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In Karagwe district in northwest Tanzania, the non-governmental organisation Mavuno is working with water scarcity and drinking-water quality issues within the local community. Mavuno started a boarding school for girls in Chonyonyo village in the district in January 2016. The school is self-sufficient of water and relies on rainwater as a raw water source. The main objective of this study was to investigate water purification with a biosand filter, built of local materials at the Chonyonyo school. The filter performance was analysed by studying the reduction of indicator organisms in the filtered water. The knowledge obtained should be transferred to employees at Mavuno, allowing them to independently operate the biosand filter and analyse the water quality. The study was done as a Minor field study with support from SIDA, in collaboration with Swedish Engineers without borders and Mavuno. The study includes a literature review and a field study where a biosand filter was built and tested. After construction, the biosand filter was operated for 6 weeks during which water samples were collected and analysed. Water samples were collected from the influent raw rainwater and effluent filtered water. The microbial properties of the water samples were investigated by analysis of the indicator organisms total coliform bacteria, e.coli and enterococci. The analysis was performed with the QuantiTray-2000® and most probable number (MPN) method from IDEXX Laboratories. Additionally, temperature and pH was measured on all water samples. The water quality of the samples was compared to Swedish, Tanzanian and WHO guideline values. The water quality analysis showed that the raw water in the rainwater tank at Chonyonyo school did not have safe drinking-water quality with regards to the levels of total coliform bacteria and enterococci. The pH in the rainwater tank was high (mean value 10.1) and did not meet the Tanzanian standards for drinking-water, while it was satisfactory according to Swedish standards. The quality of the filtered water improved over time, which is believed to be linked to the ripening of the biolayer. During week 5-6 the levels of indicator organisms declined to a satisfactory level for drinking water according to Swedish guideline values for household scale water supply, but did not meet Tanzanian standards. This indicates that the ripening time for the biolayer was approximately 5 weeks. Power cuts interrupted three incubations of samples during week 5-6. If the concerned samples were excluded, the mean values of indicator organisms in the samples remain satisfactory. The pH in the filtered water decreased to an acceptable level (pH 9.5) according to Swedish, but not Tanzanian, standards for drinking-water. This study concluded that the biosand filter can reduce levels of indicator organisms in the filtered water. Employees at Mavuno were able to operate the biosand filter and laboratory equipment independently at the end of the field study. The biosand filter can be part of a drinking-water treatment chain but should be coupled with additional disinfection, safe storage and regular microbial analysis to ensure a continued safe drinking-water quality. biosand filter, water purification, Tanzania, Karagwe