SECTION 317.4. Wastewater Treatment Facilities

(a) General requirements. Whenever possible, existing data of flows and raw waste strength from the same plant or nearby plants with similar service areas should be used in design of treatment facilities. When using such data for design purposes, the variability of data should be considered and the design based on the highest flows and strengths encountered during normal operating periods taking into consideration possible infiltration/inflow. In the absence of existing data, the following are generally acceptable parameters to which must be added appropriate allowances for inflow and infiltration into the collection system to obtain plant influent characteristics.

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(1) Effluent quality. Wastewater treatment plants shall be designed to consistently meet the effluent concentration and loading requirements of the applicable waste disposal permit.

(2) Effluent quantity. The design flow of a treatment plant is defined as the wet weather, maximum 30-day average flow. The design basis shall include industrial wastewaters which will enter the sewerage system. The engineering report shall state the flow and strength of wastewaters from industries which individually contribute 5.0% or more of plant flow or loading and discuss the aspect of hazardous or toxic wastes. It is the intent of these design criteria that the permit conditions not be violated. The engineering report shall list the design influent flow and concentration of five-day biochemical oxygen demand (BOD₅ ), total suspended solids (TSS), or other parameters for the following:

(A) dry weather 30-day average (Q_D W);

(B) wet weather maximum 30-day average (Q_D W); and

(C) two-hour peak flow (Q_p W).

(3) Piping. The piping within all plants shall be arranged so that when one unit is out of service for repairs, plant operation will continue and emergency treatment can be accomplished. Valves and piping shall be provided and sized to allow dewatering of any unit, in order that repairs of the unit can be completed in as short a period of time as possible. Portable pumping units may be used for dewatering small treatment plants (design flow of less than 100,000 gallons per day) or interim facilities. Removed wastes must be stored for retreatment or delivered to another treatment facility for processing. Consideration shall be given in design for means to clean piping, especially piping carrying raw wastewater, sludges, scum, and grit.

(4) Peak flow. For treatment unit design purposes, peak flow is defined as the highest two-hour average flow rate expected to be delivered to the treatment units under any operational condition, including periods of high rainfall (generally the two-year, 24-hour storm is assumed) and prolonged periods of wet weather. With pumped inflow, clarifiers shall have the capacity of all pumps operating at maximum wet well level unless a control system is provided that will limit the pumping rate to the firm capacity. This flow rate may also include skimmer flow, thickener overflow, filter backwash, etc. All treatment plants must be designed to hydraulically accommodate peak flows without adversely affecting the treatment processes. The engineer shall determine, by methods acceptable to the commission, the appropriate peak flow rate, including the possibility of utilizing standby pumps. The proposed two-hour peak flow rate, together with a discussion of rationale, calculations, and all supporting flow rate data shall be, unless presented in the preliminary engineering report, included in the final engineering design report. Special storm flow holding basins or flow equalization facilities can be specified to partially satisfy the requirements of this section where all treatment units within a plant are not sized for peak flow. See §317.9 of this title (relating to Appendix A) for referencing a two-year 24-hour rainfall event.

(5) Auxiliary power. The need for auxiliary power facilities shall be evaluated for each plant and discussed in the preliminary and final engineering reports. Auxiliary power facilities are required for all plants, unless dual power supply arrangements can be made or unless it can be demonstrated that the plant is located in an area where electric power reliability is such that power failure for a period to cause deterioration of effluent quality is unlikely. Acceptable alternatives to auxiliary power include the ability to store influent flow or partially treated wastewater during power outage. Auxiliary power may be required by the commission for plants discharging near drinking water reservoirs, shellfish waters, or areas used for contact recreation, and for plants discharging into waters that could be unacceptably damaged by untreated or partially treated effluent. For more information on power reliability determination and emergency power alternatives, refer to §317.3(e) of this title (relating to Lift Stations).

(6) Component reliability. Multiple units may be required based upon the uses of the receiving waters and the significance of the treatment units to the treatment processes.

(7) Stairways, walkways, and guard rails. Basins having vertical walls terminating four or more feet above or below ground level shall provide a stairway to the walkway. Guard rails on walkways shall have adequate clearance space for maintenance operations (see §317.7 of this title (relating to Safety)).

(8) Public drinking water supply connections. There shall be no water connection from any public drinking water supply system to a wastewater treatment plant facility unless made through an air gap or a backflow prevention device, in accordance with American Water Works Association (AWWA) Standard C506 (latest revision) and AWWA Manual M14. All backflow prevention devices shall be tested annually with their test and maintenance report forms retained for a minimum of three years. All washdown hoses using potable water must be equipped with atmospheric vacuum breakers located above the overflow level of the washdown area.

