1. (a) Design Requirements. The design requirements for the stabilization processes in this section are based on the assumption that the process is the sole stabilization process employed at the facility.
2. (b) Variance. An owner must request a variance in accordance with §217.4 of this title (relating to Variances), if a design employs a series of two or more stabilization processes or methods.

3. (c) Anaerobic Digestion.

   1. (1) A facility with a design flow exceeding 0.4 million gallons per day must have at least two anaerobic digesters.
   2. (2) Each digester may be used as a first stage or primary reactor for treating primary and secondary sludge flows.
   3. (3) Each digester must have a means for transferring a portion of its contents to another digester.
   4. (4) A facility that has been granted a variance to operate without multiple digesters must have an emergency storage basin so the digester may be taken out of service.
4. (d) Depth. An anaerobic digester must provide a minimum of 6.0 feet of storage depth for supernatant liquor.
5. (e) Maintenance Provisions. A design must allow access to each unit for maintenance.

6. (f) Digester Configuration.

   1. (1) The bottom of a digester must slope towards a drainpipe.
   2. (2) A flat-bottomed digestion chamber is prohibited.

7. (g) Access Manholes.

   1. (1) The top of a digester must have at least two access manholes and a gas dome.
   2. (2) One manhole must have a sufficient diameter to permit the use of mechanical equipment to remove grit and sand.
   3. (3) A digester system must have a separate sidewall manhole at ground level.

8. (h) Safety.

   1. (1) The 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. (2) At least one self-contained breathing apparatus must be maintained in operational condition and kept on site.

9. (i) Sludge Inlets and Outlets. To facilitate effective mixing of the digester contents a digester must have:

   1. (1) multiple sludge inlets located to minimize short-circuiting and at least one inlet located in the center of a digester above the liquid level at design flow;
   2. (2) at least three recirculation sections; and
   3. (3) at least three outlets.

10. (j) Digester Capacity.

    1. (1) The digester capacity must be calculated using the expected volume and character of the sludge. The report must include the calculations used to justify the design.

    2. (2) The total digester volume must based upon:

       1. (A) the volume of sludge added;
       2. (B) the percent solids and character of the sludge;
       3. (C) the temperature to be maintained in the digester;
       4. (D) the degree or extent of mixing to be obtained; and
       5. (E) the size of the installation with appropriate allowance for sludge and supernatant storage.
    3. (3) A digester must able maintain a minimum daily average sludge digestion temperature of 35 degrees Celsius (95 degrees Fahrenheit) and maintain the temperature within a 4 degrees Celsius (+/-) range.
    4. (4) The minimum detention time for sludge undergoing digestion for stabilization is 15 days in the primary digester for sludge to be landfilled, 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), if sludge is to be land applied.
    5. (5) An unheated digester must provide a minimum detention time of 60 days and maintain a temperature of at least 20 degrees Celsius (68 degrees Fahrenheit), 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.

    6. (6) A Completely Mixed System.

       1. (A) A digester must have an average feed loading rate of less than 200 pounds (lbs) of volatile solids per 1,000 cubic feet (cf) of volume per day in the active digestion volume.

       2. (B) Complete mixing in 30 minutes or less is required for:

          1. (i) a confined mixing system if gas or sludge flow is directed through a vertical channel;
          2. (ii) a mechanical stirring or pumping system; and
          3. (iii) an unconfined continuously discharging gas mixing system.
       3. (C) A tank over 60 feet in diameter must have multiple mixing devices.
       4. (D) The minimum gas flow supplied for complete mixing must be 15 cubic feet per minute (cfm) per 1,000 cf of digestion volume.
       5. (E) A complete mixing system must have a flow-measuring device and a throttling valve.
       6. (F) The minimum power supply for a mechanical stirring or pumping complete mixing system is 0.5 horsepower per 1,000 cf of digestion volume.

    7. (7) Moderately Mixed Systems.

