Coagulation and Corrosion Control for Soft and Coloured Drinking Water

Abstract

Soft, acidic, and coloured (humic substances) surface waters with a low turbidity are commonly used for the drinking water supply in Norway. For these waters, coagulation/direct filtration combined with a process for corrosion control is one of the most interesting water treatment alternatives.

In the experimental work described here, coagulation/direct filtration of water from Lake Jonsvatnet was investigated in a pilot plant using three different coagulants, alum (ALG), prepolymerized aluminium chloride (PAX14), and a calcium containing prepolymerized aluminium chloride (Ca-PAX). Magnafloc LT20 as a filter aid and three different combinations of chemicals for corrosion control purposes were also included in the pilot plant experiments: 1) lime/CO₂, 2) CaCO₃-slurry/CO₂/NaOH, and 3) CO₂/NaOH (using Ca-PAX).

A dual media (hydroanthracite on top of sand) gravity filter with a total depth of 0.85 m was used in the experiments. Headloss and effluent quality in terms of pH and turbidity were monitored continuously, and colour, Ca, alkalinity, and Al were analyzed from water samples taken throughout each filter run.

Optimum type of coagulant and combinations of chemicals for corrosion control were evaluated in terms of effluent quality, utilization of chemicals, cost of treatment, headloss, and length of filter run versus filter load.

Satisfactory treated water quality with respect to the Norwegian water quality standards was obtained with all the applied combinations of coagulants and chemicals for corrosion control. Lake Jonsvatnet is relatively low in humic content and moderate in corrosivity. The treated water quality was approximately: colour = 3 mgPt/1, pH = 8.0, alkalinity = 1.0 meq/1, and Ca = 20 mg Ca/1. However, the utilization of chemicals, filter performance and treatment costs were largely affected by the applied combinations of coagulants and chemicals for corrosion control.

The results from the investigation show that the most cost effective choice of coagulant, coagulant dosage, and coagulation pH is largely affected by the type of corrosion control process and can be considerably different for a process without water treatment for corrosion control. The results may be used to design and optimize a more cost effective overall treatment process for these types of waters.