SECTION 217.249. Sludge Stabilization

(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.

(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.

(A) A digester must be constructed above the shallowest groundwater table, including any perched water tables.

(B) A digester must be insulated to minimize heat loss.

(2) Heating Facilities.

(A) The sludge must be heated by circulating the sludge through an external heater.

(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.

(C) A pipe and valve layout must facilitate cleaning.

(D) The size of a heat exchanger sludge pipe must be based on the heat transfer requirements.

(3) Heating Capacity.

(A) A digester system must have the heating capacity to maintain the temperature required for sludge stabilization established in subsection (j)(3) of this section.

(B) A digester system must be able to use an alternate source of fuel and must have an alternate source of fuel available for emergency use.

(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.

(A) The supernatant pipes must be arranged to allow withdrawal from three or more levels in a tank.

(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.

(C) A supernatant withdrawal system must have a positive, unvalved, vented emergency overflow. The engineer must specify where overflow is routed in the engineering report.

(D) A supernatant withdrawal level design must be based on a fixed cover digester design.

(E) Supernatant withdrawal must be by means of interchangeable extensions at the discharge end of a withdrawal pipe.

(F) A supernatant piping system must have high-pressure backwash equipment.

(3) Sampling.

(A) A supernatant pipe must have sampling points at each supernatant draw-off level.

(B) The minimum diameter for a sampling pipe is 1.5 inches.

(4) Supernatant Handling.

(A) The engineering report must include how the treatment units are designed to handle shock organic loads associated with digester supernatant.

(B) Supernatant liquor from an anaerobic digester must either be returned directly to the headworks of the wastewater treatment facility for treatment, or may be chemically treated before being returned to the headworks for treatment. Any other method of treating supernatant liquor must be approved in writing by the executive director.

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

(i) Lime must be applied to obtain a pH of at least 11.5 standard units.

(ii) A lime feeder must be capable of feeding 2,000 milligrams per liter of hydrated lime or its equivalent.

(iii) Lime must be mixed with the supernatant liquor by a rapid mixer or by agitation with air in a mixing chamber.

(iv) After adequate mixing, the solids must be allowed to settle.

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

(i) A batch process may have both the mixing and the settling processes in the same tank.

(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 based on the design population of the service area.

(iii) A sedimentation tank for a continuous process must have a detention time of not less than 8.0 hours.

(E) The solids from the supernatant liquor treatment must be returned to a digester or conveyed to a sludge handling unit.

(F) The clarified supernatant liquor must be returned to the headworks of the wastewater treatment facility in accordance with §217.242 of this title (relating to Control of Sludge and Supernatant Volumes).

(s) Anaerobic Digester Covers.

(1) An uncovered anaerobic digester is prohibited.

(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) A digester cover must include a gas chamber.

(4) A digester cover must be gas tight. The specifications must include a test of each digester cover for gas leakage.

(5) A digester cover must be equipped with an air vent with a flame trap, a vacuum breaker, and a pressure relief valve.

(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 (relating to Sludge Use, Disposal, and Transportation).

(1) Solids Management. The engineering report must include a solids management plan.

(2) Detention Time. The design temperature of an aerobic digester system must be based on the average of the lowest consecutive seven-day water temperature from an aerobic digester at a wastewater treatment facility located within 50 miles of the proposed site.

(3) Mass Balance Requirements. Mass balance calculations must be included in the engineering report. The mass balance calculations must take into account design sludge age, wastestream concentration, operational hours, operational volume in the tanks, decant or dewatering volumes and characteristics, time needed for decanting or dewatering, and the volume needed for storage and sampling.

(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.

(A) 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.

(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) Multiple Stage. Multiple stage aerobic digestion consists of two or more completely mixed digesters operating in series.

(6) Field Data.

(A) Any increase in flow or organic loading, or any change in process requires new testing and verification of time and temperature operating parameters.

(B) An expansion of an existing wastewater treatment facility may be designed and operated according to previously established time and temperature operating parameters.

(C) The executive director may re-rate a wastewater treatment facility under Subchapter B of this chapter (relating to Wastewater Treatment Facility Design Requirements), if an owner requests a re-rating and submits sufficient supporting data.

(7) Design Requirements.

(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 2.0% solids concentration, unless:

(i) supporting data is submitted in the engineering report to increase the solids concentration to 3.0%; or

(ii) a higher concentration is justified by the use of a sludge thickening unit upstream of a digester.

(B) A diffuser must be designed to minimize clogging.

(C) A diffuser must be removable without dewatering a tank for inspection, maintenance, and replacement. Removable diffusers are not required if the wastewater treatment facility is designed with a redundant basin.

