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Blackwater from holding tanks in small sewage systems has a potential as fertilizer on agricultural land. However, knowledge about its usefulness as a fertilizer is limited compared to that regarding sludge from sewage treatment plants. In this paper, a method to evaluate the quality (relationship between nutrients and heavy metals) of blackwater and sludge is discussed. The quality, i.e. usefulness, of these products as fertilizers depends on their contents of plant nutrients as well as their contents of heavy metals. The use of sewage sludge and blackwater on agricultural land is limited in Swedish regulations according to the content of nutrients and heavy metals in the products. The nutrient or heavy metal that first reaches its limit decides how much sludge or blackwater can be applied per hectare (SNFS, 1994). The contents of nutrients and heavy metals were analyzed in blackwater from holding tanks in allotment gardens in the city of Lund and compared to those in sludge from the sewage treatment plant Kllby in Lund. The samples of blackwater analyzed had a low content of heavy metals and a high content of plant nutrients, i.e. nitrogen, phosphorus and potassium. The quality of the blackwater was found to be sufficiently high to allow it to be used as a fertilizer on agricultural land. The relationship between nutrients and heavy metals was more favorable in blackwater from holding tanks than in sludge from the public sewage treatment plant. The suggested method, to relate the content of nutrients to the content of heavy metals, was found to be useful for evaluating blackwater as a fertilizer in agriculture.