Assessment of the Efficiency of Onsite Wastewater Management Systems Peri-Urban Areas, Case Study

Utawala, Nairobi County

Abstract

This paper studies the efficiency of onsite wastewater management systems in a developing peri urban area, Utawala, on the outskirts of Nairobi city, Kenya, currently used due to lack of a centralised waste management system. It relates the system efficiency with environmental considerations, sizing, construction, and maintenance of the systems which are some of the factors considered during selection. A total of 25 systems were studied ranging from pit latrines, biodigesters and septic tanks. The total efficiency for the systems studied was 44% with 56% considered to be failing. When considering the systems serving 25 – 100 persons only, the efficiency rate was 22.22% with 77.78% considered to be failing. The area studied had clayey black cotton soils and a hard phonolite layer beneath as its geological make up. This study demonstrated that the study area with poor soil absorption capacity was not suitable for handling absorption-based systems for large developments. While maintenance is critical to improve the functioning of these systems, especially in areas with absorption difficulties, it is often expensive and can be neglected leading to failure. Sizing of absorption-based systems in less absorbent areas should include a soak pit detail based on the actual geophysical properties to improve the working of these systems. Thinking about waste as a resource is helpful and can promote the use of recycling systems but they could be expensive install and therefore decentralised systems to serve a larger group of developments should be considered where the centralised sewer treatment option is not available.