Course: BFM 203 Water Treatment and Control

Lecture

How Do Coagulants and Polymers Work?

When coagulants are added to water, a series of reactions occur with ions in the water. When sufficient chemical is added to exceed the solubility limit of the metal hydroxide, floc is formed. This flock with adsorb the turbidity in the water.

Polymers consist of a long chain of small subunits called monomers. The polymers contain ionizeable groups on the monomeric units and are commonly referred to as polyelectrolytes. Polymers can be used as a primary coagulant or coagulant aid. Overdosing coagulant and coagulant aid chemicals can cause problems with subsequent settling and filtration processes.

Chemical Application

Through multiple feed injection parts, any combination of chemical can be added into the influent flow. All injection-fed chemicals are fed in a liquid form. Ferric Chloride is used as a primary coagulant; a polymer is used as a coagulant aid; carbon may be used for taste and odor; and salt may be added to increase sodium levels. pH adjustment is often necessary for coagulants to work effectively.

The following photo album contains images of various equipment used in coagulant and flocculation processes.

Problems With Polymers

While there are many polymers on the market, not all water supplies can be treated with equal success. Some manufacturers have dosage limits; USEPA publishes a list of approved polymers and maximum concentration recommendations. Overdosing will affect coagulation efficiency and accelerate head loss buildup in in filters. Chlorine can reduce the effectiveness of polymers.

Basic Coagulation Chemistry

Coagulation is a physical and chemical reaction between the alkalinity of the water being treated and the coagulant added to the water which results in the formation of insoluble flocs. The pH of the water affects which chemical compounds predominate. Lower pH tends to favor positively charged chemicals that react with negatively charged colloids and particulates. A pH level of 5 to 7 is usually best for coagulation. Sulfuric acide is regularly used to adjust pH when necessary. Great care and safety must be used when handling acids and related equipment.

Residual alkalinity helps to buffer the system and aid in forming floc. Alkalinity can be increased by the addition of lime or soda ash.

When mixing chemicals, it is desirable to completely mix the coagulant in as short a time as possible. Methods of mixing include 1) hydraulic mixing using flow energy in the system, 2) mechanical mixing, 3) diffusers and grid systems, and 4) pumped blenders.