Raja Chowdhury

This study aimed to assess the potential of soil organic carbon (SOC) production through organic amendments. SOC sequestration would help to achieve the net-zero emissions targets set by the Intergovernmental Panel on Climate Change (IPCC). Given the urgency to reduce greenhouse gas emissions, traditional methods that estimate SOC over 100 years must be revised. Hence, a novel fate transport numerical model was developed to forecast SOC levels relevant to individual countries’ net-zero targets in various time frames. The simulation results revealed that most countries had sufficient organic amendment to mitigate the CO₂ emission of that country for a year if the organic amendment was applied on 20% of the arable land. However, if a significant fraction of the total CO₂ emissions needs to be mitigated before reaching the net zero target, the requirements of organic amendments need to be increased several folds. All the available agricultural land should also be brought under the organic amendment regime. Later, the dynamic LCA approach was undertaken for estimating Global Warming (GWP) from land-applied organic residue. It was observed that, depending on the dynamic LCA model, the estimated GWP was different. However, the estimated dynamic GWP was very close to the residual SOC calculated through the fate transport model. The mass of organic residues generated from a biorefinery was examined by employing a waste biorefinery model to explore further the routes of acquiring additional organic amendment. Simulated results showed that while a waste biorefinery could not provide additional organic residue compared to the original organic waste input, it was highly efficient for nutrient recovery and its uses. This study demonstrated that organic amendment-based carbon sequestration adequately mitigated residual GHG at the net-zero target.

Life cycle-based analysis is a key to understand these biofuels’ climate benefits. This manuscript provides a state-of-the-art review of current biofuel production, primarily through algae-based routes. Standalone biofuel production has an unfavorable environmental and energy footprint. Therefore, industrial symbiosis is required to reduce the environmental impacts of biofuel. The availability of waste heat, CO₂, renewable energy, and colocation of other industries, especially renewable energy and dairy firms, have been demonstrated beneficial for producing biofuel through the algal route. Dynamic life cycle assessment (DLCA) issues were discussed in detail. DLCA is one of the highlighted areas of the Life Cycle Assessment (LCA) paradigm that can improve the applicability of climate change indicators used in the LCA. Various climate change indicators, global warming potential (GWP), global temperature change (GTP), and climate tipping point (CTP) were discussed in detail. Special emphasis was given to waste-based bioenergy production and its LCA as this route provided the lowest GHG emissions compared to the other bioenergy production pathways (e.g., from energy crops, using lignocellulosic biomass, etc.). The use of LCA results and modification of life cycle inventory (e.g., modification in the form of the regional energy mix, dynamic Life Cycle Inventory (LCI), etc.) was another highlight of this study. Such modifications need to be incorporated if one wants to improve the applicability of LCA results for net zero target analysis.

Algal biomass and Colocasia esculenta (Taro) leaves are available as waste biomass all over India. These biomasses can be used as renewable and sustainable resources for sugars and nutrients. Recovered nutrients and sugars can be used as cheap raw materials for biofuels and biomaterials production. The hydrolysis of dried algal biomass and Colocasia esculenta (Taro) leaves were investigated using 1%, 2%, and 5% solutions of ferric-chloride, nitric acid, and acetic acid for the reaction times of 30 and 60 min at 121 °C and 103.4 kPa (15 psi). 1% and 2% H₂SO₄ treatments were used as the reference. The solid: liquid ratio was kept at 1:10 for all the experiments. For algal biomass, a 5% acetic acid treatment for 60 min was found to be optimum with a total carbohydrate release of 44.2 mg/g biomass (solubilized monomers-0.82 mg/g of biomass) and N and P solubilization of 1.8 mg total nitrogen/g biomass and 7 mg total phosphorus/g biomass. Moreover, for Colocasia esculenta (Taro) leaves, the maximum carbohydrate yield of 95 mg/g biomass (solubilized monomers-43.6 mg/g of biomass) and nutrient solubilization of 5.02 mg total nitrogen/g biomass was obtained with 5% ferric chloride treatment for 60 min. The results obtained showed that various hydrolyzing agents used in this study acted differently on different types of biomasses. Acetic acid worked best in hydrolyzing the algal biomass, and for the hydrolysis of Taro leaves, ferric chloride and nitric acid were effective. Statistical analysis showed that the chemical concentration was one of the prime factors for releasing P from algal biomass. For carbohydrate release from Taro leaves, either time or concentration, or both, were the prime factors that affected the carbohydrate release.

