The Arctic nature is vulnerable to environmental contaminants because of low biological diversity, lack of nutrients and extreme seasonal variations in light. In Greenland neither industrial nor domestic wastewater is treated before it is discharged to the recipients, which in most cases is the sea. Wastewater contains a variety of substances, including anthropogenic pollutants, residues of pharmaceuticals and personal care products (PPCPs), pathogenic microorganisms and parasites as well as antibiotic resistant bacteria that can be harmful for the environment as well as human health. Due to the vulnerability of the Arctic nature, the direct release of untreated sewage may have severe consequences for the receiving aqueous environment. With increasing populations in the Arctic communities and an increased demand to the level of comfort, it becomes even more vital to improve the status of wastewater treatment in these regions. However, designing, constructing and operating wastewater collection systems in the Arctic is challenging because of e.g. permafrost conditions, hard rock surfaces, freezing, limited quantity of water and high costs of electricity, fuel and transportation, as well as a settlement pattern with limited accessibility, particularly in the rural parts of the Arctic. For those reasons bucket toilets are still used in parts of the towns and in almost all settlements in Greenland. This particular toilet solution has been considered a problem for many years with respect to uncontrolled spreading of nutrients, diseases and potential pollution issues. Due to the above mentioned challenges alternative treatment methods are needed, especially in small and remotely located communities. Decentralized solutions are well suited for Greenland. Ideal solutions should reduce the need for expensive collection systems, and be more economically and environmentally sustainable than traditional wastewater collection and treatment systems. Possible alternative wastewater treatment methods for Greenlandic communities are dry composting or anaerobic digestion of excreta, collected at household level using dry or water saving toilets. This opens up for co-treatment of organic waste fractions. Freezing and thawing has also been recognised as being a cost-effective wastewater treatment method in cold regions. Thus it was chosen to concentrate on the effect of the mentioned processes, namely freezing, anaerobic digestion and composting, in this PhD project, focusing on their hygienic effect. Laboratory experiments were conducted to test the effect of the selected processes on inoculated and indigenous microorganisms in blackwater. In the first laboratory experiments the effect of long-term freezing and repeated freezing and thawing on inoculated and indigenous microorganisms in dewatered blackwater was analyzed. The results indicated that freezing has a lethal effect on some microbial groups, such as coliforms, and sublethal on others, e.g. Salmonella. Other microorganisms, like faecal streptococci and coliphages, showed a limited reduction during the long-term freezing. Repeated freezing and thawing did, however, have an enhancing effect on both coliphages and amoxicillin resistant enteric bacteria. The effect of anaerobiosis on selected indigenous microorganisms and microbial groups in blackwater during mesophilic anaerobic digestion, using fish offal as co-substrate, was investigated. The selected microorganisms and microbial groups were all reduced substantially during the experiment, and the overall results indicated that the anaerobic environment might not be the main cause of reduction of some of the microorganisms, but rather competition with active methanogenic bacteria and carbon limitation. The third laboratory experiments analyzed the effect of composting of dewatered blackwater with different bulking materials. Even though the temperature profiles of the different composting mixtures did not reach thermophilic conditions the reduction of both Escherichia coli and faecal streptococci was substantial. None of the tested processes had the ability to completely hygienize the blackwater, but some of the microorganisms and microbial groups were reduced strongly during the laboratory experiments. Other factors also play a role when selecting a suitable treatment method, e.g. operational and maintenance cost. Combining the processes might enhance the microbial reduction. One recommendation for the settlements is to combine composting and natural freezing or alternating freezing and thawing. Another alternative could be to use small and simple biogas plants, followed by dewatering of the degassed biomass, either by utilizing possible surplus of energy from the biogas plant or natural freezing, which might be a more cost-effective way. After dewatering the liquid part can be treated by filtration and the fibre part can be composted. These combinations of relatively simple processes have the possibility of a good microbial reduction. In the non-seweraged parts of the towns, the same combination could be utilized, but more advanced biogas plants could also be used, for instance with additional heat treatment, even by utilizing waste heat from the waste incinerators. For the seweraged parts of the towns it might be most beneficial to maintain the flush toilet solutions, while introducing a treatment step prior to discharging to the recipient, such as simple mechanical treatment which might even be followed by further treatment, e.g. chemical precipitation or for smaller systems, sand filtration.