With the ever-increasing population of the world, the utilization and therefore the demand for energy resources is also increasing. Also, the quick diminishing rate of the natural resources due to their degradation has created an urgent need to find alternative resources (Sasikumar & Papinazath, 2003). In the long run, bioremediation can help fulfill the increasing demands for energy resources, pollution control and waste management. It is possible by the virtue of harvesting of energy from by-products of bioremediation processes. The harvesting of probable waste products like various gases and growth mediums makes this method eco-friendly and cost-effective.
Byproducts from bioremediation
Both ex- and in-situ bioremediation procedures currently find application in wastes issue management around the globe. Simultaneously, investigations into the innovative utilization of various organic and inorganic compounds, biomass and energy sources, continue (Kilbane, 2016). The host of applications involve harnessing energy from biomass such as biodiesel, biofuels from algal bioproducts and others. Some of the significant applications are as below:
Biomass, especially from aquatic resources, can be utilized as raw material for co-firing, to harvest electricity, for liquid fuel like bio-oil by pyrolysis, or for bio-methane generation through fermentation (Demirbasa & Demirbas, 2011). The prospective use of extracted biomass proposed as fertilizer or animal feed further increases the economic value of the process. Also, co-products like pigments, agar, carrageenan and other bioactive compounds removed before fuel conversion of biomass, act as value-added products.
Cleaning of domestic wastewater by using the bioremediation measures of stabilization ponds or lagoons yields microalgal feedstocks. The nitrogen and phosphorus found in weak domestic wastewater are at an ideal level for microalgae cultivation and growth. By assimilating these gases, microalgae grow in high densities, concurrently eliminating the inorganic nutrients from the wastewater. The feedstocks thus obtained further serve for the production of high-value bioproducts. Henceforth, the growth of algae in wastewater serves the dual purpose of fuel production and organic waste degradation (B.Cantrell et al., 2008). Different fuel products or energy compounds eventually obtained from microalgal feedstock include methane, ethanol, butanol, biodiesel, bio-oil, hydrogen and other hydrocarbon derivatives. The alteration of growth factors also allows modification of the composition of biomass. This improves the richness of targeted biomolecular fractions in subsequent biomass for the purpose of harvesting.
“Lake Winnipeg Bioeconomy Project” carried out by the International Institute for Sustainable Development’s (IISD’s) presents yet another intelligent application of biomass harvesting. IISD research initiated harvesting ecological biomass, mainly a large aquatic plant species Cattail typha spp. for watershed nutrient management and biomass for the biomass sector. The project finds its basis in the harvesting of cattail biomass as a solid fuel for bioenergy, biocarbon and higher-value biofuels and bioproducts.
Harvesting Cattail typha spp. eliminates phosphorus, the polluting agent in Lake Winnipeg. The plant species take up phosphorus during growth, resulting in its accumulation in the harvested biomass. Furthermore, the ash from the burning of Cattail biomass can help raise the pH of liquid manure to remove phosphorus. It could further resupply potassium and phosphorus back to the soil, serving as a recycling agent. Apart from phosphorus, Cattail can also remediate water bodies contaminated with other toxic products like arsenic, pharmaceuticals, and even explosives. The usage of the harvested Cattail biomass as a bioenergy feedstock finds further application for heating and electricity generation.
The Cattail biomass compressed into pellets and cubes finds usage in different biomass burners. This facilitates the displacement of fossil fuels like coal where solid fuel boilers are used. Also, the heat value of biomass cubes is comparable to commercial wood pellets. The permanent removal of nutrients bound in the harvested cattail biomass makes it a Lake-Friendly feedstock for bioenergy production (Grosshans et al., 2015).
Harvesting inorganic compounds
The acidic palm oil mill effluent (POME) mixed with the palm pressed fiber (PPF) constitutes the inorganic waste. Its conversion into useful products follows the process of vermicomposting. A significant improvement in nitrogen, phosphorus, and potassium content was monitored during the vermicomposting process. The process reveals the reduction in Carbon-nitrogen ratio of vermicompost which indicates the scale of stabilization of POME–PPF mixture. It also involves substantial improvement in nitrogen, phosphorus, and potassium content. So, the POME–PPF mixture can be successfully used as a feeding material for the earthworms. This also proves to be a cost-effective and eco-friendly green fertilizer for mung bean (Rupani et al., 2017).
Syntrophic bacteria degrade hydrocarbon substrates in petroleum reservoirs and contaminated aquifers into acetate, H2, and CO2. These methanogens produce methane from the by-products in a thermodynamically maintained environment. Active research has shown that Smithella, Methanosaeta and Methanoculleus enriched from petroleum reservoir waters can convert crude oil constituents to methane. Comparatively higher quantities of methane with more than 50% exhaustion of saturate and aromatic hydrocarbons were obtained. The research also suggested that different microorganisms can impact oil bioconversion in different phases (e.g., planktonic vs. sessile) within a subsurface crude oil reservoir(Berdugo-Clavijo & Gieg, 2014).
Olive oil extraction process results in Alphechin, a waste by-product. The microbial bioremediation of alphechin wastewaters involves degradation of the alphechin polyphenols. The by-product of the process is used as a fertilizer or soil conditioner. The fertilizer helps in stimulating the respiratory activity of the soil along with the nitrogen-fixing capacity. The composts fed with alpechin as substrate are also utilized to grow edible mushrooms. Another application involves cultivation of wide spectrum of microbes on a medium made with olive oil mill polluted water. Moreover, alpechin degradation can also allow production of biopolymeric substances such as polysaccharide and biodegradable plastics. Olive oil waste effluents can also serve as substrates for production of Ethanol. The microorganisms participating in this process isolated from such effluents include Candida wickerhamii, C. molischiana and Saccharomyces cerevisiae (Takaç & Karakaya, 2009).
The enrichment Geobacteraceae microbes by biofilms with conductivities rivaling those of synthetic polymers on energy-harvesting anodes can help conserve energy. This conservation further facilitates their growth by oxidizing organic compounds with an electrode serving as the sole electron acceptor. This places the marine sediments as a potential electricity source. It involves placing a graphite electrode (the anode) in the anoxic zone and connecting it to a graphite cathode in the overlying aerobic water. This not only provides a method for extracting energy from organic matter, but also suggests a strategy for promoting the bioremediation of organic contaminants in subsurface environments(Lovley, 2011).
Future scope of harvesting useful byproducts from bioremediation
The multitude of application areas of bioremediation by-products greatly enhances the appeal of bioremediation process. The development of more sophisticated techniques inspired by research-based solutions can open new avenues to harvest bioremediation byproducts. Hence, future field studies need to invest serious efforts towards scientifically legitimate approaches. Furthermore, identification of a plausible range of diverse microbes with decontamination capabilities is necessary. Therefore, the nutrient limitation, low population, absence or insufficient knowledge of microbes with degrading abilities, which can obstruct the success of bioremediation, need to be overcome through active research.
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