Ecological Engineering 84 (2015) 29–37 Contents lists available at ScienceDirect Ecological Engineering jo ur nal home p age: www.elsevier.com/locate/ecoleng Simultaneous recovery of silica and treatment of rice mill wastewater using rice husk ash: An economic approach Anuj Kumar a , Subhajit Singha a , Dalia Dasgupta b , Siddhartha Datta c , Tamal Mandal a,∗ a Department of Chemical Engineering, National Institute of Technology, Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India b Department of Biotechnology, National Institute of Technology, Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India c Department of Chemical Engineering, Jadavpur University, Raja S.C. Mallik Road, Kolkata 32, West Bengal, India a r t i c l e i n f o Article history: Received 12 November 2014 Received in revised form 17 June 2015 Accepted 27 July 2015 Keywords: Rice mill wastewater Silica Rice husk ash Coagulation Adsorption a b s t r a c t 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 MgCl 2 as coagulant. For this, MgCl 2 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 MgCl 2 . The proposed study was made more economical by recovering MgCl 2 from the coagulation sludge for repeated application. Reusability of MgCl 2 was explored for up to four recycles with the objective of improving the entire work economically viable. © 2015 Elsevier B.V. All rights reserved. 1. 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. Genera- tion 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 par- ticular 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 organiza- tion (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 ∗ Corresponding author. E-mail addresses: prof.tamalmandal@gmail.com, tamal mandal@yahoo.com (T. Mandal). proportion of inorganic and organic pollutants (Rajesh et al., 1999). This explains the urges to critically devise scheme toward miti- gating 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) con- tributing 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 ris- ing concern for rice mill wastewater treatment prior to discharge http://dx.doi.org/10.1016/j.ecoleng.2015.07.010 0925-8574/© 2015 Elsevier B.V. All rights reserved.