Thursday, February 8, 2007

Addition of Inorganic By-Products to Sewage Sludges to Stabilize Organic Carbon.

Marta Camps Arbestain1, Zuriñe Madinabeitia2, María Victoria Gil3, Marisa Ibargoitia4, Antonio Morán3, and Felipe Macias2. (1) SPAIN, Santiago, NEIKER, Berreago Kalea, Derio Bizkaia, 48160, SPAIN, (2) Universidad de Santiago de Compostela, Departamento de Edafología y Química Agrícola, Facultad de Biología, Santiago de Compostela, Spain, (3) Instituto de Recursos Naturales, Grupo de Ingeniería Química y Ambiental, Avenida de Portugal 41, León, Spain, (4) NEIKER-Instituto Vasco de Investigación y Desarrollo Agrario, Berreaga 1, Derio, Spain

There are numerous studies in which mixtures of organic and inorganic by-products have been elaborated with the aim of producing soil amendments with better properties than the individual ingredients, particularly in terms of  reducing  heavy-metal availability, eliminating pathogens, reducing P availability and improving nutritional balances. However, few studies have been carried out in which mixtures of residues have been elaborated with the additional aim of stabilizing the organic C (OC), which was the main purpose of the present study. For this, two municipal sewage sludges were used: a dewatered anaerobic digested sludge (AN) and a CaO-treated aerobic sludge (AE). Moreover, a mixture of the two sludges (50:50 DW basis) was also elaborated to provide a third type of sludge (AN+AE). The sludges were mixed with other types of residues - herein referred to as “conditioners” - such as combustion fly ash (FA), Linz-Donowitz slag (LD), foundry sand (FS), shot blasting machine scrap (SB), and fettling (FE), at two different ratios (85:15 and 65:35, sludge:conditioner). Barley straw (BS) was also used as a conditioner, although at different proportions (99:1 and 97:3 sludge:barley straw). The mixtures were incubated for 4 weeks, and the degree of stability of organic C forms in the different mixtures was then evaluated. Total C was analysed by combustion and inorganic C was determined with a calcimeter. The OC was then calculated by subtracting the amount of inorganic C from total C. The oxidability of OC by KMnO4 (MnoxC) was also determined. Carbon functional groups in selected samples were identified by Fourier transformed infrared (FTIR) and 13C nuclear magnetic resonance spectroscopy (RNM). Thermogravimetric analysis (TGA) was used to determine the thermal properties of the different mixtures. Other analyses were carried out to determine pollutant availability and nutrient balance, but are not discussed here.