Removal of organic matter fraction from sewage in constructed wetland soil

ABSTRACT

On constructed wetlands systems, the support media usually used are coarse sand, gravel, crushed stone and gravel. In this research, we used the natural soil (Oxisol) mixed with medium sand, planted with rice (Oryza sativa L .) and a control without plants. The objective was to evaluate the BOD removal through the immobilization of microbial biomass, basal respiration (microbial activity) and total organic carbon of the soil. We used three systems, operating with hydraulic application rates of 4, 8 e 15 cm.d^-1. Each system had three units constructed wetlands and three controls, which were fed with sewage, with vertical flow unsaturated conditions, on Mondays, Wednesdays and Fridays, leaving the system with no feed on the rest of the days, for soil aeration. The results showed the soils were able to reduce significantly the BOD concentration, with removal efficiency ranging between 97 and 99%. The removal was due to the mineralization of organic matter and its accumulation in the soil. During the system start up operation, there was reduction in native soil organic matter, which started to increase gradually by the sewage application. The activity of microbial decomposers increased proportionally to the rate of sewage application, but high rates showed a trend to inhibition of this activity. In soils units controls showed minor loss of CO₂ to the atmosphere, indicating a higher carbon immobilization in soils.

Keywords:
sewage; constructed wetlands; organic substance removal; soil; microbial activity.