(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.
Attached Graphic
  (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 (QD W);
    (B) wet weather maximum 30-day average (QD W); and
    (C) two-hour peak flow (Qp 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.
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