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Dripper clogging reduces the performance and service life of a drip irrigation system. The impact of chlorination (1.5 ppm of free chlorine during 1 h application) and pressure flushing (0.18 MPa) on the biofouling of non-pressure-compensating drippers fed by real reclaimed wastewater was studied at lab scale using Optical Coherence Tomography. The effect of these treatments on microbial composition (bacteria and eukaryotes) was also investigated by High-throughput DNA sequencing. Biofouling was mainly observed in inlet, outlet and return areas of the drippers. Chlorination limited biofilm development mainly in the mainstream of the milli-labyrinth channel. It was more efficient when combined with pressure flushing. Moreover, chlorination was more efficient in maintaining the water distribution uniformity. It reduced the bacterial concentration and the diversity of the dripper biofilms compared to the pressure flushing method. This method strongly modified the microbial communities, promoting chlorine-resistant bacteria such asComamonadaceaeorAzospira. Inversely, several bacterial groups were identified as sensitive to chlorination such as Chloroflexi and Planctomycetes. Nevertheless, one month after stopping the treatments the bacterial diversity re-increased and the chlorine-sensitive bacteria such as Chloroflexi phylum and the Saprospiraceae, Spirochaetaceae, Christensenellaceae and Hydrogenophilaceae families re-emerged with the growth of biofouling, highlighting the resilience of the bacteria from drippers. Based on PCoA analyses, the structure of the communities still clustered separately from never-chlorinated drippers, showing that the effect of chlorination was still present one month after stopping the treatment.HighlightsThe fouling of drippers is a bottleneck for drip irrigation using reclaimed wastewaterBiofouling was lowest when chlorination was combined with pressure flushingThe β-Proteobacteria and Firmicutes contain chlorine resistant bacteriaThe decrease of Chloroflexi by chlorination was transitoryThe bacterial community was resilient after the interruption of cleaning events

The clogging of drippers due to the development of biofilms reduces the benefits and is an obstacle to the implementation of drip irrigation technology. The geometry of the dripper channel has an impact on the flow behaviours and head loss. The objective of this study was to analyse the influence of hydrodynamic parameters of three types of drippers (flow rates of 1, 2 and 4 l.h-1) fed by reclaimed wastewater on biofilm development kinetics and on the bacterial community. Using optical coherence tomography, we demonstrated that the inlet of the drippers (mainly the first baffle) and vortex zones are the most sensitive area for biofouling. Drippers with the lowest flow rate (1 l.h-1) and the smallest channel section were the favourable areas to biofouling. The low inlet velocity (0.34 m.s-1) in this type of dripper compared to 2 l.h-1 (0.61 m.s-1) and 4 l.h-1 (0.78 m.s-1) drippers can favour the deposition and development of biofilms. In addition, the water velocity influenced the structure of the bacterial communities in the biofilm. Low velocity (0.34 m.s-1) favoured the presence of Hydrogenophaga and Pseudoxanthomonas genera at the early stage of biofilm formation and filamentous bacteria belonging to Chloroflexi phylum at the end. So, maintaining a high flow rate and using drippers with a large flow cross-section is an effective way to control the development of biofilms by limiting the presence of filamentous bacteria.