Small scale water recycling systems - risk assessment and modelling

This paper aims to use quantitative risk analysis, risk modelling and simul ation modelling tools to assess the performance of a proprietary single hou se grey water recycling system. A preliminary Hazard and Operability study (HAZOP) identified the main hazards, both health related and economic, asso ciated with installing the recycling system in a domestic environment. The health related consequences of system failure were associated with the pres ence of increased concentrations of micro-organisms at the point of use, du e to failure of the disinfection system and/or the pump. The risk model was used to assess the increase in the probability of infection for a particul ar genus of micro-organism, Salmonella spp, during disinfection failure. Th e increase in the number of cases of infection above a base rate rose from 0.001% during normal operation, to 4% for a recycling system with no disinf ection. The simulation model was used to examine the possible effects of pu mp failure. The model indicated that the anaerobic COD release rate in the system storage tank increases over time and dissolved oxygen decreases duri ng this failure mode. These conditions are likely to result in odour proble ms.