Simultaneous recovery of silica and treatment of rice mill wastewater using rice husk ash: An economic approach

Highlights

• Adsorptive treatment of rice mill wastewater with rice husk ash, another waste generated from rice mill.

• Enhancement of contaminants removal was achieved by the use of MgCl₂ and its reusability was explored.

• Extraction of value added silica and its characterization using SEM, XRD and FTIR.

Abstract

An economical, effective and eco-friendly route for utilizing agricultural byproduct rice husk ash (RHA) in the treatment of rice mill wastewater has been investigated in the present study. Precious silica was extracted as value added product from RHA and characterized by using SEM, FTIR and XRD. The basic carbonaceous RHA residual was then used for the abatement of rice mill wastewater as an adsorbent with increasing its pH. Adsorptive process exhibited the percentage reduction of color, lignin, phenol and COD were 69.38, 73.43, 66.07 and 67.85, respectively from rice mill effluent. The batch adsorption kinetic study of this basic adsorbent fitted well to pseudo second order model. Additional removal of pollution parameters was achieved by employing MgCl₂ as coagulant. For this, MgCl₂ was added in the wastewater at a dose of 12 g/L at pH 12. This enhanced the percentage removal of lignin, color, phenol and COD by 75.21, 75.38, 40.29 and 40.14, respectively. In further study, this coagulation process was augmented with the use of Ca(OH)₂ along with MgCl₂. The proposed study was made more economical by recovering MgCl₂ from the coagulation sludge for repeated application. Reusability of MgCl₂ was explored for up to four recycles with the objective of improving the entire work economically viable.

Introduction

Cleaner and better environment is a necessity for healthy living conditions in the present adverse environmental scenario (Pooja and Anupam, 2012, Yusoff, 2006). For the last couple of decades, rigorous attention has been concentrated in the various aspects of pollution control and its subsequent treatment strategies. Generation of waste effluent is a subject of great concern, which generally causes deleterious impact on biota owing to its mobile nature. Without any proper treatment highly contaminated effluent are often discharged in natural water bodies (Alderson et al., 2015, Gálvez et al., 2003). In this context, rice mill wastewater in particular is worth mentioning. The demand of rice is increasing for a rapidly growing human population that includes rice as a major ingredient of their staple diet. The food and agriculture organization (FAO) statistical data suggests that the world rice production is approximately 697.9 million tons in the year 2010 (Ayswarya et al., 2012). Rice mill wastewater is toxic that laden with high proportion of inorganic and organic pollutants (Rajesh et al., 1999). This explains the urges to critically devise scheme toward mitigating the intense levels of COD and dark coloration before their discharge into the environment (Yusoff, 2006).

Besides milling of rice, a conventional rice mill performs diverse operations like quality control, procurement, storage, drying and finally, it make sure that it can utilize its byproducts efficiently (Rajesh et al., 1999). Out of several activities, soaking of paddy for parboiled rice production consumes huge amount of water. It leads to the discharge of 1.0–1.2 L waste effluent per kilogram of processed paddy (Rajesh et al., 1999). Little attention has been paid about the use of anaerobic methods and other physico-chemical methods like adsorption, chemical oxidation for the treatment of rice mill wastewater (Acimovic et al., 2003, Tan et al., 2000). The real rice mill wastewater being acidic in nature (pH 4.5–5.5) requires extensive pH adjustments prior to biological treatments (Rajesh et al., 1999, Thirugnanasambandham et al., 2013).

Rice mill wastewater is characterized by its yellowish color and pungent irritating odor. Chemical oxygen demand (COD) contributing components such as phenol, lignin and humic substances present in the rice mill wastewater impart a potential threat to the environment (Behera et al., 2010). Thus, there is a rising concern for rice mill wastewater treatment prior to discharge into the environmental sink. COD can be subdued by the conventional biological treatment however, they suffer limitations regarding color removal. Thus, there is a need to explore the novel physico-chemical methods namely adsorption and chemically aided flocculation and precipitation which can concomitantly remove the color (Rajesh et al., 1999). Magnesium salts have illustrated the potential as an effective alternative over conventional coagulants such as alum, lime, ferric and ferrous sulphate to remove the pollutants from industrial wastewater, owing to its recoverability and reuse in the process (Gao et al., 2007). Nevertheless during the processing of paddy to produce rice, a large amount of solid biomass in the form of rice husk is emerged. This usually serves no beneficial purpose (Lim et al., 2012), except in some cases where the rice husk biomass is extensively used for heating of furnace resulting in the generation of abundant RHA. It consists of predominantly silica 52% and carbon 45% (Liu et al., 2012). Thus, it can be concluded that RHA could be an important source of silica that may find wide applicability in electronics, ceramic and polymer industries (An et al., 2010a, An et al., 2010b, Liou, 2004, Sun and Gong, 2001).

One of the major motives of the present study is harnessing the abundantly available RHA for not only production of silica but also as an adsorbent for the abatement of rice mill wastewater. Thus use of RHA, another waste byproduct from the same industry, was efficiently used for the rice mill wastewater treatment, is indeed novel. To the best of our knowledge, this has not been tried before. Moreover, the main concept behind employing MgCl₂ as coagulant is primarily due to its ability of being recycled that can be recovered from the precipitated sludge generated by the coagulation–flocculation procedure (Chaudhary et al., 2002). The flocculation process was demonstrated more economical with the efficient recycle of MgCl₂ which is a major coagulating aid (Gao et al., 2007, Liao and Randtke, 1986).

Thus salient attempts have been performed to develop a promising economical route (Fig. 1) for simultaneous treatment of rice mill wastewater treatment while converting abundant waste byproduct RHA into value added silica that was characterized by using SEM, FTIR and XRD. Reusability of recovered MgCl₂ from the coagulation sludge for repeated application explored for turning the whole process economically attractive.