Hello Phetla,  as Gary Brown ...

Hello Phetla,  as Gary Brown has said, it is useful to conduct jar tests with a range of poly-electrolytes.  It may also help to carry out a zeta potential assessment to help narrow down the type of poly-electrolytes that could be tested.  In many cases the suppliers of the poly-electrolytes will do these tests for you or at least advise which ones would be most likely to meet your operational requirements.

However before you do that, try to understand what is happening at the water source during the summer period - is it related to rainfall events or to temperature increases, and is there a possibility that you are experiencing algal growth in the water source - this will need another approach to address the algae before coagulation, or alternatively to modify your treatment process to incorporate a flotation clarification unit.  This is likely if there is nutrient enrichment from agriculture or wastewater treatment works upstream of your source.

Best wishes - you may want to contact Lepelle Water who also have a number of treatment works in your part of South Africa.