Following a pollution crisis in the Berg river, the Stellenbosch municipality, in the Western Cape, contracted consulting and contracting engineering services provider Prentec, in 2010 to design a new emergency sewage treatment plant at Franschhoek to eliminate the risk of any further pollution to the surrounding environment.

Prentec process director Adrian Viljoen says the construction of this temporary wastewater treatment plant, as an intervention to ensure that the environment is protected without delay, is a fine example of contributing to the Department of Water Affair’s National Water Week theme, Water is life: Respect it, conserve it, enjoy it.

He explains that the plant uses the activated sludge process with extended aeration in a sequencing batch reactor (SBR).

Two SBRs were installed in parallel, each with a capacity of 625 m3/d, with two decant cycles a day. Each decant discharges 313 m3 every twelve hours.

The SBR was constructed from preformed proprietary fusion-bonded epoxy-powder-coated steel plates and bolted in place on a concrete floor. This was done as required by the client to allow easy future dismantling of the SBR for possible deployment at another location.

Viljoen says the plant erection was completed in about 12 weeks, inclusive of long lead items such as the bolted tanks and geared motors.

“The plant is fully automated and the sequence is controlled by a programmable logic controller (PLC) while operator-plant communication is achieved with a human-machine interface (HMI). The PLC is preprogrammed and the setting of the various sequence times is performed by the HMI, which indicates the status of the plant regarding the process steps, right through to the status of valves and instrumentation,” he explains.

Disinfection is achieved by gaseous chlorination through a dedicated chlorinator for the Prentec plant.

The raw sewage enters the SBR plant through an existing raw-sewage reservoir that is fed from the discharge of the existing inlet works, which comprise a grit-removal channel followed by a mechanically raked screen. The raw-sewage reservoir is essentially an anaerobic pond and the release of gases from this pond is observed.

This indicates that methane is produced in this reservoir and that partial organic removal is taking place.

The feed to the SBRs is extracted from the lower part of the raw-sewage reservoir and pumped to the SBRs when these require feed during the filling and aeration stage.

As a result of power supply limitations being experienced, the two aerators cannot operate simultaneously and the control system has been adjusted to allow only one aerator to operate at any given time. This results in one SBR being aerated for about 50% of the time while the other one is in idle mode.

Sufficient aeration was achieved during the commis- sioning phase, in May 2011, based on chemical oxygen demand (COD) and ammonia nitrogen oxidation, which were both within general limit values, says Viljoen.

To establish a viable biomass early on in the commissioning phase, the SBRs were seeded with waste-activated sludge from the adjacent existing activated sewage treatment plant. This proved to be effective as viable biomass was quickly established in the SBRs, resulting in the required final effluent quality being met early in the commissioning phase.

Compliance was not expected until at least two to three sludge ages had elapsed if no seeding was done. The design sludge age for this plant is 30 days.

Viljoen states that general limit value-compliance was achieved within less than three weeks after start-up.

“COD, ammonia and nitrate concentrations were all well within general limit value- requirements. Phosphate concentrations were generally less than 1 mg/ℓ, with two results above 1 mg/ℓ but less than 5 mg/ℓ, ” he says.