Section 123-5-006 - Mechanical Plants006.01 Flow Measurement 006.01A Plants with an average daily design flow less than 50,000 gallons per day will have either influent or effluent flow measurement. Lift station elapsed time meters will satisfy this requirement. 006.01B Plants with an average daily design flow of 50,000 gallons per day or greater will have continuous flow measurement and totalizing equipment. 006.01C Flow measurement equipment will be sized to function accurately over the full range of flows received at the treatment plant. The flow measurement equipment will be protected from freezing. 006.02 Bar Screens & Comminutors006.02A Every mechanical plant will have either screening devices or comminutors installed. 006.02B At least two flow channels will be provided for screens or comminutors. Each channel will be designed to handle the peak hourly flow. 006.02C Comminutors will be located after grit removal facilities. If grit removal is not provided, the flow channels upstream of comminutors will have a gravel trap installed to protect the comminutor from damage. 006.02D Where a comminutor is installed, a bypass bar screen will also be provided. Flows exceeding the operating capacity of the comminutor will automatically be diverted to the bar screen channel. 006.03 Grit Removal 006.03A Grit removal facilities, when provided, will be located after screening devices, but prior to comminutors or other treatment units. 006.03B Ventilation will be provided for grit removal facilities that are covered or housed in a building. Fresh air will be continuously forced into the facility at a minimum rate of 12 air changes per hour or intermittently at a minimum rate of 30 air changes per hour. 006.03C Grit removal facilities will be provided with drains to dewater the unit for maintenance and be equipped with a bypass line. 006.03D Grit removal facilities will be provided with means to dry and store grit prior to disposal. Grit removal and grit handling facilities will be protected from freezing. 006.04 Primary Clarifiers 006.04A Plants with an average daily design flow of 100,000 gallons per day or greater will have multiple primary clarifiers, if primary clarifiers are part of the facility. Each clarifier will be capable of independent operation. When a single primary clarifier is provided, a bypass line to the secondary treatment unit will be provided. 006.04B The minimum sidewall depth of primary clarifiers will be 10 feet.006.04C The horizontal distance from the inlet to outlet will be at least 10 feet, unless the design includes a method to prevent short-circuiting. 006.04D The surface overflow rate will not exceed 1000 gallons per day per square foot at the average daily flow or 3000 gallons per day per square foot at the peak hourly flow. Returned flows from later treatment units will be included in the flow used to calculate the surface overflow rate. 006.05 Activated Sludge Basins 006.05A The dimensions of each aeration tank will be such that it maintains effective mixing and utilization of air. Vertically mixed tanks will have a liquid depth of not less than 10 feet or greater than 30 feet. Horizontally mixed tanks will have a minimum liquid depth of 5.5 feet. 006.05B All aeration tanks will have a freeboard of not less than 18 inches. Aeration tanks using mechanical surface aerators will have at least three feet of freeboard. 006.05C Inlets and outlets for each aeration basin will be suitably equipped with valves, gates, stop plates, weirs, and other devices to permit controlling the flow to any unit and maintain reasonably constant liquid levels. Where multiple aeration basins are provided, the hydraulic properties of the inlets and outlets will be sized to carry the design peak instantaneous flow with any single aeration tank out of service. 006.05D Channels and pipes that carry solids in suspension will be designed to maintain a self-cleansing velocity at all rates of flow within the design limits. Open channels will be designed to prevent splashing out of the channel. 006.05E The shape of aeration tanks, the location of influent and return sludge pipes, and the aeration equipment will be designed to prevent short-circuiting through the tanks. 006.05F The rate of sludge return will be controlled by means of variable speed motors, drives, valves, or timers to pump sludge at the required rates. If motor driven return sludge pumps are used, a minimum of two pumps is required. The pumps will be sized to meet the required pumping rates with the largest pump out of service. Pumps will have at least a 3-inch suction and discharge opening. No standby unit is required for air lift pumps, provided the design facilitates rapid and easy cleaning. Airlift pumps will be at least 3 inches in diameter. Airlift pumps used to waste or return sludge will be designed so their operation is unaffected by changes of air demand of other treatment units. 