Garbo, Francesco (2019) Phytotreatment of contaminated wastewater with energy crops. [Ph.D. thesis]
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Abstract (italian or english)
Nowadays, advanced industrial societies still depend on fossil fuels supply. However, in the last decades, industrial and scientific efforts have been made toward renewable sources. The use of energy crops, fast-growing plants, aimed to produce biofuels or generate energy, represents an important alternative to traditional sources of energy, with advantages also with respect to environment and agricultural and economic development. However, the cultivation of energy crops to produce significant amounts of biomass may lead to the dilemma "Agriculture for food or energy production?". Wastewater phytrotreatment using energy crops could be considered an optimal compromise among different needs, especially if applied in derelict areas:
• simple and low-cost wastewater treatment
• renewable energy production
• conservation of fresh water storages
• preservation of land for food production
A series of researches were carried out at the University of Padova to evaluate the performances of some oily crops (sunflower, soybean, rapeseed) and Pennisetum Purpureum (elephant grass) in the phytotreatment of source separated municipal wastewater and/or MSW landfill leachate.
The results proved that oleaginous species, cultivated in 20 L pots and irrigated with increasing nitrogen concentrations in the feedstock, can be used for the phytotreatment of grey and yellow waters from source segregation of domestic sewage [Chapter 2], displaying high removal efficiencies of nutrients and organic substances (total N ɳ >80%; total P ɳ >90%; COD nearly 90%). No inhibition was registered in the growth of plants irrigated with different mixtures.
The three oleaginous species were also used to treat MSW landfill leachate. An experimental research was carried out using 20 L volume pots irrigated with old landfill leachate under different percentages in the feeding and subsequent COD, N and P loads [Chapter 3]. Significant removal efficiencies were achieved: COD (ɳ >80%), total N (ɳ >70%) and total P (ɳ >95%). Plants irrigated with leachate, when compared to control units fed only with water and nutrient solution (Hoagland solution), developed a larger plant mass. Sunflower was the best performing species. Leachate irrigation seemed to stimulate the oil production, with a favorable Free Fatty Acids composition in view of the biodiesel production.
As a further step, sunflowers were grown in 130 L rectangular tanks irrigated with increasing dosages of old MSW landfill leachate [Chapter 4]. Two different irrigation systems were tested: vertical and horizontal sub-superficial flows, with or without effluent recirculation. The experiment revealed good removal efficiencies for COD (η >50%) up until flowering, while phosphorous removal invariably exceeded 60%. In general, horizontal flow units showed the best performances in terms of contaminant removal capacity; the effluent recirculation procedure did not improve performance. Significant evapotranspiration was observed, promoting the removal of up to 80% of the irrigation volume.
Based on the previous researches, sunflowers were grown in a waste-derived substrate, a mixture of sand from sweeping of streets and compost containing sewage sludge, and irrigated with increasing dosages of old MSW landfill leachate [Chapter 5]. Plants were grown in 300 L reactors characterized by vertical and horizontal sub-superficial flows. Vertical and horizontal flow units were connected in series to enhance nitrification and denitrification: nitrogen losses in gaseous form were approximately 40-45%. The connection in series proved to be effective in removing the influent total nitrogen (ɳ > 80%) and in reducing the influent volumes due to evapotranspiration (more than 80%). Leachate irrigation did not inhibit the biomass development and resulted in a favorable oil composition for biodiesel production.
Pennisetum Purpureum (elephant grass) was also tested [Chapter 6]. The plants were grown in lysimeters, 1 m deep, to simulate the superficial layer of landfills top covers. Plants were irrigated with MSW landfill leachate produced by a landfill in operation; the pollutants loads were increased over time. The removal efficiencies were in the range 95-99% for all the investigated contaminants. The vertical sub-superficial flow led to an almost total nitrification, as expected, but a partial denitrification was detected too. A simple mathematical model was developed to study the kinetics of nitrogen removal, which confirmed the occurrence of a fast nitrification process. Pennisetum Purpureum growth seemed to be stimulated by the leachate irrigation; no significant accumulation of heavy metals was observed in the biological tissues.
Samples of soil used as substrate in a lab-scale leachate phytotreatment test with sunflowers were analysed to provide chemical characterization before, during, and at the end of the experiment [Chapter 7]. The results showed that the phytotreatment activity did not increase initial contaminant concentrations (e.g.: heavy metals). These results were reinforced by those from ecotoxicological bioassays in which Eisenia fetida (earthworms), Lepidium sativum (cress), Folsomia candida (collembola), and Caenorhabditis elegans and Steinernema carpocapsae (nematodes) were used.
A Multi-Criteria Analysis, based on economic, energetic, and environmental aspects was developed to assess four potential scenarios of energy crops application to the top of closed landfills [Chapter 8]. In this study, the scenarios have been assessed and compared with respect to a reference case defined for northern Italy. The first three scenarios were based on energy maximisation, leachate phytotreatment capacity, and environmental impact, respectively. The fourth scenario was a combination of the characteristics emphasised by the previous scenarios. The combination scenario resulted to be the best. The economic criterion emerged as weak, as all the considered scenarios showed some limits from this point of view. The decrease of leachate production due to the presence of energy crops on the top cover, which enhances evapotranspiration, represented a favourable but problematic aspect in the definition of the results. This analysis provided important indications: the presence of energy crops on the top cover represents a positive but also critical option which must be addressed in the authorization and design phase.
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