1. (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 one of the following:

   1. (1) Activated Biological Filter (ABF) System. An ABF 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. (2) Trickling Filter/Solids Contact (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 (BOD₅), 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 may include a sludge re-aeration basin.
   3. (3) Roughing Filter/Activated Sludge (RF/AS) System. A 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 BOD₅, followed by a final clarifier. A RF/AS system circulates activated sludge to the aeration basin.
   4. (4) Activated Biological Filter/Activated Sludge (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 BOD₅, followed by a final clarifier. An ABF/AS system recirculates activated sludge to the trickling filter.
   5. (5) Trickling Filter/Activated Sludge (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 BOD₅, followed by a final clarifier. A TF/AS system circulates activated sludge to the aeration basin.

2. (b) Process Design.

   1. (1) Attached and suspended growth sub-processes in a dual system must be designed in an integrated process that includes the effluent quality from the first stage in determining the design basis of the second stage.
   2. (2) A design 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. (3) For a treatment process 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. (4) A design may include estimates of the applicable design equations and methodology used for a single stage process.

3. (c) Treatment Unit Design. The detailed 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, as well as the following items:

   1. (1) Pretreatment. Pretreatment of a dual system must conform to requirements for a first-stage process.
   2. (2) Snail Control. A dual system must include a low-velocity channel between the first stage and second stage treatment units for control of snails.

   3. (3) Return sludge.

      1. (A) A dual system that includes recirculation of activated sludge or sloughing to trickling filters must prevent recirculation of pieces larger than will pass through the distributor nozzles or the filter media voids.
      2. (B) The trickling filters in a dual system that recirculates sludge to the trickling filters must be high-rate, vertical flow, and fully corrugated media.
      3. (C) Sludge must be incorporated into 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. (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 BOD₅ per day. An aeration system for second-stage treatment units in systems designed for nitrification must transfer sufficient oxygen to meet stoichiometric requirements for:

      1. (A) biomass growth;
      2. (B) respiration for both carbonaceous material oxidation and nitrification; and
      3. (C) oxygen demand due to biomass sloughing events from the first stage.

   5. (5) Sludge Age.

      1. (A) A design of second-stage suspended growth processes must operate in a way that varies the age of the sludge.

      2. (B) The mean cell residence time must be:

         1. (i) at least 1.5 days for the suspended growth process for TF/SC systems; or
         2. (ii) at least 3.0 days if the second process is an activated sludge aeration basin.
      3. (C) A nitrifying dual 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. (6) Hydraulic Residence Time. A design of second-stage processes must have a minimum hydraulic residence time of:

      1. (A) 0.5 hour if the second process is an aerated solids contact basin; or
      2. (B) 3.0 hours if the second process is an activated sludge aeration basin.

   7. (7) Nitrification Design. A design for nitrification using dual treatment processes must include:

      1. (A) a sludge re-aeration basin if the second process is an aerated solids contact basin; or
      2. (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.