Robustness analysis of technological units for drinking water clarification: Normal and emergency operating conditions

The primary goal of a water supply system is the protection of human health by providing microbiologically and chemically safe drinking water. Significant changes in water quality require sufficiently robust systems for water preparation, performances of which are unaffected by present variations and changing operational conditions. Water turbidity is an important parameter for the water filtration control and efficiency of disinfection. The efficiency of turbidity removal in the drinking water treatment plant ?Vodovod? in Banjaluka under normal and emergency operating conditions was examined in this paper. At normal conditions the maximal detected value was 25 NTU while at emergency operating conditions it was above 240 NTU. Robustness evaluation of the water clarification system was performed separately for periods of normal and emergency operating conditions (during and after emptying the accumulation). The robustness index was calculated based on a more stringent target turbidity value (0.5 NTU) than that specified by the current legislation, which represents a new criterion in the risk analysis in the existing practice. Data processing results indicate high operational stability of technological units under normal conditions. The filtered water quality was below the target value during most of the time of filter operation in all cycles. The recorded turbidity value was ? 0.3 NTU for 92.9 % of filtered water samples. Analysis of the water turbidity data has shown that 17% of all taken measurements under emergency operating conditions (336 samples) had higher turbidity than the target value (0.5 NTU). Large variations in raw water turbidity over short periods of times during the emergency operating conditions, present a problem for prompt response in the drinking water plant. Calculated robustness index values point to inadequate efficiency of the water clarification process in a certain number of filter operating cycles. We have found a significant impact of the plant operating conditions on the filtered water turbidity under emergency conditions, such as suboptimal coagulation and flocculation conditions as well as the nature of suspended and colloid particles inducing turbidity and insufficient particle interactions with the coagulant. Along with the negative influence on water turbidity, excessive coagulant dosage leads to increased concentrations of residual aluminum in filtered water. Optimization of emergency working conditions could be performed based on adequate monitoring of water sources, which would further decrease potential risks of pathogen appearance in drinking water.