Distributed wastewater treatment is increasingly considered as an alternative to the predominantly transport based combination of sewer network and centralized wastewater treatment plant. If substantial amount of wastewater of a particular area is processed in small distributed treatment units, the sum of these units must be considered as one system providing an overall service. This paper focuses on the monitoring for centralized operation of such distributed units. We present a simple stochastic model to calculate probability distributions of process performance. We thereby simulated a fleet of simple treatment units with stochastic failure rates, unreliable sensors and regular routine inspections. All calculated performance indicators exhibit a considerable difference between the true performance and the perceived that is based on sensor observations. The results also show that there is a trade-off between investing into more reliable technology or into more frequent human interventions to achieve a certain performance. The simulations can quantify both effects and thereby support the identification of requirements for centralized monitoring of distributed treatment units. The model approach is generic and can be extended and applied to various distributed wastewater treatment technologies and contexts.