Expansion design exceeds expectations

sedimentation tank image

The new sedimentation tank under construction

The extension of the Meyerton WWTW by the construction of a new works, parallel to the existing works, designed with a capacity of 15 M/d and allowing for an extension to 36 M/d in future is more than originally projected due to a different approach to design, Barry van Es, senior associate at GIBB tells Chantelle van Schalkwyk.

“The design not only allows for an extension to 36 Mℓ/d in future, but is more than the originally projected capacity of 30 Mℓ/d due to a different approach to the design, which, instead of duplication of the 15 Mℓ/d works, will consist of the addition of primary sedimentation and an increase in aeration capacity in the biological reactor,” explains Van Es.

As an initial phase of the project, one of the two new secondary sedimentation tanks (SST) is currently under construction, together with appurtenant structures such as a sludge recycle pump station and a hydraulic flow splitter. “This was done to relieve pressure on the sedimentation tank of the existing works, which is in a poor condition and requires maintenance. However, because the old works has only a single SST, this refurbishment can only be done once the new sedimentation tank is operational,” says Van Es.

Basic background

The Meyerton WWTW is the largest of three works managed by the Midvaal Local Municipality. The final effluent from this works discharges into the FourieSpruit, which is a tributary of the Klip River. The Klip River discharges into the Vaal River at the barrage.

“In a Rand Water feasibility study report of October 2010, it was stated that at the time the works received an average of about 12 Mℓ/d of wastewater from domestic, industrial and commercial sources via two pump stations. The works was designed and constructed in the 1970s and was designed to treat 5 Mℓ/d,” continues Van Es, adding that it was subsequently extended in 2007 to a capacity of 10 Mℓ/d, which is less than the current discharge to the works. As a result, the works remains under pressure due to the lack of adequate treatment capacity.

“The project was therefore initiated based on the findings of the Rand Water feasibility study, which recommended that an additional works with a treatment capacity of 15 Mℓ/d be constructed, and that this be designed in such a way that it would be possible to increase its capacity by another 15 Mℓ/d in future.”

Project progress

Design of the 15 Mℓ/d extension is currently complete and awaiting environmental approval before implementation. Construction of the first phase of the project –consisting of the addition of the new SST, hydraulic flow splitter, sludge recycle pump station and new chlorination system– was underway when Water & Sanitation interviewed van Es and expected to be completed by the end of June 2013.

For the first phase of the project, which consists of the construction of a new SST as mentioned above, the main contractor is Lichenry/Mariri Joint Venture, which is local to the area. The primary subcontractors are Khayalami Projects for concrete work; and Millennium Pumps, which provides mechanical and electrical services.

According to Van Es, the contractor’s positive attitude and the high degree of cooperation between the main contractor and subcontractors, as well as with the construction monitoring staff on-site, contributed to good progress and a minimum of delays in the execution of the project.

“The next phase will follow as soon as approval from the environmental authorities is received, which is expected soon,” says Van Es, adding that although as yet there is no construction programme for the next part of the project, a construction period of two years will be allowed. It is expected that the new 15 Mℓ/d works will be completed in 2015.

Challenging context

The greatest challenge on-site to date has been, according to Van Es, connecting a pipe between the existing biological reactor and the existing SST, “as this involved the cutting of the pipe and the installation of three gate valves and a tee that will allow the flow from the biological reactor to be diverted to the new SST. This had to be done quickly, as the works could not be shut down.”

Van Es explains  that the connection is required to allow flow to be diverted from the existing SST to the new SST, which will make it possible to make repairs to the existing tank. “To install the interconnection between the biological reactor and the new SST, the aerated balancing tank at the inlet to the works was first drawn down completely, so that storage capacity would be created.”

In addition, the water level in the biological reactor was lowered and the last small compartment from where the flow is discharged to the SST isolated and pumped out.

Van Es adds that as soon as this had been completed the pipe was cut (excavation had been done some time before to expose the pipe over the length required to execute the work) and the water that was contained in the pipe was pumped out of the working area. “The existing pipe consists of a combination of a 450 mm diameter steel pipe and an old 450 mm diameter fibre-cement pipe, and a new 400 mm steel pipe was inserted inside the cut ends, and the surrounding gap filled with a quick-setting epoxy. The epoxy was inserted after the rest of the pipe work and valves were installed to ensure correct alignment. After the epoxy had set the flow between the reactor and the existing SST was resumed. No leaks were found and the operation had required a nine-hour interruption in flow between the reactor and the SST. The balancing tank was able to contain the incoming flow for that period and no overflows occurred,” states Van Es.

The works is not set to use any particular new technologies or processes, which eliminates a larger number of possible challenges both during the construction and operation phases. “The processes used are very similar to the existing and this was done firstly because the required effluent quality can be achieved without the need for more complex processes, and secondly to maintain the operating and maintenance staff’s familiarity with the process and the equipment installed,” says Van Es.

Positive project

There are a number of positive aspects to the project, not least of which Van Es notes as his highlight of the project, namely that the project provides the opportunity to increase wastewater treatment capacity through the design and construction of a new section of the works, with up to date technology while maintaining the simplicity of operation of the old works.

“As part of the improvement, a new head of works (HOW), which will be equipped with mechanical screens and vortex degritters, has been designed. This will replace the existing HOW with its hand-raked screens and velocity-controlled grit channels, which are awkward to clean and maintain, and unpopular with staff. This will improve working conditions of staff at the works significantly,” states Van Es.

Another aspect is the positive impact the expansion is set to have on service delivery in the region. “The existing works is overloaded and this has affected the ability of the municipality to approve residential development, as the sewage generated in such developments will add to the load on the works.”

He adds that people generally do not spend much time thinking about sewage treatment. They “flush and forget”, and in this case it is not likely to be different. “Although the benefit to the community may therefore not be obvious in a general sense, the extension of the works will allow for further growth and development in the municipal area, while also ensuring that the treated effluent discharged to the environment is of an appropriate and legally acceptable quality.”

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