Probing operational conditions of mixing and oxygen deficiency using HSV color space

In this work, the relationship between sludge color and operational conditions was studied. It was found that the coordinates H and S of the HSV color space well correlated with biological status and the operational conditions of mixing and oxygen deficiency, and a coefficient of variation (CV_H/S) of the ratio of H to S in sludge cake images was derived. A smaller CV_H/S indicated better mixing conditions based on the observations of four laboratory-scale experiments and two full scale WWTPs, which can be used as a promising index for the monitoring of mixing condition. The coordinate oxygen uptake rate (OUR_q) of the respirogram space showed similar trend as CV_H/S, and analysis of microbial community also showed that CV_H/S could indicate changes of biological community including species and richness. These findings suggested that CV_H/S is a biological meaningful index for detecting the effect of changing operational conditions, which gives a key to quantify a large number of empirical rules accumulated in the past. Furthermore, it promotes the Internet of Things (IoT) application to the management of WWTPs, as color is readily available with MEMS (Micro-Electro-Mechanical Systems) sensors such as smart phones.