The clogging of borehole systems due to contamination with iron oxide, manganese oxide and calcium carbonate is a worldwide problem. This problem causes a reduction in pump efficiency. Recent tests using ultrasonic devices supplied by the Belgium-based company, Harsonic, have shown that a long-term solution to this problem is available.
The majority of these deposits are the result of bacteriological-induced oxidation processes. In these, sometimes complex reactions, bacteria play a significant role. Bacteria need a so-called biofilm matrix to propagate. This slimy layer provides an ideal environment for the propagation of the bacteria .
In the past, chemicals with bactericidal properties were used to control these outbreaks. This method proved to be inefficient and costly.
Using an Ultrasonic Device
The ultrasound solution attacks the problem from a completely different angle. With specific ultrasound waves, the binding cells which form the biofilm matrix are subjected to an appropriate resonant frequency, which results in the breaking down of the matrix. Once the biofilm is broken down, it can be taken out easily during normal borehole operation. By washing out the unprotected bacteria, the root cause of the mineral deposits is eliminated.
The success of the application is simply related to the fact that the mineral depositing process largely relies on the initial development of biofilm in boreholes and pumps. If this process can be prevented, the clogging action of the biofilm and mineral deposits will become a thing of the past.
This method uses water as the carrying medium for the ultrasound waves. It has been proven that these waves can go through open check valves and through the pump head. In this way, ultrasound waves, which are emitted by an above-ground ultrasound transducer, do their job in the underground borehole and screen.
The tested ultrasonic devices have been applied very successfully for more than ten years worldwide to prevent the development of biofilm in various applications, such as on ship hulls, in cooling systems and small diameter water distribution systems. This led to the belief that it could also solve the borehole clogging problem.
Case Study in South Africa: Control of Fe deposits
The first tests were performed in 2017 by Parac, the South African agent for Harsonic, in the Koo Valley. Many boreholes are affected by the build-up of mainly iron oxide deposits causing severe clogging that drastically reduces pumping efficiency. These boreholes are the main source of water for the Koo Valley Irrigation Board. The tests were conducted, in cooperation with the Board, under the supervision of the chairman and local farmer, Marais Rossouw.
The clogging problem is severe and it is not unusual to experience approximately 90% reduction in pumping efficiency within three months of operation. Several products have over the years been applied to address this problem with little to no success. Pumps have had to be removed from the boreholes on a regular basis and cleaned by a pump service provider.
The Harsonic device was installed in a test borehole and after eight months of continuous pumping, there was no reduction in pumping efficiency. The pump was removed from the borehole for inspection and no evidence of deposits or clogging was found. Both the pump and delivery line were as good as new. This borehole has now been performing at the same efficiency for two years.
Summary of results
Pump capacity: 5500 litres per hour
Problem: Iron deposits resulting in lower flow and pump failure after 8 months
Harsonic unit installed 3 weeks after replacement pump was installed
Initial flow rate: 4500 litres per hour
After 8 months of operation, flow rate remained 4500 litres per hour
No deposits in the pipework
50% reduction in energy consumption per cubic metre pumped
Four additional boreholes were fitted a year later, with similar results. It is the intention to install one of these units in all the affected boreholes. The boreholes are approximately 250m deep with yields varying between 4000 and 5000 litres per hour.
Over the past year, the Harsonic devices have also been tested on larger boreholes in Bastrop in Texas, USA, with a depth of 105m and yield of 200 000 litres per hour. The problem was also iron oxide clogging with severe pumping efficiency reduction within three months and pumps having to be replaced every two years. Records show that after 5 months of operation in these conditions, there has been no deterioration in borehole yields.
Further tests have now also been completed in Burkina Faso where the deposit of calcium carbonate is a severe problem. In this case, the clogging problem was not only completely solved but water turbidity was reduced from 18.8 NTU to 0.82 NTU. The reduction in calcium carbonate content brought the quality of the water within the minimum requirements set by World Health Organisation (WHO) for potability.
Types of Ultrasonic Devices
Two sizes of ultrasonic devices are available.
For boreholes up to 250m deep and 63mm diameter, Harsonic devices are installed in line with a y-piece on the feeder pipe directly before the bend into the borehole. These units use very low power - approximately 6 Watts.
On larger boreholes up to 400m deep and 250mm diameter, a larger unit is required. Installation could be by attachment to a metal riser pipe or installed in line in a non-metallic discharge line. Power consumption is approximately 20 Watts.
There is no maintenance requirement and the devices are guaranteed to remain fully operational for at least five years. Recovery of the cost of the unit through significantly higher delivery rates, much lower energy consumption and Mean Times Between Failures (MTBF) has been shown to be as low as one to two years.
Although it is preferred that the devices are run continuously with water contact, it was found in the Koo Valley test cases that even with a four-month downtime, the results were still perfect.
All-in-all the ultrasonic solution to prevent the clogging of boreholes has now been proven to be successful as originally anticipated.
Submitted by Dr Leon Geustyn PrEng. 083 626 8491 on behalf of the author Mr Koen Kinsbergen, director of Harsonic Bvn .