MATERIALS AND METHODS
4.11 Stability Check of Biosolids
period. After completion of an ageing test, the supernatant was carefully decanted and the wet solid paste was freeze dried.
4.11.2 Toxicity Characteristics Leaching Procedure (TCLP) test
Leaching tests were outlined to estimate the leaching potential of a waste in a time period much shorter than that of disposal. Though various leaching tests are designed in time duration ranging from 1 hr to 13 weeks, the TCLP test is conducted either for 18 hours, known as standard TCLP test (USEPA, 1997) or extended period termed by several researchers as extended TCLP test (Hooper et al., 1998). It has been observed that standard TCLP test for 18 hours is not always sufficient to evaluate the toxicity characteristics of the waste. The TCLP extraction has been tested for longer period (up to 84 hours) and greater arsenic leaching has been found for longer test durations (Clancy et al., 2013; Hooper et al., 1998). Therefore, extended TCLP tests for up to 84 hours have been suggested by several researchers (Hooper et al., 1998; Sima et al., 2015). In this project extended TCLP tests were conducted on backwash solids of AGRs for an extraction period of 84 hours.
Backwash solids used in TCLP
The backwash solids used for TCLP tests were collected by following the procedure as described in section 4.10.1. Triplicate samples of 1g of dry backwash solids, after being dried at 60°C, was digested in a mixture of acids of HNO3 and HClO4 (1:4, v/v) to extract out arsenic and iron present in the solids (Jong & Parry, 2005).
Concentration of arsenic and iron in the digested extract were measured in AAS. Average concentration of arsenic or iron in backwash solids were calculated and expressed as mass (µg or mg) of arsenic or iron per kg of dry solids. Concentrated HNO3 (1 ml) was added to 1.0 g of backwash solids in a digestion tube. When effervescence of brown NO2 subsided, conc. HClO4 (4 ml) was added and digested under reflux at 160°C, 220°C and 250°C for 30, 220 and 180 min, respectively, to reduce the final volume to 2 ml. After the digested samples cooled, the contents were diluted to 20 ml with DI water, mixed using a Vortex mixer (Remi Instruments, India) and allowed to settle overnight at 4°C. The supernatant was then carefully decanted into a 10 ml polypropylene tube (Tarsons) and stored at 4°C until analysis. Total suspended solids (TSS) and volatile suspended solids
(VSS) were measured according to the methods described in Standard Methods for the Examination of Water and Wastewater (APHA, 2005).
TCLP leaching tests
The TCLP test was conducted on stored freeze dried backwash solids in accordance with SW 846 test Method 1311 (USEPA, 1997). To decide on the TCLP leachant as extraction liquid, pH of the backwash solids was measured by adding 96.5 ml of DI water to 1 g of the test material (backwash solid) in a glass beaker after vigorous stirring for 5 minutes (USEPA, 2004). The pH of the backwash solid was between 6.8 and 7.2, and therefore, glacial acetic acid buffered at pH 4.93±0.05 with 1N sodium hydroxide was selected as TCLP leachant (USEPA, 1997). The TCLP tests were conducted in Teflon screw cap bottles (test vessels) keeping a specific leachant-to-solid mass ratio (on wt/wt basis) of 20 in the mixture. All experiments were carried out in triplicate to ensure the reproducibility. The agitated mixture was centrifuged at 5000 rpm for 10 min. The supernatant was filtered through 0.45 µm filter paper, and the filtrate was analysed for arsenic and/or iron. Kinetic and extended TCLP tests were conducted either in nitrogen filled vessels (screw cap Teflon bottles) leaving 50% head space of the gas above the mixture or in vessels (beakers) exposed to air. To take more accurate account for anaerobic landfill conditions, TCLP tests were conducted in nitrogen filled vessels (screw cap Teflon bottles) whereas, vessels (beakers) exposed to air were used to represent aerobic condition. The following are the experiments conducted related to TCLP tests:
(i) Kinetic TCLP leaching test
Experiments of kinetic TCLP leaching test were conducted with the aim to determine the rate of leaching of arsenic and/or iron from the backwash solids of AGR-1 and AGR-2. Kinetic experiments were conducted by gentle mixing of TCLP extraction solution and 1g backwash solid samples (leachant-to-solid ratio of 20) in a test vessel for a period of 24 h. The test vessels were open to air or filled up with nitrogen gas during the mixing. The kinetic TCLP extractions in presence of air were carried out in a 100 ml Teflon beaker with a magnetic flea and the beakers were exposed to air during the mixing.
