Removal of arsenic and iron removal from drinking water using coagulation and biological treatment

The presence of heavy metals in the water environment has been a major concern for years because of their high acute and chronic toxicity. Approximately 140 million people throughout the world are affected by the consumption of arsenic-contaminated water (Ravenscroft et al. 2009). The major means of arsenic entering the body is through drinking water, hence humans may be affected by serious health problems including those of the gastrointestinal tract, cardiac, vascular and central nervous system (Lacasa et al. 2011). Therefore, the removal of these contaminants is required to eliminate potential health risks and threats.

Several treatments, such as membrane technology, coagulation and biological processes, have been widely used for water and wastewater treatment. It has been demonstrated that the filtration process can reduce metal contaminants to some extent. Litter et al. (2010) found that low pressure membranes such as microfiltration or ultrafiltration are not very effective for removing arsenic contamination, as the arsenical species are very small and thus traverse the membranes. On the other hand, high pressure membranes can completely reject polyvalent ions in particular, and are suitable for arsenic oxyanions (Ravenscroft et al. 2009). However, this process can consume huge amounts of power, hence economically visible, reliable, easy to operate technology is urgently required.

Chemical coagulation treatment is effective and widely used for the removal of organics and particulate matter from water and wastewater because of its availability, low cost and good treatment performance, and it can be used for iron and arsenic removal from water and wastewater. Parga et al. (2005) found that ferric coagulant could reduce up to 90% of arsenic. However, chemical coagulation produces large amounts of chemical sludge. An alternative natural coagulant such as Moringa oleifera (M. oleifera) seeds seems to be a more cost-effective, viable and environmentally-friendly approach for such an application, because the production of sludge was four to five times less using M. oleifera than alum coagulant (Abaliwano et al. 2008; Pise & Halkude 2012). Mohamed et al. (2014) found that M. oleifera seeds reduced the turbidity, COD and phosphorus of wastewater by 90%, 60% and 75%, respectively. Another study by Pramanik et al. (2015a) found that M. oleifera could reduce 23% of dissolved organic carbon (DOC), 42% of ultraviolet absorbance at 254 nm (UV₂₅₄), 76% of turbidity and 60% of colour from drinking water. The uses of this natural coagulant for arsenic and iron removal from drinking water have not yet been investigated.

Biological processes are also widely used in water and wastewater treatment processes. Biological water treatment processes are better than chemically assisted technologies and are a more cost-effective technology to remove metal content from water and wastewater. It has been suggested that the operational and capital costs of biological treatment processes are 3–10 and 5–20 times lower, respectively, than chemical processes (Marco et al. 1997). Lipton et al. (1994) found that 99% removal of arsenic from groundwater by combined anaerobic and aerobic bioreactors was observed. Biological activated carbon (BAC) treatment was highly effective in removing both arsenic and iron, with a removal efficiency of 97% and 99.8%, respectively (Pokhrel et al. 2005). They also reported that the rapid sand filter removed approximately 50% of arsenic and 99% of iron. Biological aerated filters (BAF) have been studied for water and wastewater treatment due to the large surface area available for the growth of biofilm (Ryu et al. 2008; Biplob et al. 2011a, 2011b; Pramanik et al. 2012). However, the BAF process is mainly used for removing carbonaceous organics and biological nutrients from water and wastewater. However, there is no comparison for the removal of the contaminants by different treatment processes. Moreover, the influence of the presence of organic matter in drinking water during treatment processes needs to be evaluated.

The objective of this study was to compare the removal efficiency of arsenic and iron by alum and BAC treatment processes. The natural coagulant, M. oleifera, was compared with conventional alum coagulant, and BAF was compared with the BAC treatment process. During the coagulation process, the samples were coagulated with different dosages and pH to optimize the process performance for iron and arsenic removal. The effect of contact time on biological treatment for arsenic and iron removal was investigated. An assessment of the effect of the concentration of organics during iron and arsenic removal using biological and coagulation processes was also undertaken.

The samples were collected from a reservoir. Samples were stored at 4 °C and warmed to room temperature (22 ± 2 °C) prior to all experiments. Different concentrations of dissolved organic matter (humic and fulvic acid were used, and supplied by Sigma-Aldrich) were added to feedwater to investigate the impact of the concentration of organics on arsenic and iron removal.

The chemical coagulant alum (Al₂(SO4)₃18H₂O) was used in this study. M. oleifera was used as a natural coagulant. The seeds of M. oleifera were extracted from the dry pods, ground into a fine powder using a blender and then sieved through a 0.8 mm mesh. Afterwards, Milli-Q water was added to the fine powder to make a 1% suspension. The detailed procedure can be found elsewhere (Pramanik et al. 2015a).

Both alum and M. oleifera coagulation experiments were carried out with 1 L feed water using a jar test apparatus (Phipps and Bird, PB-900). Prior to all types of coagulant experiments, the organics were added into the feed water and then mixed with a stirrer for 30 min. The samples were mixed for 5 minutes at 300 rpm and then mixed for the next 20 minutes at 40 rpm. After 1 hour, the supernatant was tested to determine the amounts of arsenic and iron. This experiment was performed with a range of doses (5–25 mg/L) and pH (4–6) conditions to optimize the conditions for the target compounds’ removal. The pH was adjusted by using 0.1 N HCl or 0.1 N NaOH.