(9) Ground movement protection. The structural design of treatment plants shall be sufficient to accommodate anticipated ground movement including any active geologic faults and allow for independent dewatering of all treatment units. Plants should not be located within 50 feet of geologic faults.

(10) Odor control facilities. The need for odor control facilities shall be evaluated for each plant. Factors to be considered are the dissolved oxygen level of the incoming sewage and the type of treatment process proposed.

(b) Preliminary treatment units. Bar screens, screens, or shredders through which all wastewater will pass should be provided at all plants with the exception of plants in which septic tanks, Imhoff tanks, facultative, aerated, or partially mixed lagoons represent the initial treatment unit. In the event bar screens, screens, or shredders are located four or more feet below ground level, appropriate equipment shall be provided to lift the screenings to ground elevation. Where mechanically cleaned bar screens or shredders are utilized, a backup unit or manually cleaned bar screen shall be provided. A means of diverting flow to the backup screen shall be included in the design.

(1) Bar screens. Manually cleaned bar screens shall be constructed having a 30-degree to 60-degree slope to a horizontal platform which will provide for drainage of the screenings. Bar screen openings shall not be less than 3/4 inch for manually cleaned bar screens and 1/2 inch for mechanically cleaned bar screens. The channel in which the screen is placed shall allow a velocity of two feet per second or more at design flow. Velocity through the screen opening should be less than three feet per second at design flow.

(2) Grit removal. Grit removal facilities should be considered for all wastewater treatment plants. Grit washing facilities shall be provided unless a burial area for the grit is provided within the plant grounds, or the grit is handled otherwise in such a manner as to prevent odors or fly breeding. Grit removal units shall have mechanical means of grit removal or other acceptable methods for grit removal. Plants which have a single grit collecting chamber shall have a bypass around the chamber. All grit collecting chambers shall be designed with the capability to be dewatered. The method of velocity control used to accomplish grit removal in gravity settling chambers shall be detailed in the final engineering report.

(3) Fine screens. Fine screens, if used, shall be preceded by a bar screen. Fine screens shall not be substituted for primary sedimentation or grit removal; however, they may be used in lieu of primary treatment if fully justified by the design engineer. A minimum of two fine screens shall be provided, each capable of independent operation at peak flow. A steam cleaner or high pressure water hose shall be provided for daily maintenance of fine screens.

(4) Screenings and grit disposal. All screenings and grit shall be disposed of in an approved manner. Suitable containers with lids shall be provided for holding screenings. Runoff control must be provided around the containers where applicable. Fine screen tailings are considered as infectious waste; therefore, containers must provide vector control if wastes are not disposed of daily at a Type 1 landfill.

(5) Preaeration. Because preaeration may be proposed when a particular problem is anticipated, evaluation of these units will be on a case-by-case basis. Diffuser equipment shall be arranged for greatest efficiency, with consideration given to maintenance and inspection.

(6) Flow equalization. Equalization should be considered to minimize random or cyclic peaking of organic or hydraulic loadings. Equalization units should be provided after screening and grit removal.

(A) Aeration. Aeration may be required for odor control. When required, air supply must be sufficient to maintain 1.0 milligrams per liter (mg/liter) of dissolved oxygen in the wastewater.

(B) Volume. A diurnal flow graph with supporting calculations used for sizing the equalization facility must be provided in the engineering report. Generally, an equalization facility requires a volume equivalent to 10% to 20% of the anticipated dry weather 30-day average flow. Tankage should be divided into separate compartments to allow for operational flexibility, repair, and cleaning.

(c) Flow measuring devices and sampling points. A means for measuring effluent flow shall be provided at all plants. Consideration should be given to providing a means to monitor influent flow. Where average influent and effluent flows are significantly different, e.g., plants with large water surfaces located in areas of high rainfall or evaporation or plants using a portion of effluent for irrigation, both influent and effluent must be measured. Consideration should be given to internal flow monitoring devices to measure returned activated sludge and/or to facilitate splitting flows between units with special attention being given when units are of unequal size. All plants shall be provided with a readily accessible area for sampling effluent.

(d) Clarifiers.

(1) Inlets. Clarifier inlets shall be designed to provide uniform flow and stilling. Vertical flow velocity through the inlet stilling well shall not exceed 0.15 feet per second at peak flow. Inlet distribution channels shall not have deadened corners and shall be designed to prevent the settling of solids in the channels. Inlet structures should be designed to allow floating material to enter the clarifier.

