The increased integration of renewable energy sources into the energy supply structure represents an important element in the climate and energy policy of the EU and Austria. Specific expansion targets for 2030 are primarily based on increasing the capacity of volatile, renewable electricity supply from photovoltaic (+11 TWhel) and wind power plants (+10 TWhel). The increasing share of volatile electricity sources in low and medium voltage grids leads to alternating periods of renewable electricity surpluses and shortfalls. The volatile renewable electricity supply therefore requires flexibility service providers that offer flexibility options (e.g. load or storage management) to balance electricity demand and supply continuously. Considering the decentralised arrangement of municipal wastewater treatment plants as well as their energy intensive treatment processes, internal energy supply, and the existing storage capacities for gas and sewage sludge, it can be derived that they can serve as decentralised flexibility service providers in low and medium voltage grids. Due to the necessity of time-resolved load and generation profiles as well as time-resolved flexibility potentials for the operational planning of flexibility options in the electricity grid, the application software OPTIEVLEX is developed. OPTIEVLEX enables static analyses of energy characteristics (e.g. degree of energy self-sufficiency) of municipal wastewater treatment plants as well as the determination of their time-resolved load profiles and theoretical and feasible flexibility potentials. The latter is examined with grid- and system-serving scenarios to be able to analyse the role of wastewater treatment plants as flexibility service providers in renewable energy communities or for the provision of balancing power. In addition, OPTIEVLEX considers decentralised sewage sludge utilisation to analyse their influence on the energy characteristics, load profiles, and flexibility potentials. The knowledge gained through OPTIEVLEX is intended to support plant operators in investment decisions and to prepare grid operators and energy suppliers for new load situations in the electricity grid. As an essential result of the analyses with OPTIEVLEX, it can be emphasised that treatment-plant-specific analyses are essential for estimating the time-resolved feasible electric flexibility potential. Furthermore, it is shown that thermal sewage sludge utilisation can positively influence both the energy and cost indicators as well as the flexibility potential. If the flexibility potentials are used, they can be associated with revenues in renewable energy communities, as well as in the provision of secondary and tertiary control power. This leads to a reduction in annual costs and therefore, to the economic viability of flexibility measures.