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In addition to energy production from the biogas plants, which are among renewable energy technologies, biogas digestate is also produced. The scientific studies related to this product have increased especially since 2000, and continue to increase. These studies show that the usage area of biogas digestate is quite wide, and it is a valuable product. According to these studies, biogas digestate is a product that can be used in plant nutrition, animal feed, obtaining irrigation water, bio-pesticide, seed pre-treatment, phosphate salt, and carbon synthesis. When the studies are examined, it is seen that biogas digestate is a product that can compete with limited fossil resources. However, it was seen that more scientific studies should be done about risk factors, environmental effects, fertilizer treatment methods and enrichment in the long-term applications.

Due to usage of fossil fuels, the depletion of world crude oil reserves and increased deteriorating climate conditions have reached a high level. These circumstances have led researches to search for alternative and efficient fuels. The main biofuels considered are bioethanol and biodiesel. In this study, ethanol and biodiesel production from cotton stalk and seed were aimed using liquid hot water (LHW) along with consecutive processes, where separate saccharification and fermentation (SHF) process was carried out. The maximum ethanol concentrations of 0.348 g/L and 0.721 g/L were achieved at 24 h and 72 h, respectively. For biodiesel conversion, cottonseed oil was subjected to transesterification, where the main interest was to utilize the by-product, glycerol. Three different glycerol concentrations were investigated in terms of ethanol fermentation using Escherichia coli K1 active culture. The maximum ethanol concentration of 0.415 g/L was achieved at 20 mL glycerol concentration for 48 h. Overall, cotton stalk and seed have the potential to be utilized on an industrial scale. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.

The objective of this current review article is to evaluate the current knowledge of the contaminated soil in the study area based on reports and the results of previous experimental studies in the literature and to discuss the feasibility of phytoremediation with biofuel production using energy crops. The results indicated that the soil contamination was related mainly to the thermal power plant and mining activities in Kutahya, high industrial activity in Izmir, heavy metal and radioactive pollution in Manisa and Mugla. Moreover, the sources of the contamination are geothermal resources and transportation in Aydin and Denizli, respectively. However, soil pollution in Afyonkarahisar and Usak provinces has not been discussed due to a lack of detailed reports and data in the literature. Besides, energy crops such as Zea mays, Ricinus communis, and Gossypium hirsitum were identified as appropriate candidates for Izmir, Denizli, Manisa, and Aydin due to being resistant to the arid climate. In Mugla province, Eucalyptus grandis and Eucalyptus bicostata can be cultivated because of having adaptation to moderate climatic conditions. Ricinus communis and Helianthus annuus were determined to be very suitable energy crops for the phytoremediation of many heavy metals in Kutahya. The review promotes the development of economic, environmental, and social benefits to regain the contaminated areas through phytoremediation. The findings of the study are important for creating sustainable solutions for remediation of polluted soils in Turkey, as well as for shedding light on the process of establishing appropriate policies to make soils contaminated suitable for agriculture.