Environmental sustainability of an energy self-sufficient sewage treatment plant: improvements through DEMON and co-digestion.
It is still not proven that treatment of sewage in a wastewater treatment plant (WWTP) is (in every case) environmentally friendly. To address this matter, we have applied a state-of-the-art life cycle assessment (LCA) to an energy self-sufficient WWTP in Strass (Austria), its supply chain and the valorization of its 'products': produced electricity out of biogas from sludge digestion and the associated stabilized digestate, applied as agricultural fertilizer. Prominent aspects of our study are: a holistic environmental impact assessment, measurement of greenhouse gas emissions (including N2O), and accounting for infrastructure, replacement of conventional fertilizers and toxicity of metals present in the stabilized digestate. Additionally, the environmental sustainability improvement by implementing one-stage partial nitritation/anammox (e.g. DEMON(®)) and co-digestion was also assessed. DEMON on the digesters reject water leads to a considerable saving of natural resources compared to nitritiation/denitritation (about 33% of the life cycle resource input), this through the lowering of sludge consumption for N-removal, and thus increasing electricity production via a higher sludge excess. However, its N2O emission could be restrained through further optimization as it represents a large share (30-66%) of the plants' damaging effect on human health, this through climate change. The co-substrate addition to the digester resulted in no significant improvement of the digestion process but induced net electricity generation. If respective amounts of conventional fertilizers are replaced, the land application of the stabilized digestate is environmentally friendly through prevention of natural resource consumption and diversity loss, but possibly not regarding human health impact due the presence of toxic heavy metals, mainly Zn, in the digestate. The outcomes show that the complete life cycle results in a prevention of resource extraction from nature and a potential mitigation of diversity loss (though for some impact categories no quantification of associated diversity loss is possible) but it also leads to a damaging effect on human health, mainly via climate change and heavy metal toxicity. Since it is for now impossible to aggregate the impact to these different aspects in a sound manner, it is not yet possible to consider in this case the studied system as environmentally friendly. Generally, the field of LCA needs further development to present a better and single outcome.
Research Group ENVOC, Ghent University, Coupure Links 653, 9000 Gent, Belgium. Electronic address: thomas.schaubroeck@ugent.be.
Thomas
T
Schaubroeck
Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, 9000 Ghent, Belgium; Department of Earth and Environmental Engineering, Columbia University, New York 10027, USA.
Haydée
H
De Clippeleir
University of Natural Resources and Life Sciences Vienna, Muthgasse 18, 1190 Vienna, Austria.
Norbert
N
Weissenbacher
Research Group ENVOC, Ghent University, Coupure Links 653, 9000 Gent, Belgium.
Jo
J
Dewulf
Laboratory of Applied Physical Chemistry (ISOFYS), Ghent University, Coupure Links 653, 9000 Gent, Belgium.
Pascal
P
Boeckx
Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, 9000 Ghent, Belgium.
Siegfried E
SE
Vlaeminck
ARAconsult, Unterbergerstr. 1, A-6020 Innsbruck, Austria.
Bernhard
B
Wett
D000397:Air Pollution; D001317:Austria; D056804:Biofuels; D058572:Carbon Footprint; D057231:Climate Change; D004777:Environment; D004787:Environmental Pollution; D005308:Fertilizers / Q000633:toxicity; D017752:Greenhouse Effect; D019216:Metals, Heavy / Q000633:toxicity; D059027:Recycling / Q000379:methods*; D012722:Sewage / Q000737:chemistry*; D014865:Waste Disposal, Fluid / Q000379:methods*
D056804:Biofuels; D005308:Fertilizers; D019216:Metals, Heavy; D012722:Sewage
Anammox; Autotrophic nitrogen removal; Life cycle assessment; Recovery; Sustainability; Wastewater treatment