(2) Scum removal. Scum baffles and a means for the collection and disposal of scum shall be provided for primary and final clarifiers. Scum collected from final clarifiers in plants utilizing the activated sludge process, or any modification thereof, and aerated lagoons may be discharged to aeration basin(s) and/or digester or disposed of by other approved methods. Scum from all other final clarifiers and from primary clarifiers shall be discharged to the sludge digester or other approved method of disposal. Discharge of scum to any open drying area is not acceptable. Mechanical skimmers shall be used in units with a design flow greater than 25,000 gallons per day. Smaller systems may use hydraulic differential skimming provided that the scum pickup is capable of removing scum from the entire operating surface of the clarifier. Scum pumps shall be specifically designed for this purpose.

(3) Effluent weirs. Effluent weirs shall be designed to prevent turbulence or localized high vertical flow velocity in the clarifiers. Weirs shall be located to prevent short circuiting flow through the clarifier and shall be adjustable for leveling. Weir loadings shall not exceed 20,000 gallons per day peak design flow per linear foot of weir length for plants with a design flow of 1.0 mgd or less. Special consideration will be given to weir loadings for plants with a design flow in excess of 1.0 million gallons per day (mgd), but such loadings shall not exceed 30,000 gallons per day peak flow per linear foot of weir.

(4) Sludge lines. Means for transfer of sludge from primary, intermediate, or final clarifiers for subsequent processing shall be provided so that treatment efficiency will not be adversely affected. Gravity sludge transfer lines shall not be less than eight inches in diameter.

(5) Basin sizing. Overflow rates are based on the surface area of clarifiers. The surface areas required shall be computed using the following criteria. The actual clarifier size shall be based on whichever is the larger size from the two surface area calculations (peak flow and design flow surface loading rates). The final clarifier solids loading for all activated sludge treatment processes shall not exceed 50 pounds of solids per day per square foot of surface area at peak flow rate. The following design criteria for clarifiers are based upon a side water depth of ten feet and shall be considered acceptable.

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(6) Sidewater depth (SWD). The minimum SWD for conventional primary and intermediate clarifiers is seven feet. All final clarifiers shall have a minimum SWD of eight feet. Final clarifiers having a surface area equal to or greater than 1,250 square feet (diameter equal to or greater than 40 feet) must be provided with a minimum SWD of 10 feet.

(7) Hopper bottom clarifiers. Hopper bottom clarifiers without mechanical sludge collecting equipment will only be approved for those facilities with a permitted design flow of less than 25,000 gallons per day. The required SWD for hopper bottom clarifiers may be computed using the following equation: SWD = 160 QD + 4, where SWD equals required SWD in feet and QD equals design flow in million gallons per day. Furthermore, SWD as computed previously for any flow may be reduced by crediting the upper one-third of the hopper as effective SWD if the following conditions are met:

(A) clarifier surface loading rate is reduced by at least 15% from maximum loading rate as per paragraph (5) of this subsection;

(B) influent stilling baffle and effluent weir are designed to prevent short circuiting;

(C) detention time at peak flow is at least 1.8 hours for secondary treatment and 2.4 hours for advanced treatment; and

(D) an appropriate form of flow equalization is used.

(8) Sludge collection equipment. All conventional clarifier units that treat flow from a treatment plant facility with a design flow of 25,000 gallons per day or greater shall be provided with mechanical sludge collecting equipment. Hopper bottom clarifiers must have a smooth wall finish and a hopper slope of not less than 60 degrees.

(9) BOD₅ removal. It shall be assumed that the BOD₅ removal in a primary clarifier is 35%, unless satisfactory evidence is presented to indicate that the efficiency will be otherwise. In plant efficiency calculations, it shall be assumed that the BOD₅ removal in intermediate and final clarifiers is included in the calculation for the efficiency of the treatment unit preceding the intermediate or final clarifier.

(e) Trickling filters.

(1) General. Trickling filters are secondary aerobic biological processes which are used for treatment of sewage.

(2) Basic design parameters. Trickling filters are classified according to applied hydraulic loading in million gallons per day per acre (mgd/acre) of filter media surface area, and organic loadings in pounds of biochemical oxygen demand (BOD) per day per 1,000 cubic feet of filter media (lb BOD/day-1,000 cu ft). The following factors should be considered in the selection of the design hydraulic and organic loadings: strength of the influent sewage, effectiveness of pretreatment, type of filter media, and treatment efficiency required. Typical ranges of applied hydraulic and organic loadings for the different classes of trickling filters are presented in the following table for illustrative purposes. The design engineer shall submit sufficient operating data from existing trickling filters of similar construction and operation to justify his efficiency calculations for the filters, and a filter efficiency formula from a reliable source acceptable to the commission. The formula of the National Research Council may be used when rock media is used in the trickling filter(s).

