D. The outlet must be wide to skim off the top clear water and maintain a 

 low horizontal velocity. 



Filter Ponds - pond without an outlet 



A. Used where there is no discharge or recirculation, 



B. Water table must be low enough that water will filter out, not into 

 the pond. Pond berms must be high enough to guard against floods. 



C. Walls and bottom of the pond must be porous to allow outflow. Ponds 

 seal more slowly if they are kept full so all the area of walls and 

 bottom are working. 



D. Pond must be large enough so ii will not seal. 



E. Coagulants should not be used in filter ponds because they shorten the 

 life of these ponds. 



F. It is best to precede the filter pond with a settling pond for heavy 

 particle settlement. 



Coagu lat ion 



A. Used when there is a high concentration of solids that will not settle 

 or there is limited area for settling ponds, or both. 



B. Must be thoroughly mixed to be efficient. 



C. Works better in warm water; settlement rate is doubled for every 

 35 C increase in temperature. 



D. Commonly used coagulants are: aluminum sulphate (alum), ferrous sul- 

 phate (copperas), calcium hydroxide (hydrated lime), calcium oxide 

 (quick lime), sodium aluminate, sodium carbonate (soda ash), ferric 

 chloride (ferrisul), sodium silicate. 



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