Removal of the typical isotopes of the Chernobyl fall-out by conventional water treatment

A classical full-scale treatment scheme including coagulation-flocculation-upflow settling and rapid sand filtration is investigated for its efficiency in removal of trace concentrations of radioactivity, demonstrated by four isotopes. Known activity of each isotope is added to the raw water and a mass balance is established by measuring the activities in the sludge and in the treated water. Maximum removal efficiencies found were: 73, 61, 17 and 56% for 103Ru, 58Co, 131I and 134Cs accordingly. A general equation corresponding to a mass transfer mechanism into the sludge has been found as In Ass - A/As = -kt + (rest) in which A and As are respectively the activity of the sludge and the value at saturation. Radiocesium remains fixed on the filtration sand for several months. Decrease of the activity of the isotopes fixed on the sludge occurs according to two mechanisms: bleeding of the sludge and elution of the sludge by uncontaminated water. The latter is a linear function of time depending also on the type of isotope. The results depend on the water treatment scheme, and they enable formulation of general models and prediction of the activity accumulated in the sludge. The model is also applicable in predicting the quality of the treated water when a nuclear accident is occurring and in optimizing the operation of the flocculation-settling during such periods. (author)