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

Chemosphere 164 (2016) 404e412

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Review

Current techniques in rice mill effluent treatment: Emerging opportunities for waste reuse and waste-to-energy conversion Anuj Kumar a, Rashmi Priyadarshinee b, Abhishek Roy a, Dalia Dasgupta b, Tamal Mandal a, * a b

Department of Chemical Engineering, National Institute of Technology, Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India Department of Biotechnology, National Institute of Technology, Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Possible solutions for the minimization of waste and harvesting of resources.
Emergence of techniques for the generation of energy and valueadded products.
Co-existence of rice mill wastewater treatment along with solid waste management.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 November 2015 Received in revised form 25 August 2016 Accepted 25 August 2016

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. © 2016 Elsevier Ltd. All rights reserved.

Handling Editor: Xiangru Zhang Keywords: Rice mill wastewater Value-added products Parboiled rice Bio-hydrogen

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Parboiling process and rice mill wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 2.1. Parboiling process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 2.2. Characteristics of rice mill wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406

* Corresponding author. E-mail addresses: [email protected] (A. Kumar), [email protected], [email protected] (T. Mandal). http://dx.doi.org/10.1016/j.chemosphere.2016.08.118 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

A. Kumar et al. / Chemosphere 164 (2016) 404e412

3.

4.

5. 6.

405

Various treatment processes for rice mill effluent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 3.1. Physicochemical treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 3.2. Microbial treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 3.3. Phytoremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 Valorisation of wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 4.1. Waste to energy and nutrient recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 4.1.1. Microbial fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 4.1.2. Hydrogen and methane production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 4.2. Waste-to-waste treatment technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

1. Introduction The social, environmental and economic impacts of various industries have led to inevitable conflicts between industrial progress and environmental sustainability (Manogari et al., 2008; Mukherjee et al., 2015a). Waste effluents have detrimental impacts on biodiversity, primarily due to their mobile nature. When they are discharged into natural water bodies without substantial remediation, there can be serious, adverse impacts (G
alvez et al., 2003; Alderson et al., 2015). Rice production is a rapidly growing industry that plays a dominant role in the world economy. Accordingly, rice mill wastewater remediation is an important issue. Rice is the most common staple food consumed throughout the world. Rice mill numbers are expanding to meet the increasing demand for food resulting from human population growth. Paddy processing mills generate huge quantities of liquid waste in the form of rice mill effluent. Discharge of this effluent poses potential threats to the environment and, therefore, is a problem that requires a solution. Rice (Oryza sativa) is an integral part of the diet of about one half of the global population. It contributes about 21% of global human per capita food energyand 15% per capita protein. Paddy fields occupyabout 9% of the planet's total arable land, which is indicative of the importance of rice (Maclean et al., 2002). According to the Food and Agriculture Organization (FAO), global rice production has been monotonically increasing for many decades. Some 599 million tonnes were produced in 2000, increasing to 672 million tonnes in 2010 (IRRI, 2012) and 721.4 million tonnes in 2011e2012. The latter figure is equivalent to 481.2 million tonnes of milled rice (FAO, 2012). In order to produce edible rice, paddy (rice with husk) is milled a process which removes the husk and part of the bran. Parboiled rice is prepared by a process of soaking, cooking and drying of paddy before milling. Preferences for parboiled rice result from its traditional taste, non-sticky texture and rich nutritional characteristics (Kato et al., 1983; Unnikrishnan and Bhattacharya, 1987; Heinemann et al., 2005). Paddy crops that have undergone flooding while harvesting exhibit breakage during milling (Bhattacharya, 2011). In order to combat this, parboiling is used to increase head rice yield (Oli et al., 2014). Parboiled rice production usually consumes huge amounts of water, as the paddy requires soaking. In this technique, paddy is partially boiled at 70  Ce100  C. Water soaked up during the process is about 1.25 times the weight of parboiled paddy. Wastewater is generated at about 1e1.2 L per kg of paddy (Rajesh et al., 1999). Disposal of this wastewater is a serious environmental concern. Disposal on land is a common practice which results in nutrient overload in soil (Kim et al., 2008) and substantial surface and ground water quality degradation. Improper treatment, followed by discharge into nearby water bodies such as rivers, lakes and

