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Graphical abstract Drastic measures such as lockdown imposed in the countries worldwide to control the extent of COVID-19 have influenced environmental parameters substantially. The aim of the present study was to investigate the impact of lockdown on the air quality in Hyderabad city by comparing the pollutants concentration during lockdown and pre-lockdown periods. A comparative study was also made on the pollutant concentrations observed during the pre-lockdown (1st February – 23rd March 2020) and lockdown period (24th March - 30th April 2020) to those of the pollutants in the previous years (2018 and 2019). The Pearson correlation coefficient was employed to correlate the ozone (O 3 ) concentration with other pollutants. Carbon monoxide (CO), nitrogen oxides (NO X ) and O 3 were monitored along with meteorological parameters like temperature, relative humidity and solar radiation. It was observed that the O 3 concentration increased from 26 ppb (by volume) to 56.4 ppb during pre-lockdown and lockdown period, respectively, due to the decrease in CO and NO X concentration. The concentration of NO 2, NO and CO were also reduced during the lockdown period by 33.7%, 53.8% and 27.25%, respectively. To identify the statistical significance of the parameters, analysis of variance (ANOVA) was used. The present study provides new insights on the ambient air pollution in terms of the aforesaid parameters and could pave the way for regulatory authorities to implement control measures to curb the air pollution. Highlights • COVID-19 lockdown improved the air quality by the reduction in air pollutants leading to increased ozone concentration. • Pearson model was used to correlate the ozone (O 3 ) with oxides of nitrogen (NO X ), CO and other meteorological parameters. • NO X and carbon monoxide (CO) concentrations were reduced by 33.7 % and 27.25 %, respectively, due to COVID lockdown.

Different agricultural residue was evaluated as a biofilter material for the removal of Hydrogen sulfide (H2S) using Thiobacillus sp. A Combination of four different agricultural residues, viz., sugarcane bagasse, coconut coir pith, rice husk and saw dust were evaluated as biofilter material. From results, it was observed that filter material having composition of 2:4:2:2, 4:4:2:2 and 4:2:2:4 has resulted in retaining 70% moisture content at the end of 10 days. The biofilter was operated for 128 days in three phases by varying inlet H2S concentration from 321 to 2020 ppmv. Biofilter exhibited 100% removal efficiency (RE) at an inlet concentration of 570 ppmv, 99% RE at an inlet concentration of 1416 ppmv and 66% RE at a maximum inlet concentration of 2020 ppmv. The filter performance in terms of RE, dropped to 48% when the air was not humidified before sending to the filter, which has direct relation with the MC of the filter material. The RE recovered to 66% with recovery of moisture content (MC) to 57% upon subsequent re-introduction of the humidifier in to the circuit. The pH has dropped from 7.8 to 4.8 during the course of operation of the filter. H2S was effectively removed under different operating conditions using mixed agricultural residue as filter material. However humidification was most essential to maintain the required moisture content in the biofilter. SEM analysis has shown the good growth of Thiobacillus sp. in the filter bed and hence the new material proved to be good support for the immobilization of Thiobacillus sp.

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