Abstract
The present study aimed to determine the effect of a 17.6 mT static magnetic field (SMF) on the efficiency of anaerobic digestion (AD) of municipal sewage sludge (MSS). The SMF had a significant impact on methane (CH4) production efficiency, the levels of fermentation rate (ηFMSS) vs. removal rate (ηVS), and the structure of the anaerobic bacteria consortium, but it did not affect cumulative biogas production. The highest CH4 yield (431 ± 22 dm3CH4/kgVS) and the highest methane content in the biogas (66.1% ± 1.9%) were found in the variant in which the SMF exposure time was 144 min/day. This variant also produced the highest ηFMSS and ηVS values, reaching 73.8% ± 2.3% and ηVS 36.9% ± 1.6%, respectively. Longer anaerobic sludge retention time in the SMF area significantly decreased AD efficiency and caused a significant reduction in the number of methanogens in the anaerobic bacteria community. The lowest values were observed for SMF exposure time of 432 min/day, which produced only 54.8 ± 1.9% CH4 in the biogas. A pronounced reduction was recorded in the Archaea (ARC915) and Methanosaeta (MX825) populations in the anaerobic sludge, i.e., to 20% ± 11% and 6% ± 2%, respectively.
Highlights
The operation of sewage treatment plants directly necessitates optimal sludge management
Chacón Alvarez et al (2006) determined that static magnetic field (SMF) (5 mT) exerted a positive effect on the production of nisin by Lactococcuslactis [51], whereas Yavauz and Celabi (2000) found that the use of SMF in the range from 8.9 to 46.6 mT in an activated sludge process increased the biodegradation of organic compounds in the effluent by 44% [52]
As the SMF induction levels increased, the efficiency of organic compound biodegradation decreased significantly [52]. This was corroborated by Ji et al (2010), who examined the effect of 0–20 mT SMF on the bio-treatment of wastewater, and determined an optimum SMF induction for stimulating microbial metabolism and improving treatment efficiency, peaking at 17.8 mT [53]
Summary
The operation of sewage treatment plants directly necessitates optimal sludge management. The dynamically growing number of new wastewater treatment plants has a direct impact on the increasing amount of sewage sludge, while stringent standards related to sludge management make it necessary to use complex technologies to limit their impact on the environment [3,4]. Natural use is widely held to be the simplest and cheapest form of MSS management This method is widely promoted both in scientific/technical literature and in national/international legal acts [5]. MSS poses a sanitary threat due to high concentrations of pathogenic and parasitic organisms [8] It is often a source of heavy metals and other toxic substances derived from wastewater [9], and its improper management and neutralization frequently lead to the emission of gaseous contaminants and aerosols to the atmosphere [10]. There is a pressing need to alleviate the environmentally harmful properties of MSS by improving current technologies and exploring new ones [11]
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