This chapter deals with the conclusion achieved from all the phases conducted with the rotary drum composting of water hyacinth, isolation and identification of microbes from water hyacinth compost, biosorption studies of Pb(II) and Cd(II) with the isolated bacterial strain in batch and column system
6.1 Conclusions
Rotary drum composting of water hyacinth was successful with combination of cow dung and sawdust. Highest temperature and degradation was found in trial 3 (6:3:1). Bacterial count was highest among all microbes varying from mesophilic to spore forming form ac- cording to temperature. Their count remained substantial at the end of composting period.
Actinomycetes and streptomycetes count remained higher due to presence of lignocellu- loses in water hyacinth. Fungal count reduced as degradation proceeded but it remained till the end. Lower CO2 evolution rate and OUR in the end signified stability of compost.
Volatile solids (%) at end of composting period indicated the metabolic action of microbes on complex lignocellulosic material forming monomeric compounds but not completely into humic substances. Hence, the compost required more maturation and could not be applied to field directly. Further study was carried out on trial 3 (6:3:1) as it was found to be the best amongst all other trials. Also mesophilic bacteria were in abundance through- out the composting period. Therefore, they were taken as target microbes for isolation and metal interaction study. This made it possible to understand the microbial dynamics of water hyacinth composting more profoundly. 16S metagenome sequencing of best trial (6:3:1) of water hyacinth compost was observed with 39.08% ofBacteroidetes, 24.21% of Flavobacteriafollowed by 24.21% of Flavobacteriales. Flavobacterium genus was found to be most abundant in the composting process. The phylumBacteroidetesplays a crucial role
during the initial and final stages of composting in maturation of compost by degrading complex polymers. TheFlavobacteriaand Sphingobacteriabelong toBacteroidetes species, they contribute for the degradation of starch, cellulose, proteins, chitin, and the pheno- lic and chlorinated compounds. All the bacterial communities belonging to Bacteroidetes are known for degrading lingo-cellulosic compost material, being present during curing of compost as well as in noncured compost.
Glucose minimal medium was found to be the most efficient media for culturing bac- terial isolates as compared to the other three media. Twelve new robust bacterial strains belonging to theBacillusandEnterobacterfamily were isolated as being the most consistent throughout the 20 days of the composting period. Both the culture dependent and culture independent approach gave a spectacle of new bacterial strains. The bacterial strains not only survived the rigorous variations of composting process, but also the presence of the toxic heavy metals of the compost. The reduction in water soluble, leachability, and DTPA values of heavy metals in compost indicated the efficient degradation of the organic mat- ter in the presence of microbial community during the composting process. The bacterial species were investigated further for their capability of heavy metal removal for the purpose of micro-bioremediation.
A bacterial strainBacillus badiusAK, isolated from rotary drum compost of water hyacinth was investigated for biosorption of Pb(II) and Cd(II) in live (non-pretreated) state and dried (pretreated) state. Batch biosorption study of live (non-pretreated) biomass of Bacillus badiusAK demonstrated the optimum conditions of pH at 4, temperature at 30◦C, 150 rpm of rotational speed at biomass concentration of 20 mL with 1.7×1016CFU/mL value, at 100- 150 mg/L concentration of Pb(II). The biosorption followed pseudo second order kinetics and isotherm fitted well to the Langmuir model. The specific growth rate and maximum specific growth rate of bacterial cells under the influence of Pb(II) were determined as 0.05 h−1 and 2.54 h−1respectively, with a biomass yield coefficient of 11.81.
The batch biosorption study with dried (pretreated) biomass of Bacillus badius AK was rapid in the first 30 min, and further studies indicated the optimum batch conditions to be pH at 5, equilibrium contact time of 2.5 h at constant temperature of 40 ¡rC, initial metal concentration of 100 mg/L and rotational speed of 150 rpm. The maximum adsorption capacity was 138.88 mg/g. The loading rate test proved that the optimum dosage for the biomass is 2 g/L.The biosorption followed pseudo second order kinetics, langmuir isotherm fitted better than the fruendlich isotherm indicating possibility of monolayer adsorption.
