Final Activity Report Summary - ONUREM-COWSIM (Optimisation of nutrient removal in constructed wetlands using special substrates and numerical simulation) As stated in the "Urban Wastewater Treatment Directive" [91/271/EEC], small communities with a population of less than 2000 persons are obliged to collect and treat their wastewater in appropriate ways. "Extensive Treatment Techniques" like constructed wetlands (CWs) are more economical and ecological treatment solutions if compared to the more complicated "Conventional Treatment Systems" and therefore a possible technological option for these communities. Due to their several operational advantages over other designs, Vertical Subsurface Flow (VSSF) CWs with intermittent loading became state-of-the-art during last 20 years. VSSF CWs have high removal efficiencies for organics, suspended solids, ammonium and pathogens. However, the large surface area requirement (3-10 m2/person), as well as high total nitrogen (N) and phosphorus (P) removal efficiency to be met makes it sometimes impossible to implement these systems in small/medium communities, where land is at a premium. CW researchers recently have focused on the optimisation of SSF CWs and in particular on the use of special substrates for enhanced N and P removal.Furthermore, even there are many experiences on constructing and operating the CWs; their design is still based mostly on "rules and thumbs". Thus, development of reliable simulation tools is very important. However, only few simulations models like the multi-component reactive transport module CW2D (Langergraber, 2001) exists up to now. CW2D is able to describe the transformation and biochemical elimination processes of organic matter, N&P compounds of wastewater. For a successful application of CW2D, it was shown that the proper and detailed characterisation of the properties of the substrate used in CWs is a cornerstone.In literature a lot of experiments and methods are available describing the potential use of special substrates for use as support media in VSSF CWs. To a very large extent, however, experimentation and data published are not comparable to each other since conditions, experimental analysis and approaches are differing from each other significantly.To close this gap this project ONUREM-COWSIM has been developed. ONUREM-COWSIM aimed to compare the N and P elimination performances of different substrates tested in lab-scale VSSF CWs and operated/analysed under the controlled conditions. The substrates (natural or artificial filter materials or co-products of several industries) were: 1) Sand, 2&3) Zeolite (two kinds), 4) Crushed-concrete, 5) Ferrosorp originating from Austria and 6) Sand, 7) Zeolite, 8) Pumice, 9) Perlite and 10) Blast furnace granulated slag originating from Turkey. Lab-scale VSSF CWs filled with those substrates have been constructed in the technical hall at BOKU. The columns have been loaded with primarily treated real municipal wastewater provided from the combined sewer system of Vienna. Monitoring has carried out for a period of 8 months. The data have been analysed using the same scientific approaches. A database including the physico-chemical and hydraulic characteristics of the substrates used in this research has been prepared. Results of this research were more realistic if compared to the experiments where synthetic wastewater or only N or P solutions were used.A simulation study has been performed putting the outcomes of this research (e.g. monitoring results, tracer studies and database information) into the model CW2D. Those findings have increased number and quantity of available input model data for different substrates for CW2D. It is also believed that the outcomes and results of simulation study of the project ONUREM-COWSIM has brought the model CW2D few steps further on the way for using CW2D as a practical design tool for optimal sizing of SSF CWs.