006.05G Return sludge piping will be at least four inches in diameter and designed to maintain a velocity of not less than two feet per second when return facilities are operating at normal return rates. Suitable devices will be provided for measuring and controlling the return rate. The flow swill be returned near the inlet or head end of the aeration tank, so that it rapidly mixes with the influent. Return sludge piping will be designed to provide the operator easy access for collection of return sludge samples. 006.05H Sludge wasting facilities will be provided for all activated sludge facilities. Sludge wasting facilities include units such as sludge thickening tanks, sludge digestion facilities, sludge storage tanks, dewatering facilities or any practical combination of these units. A means of measuring, controlling, and sampling of waste activated sludge must be provided. 006.05I Aeration equipment will be capable of maintaining a minimum dissolved oxygen concentration of 2.0 milligrams per liter in the mixed liquor at all times and provide thorough mixing of the mixed liquor. For all activated sludge processes, except extended aeration, the minimum oxygen requirements will be 1.1 pounds of oxygen per pound of the design peak hourly five-day biochemical oxygen demand (BOD5) applied to the aeration tank. For the extended aeration process the minimum oxygen requirement will be 1.5 pounds of oxygen per pound of the design peak hourly BOD5 applied to the aeration tank. Where nitrification is required, the oxygen requirement for oxidizing ammonia must be added to the above requirement for oxidation of five-day biochemical oxygen demand. The minimum nitrogenous oxygen requirement will be 4.6 pounds of oxygen per pound of the design peak hourly total Kjeldahl nitrogen applied to the aeration tank.
006.05J The design of the diffused air system will be based on either standard engineering equations that calculate the transfer efficiency using such factors as tank depth, alpha and beta factors, device transfer efficiencies, wastewater temperature and altitude of the plant or empirically determined air requirements. Designs based on standard engineering equations will include the transfer efficiency in the specifications. Empirical air requirements for all activated sludge processes, except extended aeration, will be a minimum of 1500 cubic feet of air (at standard conditions of pressure, temperature, and humidity) per pound of five-day biochemical oxygen demand (BODg) applied to the aeration tank. For the extended aeration process the air requirements will be 2050 cubic feet of air per pound of BODs applied to the aeration tank.
006.05K The specified capacity of blowers or air compressors will take into account the range of ambient air conditions that may occur at the site. 006.05L A minimum of two blowers will be provided for any activated sludge facility. The capacities of blowers or air compressors will be able to meet the air demand of the system with the largest unit out of service. 006.05M Plants with less than four independent aeration tanks will be designed to allow removal of the diffusers for service or replacement without dewatering the tank. 006.05N In the absence of specific design information, the oxygen requirements will be calculated using a transfer rate not to exceed two pounds of oxygen per horsepower per hour. The design requirements of a mechanical aeration system will maintain a dissolved oxygen concentration of two milligrams per liter in the mixed liquor at all times throughout the tank, maintain solids in suspension, and meet the oxygen demand requirements with the largest unit out of service. 006.06 Trickling Filters 006.06A Trickling filters will be preceded by primary sedimentation to remove scum, grease, and floatable debris or by other suitable treatment facilities. 006.06B The volume of trickling filter media will be based upon pilot testing with the particular wastewater or by using design equations. The volume of media will be calculated on the design maximum day organic loading. The design volume will also include treatment of the organic loading of recycle flows from other treatment units such as, digester supernatant, mechanical dewatering supernatant, or others. 006.06C Media will have a minimum depth of six feet above the underdrains. Rock or slag media will not exceed 10 feet in depth. Manufactured media will not exceed the depth recommended by the manufacturer.
