Hi Muhammamd,

First, aluminium chloride is a coagulant rather than being a flocculant. Second, practically since I am working in desalination field over year, I don't recommend Al & Fe coagulants in the pre-treatment processes of RO feed water, because small amounts (0.5-1.0 ppm) may leach and reach the membranes. This will reduce dramatically the efficiency of your antiscalant and enhance the fouling potential noticeably - either colloidal fouling or scaling. 

So, adding Al & Fe to your RO feed water may damage the membranes and needs very special attention. 

If you need some papers about this point, I can give you their links. 

Finally, if you want to dose AlCl3, Jar testing is the best technique to calculate the dose cause no threshold dose of coagulants/flocculants. 

Regards

Dear Aasem Zeino,

Thanks for your valuable suggestion.

I have tested these chemicals on Lab scale as well as on Ponds level.

So Both have shown very good results on both scales.

Your suggestion to determine the THL is valuable, and I will work on it.

Thanks & Regards.

Muhammad,

You should examine the literature (or a good text book) on drinking water treatment as in this case, generally alum (aluminum sulfate, rather than aluminum chloride) is added. The water goes through rapid mixing, gentle mixing, sedimentation and then rapid sand filtration.  Aasem is correct that the floculated particles need to be removed, and they are in this method, so that dissolved ions can then be removed by R/O.  Adding either  the sulfate or the chloride does the same thing, as it is the Al(+3) that associates with hydroxide, and at high enough doses, precipitates with particles and even dissolved organics in the water.

I strongly recommend that you consult the book "Dissolved Air Flotation for Water Clarification" by Edzwald and Haarhoff. You will find in it, detailed discussion of coagulation/flocculation in preparation for algae removal by dissolved air flotation, the process of choice for removal of algae, especially as pretreatment to RO. You will also find out why ferric chloride and not alum is used to coagulate sea water.