Efficacy of well disinfection in the prevention of clogging in production boreholes

Abstract

In Uganda, the proportion of people served by groundwater is 60.6%. However, the greatest concern in the operation of groundwater supply schemes is clogging of production boreholes as it jeopardizes their sustainability due to reduced operation and borehole failure. Hoima City relies on groundwater for its supply fed by seven production boreholes. In 2015, to rejuvenate production, four collapsed boreholes were re-drilled as result of iron oxidizing bacteria infestations that led to the clogging of screens, riser pipes and submersible pump break downs. However, some of the boreholes re-drilled in 2016 were already showing signs of iron bacteria related clogging. This study was conducted to evaluate the effectiveness of borehole disinfection with chlorine in preventing clogging in production boreholes. Shock chlorination was carried out with an aim of controlling bacteria populations in the boreholes and subsequent clogging. Two boreholes fed by different well fields, were subjected to two successive shock chlorination treatments with contact times of 24-hours and 48 hours using 3.36kg and 2.35kg of calcium hypochlorite respectively, determined through the chlorine demand test. Before each treatment, the observed biofilm was quantified by measuring its mass per square meter. Water quality analysis results showed that the groundwater from the two boreholes had a low average alkalinity of 138 ± 4.36 and an aggressive index of 10.92 which indicated that the water was moderately aggressive. The risers being made of PVC and steel materials, indicated that corrosiveness was not the source of clogging, and neither were carbonates. The high iron concentration (1.751 ± 0.48 mg/L) coupled with the slime observed in both the riser pipe materials was an indicator of clogging by iron oxidizing bacteria. Increase in the ferrous iron concentration (from 12% to 78%) and the quantity of water abstracted by 14% for BH5 and 34% for BH2 was attributed to the inactivation of iron oxidizing bacteria and to the reductive dissolution of iron from the clay soils in the weathered rock and the fractured granitic bedrock. The 30% decrease in the chlorine dosage required in the second treatment, coupled with a 41.3% reduction in the amount of biofilm formed and 72% reduction of total iron concentration, were indicators of the effectiveness of disinfection. For disinfection to be effective, pumping hours should be constant with , a contact time ranging between 24 and36 hours and quarterly disinfection undertaken due to continuous resettlement of iron oxidizing bacteria.