Monitoring of the composting process using a mediator-type biochemical oxygen demand sensor

The pH, electrical conductivity, ammonia-nitrogen, chemical oxygen demand, carbon-to-nitrogen ratio, total organic carbon and biochemical oxygen demand (BOD5) of compost extract solution and the moisture content and organic matter content of solid compost samples were investigated as parameters for monitoring the composting process. It was observed that the BOD5 value of the compost sample decreased dramatically from 355 to 33 mg L-1 during the process. Compared with other parameters, the BOD5 value showed the largest change. However, the conventional method of BOD5 determination is not suitable for process control owing to its time-consuming nature and the complicated procedures required. Thus, compost monitoring using a mediator-type BOD sensor consisting of Pseudomonas fluorescens biovar. V and potassium ferricyanide was investigated. In order to realize rapid and reproducible measurements, we developed a 10-channel potentiostat system incorporating screen-printed disposable chips incorporating microorganisms immobilized in sodium alginate gel. Using sludge extract solutions as standard solutions for calibration, it was found that aqueous extracts of the compost were most suitable for BOD determination. In conclusion, the relative change in BOD sensor values determined using our system corresponded well with the BOD5 values obtained using the standard 5 d method during 58 d of composting.