Motivation. On-site wastewater treatment plants (OSTs) treat wastewater from a single household or dwelling directly on site. Because of the low initial investment costs, high flexibility, and resource recovery potential, they are used worldwide. Their core problem is that they are mostly operated without constant supervision. To keep costs low, common practice is to have one to four annual inspections. But with no online monitoring in place, treatment performance of such plants is low. Furthermore, failures are not noticed promptly after their occurrence, but only during the next inspection, if at all. This leads to reduced overall treatment performance and reinforces the widespread prejudice that OSTs are a makeshift solution.
Aim. The overall goal of this work is to see if OST systems can achieve treatment performances that are comparable to conventional centralised systems. The vision is that treatment performance should no longer be a criterion to rule out OSTs in the future any more. To get closer to this overall objective, two main objectives were defined:
• To identify a performance sensor that only requires maintenance annually or less frequent. This could greatly reduce maintenance costs, therefore enabling large-scale monitoring of OSTs.
• To investigate how such a nominally inaccurate sensor influences the treatment performance of a single OST unit as well as a system of OSTs and its quantification.
A specific hurdle when monitoring individual OSTs is sensor accuracy. In a nutrient-rich wet environment such as wastewater, biofilm grows everywhere within a few days. Therefore, manufacturers usually recommend monthly or even weekly sensor maintenance routines. Our idea was to use established, but unmaintained sensors for monitoring. [...]