REMOVAL OF ORGANIC MATERIAL IN FREE WATER SURFACE GRAVEL BED FILTERS

Abstract

The removal of organic material in free surface gravel bed filters was investigated
and modelled. Two pilot wetland units of 7.5m x 1.5m x 0.7m were constructed
adjacent to the University of Dar es Salaam waste stabilization ponds to receive
maximum organic and hydraulic loading rates of less than 185 kg/ha/d and 110
l/m 2 /d, respectively. These units received wastewater of domestic characteristics
from primary stabilization pond. The entire depth of 0.7 m of the wetland units
were packed with gravel of size 8 ~ 25 mm and porosity of 35%. Samples were
collected from the influent and effluent of wetland units and were tested for
chemical oxygen demand (COD), temperature, dissolved oxygen, pH and other
parameters useful for testing of the model. A semi-empirical model representing
the major processes taking place in the wetland units was developed by
incorporating suspended biomass, biofilm biomass, and sedimentation and
filtration components. The model was optimized using computer program. The
model was successfully developed and showed that biofilm of gravel media
contributed significantly to COD removal in wetlands.