condensation or drip traps at low
points.
6. The use of float
controlled condensate traps is not permitted. Condensation traps shall be
placed in accessible locations for daily servicing and draining.
7. Electrical fixtures and equipment located
in enclosed places where gas may accumulate will be evaluated in accordance
with the National Board of Fire Underwriters' specifications for hazardous
conditions and other applicable codes and regulations.
8. The electrical equipment provided in
sludge-digester pipe galleries containing gas piping should be designed and
installed to eliminate potential explosive conditions. The design of electrical
equipment located in any location where gas or digested sludge leakage is
possible will be evaluated in accordance with applicable codes and
regulations.
9. Waste gas burners
shall be readily accessible and should be located at least 50 feet away from
any structure, if placed at ground level. Gas burners may be located on the
roof of the control building if sufficiently removed from the digester and gas
storage tank and will comply with all applicable state and federal air
pollution control requirements. Waste gas burners shall not be located on top
of the digester or gas storage tank.
10. In remote locations it may be permissible
to discharge small quantities of digester gas (less than 100 CFH) to the
atmosphere through a return bend screened vent terminating at least 10 feet
above the walking surface, provided the assembly incorporates a flame trap and
is in compliance with all applicable state and federal regulations.
11. A gas meter with bypass shall be provided
to meter total gas production. Gas piping lines for anaerobic digesters shall
be equipped with closed type indicating gauges. These gauges shall read
directly in inches of water. Normally, three gauges will be provided:
(i) one to measure the main line pressure;
(ii) a second to measure the
pressure to gas-utilization equipment; and
(iii) the third to measure pressure to waste
burners. Gas tight shut-off and vent cocks shall be provided. The vent piping
shall be extended outside the building, and the opening shall be screened and
arranged to prevent the entrance of rainwater. All piping of the manometer
system shall be protected with safety equipment.
12. Any underground enclosures connecting
with anaerobic digester tanks or containing sludge or gas piping or equipment
shall be provided with forced ventilation in accordance with VOSH requirements
and this chapter and standards contained in this chapter. Tightly fitting, self
closing doors shall be provided at connecting passageways and tunnels to
minimize the spread of gas.
F. Energy control. If practical, digesters
should be constructed above the water table and should be suitably insulated to
minimize heat loss. The use of digester gas as a heating fuel source is
encouraged.
1. Sludge shall be heated by
circulating the sludge through external heaters unless effective mixing is
provided. Piping shall be designed to provide for the preheating of feed sludge
before introduction to the digesters. Provisions shall be made in the layout of
the piping and valving to facilitate cleaning of these lines. Heat exchanger
sludge piping shall be sized for design heat transfer requirements.
2. Sufficient heating capacity shall be
provided to maintain consistently the design temperature required for sludge
stabilization. For emergency usage, an alternate source of fuel shall be
available and the boiler or other heat source shall be capable of using the
alternate fuel.
3. The heating
system design shall provide for all controls necessary to ensure effective and
safe operation. Facilities for optimizing mixing of the digester contents for
effective heating shall be provided.
4. Sludge heating devices with open flames
should be located above grade in areas separate from locations of gas
production or storage.
G. Supernatant handling. Supernatant
withdrawal piping shall not be less than six inches in diameter, and piping
shall be arranged so that withdrawal can be made from three or more levels in
the tank. A positive, unvalved, vented overflow shall be provided.
1. On fixed cover digesters the supernatant
withdrawal level should preferably be selected by means of interchangeable
extensions at the discharge end of the piping.
2. If a supernatant selector is provided,
provision shall be made for at least two other draw-off levels located in the
supernatant zone of the digester in addition to the unvalved emergency
supernatant draw-off pipe. High pressure backwash facilities shall be
provided.
3. Provisions shall be
made for sampling at each supernatant draw-off level. Sampling pipes shall be
at least 1-1/2 inches in diameter.
4. Management of digester supernatant must be
addressed in the treatment works design. Also, sidestream treatment
alternatives for digester supernatant shall be considered in the preliminary
engineering design.
Statutory Authority
§ 62.1-44.15 of the Code of Virginia.
