<<Prev Rule

Texas Administrative Code

Next Rule>>
TITLE 30ENVIRONMENTAL QUALITY
PART 1TEXAS COMMISSION ON ENVIRONMENTAL QUALITY
CHAPTER 317DESIGN CRITERIA PRIOR TO 2008
RULE §317.5Sludge Processing

(a) General requirements.

  (1) Disposal requirements, agreement with. Sludge processing and treatment shall be in agreement with the requirements of the ultimate form of disposal.

  (2) Control of sludge and supernatant volumes. Provisions shall be made to insure that waste sludge will be discharged to the sludge digester in such a manner so as to minimize the volume of digester supernatant liquor. Provisions shall be made for the return of supernatant from sludge thickeners and digesters to the head of the treatment works or to the aeration system accounting for the impact on the treatment units.

  (3) Piping. All piping from clarifiers to thickeners, digesters, or other sludge processing facilities shall be arranged for ease of maintenance and with sufficient hydraulic gradient to insure the flow of sludge. Piping under stationary structures shall be arranged so that stoppages can be readily eliminated by rodding or with sewer cleaning devices. The sludge piping within the digester, including the sludge drain line, shall be a minimum of four inches in diameter. Appropriate facilities for transfer of supernatant liquor shall be provided. Piping shall include means to observe the quality of the supernatant from each of the withdrawal outlets provided. All units shall be capable of being drained independently of one another.

  (4) Sludge pumps. Selection of sludge transfer pumps shall be based on both the quantity and character of the anticipated solids load to be handled by them. Where mechanical pumps are used, a sufficient number of pumps shall be provided so that the design pumping capacity is available with the largest sludge pump out of service. Air lift pumps are an acceptable mechanism for sludge transfer. Duplicate design pumping capacity is not required when air lift pumps are used. Pumps used for pumping sludge shall be specifically designed for that purpose. Centrifugal sludge pumps shall have a positive suction head unless they are self-priming or equipped with some other priming device acceptable to the commission.

  (5) Sludge stabilization. Sludge stabilization is required for all biological treatment processes with the exception of extended aeration processes (with a solids retention time (SRT) of 20 days or more) in which case the sludge may be drawn directly to a sludge dewatering facility.

  (6) Sizing. Sizing requirements must be determined using the five-day biochemical oxygen demand (BOD5 ) and design flow of the raw sewage influent to the plant.

(b) Aerobic digesters.

  (1) Sludge thickening. Aerobic digesters should be provided with sludge thickening capability.

  (2) Aeration. Air supplied from air compressors or blowers through diffusers shall be not less than 30 standard cubic feet per meter (scfm) per 1,000 cubic feet of aerobic digester volume. If a separate system of air compressors or blowers will supply air to the digester, then the compressor or blower system shall be designed so that the air requirements can be met with the largest single unit out of service. If mechanical aerators are used, a minimum of 1.5 horsepower per 1,000 cubic feet must be provided.

  (3) Mixing. Adequate mixing of the sludge shall be provided to keep the solids in suspension and to bring the deoxygenated liquid continuously to the aeration device. The amount of mixing shall be based upon the sludge characteristics, the tank geometry, and type of aeration mixing devices.

  (4) Volume. A digester shall provide a minimum sludge retention time of 15 days. The design volume of the aerobic digesters may be calculated using 20 cubic feet per pound BOD 5 per day. This volume should be provided in two cells capable of operating as a single or two-step unit.

  (5) Sludge withdrawal. Provisions shall be made to include an effective means of removing solids from the digester.

(c) Anaerobic digesters.

