Biochemical characterization of hydrochars and pyrochars derived from sewage sludge and their potential as n and p slow release fertilizers and as soil amendment in agriculture

Resumo

The current demand for N and P fertilizers to meet the needs of agriculture is not sustainable. On the other hand, the huge amount of organic waste derived from human activity is a current environmental problem that needs to be solved urgently. Therefore, its recycling should be favored over disposal at landfills. Using sewage sludge (SS) as an alternative N and P sources in agriculture can alleviate both issues and may help to reduce our dependence on chemical fertilizers. However, aside of hygienic issues and possible input of organic and inorganic contaminants, nutrient leaching has been observed after SS application to soil, both directly and after stabilization by composting. Current knowledge indicates that pyrolysis and hydrothermal carbonization (HTC) can convert organic residues into recalcitrant amendments that may add to the C sequestration potential of soils. Bearing further in mind the high N and P content of SS, charring of the latter is likely to result in a product which slowly releases its N and P after its amendment to soils for plant growth. Considering that the chemical structure of a product defines its function, the present PhD thesis first focused on the characterization of pyrolyzed and HTC treated SS. Subsequently its potential to act as slow-release N and P fertilizer and its C sequestration potential was evaluated. Therefore, the first task of this work was the HTC and pyrolysis of two different SS derived from different steps of the water clean-up process of a rural waste water cleaning plant in Carrión de los Céspedes (Chapter 3). The HTC was performed at 200 and 260 ºC with residence times in the reactor of 0.5 and 3 h, whereas pyrolysis was carried out at 600 ºC for 1 h. The changes of the chemical composition of the feedstock during the thermal treatments were studied through a detailed characterization of the products by not only routine analyses but also more advanced techniques such as solid-state 13C and 15N nuclear magnetic resonance (NMR) spectroscopy. It was evidenced that HTC reduced the carboxyl and O/N alkyl C contents and slightly increased the proportion of aromatic C, whereas pyrolysis transformed most of the organic matter (OM) into aromatic structures. In addition, HTC reserved part of the formed inorganic N (Ni) and stabilized the remaining N as N-heterocyclic aromatic compounds (Black Nitrogen, BN). In contrast, the pyrochars revealed organic BN. These results suggest that hydrochars have immediate and medium-term N-fertilizer properties, whereas pyrochars seems to be more useful for supplying lower doses of N over a longer time period and for increasing the C sequestration potential of soils. The current demand for N and P fertilizers to meet the needs of agriculture is not sustainable. On the other hand, the huge amount of organic waste derived from human activity is a current environmental problem that needs to be solved urgently. Therefore, its recycling should be favored over disposal at landfills. Using sewage sludge (SS) as an alternative N and P sources in agriculture can alleviate both issues and may help to reduce our dependence on chemical fertilizers. However, aside of hygienic issues and possible input of organic and inorganic contaminants, nutrient leaching has been observed after SS application to soil, both directly and after stabilization by composting. Current knowledge indicates that pyrolysis and hydrothermal carbonization (HTC) can convert organic residues into recalcitrant amendments that may add to the C sequestration potential of soils. Bearing further in mind the high N and P content of SS, charring of the latter is likely to result in a product which slowly releases its N and P after its amendment to soils for plant growth. Considering that the chemical structure of a product defines its function, the present PhD thesis first focused on the characterization of pyrolyzed and HTC treated SS. Subsequently its potential to act as slow-release N and P fertilizer and its C sequestration potential was evaluated. Therefore, the first task of this work was the HTC and pyrolysis of two different SS derived from different steps of the water clean-up process of a rural waste water cleaning plant in Carrión de los Céspedes (Chapter 3). The HTC was performed at 200 and 260 ºC with residence times in the reactor of 0.5 and 3 h, whereas pyrolysis was carried out at 600 ºC for 1 h. The changes of the chemical composition of the feedstock during the thermal treatments were studied through a detailed characterization of the products by not only routine analyses but also more advanced techniques such as solid-state 13C and 15N nuclear magnetic resonance (NMR) spectroscopy. It was evidenced that HTC reduced the carboxyl and O/N alkyl C contents and slightly increased the proportion of aromatic C, whereas pyrolysis transformed most of the organic matter (OM) into aromatic structures. In addition, HTC preserved part of the formed inorganic N (Ni) and stabilized the remaining N as N-heterocyclic aromatic compounds (Black Nitrogen, BN). In contrast, the pyrochars revealed organic BN. These results suggest that hydrochars have immediate and medium-term N-fertilizer properties, whereas pyrochars seems to be more useful for supplying lower doses of N over a longer time period and for increasing the C sequestration potential of soils. sequestration potential of soils. Regarding the N fertilization potential, HTC provided a slightly lower amount of N for plant growth than the non-treated SS. Pyrochars released the lowest amount of N usable for plant growth. Both the non-treated and HTC-treated SS showed the highest N loss at the beginning of the experiment, which decreased with increasing experiment time. In contrast, pyrochars depicted a low but constant N release only after the first month of experiment. Anyway, both HTC and pyrolysis avoided the NO3- leaching observed for the non-treated SS. The results of this experiment demonstrated the suitability of hydrochars as short and medium-term N-fertilizers. However, although the pyrochar was less successful for a short-term fertilization, the slow release of N of this product may be favorable if low-dose but long-term N fertilization is needed. Summarizing the results of the present study, it was shown that addition of SS-chars to soils, can increase plant biomass production if compared to un-amended soils. However, it is unlikely that SS-chars can completely substitute mineral fertilizers. Anyway, the net balance of this approach is clearly positive due to: i) the remarkable reduction of the N and P losses by avoiding the disposal of this organic waste and ii) the replacement of mineral fertilizers, which are limited and demand high-energy. Thus, the transformation of SS into pyrochar or hydrochar for its use as soil amendment can certainly be seen as an efficient tool to recycle N and P from organic waste.