Phytoremediation of parboiled rice mill wastewater using water lettuce (Pistia stratiotes)

Rice mill wastewater management in the era of circular economy

Several nations around the world use rice as their primary food staple because of its tremendous nutritional value. India's expanding population has sparked a proliferation of rice mills as a result of the country's growing rice demand. However, small and medium-scale industries lack adequate facilities for processing effluents and other waste generated. Paddy is typically processed by parboiling, which involves soaking it in water, boiling it with steam, and then drying and milling. Around 1-1.5 L of water is necessary to partially cook 1 kg of unhusked rice, with approximately half of this water being discharged as effluent. Disposal of rice mill effluent (RME) in water bodies or on the land causes severe damage to soil and water. An inclusive examination of diverse approaches for the treatment and stabilization of partially cooked rice milling effluents is provided. Moreover, the document provides a concise overview of contemporary and environmentally friendly technologies for treating RME. Adsorption, electrocoagulation, chemical coagulation, and bioremediation using microbes, plants, and microalgae are all included in these methods. This manuscript discusses the concept of a circular economy, which is focused on enhancing environmental sustainability through the recycling and repurposing of generated waste into raw materials for the creation of new products. In addition, this review aims to focus on the impact of RME on soils and water species and the status of sustainable management at the point of circular economy with RME bioenergy production (bioelectricity, biomethane, and bio-hydrogen).

Phytosphere purification of urban domestic wastewater

Industrialization and overpopulation have polluted aquatic environments with significant impacts on human health and wildlife. The main pollutants in urban sewage are nitrogen, phosphorus, heavy metals and organic pollutants, which need to be treated with sewage, and the use of aquatic plants to purify wastewater has high efficiency and low cost. However, the effectiveness and efficiency of phytoremediation are also affected by temperature, pH, microorganisms and other factors. The use of biochar can reduce the cost of wastewater purification, and the combination of biochar and nanotechnology can improve the efficiency of wastewater purification. Some aquatic plants can enrich pollutants in wastewater, so it can be considered to plant these aquatic plants in constructed wetlands to achieve the effect of purifying wastewater. Biochar treatment technology can purify wastewater with high efficiency and low cost, and can be further applied to constructed wetlands. In this paper, the latest research progress of various pollutants in wastewater purification by aquatic plants is reviewed, and the efficient treatment technology of wastewater by biochar is discussed. It provides theoretical basis for phytoremediation of urban sewage pollution in the future.

Role of dominant macrophytes to treat Nambul river, the main polluter of Loktak - a dying Ramsar site in the Indo Burma hot spot (Manipur, India)

As a part of the monitoring process of the main polluting tributary of Loktak, the paper analyzed the water characteristics of the main polluted stretch of Nambul river i.e. Imphal Urban region. The results showed low pH (6.23 ± 0.32), high electrical conductivity (1373 ± 50.33 µSm^-1), high turbidity (25.40 ± 0.90 NTU), poor dissolved oxygen content (0.53 ± 0.24 mgL^-1), high nitrate, and phosphate concentrations (9.27 ± 0.40 and 2.05 ± 0.10 mgL^-1, respectively). The Water Quality Index values indicate that Nambul river is unfit for drinking. Moreover, the Sodium Percentage, Sodium Adsorption Ratio, and Kelly's Ratio revealed the unsuitable nature of the river for irrigation. Therefore, in order to clean the river water, lab scale microcosm experimentation was carried out to highlight the potential of the dominant macrophytes viz. Eichhornia crassipes (Mart) Solms and Pistia stratiotes (L.) as phytofilters. The results showed increased pH (7.6 ± 0.11 and 7.5 ± 0.1), higher dissolved oxygen content (5.87 ± 0.30 and 3.28 ± 0.30 mgL^-1), less turbidity (0.60 ± 0.25 and 0.40 ± 0.15 NTU) and reduced nitrate and phosphate concentrations (0.37 ± 0.03 and 0.52 ± 0.01 mgL^-1, and 0.534 ± 0.025 and 0.346 ± 0.01 mgL^-1, respectively).Novelty statementThe present study showed the deteriorating water quality of Nambul river, the main polluting source of Loktak - a Ramsar site in the Montreux records.Water Quality Index calculated was above the unsuitable level for drinking at all the sampling locations in the Imphal Urban region.Sodium percentage, sodium adsorption ratio and Kelly's ratio were above the desired limit, making the river water unfit for irrigation.Further, the microcosm experimentation showed that the dominant macrophytes viz. Eichhornia crassipes (Mart) Solms and Pistia stratiotes (L.) can control the pollution level of Nambul.