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(3) Pretreatment. The trickling filter treatment facility shall be preceded by primary clarifiers equipped with scum and grease removal devices. Design engineers may submit operating data as justification of other alternative pretreatment devices which provide for effective removal of grit, debris, suspended solids, and excess oil and grease. Preaeration shall be provided where influent wastewater contains harmful levels of hydrogen sulfide concentrations.

(4) Filter media.

(A) Material specifications for rock media. The following are minimum requirements.

(i) Crushed rock, slag, or similar media should not contain more than 5.0% by weight of pieces whose longest dimension is greater than three times its least dimension. The rock media should be free from thin, elongated, and flat pieces and should be free from dust, clay, sand, or fine material. Rock media should conform to the following size distribution and grading when mechanically graded over a vibrating screen with square openings:

(I) passing five-inch sieve--100% by weight;

(II) retained on three-inch sieve--95% to 100% by weight;

(III) passing two-inch sieve--0.2% by weight;

(IV) passing one-inch sieve--0.1% by weight;

(V) the loss of weight by a 20-cycle sodium sulphate test, as described in the American Society of Civil Engineers Manual of Engineering and Engineering Practice Number 13, shall be less than 10%.

(ii) Rock media shall not be less than four feet in depth (at the shallowest point) nor deeper than eight feet (at the deepest point of the filter).

(B) Synthetic (manufactured or prefabricated) media.

(i) Application of synthetic media shall be evaluated on a case-by-case basis. Suitability should be evaluated on the basis of experience with installations treating similar strength wastewater under similar hydraulic and organic loading conditions. The manufacturer's recommendations shall be included, as well as case histories involving the use of the media.

(ii) Media shall be relatively insoluble in sewage and resistant to flaking or spalling, ultraviolet degradation, disintegration, erosion, aging, all common acids and alkalies, organic compounds, biological attack, and shall support the weight of a person when the media is in operation.

(iii) Media depths should be consistent with the recommendations of the manufacturer.

(C) Placing of media.

(i) The dumping of media directly on the filter is unacceptable. Instructions for placing media shall be included in the specifications.

(ii) Crushed rock, slag, and similar media shall be washed and screened or forked to remove clays, organic material, and fines.

(iii) Such materials should be placed by hand to a depth of 12 inches above the underdrains and all material should be carefully placed in a manner which will not damage the underdrains. The remainder of the material may be placed by means of belt conveyors or equally effective methods approved by the engineers. Trucks, tractors, or other heavy equipment should not be driven over the filter media during or after construction.

(iv) Prefabricated filter media shall be placed in accordance with recommendations provided by the manufacturer.

(5) Filter hydraulics.

(A) Dosing. Wastewater may be applied to the filters by siphons, pumps, or by gravity discharge from preceding treatment units when suitable flow characteristics have been developed.

(B) Distribution equipment. Settled wastewater may be distributed over the filter media by rotary, horizontal, or travelling distributors, provided the equipment proposed is capable of producing the required continuity and uniformity of distribution over the entire surface of the filter. Deviation from a calculated uniformly distributed volume per unit surface area shall not exceed 10% at any portion of the filter. Filter distributors shall be designed to operate properly at all flow rates. Excessive head in the center column of rotary distributors shall be avoided, and all center columns shall have adequately sized overflow ports to prevent the head from building up sufficiently for the water to reach the bearings in the center column. Distributors shall include cleanout gates on the ends of the arms and shall also include an end nozzle to spray water on the wall of the filter to keep the edge of the media continuously wet. The filter walls shall extend at least 12 inches above the top of the ends of the distributor arms.

(C) Seals. The use of mercury seals is prohibited in the distributors of newly constructed trickling filters. If an existing treatment facility is to be modified, any mercury seals in the trickling filters shall be replaced with oil or mechanical seals.

(D) Distributor clearance. A minimum clearance of six inches shall be provided between the top of the filter media and the distributing nozzles.

(E) Recirculation. In order to insure that the biological growth on the filter media remains active at all times, provisions shall be included in all designs for minimum recirculation during periods of low flow. This minimum recirculation shall not be considered in the evaluation of the efficiency of the filter unless it is part of the proposed specified continuous recirculation rate. Minimum flow to the filters shall not be less than 1.0 mgd/acre of filter surface. In addition, the minimum flow rate must be great enough to keep rotary distributors turning and the distribution nozzles operating properly. For facilities with a design capacity greater than or equal to 0.5 mgd and in which recirculation is included in design computations for BOD₅ removal, recirculation shall be provided by variable speed pumps and a method of conveniently measuring the recycle flow rate shall be provided.

(F) Surface loading. The engineering report shall include calculations of the maximum, design, and minimum surface loadings on the filter(s) in terms of mgd/acre of filter area per day (for the initial year and design year). Hydraulic loadings of filters with crushed rock, slag, or similar media shall not exceed 40 mgd/acre based on design flow. The minimum surface loading shall not be less than 1.0 mgd/acre. Loadings on synthetic (manufactured or prefabricated) filter media shall be within the ranges specified by the manufacturer.

