Montana DEQ - Circular DEQ 2 - Design Standards for ... - Force Flow

Supplemental Treatment Process Chapter 110112.43 Backwash Surge ControlThe rate of return of waste filter backwash water to treatment units must becontrolled so that the rate does not exceed 15 percent of the design averagedaily flow rate to the treatment unit. The hydraulic and organic load fromwaste backwash water must be considered in the overall design of thetreatment plant. Surge tanks must have a capacity of at least two backwashvolumes, although additional capacity should be considered to allow foroperational flexibility. Where waste backwash water is returned fortreatment by pumping, adequate pumping capacity must be provided withthe largest unit out of service.112.44 Backwash Water Storage112.5 Filter Media SelectionTotal backwash water storage capacity provided in an effluent clearwell orother unit must equal or exceed the volume required for two completebackwash cycles.Selection of proper media type and size will depend on required effluent quality, thetype of treatment provided prior to filtration, the filtration rate selected, and filterconfiguration. In dual or multi-media filters, media size selection must considercompatibility among media. Media must be selected and provided to meet specificconditions and requirements relative to the project under consideration. Theselection and sizing of the media must be based on demonstrated satisfactory fieldexperience under similar conditions. All media must have a uniformity coefficientof 1.7 or less. The uniformity coefficient, effective size, depth, and type of mediamust be set forth in the specifications.112.6 Filter AppurtenancesThe filters must be equipped with wash-water troughs, surface wash or air scouringequipment, means of measurement and positive control of the backwash rate,equipment for measuring filter head loss, positive means of shutting off flow to afilter being backwashed, and filter influent and effluent sampling points. Ifautomatic controls are provided, there must be a manual override for operatingequipment, including each individual valve essential to the filter operation. Theunderdrain system must be designed for uniform distribution of backwash water(and air, if provided) without danger of clogging from solids in the backwash water.If air is to be used for filter backwash, separate backwash blower(s) must beprovided. Provision must be made to allow periodic chlorination of the filterinfluent or backwash water to control slime growths. When chemical disinfection isnot provided at the plant, manual dosage of chlorine compounds is acceptable.159