Risk assessment of contaminant intrusion into water distribution system
2011-01-18T10:16:42Z (GMT) by
It is recognised through current literature that contaminantin trusion is a primary threat leading to degradation of water quality and threat to human health. The problem is more serious in developing countries where the water supply is intermittent and water distribution system crisscross with sanitary systems. Therefore there is a need to develop the methodology that enables the decision makers and engineers to undertake actions to minimise the risk of contamination of water. The researchs tudy presentedin this thesis addresses these water quality issues by developing the appropriate modelling tools to minimize the risk of contaminant intrusion. The conceptual framework proposed in this study consists of a risk based approach where the process of contaminant intrusion into the systems is traced to know the hazards of contaminant intrusion and vulnerability of the system. The risk of contaminant intrusion into the pipes of a water distribution system is then estimated as the function of hazards and vulnerability. A suit of four models is developed based on this framework. The first model is a water distribution pipe condition assessment model that simulates the potential pathway for contaminant ingress into water pipes by relating it to the deterioration/condition of the pipes. The condition of each pipe is assessed by means of numerous factors related to physical, environmental and operational aspects of the water distribution system. These factors are grouped into different indicators at three levels, depending on the nature of influence of each factor on the deterioration process of the pipe. The uncertainties inherent in these pipe condition indicators are described with fuzzy set theory. A distance based multi-criteria decision making method-fuzzy composite programming has been applied to combine the multilevel pipe condition indicators to form a single indicator to rank the condition of the pipes. The second model is a water flow and contaminant transport modelling tool. This model predicts the envelope of pollution emanating from pollution sources (contaminant zone) and simulates the seepage and contaminant transport in this zone. It is assumed that the seepage of contaminant from pollution sources such as unlined canal/drains and surface water bodies follow saturated flow while from pollution sources such as sewer pipelines, lined canals/drains follow unsaturated flow. Accordingly Richard/Green Ampt equations (unsaturated flow) and Darcy's equation (saturatedf lows) are coupled with advection-diffusion equations that account for water flow and contaminant transport respectively. The third model, the contaminant ingress model, identifies sections of pipe of water distribution system within contaminant zone by combining the outputs from the contaminant seepage model with spatial analysis. The fourth model, the risk assessment model, identifies the risk of contaminant intrusion into a water distribution system from the outputs of the above three models, namely the vulnerability of the water distribution pipe (pipe condition assessment model), the contaminant concentration(contaminant seepage model) and section of pipe in contaminant zone (contaminant ingress model). All these models have been integrated into ArcView GIS to form a decision support system (Improved Risk Assessment of Water Distribution System) and applied to a real water distribution system in Guntur, India for which water pipe network data and data for pollution sources were collected. The modelling results are presented as risk maps that show the potential areas that are under threat of contamination with relative risk scores. It is envisaged that the developed modelling tools will be used by water utilities in developing countries to improve the water quality management by identifying vulnerable areas and understanding threats that exist to the water distribution systems.