For most of us our wastewater joins the labyrinth of sewer pipes under the city streets, and wends its way to the nearest sewage plant. But for some people, particularly in more rural areas, their wastewater may not travel far at all, perhaps just down to the septic tank at the bottom of the garden.

These kind of small-scale decentralized systems are cheaper to install, cost less to maintain, use less water and have greater flexibility in planning for future growth. For this reason they have become popular in recent years and are often seen as the 'eco-friendly' option. However, a new study in Environmental Research Letterss indicates that centralized systems may have the smaller environmental footprint.

Arman Shehabi from Lawrence Berkeley National Laboratory, US, and colleagues directly compared energy use, greenhouse-gas emissions and air pollution at two water-processing plants in California – one centralized and one decentralized.

The centralized system serves around half a million people, over a 200 square kilometre area. The decentralized system, meanwhile, treats the water for a community of 47 houses in Stonehurst, Martinez in northern California. In this case each house has a septic tank, which is connected via a sewerage pipe to a small, local treatment plant.

Using a model known as the Wastewater-Energy Sustainability Tool (WWEST), Shehabi and the team were able to compare these systems using a full life-cycle assessment. They found that the economy of scale of the centralized system made it far more energy-efficient than the decentralized system. In this particular case the centralized system used five times less energy than the decentralized system per volume of wastewater processed.

"The centralized system requires an enormous amount of infrastructure and operational energy in absolute terms, but the energy impact from this resource demand is reduced when normalized against the enormous volume of wastewater being treated," Shehabi told environmentalresearchweb.

In terms of energy efficiency, conventional centralized wastewater treatment systems appear to be the best choice. However, there are some advantages to decentralized systems.

"Decentralized systems have the advantage of being able to be tailored to the specific needs of a small community," explained Shehabi. "They can be added incrementally rather than requiring a major public works project, and because of their smaller size are more amenable to wastewater separation strategies. This separation capability allows for water reuse, which becomes increasingly important in areas with limited water supplies, not only because water itself is a scare resource but also due to the embodied energy of delivered water."

In an area where water is very scarce and sourced from energy-intensive desalination for example, decentralized systems are likely to have a significantly lower energy impact.

Although this study was specific to California, Shehabi and his colleagues believe that the findings are applicable to many parts of the world. "Planners should not automatically assume that any decentralized system will be the low energy-impact option," said Shehabi. Instead they suggest that planners carry out a life-cycle analysis before installing new water treatment works, to ensure that any hidden impacts are revealed.