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News article30 November 2022Directorate-General for Environment3 min read

Extreme decentralisation of water production: environmentally sustainable and economically feasible?

Issue 590: Water from ‘off-grid’ supply and treatment systems for individual buildings may be cheaper to supply than water from conventional systems, new research suggests.

Extreme decentralisation of water production: environmentally sustainable and economically feasible?
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These decentralised systems also cut water consumption by continuously recycling wastewater onsite. Current centralised systems for supplying and cleaning water are increasingly considered inefficient and unsustainable. Water utilities take water from rivers, reservoirs and underground sources, treat it at waterworks using energy-intensive technologies, and then transport it to homes and businesses through a vast network of pipes. They then pipe the wastewater back for treatment before returning it to water bodies.

Attention is turning to the potential of small-scale systems that manage water much more locally. Not yet widely used, cities could set up these decentralised systems to serve a number of buildings across a district, for example. However, extreme decentralisation, where each building has its own system, may be easier to adjust to.  Building-level systems would avoid the need to abandon existing water infrastructure or make large investments to rapidly move away from existing systems.

This study provides a detailed economic and technical evaluation of five decentralised water systems for individual buildings. It assesses existing technologies for supplying and treating water, as well as those likely to become mature and more affordable within the next two decades.

The researchers considered five systems. These all collect rainwater on rooftops for potable (drinkable) water and clean it using RO/UV (reverse osmosis and ultraviolet lamp treatment). This source is supplemented by water from conventional systems, where needed. All five systems also recycle water from toilets, sinks and baths for home appliances, toilet flushing and bathing, and most treat it using bacterial methods – biological anaerobic treatment (a biological process where microorganisms degrade organic contaminants in the absence of oxygen) – plus RO/UV. The systems differ in the exact form and combination of treatment technologies and type of toilet.

The evaluation suggests that most of the systems are effective at providing water to residents. Moreover, they can provide water more cheaply than centralised systems, despite initial installation and ongoing operating costs. Their costs and effectiveness vary not only according to the technologies and toilets used, but also by the size of the building and the local climate.

All five systems could meet 100% of indoor water needs (108 litres per person per day) for single-dwelling detached houses (average of 2.3 residents), and small blocks of terraced houses and flats (up to 12 residents), even in hot Mediterranean climates. But the researchers say the systems would not be able to meet full demand in more arid climates. 

They are also less effective at meeting all residents’ water needs in high-rise blocks (120–300 residents), and especially in very arid climates. These conditions offer little rainwater which then has to be shared sparsely between many residents. Nonetheless, they could still supply 74% of all residents’ water needs under these circumstances.

The costs of the five systems are calculated by adding up the costs of the equipment, installation, operation and water-quality monitoring over a 30-year period.

The most expensive system – costing 50% more than each of the others – uses anaerobic UASB (upflow anaerobic sludge blanket – a form of anaerobic digester) and aerobic MBR (a membrane bioreactor – where microfiltration or ultrafiltration, is integrated with a biological process) technologies for waste-water treatment and vacuum toilets, which consume less water by sucking away sewage with air. This system could cost around $200,000 (€202,341.68) for a low-rise building (12 residents), and around $1 million (€1.01 million) for a 300-resident high-rise.

Despite being the most expensive system, in larger buildings it can provide water at similar costs to customers as conventional systems. For buildings with over 100 people, the water costs of this system work out around the same as average water tariffs, $3.9 (€3.94) per m3 of water, for conventional systems in western Europe.

The larger the building, the more cost-effective the system. For buildings with over 300 people, the price of providing on-site water across all five systems ranged from $1.5/m3 (€1.51/ m3) to $2.7/m3 (€2.73/m3) – much less than currently charged by utilities in western Europe.


Garrido-Baserba, M., Barnosell, I., Molinos-Senante, M., Sedlak, D.L., Babaey, K., Schraa, O., Verdaguer, M., Rosso, D. and Poch, M. (2022). The third route: A techno-economic evaluation of extreme water and wastewater decentralization. Water Research 218: 118408.

To cite this article/service:

Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions


Publication date
30 November 2022
Directorate-General for Environment

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