(6) Underdrain system.

(A) Underdrains. Underdrains with semicircular inverts or equivalent shall be provided and the underdrainage system shall cover the entire floor of the trickling filter. Inlet openings into the underdrains shall provide an unsubmerged gross combined area of at least 15% of the surface area of the filter.

(B) Hydraulics. Underdrains and the filter effluent channel floor shall have a minimum slope of 1.0%. Effluent channels shall be designed to produce a minimum velocity of two feet per second at average daily flow rate of application to the trickling filter.

(C) Drain tile. Underdrains for rock media trickling filters shall be either vitrified clay or precast reinforced concrete. The use of half tile for underdrain systems is unacceptable.

(D) Corrosion. Underdrain systems for synthetic media trickling filters shall be resistant to corrosion.

(E) Ventilation. The underdrain system, effluent channels, and effluent pipe shall be designed to permit free passage of air. Drains, channels, and effluent pipes shall have a cross-sectional area such that not more than 50% of the cross-sectional area will be submerged at peak flow plus recirculation. Provision shall be made in the design of the effluent channels to allow the possibility of increased hydraulic loading. The underdrain system shall provide at least one square foot of ventilating area (vent stacks, ventilating holes, ventilating ports) for every 250 square feet of rock media filter plan area. Ventilating area for synthetic media underdrains will be provided as recommended by the manufacturer, but shall be at least one square foot for every 175 square feet of synthetic media trickling filter plan area.

(F) Maintenance. All flow distribution devices, underdrains, channels, and pipes shall be designed so they may be maintained, flushed, and properly drained. The units shall be designed to facilitate cleaning of the distributor arms. A gate shall be provided in the wall to facilitate rodding of the distributor arms.

(G) Flooding. Provisions shall be made to enable flooding of the trickling filter for filter fly control; however, consideration will be given by the commission to alternate methods of filter fly control provided that the effectiveness of the alternate method is verified at a full scale installation. This information shall be submitted with the plans and specifications.

(H) Flow measurements. Means shall be provided to measure flow to the filter and recirculation flows.

(f) Rotating biological contactors (RBC).

(1) General.

(A) RBC units shall be covered and ample ventilation provided. Working clearance of approximately 30 inches should be provided within the cover unless the covers are removable, utilizing equipment normally available on site. Enclosures shall be constructed of a suitable corrosion-resistant material.

(B) The design of the RBC media shall provide for self-cleaning action due to the flow of water and air through the media. Careful selection of media that will not entrap solids should be made.

(C) The RBC tank should be designed to minimize zones in which solids will settle out.

(D) RBC media should be selected which is compatible with the wastewater. Selection of media can be critical where the wastewater has an industrial waste portion which either significantly increases the wastewater temperature or contains a chemical constituent which may decrease the life of the RBC media.

(2) Design.

(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 transfer, the mechanical aeration devices shall also be required to provide sufficient mixing to prevent deposition of mixed liquor suspended solids under any flow condition. A minimum of 100 horsepower per million gallons of aeration basin volume shall be furnished.

(h) Nutrient removal.

(1) Nitrogen removal. Biological systems designed for nitrification and denitrification may be utilized for the conversion/removal of nitrogen. Various physical/chemical processes may be considered on a case-by-case basis.

(2) Phosphorus removal.

(A) Chemical treatment. Addition of lime or the salts of aluminum, or iron may be used for the chemical removal of soluble phosphorus. The phosphorus reacts with the calcium, aluminum, or iron ions to form insoluble compounds. These insoluble compounds may be flocculated with or without the addition of a coagulant aid such as a polyelectrolyte to facilitate separation by sedimentation. When adding salts of aluminum or iron, the designer should evaluate the wastewater to ensure sufficient alkalinity is available to prevent excessive depression of the wastewater or effluent pH. This is of particular importance when the system will also be required to achieve nitrification. The designer is referred to Nutrient Control, Manual of Practice FD-7 Facilities Design, published by the Water Pollution Control Federation and the Process Design Manual for Phosphorus Removal, published by the Environmental Protection Agency, for additional information.

(B) Biological phosphorus removal. Biological phosphorus removal systems will be considered on a case-by-case basis for systems which can produce operating data which demonstrate the capability to remove phosphorus to the required levels. All biological systems which are required to meet a 1.0 mg/liter effluent phosphorus concentration shall make provision for standby chemical treatment to ensure the 1.0 mg/liter is achieved.

(i) Aerated lagoon.

