The current installations of sensors in the liquid phase in digesters are usually not sufficient for the fast detection of disturbances. Thus, the application of strategies for process control is restricted. In this paper, multiparameter (micro-) sensors as novel process analytical tools are introduced, which are coupled with wireless data transmission and are designed for an easy integration into digesters. Furthermore, the combination with a lance-based sampling system, which enables sampling in the core of the liquid phase, is described. The monitoring and sampling devices allow for the measurement at various positions in the liquid phase for the purpose of the identification of gradients. Three case studies were performed: (i) at an open hydrolysis basin, (ii) a mid-sized biogas plant with a concrete roof, and (iii) a larger biogas plant equipped with a flexible roof. Online measurements of the redox potential and offline analysis of the dry matter, ammonia, and acetic acid concentrations indicate plant-specific gradients, which appear in the liquid phase. Thus, the concept is not only suitable to improve the monitoring in digestion processes but also to identify the most suitable locations for installing sensors and sampling ports for a fast detection of process disturbances.