Current regulations on disinfectant residuals and disinfection by-products (DBP) limits in drinking water distribution systems (DWDS), severely constrain the decisions that influence residual maintenance. However, there is a growing debate within the research and practice communities about real public health benefits of residual maintenance in DWDS.
Here, a simulation framework is developed to assess the vulnerability of a water system to microbiological contamination. Results on network examples show that the risk of consumer exposure is affected by the residual maintenance strategy employed. The common strategy that maintains a "detectable" disinfectant residual may not provide effective consumer protection against microbial contamination. A chloramine residual, instead of free chlorine, may significantly weaken this final barrier against pathogen intrusions. Moreover, booster disinfection practice, an alternative strategy where disinfectant is reapplied within the network, may provide greater operational flexibility to determine the proper disinfectant levels that guarantee sufficient consumer protection against pathogens without requiring excessive dose applications (which may lead to reduced DBP formation) and to select the adequate response strategy to control the spread of disease in a contamination event. Design methods for operation of booster disinfection systems are presented.
An offline design method for residual maintenance control is initially developed with assigned booster station locations as a linear least-squares optimization problem to determine optimal disinfectant injection rates that minimize variation in the system residual space-time distribution. Results on an example network show that booster disinfection can be effective in reducing network-wide disinfectant residual variations, while reducing the total mass of disinfectant used. A mixed integer quadratic programming problem is then proposed to optimally locate booster stations and to identify their dosage schedules. Results on an application network show that by increasing the number of well-placed stations, residual spatial and temporal variability is decreased. Thus, greater flexibility in booster system operation leads to more efficient maintenance of residuals at point of water consumption.
A different approach is to replace offline control design by automatic feedback controllers that continuously monitor and regulate in real time the ever changing water quality dynamics at few sensed network locations. Relying on such control scheme to maintain system water quality could bring great benefit not only when booster chlorination is used, since injections at remote locations are more likely to require automation, but also to conventional residual maintenance practice, when disinfectant is added only within the treatment plant. First, a design method is developed to identify optimal input/output (I/O) locations and to determine online control goals to be achieved at the sensor nodes. Application of the method to an example network shows that by using flow-paced booster stations, seasonal I/O dynamical changes can be managed by proper control inputs dynamics. Subsequently, an indirect model reference adaptive control algorithm is developed to regulate in real time sensor residual dynamics. Computer simulations based on water quality model illustrate the proposed adaptive control strategy.