A solids-free sewer is a network of small-diameter pipes that transports solids-free or pre-settled wastewater (such as the effluent from septic tank or biogas settlers) to a treatment facility for further treatment or to a discharge point. They can be built for new areas or where soil infiltration of septic tanks effluents (e.g. via leach fields) is not appropriate anymore (i.e. densely populated areas; clogging of sub-surface). Due to the simplified design, solids-free sewers can be built for 20% to 50% less than conventional sewerage. However, expert design and construction supervision is essential and repairs and removal of blockages may be required more frequently than for a conventional gravity sewer. Moreover, effluent and sludge (from interceptors) requires secondary treatment and/or appropriate discharge. Small-bore sewers also require a certain level of responsibility of users, because maintenance is high due to the high risk of clogging.