Abstract
Dynamics of the microbiological indicators of fresh sludge from wastewater treatment plants with a concentration of CaO, 10% and 20%, and ash, −30% and 50%, and treated with quicklime, ash, and microbial fertilizer for a 50-day period of composting were studied. The influence of temperature, water content, and oxygen on the development of microbes was established in laboratory conditions. Microbiological analysis included the determination of non-pathogenic (non-spore-forming bacteria, bacilli, actinomycetes, micromycetes, bacteria digesting mineral nitrogen), and pathogenic (Salmonella, Listeria, Escherichia coli, Enterococcus, Clostridium perfringens) microorganisms. Of the beneficial microflora in the sludge before treating, the amount of non-spore-forming bacteria was the highest, followed by bacilli and micromycetes. It was found that actinomycetes were absent in the untreated sludge. Clostridium perfringens occupied a major share in the composition of the pathogenic microflora, followed by Escherichia coli, Enterococcus, and Listeria. The best results for decontamination of the sludge were obtained by adding 20% quicklime and 50% ash. Alkalization of the sludge after treatment led to the destruction of pathogenic microflora but also reduced the number of beneficial microorganisms. The decrease in pH during the study period determined the redevelopment of pathogenic microflora. Combined variants with lime or ash and microbial fertilizer showed better results for the development of non-pathogenic microflora and the destruction of pathogens.
Highlights
According to a study by Dermendzhieva [3], in sludge from Bulgarian treatment plants, the number of fecal and intestinal pathogenic (Enterobacteriaceae, Salmonella) microorganisms was above the permissible norms in the Ordinance on the procedure for utilization of sewage sludge through their use in agriculture [13], which makes them unusable for fertilization in agriculture without applying methods for their decontamination
Sludge obtained from the belt filter presses of the wastewater treatment plants (WWTPs)-Blagoevgrad was studied according to microbiological indicators, in dynamics, composted in greenhouse conditions, in plastic packages from 2 L, and treated with quicklime and ash in different concentrations
Of the beneficial microflora in the sludge, the highest was the number of non-spore-forming bacteria and bacteria digesting
Summary
Its decision is directly related to the construction of wastewater treatment plants and to the use of sludge as an alternative energy source (energy resource) and/or soil improver (raw material resource) [1]. Isolated in the sludge are pathogenic microorganisms such as: Salmonella sp., Listeria sp., Escherichia coli, Campylobacter sp., Clostridium sp., Yersinia sp., and others [4,7,8,9,10]. According to a study by Dermendzhieva [3], in sludge from Bulgarian treatment plants, the number of fecal (coliforms, Escherichia coli) and intestinal pathogenic (Enterobacteriaceae, Salmonella) microorganisms was above the permissible norms in the Ordinance on the procedure for utilization of sewage sludge through their use in agriculture [13], which makes them unusable for fertilization in agriculture without applying methods for their decontamination. Co-composting of sewage sludge with lime effectively reduces and even destroys pathogens in the sludge [14,15,16]
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