Who’s More Efficient

A research report says onsite cluster systems use much more energy per gallon treated than a large-scale wastewater plant. Is it true? And does it matter?

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No less than the U.S. EPA has stated that onsite treatment systems should be a permanent part of the nation's wastewater management infrastructure.

The advantages of onsite are clear: Lower cost than "big pipe" systems, flexibility, adaptability to site conditions, suitability for small communities or clusters of homes and businesses and, perhaps most important, effectiveness (especially given the array of advanced treatment and dispersal technologies available).

Now comes a study by researchers at Lawrence Berkeley National Laboratory, with a surprising finding that a central wastewater treatment system had a smaller environmental footprint per gallon treated than an onsite cluster.

Less energy?

The report, "Energy and air emission implications of a decentralized wastewater system," acknowledges that centralized and decentralized wastewater systems have their own "distinct engineering, financial and societal benefits."

It looked at one centralized treatment facility serving about half a million people against one decentralized residential cluster serving a community of 47 homes. It compared energy use, greenhouse gas emissions and criteria air pollutant emissions from the two and found that "the scale economies of the centralized plant help lower the environmental burden to less than a fifth of that of the decentralized utility for the same volume treated."

The report goes on to say: "The centralized facility also reduces greenhouse gases by flaring methane generated during the treatment process, while methane is directly emitted from the decentralized system. The results are compelling enough to indicate that the life-cycle environmental impacts of decentralized designs should be carefully evaluated as part of the design process."

Gallon for gallon

The researchers used a computer model to make a full life-cycle comparison of the systems and found that economies of scale made the centralized system far more energy efficient. "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," lead researcher Arman Shehabi stated.

He also pointed out that decentralized systems have advantages: They enable a developer to tailor treatment specifically to a small community, and provide flexibility to add capacity incrementally instead of having to undertake a huge construction project.

Shehabi concludes that planners should not assume that a decentralized system will have a lower energy impact than a centralized system – that they should conduct a life-cycle analysis before installing any wastewater treatment facilities to discover and account for hidden impacts.

But is it reality?

While I mean no disrespect to Lawrence Berkeley lab or its researchers, I have to question what seems to be an underlying assumption of this study: That at some given juncture in a community's development there comes a time to choose between building either a set of cluster systems or a centralized treatment plant and collection system.

It strikes me that more often the question is whether to extend sewers to somewhat remote areas or develop those areas with onsite treatment. And then, it seems to me, the driving factor is the pure cost of each option, not whether one is more or less energy intensive than the other.

In any case, it appears to me that the energy used to install a collection system for a central treatment plant – with all the excavating, blasting and earth moving that it entails – would outweigh any energy savings from a central plant's treatment process itself.

Then again, I wasn't the one doing the study, and it doesn't make much sense to look at this as an either-or proposition – one type of system good, the other bad. Both central and decentralized systems have their places, and the choice will always come down to balancing many factors.

You can read the full report at http://iopscience.iop.org/1748-9326/7/2/024007/article. Let me know what you think by dropping a note to editor@onsiteinstaller.com.


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