Coagulation and settling in ...

Coagulation and settling in irrigation channels

Coagulation usually involves addition of chemical products (coagulants such as ferric chloride, ferric sulfate and alum) which neutralize the charge of the naturally occurring particles in the water and attach these particles to the coagulant thereby forming larger particles that can easily settle in quiescent conditions.  The problem with applying such coagulants directly to the irrigation channels is that the coagulated particles may not settle because of the relatively large flow through velocity in the channels. In order for the particles to settle well, the velocity in the channel should be less than 0.1 m/s, which may not be practical.  In addition, the introduction of coagulant to the irrigation water will create significant amount of extra solids - for every 1 mg/l of coagulant approximately 0.8 mg/L of extra solids are created.  So coagulant addition, even if the coagulated solids can settle, would fill up the irrigation channels after a while.  In order to coagulate irrigation water it is recommended the coagulation process to be completed in a separate settling tank/pond where coagulant would be added.  The clarified water exiting this tank will be used for irrigation and the solids retained in the tank will need to be removed for offsite disposal.