Are flocculants required in a flocculant process?

Abstract

The effectiveness of solids abatement by pH increase was investigated using the jar test procedure with a bentonite tap water suspension and an urban wastewater and an oxidation pond effluent. The results indicated that, depending on the suspended particles and on the dissolved ions, pH values between 9.5 and 12 induced extensive solids elimination without adding any other chemical than a base, i.e. sodium hydroxide or lime. The major effective reactions are then the calcium carbonate precipitation and the magnesium hydroxide precipitation. Moreover, this process does not require a flocculation step but only a precipitation step where the particles are entrapped by sweep coagulation and adsorption-coagulation. A continuous reactor was operated with an oxidation pond effluent. A suspended solids concentration less than 30 mg/l was obtained by adjusting pH between 11 and 11.5 while the reactor was operated up to 20 m/h superficial upflow velocity corresponding to a residence time through the whole unit of only 5 minutes. The sludge settling velocity depends on pH and on the primary particles but a maximum settling velocity larger than 1 m/h is easily reached. The concentration factor is then about 100. Environmental policy implications of this technique are that it allows to significantly upgrade a stabilization pond effluent and can be used when a high pH situation is acceptable.