De Simone, Simeone (2022) The fate of C10-C40 hydrocarbons during anaerobic digestion of sewage sludge destined for landspreading. [Tesi di dottorato]

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Abstract

Among the countless consequences of world population growth, the increase in sewage sludge (SS) production, a waste product of urban and industrial wastewater treatment cycles, is acquiring increasingly essential proportions. In addition to the high-cost items that the management of wastewater treatment plants (WWTPs) has to budget for the disposal of sludge, it should also be added that this latter contains such high quantities of nutrients and micronutrients that it is necessary to consider them as a resource to reintroduce into the production system, instead of waste material. From the perspective of the new paradigm of the Circular Economy, the use of SS as a source of nutrients based on nitrogen and phosphorus, essential for the agricultural sector, constitutes an important tool to address the other major problem caused by overpopulation, that is, the scarcity of raw material. Since 1986 in Europe and 1992 in Italy, there has been the possibility of reusing SS for spreading on agricultural soil to use the precious nutrients and micronutrients contained therein for food production. To operate in this way, the sector operators must carry out treatments on the sludge to respect certain limits on the concentrations of some characteristic parameters imposed by law to protect the environment and public health. Since 2018, with the so-called “Genoa decree”, the Italian legislator has introduced, for SS landspreading, the compliance with a parameter previously regulated at a regional level, namely the C10-C40 hydrocarbon content, also known as petroleum hydrocarbons. The C10-C40 hydrocarbons, as well as heavy metals and carcinogenic substances, represent an additional source of pollution that can accumulate in the soil and penetrate the aquifers. Before landspreading, SS must be made stable and dense enough to be moved with a shovel and brought to a concentration of C10-C40 hydrocarbons below the legal limit of 1,000 mg per kg of raw sludge. Therefore, it becomes necessary to verify whether the current treatment systems for stabilizing sludge can adequately reduce the hydrocarbon concentration values in SS. The presence of hydrocarbons in SS is undeniable, whether they are of natural origin or petroleum derivation. Given the low affinity of hydrocarbons with water, when the urban drainage system is of a mixed type, almost the entire hydrocarbon component in the sewage treated in WWTP ends up in the sludge line. In this circumstance, thanks to the stormwater, urban wastewater is enriched with debris and slag deriving from the washing away road surfaces, vehicles, and building facades. Moreover, there is the water coming from contact with vegetation, which releases waxes, called cuticular, and rich in hydrocarbons of natural origin, used by the leaves as a protective layer. In addition to what is released by the water treated in the WWTP’s sewage line, the sludge can receive a further contribution of hydrocarbons in the conditioning phase, used for settling and dewatering operations, a stage in which chemicals derived from petroleum synthesis products are very often used. Regarding industrial waters, the presence of hydrocarbons depends on the production process from which the wastewater derives. Another fundamental point to consider is the non-availability, at the time of the start of the PhD, of a tailored, reliable, and validated measurement methodology that can uniquely quantify the hydrocarbon concentrations in SS. It was not uncommon for two different laboratories to give different results for the same sample, even of some order of magnitude. Therefore, many issues to overcome were mainly due to the high affinity between a lipophilic analyte with a complex matrix such as SS. OBJECTIVES The PhD research project aimed to overcome the limits of a not yet standardized method of measurement, set up an easy-to-use and reliable method, and investigate the ability of the anaerobic digestion (AD) treatments of SS to degrade C10-C40 hydrocarbons and keep their concentration below the legal limits. The latter is a problem of national impact, which could soon take on an international dimension, given the ever-increasing reliance on the landspreading of SS. Two experimental lines have been set up: - An analytical one, which lasted over two years and made it possible to develop a flexible and efficient analytical methodology with different operational solutions in the various analytical phases, depending on the complexity of the specific sample to be analyzed, capable of returning two distinct parameters, i.e., total (TH) and C10-C40 hydrocarbon concentrations (C10-C40), which better identify the nature of the hydrocarbons under study, thanks to the use of a mixed gravimetric/gas-chromatographic determination. - Reactor experiments to study the hydrocarbon degradation capacity of the AD processes used for sludge stabilization operations, especially those destined for landspreading. With the refinement of the analytical method developed, it was possible at this stage also to exploit the great potential of mass spectrometry associated with gas chromatography for carrying out speciation and characterization of each subgroup identified in the hydrocarbon mixture and better understand the nature and the degradability of the hydrocarbon mixture analyzed. A case study also investigated whether using polyelectrolytes in chemical conditioning could affect SS’s anaerobic degradation and hydrocarbon content. The strong interaction of lipophilic substances such as hydrocarbons with natural organic matter (NOM), which constitutes the solid matrix of SS, is critical in understanding the processes underlying the phenomena object of our research. The compaction of NOM induced by chemical conditioning was indeed targeted as a potential factor affecting the fate of hydrocarbons during the biological treatments to which SS is subjected.

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