French vertical flow constructed wetlands: a need of a better understanding of the role of the deposit layer

Abstract

French vertical flow constructed wetlands, treating directly raw wastewater, have become the main systems implemented for communities under 2,000 population equivalent in France. Like in sludge drying reed beds, an organic deposit layer is formed over time at the top surface of the filter. This deposit layer is a key factor in the performance of the system as it impacts hydraulic, gas transfers, filtration efficiency and water retention time. The paper discusses the role of this deposit layer on the hydraulic and biological behaviour of the system. It presents results from different studies to highlight the positive role of the layer but, as well, the difficulties in modelling this organic layer. As hydraulic, oxygen transfers, and biological activity are interlinked and impacted by the deposit layer, it seems essential to focus on its role (and its quantification) to find new developments of vertical flow constructed wetlands fed with raw wastewater.

Performance evaluation of hybrid treatment wetland for six years of operation in cold climate

“…VBA may have performed effectively for TN, SS, COD and BOD 5 due to the deposition of organic layer on the surface of the bed. As described by Molle (2014), the deposited organic layer helped filtration efficiency. This layer contributed to removal of large particulate matter and the low permeability allowed the wastewater to be distributed equally on to the surface.…”

“…5). In addition, the accumulating organic layer improves filtration efficiency, increases water retention time, favors ammonia adsorption onto the layer and promotes biological activity (Molle 2014). Although this system was receiving pollutant at very high concentrations and loads, the same removal mechanisms as HSCW fed with low concentrations and loads were functioning.…”

“…According to several studies, the removal efficiencies for matured systems are relatively higher than their startup period. The growth and development of vegetation and associated activity of micro and macroorganisms are playing major roles (Molle 2014) and there are evidences that planted constructed wetlands perform better than the unplanted. This is because the plants root systems provide aeration, inhibits clogging of the bed material by root penetrations and development of bacterial colonies in the rhizomes.…”

Comportamento diário ao longo do período de alimentação do primeiro estágio do sistema francês de wetland vertical, em termos de remoção de matéria orgânica e amônia

“…Além desse critério, Molle et al [4] reforçam que as cargas hidráulicas e orgânicas, devem ser bem controladas a fim de favorecer a mineralização dessa camada. Caso contrário, temse limitações dos processos de transferência de oxigênio (convecção e difusão) pelo excesso de água na superfície, influenciando na performance do sistema.…”

“…Alternância de ciclos de alimentação por 3,5 a 4 dias e repouso por 7 dias é considerada importante para o controle do crescimento bacteriano e necessário para manter as condições aeróbias no filtro, permitindo assim, a mineralização da matéria orgânica proveniente da retenção dos sólidos suspensos contidos no afluente bruto [1] que se acumulam na superfície do sistema, formando uma camada de cerca de 2,5 cm ano -1 [4]. Em termos de nitrogênio, o sistema apresenta algumas limitações, sendo identificada nitrificação em torno de 50% para cargas variando entre 25 a 30 g nitrogênio total Kjedahl (NTK) m 2 d -1 [1].…”