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

Danieli Soares De Oliveira,Edmilson Costa Teixeira

doi:10.4314/wsa.v43i3.02

Danieli Soares De Oliveira, Edmilson Costa Teixeira

Open Access

https://doi.org/10.4314/wsa.v43i3.02

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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. Keywords: flocculation, drinking water, turbidity, turbidity removal efficiency, helically coiled tubes

Highlights

Suspended solids removal is an essential component of drinking water treatment
There is a need to obtain models that can adequately predict the behaviour of values for turbidity removal efficiency in HCTFs, since in both cases the models tested were not adequate to estimate flocculation process efficiency
The use of helically coiled tubes as a coagulation-flocculation reactor coupled to a conventional decanter system was evaluated experimentally in this study, with the aim of water clarification

Summary

Introduction

Suspended solids removal is an essential component of drinking water treatment. Such removal is commonly done in three steps which together comprise the clarification process: coagulation, flocculation and sedimentation/flotation.Coagulation involves the dispersal of a coagulant into the mass flow by an intense agitation, usually measured by G, the mean velocity gradient, and producing particle destabilisation. Suspended solids removal is an essential component of drinking water treatment. Such removal is commonly done in three steps which together comprise the clarification process: coagulation, flocculation and sedimentation/flotation. Coagulation involves the dispersal of a coagulant into the mass flow by an intense agitation, usually measured by G, the mean velocity gradient, and producing particle destabilisation. The destabilised particles are submitted to weak agitation to promote floc formation; this step is called flocculation. The flocs are separated from the water by sedimentation/flotation. There are interesting studies dealing with the effect of G on the size and structure of flocs formed during destabilisation/ aggregation (e.g., Bubakova and Pivokonsky, 2012; Lin et al, 2013; Polasek, 2007; Spicer et al, 1998). Some studies evaluate the hydrodynamic behaviour in flocculators, aiming to better understand the process (Haarhoff and Van Der Walt, 2001; Sartori et al, 2015)

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