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

Unraveling the multifaceted mechanisms and untapped potential of activated carbon in remediation of emerging pollutants: A comprehensive review and critical appraisal of advanced techniques

The rapid global expansion of industrialization has resulted in the discharge of a diverse range of hazardous contaminants into the ecosystem, leading to extensive environmental contamination and posing a pressing ecological concern. In this context, activated carbon (AC) has emerged as a highly promising adsorbent, offering significant advantages over conventional forms. For instance, AC has demonstrated remarkable adsorption capabilities, as evidenced by the successful removal of atrazine and ibuprofen using KOH and KOH-CO2-activated char, achieving impressive adsorption rates of 90% and 95%, respectively, at an initial dosage of 10 mg L-1. Moreover, AC can effectively adsorb aromatic compounds through π-π stacking interactions. The aromatic rings in organic molecules can align and interact with the carbon atoms in AC's structure, leading to effective adsorption. In this review, by employing a systematic analysis of recent research findings (majorly from 2015 to 2023), an in-depth exploration of AC's evolution and its wide-ranging applications in adsorbing and remediating emerging pollutants, including dyes, organic contaminants, and hazardous gases and mitigating the adverse impacts of such emerging pollutants on ecosystems have been discussed. It serves as a valuable resource for researchers, professionals, and policymakers involved in environmental remediation and pollution control, facilitating the development of sustainable and effective strategies for mitigating the global impact of emerging pollutants.

Valorization of Delonix regia Pods for Bioethanol Production

Delonix regia (common name: Flame tree) pods, an inexpensive lignocellulosic waste matrix, were successfully used to produce value-added bioethanol. Initially, the potentiality of D. regia pods as a lignocellulosic biomass was assessed by Fourier-transform infrared spectroscopy (FTIR), which revealed the presence of several functional groups belonging to cellulose, hemicellulose, and lignin, implying that D. regia pods could serve as an excellent lignocellulosic biomass. Response Surface Methodology (RSM) and Central Composite Design (CCD) were used to optimize pretreatment conditions of incubation time (10–70 min), H2SO4 concentration (0.5–3%), amount of substrate (0.02–0.22 g), and temperature (45–100 °C). Then, RSM-suggested 30 trials of pretreatment conditions experimented in the laboratory, and a trial using 0.16 g substrate, 3% H2SO4, 70 min incubation at 90 °C, yielded the highest amount of glucose (0.296 mg·mL−1), and xylose (0.477 mg·mL−1). Subsequently, the same trial conditions were chosen in the downstream process, and pretreated D. regia pods were subjected to enzymatic hydrolysis with 5 mL of indigenously produced cellulase enzyme (74 filter per unit [FPU]) at 50 °C for 72 h to augment the yield of fermentable sugars, yielding up to 55.57 mg·mL−1 of glucose. Finally, the released sugars were fermented to ethanol by Saccharomyces cerevisiae, yielding a maximum of 7.771% ethanol after 72 h of incubation at 30 °C. Conclusively, this study entails the successful valorization of D. regia pods for bioethanol production.

Challenges in developing strategies for the valorization of lignin-a major pollutant of the paper mill industry

Apart from protecting the environment from undesired waste impacts, wastewater treatment is a crucial platform for recovery. The exploitation of suitable technology to transform the wastes from pulp and paper industries (PPI) to value-added products is vital from an environmental and socio-economic point of view that will impact everyday life. As the volume and complexity of wastewater increase in a rapidly urbanizing world, the challenge of maintaining efficient wastewater treatment in a cost-effective and environmentally friendly manner must be met. In addition to producing treated water, the wastewater treatment plant (WWTP) has a large amount of paper mill sludge (PMS) daily. Sludge management and disposal are significant problems associated with wastewater treatment plants. Applying the biorefinery concept is necessary for PPI from an environmental point of view and because of the piles of valuables contained therein in the form of waste. This will provide a renewable source for producing valuables and bio-energy and aid in making the overall process more economical and environmentally sustainable. Therefore, it is compulsory to continue inquiry on different applications of wastes, with proper justification of the environmental and economic factors. This review discusses current trends and challenges in wastewater management and the bio-valorization of paper mills. Lignin has been highlighted as a critical component for generating valuables, and its recovery prospects from solid and liquid PPI waste have been suggested.

