Behaviour evaluation of vertical flow constructed wetlands for treatment of domestic sewage and septic tank sludge

Abstract

The research aimed to evaluate the performance and behaviour of Vertical Flow Constructed Wetlands (VFCW) for treating sewage and septic tank sludge. The facility is comprised of a typical first stage of the French system, built and planted with Cynodon dactylon Pers in Belo Horizonte - Brazil, according to the specifications and recommendations of CEMAGREF French Institute. Regarding sewage treatment, planted and unplanted units were evaluated operating with a daily flow of 13 m³ d-1 and a Hydraulic Loading Rate (HLR) on the full system of 0.22 m³ m-2 d-1. In relation to septic tank sludge treatment, three operational strategies were implemented and evaluated. In operational strategy 1 (OS1) the sludge unit received septic tank sludge without any retention of the percolate, followed by sending the effluent to the post-treatment unit. During operational strategy 2 (OS2), the percolate was retained during seven days. After that, the effluent was sent to the post-treatment unit. Operational strategy 3 (OS3) was implemented, where the sludge was discharged onto the sludge unit and the percolate was retained during seven days. The percolate from the sludge unit was then sent to the post-treatment unit for retention during seven days. In terms of sewage treatment, the medians of removal efficiencies obtained were of 72% for BOD5, 76% for COD, 85% for TSS, 63% for NH4+-N and 49% for TKN. The removal efficiencies of COD and TSS decreased with an increase in HLR. Treating septic tank sludge based on operational strategy 1, the following median removal efficiencies were obtained: 73% for BOD5, 81% for COD, 42% for TS, 55% for TVS, 66% for NH4+-N, 67% for TKN and 0.2 log units for E. coli removal. During operational strategy 2, the system increased its performance leading to global median removal efficiencies of 97% for BOD5, 90% for COD, 73% for TS, 73% for TVS, 75% for NH4+-N, 78% for TKN and 2.3 log units for E .coli removal. Implementing operational strategy 3, the global median of removal efficiencies obtained were of 90% for BOD5, 94% for COD, 68% for TS, 80% for TVS, 68% for NH4+-N, 87% for TKN and approximately 4 log units for E. coli removal. The dewatering of the accumulated sludge on the top occurred well and satisfactory sludge stabilization and mineralization were observed.