Background Many areas, including urban areas, face problems like dehydration of the soil and a low quality of surface waters and water sediments due to the discharge of sewage water. Also the rapid carry off of rainwater from paved urban areas into sewers, partly through Wastewater Treatment Plants (WWTP’s) is recognised as a serious problem. Usually, these problems are dealt with individually, e.g. with a buffer capacity of sewerage systems, measures to prevent dehydration of the soil or measures to enlarge the capacity of river basins. In the Netherlands the most commonly accepted improvement of the sewerage system is the Improved Separated Sewerage System (ISSS). However, ISSS does not deliver optimally: with this system, still 80% to 90% of rainwater (even more in areas with limited rainfall) from paved areas is carried off to the sewer system. In this process clean rainwater is unnecessarily mixed with low quality water. There is a growing notion that all of these problems are interrelated, and that it would be better to use an integral approach when solving them. Such an integral approach is also at the heart of the Water Framework Directive (Directive 2000/60/EC). Sustainable water management and disconnection of clean rainwater from paved areas and roofs, in order to let it infiltrate or run off into surface water is thus desirable. But, this is considered very difficult within the urban areas in inner cities due to the fact that pollution by heavy traffic renders conventional techniques of disconnecting clean rainwater from streets unpractical because too much pollution is discharged into the surface water. The municipality of Stadskanaal decided to apply integral water management using the Smart-Drain technique while restructuring its city centre. The municipality initiated to this purpose Drop-wise: a demonstration project bringing into practice the disconnection of several building blocks, the intelligent separation of clean and dirty rainwater, using smart drains, in order to solve water management problems especially in the inner city areas of small and medium sized towns. Objectives The Drop-wise project aimed to demonstrate that sustainable water management through disconnecting clean rainwater from roofs and paved areas is feasible in inner city areas and can be implemented successfully by small and medium sized towns. Results Within the larger project of restructuring the roads and car parks of Stadskanaals’s city centre, the municipality implemented the Drop-wise project. The Drop-wise project mainly consisted of disconnecting existing buildings and traffic-intensive roads from the sewer system by collecting rainwater and leading it into the inner city surface water system. The project ran from January 2001 to December 2003. 2001 was dedicated to preparation and planning; the infrastructure works and the disconnection of roofs and roads were for the biggest part realised in 2002; the system was operational in May 2003. The remainder of 2003 was used to monitor (water quantity, water quality and social acceptance) and to disseminate the results of the project. The central innovative aspect of the Drop-wise project was that it applied an integrated approach to inner city water management for a large area. Furthermore, the use of the smart drain system to disconnect roads with medium to high traffic density was never done on this scale until then. The combination of activities was also unique: construction of a new sewerage system, cleaning of the ponds, disconnecting of streets and roofs, installation of smart drains. A smart drain is a system for local separation of relatively clean and dirt water. A smart drain is based on a special designed collection drain on which 10 to 20 street wells are connected. The first flush of rainwater is polluted with the dirt of streets and roofs, comes into the smart drain and is sent to the sewer system. The second flush of rainwater is clean water and is sent to a higher drain that is connected to the surface water or an infiltration system. The following works were realised during the project: - Disconnection of approximately 3 km of traffic-intensive roads from the sewer system. The total road surface disconnected was 31 000 m2. The run-off water now drains into 12 “smart drains”. - Disconnection of 10 existing buildings from the sewer system. These buildings represent a total roof area of 16.000 m2. - Creation of 2 retention ponds within the city surface water system in which the 2nd flush of the disconnected smart drains and the roof water ends up. - Installation of specific pollution prevention measures. Specific measures to prevent pollution on the roads and car parks were taken; sharp angels in the road and parking designs were for example avoided in order to prevent that dirt accumulates in the corners. The municipality also purchased a special street cleaning machine and other cleaning material to prevent pollution. Water quantity calculations showed that the project realised a considerable retention of rain water (6.200 m3 of street water and 8.000 m3 of roof water per annum) and that this lead to a significant reduction of overflows to the water system (from 7 to 5 per annum) and thus to lower pollution through these overflows. Water quality monitoring showed that the surface water was cleaner after the discharge of the smart drains but this did not prove with certainty that the smart drains function properly. Additional research carried out in 2004/2005 should determine this. Social monitoring showed that a significant number of the visitors knew the project and that they were very positive about the project. The demonstration value of the project was high particularly in the Netherlands. The beneficiary demonstrated that it is feasible in the medium sized city of Stadskanaal to apply integral water management by disconnecting road and roof surfaces. Direct benefits were higher water retention capacity, better water qualities through lower number of overflows, and lower operational costs in the wastewater treatment plant (WWTP). Moreover, these benefits were combined with a clear enhancement of the appeal of the inner city area through several retention ponds. The use of walkways, water fountains, etc has rendered an attractive area, which provides recreational opportunities for inhabitants and visitors of Stadskanaal. The project showed that the implementation of a disconnected infrastructure can best be combined with other infrastructure works. The benefits were realised at equal cost of traditional systems. Calculations were made for the Stadskanaal situation comparing the cost level of (i) a traditional sewer system, (ii) the Improved Separated Sewerage System (ISSS) commonly applied, and (iii) the infrastructure implemented in Stadskanaal based on smart drains and disconnected roads and roofs. The costs of the traditional system were evaluated at € 1 000 000 and that of the ISSS system €975 000 while the costs of the infrastructure implemented in Stadskanaal was €992 000. The cost of the smart drain system is thus comparable to the traditional sewer system and to ISSS; this allows policy makers to make an open decision on the system to be used. In the eyes of the project team, the major benefits point towards the smart drain system as being the only right choice for the Stadskanaal situation. In addition to this, benefits have been achieved regarding the attractiveness of the city centre of Stadskanaal that would not have been realised in either of the other systems. The Drop-wise approach can be applied in other cities. Disconnection is possible for any organisation that governs road surfaces and parking spaces from around 1 500 m2 connected to the sewer system. Application possibilities include both the public and private sector. The key group are of course local and regional government organisations administrating the public roads. However, companies with large facilities could also benefit from small to medium scale disconnection which is possible with Smart-Drain. Application possibilities can be found across Europe. Any place where the retention of rainwater is of importance could be suitable for disconnection through the Drop-wise approach. Strong point of this system is that it provides a scalable solution that can be used from small scale to virtually any scale needed. Also, the Smart-Drain principle of separating the polluted first flush from the clean second flush could be used to relieve the strain on existing sewer systems, as it reduces the need for peak capacity in heavy rains, without emitting pollutants to the environment. This could provide practical and cost effective solutions in cases were enlargement of traditional sewers is problematic. However, because there are significant differences in the layout of Dutch sewers vis-à-vis many other European countries, the smart drain system has application limitations. Application across Europe therefore requires additional engineering to prevent premature closing of the connection to the sewer system. The water management in Stadskanaal will be further improved along the lines of the Drop-wise project in the years after the project ends. More roof area in the city centre and neighbouring residential areas will be disconnected from the sewer system, rendering a total disconnected area far exceeding the targets for the project. More ponds will be cleaned for the retention of rainwater. The beneficiary disseminated the results of the project. A major event was organised on 13/11/03 in the form of a symposium. Stakeholders showed their interest: about 90 persons attended the symposium, some 20 local, national and international stakeholders requested additional information on the project. All in all, the Drop-wise project was the starting point for a new and improved water management in Stadskanaal, which will have lasting effects in coming years. Important improvements have been reached for both the environment and the city of Stadskanaal. Moreover these improvements were shown to be both practically feasible and economically viable.