(1) Horsepower. Mechanical aeration units in aerated lagoons shall have sufficient power to provide a minimum of 1.6 pounds of oxygen per pound of BOD₅ applied with the largest unit out of service. If oxygen requirements control the amount of horsepower needed, proposed oxygen transfer rates in excess of two pounds per horsepower-hour must be justified by actual performance data. The amount of oxygen supplied or the pounds of BOD₅ per hour that may be applied per horsepower-hour may be calculated by the use of any acceptable formula. The combined horsepower rating of the aeration units shall not be less than 30 horsepower per million gallons of aerated lagoon volume.

(2) Construction. Earthen ponds shall have large sections of concrete slabs or equivalent protection under each aeration unit to prevent scouring of the earth. Concrete scour pads shall be used in all areas where the velocity exceeds one foot per second. Earthen ponds shall have protection on the slopes of the embankment at the water line to prevent erosion of the slopes from the turbulence in the lagoon. Where the horsepower level is more than 200 horsepower per million gallons of lagoon volume, the pond embankment at the water line shall be protected from erosion with riprap which may be concrete, gunite, a six-inch thick layer of asphalt-saturated or cement-stabilized earth rolled and compacted into place, or suitable rock riprap. The crest and dry slopes of embankments shall be protected from erosion by planting of grass.

(3) Subsequent treatment, discharge systems. Aerated lagoon effluent will normally be routed to additional ponds for secondary treatment and to provide sufficient detention time for disinfection. The secondary ponds system shall consist of two or more ponds. Secondary pond sizing shall not exceed 35 pounds of BOD₅ per acre per day. Hydraulic detention time in a combined aerated lagoon and secondary pond system shall be a minimum of 21 days (based on design flow) in order to provide adequate disinfection. In designing the secondary ponds, BOD₅ removal efficiency in the aerated lagoon(s) may be calculated using the following formula.

Attached Graphic

(j) Wastewater stabilization ponds (secondary treatment ponds).

(1) Pretreatment. Wastewater stabilization ponds shall be preceded by facilities for primary sedimentation of the raw sewage. Aerated lagoons or facultative lagoons may be utilized in place of conventional primary treatment facilities.

(2) Imperviousness. All earthen structures proposed for use in domestic wastewater treatment or storage shall be constructed to protect groundwater resources. Where linings are necessary, the following methods are acceptable:

(A) in-situ or placed clay soils having the following qualities may be utilized for pond lining:

(i) more than 30% passing a 200-mesh sieve;

(ii) liquid limit greater than 30%;

(iii) plasticity index greater than 15; and

(iv) a minimum thickness of two feet;

(B) membrane lining with a minimum thickness of 20 mils, and an underdrain leak detection system;

(C) other methods with commission approval.

(3) Distribution of flow. Stabilization ponds shall be of such shape and size to insure even distribution of the wastewater flow throughout the entire pond. While the shapes of ponds may be dictated to some extent by the topography of the location, long narrow ponds are preferable and they should be oriented in the direction of the prevailing wind such that debris is blown toward the inlet. Ponds with narrow inlets or sloughs should be avoided.

(4) Access area. Storm water drainage shall be excluded from all ponds. All vegetation shall be removed from within the pond area during construction. Access areas shall be cleared and maintained for a distance of at least 20 feet from the outside toes of the pond embankment walls.

(5) Multiple ponds. The use of multiple ponds in pond systems is required. The operation of the ponds shall be flexible, enabling one or more ponds to be taken out of service without affecting the operation of the remaining ponds. The ponds shall be operated in series during routine operation periods.

(6) Organic loading. The organic loading on the stabilization ponds, based on the total surface area of the ponds, shall not exceed 35 pounds of BOD₅ per acre per day. The loading on the initial stabilization pond shall not exceed 75 pounds of BOD₅ per acre per day.

(7) Depth. The stabilization ponds or cells shall have a normal water depth of three to five feet.

(8) Inlets and outlets. Multiple inlets and multiple outlets are required. The inlets and outlets shall be arranged to prevent short circuiting within the pond so that the flow of wastewater is distributed evenly throughout the pond. Multiple inlets and outlets shall be spaced evenly. All outlets shall be baffled with removable baffles to prevent floating material from being discharged, and shall be constructed so that the level of the pond surface may be varied under normal operating conditions. Submerged outlets shall be used to prevent the discharge of algae.

(9) Embankment walls. The embankment walls should be compacted thoroughly and compaction details shall be covered in the specifications. Soil used in the embankment shall be free of foreign material such as paper, brush, and fallen trees. The embankment walls shall have a top width of at least 10 feet. Interior and exterior slope of the embankment wall should be one foot vertical to three feet horizontal. There shall be a freeboard of not less than two feet nor more than three feet based on the normal operating depth. All embankment walls shall be protected by planting grass or riprapping. Where embankment walls are subject to wave action, riprapping should be installed. Erosion stops and water seals shall be installed on all piping penetrating the embankments. Provisions should be made to change the operating level of the pond so the pond surface can be raised or lowered at least six inches.

