Assessment And Management Of The Water Quality Risk Of Reclaimed Water Disinfection And Distribution System

The proportion of reclaimed water in the total urban water supply is keptincreasing in China, and the public concerns on the quality of the reclaimed water arealso growing. However, the highly complex water quality and the traditional passivemanagement approach which is based on the end detection have made it difficult toguarantee the water quality in the long process of wastewater reclamation, recyclingand reuse. In order to solve the present problems, the mathematic description of thewater quality change mechanism and the complete assessment of the potential as wellas the revealed water quality risk were developed, and then the determination of thecritical factors influencing the risk was conducted on the basis of these works. Therisk management framework would be designed focused on the critical factors toprevent the incident factually occurring or minimize the occurring probability. Themethodology above was practiced on the actual disinfection and network in theresearch area to test its feasibility and rationality.According to the actual wastewater reuse conditions in the research area, the riskwhich should be concerned in the urban miscellaneous, industrial and scenicenvironment reuse was discussed, and the COD, BOD5, ammonia, fecal coliform,residual chlorine and turbidity were regarded as the concerned indexes. The waterquality models for the disinfection process and distribution system were developedrespectively; the dynamic equations involved in the models described the reactionsamong the concerned indexes and the mass transfer between the two phases, bulkwater phase and biofilm (as well as pipe wall) phase, occurring in the disinfection andnetwork part of the recycling process.Considering the uncertainty of the parameters in the model describing the factualreaction system which is commonly influenced by a great many factors, the MCMCalgorithm (based on Bayesian theory) and the HSY algorithm (based on stochasticprocesses theory) were integrated with numerical method, and respectively used in theparameter calibration of disinfection and network model. On principle of structuralvalidity check, the parameter calibration results and the developed models were testedand proven reliable. In the risk assessment, the variability of the model parameters, initial variablevalues and process running conditions were described in a set of distribution. MCalgorithm was applied to conduct the stochastic sampling in the distributions, and thepredictive water quality of disinfection process effluent and network nodes werecalculated on basis of a great number of MC iterations. In this way, the predictivedistributions of disinfection process effluent and network nodes’ water quality wouldbe obtained from the statistics of the MC modeling results, and those could beconsidered as the complete reflect of the current water quality condition of thedisinfection and network parts. The risk of the disinfection effluent was expressed bythe probabilities that the predicted water quality data violating with the set guidelines;the risk of the whole network was shown by the contour maps of the predicted waterquality data, and the factual influence of the water quality risk was assessed by theintegration of node risk and water demand. Compared with the traditional waterquality risk assessment which is dependent on several discrete detected data, theproposed methodology is much better on the complete risk description shown by theviolating probabilities of predictive water quality distributions as well as the reliabledecision support from the abundant water quality modeling data.Based on the results of water quality modeling, RSA method was applied in thedetermination of the cause of the water quality risk. The influence of the raw waterqualities and process running conditions were quantified, ordered in descendingsequence and analyzed, in order to define the key factors affecting the water qualityrisk on disinfection and network parts. Some risk control methods were discussedfocused on the critical factors, and the needed adjustment at any unit in thewastewater reuse process was regarded as belonging to the optimal strategy. In thisway, the risk occurred at any part of the water reuse process could be efficientlysolved through the management strategy focused on the whole reuse process.