In algal pond used for treating wastewater, a part of the solar radiation (PAR) is used by algae for photosynthesis, and rest of the solar energy is wasted. To date, no studies have been conducted that optimize these aspects (wasted solar radiation for heat production) from an operational cost perspective. Therefore, a model is developed for the estimation of photosynthetically active radiation (PAR) from solar radiation. Subsequently, derived PAR was utilized in the optimization algorithm. Experimental data on PAR and solar radiation were used to obtain empirical parameters of the developed model. Using empirical parameters, diurnal PAR was estimated for other locations for which the diurnal variation of solar radiation was not available. Afterwards, the estimated solar radiation was used to obtain the cost of algal biomass production using wastewater. For this purpose, a cost function was minimized. The cost function contains various cost components of algae-based wastewater treatment. The major costs of the treatment were incurred by the sunlight harvesting, and distribution equipment, whereas the major income was registered through the conversion of sunlight to heat and biomass production. The yearly cost of treating 1m³ wastewater in the proposed wastewater treatment plant could be varied from 186 to −44 (Rs, INR). The capital cost of constructing a proposed treatment plant having a capacity of 1000 m³/day varied from 11–45 crores INR.

The microalgal strain Chlorella sorokiniana isolated from a waste stabilization pond was used for tertiary treatment of municipal wastewater. Three light:dark (L:D) regimes of 12:12, 16:8, and 24:0 were used for treating wastewater in microalga (A), microalga + sludge (A + S), and sludge (S) reactors. The removal of nutrients (N and P) was found to be the highest in the microalga-based reactor, with more than 80% removal of biochemical oxygen demand (BOD) and 1.2–5.6 log unit removal of pathogens. The addition of sludge improved chemical oxygen demand (COD) removal. Nitrifiers were found to be predominant in the A + S reactor. Algal biomass productivity was more than 280 mg/L/d in all the L:D regimes. The increase in light regime improved nutrient removal and biomass productivity in the algal reactor. Results of the kinetic study showed that (i) nitrifiers had more affinity for ammonium than microalga, and hence, most of the ammonia was oxidized to nitrate, (ii) microalga assimilated nitrate as the primary nitrogen source in the A + S reactor, and (iii) solubilization of particulate organic nitrogen originated from dead cells reduced the nitrogen removal efficiency. However, in the microalga-based reactor, the ammonium uptake was higher than nitrate uptake. Among pathogens, the removal of Salmonella and Shigella was better in the A + S reactor than in the other two reactors (microalga and sludge reactor). Additionally, the heterotrophic plate count was drastically reduced in the presence of microalga. No such drastic reduction was observed in the stand-alone sludge reactor. Kinetic modeling revealed that microalga–pathogen competition and pH-induced die-off were the two predominant factors for pathogen inactivation.

Several algal species were found in the samples collected from a wastewater treatment plant located in Uttarakhand, India. Several media and their modifications were employed to understand the role of these elements and compounds in the species composition and proliferation. Euglena, Chlamydomonas-reinhardtii, Chlorella-sorokiniana, Chlorococcum, several diatoms and cyanobacteria were found in the wastewater samples. In stagnant and flowing municipal wastewater, Euglena, Chlorococcum, cyanobacteria and Volvox were predominant. In a bold basal medium, Scenedesmus and several diatom species were observed along with Chlorococcum. In the presence of acetate Selenastrum species were abundant. Metagenomic analysis of wastewater collected during winter revealed that Tetradesmus wisconsinensis (21.23%) and Actinastrum hantzchii (11.48%) were the dominant species. Some cyanobacterial species accumulated near the high-intensity light region. Hence, subsequent phototaxis experiments were performed. Under the white light, most species showed positive phototaxis. However, under the blue and red light, all the algal species present in the sample showed negative phototaxis. Under the green light, Chlorococcum and Chlamydomonas showed positive phototaxis and Euglena showed negative phototaxis. However, under the pink light, neither negative nor positive phototaxis was observed. Hence, the obtained results provided new insights into the effects of the studied parameters on the species’ diversity and proliferation.

