Polymer mixing and flocculation system 



48. Weir box. The primary containment area should have a retention 

 time greater than 24 hr at the 2-ft ponding depth. This will allow most of 

 the settleable solids to settle by gravity, leaving the nonsettleable solids 

 suspended in the effluent. The effluent from the primary containment area 

 will drop over a weir into a weir box (see Figure 17). As the effluent drops 

 into the weir box, a polymer will be injected uniformly over the entire weir 

 length. The 0.8-ft drop into the weir box will provide adequate rapid mixing 

 for the polymer and effluent. A weir box of the dimensions 9 (L) by 3 (W) by 

 5 ft (D) will provide adequate mixing and retention time. This will provide 

 adequate mixing with a mean velocity gradient G of 178 per second, which is 

 calculated using a flow rate of 3 cfs and a Gt of 8,000 (Schroeder 1983). 

 Discharge from the weir box will be through a culvert. 



49. Discharge culvert. The discharge culvert is designed to provide 

 the required slow mixing. Design parameters include length, diameter, and 

 number of culverts that will maximize slow mixing. A detailed procedure is 

 outlined in Schroeder (1983). For an 800-cu yd per day production rate, a 

 15-in.-diam, 100-ft-long culvert is needed. The culvert will provide adequate 

 mixing with a Gt value of approximately 8,000. The head difference between 

 the primary and secondary containment area is 1.7 ft (see calculations in the 

 next section) . This head must be maintained so that the culvert will provide 

 adequate mixing. 



50. The design approach is to size the culvert for the maximum flow 

 rate and the minimum available head and then to calculate the available mixing 



SECONDARY 



CONTAINMENT 



AREA 



Figure 17. Weir mixing system 



v* 



3.3' 



33 



