Flottweg decanter technology is transforming how municipal wastewater plants manage sludge – making operations cleaner, more compact, and more cost-effective. With two decades of industrial experience, Porex SA is now helping municipalities tackle long-standing challenges in sludge treatment.

Building on extensive industrial-scale wastewater experience in wineries, breweries, and beverage production, filtration and separation specialists Porex SA have now extended their services to the municipal wastewater sector.

“We are the supplier and agent for Flottweg, a German-based company that is a leading manufacturer of mechanical separation technology and have had great success with their decanters over the years with our industrial clients, and have found that they are well suited for municipal wastewater,” explains Johan de Jager, technical sales manager, Porex SA.

He adds that the Flottweg OSE (optimum sludge thickening) decanters achieves considerably better separation qualities than other centrifuges.

“This is not new technology, it has been successfully implemented in the United States, Europe and the Middle East for over 30 years.”

Decanters vs belt presses for municipal wastewater plants

Flottweg decanters use centrifugal forces to separate solids from liquids. The new patented design Flottweg Xelletor deep pond decanter is now also available. They are more efficient than most belt presses at separating solids from liquids, typically achieving higher dry solids content in the sludge cake. This means less volume to dispose of, which can reduce transport and disposal costs.

“The Flottweg OSE decanter is a closed system, minimising odour and exposure to pathogens while belt presses are open systems and require more attention to cleaning and hygiene. Furthermore, the Flottweg OSE is compact, about 1/3 of the size of a belt press system, which is a major advantage in plants where space is limited. Lastly, the Flottweg OSE decanter is easier to maintain as there are less moving parts (four bearings) than a belt press (between 30 and 40 bearings). This decreases the time needed to maintain the decanter,” states de Jager.

Other advantages include the fact that the Flottweg OSE decanter consumes less power and needs less polymer than a belt press. Programmable logic controllers can facilitate the fully automatic start-up, operation, and shutdown of the decanter.

Operators don’t need to manually adjust parameters, which reduces human error and labour costs. The decanter can easily work 24/7 without supervision.

“Furthermore, Porex SA has a team of six millwrights that service all equipment and can attend to any breakdowns. While purchasing new equipment may be easy, maintaining it over time is far more challenging. That’s why we go the extra mile to support our customers – offering reliable backup and technical assistance to minimise downtime, because we understand that our clients can’t afford interruptions to their operations,” says Sakkie Smit, responsible for municipal wastewater treatment, Porex SA.

How the Flottweg OSE decanter works

A decanter centrifuge can be viewed as a settling pond wrapped around an axis.

In the settling pond, solid particles, which are heavier than the liquid, settle to the bottom driven by gravity and build up a sediment layer on the bottom of the pond.

In the rotating bowl of the centrifuge, solid particles, which are heavier than the liquid, move to the inner diameter of the bowl driven by centrifugal force and build up another sediment layer on the inner surface of the centrifugal bowl.

Since the centrifugal force in the decanter is approximately 3000 x g instead of 1 x g in the gravitation field, separating solids from a liquid in a centrifuge becomes much faster and more efficient.

Flottweg OSE decanters are available for capacities ranging from 8 to 250 m³h (90 to 1100 gpm) per unit.

Management of sludge

Sludge mismanagement is a significant challenge in many municipal wastewater treatment plants across South Africa. Inadequate infrastructure, aging equipment, and limited technical capacity often lead to poor handling, treatment, and disposal of sludge. As a result, untreated or partially treated sludge is sometimes dumped in landfills, released into the environment, or stored in overflowing settling plants – posing serious environmental and health risks. Often sludge is transferred from one wastewater treatment plant to another plant.

“By using a decanter, sludge can eventually be reworked into dry solids and then further treated, turning it into a valuable resource (biogas, soil conditioner, building material) rather than just a waste product,” explains Smit.

Porex provides both dewatering decanters and thickening decanters.

“The thickening of excess sludge is often not given the attention it actually deserves. Thickening is an important process stage that affects the removal of sludge from the secondary clarification plant as well as the feeding of a digester.

“Thickening is usually done before digestion, dewatering, or final disposal to reduce the volume of sludge handled in downstream processes. It is done to increase the bacterial count in the sludge, increasing its efficiency in the digester. With the Flottweg OSE decanter, surplus sludge generated during the biological stages is concentrated from 1% or less dry substance content to 5 – 8% before it is pumped into the digester. As a result, the amount of sludge is reduced by 90 to 95%,” adds de Jager.

As municipalities across South Africa face mounting pressure to modernise wastewater infrastructure and improve environmental compliance, efficient sludge management has become a critical priority. Advanced decanter technology offers a compact, hygienic, and cost-effective alternative to traditional systems – helping plants reduce operating costs, improve sludge quality, and unlock new value from waste.