Brussels - A recent study has compared the carbon footprints of different treatment and disposal options for sewage sludge. The lowest carbon footprints were associated with the advanced anaerobic digestion of sludge, regardless of the final use of the sludge.

Biological treatment of urban wastewater produces large quantities of sludge as a waste by-product. The sludge is frequently treated further (e.g. by bacterial digestion) before final use or disposal.

The most common treatment process is anaerobic digestion (the breakdown by bacteria of organic matter in the sludge in the absence of oxygen), which generates a methane-enriched biogas. This can be converted to a renewable source of energy in a combined heat and power plant. More efficient breakdown of the sludge, through advanced digestion, generates greater quantities of renewable energy and can be obtained by pre-treating the sludge before anaerobic digestion.

The study calculated the carbon footprints of different treatment options for raw, digested and advanced digested sludge. Carbon footprints determine the amount of greenhouse gases (GHGs) emitted during processing and recycling of sludge, expressed in carbon dioxide equivalents. These units are the amount of global warming potential a particular GHG has in terms of the quantity of carbon dioxide that would have an identical impact. Sludge treatment options were:
* applying dewatered cake (sludge with excess fluid removed) to land;
* drying, followed by land application or cofiring for fuel;
* and incineration.
Calculations were based on data specific to the water industry in the UK.

Two sets of results were modelled: a 'complete' footprint and a water industry 'gate' carbon footprint. The 'complete' footprint includes all processes up to the sludge endpoint. For example, sludge used as a fertiliser would include land application and take into account the energy saved from the manufacture of a comparible fertiliser. The 'gate' carbon footprint is calculated up to the point where the sludge is removed from the sewage works gate: for sludge used as a fertiliser, the carbon footprint would not include the emissions from land application or the fertiliser replacement benefit.

Overall, advanced pre-treatment of sludge before anaerobic digestion has the lowest carbon footprint, both complete or gate. One of the pre-treatment technologies studied could also significantly improve dewatering, which would reduce the energy needed for transport and further processing.

Raw sludge treatment has a higher carbon footprint than any other option involving anaerobic digestion regardless of the final destination of the sludge.

For standard anaerobic treatment of sludge (without pre-treatment), any carbon footprint benefit (compared with raw treatment) is lost because methane is released during the process. However, the carbon footprint is substantially reduced if the methane is collected and burnt. For all options where sludge was applied to the land, the gate carbon footprints were lower than the complete carbon footprints. However, as sludge contains nutrients its application on land can also reduce the carbon footprint associated with the manufacture of fertiliser with a similar nutritional content.

The study suggests that pre-treatment technology should be included in all new digestion plants to reduce the carbon footprint and to maximise renewable energy generation. Any treatment of sludge without anaerobic digestion appears to be unsustainable in the majority of cases.

Original source: Barber, W.P.F. (2009). Influence of anaerobic digestion on the carbon footprint of various sewage sludge treatment options. Water and Environment Journal. 23: 170-179.

Quelle: EU commission

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