Virginia Administrative Code › Title 9 - ENVIRONMENT › Agency 25 - STATE WATER CONTROL BOARD › Chapter 790 - VIRGINIA POLLUTANT DISCHARGE ELIMINATION SYSTEM (VPDES) PERMIT REGULATION › Part III - Manual of Practice for Sewerage Systems and Treatment Works › Article 5 - Sludge Processing and Management › Section 9VAC25-790-550 - Anaerobic digestion

Current through Register Vol. 40, No. 24, July 15, 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.