This paper aims to show some applications of microbial fuel cells (MFCs), a suitable energy harvesting technique, as clean power source to supply a wireless sensor network for environmental monitoring or wastewater treatment or biosensor. An MFC is a bioreactor that converts energy stored in chemical bonds of organic matter into electrical energy. The performance of MFC is limited by a low output voltage and low output power. However this energy may be enough to supply low power applications to replace their batteries. Moreover considering that the performances of MFCs are related to many operating conditions, MFCs can be employed as a biosensor to measure some parameters (Organic matter concentration, temperature, and pH). The applications targets inaccessible environments where soil and waste-water are nonetheless present. In this case, a terrestrial microbial fuel cell (TMFC), based on soil that acts as membrane and as flow of nutrient, it can function as a pH sensor. Instead a wastewater microbial fuel cell (WWMFC), where waste-water acts as flow of nutrient and bacteria, it can operate as a temperature sensor. Other low power devices can be supplied by the harvested energy from MFCs.