(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...