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

► 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.