(a) Classification. A trickling filter or other attached-growth treatment unit in series with a suspended-growth process is considered a dual treatment process that is classified as an Activated Biological Filter (ABF) System, a Trickling Filter/Solids Contact (TF/SC) System, a Roughing Filter/Activated Sludge (RF/AS) System, an Activated Biological Filter/Activated Sludge (ABF/AS) System, or a Trickling Filter/Activated Sludge (TF/AS) System.      
        (1) ABF System. An ABF system consists of a tricking filter and a final clarifier. An ABF system recirculates settled solids from the final clarifier through the trickling filter with no separate aeration basin or solids contact basin.       
        (2) TF/SC System. A TF/SC system consists of a trickling filter sized to remove the majority of the soluble five-day biochemical oxygen demand (BOD                 5                ), followed by an aerated solids contact basin sized to provide polishing and improved sludge settleability, followed by a final clarifier. A TF/SC system recirculates activated sludge to a solids contact basin. The design of a TF/SC system may include a sludge re-aeration basin.       
        (3) RF/AS System. An RF/AS system consists of a trickling filter sized to perform primary treatment, followed by an aeration basin sized to remove the majority of the soluble BOD                 5                , followed by a final clarifier. An RF/AS system circulates activated sludge to the aeration basin.       
        (4) ABF/AS System. An ABF/AS system consists of a trickling filter sized to perform primary treatment, followed by an aeration basin sized to remove the majority of the soluble BOD                 5                , followed by a final clarifier. An ABF/AS system recirculates activated sludge to the trickling filter.       
        (5) TF/AS System. A TF/AS system consists of a trickling filter sized to perform roughing and concentration dampening, followed by an intermediate clarifier, followed by an aeration basin sized to remove the majority of the soluble BOD                 5                , followed by a final clarifier. A TF/AS system circulates activated sludge to the aeration basin.       
      (b) Process Design.      
        (1) Attached and suspended growth sub-processes in a dual treatment system must be designed through an integrated process that includes the effluent quality from the first stage in determining the design basis of the second stage.       
        (2) The design of a dual treatment system must include an estimate of the performance of the second stage of a dual system using data from existing similar installations or applicable pilot studies.       
        (3) For a dual treatment system design in which activated sludge is recycled to first-stage trickling filters, the design must not include the reduction of oxygen demand to the second-stage aeration basin because of sludge recirculation to the trickling filters.       
        (4) The design of a dual treatment system may include estimates of the applicable design equations and methodology used for a single stage process.       
      (c) Treatment Unit Design. The design of a suspended and attached growth system must include all of the features and operational capabilities required for the same treatment unit used for single-process treatment. The design of a suspended and attached growth system must also include the design for pretreatment, snail control, return sludge, aeration, sludge age, hydraulic residence time, and nitrification design.      
        (1) Pretreatment. Pretreatment for a dual treatment system must conform to the requirements for a first-stage process.       
        (2) Snail Control. A dual treatment system must include a low-velocity channel between the first-stage and second-stage treatment units for control of snails.       
        (3) Return sludge.                  (A) A dual treatment system that includes recirculation of activated sludge or sloughing to trickling filters must prevent recirculation of pieces too large to pass through the distributor nozzles or the trickling filter media voids.                   (B) The trickling filters in a dual treatment system that recirculates sludge to the trickling filters must be of a high-rate, vertical flow design that uses fully corrugated media.                   (C) Sludge must be incorporated into the influent prior to application to trickling filters, and must be incorporated into the effluent from first-stage processes prior to being introduced into second-stage aeration basins.         
        (4) Aeration. An aeration system for second-stage treatment units in a dual system not designed for nitrification must transfer at least 1.2 pounds of oxygen per pound of first stage effluent BOD                 5                per day. An aeration system for second-stage treatment units in systems designed for nitrification must transfer sufficient oxygen to meet stoichiometric requirements for:                  (A) biomass growth;                   (B) respiration for both carbonaceous material oxidation and nitrification; and                   (C) oxygen demand due to biomass sloughing events from the first-stage.         
        (5) Sludge Age.                  (A) A second-stage suspended growth process must operate in a way that varies the age of the sludge.                   (B) The mean cell residence time must be:                      (i) at least 1.5 days for the suspended growth process for TF/SC systems; or                       (ii) at least 3.0 days if the second process is an activated sludge aeration basin.                     (C) A nitrifying dual treatment system must maintain a total combined mean cell residence time in the attached and suspended growth systems of at least 10.0 days with capability to provide at least 6.0 days mean cell residence time in the suspended growth process alone.         
        (6) Hydraulic Residence Time. A design of second-stage processes must have a minimum hydraulic residence time of:                  (A) 0.5 hour if the second process is an aerated solids contact basin; or                   (B) 3.0 hours if the second process is an activated sludge aeration basin.         
        (7) Nitrification Design. A wastewater treatment facility designed for nitrification using a dual treatment system must include:                  (A) a sludge re-aeration basin if the second process is an aerated solids contact basin; or                   (B) an intermediate clarifier if the second process is an activated sludge aeration basin.         
     Source Note:        The provisions of this §217.184 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254