Optimization of the Clarification System for Raw Water from the Pakra Reservoir Lake

• Main Author: Zečević, N.
• Format: Article
• Language: English
• Published: Croatian Society of Chemical Engineers 2011-10-01
• Series: Kemija u Industriji
• Subjects: raw water; free Cl₂
• Online Access: http://pierre.fkit.hr/hdki/kui/vol60/broj10/497.pdf

The first step in processing raw water from the Pakra lake for use in fertilizer production at Petrokemija is oxidation of total organic carbon matter with gaseous chlorine, Cl₂. Thereupon it is clarified and filtered with the help of a clarification reactor and sand filters. Construction of the clarification reactor and process sand filters enables only the removal of the suspended matter from the raw water, without affecting its overall hardness. Process control of the clarification reactor and removal of the suspended matter from the raw water is achieved by adding corresponding mass concentration water solutions of aluminum sulphate, Al₂(SO₄)₃ · 18 H₂O and organic polyelectrolyte. The effectiveness of flocculation is carried out by laboratory determination of the m-alkalinity difference between inlet and outlet of raw water from the clarification reactor. For the most effective clarification of raw water, the optimal empirical value of the m-alkalinity difference is 0.65 mmol L^-1 in the pH range of raw water from 7.0 to 8.0. Prior to processing clarified water by ionic decarbonatisation and demineralisation for protection of the ionic exchange resin from excess free Cl₂, a corresponding mass concentration of a sodium bisulfite water solution, NaHSO₃, is added. An improved system is proposed for continuous measurement of mass concentrations of free Cl₂ in raw and clarified water, and pH difference value at the inlet and outlet of the clarification reactor. The proposed system can achieve optimal dosage of gaseous Cl₂ in the raw water, improving the clarification process in the reactor as well as optimal dosage of water solution of NaHSO₃. It is shown that the average pH difference from 0.65 to 0.75 at the inlet and outlet of the clarification reactor in the pH range of the raw water from 7.0 to 8.0 is an equally effective replacement for the laboratory determination of m-alkalinity. Also shown is the connection between dosage mass of the gaseous Cl₂ in the raw water, pH difference value at the inlet and outlet of the clarification reactor and dosage mass of the NaHSO₃. The proposed system for the continuous measurement of mass concentration of free Cl₂ and pH values in the raw and clarified water achieved a better and safer system of processing raw water with annual savings of gaseous Cl₂ of 15 % and NaHSO₃ of 50 %.