216 Figure 5.113 Effects of Vair/VL ratios on iron leaching from AGR-2 biosolids.
ABSTRACT
However, in the attached growth state, AGR-1 could reduce arsenic to below 10 µg/L, from an initial up to 750 µg/L in simulated groundwater. In the presence of iron, arsenic was reduced to below 10 µg/L, from an initial up to 1000 µg/L and 1500 µg/L in suspended growth (SmBR-2) and attached growth systems (AGR-2), respectively.
INTRODUCTION
Nitrate (NO3−) is also a major groundwater pollutant in most parts of the world due to its high water solubility (Mohseni-Bandpi et al., 2013). Along with arsenic and iron, the co-occurrence of nitrate and fluoride in groundwater is reported from many places in the world (Rezaie-Boroon et al., 2014; Smedley et al., 2008).
LITERATURE REVIEW
Introduction, Source and Environmental Impact/Health Effects of Contaminants
- Arsenic
- Nitrate
- Iron
- Fluoride
Nowadays, NO3 contamination of groundwater has become a serious environmental problem in many parts of the world (Mohseni-Bandpi et al., 2013). Several public water supply wells in Huntington Town, New York, have been closed due to high nitrate concentrations in drinking water (Bleifuss et al., 1998).
Simultaneous Co-occurrence of Multiple Pollutants in Groundwater
- Co-occurrence due to Various Industrial Activities
- Co-occurrence due to Geogenic Sources
- Co-existence of Multiple Contaminants
- Environmental Impact of Co-occurrence of Multiple Contaminants
The co-occurrence of As and F was explained as the desorption of As and F from Fe (hydr)oxides due to increased pH in aquifers (Kim et al., 2012b). This process is known as denitrification using pyrite (as an electron donor) or pyrite oxidation by NO3− (Schwientek et al., 2008).
Review of Removal Techniques
- Oxidation
- Coagulation-Flocculation
- Ion Exchange
- Membrane Technology
- Adsorption Processes
- Small Scale/Household Technologies
Solar oxidation (Hug et al., 2001), in-situ oxidation (DPHE, 2001) and biological oxidation also received much attention in connection with arsenic removal from drinking water. Donnan dialysis was shown to remove high fluoride (>30 mg/L fluoride) from groundwater to below 1.5 mg/L, even in the presence of other ions (Mohapatra et al., 2009).
Biological Processes
- Aerobic Respiration
- Biological Denitrification
- Biological Iron Removal
- Microbial/Biological Arsenic Removal Mechanisms
The biological oxidation of As(III) to As(V) by iron-manganese-oxidizing microbes is widely reported and well-established for efficient As(III) removal without any additional use of chemicals in this bioprocess (Casiot et al., 2003 ). The ArsC gene is responsible for the reduction of As(V), while arsA and arsB control the release of As(III) from the cytoplasm (Villegas-Torres et al., 2011). Dissimilatory sulfate-reducing prokaryotes (SRPs) are a widespread heterogeneous group of archaea and bacteria that can reduce sulfate to sulfide (H2S, HS−) using sulfate as an electron acceptor (Castro et al. 2000; Garrity et al., 2003). ).
In ethanol batch reactors supplemented with As(v) and sulfate, biogenic minerals formed as a result of the combined reduction of As(v) and sulfate (Rodriguez-Freire et al., 2014).
Arsenic Containing Solids Treatment and Management
2013) suggested the formation of realgar-like surface precipitates on the surface of FeS or of arsenic sulfide mineralization under prolonged sulfidogenic environments, which may lead to more efficient arsenic removal. 2014) also reported the arsenic precipitation as As2S3 and arsenic co-precipitation with FeS, FeS2 or FeAsS as arsenic removal mechanisms in the AMD treating bioreactor. Arsenic leaching from stabilized wastes is affected by environmental factors such as pH, relative humidity, and wetting and drying cycles.
