Design and evaluation of a horizontal filter unit for ground water recharge through abandoned tube well

Abstract

The groundwater table is declining at an alarming rate and it is essential to replenish the dried out aquifers by adopting proper artificial recharge methods. Field experiment on the design and evaluation of a horizontal filter unit for groundwater recharge through abandoned tube well was conducted in the research field of Nodal Water Technology Centre, College of Horticulture, Vellanikkara. The specific objectives of the study were to design and develop a horizontal filter unit with alternate filter media for treating storm water runoff, and to evaluate the developed filter for hydraulic and pollutant removal efficiencies. Suitability of the tube well for recharging and availability of adequate amount of source water were analysed in the primary stages of study. The peak runoff expected from the area was computed and compared with the carrying capacity of the existing conveyance channel. Thus the existing channel was modified and a masonry structure with a filter unit for treating runoff and a recharging section was constructed. The filter unit had five compartments filled with Gravel, Sand, Charcoal, Synthetic fibre and Coir fibre combinations as treatments, T1: Gravel, Sand, Coir fibre, Gravel; T2: Gravel, Sand, Synthetic fibre, Gravel; T3: Gravel, Charcoal, Sand and Gravel. Three sets of length variation in three different media combinations were selected as factors, F1: 80 cm, 80 cm, 80 cm, 80 cm; F2: 80 cm, 100 cm, 60 cm, 80 cm; F3: 80 cm, 60 cm, 100 cm, 80 cm Inflow and outflow water quality was analysed for evaluating filter hydraulic and pollutant removal efficiencies in simulated and actual runoff conditions. Treatment T1F1 was better in pH normalising efficiency, T1F2 removed Nitrate and Sulphates effectively, T1F3 was better in Total hardness and Calcium removal efficiency, T2F1 performed well for Magnesium and Chloride removal efficiencies. Treatments T2F2 and T3F3 were effective for EC, TDS, Salinity, TSS, Turbidity, Acidity, Fluoride, Alkalinity, Iron removal and Hydraulic efficiencies. 135

The Gravel (80 cm), Charcoal (100 cm), Sand (60 cm) and Gravel (80 cm) combination was selected as best filter media combination and it was installed in the field for recharging. With an average annual rainfall of 2795 mm in the study area, a runoff depth of 1118 mm can be expected. Accordingly a runoff volume of 2.3 million litres of water can be diverted from the study area to the recharge well annually. Comparison of this enormous benefit from the artificial recharge structure with the cost incurred is indicating the promising future of the artificial recharge schemes. If the abandoned open wells and tube wells in Kerala are utilised as recharge wells, a large quantity of water can be recharged annually and it will improve the groundwater potential of the state for future benefits.