Analytical campaigns were conducted on different drinking water treatment lines in order to characterize filter backwash water and assess the impact of recycling this water at the head of the plant. The pollutants identified in this water are essentially in the form of particles. Recycling this water may consequently increase the concentration of parameters such as turbidity, suspended solids, metals from coagulants and protozoa. On the other hand, no release of pesticides nor significant generation of disinfection by-products was observed during filter backwash with chlorinated water, in the conditions applied in France for chlorination.

A modeling approach based on the mass balance of Cryptosporidium oocysts was applied to estimate the impact of recycling on oocysts concentration in the inlet water. A risk of infection was then assessed for each recycling scenario. A similar approach was also applied for amoebae, which have the capacity to colonize filter media, and for metal residues from coagulants.

The results of this study demonstrate that two different situations have to be considered separately:

• In the case of treatment lines composed of separate sedimentation and filtration steps, recycling at the head of the treatment process, even with no treatment, has no significant consequence on the microbial quality of the inlet water, and generates no additional health risk for the consumer.

• In the case of treatment lines with no sedimentation step (direct filtration or UF used alone), recycling untreated water generates an excess of risk for the consumer which is not acceptable. Adding a coagulation / sedimentation step in the recycling circuit is sufficient in that case to keep the risk within acceptable limits.

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