This work aims to find ways to improve the environmental and energy performance of three small wastewater treatment plants (WWTP). These WWTP, placed in Sintra municipality, were the object of case studies regarding corrective measures or solutions that could improve their environmental and energy performance. The evaluation of the settings of these three WWTP was obtained by an audit exercise performed with a specific methodology for small WWTP. After the evaluation were proposed corrections and solutions for all improvement opportunities. A state of art on wastewater treatment technology and sludge treatment and valorisation options were also objectives of this work, as well as an overview of the main legislations and policies of E.U. in particular the situation of Portugal on this matters.