Decentralized two-stage sewage treatment by chemical–biological flocculation combined with microalgae biofilm for nutrient immobilization in a roof installed parallel plate reactor

[Display omitted] ► A two-stage treatment system for domestic wastewater treatment in situ is proposed. ► In the first stage, COD and P are removed by chemical biological adsorption. ► In the second stage, N and P are removed by immobilization in microalgae biomass biofilm. ► Algal biofilm grown in an innovative parallel plate reactor installed on a roof. ► COD and P discharge limits meet EU regulation but N is just below these limits. In this lab-scale study, domestic wastewater is subjected to a chemical biological adsorption (A-stage), followed by treatment in an innovative roof installed parallel plate microalgae biofilm reactor for nutrient immobilization (I-stage). The A-stage process was operated at a hydraulic retention time (HRT) of 1h and a solid retention time of 1day (FeSO4 as flocculant). The I-stage, which consequently received the effluent of the A-stage process, was operated at an HRT of 1day and exposed to natural light. The overall system removed on average 74% of the total chemical oxygen demand, 82% of the total suspended solids, 67% of the total nitrogen and 96% of the total phosphorous in the wastewater. The design involves a relatively low capital and operating cost which is in the order of 0.5€/m3 wastewater treated. These aspects suggest that the A/I process can be used as a decentralized domestic wastewater treatment system.