(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:
(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.
(6) Moderately Mixed Systems.
(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.
(3) Gas Pipes and Condensate.
(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.
(4) A manometer design must specify all safety devices that are needed for a manometer pipe system and must list the safety items in the engineering report.
(p) Gas Piping. The gas piping for an anaerobic digester must be equipped with gauges that measure the following in inches:
(1) the pressure of the main pipe;
(2) the pressure to gas-utilization equipment; and
(3) the pressure to waste burners.
(q) Digestion Temperature Control.
(1) Passive Temperature Control.
(2) Heating Facilities.
(3) Heating Capacity.
(4) Mixing. A digester system must have equipment to mix the sludge.
(5) Location of a Sludge Heating Device. A sludge heating device with an open flame must be located above grade and in an area separate from gas production and any storage area.
(r) Supernatant Withdrawal.
(1) Pipe Size. The minimum diameter for a supernatant pipe is 6.0 inches.
(2) Withdrawal Arrangements.
(3) Sampling.
(4) Supernatant Handling.