Smartphone-based colorimetric method for decentralized wastewater treatment monitoring by inexperienced users

Rural areas often rely on decentralized wastewater treatment systems, including individual sewage treatment systems (ISTS), for remote houses, farms, and other establishments. For practical and financial reasons, ISTSs are often exempt from routine monitoring of effluent quality. As a result, they cannot guarantee the same level of treatment performance as their large-scale counterparts. A smartphone-based colorimetric monitoring method has been developed and tested to address this issue. This method can be easily adopted by untrained system owners. It uses modified test strips with built-in color reference zones and fiducial markers, which simplifies data acquisition and interpretation, eliminating the need for positioning accessories or controlled lighting. The proposed method ensures reliable results even when using a regular smartphone with default picture quality settings. The method has been tested with test strips measuring five different relevant wastewater quality parameters: pH, phosphates, ammonium, nitrates, and nitrites. For each type of test strip, the software installed on a smartphone provides a correlation for interpolating the measurements. The R2 -values ranged from 0.881 to 0.993 for different reagents. The accuracy of this method was verified using lab -grade reference sensors and was found to be sufficient for the intended purpose.