Sodium nitrate as a methanogenesis suppressor in earthen separator microbial fuel cell treating rice mill wastewater

The microbial fuel cell (MFC) is one of the sustainable technologies, which alongside treating wastewater, can generate electricity. However, its performance is limited by factors like methanogenesis where methanogens compete with the anode respiring bacteria for substrate, reducing the power output. Thus, sodium nitrate, which has been previously reported to target the hydrogenotrophic methanogens, was used as a methanogenic suppressor in this study. The performance of MFC with and without sodium nitrate was studied during the treatment of rice mill wastewater. A significantly higher power density and coulombic efficiency (CE) were noted in the MFC with sodium nitrate (MFC_T) (271.26 mW/m³) as compared to the control MFC (MFC_C) (107.95 mW/m³). Polarization studies showed lower internal resistance for the MFC_T (330 Ω) as compared to MFC_C (390 Ω). Linear sweep voltammetry and cyclic voltammetry indicated a higher electron discharge on the anode surface due to enhancement of electrogenic activity. Considerable reduction (76.8%) in specific methanogenic activity was also observed in anaerobic sewage sludge mixed with sodium nitrate compared to the activity of anaerobic sewage sludge without any treatment. Due to the inhibition of methanogens, a lower chemical oxygen demand (COD) and phenol removal efficiency were observed in MFC_T as compared to MFC_C. The COD balance study showed an increase in substrate conversion to electricity despite the increase in nitrate concentration. Therefore, selective inhibition of methanogenesis had been achieved with the addition of sodium nitrate, thus enhancing the power generation by MFCs.

Regulation of volatile fatty acid accumulation from waste: Effect of inoculum pretreatment

The study investigates the implications of waste feedstock, inoculum origin, and pretreatment on volatile fatty acids accumulation (VFA). The acidogenic fermentation of the feedstocks, rice mill effluent (RME), and brewery effluent (BE) was studied using untreated and pretreated (cyclic heat-acid shock) brewery anaerobic sludge as inoculum. The pretreatment was successful in refining and stabilizing VFA production from the feedstocks. The fermentation of RME with pretreated sludge had an enhanced acetate yield of 0.37 ± 0.02 mgCOD/mgCOD, even to odd ratio of 20.97 ± 0.08 mg/mg and the highest butyrate yield of 0.39 ± 0.01 mgCOD/mgCOD compared to untreated system. The pretreated system had stability in COD and pH profile, while VFA content depends on the origin of inoculum. Pretreatment inhibited the carbon sinks and augmented acetate-butyrate type metabolism with stable performance. The fermentation of RME by pretreated sludge produced a higher even-numbered VFAs and enhanced even to odd ratio in comparison with fermentation of BE, thereby affecting polymer composition and property. PRACTITIONER POINTS: The pretreated system had stable acidification, chemical oxygen demand, and pH profile. The pretreated system had higher acetate and butyrate yield compared to the untreated system. Rice mill effluent acidified with pretreated sludge had the highest even to odd ratio, 20.97 mg/mg. The even to odd ratio for acidification of brewery effluent was insignificant. Pretreatment, the origin of sludge, and the effluent had a regulatory effect on acidification.

