A system and method for purifying an aqueous contaminated fluid from colloidal contaminants using a closed-loop feedback control system are disclosed. Contaminated fluid flows into a mixer and mixed with a purifying agent, such as diatomaceous earth, to facilitate metathesis reaction between the purifying agent and the contaminants. The purifying agent entraps the contaminants, and is subsequently removed in a separator into sediments. An estimation of the amount of the purifying agent to dispense is performed using pre-defined equations or look-up tables stored in the database, including relationship between the concentration of contaminants in the fluid and the concentration of the added purifying agent. An automated feedback control is applied to fine tune the purification process into compliance with regulations. High accuracy of measurements is ensured by selecting the same material for both the purifying agent and the trapping medium of analyzers, and additionally by the design of the analyzers.