8.2.4 Carry out a risk assessment and prioritise the risks
Risk is the likelihood of a hazard affecting the water supply system. A risk assessment can be carried out if the risk and the severity of the associated hazard are known. A method of undertaking a risk assessment quantitatively is to use a risk matrix as shown in Figure 8.5 (Bartram et al, 2009). The severity of the impact of a hazard and its likelihood can be multiplied together to arrive at a figure that indicates its risk score. The risk rating will be determined by this risk score, as shown at the bottom of Figure 8.5.
To illustrate this application of a risk matrix, suppose you wanted to carry out a risk assessment for the case where a treatment plant runs out of chlorine disinfectant (Figure 8.6). In a properly managed plant, with a system of stock control in place, such an occurrence may be rare (with a rating of 1). The public health impact of it, however, will be catastrophic (with a rating of 5). Using the matrix in Figure 8.5, you can calculate a risk score, which will be the product of 1 and 5. This risk score of 5 can then be given a risk rating. From the scale in Figure 8.5, this would be ‘low’ (being less than 6).
By repeating the exercise for other hazards and risks and comparing the risk scores, a prioritised list can be drawn up that ranks the risks in order of importance. Unfortunately, budgets are often limited and because of this many of the smaller risks are disregarded.