Abstract
In Mexico, as in other developing countries, the most important pollution and management problems of food-processing sludge are the high levels of pathogen microorganisms within the sludge and the lack of sites for its disposal. The aims of this study were to evaluate the effect of calcium oxide in the inactivation of pathogenic microorganisms and the subsequent use of the resulting product in an agricultural application at various agronomic rates. Stabilisation tests were done in a hermetically closed fibreglass reactor with 1.5 . capacity, using physicochemical sludge with concentrations of 4, 8 and 12% of total solids (TS) and contact times of 30, 60 and 90 min. At the end of each treatment, the raw and treated sludge quality was evaluated. Recommended doses for Class A biosolids production were 20, 10 and 8% m/m of CaO for 4, 8 and 12% of TS respectively with a minimum contact time of 90 min. The land-application test was done using sludge with 8% TS treated with a quicklime dose of 10% m/m. Nitrogen-based Agronomic Rates (AR) of 0, 1, 5, 10 and 15 were evaluated in the cultivation and production of chayote (Sechium edule). The results with 5 AR showed an estimated total production of 70 kg of the vegetable species over a period of 90 d, which is higher than that reported for the same crop grown without biosolids application.
Highlights
Mexico generates approximately 12 000 000 t/a of sludge containing high levels of organic matter, pathogen microorganisms and parasites (Ortega, 2003)
The concentrations of microorganisms are higher than those reported in countries like the United States (faecal coliforms 2 x MPN/g TS and Salmonella spp. 2.1 x 102 MPN/g total solids (TS)) and the United Kingdom where faecal coliform concentrations varying between 3.6 x 104 and 1.4 x 106 MPN/g TS and Salmonella spp. varying between 1.4 x 102 and up to 4.2 x 104 MPN/g TS are reported (Crewe, 1994; Jimenez et al, 2000; Jimenez et al, 2004; Reimers, 1986)
A unique characteristic of this type of sludge is the high concentration of pathogenic microorganisms which are above the permissible limits of the NOM-004-SEMARNAT-2002 regulation because of the nature of the sludge; concentrations of between 2.90 x 1010 and 1.23 x 108 (Salmonella spp.) were quantified; this finding is in agreement with information given in reports on the sludge produced in a bird slaughterhouse (Salmonella are generally present on bird farms and concentrations of between 1.23 x 108 (Salmonella spp.) and 2.90 x 1010 are found)
Summary
Mexico generates approximately 12 000 000 t/a of sludge containing high levels of organic matter, pathogen microorganisms and parasites (Ortega, 2003). Literature regarding the development of technologies for sludge stabilisation with extremely high pathogen content is limited and a large amount of information deals with conventional stabilisation processes applied to sludge with ‘typical’ characteristics Some of these technologies need to demonstrate their ability to significantly reduce those microorganisms that are present in sludge generated in developing countries. Due to the simplicity of the operation and its low cost, one of the most efficient alternatives to treat this type of sludge is by means of alkaline stabilisation (Girovich, 1996) This process has high effectiveness in the inactivation of pathogenic microorganisms and it is capable to inactive up to 7 and 6 log of faecal coliforms and Salmonella spp., Mendez et al, (2002; 2004), but the effect of these biosolids when applied as organic fertilisers should be evaluated
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