Optimization of production conditions of hydrochars from sewage sludge for germination and growth of Lolium perenne

Abstract

Nowadays, EU countries produce more than 10 million tonnes of dried sewage sludge (SS) per year (Laturnus et al., 2007). This residue is rich in N, P and other micronutrients, which may turn it into a useful fertilizer, if subsequent pollution with heavy metals and pathogens can be avoided. Pyrolysis of this material prior to its land application can help to eliminate such pathogens and may have an additional benefit to increase the carbon sequestration in soils after its application. The main goal of this study was to determine the conditions for the hydrothermal carbonization of sewage sludge which are the most beneficial for the plant growth on a typical soil of the Mediterranean area. The hydrochars derived from sewage sludge collected from the Experimental Wastewater Treatment plant CENTA (http://www.centa.es/), located near Seville, Southern Spain. Hydrothermal carbonization was carried out at 200°C and 260°C with residence times of 30 min and 1h. They were used as soil amendment in a greenhouse incubation study which was carried out for 80 days using a Calcic Cambisol (IUSS Working Group WRB, 2007) and an amount of hydrochar equivalent to 5 and 25 t ha-1 (n = 4). Control pots, without hydrochar addition, were also settled (n = 4). In each pot, 25 seeds of Lolium perenne were planted and their germination, the survival rate and biomass production of the plants per pot were measured periodically. In addition, a characterization of the hydrochars was carried out to determine their main chemical and physical properties as well as the presence of heavy metals and organic pollutants. Although hydrochar amended pots exhibited lower germination rates than the unamended pots, the total biomass production per pot raised significantly for all tested hydrochars. The first may be due to the presence of and PAHs in the hydrochars. Whereas hydrochars produced at 260°C resulted in a significantly higher amount of harvested plant biomass than those heated at 200°C, the residence time of the hydrochars in the reactor had no impact on plant growth. The higher nitrogen amount of the hydrochars produced at 200¿C compared to those produced at 260¿ may explain the better effect on plant growth of the former compare to the latter. Hydrothermal carbonization of sewage sludge at 200¿C, regardless the residence time, seems to be the most suitable production conditons for Lolium perenne growth. In addition, a washing with water and acetone of the hydrochar before its use may be appropriated to avoid the decrease in the germination rate observed. Laturnus F, von Arnold K, Grøn C (2007). Organic Contaminants from Sewage Sludge Applied to Agricultural Soils. Env Sci Pollut Res 14, Special Issue 1, 53¿60