CHAPTER 1
Nitrate (NO3−) is also a major groundwater contaminant in the most parts of the world due to its high water solubility (Mohseni-Bandpi et al., 2013). High NO3− contamination can be linked to increased risks of ―Blue-baby syndrome‖, various type of cancers, formation of carcinogenic nitrosamines, reproductive disorders, and other adverse health effects on humans (Horing & Chapman, 2004; WHO, 2004b). High nitrate concentration also causes acute poisoning in cattle. Environmental impact of high nitrate includes eutrophication of surface waters due to excess nutrients (Bhatnagar & Sillanpaa, 2011; Calderer et al., 2010). Due to severity of the health problems associated with excess nitrate in potable water, the European Union, WHO and Bureau of Indian Standard set the drinking standard for nitrate at 50 mg-NO3− /L and 45 mg-NO3− /L respectively (BIS:10500, 2012; EC, 1998; WHO, 2004b). In addition to arsenic, nitrate and iron, fluoride is also one of the most abundant anions present in groundwater worldwide and creates a major problem in safe drinking water supply. It is classified as one of the water contaminants by the World Health Organization (WHO), which cause adverse health effects on humans and animals (Jagtap et al., 2012; WHO, 2006).
The co-existence of multiple contaminants has been reported in many areas because of special geochemical conditions or industrial activities, (Velizarov et al., 2005).
The co-existence of multiple contaminants is adding complexity to the problem of groundwater contamination. There are several reports on co-occurrence of arsenic and iron in groundwater. Along with arsenic and iron, the co-occurrence of nitrate and fluoride in groundwater is also reported from many locations of the world (Rezaie-Boroon et al., 2014; Smedley et al., 2008). The presence of one or a combination of these contaminants in drinking water sources often needs an expensive, multi-step treatment or abandonment of wells and other water bodies (Mazumder et al., 2010; Rosen et al., 2004).
Simultaneous removal of multiple number of contaminants from drinking water have been tried by adsorption, coagulation-flocculation, ion exchange, membrane separation, precipitation and cementation (Liu et al., 2012; Matos et al., 2006; Velizarov et al., 2004). However, these methods suffers from drawbacks in terms of application, high operational, maintenance costs and effectiveness as it usually results in the production of unstable sludge, which leads to a greater disposal expense.
In recent years, biological processes have gained increasing interest in drinking water treatment, mainly due to the conversion of many organic and inorganic contaminants to innocuous by-products. In addition, biological treatment achieves multiple contaminant removal in a single system in lesser contact times, potentially minimizing costs and suitable for large community scale operations, while avoiding the need for regeneration of solid phase sorbents or treatment of the generated wastes (Brown, 2008).
In groundwater systems, under anoxic conditions the ecological succession of terminal electron-accepting processes is O2 followed by NO32−, Mn(IV), Fe(III), SO42−
and finally CO2. Other oxyanions, including arsenate, selenate, chlorate and chromate, may also be used as terminal electron acceptors during microbial respiration depending on their availabilities in an aquatic system (Narasingarao & Haggblom, 2007). Thus in a biologically active aquatic environment where arsenic, iron and nitrate are present along with sulphate, the nitrate reducers reduces nitrate to nitrogen gas and sulphate-reducing bacteria (SRB) produces biogenic sulphides, which in turn lead to the precipitation of arsenic and iron sulphides. Hence, arsenic is removed from the water simultaneously as precipitates of arsenosulphides (orpiment and realgar) as well as adsorption/co- precipitation with iron sulphides (Altun et al., 2014; Battaglia-Brunet et al., 2012).
Recently, development of several bioreactor configurations either of attached or suspended growth types operated in up-flow or down-flow modes have led to effective removal of arsenic from mine waters (Altun et al., 2014; Battaglia-Brunet et al., 2012;
Rodriguez-Freire et al., 2014). However, report on sulphidogenic arsenic removal and multi contaminant removal from drinking water/groundwater sources is scanty. In the scientific literature, so far, only a few efforts have been made to study the arsenic removal in sulphidogenic bioreactor from drinking water sources. Even the most relevant report on biological treatment of groundwater for simultaneous removal of multi-pollutants available so far is the outcome of the experiments carried out on simulated groundwater spiked with a fixed arsenic (200 µg/L), iron (2 mg/L), nitrate (50 mg/L) and sulphate (22 mg/L) concentration operated at a fixed EBCT (30 min) at 22oC (Upadhyaya et al., 2010).
However, occurrences of such contaminants in real groundwater even at higher concentrations have been reported. There are no reports available so far, on simultaneous removal of multi-contaminant in a single bioreactor at varying arsenic, nitrate, iron and
sulphate concentrations; effects of EBCT, pH and temperature. Also, there is no study reported so far on arsenic removal from real groundwater. With this background, the purpose of this research was to develop sulphidogenic anaerobic bioreactor systems for simultaneous removal of arsenic, iron and nitrate from simulated and real groundwater at varying feeding and operating conditions.
In this study, mixed microbial culture was collected from a wastewater treatment plant and acclimatized in presence of arsenic, nitrate and sulphate. The acclimatized sludge was used to evaluate its performance on simultaneous removal of target pollutants, arsenic, nitrate, iron and fluoride as well as effects of one pollutant on the others in batch, semi-batch and flow through reactor systems. The reactors were operated in absence as well as in presence of iron.
Firstly, the simultaneous removal of multipollutants was assessed in batch shake flasks and semi-batch reactors. The effect of arsenic, nitrate and/or iron concentration on the arsenic removal was investigated in batch as well as semi batch reactors. The experiments were also carried out with different carbon sources in semi-batch reactors in order to study effect of carbon sources on multipollutants removal. Furthermore, biological arsenic and nitrate-removing processes were performed in presence and absence of iron in anaerobic flow-through attached growth reactors (AGRs). Therefore, two sulphidogenic AGRs enriched with mixed bacterial culture, where waste activated carbon (WAC) was used as supporting material for bacterial growth were operated. In particular, the AGRs operation was directed to: 1) evaluate the arsenic removal efficiencies under different arsenic, nitrate and sulphate concentrations in absence and presence of iron from synthetic as well as real contaminated groundwater; 2) evaluating the bioreactor performance for simultaneous removal of pollutants in diverse range of temperature; 3) test the reactor performance under pH variations in the feed solution; 4) check the acute toxicity of the treated water; 5) collection, preparation, characterization and performance evaluation of an adsorbent from water treatment plant residues (WTR) on fluoride removal from AGR treated water; 6) identification of microbial population dynamics; 7) characterization of biosolids formed in AGRs; and finally, the experimentation for stability check of biosolids, carried out both in batch and continuous mode, aimed to study the leachability of arsenic from biosolids.
Organization of the Thesis
The thesis has been organized into six chapters. The current Chapter 1 presents the general introduction to the present work while the literature that supports the present study is presented in Chapter 2. The primary objective and the scopes of the study are given in Chapter 3. Details of the materials and methods adopted in the present study along with the reactor configurations and operating conditions are discussed in details in Chapter 4. Chapter 5 presents the results and discussions of sequential studies carried out on batch reactors, semi-batch reactors and flow through reactors. The key conclusions drawn from these studies and discussion on the future scope of work are presented in Chapter 6.
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