(D) The volatile solids loading rate must be designed to be at least 100 pounds but not more than 200 pounds of volatile solids per 1,000 cubic feet per day, unless otherwise justified in the engineering report.

(E) The dissolved oxygen concentration maintained in the liquid in an aerobic digester must be at least 0.5 milligrams per liter.

(F) If mechanical aerators are used, the energy input for mixing must be at least 0.5 horsepower per 1,000 cubic feet.

(G) If diffused air mixing is used, the energy input for mixing must be at least 20 standard cubic feet per minute per 1,000 cubic feet of aeration tank.

(H) An aerobic digester must be able to separate and withdraw solids, or must decant the supernatant.

(u) Heat Stabilization.

(1) The design of a heat treatment system must be based on the anticipated sludge flow, sludge characteristics, and sludge concentration.

(2) A heat treatment system must operate continuously to minimize the additional heat input necessary to start up the system, unless justified in the engineering report.

(3) A heat treatment system must have multiple units, unless storage or an alternate stabilization method is available.

(4) A single unit heat treatment system must have a standby grinder, a fuel pump, an air compressor, and dual sludge pumps.

(5) The engineering report must identify the expected downtime for maintenance and repair, based on data from a comparable wastewater treatment facility.

(6) The engineering report must include a design for adequate storage for process feed and downtime.

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

(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 pounds per square inch, gauge, but not more than 400 pounds per square inch, gauge; or

(B) provide for pasteurization at temperatures of 30 degrees Celsius (85 degrees Fahrenheit) or more and gauge pressure of more than 1.0 standard atmosphere (14.7 pounds per square inch) for a period of at least 25 days.

(8) A heat treatment system must have a sludge grinder to protect a heat exchanger from damage or clogging caused by rags or other debris.

(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) A decant tank must have a sludge scraper mechanism and must be covered.

(11) A heat exchanger must be constructed of corrosion-resistant material.

(12) A heat treatment system must have a continuous temperature recorder.

(v) Recycle Loads.

(1) The engineering report must identify a method of treatment for the recycle stream from the heat treatment system.

(2) A recycle stream must not impact effluent quality or the wastewater treatment facility's treatment processes.

(w) Alkaline Stabilization.

(1) Design Basis.

(A) Alkaline Dosage. The engineering 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 in the sludge. Performance data taken from a pilot test program or from a comparable wastewater treatment facility must be used to determine the proper dosage.

(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) Reliability.

(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.

(B) A single unit that has adequate storage or an alternate stabilization method must have a standby conveyance and mixer, a backup heat source, and dual blowers.

(C) A design must include:

(i) the expected downtime for maintenance and repair based on data from a comparable wastewater treatment facility; and

(ii) adequate storage for process, feed, and downtime.

(3) Alkaline Stabilization Housing Unit.

(A) A housing unit must meet the requirements in §217.247(r)(1) of this title (relating to Chemical Pretreatment of Sludge).

(B) A housing unit must have mechanical or air agitation to ensure uniform discharge from the storage bins.

(4) Feeding Equipment.

(A) The alkaline additive feeding equipment must meet the requirements of §217.247(r)(1) of this title.

(B) Hydrated lime must be fed as a slurry consisting of at least 6% calcium hydroxide (Ca(OH)₂) by weight but not more than 18% Ca(OH)₂ by weight, unless otherwise justified in the engineering report.

(C) The engineering report must identify a means for controlling the feed rate of any other dry additive.

(5) Mixing Equipment.

(A) An additive and sludge blending or mixing vessel must be large enough to hold the mixture for a minimum of 30 minutes at the maximum feed rate.

(B) A batch process must maintain a pH greater than 12 standard units in a mixing tank during the blending period.

(C) A continuous flow process must maintain a pH greater than 12 standard units in an exit pipe.

(D) A continuous flow process must be designed for a detention time that is the tank volume divided by the volumetric input flow rate.

(E) A slurry mixture may be mixed with either a diffused air mixer or a mechanical mixer.

(F) The mixing equipment must maintain an alkaline slurry mixture in complete suspension.

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

(i) a coarse bubble diffuser must have a minimum air supply of 20 standard cubic feet per minute per 1,000 cubic feet (cf) of tank volume; and

(ii) a mixing tank must be ventilated and include odor control equipment.

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

(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; and

(ii) the impellers must minimize debris fouling in the sludge.

(6) Detention Time. A pasteurization vessel must provide a minimum detention period of 30 minutes.

(7) External Heat. The engineering report must include specifications about any supplemental external heat necessary for sludge stabilization.

Source Note: The provisions of this §217.249 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254