canals with sluggish movement can cause eutrophication, odour problems and many other adverse effects (Karunaratne, 2010; Shrivastava and Soni, 2011). Globally, rice is an important staple food for most people. Some 90% of rice is produced and consumed in Asia alone. In 2011e2012, India produced roughly 22% of the world's rice. Brazil is one of the largest producers of rice in the world. Parboiling processes account for 20% of the total processing practices employed in Brazilian rice mills. After parboiling, the effluents are generally loaded with high pollutant content, particularly organic materials and nutrients (Santos et al., 2012). Sri Lankan rice mills discharge nearly 75.5  103 L of rice mill effluent per metric tonne of soaked paddy without carrying out any treatment of the effluent (Mukherjee et al., 2015b). In India, the total volume of effluent released from each rice mill reaches 20  106 L per annum (Varshney, 2012). Therefore, the approximately 57,850 parboiled rice mills in India discharge up to 11.57  109 L per annum (CPCB, 2008). Rice mills in India vary widely in their capacities (Choudhary et al., 2015). Indian pollution control agencies have provided norms regarding discharge parameters for rice mill wastewater (CPCB, 2008). Meeting the disposal standards is of utmost importance, so the necessary treatment is carried out (Mukherjee et al., 2015a). The Environment (Protection) Act of India, 1986, under the Central Pollution Control Board (CPCB), proposed that every rice mill should have a fully functional Effluent Treatment Plant (ETP). This should essentially consist of a biological treatment process (CPCB, 2007). However, due to the high cost of establishment and maintenance of ETPs, the majority of rice mills in India openly disregard the CPCB guidelines (Business Standard, 2015; Paul et al., 2015). It is unfortunate that the treatment of contaminant-laden rice mill wastewater has scarcely been investigated. Rice mill effluent is primarily yellowish in colour, has a repulsive, pungent odour and consists of organic matter and other impurities. Chemical oxygen demand (COD) components such as phenol, lignin, cellulose and other humic substances are invariably present in the rice mill wastewater and are considered as potential threats to the environment (Yusoff, 2006). This article critically reviews existing technologies for the abatement of rice mill wastewater. While considering rice production and its environmental and economic aspects, the process and design improvements that could potentially resolve the problems of rice mill effluent are highlighted. 2. Parboiling process and rice mill wastewater 2.1. Parboiling process Traditional methods of parboiling vary from one geographical

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Fig. 1. Process of parboiled rice production and source of wastewater (Asati, 2013).

region to another, and also depend on the operational technology employed (Araullo et al., 1976). Rice in parboiled form is produced, consumed and exported from different Asian countries. Nigeria, Ghana, Egypt, Niger and Benin, in Africa, are also important producers of rice. They commonly use parboiling as a technique for avoiding malnutrition, food shortages and food losses (Tomlins et al., 2005). There are several key steps involved in the production of rice. Paddy is hydrated to a moisture level of 24e30%. Thermal treatment completes gelatinisation and dehydrates the paddy to the optimum level for milling. Varying degrees of thermal treatment result in different parboiling techniques, thereby lead to production of rice with diverse qualities. The process needs to be optimised to achieve maximum yield and enhance product quality before the establishment of a mill (Bhattacharya, 1985; Oli et al., 2014). Parboiled rice generally has a yellowish-brown colour which may be due to the dispersion of colour from husks. In addition, it has a high mechanical strength, making it less prone to breakage during milling. It also has enhanced vitamin B content (Oli et al., 2014). As shown in Fig. 1, raw paddy is washed to remove impurities like sand, gravel and dirt. Screened paddy is then put into a waterfilled cylindrical container for parboiling. Steam is passed and the hot water is continuously recycled to ensure that a temperature of 70e100  C is maintained for up to 4 h (Asati, 2013). The process of parboiling is season dependent. It is done at 90e100  C during the rainy season and at 70e80  C during the dry summer season. Remaining water is drained off which contains organic and

inorganic impurities. Hence, drained water requires remediation before discharge.

