The main components of the filter unit are Ceramic filter, Iron net/ Scrap iron/ Iron rod, Iron bacteria sludge, Reactor (14 16 L Clay pot was used), Effluent storage bucket, Wooden stand etc.
2.1 Manufacturing of Ceramic Filter The filter was made with locally available and cheap materials as rice bran, clay soil and water.
— Oven-dry soil ground with the hammer. Then soil and rice bran were screened through 0.5 mm and 1 mm sieve respectively. Soil (640g for 1 filter) and rice bran (160g for 1 filter) was taken in a ratio of 80:20.
— Soil and rice bran was mixed homogeneously with water to make dough. Then dough was placed around the bar of the dice and two pieces of PVC pipe were pushed by hand from both sides to make a cylindrical shape.
— Next, the pipes were taken off and the surface of the filter was polished with water. The total frame was then toppled down to remove the dice (Figure 5).
— The resulting cylindrical ceramic filters were hollow with one side open. This soft filter was then dried in the sun for at least 3 days.
— The air-dried filters were burnt in a potter kiln at 900 to 1000. After continuous burning for 6 to 8 hours, the kiln was kept to cool down. After some hours, the filters were taken out from the kiln. The final ceramic filters had a height of 10 cm and a thickness of 2 cm.
2.2 Preparation of Iron Net 600 gm commercially available iron net without coating was taken and an 11cm×11cm×11cm cube with one side open by the iron net was made.
2.3 Preparation of Iron Sludge Tap water from KUET was filled in a big drum of a capacity 100 L. Some iron net, iron bar and other iron materials were added in the drum. No other additional nutrients were added to the water. This was aerated with a stick for 5 minutes daily to ensure sufficient dissolved oxygen in the water for the biological oxidation of iron. The iron bacteria layer will be deposited at the bottom of the drum after 10-15 days.
2.4 Double Unit Filter System Set up Raw influent was poured into the first reactor and the filtrated effluent was automatically poured into the second reactor and the final effluent was found in the storage bucket.
2.5 Conceptual Arsenic Removal Mechanism This removal of arsenic occurred due to the oxidation of iron and arsenic followed by their subsequent adsorption and precipitation on and with biologically produced iron hydroxides. Biological oxidation of iron by iron bacteria is the main mechanism in respect to the removal of arsenic in this study. Both forms of inorganic arsenic (AS (III) and As (V)) could be removed more efficiently during iron oxidation than formed iron precipitation. This might be because a very fine iron hydroxide floc is produced which had a high adsorptive surface area and high binding energy resulting in the effective removal of both forms of arsenic at the beginning of biological iron oxidation. Firstly Fe (II) oxidation is catalyzed by the iron bacteria and transformed to Fe (III). Secondly, a part of As (III) is oxidized to As (V) in the presence of Fe (II) and the iron bacteria. Finally, adsorption of As (V) on iron hydroxides occurred.