Real-Time Determination of Swine Manure Liquid Compost Maturity by Oxidation-Reduction Potential and pH Monitoring

The aim of this study was to develop a real-time diagnostic method for assessing the maturity of liquid compost from swine manure by monitoring the oxidation-reduction potential (ORP) and pH, two key indicators of the oxidative status of organic matter and nitrogen compounds. The experimental system comprised a cylindrical bioreactor equipped with an aeration and mixing device. During the liquid composting process of swine manure in the bioreactor, the ORP and pH were continuously monitored, while changes in the liquid compost maturity and concentrations of soluble nitrogen and organic matter were observed. In addition, the physicochemical differences between mature liquid compost (MLC) and non-mature liquid compost (NMLC) were comparatively analyzed. The MLC exhibited significantly higher ORP and lower pH with lower ammonium nitrogen concentrations and higher concentrations of oxidized nitrogen compounds (NOₓ-N) compared to NMLC. A diagnostic method combining ORP and pH thresholds was proposed, where samples with an ORP greater than or equal to 180 mV were classified as MLC. In addition, samples with an ORP between 0 and 180 mV and pH < 8 were classified as MLC. Field validation at a swine manure liquid composting facility confirmed the accuracy of the diagnostic method, achieving 100% agreement with liquid compost maturity analysis using an automatic liquid compost maturity tester. This study provides a simple and effective tool for optimizing liquid compost production, minimizing ammonia emissions, and enhancing liquid compost maturity in swine manure recycling systems.