This study evaluated the performance of two open-photobioreactors operated with 18 microalgae-bacteria (PBR-AB) and purple photosynthetic bacteria (PBR-PPB) consortia 19 during the continuous treatment of diluted (5%) piggery wastewater (PWW) at multiple 20 hydraulic retention times (HRT). At a HRT of 10.6 days, PBR-AB supported the 21 highest removal efficiencies of nitrogen, phosphorus and zinc (87±2, 91±3 and 98±1%), 22 while the highest organic carbon removals were achieved in PBR-PPB (87±4%). The 23 decrease in HRT from 10.6, to 7.6 and 4.1 day caused a gradual deterioration in organic 24 material and nitrogen removal, but did not influence the removal of phosphorus and Zn. 25 The decrease in HRT caused a severe wash-out of microalgae in PBR-AB and played a 26 *Manuscript Click here to view linked References key role in the structure of bacterial population in both photobioreactors. In addition, 27 batch biodegradation tests at multiple PWW dilutions (5, 10 and 15%) confirmed the 28 slightly better performance of algal-bacterial systems regardless of PWW dilution.

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