Emissions of insufficiently treated wastewater, mainly blackwater (i.e. wastewater from toilets), contributes to the eutrophication of lakes and seas. At the same time, blackwater is a fraction with a low content of heavy metals and, after sanitization, it can recirculate plant nutrients to arable land using the liquid composting treatment method. Liquid composting means that organic material is degraded under aerobic conditions. For sanitization, the blackwater needs to be treated together with energy-rich complementary material. A requirement for the sanitization in a liquid compost is that the substrate is energy rich, that the energy content is high enough and that the material is well dispersed and pumpable. The aim of this master thesis has been to find appropriate complementary material for treating blackwater in the wet compost treatment plant located in Karby, Norrtälje municipality, Sweden. An inventory was made of the organic waste produced in Norrtälje municipality and two nearby municipalities. Samples of the organic waste were collected and analysed for dry matter (as an indicator of energy content), plant nutrients, heavy metals and biochemical oxygen demand (BOD7). The suitability and availability of different organic waste fractions as complementary material was decided upon analyses results, calculations of estimated amounts of organic waste as well as some assumptions. Food waste is a relative clean fraction with a high energy content that would be a good complementary material for liquid composting. Fruit, vegetables and bread from stores are also clean fractions, but the availability is uncertain and depends on different factors. Based on results from this project, grease from grease trap is an uncertain fraction that is not recommended for complementary material for the liquid compost. The results show that Norrtälje municipality has enough organic waste to make it possible to use the whole capacity of the liquid compost plant in the future and to treat larger quantities of blackwater than today. The quantities of blackwater that can be treated are uncertain because a number of assumptions have been made and also because availability varies between different fractions of organic waste. Before it will be possible to add new organic waste fractions and more black water into the liquid compost plant, some kind of pretreatment plant is needed.