2.2. Characteristics of rice mill wastewater The wastewater generated contains a wide range of organic and inorganic toxic contaminants. Table 1 lists the characteristics of rice mill effluent as reported by earlier studies. Variations of pH (4.5e8.5) are encountered owing to different paddy characteristics, the parboiling process and the quality of water used. Suspended solids (0.3e166 mgL1) increase both biochemical oxygen demand (BOD) and chemical oxygen demand (COD), which pose a serious threat to aquatic life by occluding sunlight and hampering photosynthesis (Chowdhury et al., 2010). Rice mill effluent is mainly comprised of lignin, phenol and colour components that enhance the COD of the effluent (Behera et al., 2010; Kumar et al., 2015a; Kumar et al., 2016b). Lignin is a complex, heterogeneous and biorefractory organic pollutant present in the effluent which poses a major challenge for chemical and biological processes (Kumar et al., 2015b). Phenol is also a noxious organic toxin even at relatively low concentrations. Phytotoxicity assays of rice mill wastewater have been performed on the seeds of Vigna radiate, which exhibited inhibited root and shoot lengths. Acute toxicity has been assessed in the fish Lebistes reticulatus in terms of LC50 values (lethal concentration required to kill 50% of the population (Giri et al., 2016). Furthermore, parboiled rice effluent has been shown to lower the sperm quality of zebrafish,

Table 1 Reported qualitative characteristics of rice mill wastewater. pH

BOD (mgL1)

COD (mgL1)

TDS (mgL1)

TSS (mgL1)

Lignin (mgL1)

Phenol (mgL1)

Nitrogen (mgL1)

References

4.22e5.51 5.1 7.6 4.8 7.5 8.4 4.7 4.0e4.3 4.5 7.1

e 6900 e e 1200 36 1089 e 2401.2 510

2578e6480 18,600 2578e5022 1708 1350 1400 1931 2200e2250 2886.2 1600

e 24,720 e 1578 700 4.64E-009 3010 e 1773 1.4

e 49,140 e e 1100 e e e e 1.7

e e e 182 e e e 80.3e87.6 e e

e e e 16.21 e e e 14.8e16.5 e e

25.4e95.04 3100 25.4e50.4 e e e 36.7 e e 154

Queiroz et al., 2007 Ramprakash and Muthukumar, 2015a Bastos et al., 2015 Kumar et al., 2015a Asati, 2013 Manogari et al., 2008 Mukherjee et al., 2015a Behera et al., 2010 Choudhary et al., 2015 Abinandan et al., 2015

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407

Table 2 Performance of different treatment systems for rice mill wastewater treatment. Process

Adsorption a. Chitosan b. ADC (Rice husk ash without silica) c. ADS (Rice husk ash containing silica) Biodegradation Electrocoagulation a.Electrocoagulation using aluminium electrodes b.Continuous electrocoagulation method (CEC) using stainless steel electrodes c.Electrocoagulation using Fe-Fe electrode. Biomethanation Phytoremediation Microbial Fuel cells

Parameter

Other parameters

Removal (%)

References

768

95

e e BOD

e e 36

e e 55.34

Thirugnanasambandham et al., 2013 Kumar et al., 2016b Kumar et al., 2016b Manogari et al., 2008

TSS TSS

3888 768

97 89

Shrivastava and Soni, 2011 Karichappan et al., 2013

BOD e BOD

2401.24 e 1089 ± 122

Choudhary et al., 2015 Rajesh et al., 1999 Mukherjee et al., 2015a

Lignin

80.3e87.6

phenol

14.8e16.5

76.27 e 85(RFPa) 83(RTWb) 84(MFC1) 83(MFC2) 81(MFC1) 77(MFC2) 97.6(a) 98.3(b) 90.3(a) 92(b) 96.2 81.4

