Tech Talk 1: overview to coagulants and flocculants

Each month we will be providing a brief technical talk aimed at clearing up some industry myths and misinformation as well as providing some basic guidance on key topics. Our first tech talk is an overview to coagulants and flocculants, which are very topical given their increased use and reliance in treating sediment laden runoff captured within sediment basins, in particular Type A & B basins.

Firstly, what are coagulants and flocculants… While they are often referred to interchangeably when talking about water treatment they operate in a very different way. To add a level of complexity some products available are a blend of a coagulant and flocculant. Simply put, coagulants neutralise or destabilise the charge on clay particles which is causing them to repel and hence stay suspended in solution. As most clay particles in water are negatively charged, a cationic (positive) coagulant will be used. Some key considerations regarding use of coagulants include:

• Ensure rapid mixing of coagulant throughout the water body (i.e. within the forebay), before passing into the settling zone of the basin;
• Flocs formed by coagulation are generally very small, typically referred to as pin flocs. As a result, they can be re-suspended by high velocity and shearing forces between the settlement and storage zones. Maximising the width of your spillway and using flow control baffles along the length of the basin will reduce the potential of this occurring.

In contrast, flocculants work by physically forming larger particles through contact and adhesion to effectively increase particle size to a point allowing flocculation (i.e. settlement) to occur. Flocculants can be used alone, however may prove more effective in combination with coagulants. Key considerations with flocculants include:

• Flocculants require even distribution throughout the water body, ideally followed by slow continuous mixing to encourage contact and adhesion of suspended clay particles. This may be difficult to achieve within a sediment basin and consideration will need to be given to the method of application;
• Flocs formed by the use of flocculants are typically larger and more stable, given their relative size in comparison with those formed by coagulation. If flocs form too rapidly, they may settle before binding all suspended clay and colloidal particles, leaving a cloudy water column.

The points above will be expanded upon in our next Tech Talk, within our September newsletter addition. Discussion will include how to select the most effective product and determining the optimum dose rate to apply.