006.06D Rock media swill not contain more than five percent by weight of pieces whose longest dimension is three times the least dimension. It will be free of thin, elongated and flat pieces, dust, clay, sand, or fine material and will conform to the following size and grading. Passing 4 1/2 inch screen - 100%
Retained on 3 inch screen - 95-100% by weight
Passing 2 inch screen - 0 - 2% by weight
Passing 1 inch screen - 0-1% by weight
006.06E Manufactured media will be resistant to ultraviolet degradation, disintegration, erosion, aging, all common acids and alkalis, organic compounds, and fungus or biological attack. Media will be structurally capable of supporting the weight of a person or a suitable access walkway will be provided for maintenance of the distributor. Manufactured media for carbonaceous treatment will not have a specific area greater than 30 square feet per cubic foot. Manufactured media for second stage ammonia treatment will not have a specific area greater than 45 square feet per cubic foot. 006.06F The piping system, including dosing and distributor equipment, will be designed to provide capacity for the design peak hourly flow including the recirculation rate. The design recirculation ratio (ratio of recirculation flow to design average flow) will be based on the amount of variation in plant flow between design minimum daily flow and design peak hourly flow. 006.06G The piping system will be designed for recirculation as required to achieve the design wetting rate, or design dosing rate, and to achieve the design treatment efficiencies. The design of recirculation facilities will provide for continuity of service, variable recirculation rates, measurement of the recirculation rate, and subject to control by the plant operator. A minimum of two recirculation pumps must be provided. 006.06H The wastewater will be distributed by rotary, fixed nozzle, or other suitable distributor that ensures uniform distribution over the entire surface area. The distribution equipment will be able to apply wastewater at the design hydraulic loading rate (gallons per minute per square foot) or dosing rate (inches per revolution). At design average flow, the deviation from a calculated uniformly distributed volume per square foot of the filter surface will not exceed plus or minus 10 percent at any point. 006.06I There will be a minimum of 12 inches of clearance between the media and the distribution arms. 006.06J A freeboard of four feet or more will be provided for tall manufactured uncovered trickling filters to contain windblown spray. 006.06K Appropriate valves, sluice gates, or other structures will be provided to enable flooding of the filter. Access will be provided around the periphery of the underdrain system to allow flushing of the underdrains. 006.06L The underdrain system will cover the entire floor of the filter. Inlet openings into the underdrains will have an unsubmerged combined area equal to at least 15 percent of the surface area of the filter. The underdrains will have a minimum slope of one percent. Effluent channels will be designed to produce a minimum velocity of two feet per second at design average daily flow rates including recirculated flows. 006.06M The underdrain system and effluent channels will be designed to permit free passage of air. The size of the drains and channels will be designed such that no more than 50 percent of their cross-sectional area will be submerged under design peak flow conditions. Forced ventilation will be provided for covered trickling filters to insure adequate oxygen for process requirements. The design of ventilation equipment will provide for operator control of air-flow in accordance with outdoor seasonal temperatures. 006.06N All distribution devices, underdrains, channels, and pipes will be installed so that they allow for proper maintenance. 006.07 Intermediate and Final Clarifiers 006.07A Facilities with an average daily design flow of 100,000 gallons per day or greater will have multiple final clarifiers. Effective flow splitting devices and control appurtenances will be provided to permit proper proportioning of flows and solids to each unit, throughout the expected range of flows. 006.07B The minimum distance between inlet and outlet will be 10 feet, unless the design includes a method of preventing short-circuiting. The minimum side water depth ill be 12 feet. Walls of settling tanks will extend at least six inches above the surrounding ground surface and will have a minimum of 12 inches of freeboard. 006.07C Surface overflow rates will be calculated based on design average and design peak hourly influent flow conditions. The solids loading rate will be calculated based on the design average daily flow or design maximum day flow plus the design maximum return sludge rate plus the design mixed liquor suspended solids (MLSS). 006.07D Influent pipes and center columns of circular clarifiers will have a maximum velocity of 2.0 feet per second. Inlet structures will be designed to dissipate the inlet velocity, to distribute the flow equally both horizontally and vertically and to prevent short-circuiting. Corner pockets and dead ends will be eliminated and corners fillets or channeling will be used where necessary. Provisions will be made to eliminate or remove floating materials from inlet structures. 