OPERATION EVALUATION OF THE WASTEWATER TREATMENT PLANT OF MYTILENE, LESVOS - GREECE

摘要

The aim of the present study is to evaluate the efficiency of the operation of the Wastewater Treatment Plant (WWTP) of Mytilene, on Lesvos Island, north-eastern Greece. The Mytilene's WWTP is in operation since 2001 and treats approximately 4,000 m~3/day of municipal wastewater and roughly 100 m~3/day septage. The plant has been designed for a population equivalent of 27,000 citizens during the winter season and 40,000 citizens in the summer, until the year 2015.The unit operates in the extended aeration activated sludge process which achieves a high removal efficiency of pollutants and simultaneously presents large flexibility and resistance in the fluctuations of hydraulic and polluting loads. The treatment system provides the possibility for biological removal of nitrogen (nitrification - denitrification) and phosphorus besides the stabilization of produced sludge. According to the design of the unit, the characteristics of treated wastewater fulfil minimally the discharge limits of Directive 91/271/EEC for "Urban waste water treatment". The measurements from July 2005 to May 2006 show that Mytilene's WWTP operates efficiently ensuring satisfactory quality of flow. Exceptional is the operation of the plant concerning the removal of these parameters achieving removal 97.41% of BOD5, 95.12% of COD and 93.12 % of SS. The concentrations of BOD_5, COD and SS in the effluent are approximately 5 mg/l, 18 mg/l and 8 mg/l, respectively. Also, the concentration of NO_3-N and NH_4-N and PO_4-P in the output of the plant are quite satisfactory. According to the annual data, there is a change of F/M loading year after year during the period 2002 -2006, although it does not imply any particular change in WWTP efficiency, which is relatively stable. The analysis shows a significant correlation of BOD5 removal with F/M loading. Septage is more polluted than the urban sewage while at the same time it presents bigger fluctuation in the samplings for the examined parameters. In comparison to other nine Greek cities, septage of Mytilene is not as much polluted and does not overload particularly the quality of the influent wastewater.