COD (mgL1)

Removal (%)

Type

Initial (mgL1)

2200

98

TSS

3388 3388 1400

48.1 39.3 78

1628.8 2200

96 97

2886.24 70.2 2400 78 1931 ± 212 79(RFP) 76(RFW) 2200e2050 96.5(MFC1) 92.6(MFC2)

Algal treatment a.(C. pyrenoidosa) b.(S. abundans)

e e e

e e e

phosphate 360

Integrated treatment

1708

86

lignin phenol

a b

182 16.21

Behera et al., 2010

Abinandan et al., 2015

Kumar et al., 2015a

(Rice mill effluent: Facultative Pond water), termed as RFP. (Rice mill effluent: Tap water), termed as RTW.

subsequently reducing reproductive rates (Gerber et al., 2016). Therefore, effective and efficient treatment technology for rice mill effluent is essential for its safe and sustainable release to the environment. 3. Various treatment processes for rice mill effluent Rice mill wastewater is characterised by high concentrations of BOD, COD and colour, which adversely affect natural waterbodies. Table 2 summarises the performance of different rice mill wastewater treatment systems that have been reported in recent studies. In the present study, several physicochemical treatment technologies pertaining to rice mill wastewater mitigation are reviewed.

disadvantage of generating sludge. The performance of electrocoagulation is superior compared to conventional treatment processes. Choudhary et al. (2015) reported that the use of a Fe-Fe electrode combination resulted in decent rates of organic matter removal. The process involves electrolysiselectrolytic reactions at electrode surfaces, formation of coagulants in an aqueous phase, followed by colloidal adsorption of pollutant particles. The removal of pollutants occurs by electrofloatation, adherence to bubbles and sedimentation (Fig. 2). In

3.1. Physicochemical treatment Contaminant-laden discharge needs to be mitigated, or else may cause severe damage at the release site. Rice mill wastewater is mainly composed of organic matter such as BOD, COD, phenol, lignin and cellulose that may cause serious damage to the environment. Several remediation techniques are employed, which include chemical treatments such as adsorption. In an investigation by Thirugnanasambandham et al. (2013), significant reductions in COD (98%) and TSS (Total suspended solids; 95%) were achieved through a chitosan-mediated adsorption process. However, the high price of chitosan is a serious drawback to its practical implementation. The utilisation of rice husk ash, which is also a waste product of rice mills and is abundantly available for the treatment of rice mill wastewater, is a potentially sustainable solution. However, the treatment of wastewater from carbonised and modified rice husk ash only showed moderate efficiency (48.1%) (Kumar et al., 2016b). Asati (2013) remarked that treatment of rice mill wastewater could also be carried out by means of a coagulant and aeration system. However, the aforementioned treatments have the critical

Fig. 2. Schematic diagram of a typical electrocoagulation unit (Choudhary et al., 2015).

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Fig. 3. Experimental set up of UASB reactor and two-stage biomethanation process (Rajesh et al., 1999).