(10) Partially mixed aerated lagoons.

(A) Horsepower. With partially mixed aerated lagoons, no attempt is made to keep all pond solids in suspension. Mechanical or diffused aeration equipment should be sized to provide a minimum of 1.6 pounds of oxygen per pound of BOD₅ applied with the largest unit out of service. Where multiple ponds are used in series, the power input may be reduced as the influent BOD₅ to each pond decreases. proposed oxygen transfer rates in excess of two pounds per horsepower-hour must be justified by actual performance data.

(B) Pond sizing. Partially mixed aerated lagoons should be sized in accordance with the formula in subsection (i)(3) of this section using K-0.28. Pond length to width ratios should be three to one or four to one.

(C) Imperviousness. Requirements for imperviousness, multiple cells, embankment walls, and inlets and outlets shall be the same as for other secondary treatment ponds.

(k) Facultative lagoon (raw wastewater stabilization pond).

(1) Configuration. The length to width ratio of the lagoon should be three to one, with flow along the length from inlets near one end to outlets at the opposite end (other configurations may be approved if adequate means of prevention of short circuiting are provided). The length should be oriented in the direction of the prevailing winds with the inlet side located such that debris will be blown toward the inlet (generally, the north-northwest side). Inlet baffles shall be provided to collect flotable material. The outlets shall be constructed so that the water level of the lagoon may be varied under normal operating conditions. Storm water drainage shall be prevented from entering the lagoon. The design engineer may wish to locate the facultative lagoon in a central location with regard to the surrounding secondary ponds to facilitate compliance with the buffer zone requirement specified in Chapter 309 of this title (relating to Domestic Wastewater Effluent Limitations and Plant Siting).

(2) Imperviousness. Requirements for imperviousness shall be the same as those for secondary treatment ponds.

(3) Depth. The portion of the lagoon near the inlets shall have a 10 to 12 foot depth to provide sludge storage and anaerobic treatment. This deeper portion should be approximately 25% of the area of the lagoon bottom. The remainder of the pond should have a depth of five to eight feet.

(4) Organic loading. The organic loading, based on the surface area of the facultative lagoon, shall not exceed 150 pounds of BOD₅ per acre per day.

(5) Odor control. The facultative lagoon shall have multiple inlets and the inlets should be submerged approximately 24 inches below the water surface to minimize odor but not disturb the anaerobic zone. Capabilities for recirculation at 50% to 100% of the design flow should be provided. Care should be taken to avoid situations where siphoning of lagoon contents through submerged inlets can occur.

(6) Embankment walls. Refer to subsection (j)(9) of this section.

(7) Subsequent treatment. The facultative lagoon effluent will normally be routed to a wastewater stabilization pond system for secondary treatment. In designing the stabilization pond system, it may be assumed that BOD removal in the facultative lagoon is 50%. The stabilization pond system shall contain two or more ponds.

(l) Filtration. Filtration must be employed as a unit operation to supplement suspended solids removal for those treatment facilities with tertiary effluent limitations (suspended solids effluent quality equal to or less than 10 mg/liter). Filtration may be employed as a unit operation for those treatment facilities with secondary or advanced secondary effluent limitations. The utilization of filtration in the design of the treatment facility normally provides effective removal of suspended biological floc and neutral density trash material which may remain in secondary clarifier effluent. Intermittent filter operation is acceptable where on line controls monitor plant performance or filters are not necessary to meet a specific discharge limitation.

(1) General requirements.

(A) Filter units shall be preceded by final clarifiers designed in accordance with subsection (d) of this section for secondary treatment criteria.

(B) Filtered effluent, and not potable water, shall be utilized as the source of backwash water.

(2) Deep bed, intermittently backwashed granular media filters.

(A) Single media (sand filters), dual media (anthracite-sand filters), or mixed media filter types (nonstratified anthracite, sand, garnet, or other media) are acceptable for application; however, single media filters shall be designed for maximum filtration runs of six hours between backwash periods.

(B) Design filtration rates shall not exceed three gpm/square foot for single media filters, four gpm/square foot for dual media filters, and five gpm/square foot for mixed media filters. The filter area required shall be calculated utilizing the previously listed specified rates at the design flow of the facility. A minimum of two filter units shall be provided with the required filter area calculated with one unit out of service.

(C) Facilities to provide periodic treatment utilizing chlorine or other suitable agents, introduced to the influent stream of the filter units, shall be provided as an operational technique to control slime growth on the filter surface and the backwash storage basin.