On the other hand, the kinetic TCLP extractions were carried out in 100 ml Teflon screw cap bottles filled up with nitrogen gas. The mixture in the screw cap bottles was agitated
in an end-over-end rotary shaker at 30±1 rpm. Both sets of experiments were conducted at a temperature of 30±2°C. Samples were taken at the end of the desirable extraction period and filtered samples were analyzed for arsenic and/or iron. A duplicate blank extraction, containing extraction solution but no test solids was also run.
(ii) Extended TCLP test
The extended TCLP test (for 84±1 h) was conducted by extracting the contaminants from a 1g sample of test material with an appropriate extraction fluid.
(iii) Effects of dissolved oxygen (DO) on TCLP leaching characteristics
TCLP test was also conducted to determine the effects of oxygen on arsenic leaching. The headspace in the vessels was varied by varying the headspace-to-leachant volume ratio (Vair/VL) at a constant leachant-to-solid ratio of 20. The Vair/VL ratios of 0.25, 0.50, 1.0 and 1.25 were used. As the size of the extraction vessels were not specified in TCLP test protocol, thus extraction vessels of different sizes were used in order to maintain Vair/VL ratio of 0.25, 0.50, 1.0 and 1.25. Filtered samples were taken at the end of the extraction period and analysed for arsenic and/or iron. A duplicate blank extraction, containing extraction solution but no test solids was also taken.
4.11.3 Long term Aerobic Leaching Test
After useful life of WAC filter bed it needs to be disposed off and replaced by fresh WAC bed. The exhausted WAC bed may behave like a layer of soil. There are several reports on leaching of arsenic from exhausted resins and adsorbents (Henke, 2009;
Jing et al., 2005). After successful operation of flow through reactors for approximately 1000 days, the experiments on long term stability check was conducted on the arsenic and/or iron bearing solid-precipitates and exhausted WAC filter bed, whilst it was still intact in the flow-through reactors. The experiment was intended to simulate the in situ leaching of uncontaminated groundwater through the arsenic bearing solid-precipitate with virtually no physical disturbance in a subsurface environment. The AGRs were operated for 20 days without backwashing prior to this test in order to keep solid phase sulphides remain in the system. The feeding and operating conditions of the reactors were as follows: Arsenic = 500µg/L; Nitrate = 50 mg/L; Sulphate = 25 mg/L; iron = 3.0 mg/L (AGR-2 only) with other components as mentioned in table 4.2. After 20 days of
operation, the original influent media was drained to maximum possible extent and the columns were subjected to a sequential leaching process by passing aerobic water through it. The new influent media of pH 7.0±1, prepared in DI water without addition of any metal was charged into the AGRs through the influent port using a syringe pump (200 µL/min). The dissolved oxygen was maintained between 4.6-5.4 mg/L in the new influent and intermittent supply of oxygen was also done in order to maintain the DO level constant in the aerobic water. The sample was collected in every 24 hr and the parameters were analyzed for pH, arsenic and iron concentrations in the leachate. Aerobic leaching test was stopped after 110 days, which is equivalent to 220 pore volumes. After completion of test, the solids were collected under a flow of nitrogen by backwashing and freeze dried. The freeze dried solids were stored at 4°C and subjected to XAS analysis for their characterization.