Self-cleaning Preliminary Screening Mechanisms to Improve Health and Wastewater Treatment Efficiency in Low-income Countries

Abstract

The first process step in a wastewater treatment plant is generally screening. A screen can be described as a coarse sieve, designed to prevent large solids (such as pieces of wood, plastic waste and hygiene products) from entering the plant and possibly damaging subsequent treatment equipment. Screens can be classified regarding their aperture size and cleaning mechanisms. Mechanically-cleaned screens are most common in industrialized countries, whereas manually-cleaned screens, which are less expensive, are common across the global south. The problem regarding the latter is that while removing accumulated materials, the operators are exposed to the pathogenic wastewater, as well as physical risks such as cuts inflicted by the screenings. Furthermore, if manual cleaning is not properly enforced and managed, screenings might accumulate to such a degree that the wastewater flow is reduced and the plant cannot operate at its design capacity. The deployment of mechanically-cleaned screens in low-income countries is hindered by their comparatively higher investment and operating cost and their requirement for a reliable source of electricity. This thesis assesses and pilots concepts of low-cost mechanically cleaned alternatives to conventional devices. Mechanically cleaned screens increase the protection of operators against harmful substances as they only access the screen in case of a blockage, other mechanical failures or maintenance. Low-cost in this context means that it has to be feasible for low-income countries without the reliability of constant electricity. During the concept generation phase, several alternatives for different sub-functions of screening devices were developed. The majority of developed concepts relied on water wheels as an energy source. The two most promising concepts - namely a rotating rake mechanism and a horizontally mounted screen - were realised on a scale of 1:11 in order to validate the concepts and gain first insights. Neither of the two concepts could be fully validated. This underlines the need for further development and testing until the potential deployment of mechanically cleaned screens in low-income countries.