  (1) Volume. The following minimum design criteria shall be used in computing the capacity of digesters with and without facilities for heating the sludge undergoing digestion and without sludge thickening ahead of the digester. Variances to the table referenced as follows for minimum digester volume may be granted provided that it can be demonstrated to the satisfaction of the commission that a minimum SRT of 30 days will be provided for unheated digesters and a minimum SRT of 15 days will be provided for heated digesters. Heating of the digester means that adequate facilities shall be provided for heating and mixing the sludge and maintaining a year-round temperature of at least 95 degrees Fahrenheit. Heating coils inside the digester are not acceptable. All heated digesters shall include a thermometer with not less than a four-inch dial to indicate the temperature of digester contents. The use of flat-bottomed digestion chambers is not acceptable. In sewage treatment plants employing sludge thickeners, the volume of the digester may be reduced, with sufficient justification, as a result of the thickeners reducing the volume of sludge going to the digester. The calculations for the required sludge digestion volume shall be based on the minimum percent solids in the sludge expected to be encountered.

Attached Graphic

  (2) Mixing. Adequate mixing of digester contents is required for all first-stage and all single-stage digesters. Mixing may be performed by mechanical equipment, including external pumps, or by gas recirculation. The rate of mixing shall be such that the flow created in the digester is sufficient to completely mix the incoming sludge with the digester contents and prevent the formation of a scum layer.

  (3) Digester covers. Uncovered anaerobic digesters are not acceptable. The sludge and supernatant withdrawal piping for all single-stage and first-stage digesters with fixed covers shall be arranged in such a manner so as to minimize the possibility of air being drawn into the gas chamber above the liquid in the digester. All digester covers shall include a gas chamber adequate for the gas production anticipated. Digester covers shall be gas tight and the specifications shall require a test of every digester cover for gas leakage.

  (4) Gas piping and safety equipment. The gas piping shall be adequate for the volume of gas to be handled and shall be pressure tested for leakage (at 1.5 times the design pressure) before the digester is placed into operation. All gas piping shall slope at least 1/8 inch per foot to provide drainage of condensation in the gas piping. The main gas line from the digester shall have a sediment trap equipped with a drip trap. Drip traps shall be provided at all other low points in gas piping. The gas piping to every gas outlet including the pilot line to the waste gas burner shall be equipped with flame checks or flame traps. A natural or bottled gas source shall be utilized for the burner pilot. Flame traps with fusible shutoffs shall be included in all main gas lines. The gas line to the waste gas burner shall include a suitable pressure, vacuum and relief valve. Digester covers shall be equipped with an air vent which includes a flame trap, a vacuum breaker, and pressure relief valve. The main gas line shall be provided with a manometer or other acceptable device which measures the gas pressure in inches of water. Manometers may be used to measure the gas pressure in other gas lines. All manometers shall be vented to the atmosphere outside digester buildings. A gas meter to measure the rate of gas production is desirable and is mandatory on all anaerobic digester systems designed for 1.0 mgd facilities or larger. All rooms in digester buildings with floor level below grade shall be ventilated. Ventilation may be either continuous or intermittent. Ventilation, if continuous, shall provide at least six complete air changes per hour; if intermittent, at least 30 complete air changes per hour.

  (5) Other requirements. The discharge end of sludge inlet piping shall be separated from the overflow of the supernatant liquor withdrawal point by a minimum distance equal to the radius of the digester tank. Every digester shall be provided with an overflow. A means shall be provided by which the level can be varied from which supernatant liquor is withdrawn either automatically or by the operator. If this means is by withdrawal pipes at different levels in the digester, at least three different levels of supernatant liquor withdrawal shall be provided. All supernatant liquor withdrawal systems shall be provided with sampling cocks or other means of inspecting and testing the supernatant liquor from each level. Piping for hot water heating systems may be of any size adequate for the flow. The fresh water supply to hot water heating systems shall be from a tank with an air gap between the top of the tank and the fresh water supply pipe to prevent a cross connection between the digester hot water system and the fresh water supply system.

Cont'd...

Next Page

Link to Texas Secretary of State Home Page | link to Texas Register home page | link to Texas Administrative Code home page | link to Open Meetings home page