Choice of a Coagulant is ...

Choice of a Coagulant is dependent on many things: The availability of the coagulant, the cost of the product, the storage of the product  etc.  Since it is alum they are using and the problem is the residual alumina my suggestion is; determine the optimum dose of that Alum where the residual alumina is within the standards you are looking for and teach/show the users what quantity Aluminum Sulphate to use that would not leave a very high residual alumina this can be achieved by determining  the alkalinity of the water, then conduct a jar test to determine the optimum dose of the alum that gives you a residual within the limit you're looking for. Do you need to change a coagulant, because the problem you described above is not impossible to solve, easily too.