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Development of an electronic monitoring system for the operation of solid state anaerobic bioreactors

Viskadouraki Evangelia

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Summary

Energy production from residues produced in the arid or semi-arid regions of theMediterranean basin, as well as from food residues, is a major field of research with promising results. A major objective of this research field is the development of a dry anaerobic bio-reactor (Solid State Anaerobic Bioreactor), which will be able to manage all the residues of the Mediterranean countryside (agricultural, livestock, agro-industrial and urban origin) in an automated and optimal way, in order to produce the maximum possible volume and optimal composition of biogas. The main operational parameters of such a bioreactor need to be closely monitored and actively controlled in order to be both safe and optimally tuned for maximum biogas production.The development of an electronic circuit, based on the Arduino platform, for themonitoring and control of the anaerobic solid state bioreactor was the main subject of this master's thesis. The system developed was based on the Arduino nano (ATmega328) microcontroller, combined with various sensors suitable for monitoring the various operating parameters. More specifically, the integrated circuit LM35DZ was used to measure the temperature of the bioreactor, the ambient temperature, as well as the temperature of the heating water of the bioreactor, with an accuracy of 0.1 °C.For the measurement of humidity inside the bioreactor, the DHT22 sensor was used, which provides a humidity measurement accuracy of ±2% and communicates digitally with the Arduino. To measure the acidity (pH) of the material, inside the bioreactor, a glass pH electrode was used. The output of this glass electrode was amplified with a suitable circuit so that it could be read by the Arduino. Using prepared solutions of known pH, the entire acidity measurement system was calibrated and the Arduino was programmed to display the actual pH values.For the real-time display of the data collected by the sensors, initially, a small OLED type screen was connected, in which all the information related to the bioreactor operation was presented. Error or warning messages were also displayed on this screen, in case the measured values exceeded user-defined limits. Data were recorded to an SD card at 30 second intervals to allow for further analysis. The connection of the SD card to the Arduino was made using the SPI protocol, with a suitable expansion board.In order to be able to monitor and control the system from a distance, the possibility of remote connection via Bluetooth and the transmission of the data to a portable device (mobile phone or tablet) was developed, using a suitable two-way Bluetooth communication board and the Virtuino platform. In this way, it became possible to monitor the system from a distance, even outside the bioreactor building, while the large screen size of the mobile phone or tablet allows for a better presentation of the measured quantities in a real-time graphic display. For the scope of this work, a graphical representation screen was designed in the Virtuino environment to clearly show all the measured values, as well as the total elapsed time of system operation.After testing all the sensors and individual components of the system on a breadboard, the final assembly was built and attached to the bioreactor. In the present work, two separate series of experiments were carried out, lasting 6 days and approximately one month, respectively. As was observed, the value of the relative humidity inside the bioreactor was constantly equal to 100%, which was expected to happen since the bioreactor content is in aqueous form. The temperature inside the bioreactor was relatively constant with small fluctuations of the order of 1-2 o C, due to the variation of the heating water temperature. The temperature of the heating water was shown to be slightly affected by the ambient temperature, but was kept constant with small fluctuations of about 2 oC. The acidity slightly decreased with time and stabilized just below the ideal value of 7, after a time period of about 4 days.Moreover, a system was developed to actively control the acidity inside the bioreactor, by using a peristaltic pump to inject small amounts of basic solution, in order to maintain the desired pH level very close to the ideal value, for optimal performance and maximum biogas production. The peristaltic pump was powered by a separate 12 V power supply, while its operation was controlled by a relay, controlled by the Arduino.Appropriate code was used to effectively adjust the acidity to the desired levels, which checks the pH value every 5 minutes and decides if a correction is needed. If required, it injects a small amount of base solution into the bioreactor. Two different injection rates were tested in order to reduce the overshoot and achieve effective pH regulation, very close to the optimal value of 7, even if the initial pH deviation was large.

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