The heat of adsorption was very low indicating possibility of physical adsorption. The data from FTIR and EDX analysis indicated the presence of several functional groups on the surface of the biomass and their participation in the biosorption process. A 100% desorption of Pb(II) was achieved using 0.1M HCl within 2.5 h but negligible amount of Pb(II) was
6.1. Conclusions
recovered using water which indicated that the biosorbent was fit for treating drinking water. Pb(II) removal efficiency was 60% after desorption. Storage study also proved that the biosorbent can be stored for at least 3 months with very less changes in the metal uptake and removal efficiency. There was very little variation in removal efficiency using sulphates, phosphates and chloride, uptake decreased only at very high concentration of nitrates.
The biomass of Bacillus badius AK showed promising results when it was tested with actual water sample with removal efficiency reaching as high as 97% with initial lead con- centration of 40 mg/L. Biosorption of Cd(II) by dried biomass of Bacillus badius AK was investigated for its potential application in biosorption of heavy metal other than Pb(II) in batch system. The optimum conditions of biosorption were determined to as pH adjusted to 7, contact time of 30 min, initial biomass dosage of 2 g/L at a constant temperature of 40◦C, the initial metal concentration of 100 mg/L and agitation at 150 rpm. The maximum biosorption capacities of Cd(II) onBacillus badius AK was found to be 131.58 mg/g. The pseudo-second order model fitted better than the first order model. Langmuir isotherm fit- ting depicted the monolayer adsorption behavior. The low adsorption heat portrayed that the nature of adsorption was physical adsorption. The recovery of biosorbent after biosorp- tion was 39% of the initial value and the recovery after desorption was 68.2%. Reuse of biosorbent after desorption study showed 52% of Cd (II) removal this was due to the destruction of sites at the surface of the biosorbent during desorption. The biosorption capacity ofBacillus badiusAK for Pb(II) removal was more than that for Cd(II).
The continuous column mode operation of dried biomass ofBacillus badiusAK in biosorp- tion of Pb(II) was observed with higher breakthrough capacity as compared to the batch process. But at a high flow rate the adsorbent got exhausted rapidly, while a low flow rate continuous system is impracticable for situation such as industrial wastewater treatment plant. This showed that replacement of biosorbent in the column was required to be fre- quent which is not feasible for a upscale industrial application. Thus, the approach of a batch system for metal removal by biosorbentBacillus badiusAK is better than the column mode operation.
Therefore, it can be concluded that rotary drum compost of water hyacinth being a pro- cess of microbial degradation of waste organic material serves as an efficient source of robust and vigorous microbes. The isolated microbes justified their potential of surviving in the unfavourable metal loaded conditions of water hyacinth compost by being utilised in biosorption technology for micro-bioremediation. Bacillus andEnterobacterfamily were prominent among the isolated bacterial species. Bacillus badiusAK was found to be most efficient in surviving with its higher growth rate. Biosorption of heavy metals such as Pb(II) and Cd(II) was accomplished in the living (non-pretreated) as well as dried (pretreated) state of bacterial biomass. It was observed that the dried (pretreated) bacterial biomass
performed well in Pb(II) and Cd(II) batch biosorption. The column mode operation gave a spectacle of the application of biosorbent in the industrial process. The efficiency of biosorp- tion was faster and higher as compared to the batch system but due to its higher exhaustion rate it becomes non-feasible and impracticable to use this biosorbent for upscale process.
The other technologies for pretreatment of biomass can make the modification of biomass in favour of its application at the industrial level.
6.2 Future Recommendations
• Efficiency of removal for other heavy metals.
• Biomass immobilization ofBacillus badiusAK with different techniques.
• Application of immobilized biomass in column mode operation.
• Studies on availability and speciation of heavy metals in wastewater due to bacterial biomass.
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