       1. (A) A digestion system where mixing is accomplished only by circulating sludge through an external heat exchanger must be loaded at less than 40 lbs of volatile solids per 1,000 cf 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.
       2. (B) The report must include a justification for the loading rates, if mixing is accomplished by another method.

11. (k) Gas Collection, Pipes, Storage, and Appurtenances.

    1. (1) General Requirements. Each portion of a gas system must maintain positive gas pressure under all normal operating conditions, including sludge withdrawal.

    2. (2) Safety Equipment.

       1. (A) A gas system must include a pressure valve, vacuum relief valve, a flame trap, and an automatic safety shut-off valve.
       2. (B) An installation of water seal equipment on a gas pipe is prohibited.

    3. (3) Gas Pipes and Condensate.

       1. (A) The gas pipe system must be designed for the volume of gas expected.
       2. (B) A gas pipe must be pressure tested for leakage at 1.5 times the design pressure before a digester is placed into service.
       3. (C) A gas pipe must slope at least 1/8 inch per foot to drain condensate.
       4. (D) The main gas pipe from a digester must have a sediment trap and a drip trap.
       5. (E) A float controlled condensate trap is prohibited.
       6. (F) A condensation trap must be accessible for daily servicing and draining.
       7. (G) A drip trap must be located at each low point in the pipes.
       8. (H) A gas pipe to each gas outlet must have a flame check or a flame trap.
       9. (I) A burner pilot must use natural or bottled gas.
       10. (J) Each main gas pipe must have a flame trap with a fusible shut-off.
       11. (K) A gas pipe to a waste gas burner must have a pressure valve and a vacuum relief valve.
    4. (4) Electrical Fixtures and Equipment. The electrical equipment near sludge digester pipe containing gas must be designed to prevent potentially explosive conditions.

12. (l) Waste gas.

    1. (1) A waste gas burner must be accessible and must be located at least 50 feet away from any structure, if placed at ground level.
    2. (2) A waste gas burner may be located on the roof of the control building.
    3. (3) A waste gas burner must not be located on top of a digester.
    4. (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.

13. (m) Ventilation.

    1. (1) An underground enclosure connected to an anaerobic digesters tank, gas pipe, or sludge equipment must have forced ventilation in accordance §217.246 of this title (relating to Ventilation and Odor Control).
    2. (2) An underground enclosure must have a tight-fitting, self-closing door to minimize the spread of gas.

14. (n) Gas Meter.

    1. (1) A system must have a gas meter to measure total gas production.
    2. (2) A meter must have a bypass.

15. (o) Manometer.

    1. (1) A gas manometer must have a tight shut-off vent and vent cock.
    2. (2) A vent pipe must be extended from a manometer to the outside of the building.
    3. (3) A vent pipe opening must have a screen and be designed to prevent the entrance of rainwater.
    4. (4) A design must specify all safety devices that are needed for a manometer pipe system and must list the safety items in the report.

16. (p) Gas Piping. The gas piping for an anaerobic digester must be equipped with gauges that measure the following in inches:

    1. (1) the pressure of the main pipe;
    2. (2) the pressure to gas-utilization equipment; and
    3. (3) pressure to waste burners.

17. (q) Digestion Temperature Control.

    1. (1) Passive Temperature Control.

       1. (A) A digester must be constructed above the shallowest ground water table.
       2. (B) A digester must be insulated to minimize heat loss.

    2. (2) Heating Facilities.

       1. (A) The sludge must be heated by circulating the sludge through an external heater.
       2. (B) A piping system must allow for the preheating of feed sludge before introduction to the digesters, unless effective mixing is provided within a digester.
       3. (C) A pipe and valve layout must facilitate cleaning.
       4. (D) The size of a heat exchanger sludge pipe must be based on the heat transfer requirements.

    3. (3) Heating Capacity.

