Dala Vatten AB has been operating two municipal wastewater treatment plant namely Gagnef wastewater treatment plant and Tällberg wastewater treatment plant since 1970’s and 1950’s respectively in Dalarna, middle of Sweden. These both traditional treatment plants have been updated with continuous and intermittent aerated biological treatment facilities: Moving bed biofilm reactor (MBBR) in 2012 and 2007 respectively. Recently, more detailed process performance and evaluation of both plants is required focusing on energy consumption. The objective of this master thesis is thus to investigate the opportunities in reducing energy consumption from both plants to save operation cost, identify the opportunities for chemical saving, if possible reduce the excess sludge so produced from the plant and potential optimization of the process for its plant’s sustainability. The laboratory study was made in May and August, 2015 with grab sampling and flow proportional sampling method. The analysation was carried out with several wastewater parameters: BOD7, COD, TOC, TP, NH4-N and TN with Hach Lange analysing method. Results showed that both continue and intermittent aerated plants have higher process performance with lower effluent organic (BOD7, COD) and TP loading to the recipient. Gagnef WWTP with continuous aeration has demonstrated an excessive use of chemical, sludge production and high-energy consumption by the blower serving MBBR during the studied period. By contrast, Tällberg WWTP with intermittent aeration has proved to be successful in terms of lower energy consumption by the blower serving the MBBR but failed to show improved specific energy efficiency for each pollutant load during the studied period. The recommendation in improvising energy saving and saving operation cost at both treatment plants was put forward.