If you are getting algal ...

If you are getting algal growth in finished water, it is either because you are not removing all of cells in the treatment process, or it is being reseeded after treatment, somehow (e.g. open tank?). If they are not all being removed by treatment you should improve filtration. Standard treatment for a bloom problem is conventional coagulation, sedimentation, filtration treatment plus permanganate and powdered activated carbon to remove the algal toxins, if they are present in excess.  If you have significant and regular algal growth in the source water, you should filter in teh treatment plant to remove cells before adding a disinfectant, because that will lyse the algal cells and release toxins into the water, if they are being formed. Also, if you are having blooms you should check to see if toxins are being produced. Also, you should try to restrict nitrogen and phosphorous entry to the source water to reduce the growth. There are ELISA methods and LC/MS methods of analysis for toxins.  They are not always present. Also, if you have algal growth it is usually noticed by taste from isoborneol and geosmin. The WHO long term (years to lifetime)  Guideline for Drinking Water Quality is 1 ppb. The Australian short term (couple of weeks) guideline is 13 ppb, and it is based upon the same study that WHO used for long term and using a modified calculation for short term exposure. The equivalent WHO value would probably be 10 ppb.  WHO is working on a short term guideline.