       1. (A) A digester system must have the heating capacity to maintain the temperature required for sludge stabilization.
       2. (B) A digester system must be designed to use an alternate source of fuel and have an alternate source of fuel available for emergency use.
    4. (4) Mixing. A digester system must have equipment to mix the sludge.
    5. (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.

18. (r) Supernatant Withdrawal.

    1. (1) Pipe Size. The minimum diameter for a supernatant pipe is 6.0 inches.

    2. (2) Withdrawal Arrangements.

       1. (A) The supernatant pipes must be arranged to allow withdrawal from three or more levels in a tank.
       2. (B) A supernatant selector must have at least two draw-off levels located in the digester's supernatant zone, in addition to an unvalved emergency supernatant draw-off pipe.
       3. (C) A system must have a positive, unvalved, vented overflow.
       4. (D) A supernatant withdrawal level design must be based on a fixed cover digester design.
       5. (E) Supernatant withdrawal must be by means of interchangeable extensions at the discharge end of a withdrawal pipe.
       6. (F) A supernatant piping system must have high-pressure backwash equipment.

    3. (3) Sampling.

       1. (A) A supernatant pipe must have sampling points at each supernatant draw-off level.
       2. (B) The minimum diameter for a sampling pipe is 1.5 inches.

    4. (4) Supernatant Handling.

       1. (A) The report must include how the treatment units are designed to handle shock organic loads associated with digester supernatant.
       2. (B) Supernatant liquor from an anaerobic digester may be returned directly to the facility for treatment or chemically treated before being returned to the facility for treatment. Any other method of treating supernatant liquor must be approved by the executive director.

       3. (C) If treating the supernatant liquor with lime, each of the following requirements must be met:

          1. (i) Lime must be applied to obtain a pH of at least 11.5 standard units (su).
          2. (ii) A lime feeder must be capable of feeding 2,000 milligrams per liter (mg/l) of hydrated lime or its equivalent.
          3. (iii) Lime must be mixed with the supernatant liquor by a rapid mixer or by agitation with air in a mixing chamber.
          4. (iv) After adequate mixing, the solids must be allowed to settle.

       4. (D) A supernatant liquor treatment system may be a batch or a continuous process.

          1. (i) A batch process may have the mixing and settling processes in the same tank.
          2. (ii) A sedimentation tank for a batch process must have the capacity to hold at least 36 hours of supernatant liquor at design flow, but not less than 1.5 gallons per capita.
          3. (iii) A sedimentation tank for a continuous process must have a detention time of not less than 8.0 hours.
       5. (E) The solids from the supernatant liquor treatment must be returned to a digester or conveyed to a sludge handling unit.
       6. (F) The clarified supernatant liquor must be returned to the head of the treatment works in accordance with §217.242 of this title (relating to Control of Sludge and Supernatant Volumes).

19. (s) Anaerobic Digester Covers.

    1. (1) An uncovered anaerobic digester is prohibited.
    2. (2) The sludge and supernatant withdrawal pipes for a single-stage or a first-stage digester with a fixed cover must be arranged to minimize the possibility of air being drawn into a gas chamber above the liquid in a digester.
    3. (3) A digester cover must include a gas chamber.
    4. (4) A digester cover must be gas tight. The specifications must include a test of each digester cover for gas leakage.
    5. (5) A digester cover must be equipped with an air vent with a flame trap, a vacuum breaker, and a pressure relief valve.

20. (t) Aerobic Sludge Digestion. This subsection applies to the stabilization by aerobic digestion of waste sludge to Class B biosolids as defined in Chapter 312 of this title.

    1. (1) Solids Management. The report must include a solids management plan.
    2. (2) Detention Time. The design temperature of an aerobic digester system must be based the average of the lowest consecutive seven-day low temperature at a similar wastewater treatment facility located within 50 miles of a proposed site must be used.
    3. (3) Mass Balance Requirements. Mass balance calculations must be included in report. The mass balance calculations must take into account design sludge age, wastestream concentration, operational hours, operational volume in tanks, decant or dewatering volumes and characteristics, time needed for decanting or dewatering, and the volume needed for storage and sampling.