batch experiments, electrocoagulation techniques using aluminium electrodes removed 96% of COD and 97% of TSS from rice mill effluent (Shrivastava and Soni, 2011). In a continuous experiment, stainless steel electrodes removed 70.2% of COD and 76.27% of BOD (Karichappan et al., 2013). However, these methods suffered from serious drawbacks, such as sludge generation, which would complicate the large-scale operations. 3.2. Microbial treatment Biological wastewater treatment systems are less expensive, require less energy, and produce lower amounts of secondary pollutants than the previously discussed techniques. A study conducted by Manogari et al. (2008) investigated the biodegradation of rice mill effluent by Pseudomonas sp. It achieved a reduction of 86.4% in COD, using a packed-bed system of immobilised cells with a cell loading of 1.7e2.2 mg cells per bead. Rajesh et al. (1999) evaluated the performance of bioreactors for the remediation of rice mill wastewater. The bioreactor employed a two stage biomethanation process using upflow anaerobic sludge blanket (UASB) reactors for the remediation of rice mill wastewater. Fig. 3 illustrates the tremendous capacity for wastewater stabilisation, with removal efficiencies of 89% BOD and 78% COD, and organic and hydraulic loading of 3 kgm3d1 and 22 mL min1, respectively. However, this microbial-based bioremediation often failed to fulfil the requirements of practical on-field administration. Treatment of parboiled rice mill effluents using cyanobacteria Aphanothece microscopica Nageli in a stirred batch reactor have been determined (Queiroz et al., 2007). The kinetic investigation estimated 83.44% and 72.74% reductions in COD and total nitrogen (N-TKN), respectively, after 15 h, which reflects the remediation efficiency. Santos et al. (2012) analysed the bioremediation potential of a methylotrophic yeast Pichiapastoris X-33 to treat parboiled rice effluent. The cultivation media was supplemented with glycerol which was collected as a by-product from the biodiesel industry. This strategy showed comprehensive mitigation of COD, total Kjeldahl nitrogen (TKN) and phosphorus (in phosphate form). In addition to remediation, the study reported the fruitful utilisation of a by-product of the biodiesel industry. Lopes et al. (2001) described the successful denitrification of parboiled rice effluent on top of a UASB, which constituted an anoxic zone. This eliminated the requirement of an additional denitrification reactor. The treatment process demonstrated 80% removal of total nitrogen (N-TKN).

3.3. Phytoremediation In addition to chemical and microbial treatments, another mode of treatment called phytoremediation is an emerging technology for the treatment of wastewater. It is effective and has been used in developing countries due to its inherent qualities of being simple, suitable, sustainable and economical. A laboratory-based batch study of the free-floating aquatic plant, water lettuce (Pistia stratiotes), showed tremendous capability for COD, BOD, ammoniacal nitrogen, nitrate nitrogen and soluble fertiliser removal. The only drawback is the requirement of a relatively large area of land for purification, and a relatively long processing time (Mukherjee et al., 2015a). In a similar effort, Abinandan et al. (2015) worked on nutrient removal using microalgae Chlorella pyrenoidosa and Scenedesmus abundans, which displayed growth (measured in terms of chlorophyll content) of 3.88 mgL1 and 5.55 mgL1, respectively. They reduced phosphate by up to 98.3% and 97.6%, and ammoniacal nitrogen by 92% and 90.3%, respectively (Chowdhury et al., 2010). In summary, there are several wastewater remediation systems which vary in their efficiency, economic viability and ability to recover valuable by-products. 4. Valorisation of wastewater Wastewater treatment systems have experienced a paradigm shift. Whereas they once merely aimed to remove pollutants, they now also attempt to recover resources while minimising energy consumption. These recovery technologies are known as water resource recovery facilities (WRRFs) (Gude, 2016). Environmental concerns emphasise the recovery or reuse of wastes generated from industry. Waste recycling is not only important environmentally, but also economically, as it affects the running costs of treatment plants. Additionally, the reuse of wastes furnishes a viable solution to the raw material availability that could be harnessed for the generation of energy, fuel and chemicals. The potential utility of wastewater for the recovery of various value-added resources and waste-to-waste green technology has been reviewed and summarised. 4.1. Waste to energy and nutrient recovery The primary aim of conventional wastewater treatment is to remove pollutants in order to meet discharge standards, and subsequent stabilisation of the sludge. Continuous depletion of fossil

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409

Fig. 4. Strategy designed for silica extraction and simultaneous rice mill wastewater treatment procedure (Kumar et al., 2015a).

