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TITLE 30ENVIRONMENTAL QUALITY
PART 1TEXAS COMMISSION ON ENVIRONMENTAL QUALITY
CHAPTER 317DESIGN CRITERIA PRIOR TO 2008
RULE §317.4Wastewater Treatment Facilities

    (A) Pretreatment. RBC units shall be preceded by pretreatment to remove any grit, debris, and excess oil and grease which may hinder the treatment process or damage the RBC units. The design engineer should consider primary clarifiers with scum and grease collecting devices, fine screens, and oil separators. For wastes with high hydrogen sulfide concentrations, preaeration shall be provided.

    (B) Organic loading. The organic loading for the design of RBC units shall be based on total BOD5 in the waste going to the RBC, including any side streams. The design engineer should consider a maximum loading rate of five pounds BOD5 per day per 1,000 square feet of media in any stage, depending on the character of the influent wastewater. The maximum loading rate shall not exceed eight pounds BOD5 per day per 1,000 square feet of media in any stage. The design engineer should also consider the ratio of soluble BOD5 to total BOD5 and its possible effect on required RBC media area. Allowable organic loading for the entire RBC system shall not exceed the following criteria.

Attached Graphic

    (C) Stages of treatment. The number of RBC units in series (stages) for BOD removal only shall be a minimum of three stages. For BOD removal and nitrification, there shall be a minimum of four stages. If the plant is designed with less stages than noted in the previous sentences of this subparagraph, the engineer must provide justification based on either full-scale operating facilities or pilot unit operational data. Any pilot unit data used in the justification must take into consideration an appropriate scale-up factor.

    (D) Drive system. The drive system for each RBC unit shall be selected for the maximum anticipated media load. A variable speed system should be considered to provide additional operator flexibility. The RBC units may be mechanically driven or air driven.

      (i) Mechanical drives.

        (I) Each RBC unit shall have a positively connected mechanical drive with motor and speed reduction unit to maintain the required rpm.

        (II) A fully assembled spare mechanical drive unit for each size shall be provided on-site.

        (III) Supplemental diffused air should be considered for mechanical drive systems to help remove excess biomass from the media and to help maintain the minimum dissolved oxygen concentration.

      (ii) Air drives.

        (I) Each RBC unit shall have air diffusers mounted below the media and off-center from the vertical axis of the RBC unit. Air cups mounted on the outside of the media shall collect the air to provide the driving force and maintain the required rpm.

        (II) Blowers shall provide enough air flow for each RBC unit plus additional capacity to double the air flow rate to any one unit while the others are running normally.

        (III) The blowers shall be capable of providing the required air flow with the largest unit out of service.

        (IV) The air diffuser line to each unit shall be mounted such that it can be removed without draining the tank or removing the RBC media.

        (V) An air control valve shall be installed on the air diffuser line to each RBC unit.

    (E) Dissolved oxygen. The RBC plant shall be designed to maintain a minimum dissolved oxygen concentration of one milligram per liter at all stages during the peak organic flow rate. Supplemental aeration may be required.

    (F) Nitrification. The design of an RBC plant to achieve nitrification is dependent upon a number of factors, including the concentration of ammonia in the influent, effluent ammonia concentration required, BOD5 removal required, minimum operational temperatures, and ratio of peak to design hydraulic flow. Each of these factors will impact the number of stages of treatment required and the allowable ammonia nitrogen loading (lb NH3 /day/1,000 ft2 media) required to achieve the desired levels of nitrification for a given facility. The engineer shall submit appropriate data supporting the design.

    (G) Design flexibility. The designer of an RBC plant should consider provisions to provide additional operational flexibility such as controlled flow to multiple first stages, alternate flow and staging arrangements, removable baffles between stages, and provision for step feed and supplemental aeration.

(g) Activated sludge facilities.

  (1) Organic loading rates. Aeration tank volumes should be based upon full scale experience, pilot scale studies, or rational calculations based upon commonly accepted design parameters such as food to microorganism ratio, mixed liquor suspended solids, and the solids retention time. Other factors to be considered include size of the treatment plant, diurnal load variations, return flows and soluble organic loads from digesters, or sludge dewatering operations and degree of treatment required. Temperature, pH, and dissolved oxygen concentration are particularly important to consider when designing for nitrification. As a general rate, minimum aeration tank volumes shall be as set forth in the following table. Calculations must be submitted to fully justify the basis of design for any aeration basins not conforming to these minimum recommendations.

Attached Graphic

    (A) The conventional activated sludge process is characterized by having a plug flow hydraulic regime wherein particles are discharged in the same sequence in which they enter the aeration basin. Plug flow may be approximated in long tanks with a high length-to-width ratio.

    (B) The contact stabilization process divides the aeration tank volume between the reaeration zone and the contact zone. The ratio of reaeration volume to contact volume ranges from 1:1 to 2:1. The hydraulic detention time in the contact zone shall be sufficient to provide removals of soluble substrates to the required levels. For domestic flows normally two hours is sufficient in the contact zone. Contact zone volume shall be based upon acceptable removal kinetics for soluble BOD5 and ammonia nitrogen.

    (C) Oxidation ditches (which are organically loaded consistent with this paragraph) shall have a minimum hydraulic retention time of 20 hours based on design flow. These oxidation ditch systems shall provide final clarification and return sludge capability equal to that required for the extended aeration process. There shall be a minimum of two rotors per ditch, each capable of supplying the required oxygenation capacity and maintaining a minimum channel velocity of 1.0 foot per second with one rotor out of service. The ditch shall be lined with reinforced concrete or other acceptable erosion-resistant liner material. Provision shall be made to easily vary the liquid level in the ditch to control the immersion depth of the rotor for flexibility of operation. A motor of sufficient size to maintain the proper rotor speed for continuous operation shall be provided. Rotor bearings should have grease fittings that are readily accessible to maintenance personnel. Gear housing and outboard bearings should be shielded from rotor splash.

