Volume 44 Number 11
World Spotlight: Electrolytically Active Water -- Using Chemical-Free Technology to Clean Up Wells in Botswana
Summary: Electrochemically activated water (EAW) technology is allowing Botswana residents greater access to drinking water from their wells. By using anolyte, EAW is gaining wider acceptance because of its simplicity, efficiency and relatively low costs.
Botswana is a dry country with an annual average rainfall of 400 millimeters (15.7 inches). Lack of surface water in most parts of the country means that wells are required to cover domestic and industrial water needs. Currently, there are more than 10,000 water wells in operation, covering the needs of rural communities and agriculture and including 29 major wellfields.
Many wells show a marked and continuous decline in yield from inception. In 2001, the government of Botswana through the Department of Water Affairs (DWA) started to investigate the causes of well deterioration and identify and test appropriate technologies to improve the maintenance of borehole infrastucture.
Historically, the declined yield was attributed to corrosion and scaling, caused by unfavorable chemical characteristics of groundwater. Corrosion is so severe in some wells that pumping equipment and rising mains have to be regularly replaced, often at intervals of less than 12 months.
Avoiding the ‘slime’
Many methods for successful treatment of wells affected by bacteriological growth have been documented, such as treatment with hydrochloric acid or other strong chemicals. These methods necessitate using chemicals, however, which are hazardous to the semi-skilled staff involved at the DWA.
To minimize these problems, the DWA along with a company developing electrolytically active water (EAW) treatment technology (see Principles of EAW Technology) initiated a water well rehabilitation program using anolyte. Anolyte, a super-oxidant solution, has proven to be at least as effective as hypochlorite, and completely safe and non-polluting.
Well problems in Botswana
Groundwater in Botswana occurs mainly in sedimentary aquifers and is restricted to semi-confined conditions and some basement aquifers. Wellfields most affected by yield and corrosion problems are within sedimentary environments. Explosive bacteriological growth is likely caused by widespread organic layers such as coal -- containing organic carbon as the prime source of energy -- and allowing microorganisms to flourish within the porous structures of the aquifer. With time, the flow conditions within the system deteriorate. It’s not yet understood why a sudden burst of biofilm growth affects the well and pumping gear. It’s believed that short pumping intervals, typical for most wells in Botswana, play an important role.
In non-sedimentary basement aquifers, nutrient flow within fractures is probably influenced by weathered layers containing important by-products such as iron or manganese. Semi- to unconfined flow conditions within the fracture system permit continuous oxygen inflow and buildup of biofilm.
In Botswana, biofilm -- caused by microorganisms -- is now acknowledged as the main cause of well performance problems. As it develops within the screen section, biofilm grabs silt, clay and mineral particles, which originate from shale layers typical of many sedimentary aquifers in Botswana. They move with the groundwater flow into the well. Some by-products associated with bacterial growth, such as oxidized iron and manganese, also accumulate. Finally, the well and often the pump components are blocked by a thick biofilm while mineral mass obstructs the groundwater flow from the aquifer into the well.
1. Many boreholes have a considerable amount of stagnant water below the lowest inflow zone. These boreholes are constructed as open holes where even significant inflows were considered useful. The unproductive lower zones have rarely been refilled, and wells with more than 50 meters depth of stagnant water are not uncommon.
2. Many boreholes have unfavorable pumping cycles of 10 hours pumping and 14 hours non-pumping per day. The main reason for the short pumping cycle in Botswana is the remoteness of wells and wellfields, and the need to supervise the equipment during daylight hours. Therefore, wells are pumped at a higher-than-normal rate. This production cycle results in a large daily decline of water levels and large expansion of the drawdown cones and inflow of oxygen to the temporarily denuded aquifer zones. During the non-pumping hour, bacterial cells have ample time to settle and anchor to surfaces in the aquifer around the well intake, the well itself, inside the pumping equipment, and strengthen and harden their cell coatings. This hardening makes standard treatment chemicals ineffective.
3. The traditional use of mid-steel for casings and screening favors the settlement of bacteria and development of biofilms.
The full complexity of the interactions of the hydrogeological environment borehole design, and pumping schedules on bacterial activity in wells isn’t yet fully understood and needs further investigation.
