Abstract
Abstract. The size and structural characteristics of floc particles are important design and control parameters in water treatment and should be rapidly monitored with a reasonable amount of accuracy. In this study, a photometric dispersion analyser (PDA) coupled to standard jar test experiments was used to optimize coagulation-flocculation parameters while monitoring floc size and structure as well as the rate of floc formation during coagulation using alum. The optimal coagulation conditions were as follows: sample pH 8; alum dosage, 3 mg L−1 as Al3+; G value, 172 s−1; rapid mixing time, 20 s. These conditions resulted in unstable treated water having a calcium carbonate precipitation potential (CCPP) of −15 mg L−1 as CaCO3 and required a slaked lime dosage of 17 mg L−1 as CaCO3 to equilibrate CCPP to acceptable levels. PDA data revealed that aggregation rate and steady-state variance are primary parameters as both have substantial influence on coagulation-flocculation efficiency. However, the average steady state ratio, although an important parameter, had a lessened impact on coagulation-flocculation efficiency. The results of this study showed that the PDA instrument is an important tool in coagulation kinetic studies and can be employed as an additional tool in the optimization of coagulation conditions.
Highlights
IntroductionThe treatability of a water source, including the determination of optimal conditions for specific parameters (most effective coagulant, dosage rates, pH, velocity gradient and mixing time), is conventionally performed at bench-scale using standard jar test experiments
The treatability of a water source, including the determination of optimal conditions for specific parameters, is conventionally performed at bench-scale using standard jar test experiments
For each jar test the following procedure was performed: one litre of raw water was added to each Perspex jar; the pH of samples were adjusted to the relevant pH using either 0.1 M hydrochloric acid or 0.1 M sodium hydroxide; appropriate dosages of alum were dosed into each jar using plastic syringes; various velocity gradients were applied to the sample using electronically controlled programmes; upon completion of electronically controlled programme samples were left to settle for 15.5 min
Summary
The treatability of a water source, including the determination of optimal conditions for specific parameters (most effective coagulant, dosage rates, pH, velocity gradient and mixing time), is conventionally performed at bench-scale using standard jar test experiments. The performance of these experiments is generally determined through measuring turbidity and/or colour removal. The size distribution of floc particles is an important design and control parameter during coagulation-flocculation (Spicer et al, 1996). Floc size and structure are influenced by several operational parameters and directly impacts floc density which controls solid removal efficiency during sedimentation (Spicer and Pratsinis, 1996)
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