Optimization of coagulation-flocculation parameters using a photometric dispersion analyser

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.

Optimization of coagulation-flocculation parameters using a photometric dispersion analyser

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, 6 mg L−1 as Al3+; G value, 116 s−1; rapid mixing time, 20 s. These conditions resulted in unstable treated water having a calcium carbonate precipitation potential of −15 mg L−1 as CaCO3 and required a slaked lime dosage of 17 mg L−1 to equilibrate CCPP to acceptable levels. PDA data revealed that aggregation rate and steady-state variance are primary parameters as both have significant 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.

Natural organic matter removal by coagulation: effect of kinetics and hydraulic power

In this paper, the mechanism of NOM removal by coagulation is investigated using coagulant polyaluminium chloride (PACl), compared with AlCl3. The kinetics of Al ion hydrolysis, interaction of hydrolyzed species of Al and NOM, and hydraulic power condition in coagulation process are investigated by programmable jar test, photometric dispersion analyzer and ferron assaying. After the coagulant dosing, Al ion would hydrolyze very quickly, and then monomeric Al and small polymeric Al would further hydrolyze to form larger polymer and precipitate, based on pH condition, while the further hydrolyzed process is relative slow. Although complexation between NOM and Al appears to be several orders of magnitude slower than hydrolysis of monomeric forms of Al, it would be faster than further polymerization and precipitation of Al during aging. However the further polymerization of fresh Al would benefit to Al-NOM complex aggregation and settlement. Therefore, PACl, with stabile preformed polymer shows significantly different performance in NOM removal compared with AlCl3. Although the hydraulic power condition plays significant role in collision frequency and efficiency of particles removal, the process of the aggregation to form larger floc to settle down is relative slower than interaction of Al and NOM, and it is not very correlative to the performance of NOM removal if only it can provide enough mixture.

The effect of polymeric flocculants on floc strength and filter performance

Polymeric flocculants are widely used throughout the water industry as flocculant aids, they are known to increase floc density and aid settlement in the clarification stage of the water treatment process. In this research, polymeric flocculants were used to improve floc strength prior to filtration on a dissolved air flotation (DAF) plant in an attempt to prevent filter breakthrough. A modified jar test procedure using a PDA (photometric dispersion analyser) optical flocculation monitor was developed in order to evaluate the system floc strength. Filtration trials were carried out on a pilot filter rig situated on a surface water treatment works in Yorkshire. The filter feed originated from the main plant filter channel. Filter performance was assessed by continuous online monitoring of effluent particle counts, turbidity and headloss over the period of the filter run. Results indicated that low doses of polymeric flocculants had a beneficial effect on filtered water quality, as measured by particle counts, turbidity, UV254 absorption and dissolved organic carbon (DOC). Polymeric flocculants also had the effect of extending filter run length. The modified jar test results indicated that the flocculants used improved the floc strength and enhanced reflocculation of the micro flocs present after the flotation process.

Temperature effects on flocculation, using different coagulants

Temperature is known to affect flocculation and filter performance. Jar tests have been conducted in the laboratory, using a photometric dispersion analyser (PDA) to assess the effects of temperature on floc formation, breakage and reformation. Alum, ferric sulphate and three polyaluminium chloride (PACl) coagulants have been investigated for temperatures ranging between 6 and 29°C for a suspension of kaolin clay in London tap water. Results confirm that floc formation is slower at lower temperatures for all coagulants. A commercial PACl product, PAX XL 19, produces the largest flocs for all temperatures; and alum the smallest. Increasing the shear rate results in floc breakage in all cases and the flocs never reform to their original size. This effect is most notable for temperatures around 15°C. Breakage, in terms of floc size reduction, is greater for higher temperatures, suggesting a weaker floc. Recovery after increased shear is greater at lower temperatures implying that floc break-up is more reversible for lower temperatures.

Respirometric measurement of kinetic parameters: effect of activated sludge floc size

The variation of activated sludge floc size with the mixing intensity of a mechanically stirred respirometer, expressed in terms of the mean energy dissipation rate, was characterized using a photometric dispersion analyzer. The floc size decreased rapidly when the energy dissipation rate was increased from 1.33 × 10-3 to 2.68 × 10-3 W/kg. Experiments were performed to investigate the effect of floc size on the oxygen saturation coefficient measured under the condition of acetate oxidation. The respirometric data were interpreted by considering only the kinetics of biochemical reactions. The variation of the oxygen saturation coefficient with mixing intensity was found to correlate with the variation of floc size with mixing intensity. The oxygen saturation coefficient was found to decrease from 0.23 to 0.08 mg/L when the mean energy dissipation rate was increased from 1.33 × 10-3 to 2.68 × 10-3 W/kg. The dependence of the oxygen saturation coefficient on floc size or mixing intensity suggests the presence of mass transfer resistances in large flocs.

Analysis of a clariflocculation process with a photometric dispersion analyser (PDA2000)

In this experimental study the clarification process of the drinking water treatment plant (WTP) of Florence (Italy) has been evaluated. At present, the most common way to optimise the clariflocculation process (best type and dosage of coagulants and mixing conditions) is the jar-test procedure which can give information about the final turbidity, and consequently the process efficiency, after a settling period in a batch procedure at laboratory-scale. An alternative method with a Photometric Dispersion Analyser (PDA2000) was recently introduced at the WTP to provide quick and continuous information (flocculation index, correlated with the flocs size) about the aggregation state of particles during a modified jar-test procedure. The PDA2000 was applied to a real suspension (namely Arno river water) providing useful data for the determination of best type and optimum dosage of coagulants. Furthermore a strong correlation between the removal efficiency of the turbidity and PDA a parameter derived from the PDA 2000 data (defined as the rate of the square root of the flocs size index and the maximum slope of the growing curve) was observed.