Design Guide for Community Water Systems - The Water, Sanitation ...

d. Be automatic. Timers on automatic valves shall be designed toallow frequency and duration intervals to be set to providefrequent operation for very short intervals. Operation of automaticvalves shall provide for fast opening and closing that will becompatible with the required short intervals residuals removal,shall discharge into a facility that is approved and permitted bythe Water Pollution Control Program of this department, andshould include provisions for the operator to observe and sampleresiduals being withdrawn from the unit.4.1.5.8. Cross-connections.a. Blow-off outlets and drains must terminate and discharge atplaces satisfactory to the department.b. Cross-connection control must be included for the potable waterlines used to backflush residuals lines.4.1.5.9. Detention period.The detention time shall be established on the basis of the raw watercharacteristics and other local conditions that affect the operation ofthe unit. Based on design flow rates, the detention time should be:a. Two and one-half to four hours for suspended solids contactsofteners treating surface water; andb. One and one-half to two hours for the suspended solids contactsofteners treating only ground water.4.1.5.10. Suspended slurry concentrate.Softening units should be designed so that continuous slurryconcentrates of one percent (1%) or more, by weight, can besatisfactorily maintained. In general, softening efficiency improves assuspended slurry concentration increases, although with very highslurry concentration, carry-over is a problem.4.1.5.11. Water losses.a. Units shall be provided with suitable controls for residualswithdrawal.b. The total water losses should not exceed:1. Five percent for clarifiers; or2. Three percent for softening units.c. Solids concentration of residuals bled to waste should be--1. Three percent by weight for clarifiers; or2. Five percent by weight for softeners.Aug. 29, 2003Page 53