Long term performance of an urban decentralised greywater treatment system in Oslo, Norway
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- Master's theses (IMV) 
Source separation of wastewater keeps the nutrient-rich fraction from the toilet (blackwater) separated from the remaining main volume (greywater). This separation optimises the recycling of both nutrients and water. A pilot system for decentralised greywater treatment was constructed in 2001 in a courtyard at Klosterenga (KL) in Oslo, Norway, which serves an apartment building of 100 persons. The system consists of a septic tank followed by pre-treatment vertical flow aerobic biofilters and a sub-surface horizontal flow constructed wetland. The scope of this thesis was to investigate the long-term treatment performance of the system. Water samples from the influent (i.e. septic tank effluent) and effluent were analysed with regards to: phosphorus, orthophosphate, nitrogen, nitrate, ammonia, pH, conductivity and indicator bacteria. The average phosphorus effluent was 0,27 mg P/l, which is substantially below the 1 mg P/l guideline. The BOD treatment efficiency was as high as 98 % and resulted in an average effluent of less than 5 mg O/l. Average total nitrogen effluent concentration was 2,2 mg N/l and thus achieves drinking water quality with respect to nitrogen. All parameters fulfilled discharge limits from the time of construction until today. There were no significant change in the effluent from 2001-2013 compared to 2014, except for pH and phosphorus. This is also as expected when the filter material is functioning as intended, and it was calculated that the wetland filter would have a total service time of 45 years with regards to phosphorus removal. The highest registered amount of bacteria in the effluent was 19 E. coli per 100 ml which means that the water fulfils the Norwegian requirements for good bathing water quality (<100 per 100ml), and that the effluent can be re-used for irrigation of edible crops. When the KL system was compared to the average effluent concentrations of other large-scale constructed wetlands systems, it performed better with regards to all of the parameters, and it also had higher treatment efficiencies with regards to BOD and nitrogen. An investigation of the plans and policies regarding wastewater in Oslo revealed that expanding the use of systems as KL can contribute to achieve the current goals, and that the KL system is becoming increasingly relevant. An economic estimate indicates that treating greywater in systems such as KL would be 65% cheaper than using the centralised system of Oslo municipality. For KL and the centralised system the final recipient is in a state where nutrient loading should be reduced, and a substantial (70-95 %) reduction of this loading, per person, can be achieved by source separation and treating the greywater at KL. The system at KL shows how decentralised sanitation solutions can offer cross-sectorial benefits, and thus can contribute to making urban areas more sustainable. The constructed wetland system not only treats wastewater, it contributes to urban greening, increased environmental awareness, reduced pollution, recycling of resources – and it can be easily integrated with existing centralised infrastructure. Overall the system demonstrates that successful greywater treatment by constructed wetlands is possible in urban settings where space is limited, and that a high effluent quality can be achieved, even after more than a decade of operation.