2014) reported that there was no significant arsenic leaching from the arsenic residues stored on the coarse sand filters of a community arsenic removal unit in West Bengal, India.
AIM AND SCOPES OF THE STUDY
Aim of study
Scopes of the Study
Performance evaluation of AGR-1 on simultaneous removal of pollutants in the absence of iron at varying feeding and operating conditions. Performance evaluation of AGR-2 on simultaneous removal of pollutants in the presence of iron at different feeding and operating conditions. Characterization of AGR treated water by "chemical analysis", "Whole Effluent Toxicity (NAT)" and "Most Probable Number (MPN)" tests.
Stability control of biosolids and spent WAC under aerobic as well as anoxic conditions through ―aging test‖, ―toxicity characteristics leaching procedure test (TCLP)‖ and ―long-term washing test‖.
MATERIALS AND METHODS
Materials
Waste Activated Carbon (WAC)
- Microscopic Analysis of WAC
- Bulk Density of WAC
- Evaluation of WAC Adsorption Capacity on Arsenic Removal
This type of water purifier generally contains 200-300g of activated carbon depending on its size and capacity. After its useful life of about 6 months to 1 year, the spent activated carbon is replaced with fresh activated carbon. The dried waste activated carbon (WAC) granules were subjected to FESEM/EDX analysis before seeding.
The characterization of waste activated carbon (WAC) and its adsorption behavior in arsenic removal from simulated groundwater in the absence of iron was evaluated.
Analysis of Liquid Samples
The required amount of distilled water was added to the standards and the samples to obtain the appropriate dilution and the arsenic concentrations of the solutions were quantified using AAS. The arsenic concentrations of the solutions were estimated according to the guidance of the AAS user manual.
Experimental Methodologies
- Seed sludge and Its Acclimatization in Reactor BR-0
- Experimental Set-up and Bio-reactors
- Batch Studies
- Semi-Batch Reactors (SmBR-1 & SmBR-2)
- Flow through Reactors (AGR-1 and AGR-2)
- Operation of AGR-1
- Operation of AGR-2
Experimental work carried out in various laboratory-scale reactors is generally classified into two categories, namely reactors operating without iron and reactors operating in the presence of iron (in contaminated water). In addition to the peristaltic pumps, a syringe pump (model: SP10, Miclins India) was used to supply the iron solution to AGR-2 so that the carbon source (acetic acid) and the Fe(II) supply could be varied independently and without interference. The iron stock solution was loaded into a syringe by filtering through a 0.2 µm filter. The performance of a reactor loaded with 10.0 mg/L iron was also evaluated at an EBCT of 90 min.
The minimum concentration of 500 µg/L was chosen based on reactor performance under the lowest influent arsenic concentration.
AGR-2 operation with Real Groundwater
30 °C to bring the AGR-2 back to normal for future operation with truly contaminated groundwater. Throughout the operation of AGR-1, synthetic groundwater and nutrient solution were supplied to the bioreactor at night, on weekends and on holidays. From day 935 to day 941, AGR-2 was shut down for seven days, during which no synthetic groundwater and nutrient solution were supplied to the bioreactor.
The AGR-2 was used for the next 19 days (from day 837 to day 856) at an elevated sulfate and COD level of 100 mg/l and 150 mg/l at an elevated EBCT of 120 minutes. The effluent was analyzed for its arsenic and iron removal efficiency.
Microbial Population Identification and Diversity Analyses
- Biofilm formation on WAC
- T-RFLP analysis
- Metagenomic analysis
Metagenomic analyzes of mixed bacterial populations of AGRs were performed on V3-V4 variant regions of the 16 S rRNA genes. The primary aim of metagenomic analysis was to identify microbial diversity present in complex polybacterial populations of AGRs. WAC pellets containing biofilm were collected from the AGRs after backwashing, to give a complete representation of bacterial population in the reactors.
WAC granules were collected in polyethylene bottles in an anoxic environment and immediately sealed on day 752 and 720 of AGR-1 and AGR-2 operation, respectively.