A review on current techniques used in India for rice mill wastewater treatment and emerging techniques with valuable by-products

Parboiling rice mills produce a large amount of wastewater. The effluent from the rice mill contains high concentration of organic and inorganic compounds. Continuous discharge of this wastewater from rice mills could be a major cause of eutrophication of the water bodies, leading to a robust and undesirable growth of vegetation and algal bloom. Eutrophication leads to deoxygenation of water bodies, resulting in the mortality of aquatic flora and fauna. It is therefore very important to treat wastewater and ensure safe disposal. There are various types of processes already in existence, but the most important thing is to select a cost-effective technique. Phytoremediation technique has shown promising results for low-income countries like India. It is useful for the small-scale industrial and domestic wastewater treatment. It is a slow process, requiring a large area of land. It is an eco-friendly approach and ideal for rural areas where there is ample land availability. This paper addresses the techniques used by various researchers for rice mill wastewater treatment and also focuses on reusing and recycling of waste from rice mills. The performance of every technique is discussed in detail with its specific advantages and disadvantages. This paper also focuses on the advanced techniques of water treatment with valuable by-products such as silica, activated adsorbent, electricity and methane gas.

Assessing the impact of vegetation coverage ratio in a floating water treatment bed of Pistia stratiotes

Floating bed (FB) is one of the low cost and efficient system to treat polluted water bodies in rural as well as urban area. It requires less space compared to other macrophyte based treatment system. Vegetation coverage is an effective factor as atmospheric oxygen diffusion to water body depends upon this. When a water body is fully covered by vegetation, it prevents the diffusion of atmospheric oxygen, which is a major source of dissolved oxygen (DO). An optimum vegetation coverage is necessary to obtain maximum treatment efficiency and to verify this a laboratory scale experiment was performed with various vegetation coverage ratio (VCR) of 0, 0.25, 0.5, 0.75, 1. Organics and nutrient removal were measured corresponding to initial floating bed VCR. COD removal for VCR 1.0, 0.75, 0.5, 0.25 and 0 were 86.4%, 93.6%, 96.2%, 85.4% and 60.8%, respectively. The roots of macrophyte (Pistia stratiotes) supports growth of microorganism, which perform biodegradation of organics as well as uptake nutrients. Maximum NH4+-N and PO43−-P removal were observed at VCR of 0.75 and 1.0, respectively. In the tanks with VCR of 0.25, 0.5 and 0.75, the final DO was more than 4 mg/L, which is the requisite DO value for survival of aquatic organism.

Aquatic Plants, Landoltia punctata, and Azolla filiculoides as Bio-Converters of Wastewater to Biofuel

The aquatic plants, Azolla filiculoides, and Landoltia punctate, were used as complementing phytoremediators of wastewater containing high levels of phosphate, which simulates the effluents from textile, dyeing, and laundry detergent industries. Their complementarities are based on differences in capacities to uptake nitrogen and phosphate components from wastewater. Sequential treatment by L. punctata followed by A. filiculoides led to complete removal of NH4, NO3, and up to 93% reduction of PO4. In experiments where L. punctata treatment was followed by fresh L. punctata, PO4 concentration was reduced by 65%. The toxicity of wastewater assessed by shrimps, Paratya australiensis, showed a four-fold reduction of their mortality (LC50 value) after treatment. Collected dry biomass was used as an alternative carbon source for heterotrophic marine protists, thraustochytrids, which produced up to 35% dry weight of lipids rich in palmitic acid (50% of total fatty acids), the key fatty acid for biodiesel production. The fermentation of treated L. punctata biomass by Enterobacter cloacae yielded up to 2.14 mol H2/mole of reduced sugar, which is comparable with leading terrestrial feedstocks. A. filiculoides and L. punctata can be used as a new generation of feedstock, which can treat different types of wastewater and represent renewable and sustainable feedstock for bioenergy production.

