Decentralised water reuse as a building block of municipal resilience

Municipal drinking water supply and wastewater disposal are part of the critical infrastructure: they ensure basic services of general interest and must function reliably even in the event of disruptions. (1) In the CRITIS water sector, this includes drinking water supply (extraction, treatment, distribution) as well as wastewater disposal, wastewater treatment, urban drainage and the control and monitoring of these systems. (2)

At the same time, physical and digital risks are becoming more of a focus. The EU has defined minimum requirements and supervision to strengthen the resilience of critical facilities; in Germany, the protection of critical infrastructure is also being further tightened. (3) (4)

In the energy supply sector, resilience has been a guiding principle for years (redundancy, island operation, decentralised feeders). The same principle applies to the water industry: the greater the dependency on a few centralised nodes, the greater the system vulnerability - and the more important it is to have supplements that distribute loads and create local reserves.

The security situation in Europe - including the Russian war of aggression against Ukraine - has significantly accelerated the discussion about the protection and functionality of critical infrastructures. (5) For local authorities, this means that in addition to climate adaptation and economic efficiency, the question of how quickly supply and disposal can be stabilised after disruptions - and the extent to which individual failures have an impact - is becoming increasingly important. Decentralised systems contribute to this if they are correctly integrated in terms of planning and operation. They do not replace municipal responsibility for security of supply, but expand the range of instruments - similar to emergency power, redundant pumping stations or additional storage volumes:

• Risk diversification: Several local units reduce dependence on individual centralised components.
• Emergency operation capability for partial functions: Non-drinking water-related applications can be backed up locally (e.g. process water, irrigation, toilet flushing).

Operational transparency: Measuring and monitoring concepts support safe operation - especially where water quality and hygiene requirements must be met.

In addition to the safety aspect, heavy rainfall, overloading of drainage systems and urban heat remain risks at municipal level. Decentralised rainwater management (retention, infiltration, use) works where the pollution occurs - on the property and in the neighbourhood.
Sponge city approaches aim to retain, evaporate and infiltrate rainwater in a targeted manner; this can reduce runoff peaks, reduce substance inputs and support groundwater recharge - and at the same time contribute to cooling the urban climate through additional evaporation. (6) Traditional network expansion (larger channels, retention basins) solves symptoms "downstream". Decentralised measures start "upstream" and can - depending on the local situation - postpone, reduce or target the expansion. In many municipalities, this results in a practicable combination: central infrastructure remains the backbone, decentralised measures are systematically implemented as a second level.

Clear roles and reliable operating models are required for decentralised solutions to become viable at a municipal level. Neighbourhood solutions have proved particularly successful:

1. Neighbourhood solutions instead of individual systems (scaling, standardised operation)
2. Operator/contracting models (maintenance, monitoring, responsibilities regulated)
3. Incentive systems in the rainwater sector (e.g. via retention or runoff effect) - always within the framework of local bylaws and fee calculation
4. Pilot programmes with measurable targets (relief effect, volume, operating experience)

Decentralised water reuse and rainwater management are therefore not a counter-design to municipal infrastructure, but a necessary, resilience-oriented addition - technically, organisationally and increasingly also strategically. The Aachen-based research and development alliance AIX-Net-WWR is an example of an integrative overall solution (7).