Whole Effluent Toxicity (WET) Test of Treated Water
Freshwater fish, Puntius, were collected from the local market as a freshwater species that can be commonly used for the acute freshwater WET test according to EPA guidelines. The test species is of local importance and is abundant in the Brahmaputra River and water bodies in and around Kamrup district of Assam. The test was performed on absolute and 50% diluted treated water along with the control containing only river water.
Fish mortality rate along with other physiological responses (change in behavior, growth) was recorded at and 96 hours of exposure to treated water as a measure of toxicity.
MPN test of Treated Water
Characterization of Biosolids
- Collection and Preservation of Biosolids
- X-ray Fluorescence (XRF)
- Microscopic Methods FESEM and EDX
- X-Ray Diffraction (XRD)
- X-ray Absorption Spectroscopy (XAS)
Field emission scanning electron microscopy (FESEM) (Zeiss, Sigma, Germany) equipped with energy dispersive X-ray microanalysis (EDX) system (INCA 300, Oxford, UK) was used for topographical characterization and elemental confirmation of arsenic, iron and sulfur in the biosolids . Prior to examination, the lyophilized backwash solids were lightly dusted onto the carbon ribbon of the SEM stub surface and coated with gold using a Scancoat Six SEM sputter coating system. Then one drop of the suspension was dropped with a micropipette onto hole-like carbon supporting film (TEM Grids).
The analysis was performed with a PANalytical X'pert PRO-MPD diffractometer equipped with a diffracted beam monochromator.
Stability Check of Biosolids
- Batch Ageing Test of Backwash Suspension
- Toxicity Characteristics Leaching Procedure (TCLP) test
- Long term Aerobic Leaching Test
In this project extended TCLP tests were performed on AGR backwash solids for an 84 hour extraction period. TCLP tests were performed in Teflon screw cap bottles (test vessels) maintaining a specific liquid to solid mass ratio (on a wt/wt basis) of 20 in the mixture. After successful flow-through operation of the reactors for approximately 1000 days, long-term stability control experiments were conducted on the solid arsenic and/or iron precipitates and the spent WAC filter bed while it was still intact in the flow-through reactors. .
The sample was collected every 24 hours and the parameters were analyzed for pH, arsenic and iron concentrations in the leachate.
Fluoride Removal by Water Treatment Residues (WTR)
- Water Treatment Residues (WTR)
- WTR Characterization
- Batch Adsorption Experiments
- Adsorption Equilibrium Study
The pH of the solution was initially maintained without any adjustment except for the experiments conducted to study the effect of pH where the pH of the solution was adjusted using either HCl or NaOH. The initial fluoride concentration was maintained at 5.0 mg/L for all experiments except during studies on the effects of initial fluoride concentration. Fluoride removal efficiency (%) by WTR in batch reactors was determined as:. initial fluoride mg/L – final fluoride mg/L.
The Langmuir equations are mainly based on the assumptions that molecules are adsorbed at specific locations on the surface of the adsorbent, each location can accommodate only one molecule (monolayer) and the adsorption energy is the same at all locations.
RESULTS AND DISCUSSION
Seed Sludge Collection and Acclimatization
Enrichment of the mixed microbial consortium was carried out in the BR-0 reactor by adding a fixed amount of 200 µg/L arsenate and 50 mg/L NO3− and SO42− respectively in simulated groundwater. It was observed that NO3− was completely removed, while the removal rate of SO42− improved at each stage of acclimation.
Performance Evaluation of Batch Shakes Flasks
- Adsorption Studies in Batch Shake Flasks
- Adsorption of Arsenic by WAC
- Bioremoval Studies in Batch Shake Flasks .1 Batch Studies in Absence of Iron
Reduction of arsenic concentration in control‖ may be due to loss of arsenic by adsorption to the flask material and/or volatilization. This may be due to the lack of sulfate and iron in the media used in this study. The slow rate of COD reduction may be due to the lack of sulfate and/or methanogens in the inoculum.