fuels, environmental degradation due to pollution, and water and resource scarcity require significant efforts to meet modern sustainability requirement. Waste-to-energy conversion is a process of generating energy from waste, ultimately reducing waste disposal and providing an alternative form of energy. 4.1.1. Microbial fuel cells Microbial fuel cells (MFCs) are a sustainable technology with the potential to generate electricity while eliminating contaminants from organic wastewaters (Pant et al., 2010; He et al., 2015). MFCs are bio-electrochemical devices in which electricity is produced via microorganisms that perform single-step conversion of organic substances in an anaerobic environment. The microbes in the anodic compartment generate protons and electrons from the oxidation of organic matter, releasing CO2 and biomass as waste products. Behera et al. (2010) utilised a rice mill wastewater substrate to develop microbial fuel cells for the abatement of contaminants in rice mill effluent. The study illustrated the promising features of earthen pot MFCs in terms of electricity production and simultaneous reduction of organic matter. They operated at an industrially-relevant feed pH which effectively contributed to a MFC fabrication alternative for the treatment of industrial wastewaters in an economical manner. 4.1.2. Hydrogen and methane production Hydrogen is a non-polluting, high energy and recyclable fuel. The combustion of hydrogen generates 2.75 times more energy than hydrocarbon fuels, with water as the only end product

(Demirbas et al., 2011). In a recent study, Ramprakash and Muthukumar (2014) demonstrated bio-hydrogen production from carbohydrate-rich rice mill effluent using E. aerogenes and C. ferundii. The results revealed efficient enzymatic hydrolysis. Production of hydrogen gas from rice mill wastewater reduced COD by 71.8%. Furthermore, enhanced hydrogen production was achieved by combined acid and enzymatic processes (Ramprakash and Muthukumar, 2015a). However, in another study by Ramprakash and Muthukumar (2015b), improved bio-hydrogen production was achieved with a mutant strain of E. aerogenes, compared to the wild strain. As per Wang et al. (2011), hydrogen and methane production from domestic and industrial effluents requires a twostage fermentation process that could utilise organic waste for energy production or accelerate the treatment efficiency of residues, thus facilitating improvements in the economic feasibility of waste treatment. Peixoto et al. (2012) demonstrated co-production of H2 and CH4 from rice mill effluent mixed with microflora through a two-stage batch fermentation process. The process observed adequate production of hydrogen and reasonable generation of methane from parboiled rice wastewater. Bastos et al. (2015) utilised the cyanobacterium Aphanothece €geli for the bioremediation of nitrogen- and organic microscopia Na matter-laden parboiled effluent, which led to single-cell protein production. Single-celled proteins can be important food supplements. As such, this process was beneficial in terms of nutrient recovery as well as reducing the pollution load. Henceforth, there is substantial potential to reduce contaminants while simultaneously generating energy. However, achieving

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Fig. 5. A schematic representation for the treatment of rice mill wastewater using efficient utilisation of rice husk ash (Kumar et al., 2016b).

high rates of productivity and efficiency are key challenges that need to be overcome to allow successful practical implementation.

4.2. Waste-to-waste treatment technology Besides rice mill wastewater, rice husk (RH) and rice husk ash (RHA) comprise the major waste products of paddy processing in rice mills. In current practice, most of these by-products are disposed of in landfills or burnt. These kinds of disposal processes have severe drawbacks, including increased maintenance expenses, damage to the environment and destruction of the hidden value added feature of the by-products. Henceforth, it is advantageous to search for alternative processes which reuse rice husk and rice husk

ash. The composition of RHA is approximately 95% silica (by weight). Many recent studies have identified RHA as a potential costeffective source of silica (Della et al., 2002; Kumar et al., 2016a). The extraction process involves removal of metallic impurities by refluxing RHA with either nitric acid or hydrochloric acid (Adam et al., 2013; Chang et al., 2006). The acid treated ash is directly converted to sodium silicate and then into silica by the addition of mineral acid. There are two basic reactions involved in silica extraction (Lu and Hsieh, 2012; Soltani et al., 2015):

SiO2 ðashÞ þ 2NaOH / Na2 SiO3 þ H2 O

Fig. 6. A simplified route depicting the process flow diagram for the production of Brown rice husk ash Fe catalyst (BRHF) from rice husk ash (Kumar et al., 2016c).