Hydrolysate prepared from the chemical hydrolysis of water hyacinth biomass contains a high amount of solubilised carbohydrate and nutrients. This hydrolysate was utilised as a medium for the cultivation of two strains of Chlorella sorokiniana, isolated from a municipal wastewater treatment plant using two different media, i.e., BG-11 and Knop’s medium. Different light intensities, light–dark cycles, and various concentrations of external carbon sources (monosaccharides and inorganic carbon) were used to optimise the microalgal growth. For the accumulation of lipids and carbohydrates, the microalgal strains were transferred to nutrient amended medium (N-amended and P-amended). It was observed that the combined effect of glucose, inorganic carbon, and a 12:12 h light–dark cycle proved to be the optimum parameters for high biomass productivity (~200 mg/L/day). For Chlorella sorokiniana 1 (isolated from BG-11 medium), the maximum carbohydrate content (22%) was found in P-amended medium (N = 0 mg/L, P: 3 mg/L), whereas, high lipid content (17.3%) was recorded in N-amended medium (N = 5 mg/L, P = 0 mg/L). However, for Chlorella sorokiniana 2 (isolated from the Knop’s medium), both lipid (17%) and carbohydrate accumulation (12.3%) were found to be maximum in the N-amended medium. Chlorella sorokiniana 2 showed a high saturated lipid accumulation compared to other strains. Kinetic modelling of the lipid profile revealed that the production rate of fatty acids and their various constituents were species dependent under identical conditions.

Hydrolysate prepared from water hyacinth biomass, containing a considerable amount of solubilised carbohydrate and nutrients, was utilised as a medium for the cultivation of two strains of Chlorella sorokiniana. These strains were isolated from an oxidation pond using two different media, i.e., BG-11 and Knop's media maintained at pH-9. Different light intensities, light-dark cycles, and various concentrations of external carbon sources (monosaccharides and inorganic carbon) were used to optimise the microalgal growth. It was observed that in the presence of organic carbon (glucose), biomass productivity increased significantly (~300 mgL-1day-1) as compared to that in the presence of only inorganic carbon (~100 mgL-1day-1). For the accumulation of stress products (lipids and carbohydrates), the microalgal strains were transferred to nutrient-amended media (N-amended and P-amended). The combined effects of glucose, inorganic carbon, and a 12h:12h light-dark cycle proved to be optimum for biomass productivity. For Chlorella sp. isolated from BG-11, maximum carbohydrate content (22%) was found in the P-amended medium, whereas high lipid content (17.3%) was estimated in the N-amended medium. However, for Chlorella sp. isolated from Knop's medium, both the lipid (17%) and carbohydrate accumulation (12.3%) were found maximum in the N- amended medium. Kinetic modelling of the lipid profile revealed that kinetic coefficients obtained for strain isolated from BG-11 media were statistically significant from each other.

Two microalgal strains (Chlorella sorokiniana, A&B,) grown optimally at pH 9 (A) and 7 (B) were isolated from a municipal wastewater treatment plant. These strains were used to check their efficacy for nutrients and organic carbon removal capacities from the tertiary wastewater. The strains were characterised using 18S rDNA sequencing method and both the strains closely related to Chlorella sorokiniana. Different inoculum doses (IDs) of both the strains were tried to check their efficacy on wastewater treatment. Due to the increase in the IDs, hydraulic retention time decreased from 6 d to 1 d. On an average, 12–100% of total kjeldahl nitrogen, 53–96% NO₃–N, and 59–92% PO₄–P, were removed. Despite the same growth environment, strain A with the highest ID (700 mg/L) showed the best performance in terms of overall organic carbon removal, nutrient removal, and value-added products recovery. Strain A showed an increase in carbohydrate and protein content by 42% and 13%, respectively. Both the strains showed luxury phosphorus uptake and found suitable for advanced wastewater treatment.