OCTOPUS, Low-cost stand-alone equipment to monitor thermal diffusion in static composting system

The activity has been developed within the framework of a novel static composting method set up by ISAFOM-CNR for olive mill waste (OMW) bioremediation. Usually, composting procedure requires that temperature should keep over 55°C for a minimum of three days in order to reduce human pathogens and ensure the safety of the end product. Briefly, OMW was mixed with appropriate bulking agents and packaged in polyethylene bags (Big Bag, BB); after packaging, the Active Thermal Composting (ACT) phase immediately started and it was monitored by means of a prototype equipment consisting of a grid including 45 thermistors NTC 10K Ohm 1% (MuRata Electronics) placed in Eppendorf polypropylene tubes and sealed by epoxy resin. The grid was located in a vertical section of the BB which was stored outdoor and protected from the rain. Outdoor conditions (temperature and relative humidity) were collected during the trial.In order to minimize noise coupling, the thermistors were connected with shielded twisted-pair cable for the transmission of the floating signal source to the data stand-alone acquisition device (ArduinoTM) and powered by 3.3V DC circuit. A multiplexer device and a real time clock were arranged in external junction box in order to expand the signals and synchronize the time. Appropriate software, developed in Arduino-based free platform, ensured temperature collection to a file on a SD card; acquisition of data occurred automatically every 15 minutes during the ACT phase of composting which lasted 2.5 months.Temperatures were elaborated and contour plotted with profiles by OriginPro graphing and data analysis software; it showed patterns similar to those reported for usual composting systems.Taking into account weight and volume changes during the ACT phase, results showed that over 85% of the mass put inside the BB kept over 60°C for more than 10 days, thus confirming the safety of the innovative static composting procedure carried out.