Izabela Krupińska

Effect of the type of aluminium coagulant on effectiveness at removing pollutants from groundwater in the process of coagulation

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Abstract

Coexistence of organic substances and iron in groundwater causes that a particular part of iron occurs in a form of iron-organic compounds as colloids or dissolved complexes, whereas water has an increased colour intensity and turbidity. Successful treatment of such water generates technological problems and is practically impossible by means of traditional groundwater treatment processes. Application of the coagulation process is one of the ways recommended for intensifying the removal of iron that occurs in a form of stable iron-organic compounds. The subject of research was groundwater from Quaternary formations with high concentration of total iron amounting to 7.0 mgFe/dm3 increased colour up to 41.0 mgPt/dm3 and turbidity up to 30 NTU, and also an increased amount of organic substances (TOC from 4.00 to 7.20 mgC/dm3 ) and manganese up to 1.80 mgMn/dm3 . The water samples examined differed in terms of the value of the ratio of the TOC concentration and the concentration of total iron (D) and its turbidity and intensity of colour indicated the presence of colloidal and dissolved combinations of iron with organic compounds. The article discusses the effect of the type and dose of aluminium coagulan (Dc, mgAl/dm3) and the alkalinity, as well as the alkalinity ratio value (r = OH–/Al 3+) of the tested aluminium coagulants on the efficiency of removing pollutants from groundwater containing iron-organic complexes. In order to oxidize Fe(II) to Fe(III) the groundwater samples were aerated with compressed air for 15 minutes. The efficiencies of four aluminium coagulants were compared: that of aluminium sulphate (VI) and those of three poly-aluminium chlorides (PAX XL-60, PAX-18, PAX-16) differing in alkalinity (70%, 41% and 8.2%) and the alkalinity ratio value (r = 2.10, 1.23 and 0.25). Coagulant doses expressed in mgAl/dm3 and changed within the range of 1 to 6 mgAl/dm3 . Volume coagulation was performed in 1 dm3 groundwater samples using 1 minute rapid mix (250 rpm) and 25 minute slow mix (30 rpm) followed by 1 hour sedimentation. In the interpretation of the research results for the evaluation of the level of organic pollution in the water samples and its influence on the process and effectiveness of coagulation the coefficient of co-existence of organic substances and total iron D was used, calculated as D = TOC/Fetot (mgC/mgFe). Efficiency of disposal of wastes in the process of coagulation and sedimentation of post-coagulation solids depended on a type and dose of coagulant, alkalinity and alkalinity ratio of coagulant, coefficient of coexistence of organic substances and total iron in raw water, as well as on the temperature of the treated water. Among the coagulants types under analysis the best effects of removing iron compounds, organic substances and compounds causing coloring and turbidity were found for the pre-hydrolyzed coagulant PAX XL-60 possessing the highest alkalinity and the highest value of the alkalinity ratio.

Keywords: groundwater; organic substances; volume coagulation; aluminium sulphate (VI); poly-aluminium chlorides.