Use of a simplified grayscale essay for monitoring compact decentralized sewage treatment equipment

The scarcity of sewage services is a great problem and of particular concern in rural and peri-urban areas, traditional communities, and small cities. A viable solution for expanding access to wastewater treatment services in these areas is the use of decentralized systems. However, an obstacle to the use of these systems is the lack of monitoring and operational management programs. The aim of this study was to evaluate compact wastewater treatment equipment (CWTE) using the grayscale proposed by von Sperling et al. (Water Science & Technology, 82(7):380-1392, 2020) and to develop prediction models for biochemical oxygen demand, chemical oxygen demand, and suspended solids using the grayscale and other parameters that are easy to obtain, such as color, turbidity, and electrical conductivity. Five CWTEs were monitored for 270 days, with students acting as "citizen scientists" using the grayscale essay. Of the 705 samples monitored, 157 (22%) were classified as level 1 or 2 on the grayscale, 410 (58%) as level 3, 136 (19%) as levels 4 and 5, and only two samples as level 6. Regarding the use of the grayscale, it was observed that in this case, there was a small overestimation of the real values obtained in the laboratory analyses. With the use of the grayscale, the most expensive analyses would only be accessed if the level stabilized at values equal to or greater than 3. The predictive models developed based on the grayscale and simple parameters such as color, turbidity, and electrical conductivity demonstrated good accuracy (R² > 0.8), offering a practical and efficient approach for estimating effluent quality indicators while reducing the need for complex laboratory analyses. The grayscale method can be a useful tool for (self) monitoring effluents in CWTEs, especially in contexts where resources are limited.