Small scale subsurface wetlands for onsite wastewater treatment

Abstract

Subsurface gravel bed wetland systems are now being widely used for tertiary treatment of wastewater. The commonly accepted design follows biological treatment based on first order reaction kinetic theory. The hydraulic phenomena in gravel bed wetlands is based on D'Arcy's law of laminar flow in porous media.

An investigation was conducted into a simple design of a subsurface gravel bed wetland to treat wastewater from a septic tank. The wetland was designed to treat the septic tank effluent so that the final effluent met the discharge requirements that would allow it to be irrigated.

This study was primarily concerned with design, construction, and monitoring of a subsurface wetland of simple configuration which was functional and met the discharge requirements. The wetland was constructed to serve a domestic household and the influent was pre-treated in a septic tank system.

During the monitoring of ten months, the constructed wetland was able to treating the domestic wastewater to a degree which easily allowed it to be irrigated with minimum health risk. The chemical oxygen demand of the final effluent showed a mean removal of 75%. The faecal coliform removal was of two orders of magnitude and the suspended solids removal was 88%.

From an appropriate technology point of view, it is seen that a simple configuration of subsurface wetland will be able to remove significant amount of contaminants from domestic wastewater.