Many boreholes in Botswana are remote and difficult to access with equipment and material. A mobile device was designed, which allows production of anolyte at the site itself. This eliminates the need to transport large volumes of fluids, and the mobile device uses the in situ well water to produce the anolyte. The mobile device is capable of producing 5,000 liters (L) of anolyte in a 20-hour cycle. The device ensures that anolyte is available in large quantities. Production costs are minimal and generously introduced anolyte guarantees more effective cleaning and deeper penetration of the aquifer.
The test treatments with anolyte were carried out only on boreholes that had experienced considerable decline in yields. Biofouling was identified using a downhole camera and microscopic tests, followed by visual inspection of pumps and discharge lines to confirm the presence of iron or sulfate biofilms on pumps and pump intakes. Wells most affected were selected for rehabilitation with the EAW technology. The method that has proven to be most effective for heavily clogged boreholes in Botswana is explained this way.
Then, after an exposure time of approximately 60 minutes, the wells are mechanically bailed using a surge block attached to the cable tool rig, starting at the top of the screen and progressively working down. The motion of the tool forces the anolyte through the gravel-pack into the aquifer. This process normally lasts three to four hours. The efficacy of the anolyte solution is tested at regular intervals with additional anolyte added as required. Afterwards, sediment and loosened biofilm are cleaned out with a mechanical bailer. Finally, the pump is reconnected with water pumped from the well until clear. The well is then reconnected to the system.
A quantity of 1,000 to 2,000 L anolyte was sufficient to efficiently clean most boreholes, which were between 100 and 250 meters (m) deep and had an 8-to-10 inch internal diameter. Larger volumes might be necessary for deeper boreholes or if the aquifer is heavily biofouled. In one case, 4,000 L had to be introduced.
Results of treatment
Although testing the EAW treatment process in well rehabilitation is still in its initial investigation stages, it’s proving to be an exciting and effective treatment method for Botswana. Some advantages are:
With further refinements to the treatment process, EAW treatment should prove to be effective in rehabilitating not only water wells but also biofouled aquifers. This method is therefore of potential interest for all drilling contractors with interest in the restoration of boreholes.
Editor’s note: The above article is reprinted from Water Well Journal’s August 2002 issue with permission of the National Ground Water Association. Any request for republication, in whole or part, should be directed to the association.
About the authors
Principles of EAW Technology
Electrolytically active water technology generates oxidant and antioxidant solutions -- anolyte and catholyte, respectively -- by forcing slightly saline water (0.2 percent) through an electrolytic cell. The water can be taken from the well.
The inventors describe the activation process as a change of molecular state of water from a stable to a metastable state.2 Thus, the antioxidant solution, catholyte, contains negatively charged free radicals while the superoxidant solution, anolyte, has positively charged free radicals and is a powerful bactericide. Anolyte has an oxidation-reduction potential (ORP) of +1,000 millivolts (mV) and catholyte of about -800 mV. Biocidal agents in the solutions are free radicals with oxidizing effects such as ClO, HClO, H2O2, O3 and OH.
Both solutions are nontoxic and fully biodegradable. EAW is highly microbicidal and attacks living cell components developed within and outside the well environment. The electrolytic cell is around 20 centimeters (cm) long and consists of a central rod anode (positive electrode), which is surrounded by a concentric ceramic tube membrane that acts as the cathode (negative electrode). The cell itself isn’t much thicker than a pencil.
Effectively, the ceramic membrane divides the reactor into two compartments -- the anode compartment and cathode compartment. Water enters the reactor and exits from these compartments separated by a high electric field as two streams (the anolyte and catholyte solutions, respectively), whose pH, ORP and other physiochemical properties lie outside of the range that can be achieved by conventional chemical or electrochemical means.
EAW technology isn’t new (see EXTRA). This system, however, with its ability to separate the anolyte and catholyte solutions, is unique. The solutions can be applied separately to maximum effect. This wasn’t possible with previous systems. Recent advances have further minimized the size of the electrolytic cell and associated equipment as well as increased the efficiency of anolyte and catholyte separation (see Table 1).
Table 1. Properties of Anolyte and Catholyte Solutions
EXTRA: Understanding Anolytes and Catholytes
For a better grasp of some of the concepts in this article, here are a few online resources: