Jia-Qian Jiang

LABORATORY STUDY OF ELECTRO-COAGULATION–FLOTATION FOR WATER TREATMENT

An electro-coagulation-flotation process has been developed for water treatment. This involved an electrolytic reactor with aluminium electrodes and a separation/flotation tank. The water to be treated passed through the reactor and was subjected to coagulation/flotation, by Al(III) ions dissolved from the electrodes, the resulting flocs floating after being captured by hydrogen gas bubbles generated at cathode surfaces. Apparent current efficiencies for Al dissolution as aqueous Al(III) species at pH 6.5 and 7.8 were greater than unity. This was due to additional reactions occurring in parallel with Al dissolution: oxygen reduction at anodes and cathodes, and hydrogen evolution at cathodes, resulting in net (i.e. oxidation + reduction) currents at both anodes and cathodes. The specific electrical energy consumption of the reactor for drinking water treatment was as low as 20 kWh (kg Al)(-1) for current densities of 10-20A m(-2). The water treatment performance of the electrocoagulation process was found to be superior to that of conventional coagulation with aluminium sulphate for treating a model-coloured water, with 20% more dissolved organic carbon (DOC) being removed for the same Al(III) dose. However, for a lowland surface water sample, the two processes achieved a similar performance for DOC and UV-absorbance removal. In addition, an up-flow electrocoagulator configuration performed better than a horizontal flow configuration, with both bipolar and monopolar electrodes.

Observations of the comparative hydrolysis/precipitation behaviour of polyferric sulphate and ferric sulphate

The hydrolysis/precipitation behaviour of polyferric sulphate (PFS) and ferric sulphate (FS) under conditions typical for coagulation and flocculation in water treatment are investigated by studying the rate of floc size development, the zeta potential and chemical structure of precipitates and the iso-electric point of re-suspended precipitates. The results detailed in this study strongly suggest that, under the test conditions, the hydrolysis behaviour of PFS was significantly different from that of FS, with evidence of a relatively lower rate of floc size development and colloid charge reversal in the investigated Fe(III) dose range and a different chemical structure of the precipitates formed from PFS and FS.

Preparation and characterisation of an optimal polyferric sulphate (PFS) as a coagulant for water treatment

A range of effective optimal formulations of polyferric sulphate (PFS) can be prepared by a novel procedure, recently developed by the authors, involving an oxidation stage followed by a partial hydrolysis (aging) stage. The Fe(III) concentrations, the dose and nature of the base used, and the temperature and time of partial hydrolysis influence the properties of the resulting PFS formulations and these have been carefully studied. The analytical techniques of membrane ultrafiltration, ferron-timed spectroscopy and specific electrophoretic mobility have been used to characterise the nature of the PFS species. The optimal PFS coagulating species have been found to have medium molecular weight (0·5–10 kDa) and high cationic charge. Such optimal species are believed to be responsible for the superior performance of PFS in water treatment practice.

Preliminary Evaluation of Polyferric Sulphate As a Coagulant for Surface Water Treatment

This paper is concerned with the performance of a relatively new form of Fe(III) coagulant, Polyferric Sulphate (PFS), which contains a range of pre-formed Fe(III) hydrolysis species with both a high positive charge and medium and large molecular weights. Preliminary laboratory and pilot-plant experiments were undertaken to evaluate the performance of PFS, in comparison with conventional coagulants such as ferric sulphate (FS), for the coagulation of surface waters containing substantial concentrations of algae and/or colour, and natural organic matter.

Evaluation of Poly-Alumino-Iron Sulphate (PAFS) as a Coagulant for Water Treatment

This paper is concerned with the performance of a new kind of pre-polymerised inorganic coagulant, poly-alumino-iron sulphate (PAFS), for use in drinking water treatment. Considerable laboratory work has been undertaken to evaluate the performance of PAFS in comparison with conventional coagulants such as ferric sulphate (FS) and aluminium sulphate (AS), for the coagulation of a model upland coloured water and a typical lowland surface water in the UK. The results have shown that the PAFS coagulant was superior to the FS and AS in terms of the removal of colour, UV254-absorbing substances and dissolved organic carbon (DOC). PAFS achieved the lowest residual concentrations of Fe and Al in comparison with FS and AS, respectively. In addition, to achieve an equivalent removal percentage, a lower coagulant dose (in molar units) was required for PAFS in comparison with AS and FS.