ABSTRACT
Hybrid wastewater systems can be defined as the coexistence of centralized and modular systems in the same catchment. Currently, we have no explicit knowledge if such hybrid systems can be stable over the long term or if modular systems always will be a stopgap solution.
Current evidence indicates that, depending on the settlement structure, centralized systems can have diseconomies of scale, and modelling studies show that there are conditions where hybrid systems are cost-effective. Decisive factors are the costs of modular systems and the heterogeneity of urban areas. Overall, there are good reasons to believe that fortifying centralized systems with modular systems enable overcoming some of the critical weaknesses of the one-size-fits-all centralized systems approach.
However, centralized systems show strong path dependencies. Besides a wide range of institutional and organizational barriers, current engineering economic and planning methodologies also need to be improved and adapted. From a purely engineering perspective, the following research needs can be identified: (i) long-term transition planning tools that are spatially explicit and can consider a wide range of modular technologies; (ii) cross-sectoral integration methodologies; (iii) better methods to integrate multi-criteria decision analysis (MCDA) to consider the broader range of benefits hybrid systems can provide; and (iv) improved engineering economic methodologies considering uncertainties, unused capacity, and the value of adaptability.
