Quality Assessment of sludge from Glen Valley WWTP and its potential as fertilizer

Sammanfattning: The use of sludge as fertilizers contributes to the recycling of nutrients to the environment due to its origin from wastewater and therefore relatively high content of nutrients and organic matter. However, wastewater does also contain hazardous compounds like heavy metals and micropollutants which eventually are separated to the sludge during the treatment processes at the wastewater treatment plant. In order to improve the sludge quality, source tracking is a relatively cheap and effective way to find and eliminate hazardous compounds and prevent them from ending up in the sludge. The aim with this thesis was to investigate the fertilizer potential in sludge from Glen Valley wastewater treatment plant (GWWTP) in Gaborone, Botswana. The sludge is analyzed in terms of nutrient and heavy metal contents as well as physiochemical parameters. In order to determine the potential as fertilizer, the quality of the sludge is compared with other fertilizer alternatives, other sludge types and regulations for sludge use in agriculture. The work with sludge quality improvements in Gaborone is also investigated. Sludge from different stages along the treatment processes were collected in order to see differences in quality related to the treatment. The results shows that the samples of primary anaerobically treated sludge tend to have higher heavy metal content than the secondary sludge. The quality of the dry sludge samples indicates lower nutrient content than both the primary and secondary sludge, but similar heavy metal content. The analysis of dry sludge from GWWTP indicates a low nutrient value and high heavy metal content in comparison to other selected fertilizer options and sludge from other WWTPs. The processes for removal of nutrients from the wastewater are an important factor for the nutrient content in the sludge. The sludge from GWWTP is currently sold as fertilizer and soil conditioner. In order to get a sustainable sludge production and a safe fertilizer alternative, a clearer strategy for the management and analysis of the sludge could be established. There is no found legislation of sludge quality for agricultural use in Botswana but it was found that arsenic, copper, lead and zinc exceeded the limits according to the South African regulations that are likely to be implemented in Botswana in the future. The Swedish limits were exceeded for lead and zinc. The work with source tracking in Botswana is conducted in the Trade Effluent Agreement (TEA) where the strategy is to make an agreement with industries to only discharge wastewater to the sewer system that is approved by the country’s water authority. There is a challenge in getting industries to participate in the agreement. The result from this thesis is only based on one single sampling occasion. A continuous and more frequent sludge quality analysis work must be done in order to determine the actual potential as fertilizer. The result presented in this report does though indicate that the quality of the sludge needs to be improved before it can be assured that the sludge is safe and sustainable to use as fertilizer for food production.