Design and construction of an urban storm water treatment system.

Abstract

The control of the increasing amounts of storm water runoff and its impacts on the surface and ground water sources is a serious problem faced by the many and rapidly growing urbanized areas in the world today. Storm water runoff refers rainfall that flows off the ground or impervious surfaces and is a major cause of ground and surface water pollution in urban areas nowadays; since most of them have been engineered with impervious surfaces like roofs, roads, pavements, drive ways, compounds and parking lots made of concrete, asphalt and tarmac. Unfortunately, in most cases, this runoff drains directly into streams, rivers or lakes directly carry everything along with a wide range of non-point source pollutants such as silt, rubbish, chemicals, total suspended solids, nitrogen, phosphorus, heavy metals (lead, zinc, copper), pathogens and organics such as petroleum hydrocarbons, oils and grease to the wetlands and surface water bodies. A case at hand is river Nabuyonga that passes through the city of Mbale. This river has been seriously polluted with one of the major causes of this dire situation is the immense amount of storm water that flows from the city particularly along Kumi road starting at the Clock Tower down to the Nabuyonga river. Bioretention systems were found to be most suitable solution. This was majorly attributed to the fact that they are retention and infiltration BMPs and can be retro fitted in urban centers of any side or shape as well as improving on the green spots in the urbanized areas. Bio retention systems are shallow landscaped depressions that utilize woody herbaceous plants like vetiver grass, shrubs and top soil, sand-gravel filter designed to remove pollutants and infiltrate storm water runoff. Bioretention systems are usually designed for a design life not longer than ten years which requires establishing of a design storm of ten-year return period. The amount of runoff generated from the catchment area was estimated using the rational method and the time of concentration. The volume of runoff was later used to identify the size of the bioretention systems as well as the total number of the bio-filters required. A prototype was set up using appropriate linear scale factors to associate the actual extents of the bioretention cell and the prototype. The performance of the prototype was established basing on mainly two tests. The first one was testing of the difference in concentration of raw water sample from the catchment and the filtrate from the bioretention cell prototype. The percentage removal of the colour and the total suspended solids attained was over 80%. On the other hand, the percentage removal of the other parameters was not as good as for the previous parameters mainly due to limited population of required plants species and organisms.