Phytoremediation capabilities of Salvinia molesta, water hyacinth, water lettuce, and duckweed to reduce phosphorus in rice mill wastewater

The objective of this study was to investigate the reduction of phosphorus from rice mill wastewater by using free floating aquatic plants. Four free floating aquatic plants were used for this study, namely water hyacinth, water lettuce, salvinia, and duckweed. The aquatic plants reduced the total phosphorus (TP) content up to 80% and chemical oxygen demand (COD) up to 75% within 15 days. The maximum efficiency of TP and COD reduction was observed with water lettuce followed by water hyacinth, duckweed, and salvinia. The study also aims to predict phosphorus removal by three modeling techniques, for example, linear regression (LR), artificial neural network (ANN), and M5P. Prediction has been done considering hydraulic retention time (HRT), hydraulic loading rate (HLR), and initial concentration of phosphorus (Cin) as input variables whereas the reduction rate of TP (R) has been considered as a predicted variable. ANN shows promising results as compared to M5P tree and LR modeling. The model accuracy is analyzed using three statistical evaluation parameters which are coefficient of determination (R2), root mean square error (RMSE), and means absolute error (MAE).

Use of plant materials for the bioremediation of soil from an industrial site

Bioremediation is one of the existing techniques applied for treating oil-contaminated soil, which can be improved by the incorporation of low-cost nutritional materials. This study aimed to assess the addition of two low-cost plant residues, sugarcane bagasse (SCB) and leaf litter (LL) of the forest leguminous Mimosa caesalpiniifolia plant (sabiá), either separately or combined, to a contaminated soil from a petroleum refinery area, analyzed after 90 days of treatment. Individually, both amounts of SCB (20 and 40 g kg^-1) favored the growth of total heterotrophic bacteria and total fungi, while LL at 20 g kg^-1 better stimulated the hydrocarbon-degrading microorganism's activity in the soil. However, no TPH removal was observed under any of these conditions. Higher microbial growth was detected by the application of both plant residues in multicontaminated soil. The maximum TPH removal of 30% was achieved in amended soil with 20 g kg^-1 SCB and 20 kg^-1 LL. All the experimental conditions revealed changes in the microbial community structure, related to the handling of the soil, with abundance of Alphaproteobacteria. This study demonstrates the effectiveness of the plant residues SCB and LL as low-cost nutritional materials for biodegradation of hydrocarbon in real oil contaminated soil by indigenous populations.

Floating treatment wetlands as biological buoyant filters for wastewater reclamation

Floating treatment wetlands (FTWs) are an innovative product of ecological engineering that can play a promising role in wastewater treatment. It provides low-cost, eco-friendly, and sustainable solutions for the treatment of wastewater, particularly in regions with economic constraints. Generally, FTWs comprise rooted plants that grow on the surface of water with their roots extending down into the pelagic zone rather than being embedded into the sediments. This drooping structure helps develop (1) a hydraulic flow between the root network and the bottom of the treatment system and (2) a large biologically active surface area for the physical entrapment (filtration) of contaminants, as well as their biochemical transformation and degradation. Furthermore, the rooted network allows proliferation of microorganisms that form biofilms and enhance pollutant degradation while promoting plant growth. The augmentation of bacteria in FTWs has been proven to be the most effective approach for reclamation of wastewater. This article discusses the operational parameters of FTWs for maximal remediation of wastewater and highlights the importance of plant-bacteria partnerships in a typical FTW system for enhanced cleanup of wastewater. We propose that this technology is preferable over other methods that require high energy, costs, and area to install or operate machinery.