006.07E Overflow weirs will be adjustable to correct for differential settlement. Weirs will be located to optimize hydraulic detention time and minimize short-circuiting. The loading rate at design peak hourly flow will not exceed 20,000 gallons per day per lineal foot of weir for plants with a design flow less than one million gallons per day (MGD) or 30,000 gallons per day per lineal foot of weir for plants with a design flow greater than one million gallons per day. Weir troughs will be designed to prevent submergence at design peak hourly flow and maintain a velocity of 1.0 foot per second at one-half the design average flow. 006.07F Provisions for scum collection and removal will be provided for all clarifiers. Scum will be discharged to a sludge treatment unit or scum storage unit that prevents recycling of the scum through the secondary treatment units. Scum will not be discharged to the head of the plant, unless it can be permanently removed from the system in a primary clarifier or similar treatment unit. 006.07G All final and intermediate clarifiers will have a mechanical sludge scraper or collection system. Suction withdrawal will be provided for activated sludge final clarifiers with a diameter greater than 60 feet. Each settling tank will have its own sludge withdrawal lines to insure adequate control of sludge wasting from each tank. 006.07H All settling tanks will be equipped with appropriate equipment such as machinery covers, walkways, handrails, area lighting, and slip resistance surfaces that enhance the safety of the operator. The design will provide for convenient and safe access to routinely checked items such as gear-boxes, scum removal mechanisms, baffles, weirs, inlet areas, and effluent channels. 006.08 Sludge Treatment, Storage, and Disposal006.08A Facilities for the treatment and storage of sludge will be provided at all mechanical wastewater treatment plants, unless arrangements are made for processing sludge at another treatment facility. Where sludge is hauled off site for treatment and disposal, an all-weather access road to the treatment plant will be provided. 006.08B Treatment plants using suspended growth processes (activated sludge variations) will, at a minimum, be provided with on-site sludge storage facilities to allow for proper sludge wasting and operational control of the treatment process. 006.08C Sludge withdrawal piping will have a minimum diameter of eight- inches for gravity withdrawal, and six-inches for pump suction and discharge lines. There will be a minimum of four feet of head above the discharge pipe where withdrawal is by gravity. Provisions for draining and flushing discharge lines will be provided for all gravity sludge piping. The minimum slope for gravity piping will be three percent. A minimum positive head of 24 inches will be provided at the suction side of centrifugal pumps. Suction heads for positive displacement pumps will not exceed 10 feet. 006.08D Anaerobic Sludge Digestion 006.08D1 Digester tanks will be covered with either a fixed or floating cover. Covers will be sealed to protect against gas leaks. The top of the tank will have at least two manholes, not less than 30 inches in diameter, in addition to the gas dome. 006.08D2 The tank bottom will slope towards the withdrawal pipe. A minimum bottom slope of 1:12 is required for tanks equipped with a suction mechanism for sludge withdrawal. A minimum slope of 1:4 is required where sludge is to be removed by gravity. 006.08D3 Where mixing equipment is not provided within the digester, multiple sludge inlets, outlets, and other suction or discharge points will be used to achieve mixing and allow for flexibility of operation. One inlet will discharge above the liquid level near the tank center to facilitate the breakup of scum. Raw sludge inlets will be designed to avoid short-circuiting. 006.08D4 An unvalved, vented overflow will be included to prevent damage to the digester in case of accidental overfilling. The overflow will return the sludge to a treatment unit. 006.08D5 Where rational calculations are not submitted to justify the required tank capacity for heated, anaerobic digestion of domestic wastewater sludge, the minimum volume for completely mixed systems will be 1000 cubic feet of active digester volume for each 80 pounds of volatile solids per day. Where partial mixing is provided, the minimum volume will be 1000 cubic feet of active digester volume for each 40 pounds of volatile solids per day. 006.08D6 In two stage systems, where the secondary digesters are utilized for thickening and storage, the volume of the secondary digesters will not be used to meet the volume requirement for sludge digestion. 006.08D7 The digester gas system will be designed so that gas is under positive pressure during all normal operations, including during sludge removal from the digester. All enclosed building areas where gas leaks could occur will be provided with ventilation systems. 006.08D8 Supernatant piping will not be less than six-inches in diameter.006.08E Aerobic Digestion 006.08E1 Where aerobic digesters are used, multiple units will be provided where the design average wastewater flow is at least 100,000 gallons per day. 006.08E2 Volatile suspended solids loading will not exceed 300 pounds per 1,000 cubic feet of volume per day in the digestion units. 