(6)In order to ensure the effective treatment of urban wastewater before discharge into the environment, all urban wastewater from agglomerations of 1 000 p.e. and above should be collected in collecting systems, unless Member States justify the need for a derogation for the use of individual systems under this Directive. When delineating their agglomerations, Member States should take into account the indicative reference threshold of 10 to 25 p.e. per hectare, above which the population, possibly combined with economic activities, located in a specific area is considered sufficiently concentrated. Where collecting systems are already in place, Member States should ensure that all sources of domestic wastewater are connected to them.