Lipid production in aquatic plant Azolla at vegetative and reproductive stages and in response to abiotic stress

The aquatic plant Azolla became increasingly popular as bioenergy feedstock because of its high growth rate, production of biomass with high levels of biofuel-producing molecules and ability to grow on marginal lands. In this study, we analysed the contribution of all organs of Azolla to the total yield of lipids at vegetative and reproductive stages and in response to stress. Triacylglycerol-containing lipid droplets were detected in all (vegetative and reproductive) organs with the highest level in the male microsporocarps and microspores. As a result, significantly higher total yields of lipids were detected in Azolla filiculoides and Azolla pinnata at the reproductive stage. Starving changed the yield and composition of the fatty acid as a result of re-direction of carbon flow from fatty acid to anthocyanin pathways. The composition of lipids, in regard the length and degree of unsaturation of fatty acids, in Azolla meets most of the important requirements for biodiesel standards. The ability of Azolla to grow on wastewaters, along with their high productivity rate, makes it an attractive feedstock for the production of biofuels.

Economic and sustainable management of wastes from rice industry: combating the potential threats

Rice is one of the imperative staple foods, particularly in the developing countries. The exponential boom in human population has resulted in the continuous expansion in the rice industry in order to meet the food demands. The various stages of paddy processing release huge quantity of solid wastes, mainly rice husk, rice husk ash and liquid wastes in the form of rice industry wastewater. The discharge of the rice industry wastewater imparts a substantial threat to the aquatic bodies and the nearby surrounding and, thus, consequently demands eco-benign treatment plan. As a result, different strategies are needed to enhance the effluent quality and minimize the operational cost of the treatment process. Therefore, efficient technological approach targeting the minimization of pollution as well as assuring the economic prosperity should be implemented. In this review article, several aspects related to the rice industry discussing the significant challenges involved in the generation of both solid and liquid wastes, mitigation experiments and future prospects have been meticulously elaborated. Furthermore, the article also focuses on the various processes utilized for reducing the pollution load and promoting the practice of reuse and recycle of waste rather than the discharge action for the sake of sustainability and the emergence of novel techniques for the generation of energy and value-added products.

Phytoremediation of Nitrogen as Green Chemistry for Wastewater Treatment System

It is noteworthy that ammoniacal nitrogen contamination in wastewater has reportedly posed a great threat to the environment. Although there are several conventional technologies being employed to remediate ammoniacal nitrogen contamination in wastewater, they are not sustainable and cost-effective. Along this line, the present study aims to highlight the significance of green chemistry characteristics of phytoremediation in nitrogen for wastewater treatment. Notably, ammoniacal nitrogen can be found in many types of sources and it brings harmful effects to the environment. Hence, the present study also reviews the phytoremediation of nitrogen and describes its green chemistry characteristics. Additionally, the different types of wastewater contaminants and their effects on phytoremediation and the phytoremediation consideration in wastewater treatment application and sustainable waste management of harvested aquatic macrophytes were reviewed. Finally, the present study explicates the future perspectives of phytoremediation. Based on the reviews, it can be concluded that green chemistry characteristics of phytoremediation in nitrogen have proved that it is sustainable and cost-effective in relation to other existing ammoniacal nitrogen remediation technologies. Therefore, it can be deduced that a cheaper and more environmental friendly ammoniacal nitrogen technology can be achieved with the utilization of phytoremediation in wastewater treatment.

Current techniques in rice mill effluent treatment: Emerging opportunities for waste reuse and waste-to-energy conversion

Rice mills release huge volumes of wastewater and other by-products when processing paddy rice. The wastewater often contains toxic inorganic and organic contaminants which cause environmental damage when released. Accordingly, cost-effective techniques for removing contaminants are needed. This article reviews current processes for curbing pollution and also reusing and recycling waste products. Novel techniques exist for converting waste products into energy and value-added products.