006.08E3 A minimum of 15 days of solids retention time will be provided for waste activated sludge and 20 days for a combination of primary and waste activated sludge, or for primary sludge alone. In order to qualify as a Process to Significantly Reduce Pathogens (See Title 119 - Rules and Regulations Pertaining to the Issuance of Permits Under the National Pollutant Discharge Elimination System, Chapter 12, 004), aerobic digestion processes will have a minimum mean cell residence time of 40 days at 20° Celsius and 60 days at 15° Celsius. If supernatant separation is performed in the digestion tank, additional volume is required to allow for decanting. This added volume will not be included in the determination of mean cell residence or solids retention time. 006.08E4 Sufficient air will be provided to keep solids in suspension and maintain a dissolved oxygen concentration of 1.0 milligram per liter. The minimum amount of air supply will be 30 cubic feet per minute per 1000 cubic feet of tank volume with the largest blower out of service. The minimum requirement for mechanical aerators will be 1.0 horsepower per 1000 cubic feet of digester volume. 006.08E5 Facilities will be provided for the separation and decanting of supernatant. The supernatant draw off unit will be designed to prevent recycling of scum and grease back to the wastewater treatment processes. A method for the collection and disposal of scum and grease will be provided. 006.08E6 An unvalved high-level overflow will be provided to return digester overflow back to the head of the plant or to a secondary treatment unit to protect against accidental overfilling.006.09 Disinfection 006.09A Chlorine Disinfection 006.09A1 The chlorination system and equipment will be sized to produce an effluent meeting applicable bacterial limits at design peak flow conditions. 006.09A2 Where 100 or 150 pound chlorine gas cylinders are used, the cylinders will be stored in an upright position with adequate support brackets and chains at the two-thirds cylinder height for each cylinder. Storage containers for hypo-chlorite solutions will be of sturdy, non-metallic lined construction and will be provided with secure tank tops and pressure relief and overflowing piping. Storage tanks will be vented outside. Tanks will be located where leakage will not cause corrosion or damage to other equipment. A means of secondary containment to control spills and facilitate cleanup will be provided. 006.09A3 Scales for weighing cylinders and containers will be provided at all plants using chlorine gas. 006.09A4 The disinfectant will be mixed as rapidly as possible. A minimum contact time of 15 minutes at design peak hourly flow or maximum pumping rate will be provided after thorough mixing. The chlorine contact tank swill be constructed to reduce short-circuiting. 006.09A5 Piping systems will be as simple as possible and selected and manufactured to be suitable for chlorine service. Piping will be well supported and protected against temperature extremes. The chlorine piping will be color-coded and labeled to distinguish it from other plant piping. Piping and fittings for chlorine and sulfur dioxide will be designed so that interconnection between the two systems cannot occur. 006.09A6 If gas chlorination equipment or chlorine cylinders are to be in a building used for other purposes, a gas-tight room will separate this equipment from other portions of the building. Floor drains from the chlorine room will not be connected to floor drains from other rooms. Doors to this room will open to the outside, and will be equipped with panic hardware. Rooms will be at ground level and should permit easy access to all equipment. Storage areas for one-ton cylinders will be separated from the feed area. A clear glass, gas-tight window will be installed in an exterior door or interior wall of the chlorinator room to permit the units to be viewed without entering the room. 006.09A7 Forced, mechanical ventilation will be installed that provides one complete air exchange per minute when the room is occupied. The entrance to the air exhaust duct from the room will be near the floor. The point of discharge will be located so as not to contaminate the air inlet of another building. Air inlets will be located to provide cross ventilation with air at such a temperature that will not adversely affect the chlorination equipment. The outside air inlet will be at least three feet above grade. The vent hose from the chlorinator will discharge to the outside atmosphere above grade. 006.09A8 Switches for fans and lights will be located outside the room at the entrance. If the fan can be controlled from more than one point, a labeled signal light indicating fan operation should be provided at each entrance. 006.09A9 Respiratory protection equipment, meeting the requirements of National Institute for Occupational Safety and Health (NIOSH) will be available where chlorine gas is handled, and will be stored at a convenient location, but not inside any room where chorine is used or stored. 006.09B Dechlorination 006.09B1 The dechlorination chemical swill be introduced at a point in the process where the hydraulic turbulence is adequate to assure complete and thorough mixing. If no point exists, mechanical mixing will be provided. A minimum of 30 seconds for mixing and contact time will be provided at the design peak hourly flow or maximum rate of pumping. 006.09C Ultraviolet (UV) Radiation Disinfection 006.09C1 For facilities with a design average day flow of 100,000 gallons or more, a UV disinfection system will consist of multiple banks of lamp modules in series that are capable of continuously disinfecting the effluent with one bank out of service. 006.09C2 Provisions will be made for easy removal and inspection of the UV lamps without draining the disinfection chamber. 006.09C3 The contact chamber will be designed so that the UV lamps remain submerged at all times during operation and to maintain a near constant water level at all design flow conditions.123 Neb. Admin. Code, ch. 5, § 006
Amended effective 9/4/2019