(1)

A. Kumar et al. / Chemosphere 164 (2016) 404e412

Na2 SiO3 þ H2 SO4 / SiO2 þ Na2 SO4 þ H2 O

(2)

Rice husk is traditionally burned to fuel the rice mill boilers, producing rice husk ash (Ayswarya et al., 2012). The enormous quantities of RHA produced could be used as an adsorbent for the treatment of rice mill wastewater (Kumar et al., 2015a). Kumar et al. (2015a) developed a cost-effective route for rice mill wastewater treatment while simultaneously generating silica as a value-added product (Fig. 4). They demonstrated the utilisation of one waste product for the abatement of another by using RHA for silica production and synthesis of a wastewater contaminant adsorbent. They employed MgCl2 as a coagulant, owing to its attributes of recyclability and recoverability from sludge when precipitated by a coagulationeflocculation process (Chaudhary et al., 2002). The continuous recycling of MgCl2 makes the flocculation process highly economical (Liao and Randtke, 1986; Gao et al., 2007). With a different approach, the same research group (Kumar et al., 2016b) prepared two types of adsorbents - the first from raw RHA containing naturally occurring silica and carbon (called ADS: rice husk ash containing silica) and the second synthesised from RHA leftover after silica extraction (called ADC: rice husk ash without silica; Fig. 5). Both distinctly prepared adsorbents were then individually used for the treatment of rice mill wastewater. The adsorbent without silica was found to be more effective in decreasing pollutants, making it the more useful material. The study measured overall costs and found the treatment process to be relatively eco-benign compared to ones using commercially available activated carbon. Furthermore, the successful conversion of RHA into recoverable and reusable silica supported catalyst which effectively performs the treatment of rice mill wastewater was done as depicted in Fig. 6 (Kumar et al., 2016c). The process reduced more than 75% COD and used the treated wastewater to germinate mung seeds. This study showed an economically suitable pathway by using silica to produce the catalyst, which was not only effective but also reusable (Kumar et al., 2016c).

5. Future prospects The success of emergent green technologies lies in achieving the ideals of low waste generation, reduced pollution load in effluent, minimization of hazardous chemical usage, and conversion of solid wastes into value-added products. The integration of wastewater treatment along with solid waste management and resource recovery could prove fruitful in the future. Although there are examples of processes that produce secondary energy and value-added products, these require process intensification and direct application in industry for them to become viable. Wastewater remediation approaches that consider an integrated plant for nutrient recovery are very few in number. It is well known that a process achieving nutrient recovery as well as wastewater treatment is expected to be sustainable and economical. Limited numbers of studies are available and most have insufficiently documented the strategies required to solve the problem. As such, intense effort is required to upscale experimental processes to commercial rice mills. Treatment plants extensively employ integrated techniques involving a cascade of various processes. Optimisation processes should facilitate better efficiency for the plausible combination of treatment techniques. Though lesser amounts of secondary by-products are generated, proper disposal should be taken into account. Nevertheless, further research is essential to furnish a complete solution for the abatement of rice mill wastewater while simultaneously achieving good environmental outcomes.