The BAF was constructed in a PVC chamber with an internal diameter of 8 cm and an effective media height of 60 cm. Plastic media (Kaldness) with a length, diameter, density and an internal surface area of 3 mm, 5 mm, 0.42–0.46 g/cm³ and 305 m²/m³, respectively, were used for the BAF.

The BAC column was constructed of glass, with an internal diameter and effective carbon bed height of 4.5 cm and 50 cm, respectively. The surface area, total pore volume and micropore volume of the activated carbon used were 800 m²/g, 0.865 cm³/g and 0.354 cm³/g, respectively.

Prior to packing for both processes, the medium was inoculated with activated sludge, and provided with additional nutrient sources (N, P and C) to promote the rapid growth of biofilm on the media. Afterwards, it was gently washed with tap water to remove excess biofilm and transferred to the system, and feed was commenced. DOC removal was stable after 35 and 40 days of BAC and BAF operation, respectively, indicating that equilibrium had been established.

Both processes were operated in a downflow mode with an empty bed contact time of 15, 30, 45 and 60 min. The processes were backwashed for 20 min every week to avoid physical clogging.

DOC concentration was determined using a total organic carbon (TOC) analyser (TOC-5000A, Shimadzu). Potassium hydrogen phthalate (1, 5, 10 and 25 mg C L⁻¹) was used for calibration of the TOC analyser. Before these analyses, all samples were filtered through a 0.45 μm filter. The pH of the water samples was measured using a pH meter (Mettler Toledo). The pH meter was calibrated with standard solutions of pH 4, 7 and 10. Zeta potential measurements were measured using a Zetasizer Nano-ZS (model ZEN3500).

The concentration of iron and arsenic was determined using atomic absorption spectrometry (Shimadzu AA6800). The limit of quantification for arsenic and iron was 1 μg/L and 20 μg/L, respectively. Prior to the analysis, the samples were filtered through 0.45 μm cellulose acetate filters. All filtration analyses were duplicated and reported in terms of mean value.

This was attributed to the adsorption of these contaminants into the flocs, and precipitation of some of these contaminants with precipitates (Edwards 1994; Lytle et al. 2005). Moreover, some of the arsenic (particularly As(V)) species are negatively charged (H₂AsO₄ and HAsO₄), therefore positively charged coagulant could alter or destabilize negatively charged arsenic content. There was a greater removal of these molecules by chemical coagulant than natural coagulant. This was attributed to the development of a greater settleable floc area, as confirmed by zeta potential measurement, resulting in greater adsorption. All coagulants achieved a considerably higher removal of arsenic than iron. This was possibly due to the molecular size of arsenic, which is larger than iron, therefore leading to a greater adsorption of arsenic to floc than iron.

The concentration of iron and arsenic decreased with increasing coagulant dose. With the increase in coagulation dosage from 5 to 25 mg/L of alum, the removal efficiency increased from 63% to 92% for arsenic and from 58% to 78% for iron. This was attributed to the greater dose allowing the destabilization of the colloidal suspension and the formation of flocs. Similarly to alum, M. oleifera had a similar trend in removing arsenic/iron concentrations with increasing dosage, but the removal efficiency was lower compared with alum. This was possibly related to the lower increment of the floc area, suggesting that the removal efficiency was influenced by the size of the flocs.

With the increase in contact time, both processes led to an increase in the reduction of arsenic and iron concentration, with BAC giving a greater removal than BAF. Both treatments led to a greater reduction of arsenic than iron. This was likely due to the greater solubility of arsenic and the degree of attraction by water molecules, which are prevented from being bound by the carbon surface.

The arsenic and iron content in the untreated water was 1,840 μg/L and 3,480 μg/L, respectively. The World Health Organization (WHO) has set an allowable limit for arsenic in drinking water for human consumption of 10 μg/L (World Health Organization 1993). Canadian Drinking Water Guidelines (1996) also set an allowable limit of 25 μg/L arsenic and 300 μg/L iron for drinking water. Hu et al. (2012) reported that China's newly revised drinking water standards for the maximum contaminant level of arsenic have reduced from 50 to 10 μg/L. However, only the combination of coagulation (5 mg/L) and a biological process (60 min contact time) treated water could meet the allowable limit of the arsenic and iron concentration for drinking water.

This study compared the performances of BAC, BAF and coagulation processes for removal of arsenic and iron from drinking water treatment in the presence of organic matter. Coagulation could remove arsenic and iron from drinking water, and the removal was achieved by precipitation and adsorption onto the surface of flocs via both electrostatic and hydrophobic interaction. Alum performed better in removing these compounds than M. oleifera. This was likely due to the greater floc area, resulting in greater adsorption. Both iron and arsenic removal strongly depended on coagulant dose and to a small extent on coagulation pH. There was a greater reduction of arsenic than iron by the coagulation process as the molecular size of arsenic is larger than iron. Biofilters were more effective for removing these contaminants than coagulation. BAC gave greater removal of arsenic and iron contaminant than BAF. The addition of coagulation before the biological process further reduced the concentration of the target contaminant. When the concentration of DOC increased, the removal efficiency of arsenic and iron decreased because of the steric hindrance effect of organic molecules and competitive adsorption. Overall, this study demonstrated that new BAF and a natural coagulant could reduce iron and arsenic contamination in reservoir water, and the combination of coagulation and biological process at 60 min contact time could meet the allowable limit for drinking water.