In the water and wastewater treatment industry, coagulation is a common process in removing turbidity in the form of suspended and colloidal materials. Many coagulants are widely used in conventional water treatment processes including inorganic and polymeric coagulants. In recent years because of problems associated with usage of these coagulants including cost and health related issues, there has been considerable interest in the development of usage of natural coagulants which can be produced or extracted from microorganisms, animal or plant tissues. The advantages of these coagulants over the synthetic ones include safety to human health, biodegradability and a wider effective dose range for various colloidal suspensions. Mineral coagulants are frequently used for coagulation. Because of harmful metals including Aluminum and impurities as polluted monomers and heavy metals in this kind of coagulats, the use of natural organic matter has been recently taken into consideration. In this study, ability of Starch as Coagulants with Bentonit as a Coagulants-aid was evaluated. In this study two kinds of starch (laboratory starch and industrial starch) and two kinds of turbid samples (Zayanderood river water with natural turbidity and synthetic turbid water) were used. In order to optimize the removal efficiency, four factors including pH, coagulant dosage, coagulant-aid dosage and initial turbidity were investigated and experiments were carried out in batch reactors. Ful factorial method was used. Results showed that in optimum conditions laboratory starch is capable of reducing turbidity with 84% in synthetic turbid samples. ANOVA showed that all of the experimental factors were statistically significant in Ind. ANOVA results revealed that the contribution of error term in the total results for all the samples is less than 13%. Interaction analysis were carried out to investigate the relative importance of different interactions between involved factors in this sample and it was concluded that the most important interaction in this sample happens between pH and initial turbidity. Considering the behavior of the coagulants results it was concluded that bridging and sweeping mechanisms are the most important mechanisms in turbidity removal using starch and bentonit. The jar test in different temperature showed that the turbidity omission output will be different by starch and bentonit which is dependent of raw water temperature. Also, the experiments showed that sedimentation and flocculation processes in coagulation by starch and bentonit will be completed in longer time than the frequent coagulation. Overall, this study showed that starch and bentonit is capable of decreasing turbidity and can be used as a cheap alternative for high cost common coagulants in wastewater treatment.
