Greywater originates from kitchen sinks, dishwashers, handbasins, showers, and laundry. Greywater can account for 70–90% of the domestic wastewater volume and contains organics, nutrients, pathogenic microorganisms, micropollutants, and microplastics. Effective treatment can unlock the potential of greywater for non-potable reuse purposes like urban landscaping or irrigation. The overall aim of this thesis was to investigate on-site greywater treatment systems which included on-site systems, two green walls, and a treatment wetland, and investigate the treatment in terms of organic matter, nitrogen (N), phosphorus (P), pathogenic microorganisms and microplastics (MPs), including the potential resource recovery and safe reuse of greywater.

Among the eight on-site systems (1–5 persons) investigated, commercial systems included three type A, two type B, and C system. Type D was a conventional sand filter. After the pre-treatment septic tanks, the treatment unit of type A consisted of a geotextile-fitted trickling filter over a sand bed, type B contained a mineral wool filter, and type C had fine-meshed plastic filters. The two green wall studies were conducted at a testbed facility, RecoLab, which received greywater from a newly developed urban city district (ca 1000 people). The treatment of an indoor vertical flow (VF) green wall with five filter materials (pumice, biochar, hemp fiber, spent coffee grounds, and composted fiber soil (a paper industry byproduct)) was investigated with the flow rates of 4.5, 9, and 18 L/d. The outdoor horizontal flow (HF) green wall with four levels filled with biochar, pumice, and LECA as filter material was investigated for one year, using a subsurface horizontal flow of 430 L/d. A long-term evaluation of the performance of a constructed wetland for treating greywater from a residential building (ca 100 people) in Norway was conducted, using data from the period 2001–2024. The constructed wetland consisted of a biofilter with Filtralite® material and a horizontal subsurface filter with Filtralite®P, for enhanced phosphorus removal.

The treatment efficiency of the systems was highly influenced by the filter material and flow rates, while seasonal temperature changes had a low impact. All the systems demonstrated effective treatment of greywater and met the local discharge guideline of 80% BOD reduction and <3mg/L of P in the effluent. However, only the VF green wall and constructed wetland could produce an effluent with <1 mg P/L, a limit for facilities located in sensitive areas. Among the studied filter materials, sand, biochar, and Filtralite® were the most efficient (log10 reduction up to 4) in the bacteria Escherichia coli, enterococci, Clostridium perfringens, Pseudomonas aeruginosa, Legionella spp., and met the European Commission’s guideline for reuse of reclaimed water in agriculture. The quantitative microbial risk assessment (QMRA) on effluent greywater from the constructed wetland, for multiple exposure scenarios (16 exposures/year) of accidental ingestion of 1 mL, indicated safe reuse in a water cascade during the summer season with regard to E. coli and C. perfringens. In addition, using TED-/Py-GC/MS, high variability of MPs was observed in greywater from the different sources of generation, while all the filter material of the respective treatment systems effectively retained the MPs, except for mineral wool and hemp.

The findings of this thesis could contribute to the development of a more resource-efficient wastewater management and Water-Food-Energy nexus by demonstrating the potential of decentralized greywater treatment systems.