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6. Conclusion Successful and cost-effective treatment of wastewater and the safe discharge of sludge are necessary if rice mills are to meet the increasingly stringent regulations of law, health and environment sustainability. Conventional chemical treatment techniques have several drawbacks pertaining to the generation of nondischargeable sludge, which in most cases causes environmental problems such as eutrophication and bio magnification. Research should focus on creating new processes of wastewater treatment that are integrated, economic and sophisticated enough to treat a wide range of wastewater. Additionally, it should aim to minimise sludge generation and recover value-added products. Developments in this direction will improve sustainability and promote the reuse of waste products. Acknowledgment The authors are thankful to the Ministry of Human Resources and Development (MHRD), Government of India, for financial support. The authors would also like to thank Miss Jhilly Dasgupta, Chemical Engineering Department, National Institute of Technology Durgapur, West Bengal, for immense support and cooperation during the execution of this work. References Abinandan, S., Bhattacharya, R., Shanthakumar, S., 2015. Efficacy of Chlorella pyrenoidosa and Scenedesmus abundans for Nutrient removal in rice mill effluent (Paddy soaked water). Int. J. Phytorem. 17, 377e381. http://dx.doi.org/10.1080/ 15226514.2014.910167. Adam, F., Appaturi, J.N., Khanam, Z., Thankappan, R., Nawi, M.A.M., 2013. Utilization of tin and titanium incorporated rice husk silica nanocomposite as photocatalyst and adsorbent for the removal of methylene blue in aqueous medium. Appl. Surf. Sci. 264,718e726. http://dx.doi.org/10.1016/j.apsusc.2012.10.106. Alderson, M.P., Santos, A.B.D., Filho, C.R.M., 2015. Reliability analysis of low-cost, full-scale domestic wastewater treatment plants for reuse in aquaculture and agriculture. Ecol. Eng. 82, 6e14. Araullo, E.V., De Pauda, D.B., Graham, M., 1976. Rice: Post Harvest Technology. International Development Research Centre, Ottawa, Canada K1G 3H9, pp. 163e204. Asati, S.R., 2013. Treatment of wastewater from parboiled rice mill unit by coagulation/flocculation. Int. J. Life Sci. Biotechnol. Pharma. Res. 3, 264e277. Ayswarya, E.P., Francis, K.F.V., Renju, V.S., Thachil, E.T., 2012. Rice husk ash e a valuable reinforcement for high density polyethylene. Mater. Des. 41, 1e7. Bastos, R.G., Bonini, M.A., Zepka, L.Q., Lopes, E.J., Queiroz, M.I., 2015. Treatment of rice parboiling wastewater by cyanobacterium Aphanothece microscopica Nageli with potential for biomass products. Desalin.Water. Treat. 56, 1e7. http:// dx.doi.org/10.1080/19443994.2014.937758. Behera, M., Jana, P.S., More, T.T., Ghangrekar, M.M., 2010. Rice mill wastewater treatment in microbial fuel cells fabricated using proton exchange membrane and earthen pot at different pH. Bioelectrochemistry 79, 228e233. http:// dx.doi.org/10.1016/j.bioelechem.2010.06.002. Bhattacharya, K.R., 1985. Parboiling of rice. In: Juliano BO9 (Ed.), Rice: Chemistry and Technology, 2ndedn. American Association of Cereal Chemists, St. Paul, pp. 289e348. Bhattacharya, K.R., 2011. Rice Quality, first ed. Wood head publishing Limited, Cambridge, UK. Business Standard, 2015. Rice Mills: HC Refuses to Extend Deadline for Getting Pollution. Press Trust of India, Chennai (accessed 8.03.15.). Central Pollution Control Board, 2008. Comprehensive Industry Documents on Pulse, Wheat, Rice Mills. New Delhi (India): Ministry of Environment and Forest. Govt. of India Report No. 76. www.cpcb.nic.in. Central Pollution Control, Board New Delhi, 2007. Guidelines on Effluent Characteristics (accessed 10.09.15.). Chang, F.W., Yang, H.C., Roselin, L.S., Huo, W.Y., 2006. Ethanol dehydrogenation over copper catalysts on rice husk ash prepared by ion exchange. Appl. Catal. A Gen. 304, 30e39. http://dx.doi.org/10.1016/j.apcata.2006.02.017. Chaudhary, D.S., Jollands, M.C., Cser, F., 2002. Understanding rice hull ash as fillers in polymers: a review. Silicon Chem. 1 (4), 281e289. Choudhary, M., Majumder, S., Neogi, S., 2015. Studies on the treatment of rice mill effluent by electrocoagulation. Sep. Sci. Technol. 50, 505e511. Chowdhury, P., Viraraghavan, T., Srinivasan, A., 2010. Biological Treatment processes for fish processing wastewater- A review. Bioresour. Technol. 101, 439e449. http://dx.doi.org/10.1016/j.biortech.2009.08.065. Della, V.P., Kühn, I., Hotza, D., 2002. Rice husk ash as an alternate source for active

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