    4. (4) Single Stage. Single stage aerobic digestion consists of utilizing one tank operating in continuous-mode-no-supernatant removal, continuous-mode-feeding-batch removal, or other mode detailed in a solids management plan.

       1. (A) The design of the size of an aerobic digester must be based on the minimum total detention time for the water temperature in the table located in subparagraph (B) of this paragraph based on Chapter 312 of this title and 40 Code of Federal Regulations Part 503.
       2. (B) The digester size must be sufficient to provide both the detention time in the following table and to provide for the mass load received by the unit:
          Attached Graphic
          
    5. (5) Multiple Stage. Multiple stage aerobic digestion consists of two or more completely mixed reactors operating in series.

    6. (6) Field Data.

       1. (A) Any increase in flow or organic loading or change in process requires new testing and verification of time and temperature operating parameters.
       2. (B) An expansion of an existing facility may be designed and operated according to previously established time and temperature operating parameters.
       3. (C) The executive director may re-rate a facility under Subchapter B of this chapter (relating to Treatment Facility Design Requirements), if an owner requests a re-rating and submits sufficient supporting data.

    7. (7) Design Requirements.

       1. (A) The maximum solids concentration used to calculate the total detention time for an aerobic digester that concentrates the waste sludge only in a digester tank must be:

          1. (i) 2.0% solids concentration; unless
          2. (ii) supporting data is submitted in the report to increase the solids concentration to 3.0%; or
          3. (iii) a higher concentration is justified by the use of a sludge thickening unit upstream of a digester.
       2. (B) A diffuser must be designed to minimize clogging.
       3. (C) A diffuser must be designed to permit its removal without dewatering a tank for inspection, maintenance, and replacement.
       4. (D) The volatile solids loading rate must be designed to be at least 100 lb but not more than 200 lb of volatile solids per 1,000 cf per day, unless otherwise justified in the report.
       5. (E) The dissolved oxygen concentration maintained in the liquid must be at least 0.5 mg/l.
       6. (F) The energy input for mixing must be at least 0.5 horsepower per 1,000 cf for mechanical aerators.
       7. (G) The energy input for mixing must be at least 20 standard cf per minute per 1,000 cf per 1,000 cf of aeration tank if diffused air mixing is used.
       8. (H) A unit must be designed for effective separation and withdrawal, or decanting of the supernatant.

21. (u) Heat Stabilization.

    1. (1) The design of a heat treatment system must be based on the anticipated sludge flow, characteristics, and concentration.
    2. (2) A heat treatment system must operate continuously to minimize the additional heat input necessary to start up the system, unless justified in the report.
    3. (3) A heat treatment system must have multiple units, unless storage or an alternate stabilization method is available.
    4. (4) A single unit heat treatment system must have a standby grinder, fuel pump, air compressor, if applicable, and dual sludge pumps.
    5. (5) The report must identify the expected downtime for maintenance and repair, based on data from a comparable facility.
    6. (6) The report must include a design for adequate storage for process feed and downtime.

    7. (7) A heat treatment system must provide heat stabilization in a reaction vessel:

       1. (A) at a minimum of 175 degrees Celsius (350 degrees Fahrenheit) for 40 minutes but not more than 205 degrees Celsius (400 degrees Fahrenheit) for 20 minutes and at a pressure of not less than 250 lbs per square inch gauge (psig) but not more than 400 lbs/psig; or
       2. (B) provide for pasteurization at temperatures of 30 degrees Celsius (85 degrees Fahrenheit) or more and gage pressure of more than 1.0 standard atmosphere (14.7 pounds per square inch) for a period of at least 25 days.
    8. (8) A heat treatment system must have a sludge grinder to protect a heat exchanger from rag fouling.
    9. (9) A heat treatment system must include an acid wash or high-pressure water wash system to remove scale from heat exchangers and reactors.
    10. (10) A decant tank must have a sludge scraper mechanism and must be covered.
    11. (11) A heat exchanger must be constructed of corrosion resistant material.
    12. (12) A heat treatment system must have a continuous temperature recorder.