  (2) Aeration basin general design considerations. Aeration tank geometry shall be arranged to provide optimum oxygen transfer and mixing for the type aeration device proposed. Aeration tanks must be constructed of reinforced concrete, steel with corrosion-resistant linings or coatings, or lined earthen basins. Liquid depths shall not be less than 8.0 feet when diffused air is used. All aeration tanks shall have a freeboard of not less than 18 inches at peak flow. Access walkways with properly designed safety handrails shall be provided to all areas that require routine maintenance. Where operators would be required to climb heights greater than four feet, properly designed stairways with safety handrails should be provided. The shape of the tank and the installation of aeration equipment should provide a means to control short circuiting through the tank. For plants designed for design flows greater than 2.0 mgd the total aeration basin volume shall be divided among two or more basins. Each treatment facility shall be designed to hydraulically pass the design two-hour peak flow with one basin out of service.

  (3) Sludge pumps, piping, and return sludge flow measurement. The pumps and piping for return activated sludge shall be designed to provide variable underflow rates of 200 to 400 gallons per day per square foot for each clarifier. If mechanical pumps are used, sufficient pumping units shall be provided to maintain design pumping rates with the largest single unit out of service. Sludge piping and/or channels shall be so arranged that flushing can be accomplished. A minimum pipe line velocity of three feet per second should be provided at an underflow rate of 200 gallons per day per square foot. Some method shall be provided to measure the return sludge flow from each clarifier.

  (4) Aeration system design.

    (A) General design consideration. Aeration systems shall be designed to maintain a minimum dissolved oxygen concentration of 2.0 mg/liter throughout the basin at the maximum diurnal organic loading rate and to provide thorough mixing of the mixed liquor. The design oxygen requirements for activated sludge facilities are presented in the following table. The minimum air volume requirements may be reduced with appropriate supporting performance evaluations from the manufacturer.

Attached Graphic

      (i) Minimum air volume requirements are based upon a transfer efficiency of 4.0% in wastewater for all activated sludge processes except extended aeration, for which a wastewater transfer efficiency of 4.5% is assumed.

      (ii) Value in parentheses represents the minimum oxygen requirement for ditch type systems which will achieve nitrification.

    (B) Diffused air systems.

      (i) Volumetric aeration requirements. Volumetric aeration requirements shall be as determined from the preceding table unless certified diffuser performance data is presented which demonstrates transfer efficiencies greater than those used in the preparation of the table. Wastewater transfer efficiencies may be estimated for:

        (I) coarse bubble diffusers by multiplying the clean water transfer efficiency by 0.65%;

        (II) fine bubble diffusers by multiplying the clean water transfer efficiency by 0.45%. The maximum allowable wastewater transfer efficiency shall be 12%. Plants treating greater than 10% industrial wastes shall provide data to justify actual wastewater transfer efficiencies. Wastewater oxygen transfer efficiencies greater than 12% are considered innovative technology. See §317.1(a)(2)(C) of this title (relating to General Provisions) for performance bond requirements. Clean water transfer efficiencies obtained at 20 degrees Celsius shall be adjusted to reflect field conditions (i.e., wastewater transfer efficiencies) by use of the following equation.

Attached Graphic

      (ii) Mixing requirement. Air requirements for mixing should be considered along with those required for the design organic loading. The designer is referred to Table 14-V, aerator mixing requirements in Wastewater Treatment Plant Design, a joint publication of the American Society of Civil Engineers and the Water Pollution Control Federation.

      (iii) Blowers and compressors. Blowers and compressors shall be of such capacity to provide the required aeration rate as well as the requirements of all supplemental units such as airlift pumps. Multiple compressor units shall be provided and shall be arranged so the capacity of the total air supply may be adjusted to meet the variable organic load to be placed on the treatment facility. The compressors shall be designed so that the maximum design air requirements can be met with the largest single unit out of service. The blower/compressor units shall automatically restart after a period of power outage or the operator or owner shall be notified by some method such as telemetry or an auto-dialer. The specified capacity of the blowers or air compressors, particularly centrifugal blowers, should take into account that the air intake temperature may reach 104 degrees Fahrenheit (40 degrees Celsius) or higher and the pressure may be less than standard (14.7 pounds per square inch absolute). The capacity of the motor drive should also take into account that the intake air may be 10 degrees Fahrenheit (-12 degrees Celsius) or less and may require oversizing of the motor or a means of reducing the rate of air delivery to prevent overheating or damage to the motor.

      (iv) Diffusers and piping. Each diffuser header shall include a control valve. These valves are basically for open/close operation but should be of the throttling type. The depth of each diffuser shall be adjustable. The air diffuser system, including piping, shall be capable of delivering 150% of design air requirements. The aeration system piping should be designed to minimize headlosses. Typical air velocities in air delivery piping systems are presented in the following table.

Attached Graphic

  (5) Mechanical aeration systems. Mechanical aeration devices shall be of such capacity to provide oxygen transfer to and mixing of the tank contents equivalent to that provided by compressed air. A minimum of two mechanical aeration devices shall be provided. Two speed or variable speed drive units should be considered. The oxygen transfer capability of mechanical surface aerators shall be calculated by the use of a generally accepted formula and the calculations presented in the engineering report. Proposed clean water transfer rates in excess of 2.0 pounds per horsepower-hour shall be justified by performance data. In addition to providing sufficient oxygen transfer capability for oxygen Cont'd...

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