Application of clayware ceramic separator modified with silica in microbial fuel cell for bioelectricity generation during rice mill wastewater treatment

Ceramic separators have recently been investigated as low-cost, robust, and sustainable separators for application in microbial fuel cells (MFC). In the present study, an attempt was made to develop a low-cost MFC employing a clayware ceramic separator modified with silica. The properties of separators with varying silica content (10%-40% w/w) were evaluated in terms of oxygen and proton diffusion. The membrane containing 30% silica exhibited improved performance compared to the unmodified membrane. Two identical MFCs, fabricated using ceramic separators with 30% silica content (MFC_S-30) and without silica (MFC_C), were operated at hydraulic retention time of 12 h with real rice mill wastewater with a chemical oxygen demand (COD) of 3,200 ± 50 mg/L. The maximum volumetric power density of 791.72 mW/m³ and coulombic efficiency of 35.77% was obtained in MFC_S-30, which was 60.4% and 48.5%, respectively, higher than that of MFC_C. The maximum COD and phenol removal efficiency of 76.2% and 58.2%, respectively, were obtained in MFC_S-30. MFC fabricated with modified ceramic separator demonstrated higher power generation and pollutant removal. The presence of hygroscopic silica in the ceramic separator improved its performance in terms of hydration properties and proton transport.

Cereal processing waste, an environmental impact and value addition perspectives: A comprehensive treatise

This review is to describe various methods used for the treatment of cereal processing wastes and their efficiency to reduce environmental issues. Physico-chemical and biological methods have great potential for the treatment of cereal industrial waste. These methods can be used alone or in combination with effective treatment. Physico-chemical treatments are mostly employed for the pretreatment of cereal processing wastes that helps in reduced sludge formation during biological treatments. Biological treatments are mostly used owing to their high efficiency in removing pollutants, and less expensive than physico-chemical treatments. However, these treatments consume more time as compared to physico-chemical treatments. These treatments also proved to be efficient for a high rate of cereal waste conversion into value-added products. Thus, cereal industrial waste can result in value-added products such as biohydrogen, bioethanol, butanol, biogas and biocoal as biofuels, industrial valued enzymes, biomass, biofertilizer, proteins, organic acids, polysaccharides and few others.

Bioconversion of agro-industry sourced biowaste into biomaterials via microbial factories - A viable domain of circular economy

Global increase in demand for food supply has resulted in surplus generation of wastes. What was once considered wastes, has now become a resource. Studies were carried out on the conversion of biowastes into wealth using methods such as extraction, incineration and microbial intervention. Agro-industry biowastes are promising sources of carbon for microbial fermentation to be transformed into value-added products. In the era of circular economy, the goal is to establish an economic system which aims to eliminate waste and ensure continual use of resources in a close-loop cycle. Biowaste collection is technically and economically practicable, hence it serves as a renewable carbon feedstock. Biowastes are commonly biotransformed into value-added materials such as bioethanol, bioplastics, biofuels, biohydrogen, biobutanol and biogas. This review reveals the recent developments on microbial transformation of biowastes into biotechnologically important products. This approach addresses measures taken globally to valorize waste to achieve low carbon economy. The sustainable use of these renewable resources is a positive approach towards waste management and promoting circular economy.

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.

One-step acquirement of superior microbial communities from mesophilic digested sludge to upgrade anaerobic digestion

Anaerobic digestion is a promising waste-to-energy alternative technology. However, the efficiency upgrading for conventional mesophilic digestion of organic solid waste is always indispensable. Employing hyperthermophilic or thermophilic microbial community is one of the viable upgrading alternatives. Given the unavailability of the superior microbial communities, mesophilic digested sludge was used as inoculum, and instantly controlled at 70 °C and 55 °C for acclimation of hyperthermophilic and thermophilic inocula, respectively. Waste activated sludge was continuously and synchronously fed into two digesters. After one round, thermophilic digester achieved stable biogas production rate at 0.22 L L^-1 d^-1, with a methane proportion over 60%, whereas fluctuation was observed in the hyperthermophilic digester, and approximately triple time was needed to reach a relatively stable biogas production rate 0.12 L L^-1 d^-1. Nevertheless, higher hydrolysis ratio 24.4% was observed in the hyperthermophilic digester despite the lower biogas production. Therefore, methanogenesis step limited the whole anaerobic process for the hyperthermophilic digestion, and digestion at 70 °C was appropriate as a pre-fermentation stage to enhanced hydrolysis. The genus Methanothrix proportion in the thermophilic digester gradually decreased, while another acetoclastic genus Methanosarcina ultimately was acclimated to the dominant methanogen. In addition to Methanothrix, hydrogenotrophic archaea became competitive in the hyperthermophilic digester, with Methanothermobacter dominant at 22.6%. The genus Psychrobacter, affiliated to the phylum Proteobacteria could survive better than the others at 70 °C, with a final proportion of 62.5%.