(D) A graded gravel layer of a minimum of 15 inches or variable thickness of other filter media support material shall be provided over the filter underdrain system. Filter media support material other than gravel will be reviewed on a case-by-case basis. Normal media depths for the various filter types are as specified below. Media depths significantly different than these must be justified to the commission. The justification must include an analysis of the backwash rates. The uniformity coefficient shall be 1.7 or less. The particle size distribution for dual and mixed media filters shall result in a hydraulic grading of material during backwash which will result in a filter bed with a pore space graded progressively coarse to fine from the top of the media to the supporting layer.

Attached Graphic

(E) The unit piping for the filter units shall be designed to return backwash waste to upstream treatment units. In order to minimize a hydraulic surge, a backwash tank must be included into the design for those plants that do not have some means of flow equalization or surge control. A backwash tank shall be designed to provide storage for filter backwash based upon the number of design daily backwash cycles and the volume required for each backwash. Calculations must be provided to the commission demonstrating that the performance of the plant will not diminish with the discharging of the backwash water into the treatment process. Enclosed backwash tanks shall be vented to maintain atmospheric pressure. Surge control shall be provided to the backwash system to limit flow rate variations to no more than 15% of the design flow of the treatment units that will receive the backwash water. For these calculations, an influent lift station is not considered as a treatment unit and, therefore, is not bound by the 15% design flow requirement.

(F) Pumps for backwashing filter units shall be designed to deliver the required rate with the largest pump out of service. The backup pump unit may be uninstalled provided that the commission is satisfied that the spare unit can be quickly installed and placed into operation. Valve arrangement for isolating a filter unit for backwashing shall provide ready access for the operator. Provision for manual override shall be provided for any backwash system employing automatic control.

(G) Head loss indicators shall be provided for all filter units.

(H) Backwash for dual or mixed media filters shall provide a minimum bed expansion of 20%. A surface scour shall be provided prior to or during the backwash cycle. Backwash flow rates at 15 to 20 gpm/square foot and at a cycle time of 10 to 15 minutes should be provided. The backwash cycle shall provide media fluidization at the end of the cycle to restratify the media. Backwash for single media filters should be provided by a surface air scour or combination air-water scour and washwater at recommended rates as follows.

Attached Graphic

(I) The filter underdrain system shall be of a design adaptable to wastewater treatment, providing a uniform distribution of filter backwash and freedom from excessive orifice plugging. Wash water collection trough bottoms shall be located a minimum of six inches above the maximum elevations of the expanded media. A minimum freeboard of three inches shall be provided in addition to the design upstream depth of the wash water media. A minimum freeboard of three inches shall be provided in addition to the design upstream depth of the wash water trough to prevent submerged trough conditions during filter backwashing.

(3) Multi-compartmented low head filters with continuous operation (automatic backwash). This paragraph contains the design criteria for multi-compartmented low head filters where the applicable criteria are different than those contained in paragraphs (1) and (2) of this subsection. All other criteria included in paragraphs (1) and (2) of this subsection will apply to multi-compartmented low head filters with continuous operation.

(A) Filtration rates. Filtration rates shall not exceed three gpm/square foot for single media filters and four gpm/square foot for dual media filters based on the design flow rate applied to the filters. The total filter area should be provided in two or more units and the filtration rate shall be calculated on the total available filter area with one cell of each unit out of service. Manufacturer's recommended rates should be utilized if substantiated by test data.

(B) Backwash. The backwash rate shall be adequate to fluidize and expand each media layer a minimum of 20%. Provision should be made for an approximate rate of 10 gpm/square foot over a 30 to 60 second interval. Manufacturer's recommended rates should be utilized if substantiated by test data. Pumps for backwashing filter units shall be adequate to provide the required rate with the largest pump out of service. It is permissible for the backup unit to be an uninstalled unit, provided that the installed unit can be easily removed and replaced. Waste filter backwash water shall be returned to upstream units, preferably the final clarifiers, for treatment.

(C) Backwash surge control. The rate of return of waste filter backwash water to treatment units shall be controlled such that the rate does not exceed 15% of the design flow of the treatment units. The hydraulic and organic load from waste backwash water shall be considered in the overall design of the treatment plant. Where waste backwash water is returned for treatment by pumping, adequate pumping capacity shall be provided with the largest unit out of service. It is permissible for the backup unit to be an uninstalled unit, provided that the installed unit can be easily removed and replaced.

(4) Alternative design for effluent polishing. Where filters are proposed to remove remaining visible particles, other criteria will be considered on a case-by-case basis.

Source Note: The provisions of this §317.4 adopted to be effective November 26, 2015, 40 TexReg 8341