Hi Hussein,

in theory, pH = -Log[H+], but in practice water and wastewater contain other compounds that have proton binding/releasing properties; viz. acids and bases (and especially the dissociation of weak acids and bases). Most important one is the CO2-bicarbonate-carbonate buffer system, but there are other systems as well. If there is only carbonate/bicarbonate (or one other buffer system) you should look at the Henderson-Hasselbach equation, but if there are more buffer systems present, it will be very tough to calculate this (as is the case with most wastewaters...). best way to find out is to make a titration experiment; titrate with calibrated pH sensor and acid/base of well known titer: pH as a function of meq added... This data will give you the required info

best regards,

DS