Arsenic is one among many oxyanions present in sources of drinking water as a potential contaminant. There are several reports on health effects of arsenic as well as technologies for its removal from contaminated drinking water. Although several methods of arsenic removal as sole contaminant from contaminated water are successful, simultaneous removal of co-pollutants along with arsenic is still a challenging job to the environmental engineers. Most of the highly efficient physicochemical processes are silent on simultaneous removal of multipollutants such as arsenic, nitrate, iron, sulphate, fluoride etc. Furthermore, the efficient treatment processes generate high volume of arsenic bearing unstable solids, which become a source of arsenic contamination of nearby drinking water sources. Based on thorough literature survey but limited information, reveals that biological process has a high potential on simultaneous removal of multiple number of contaminants of groundwater leaving low volume stable arsenic bearing waste which may disposed of safely in a landfill along with comingled municipal solid waste. The main objective of this research work is to develop a biological reactor for simultaneous removal of arsenic, nitrate and iron in presence of sulphate from simulated as well as real contaminated groundwater.
In this study, mixed microbial culture was collected from a wastewater treatment plant, mixed with small amount of (<5%) of bio-sludge collected from two nos.
laboratory scale bioreactors treating perchlorate and nitrate, and sulphate, respectively.
The mixed bacterial biomass thus prepared was 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. Batch studies were carried out in shake flasks whereas studies in semi batch mode were conducted in 1 L bottles, operated in suspended growth mode. Reactors operated in continuous mode were of attached growth reactors (AGR), where waste activated carbon (WAC) was used as supporting material for bacterial growth. Adsorption characteristics of the WAC were evaluated before being used in AGRs. Besides performance evaluation of mixed bacterial culture on simultaneous removal of target pollutants from groundwater,
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mechanisms of arsenic removal were also investigated through their characterization.
Collection, preparation, characterization and performance evaluation of an adsorbent from water treatment plant residues (WTR) on fluoride removal from AGR treated water was performed to evaluate its potential as a post treatment unit. In addition to this, stability of the biosolids as well as the spent WAC were checked under aerobic as well as anoxic conditions through ―Ageing test‖, ―Toxicity Characteristics Leaching Procedure (TCLP) test‖ and ―Long Term Leaching test‖.
Experimental results show that, in absence of iron, the mixed microbial culture could reduce arsenic to below permissible limit in drinking water, from an initial of 600 µg/L in suspended growth mode. However, in attached growth mode, AGR-1 could reduce arsenic to below 10 µg/L, from an initial of up to 750 µg/L in simulated groundwater. In presence of iron, arsenic was reduced to below 10 µg/L, from an initial of up to 1000 µg/L and 1500 µg/L in suspended growth (SmBR-2) and attached growth (AGR-2) systems, respectively. From an initial of up to 250 mg/L, complete removal of nitrate in all the reactor systems was noticed within 24 hours of operation. No presence of nitrite in the treated water suggests complete denitrification. From an initial of 13.2 mg/L, the AGR-2 could reduce iron to below 0.3 mg/L from real groundwater at appropriate feeding and operating conditions. At varying temperature, performance of either AGR did not get affected except for first few initial days at 20oC and 50oC. The T-RFLP and metagenomic analysis of microbial community confirmed the presence of arsenic, nitrate and sulphate reducers in AGRs. The adsorbent prepared from WTR could remove fluoride from an initial of 5 mg/L to 0.55 mg/L. FESEM/EDX, XRD, TEM and XAS analysis of biosolids in AGR-1 confirmed the presence of As(III) as orpiment and realgar in amorphous and nanocrystalline forms. Biosolids in AGR-2 contains As(III) and Fe(II) as pyrrhotite, and pyrite in addition to as orpiment and realgar in crystalline form. The results suggest that precipitation of arsenosulphide is the main mechanism of arsenic removal in AGR-1, whereas, in AGR-2 precipitation as arsenosulphide and/or co- precipitation of arsenic with biogenic iron sulphides are the main arsenic removal mechanisms. After 90 days of experiment in anoxic and oxic environment, ageing test results have shown that arsenic leaching from biosolids of either AGRs was less than or equal to 0.6%, whereas maximum iron leaching of 0.48% was noticed from AGR-2 biosolids. TCLP and long term leaching test results show that the concentration of arsenic
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leachate values of 300 and 5000 µg/L, respectively. Thus the biosolids in the AGRs, including the spent WAC, would not be classified as hazardous waste material.
In summary, the AGR systems developed in this project could remove arsenic and its co-pollutants such as nitrate and iron from simulated as well as real groundwater at wide range of temperature to meet the drinking water standards, leaving a non hazardous and stable biosolids as well as spent WAC, which can be dumped in sanitary landfill safely. However, fluoride needs to be treated separately in an additional treatment unit. A waste product, WTR, proved to be efficient for fluoride removal to meet the drinking water standards.