Parameters affecting enhanced coagulation: a review

Abstract

ABSTRACTCoagulation is a process which is extremely susceptible to the chemistry of raw water and the type of coagulant used and so is ‘Enhanced coagulation’. The speciation of the coagulant and subsequently the mechanisms involved in coagulation for removal of turbidity are fairly clearly understood. However, the same theory cannot be directly applicable for ‘Enhanced coagulation’ since it involves removal of NOM and other colour causing substances along with turbidity. The dependence of enhanced coagulation efficiency on the fractionation of NOM (Natural organic water) has been dealt in detail and reported adequately. However enhanced coagulation is influenced by many other operating conditions such as type, dose and basicity of coagulants, pH/alkalinity, presence of anions, characteristics of NOM-Al flocs along with composition and fractionation of NOM. This paper is an attempt to put together the reported isolated studies in one place for all of these parameters influencing enhanced coagulation, including fractionation of NOM. The speciation of coagulants and mechanism of NOM removal are discussed in detail. It appears that the removal of dissolved organics primarily occurs by charge neutralisation under slightly acidic conditions. But when both suspended colloidal particles as well as dissolved organics are present under different relative concentrations and different raw water conditions, the dominant mechanism and pH still needs to be explored. Modelling of enhanced coagulation involving every possible variable still needs to be done to provide new insights into underlying mechanisms of removal. As the raw water conditions and NOM characteristics are not under the control of the operator, he will need to select the best coagulant available. Reviewing and summarising the available papers, a brief guide for the selection of coagulants for different raw water conditions has been prepared.