Experimental evaluation of helically coiled tube flocculators for turbidity removal in drinking water treatment units

Abstract

The constant need to improve water treatment techniques allows for the emergence of new technologies for obtaining adequate water, both in terms of quality and quantity. In order to obtain an efficient, rapid and low-cost clarification system, this study proposes the use of helically coiled tubes (HCTs) as a coagulation-flocculation reactor coupled with a conventional decanter system. Eighty-four (84) turbidity removal tests were performed to evaluate the proposed clarification system, while varying hydraulic and geometrical parameters in HCTs. Removal efficiency values higher than 80% were obtained (with a maximum removal efficiency of 86.2%), presenting better results than systems using baffled tanks, which are traditionally applied for water treatment purposes in developing countries. In addition, significantly lower processing times (lower than 2 min, about 10% of baffled tank processing times) were observed for high efficiency process values, indicating that this clarification system can be useful in rational design of coagulation-flocculation units. It should be noted that the turbidity removal efficiency results obtained (with a rising-then-decreasing behaviour over time) differ significantly from those obtained by the commonly used models for flocculation evaluation (with asymptotic behaviour over time), presenting a maximum absolute percentage deviation of 48.9%, and indicating caution in the use of such models for alternative flocculation unit evaluation.