This may be due to insufficient feed (COD) in the media leading to a negative net growth.
Performance Evaluation of Semi-batch Bioreactor SmBR-1 in Absence of Iron
- SmBR-1 Phase-1: Effect of HRT
- SmBR-1 Phase-2: Effect of Initial Arsenic Concentration
- SmBR-1 Phase-3: Effect of Initial Nitrate Concentration
- SmBR-1 Phase-4: Effect of Different Carbon Sources
Arsenic in the treated water remained below the allowable limit of 10 µg/L when the initial arsenic was 600 µg/L or less. Regardless of the initial concentration of NO3− (up to 250 mg/L), arsenic in pure water was always below 10 µg/L. Nitrate in treated water was below detection limits for all carbon sources tested.
The pH of the treated water was in the range of 7.2-7.5 for all carbon sources studied during the entire phase.
Performance Evaluation of Semi-batch Bioreactor SmBR-2 in Presence of Iron
- SmBR-2 Phase-1: Effect of HRT
- SmBR-2 Phase-2: Effect of Initial Arsenic Concentration
- SmBR-2 Phase-3: Effect of Initial Nitrate Concentration
- SmBR-2 Phase-4: Effect of Different Carbon Sources
When the sulfate reduction was well stabilized, the iron in the treated water was always below the detection limit. Arsenic in treated water was always reduced below 10 µg/L (99.5% removal), while iron and nitrate remained below detection limits. However, SO42− reduction was improved at this stage, which may be due to the good establishment of the sulfate reducing community in the reactor.
Regardless of the initial NO3− concentration (up to 250 mg/L), arsenic in the treated water was found below 10 µg/L, with more than 99% removal efficiency.
Performance Evaluation of the Flow through Attached Growth Reactor, AGR-1
- AGR-1 Start-up
- AGR-1 Phase-1: Effects of EBCT and Backwash Frequency
- AGR-1 Phase 2: Effect of Influent Arsenic Concentration
- AGR-1 Phase 3: Effect of Influent Nitrate Concentration
- AGR-1 Phase 4: Effect of pH
- AGR-1 Phase 5: Effect of operating Temperature
- AGR-1 Phase 6: Removal and Effects of Fluoride
- AGR-1 Phase 7: Performance at Lower EBCT
- AGR-1 Phase 8: Performance after Shutdown
Arsenic removal in the reactor was dependent on sulfate removal and also a function of EBCT. The arsenic removal efficiency also remained the same, with 1-6 µg/L remaining in the treated water. Regardless of the operating temperature (up to 40 °C), arsenic in the treated water was always found below 10 µg/L.
NO3− in the treated water remained below the detection limit and, therefore, is not shown in the figure.
Performance Evaluation of the Flow through Attached Growth Reactor, AGR-2
- AGR-2 Start-up
- AGR-2 Phase-1: Effects of Influent Iron Concentration
- AGR-2 Phase 2: Effects of Arsenic
- AGR-2 Phase 3: Effects of Influent Nitrate
- AGR-2 Phase 4: Effect of Initial pH
- AGR-2 Phase 5: Effect of operating Temperature
- AGR-2 Phase 6: Treatment of Real Groundwater
- AGR-2 Phase 7: Performance at Lower EBCT
- AGR-2 Phase 8: Performance after Shutdown
The iron content in the treated water remained below the detection limits, while the arsenic content in the treated water was found to be 7.6 ± 1.2 µg/l. The increase in COD in treated water could be the result of reduced SO42 reduction in the reactor. The iron removal in the treated water was found to be below the permitted limits after the 6th day of operation in phase 8.
Arsenic in the treated water was recovered to below 10 µg/L after 4 days of operation with 99% efficiency.
AGR-2 Operation with Real Groundwater
Characterization of Bioreactor Treated Water
- MPN Test
- Whole Effluent Toxicity (WET) Test Results
