SHORT-TERM LEACHING OF ARSENIC
3.4 MATERIALS AND METHODS
conduct the appropriate analyses, and combine the results mathematically by using a simple volume-weighted average:
Where
VI =The volume of the first phase (L)
C1 =The concentration of the analyte of concern in the first phase (mg/L) V2=The volume of the second phase (L)
C2
=
The concentration of the analyte of concern in the second phase (mg/L).safe disposal of these wastes and possible contamination environment from arsenic present in the wastes. However, there is only limited data on the quantities and characteristics of these wastes (e.g., Hamel and Zinia, 2001) and possible mobilization of arsenic from these wastes.
In this study, both of the above mentioned wastes have been collected. Arsenic rich wastes, generated from coagulation-based arsenic removal systems, have been collected from two different types of ARUs. These are:
• STAR bucket treatment unit (based on iron coagulation), and
• BUET-UNU bucket treatment unit (based on ferric chloride coagulation)
Wastes from the first type of ARU have been collected from Laksmipur district, where a number of such ARUs are currently in operation. Two types of samples have been collected from the BUET-UNU bucket treatment units, slurry from the upper bucket and sand filter media from the lower bucket. These wastes have been collected from the BUET-UNU field site at Adda village in Barura thana of Comilla district.
Arsenic rich wastes have also been collected from three different types of filtration- based arsenic removal systems. These are:
• "Shapla" filter (based on filtration through iron-coated brick chips),
• BUET-UNU iron-coated sand filtration units and
• SIDKO granular ferric hydroxide units.
Spent media from "Shapla" filter have been collected from Ashwadia union in Noakhali Sadar Upazila of Noakhali District. A number of such ARUs were installed in this region and are currently in operation. Spent iron-coated sand media have been collected from Adda village in Barura Thana of Com ilia district. SIDKO filter media have been collected from Manikganj district, where a large arsenic treatment plant is currently in operation.
3.4.2 Estimation of Treatment Waste Generation
Quantity of wastes generated from arsenic removal processes was evaluated for above mentioned arsenic removal systems. In the BUET-UNU bucket treatment unit, a chemical packet (containing ferric chloride, potassium permanganate and bleaching powder) is added to 25 litres of water in the upper bucket of the unit. After stirring for a few minutes, the water is kept undisturbed for about one hour and then the water is allowed to flow to the lower bucket through a tap located at the bottom of the bucket.
Information gathered from the field suggest that after each run, about 24.5 litres of treated water is produced and about 500 ml of slurry (arsenic rich) containing mainly ferric hydroxide floes accumulate at the bottom of the upper bucket, which is later disposed of. Thus, waster generation rate for this unit is about 500 ml slurry per 24.5 Iitres of treated water. The STAR filter runs in a similar principle. However, information gathered from the field suggests that most users dispose of the slurry (accumulated at the bottom of the bucket) after treating two or three buckets of water.
It was estimated that on an average, about 250 ml of slurry waste is generated per 40 Iitres of treated water.
In each unit of 'Shapla' filter, about 20 kg of filter media (iron-coated brick chips) is used for treatment of water. This media can treat about 3000 to 6000 Iitres of water (depending on arsenic and other water quality parameters). The media is discarded after exhaustion. Thus, on an average, 20 kg of arsenic rich spent filter media is produced for about 4500 Iitres of treated water. Good estimates of some other types of arsenic removal units (e.g., SIDKO granular ferric hydroxide units, BUET-UNU iron- coated sand units) could not be obtained due to lack of reliable field data.
3.4.3 Determination of Raw Arsenic Concentration in the Sample
Arsenic rich treatment wastes (liquid or semi-solid and solid) collected from the aforementioned treatment units were stored in the laboratory and a portion of each of the sample was used for the determination of total arsenic content. For solid samples, the sample was dried in an oven at a temperature of 110° C for 24 hours to ensure that there was no moisture left in the wastes. The sample was then digested using aqua
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regia as described in the Standard Methods (Clesceri, et aI., 1998) for total extraction of heavy metals. Briefly, the sample digestion procedure consisted of the following steps: the sample was ground and 5 gram of it was taken in a volumetric flask. Then 5 ml of concentrated nitric acid and 15 ml of concentrated hydrochloric acid was added to the sample and it was kept overnight. After that, the sample was heated for 2 to 3 hours to boiling. After heating, it was allowed to cool and volume was adjusted to 500 ml by adding deionized water. Finally, the sample was filtered and stored. Following extraction, the amount of arsenic present in each extracted liquid was determined using an Atomic Absorption Spectrophotometer equipped with a graphite furnace (Shimadzu Model AA-6800). The digestion procedure for the liquid sample was same as that for solid sample with the exception that in this case 50 ml of sample was taken instead of 5 gm, which was used for solid samples.
3.4.4 Carrying out TCLP
Leaching characteristics of wastes collected from different arsenic removal units have been evaluated through Toxicity Characteristic Leaching Procedure (TCLP) developed by USEPA. The procedure was described earlier in this chapter. TCLP was performed for sixteen semi-solid (slurry) waste samples, of which fourteen samples were collected from STAR bucket treatment units while the rest two samples from BUET-UNU bucket treatment units. TCLP was also carried out for nineteen solid waste samples, of which fifteen samples were from Shapla filter, two from BUET- UNU iron coated sand units and two from SIDKO units. All TCLP tests for solid samples were performed in duplicates and averages of these results have been reported here. Detail results ofTCLP for both semi-solid and solid samples have been presented in Appendix B. In all cases, arsenic concentrations in the TCLP extracts were measured using an Atomic Absorption Spectrophotometer equipped with a graphite furnace (Shimadzu Model AA-6800).