Aeration system proposed to improve water quality of the Boksburg Lake

The Ekurhuleni metro plans to install a new aeration system in the Boksburg Lake to improve the water quality. Many critics have, however, already shared their deep reservations about the proposed plan for the lake. They described the plan as a recipe for disaster, saying it was used in many places in the past and has failed to deliver positive results. Seen at the lake is the Ward 32 councillor Marius de Vos.

To improve the quality of the Boksburg Lake water, the Ekurhuleni metro plans to install aeration units in the lake which authorities believe will improve water circulation and oxygenation.

According to metro spokesperson Themba Gadebe, the first step in the restoration of the lake will be to install three 45 kW aerators into the lake and then to monitor the results.

It was also stated that with the aeration of the water and improved dissolved oxygen content, aerobic conditions will return and this will in turn aid the digestion of the suspended solids (SS).

Consultants SRK had already completed the scientific studies and has recently drawn up the rehabilitation plan to be implemented to improve the condition of the lake.

Gadebe said SRK was after a public tender process awarded the project, adding that the full awarded tender amount is R2 231 275.

“The cost included a sub-bottom survey, specialist modelling from siltation, silt and water sampling and a range of other specialist studies, engineering studies for alternatives and environmental authorisations,” said Gadebe.

The service provider has, however, indicated that it is possible that only this intervention could restore the lake, but if the SS levels and nutrient loading are not resolved with aerators, the next step would be to filter out the SS.

The removal of the SS would require the construction of low crest weir and the establishment of a filtration plant.

Below are some of the questions posed to the metro regarding the proposed lake rehabilitation project, sub-surface water aeration and mixing.

How much will this newly proposed project costs?

The current project is exploring the alternatives and the estimated costs are part of the outcomes, and what is being considered for this project. We have some cost estimates at this time, but it would be better if the report, once finalised, is viewed in context to understand costs but also the reasoning in the weighing of alternatives.

Many critics are already saying, to them, this plan sounds like a recipe for disaster. What guarantee can you give that your plan will achieve the desired effect?

There are many contributing factors, such as the elimination of point source pollution that contributes to the state that the lake is currently in. All alternatives suggested have substantiating case studies and alternatives to refer to. The alternative that is suggested has a staggered approach, suggesting testing as each stage is implemented to verify the suggested outcomes.

How long will it take the sub-surface aeration to yield positive results?

From other case studies and similar situations, it showed that the aerator initially stirred up sediments and increased the turbidity of the water and there was an increase in the release of hydrogen sulphide and as the dissolved oxygen in the water increased, conditions improved over two to three weeks.

In the long term, it had a marked effect on improving the condition of the water. This can however not be viewed in isolation. If there is continued upstream pollution this can slow or worsen the situation.

The removal of the suspended solids from the inflows then becomes the next problem to overcome. The lake also has an attenuation effect and can also cause upstream and downstream flooding which had to be considered.

Where have the proposed sub-surface aerators been utilised before and delivered solutions to a problem similar to the present condition of the Boksburg Lake?

Aerators have previously been successfully used in Boksburg lake. These were the turbine type and were damaged by polypropylene bags, tyres and other debris from the lake and as a consequence removed. The proposed vertical shaft aerators proposed work in a different way and have an anti-fouling propeller.

These were used for approximately 10 years in the Brakpan Lake and within a week of the removal of these units the Blue-Green algae proliferated. The installation of the aeration units will include a screening pen to filter out large debris.

What’s the maximum depth of the lake?

A bathymetric survey was executed in October 2016 to establish the present bottom topography of the lake and to determine the sediment thickness of the lake bottom. The maximum depth of the lake is 4.5m below the full supply level.

Has the sludge in the lake been measured?

A study of the silt composition and depth has been done.

Should this plan fail to yield positive results another option will has to be considered. The second option to be considered is apparently filtration in order to remove the suspended solids. How much will it cost to filtrate and dispose of the SS?

There are a number of options that were considered. The filtration is, however, part of a staggered approach for the same solution. Lake rehabilitation is a dynamic process and it is not considered a failure if the aerators don’t solve the lake’s problem alone.

The objective of the aerators is to increase the dissolved oxygen in the water so that an aerobic environment is returned.

Bodies of water have a natural ability to cleanse themselves of organic matter and nutrients, if there is an overloading of these, the systems “stalls” and anaerobic conditions prevail.

Unaided oxygen dissolves into a lake through the surface and if the dissolved oxygen to the organic matter and nutrients are balanced, the lake digests them.

As the organic matter and nutrients increase so does the demand for chemical oxygen, depleting the oxygen faster than it can be acquired through the surface. One of the functions of the aerators is to increase the dissolved oxygen levels to compensate for the higher demand of oxygen and try and bring it back into balance.

This is to use natural means to rehabilitate the lake without the addition of chemicals that would have other potential side effects.

Where will the filtered waste be disposed of?

The waste will be contained and disposed of at a municipal waste disposal facility.

SRK also previously presented a report with four other remediation options to the municipality. Each of these options outlined how to improve the water quality of the lake and how to prevent further contamination. Why were these options not considered?

A whole process was gone through where alternative remediation options were considered. These options were weighed with a number of considerations, such as cost, scientific proof outcomes, impact on the surrounding areas such as the nearby hospital and residents, and sustainability. The medium and long-term implications were also considered.

What is the scope of work done on the lake?

Due to the increased sedimentation, as well as the deterioration of the water quality in the Boksburg Lake, it has become necessary to develop a rehabilitation plan for implementation to improve the condition of the lake for use as a public amenity.

This process forms part of the Lakes and Dams Flagship programme implemented in the municipality to optimise the use of these amenities by the public.

This includes phase 1 (status quo assessment), phase 2 (lake and catchment rehabilitation plan), phase three (stakeholder engagement) and phase 4 (authorisation applications).

ALSO READ: Boksburg Lake is an unhealthy, unattractive environment – councillor 

Metro to restore the Boksburg Lake to its former glory |

A ‘dam’ shame – Lake of Doom desperately needs remidiation

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  AUTHOR
Fanie Mthupha
Journalist

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