An owner who proposes to alter, or re-rate an existing wastewater treatment facility, or to obtain approval for an existing wastewater treatment facility after that wastewater treatment facility's permit has expired, is required to use the wastewater treatment facility's current operating data for flow and organic loading (biochemical oxygen demand or carbonaceous biochemical oxygen demand, total suspended solids, and ammonia nitrogen, as required by the permit) as the design basis. The compiled data must meet the criteria outlined in paragraphs (1) and (2) of this section.
  (1) Flows.
    (A) When an existing wastewater treatment facility is to be re-rated or altered, the wastewater treatment facility's data for the last five years must be used to determine the annual average flow, the maximum monthly average flow, the peak flow, the ratio of maximum monthly average flow to annual average flow, and the ratio of the peak flow to the annual average flow. All flow data for these analyses must be collected by a totalizing meter. If the wastewater treatment facility is less than five years old, all existing data must be used. All calculations and assumptions must be included in the engineering report.
    (B) An analysis of the peak flow must be based on a frequency distribution analysis using flow charts for each individual day.
    (C) The projected peak flow must be the result of collection system monitoring or modeling based on a two-year, 24-hour storm event for the service area.
    (D) For a wastewater treatment facility that will not be affected by future growth, the design flow for a re-rating or alteration must be calculated using the wastewater treatment facility's average flow plus one standard deviation.
    (E) For a wastewater treatment facility that will be affected by future growth, the design flow for a re-rating or alteration must be based on future flow, calculated using anticipated changes from the existing flow. The design analysis may use a linear regression or other appropriate statistical method for predicting the design flow when significant data exists.
  (2) Organic loadings.
    (A) When an existing wastewater treatment facility is to be re-rated or altered, the design organic loading must be calculated based on the average daily organic loading.
      (i) The data used to determine the organic loading must be from the analyses of at least three composite samples of the influent wastewater per week, taken during days with representative flow, for a period of at least one year. If samples are collected at a frequency of less than three times per week or less than a three-part composite sample, the executive director may require an owner to collect and analyze additional samples that are representative of actual conditions at the wastewater treatment facility. The data must include samples collected during both wet and dry weather conditions.
      (ii) If the samples are not 24-hour flow-weighted composite samples, the samples must be representative of the peak loading.
      (iii) Sample data must include the following parameters, at a minimum, unless monitoring of the parameter is not required by the wastewater permit:
        (I) five-day carbonaceous biochemical oxygen demand or five-day biochemical oxygen demand;
        (II) total suspended solids; and
        (III) ammonia-nitrogen.
    (B) The owner must provide an engineering analysis for the minimum sampling period in the engineering report, which must include:
      (i) a summary of the monthly data;
      (ii) the average monthly load; and
      (iii) the standard deviation of the monthly data.
    (C) For a wastewater treatment facility that will not be affected by future growth, the design organic loading for a re-rating or alteration must be calculated using the wastewater treatment facility's average organic loading plus one standard deviation.
    (D) For a wastewater treatment facility that will be affected by future growth, the design organic loading for a re-rating or alteration must be based on future loading calculated using anticipated changes from the existing loading. The design analysis may use a linear regression or other appropriate statistical method for predicting the design organic load when significant data exists.
    (E) The design organic loading must be used to determine the required size of each treatment unit that provides treatment of organic waste.

Source Note: The provisions of this §217.34 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254