Waste valorization and resource conservation in rice processing industries-an analytical study from Pakistan

The study aims to analyze and enhance the eco-efficiency of rice processing in Pakistan while focusing on pollution prevention, waste management, and valorization opportunities. Three rice mills were selected, and physical and chemical properties of their wastewater and solid waste were analyzed. It is depicted that rice husk has the highest share in the solid waste stream and possesses a significant renewable energy potential with a calorific value of 16 MJ/kg and a much lower energy cost of about 0.4 USD/MJ as compared to diesel and furnace oil, i.e., 36.8 and 26.2 USD/MJ, respectively. Recovery of commercially valuable by-products from the effluent of parboiling process is also investigated, and about 0.25 kg of starch/L from wastewater has been effectively reclaimed by wet milling method. The effluent water is also analyzed using Streeter-Phelps model which confirmed that value of contaminants in the wastewater of two of the selected rice mills exceeds the dilution capacity of the receiving body, with dissolved oxygen values at critical level as 0.54 mg/L and -20.7 mg/L respectively. Water pinch analysis and water cascade analysis (WCA) have also been employed to monitor and manage the water footprints. While the concentration composite curve depicted that about 0.8 ton/h of freshwater is saved by an application of smart water integration and pinch point was determined as 1814 ppm. While the WCA confirms that a total water demand of 45.3 kg/s at the third purity level has been reduced to 28.0 kg/s at the lowest purity level.

Phycoremediation of paddy-soaked wastewater by indigenous microalgae in open and closed culture system

Phycoremediation; biotransformation of nutrients and or pollutants by microalgae, supports sustainable wastewater treatment, coupled with biomass production, resulting in enhanced cost savings, waste minimization, and energy conservation. A major challenge in this technique involves cultivation system to be adopted, mode of treatment and harvesting methods. Three different algal cultivation systems; polybags (PB), photobioreactors (PBR) and race way ponds (RWP) were employed for culturing an indigenous microalga, Scenedesmus obliquus in rice mill paddy-soaked wastewater (PSW). The maximum biomass productivity (BP) of 340 ± 2 mg/L/d was observed in PBR with an ammonical nitrogen (NH₃-N) removal (R_N) of 96.12 ± 0.21% and the phosphates (PO₄-P) removal (R_P) of 97.58 ± 0.18%. Highest lipids (L)∼12% (% biomass), protein (P)∼40% and carbohydrates (C)∼20%) were gained in PBR culture system, followed by RWP and PB.

Phytotoxicity Evaluation of Wastewater from Rice Parboiling

The aim of this study was to evaluate the phytotoxicity of raw and treated effluents from a parboiling rice industry in the south of Brazil. The evaluation included 19 physicochemical parameters and a phytotoxicity test using lettuce and cucumber seeds as bioindicators. The germination index (GI) of both seeds was decreased when in contact with the raw effluent (p < 0.05) and with the treated effluent reduced the GI of cucumber seeds (p < 0.05) and the root length (RL) of lettuce seeds (p < 0.05). For lettuce seeds exposed to the treated effluent there was a negative correlation (p < 0.05) between RL and Mn (- 0.82) and between GI and Fe (- 0.68). For cucumber seeds in the raw effluent, the Fe showed negative correlation (p < 0.05) with seed germination (- 0.75), but, in the treated effluent, their GI presented positive correlation (0.61) with the ammoniacal-N (p < 0.05). The results indicated that both effluents showed phytotoxicity for lettuce and cucumber seeds.

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.