The water-energy nexus reveals that large amounts of energy is stored in used water and can be used as an energy source. Anaerobic fermentation is an excellent ecological waste treatment and a way of producing low cost electrical and thermal energy. By using digestate resulting from biogas production instead of synthetic fertilizers derived from natural gas, we can save energy, cut consumption of fossil fuels and reduce our carbon footprint. The aim of this study then is to determine the effect of digestate on the yield and quality of crop production as well as its effect on the immediate environment. A ratio of raw materials in biogas, pig manure and vegetable wastes of 70:30 was used on lettuce as an indicator culture. The studies were carried out on two different soil types. Digestate was applied at various percentages, tested with untreated soil as control. The results are encouraging indicating that digestate as a biomass is rich in macro and micronutrients and can be used in agriculture for the improvement of soil fertility.

The aim of this study is to establish changes that may occur after a prolonged application of wastewater sludge treated to biosolids, in the 'soil-fertilizer-plant' system. Thirteen experimental plots with different soil types planted with experimental crops were investigated in order to evaluate the suitability of these biosolids as soil conditioners and fertilizers. The biosolids were incorporated in soil starting in 2006 in different quantities (from 6 tons per ha) for various arrays. The rate of application was calculated on the basis of imported nitrogen and was consistent with the characteristics of the sludge, soil diversity, growing crop requirements, and other factors. In 2013 (after 7 years of land use) average soil samples from the same arrays were taken and analyzed. No chemical fertilizer was applied during the experimental period. The results show that the use of sewage biosolids as a soil improver in accordance with local legislation does not pose any serious environmental risks but can maintain and improve soil fertility and crop yield. A slight increase in Cu and Zn in plants was detected, however the content of heavy metals in all soil samples was below maximum allowable limits and no signs of phytotoxicity were observed.

Natural resources are elements of the environment that humans use as a means of production in order to achieve economic development. The aim of the study is based on the evaluation of the proposed 5 natural products /vermiculites, glauconite, straw fly ash, wood biomass and pig manure/ in different proportions to develop various versions of compositions for use in agricultural practice. On the basis of the chemical and agrochemical characteristics of the starting materials, 5 compost samples were prepared from mixing the natural products in a different ratio. Thеse analyzes of the five starting products found, that in vermiculites, glauconite and ash from straw the total amounts of the biogenic element nitrogen and the sum of the mineral fractions of nitrogen (ammonia and nitrate) were very small, whereas for wood biomass and pigs manure the content of the total N is high. The content of the tested heavy metals is much lower, than the ones defined in the legislation. An exception is made for vermiculite, where the total amounts of chromium and nickel are above the MRLs. The investigators found, that the pH in all the mixtures was in the alkaline region, due to the alkaline reaction of the medium with all the starting components. The sum of the mineral fractions of nitrogen for the five mixtures ranges from 163.0 to 241.3 mg/kg. Moving forms of phosphorus and potassium show high values as a result of mixing. No increase in heavy metal values is observed in the different mixtures and the reported values are below the MRLs. The five composts represent a qualitative improver in their use in agricultural practice.

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