22. (v) Recycle Loads.

    1. (1) The report must identify a method of treatment for the recycle stream from heat treatment.
    2. (2) A recycle stream must not impact effluent quality or the facility's treatment processes.

23. (w) Alkaline Stabilization.

    1. (1) Design Basis.

       1. (A) Alkaline Dosage. The report must include the calculation of the alkaline dosage required to stabilize sludge based on the type of sludge, chemical composition of sludge, and the solids concentration. Performance data taken from a pilot test program or from a comparable facility must be used to determine the proper dosage.
       2. (B) Temperature, pH, and Contact Time. An alkaline stabilization system must uniformly mix an alkaline additive-sludge mixture to maintain the pH, temperature, and contact time, as specified in §312.82 of this title (relating to Pathogen Reduction) and §312.83 of this title (relating to Vector Attraction Reduction).

    2. (2) Reliability.

       1. (A) An alkaline stabilization system must have multiple units, unless storage or an alternate stabilization method is available to continue operations when a unit is not in service.
       2. (B) A single unit that has adequate storage or an alternate stabilization method must have standby conveyance and mixer, backup heat source, and dual blowers.

       3. (C) A design must include:

          1. (i) the expected downtime for maintenance and repair based on data from a comparable facility; and
          2. (ii) adequate storage for process, feed, and downtime.

    3. (3) Alkaline Stabilization Housing Unit.

       1. (A) A housing unit must meet the requirements in §217.247(u)(1) of this title (relating to Chemical Pretreatment of Sludge).
       2. (B) A housing unit must have mechanical or air agitation to ensure uniform discharge from the storage bins.

    4. (4) Feeding Equipment.

       1. (A) The alkaline additive feeding equipment must meet the requirements of §217.247(u)(1) of this title.
       2. (B) Hydrated lime must be fed as at least 6% calcium hydroxide Ca(OH)₂ slurry by weight but not more than 18% Ca(OH)₂ slurry by weight, unless otherwise justified in the report.
       3. (C) The report must identify a means for controlling the feed rate of any other dry additive.

    5. (5) Mixing Equipment.

       1. (A) An additive and sludge blending or mixing vessel must be large enough to hold the mixture for a minimum of 30 minutes at maximum feed rate.
       2. (B) A batch process must maintain a pH greater than 12 su in a mixing tank during the blending period.
       3. (C) A continuous flow process must maintain a pH greater than 12 su in an exit pipe.
       4. (D) A continuous flow process must be designed for a detention time that is the tank volume divided by the volumetric input flow rate.
       5. (E) A slurry mixture may be mixed with either a diffused air mixer or a mechanical mixer.
       6. (F) The mixing equipment must maintain an alkaline slurry mixture in complete suspension.

       7. (G) If using a diffused air mixer, the following requirements apply.

          1. (i) A coarse bubble diffuser must have a minimum air supply of 20 standard cubic feet per minute per 1,000 cf of tank volume.
          2. (ii) A mixing tank must be ventilated and include odor control equipment.

       8. (H) If using a mechanical mixer, the following requirements apply.

          1. (i) A mechanical mixer must provide at least 5.0 horse power per 1,000 cf of tank volume but not more than 10 horse power per 1,000 cf of tank volume.
          2. (ii) The impellers must minimize debris fouling in the sludge.
    6. (6) Detention Time. A pasteurization vessel must provide a minimum detention period of 30 minutes.
    7. (7) External Heat. The report must include any supplemental external heat necessary.

Source Note:The provisions of this §217.249 adopted to be effective August 28, 2008, 33 TexReg 6843.