Document Type

Article

Publication Date

2010

Journal / Book Title

Journal of Analytical and Applied Pyrolysis

Abstract

For 20 years ending in the 1990’s the city of Barcelona discharged the products from a large primary sewage treatment plant directly into the Mediterranean Sea via underwater conduits. About ca. 3 million m3 of relict sewage sludge, rich in organic matter and heavy metals, has spread over an elongated area offshore, due to successive ruptures of the conduits. The use of the discharge pipes ceased, but he sludge deposit remains in place for the time being.

To understand the history and present state of the sludge deposit in advance of future remediation, a program of geophysical mapping, sampling, and analytical work was undertaken. Rock Eval pyrolysis, although created for use in petroleum prospecting, can also be applied to environmental contamination studies. It offers a simple means to effectively delineate the sludge deposit, with the S2 parameter and the hydrogen and oxygen indices particularly useful. On the molecular level, the sludge flash pyrolysis products notably include relatively abundant C27 and C29 sterenes and steranes, likely produced from the pyrolysis of fecal and other steroids, including coprostanol, in the sewage sludge. Linear alkylbenzenes and trialkylamines, derived from surfactant residues in the sludge, are also detected. The indoles detected are likely the pyrolysis products of proteins, while the alkylnitriles and alkylamides in the pyrolyzate likely derive from bacterial biomass. Principal components analysis aided the interpretation of the large geochemical dataset and a geographic information system enabled the three-dimensional visualization of the results in their geospatial context. The distinctive pyrolysis products and the trace elements would be geochemical markers useful in planning and assessing a future remediation program. The recognition of a distinctive sewage pyrolysis-GC/MS signature in this deposit would facilitate the use of this method in the detection of sewage-contaminated sediments in urban waterways worldwide.

Comments

High resolution seismic reflection data were used to estimate the extent and thickness of the sludge deposit, based on the contrast between its signature and that of the surrounding sediments. The sludge was further mapped based on its chemical characteristics. Elemental analysis indicated enrichment in Corg, Ca, Na, P, and S, all derived from the organic constituents of the sewage or substances added during sewage treatment, as well as in trace elements of industrial provenance such as Ag, Cr, Cu, Pb, and Zn. Rock Eval pyrolysis, normally used as a tool in petroleum prospecting, was shown to offer a simple means to effectively delineate the sludge deposit, with the S2 parameter and the hydrogen and oxygen indices particularly useful.

For more detailed characterization of the sludge deposit on the molecular level, sediment Py-GC/MS provided a direct, rapid analytical method for detection of an extensive suite of sewage marker compounds, without the need for expensive and time-consuming sample preparation procedures. Characteristic marker compounds included C16-C19 linear alkylbenzenes (LABs), indole, methylindole, and diketopiperazines, as well as long-chain alkylnitriles, alkylamides, and trialkylamines. The LABs and trialkylamines are known surfactant marker compounds, while the other nitrogen compounds are most likely pyrolysis products of biomass present in the sludge. Of particular interest, the sludge pyrolyzates contained the fecal sterol coprostanol and pyrolytic steroid derivatives, of which sterenes are the most important. We propose the use of the observed sterene carbon number distribution pattern (C27 > C29 > C28) as a Py-GC/MS signature for sewage pollution in sediments. The recognition of the pyrolysis-GC/MS and Rock Eval signatures in this deposit will facilitate the use of this method in the detection of sewage-contaminated sediments in urban waterways worldwide.

Principal components analysis aided the interpretation of the large geochemical dataset and the use of a geographic information system enabled the three-dimensional visualization of the results in their geospatial context. The distinctive pyrolysis products and the trace elements are geochemical markers which would be useful in planning and assessing the efficacy of any future remediation program.

DOI

https://doi.org/10.1016/j.jaap.2010.08.005

Published Citation

Kruge M.A., Permanyer A., Serra J., Yu D. (2010) Geochemical investigation of an offshore sewage sludge deposit, Barcelona, Catalonia, Spain. Journal of Analytical and Applied Pyrolysis 89:204-217

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