Flocculation efficiency in drinking or wastewater treatment needs to be maximised, if raw material costs are to be kept to a minimum. Chemical flocculants are not cheap and if treatment processes are not closely monitored the water quality will suffer. The best way to monitor flocculation efficiency is to measure the ‘zeta potential’, a measurement of electrical interaction between particles. Colloid stability is determined by the balance between the Van der Waals attractive forces and electrical repulsive (zeta) forces. To destabilise a suspension or increase the rate of flocculation the zeta potential must be reduced. The fastest rate of flocculation is achieved at the point of zero zeta potential (PZZP).
Plotting zeta potential over a range of electrolyte concentrations allows the critical coagulation concentration (CCC) to be determined. Below this value the suspended material will not flocculate but may sediment slowly, forming a dense deposit. Above the CCC flocculation occurs, the material settles more quickly and forms a lower density deposit.
In the case of raw water abstracted from the ground for drinking water treatment, mineral oxides usually form a large proportion of the suspended material. The point of PZZP for most mineral oxides is between pH 7-9, so adding an alkali tends to reduce the zeta potential and increase flocculation rate. However, straightforward pH adjustment by the addition of caustic soda, for example, is not sufficient to maximise the flocculation rate. And although turbidity measurement can give a broad indication of water clarity, this is not in itself an indicator of flocculation efficiency.
The addition of another positive cation can boost flocculation enormously because the suspended material is negatively charged. One of the best options is controlled dosing of aluminium sulphate, a cation which is trivalent because, believe it or not, the flocculation effect is proportional to ‘the sixth power of the valency of the positively charged ions added’. In other words, you get more flocculation for your money. The pH must be finely controlled because the degree to which aluminium ions will adsorb onto suspended particles and sink them can vary by 0-100% over a pH range of just one. Adsorption efficiency is directly related to the reduction in zeta potential, so in order to achieve the correct conditions for efficient flocculation zeta potential and pH must be regularly, if not constantly monitored. Chemical dosing rates can then be set to cope with variations in the natural pH of the incoming water and its mineral content. Groundwater pH is typically around 5-6, so a degree of pH adjustment is usually necessary.
Accurate equipment for zeta potential measurement is somewhat harder to find than pH measurement, the technology for which is well-established. Malvern Instruments makes a zeta potential monitor called the Zetasizer which can be used in a variety of applications, from water and wastewater treatment to food manufacture. The Zetasizer uses lasers to assess the electrical potential of particles in liquid samples. The results are then converted to zeta potential using the Smoluchowski equation. The system is semi-automated and analysis is simplified by the use of customised software.
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