Technologies introduced to tackle increasing sewage sludge production in Africa

31st May 2013

By: Ilan Solomons

Creamer Media Staff Writer

  

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International water-treatment plant special- ist Degrémont is in the process of introducing new water technologies into Africa to tackle the challenge of treating increased volumes of sewage sludge owing to the rapid development taking place on the continent.

Degrémont South Africa deputy MD Dumi Luthuli tells Engineering News that the company’s technologies offer sustainable solutions as quantities of sewage sludge produced are steadily on the rise because of demographic change and increasing urban sprawl, as well as the increasing rate of connections to sewage networks and wastewater treatment works.

“New international regulations – such as the European Federation for Agricultural Recycling sewage sludge directive 86/278/EEC are also increasing the need to adopt newer technologies to treat higher volumes of effluent at higher standards,” says Luthuli.

“Degrémont considers the conversion of sewage sludge into potential energy, turning the sludge from a waste product into a product with real financial value. The company’s solutions help increase the recovery of potential energy from sewage sludge in relation to the possible final disposals and the reuse potential,” he points out, adding that this is achieved, in most cases, by reducing the volume of sludge produced, through implementing stabilisation and hygienisation.

Luthuli explains that Degrémont considers the potential to extract value from sewage sludge on a case-by-case basis. Where this is not possible, the sludge is destroyed using one of several treatment techniques in its range.

He says the company has built about 10 000 plants worldwide, including three in Africa, in Morocco, Algeria and Egypt.

“Sludge treatment is one of Degrémont’s five areas of expertise, along with drinking water purification, desalination by reverse osmosis, wastewater treatment and treated wastewater reuse and industrial water treatment,” Luthuli adds.

Degrémont developed a range of technologies adapted to all the elementary processes that are part of the sludge treatment line.

One such solutuon is the anaerobic diges-tion stabilisation process, which can be boosted by preliminary thermal hydrolysis, resulting in boosted energy production. This solution also improves filterability of digested sludge, while 5% to 10% additional cake dryness may be obtained by downstream dewatering devices.

Luthuli explains that digestion reactors have to be properly mixed to efficiently process the organic matter, while avoiding the grit deposits.

The company’s Cannon Mixer product is a proven anaerobic digester mixer, which, using large distinct bubbles, combines integral mixing and heating within one tank. The result is a reduction of volatile solids and sludge volume, greater overall gas production and improved sludge dewatering.

Degrémont’s Cannon Mixer features a vertical stack pipe, which is open at each end and varies in length according to digester depth, and is the central component of the Cannon Mixer. Based on computerised modelling, multiple units are arranged to optimise mixing zones across the entire floor, achieving more than 90% total active volume operating capacity, explains Luthuli.

He says that the Cannon Mixer uses recirculated gas from the waste sludge, which is continuously fed to the bubble generator and intermittently discharged into the stack pipe as a large piston bubble.

“Then the piston bubble fills the entire cross section of the pipe, driving out liquid as it rises and creating a siphon. “As one bubble leaves the stack pipe at the top, another enters from the generator, resulting in continuous mixing and the preven- tion of solids settling. Large bubbles burst as they leave the liquid surface, creating sub- stantial turbulence that prevents scum build-up,” he explains.

The sludge drying process serves the purpose of reducing the sludge volume and, therefore, helps to decrease disposal costs, says Luthuli.

“Solar or thermal drying are the two main types of sludge drying methods employed by Degrémont, the final choice being dependent on the climate, energy costs and plant size,” he says.

Degrémont has developed and installed dozens of solar sludge dryers called Heliantis, equipped with adapted turning machines. The Heliantis is a simple, clean, ecofriendly and economical technology that uses 20 kWh/t to 100 kWh/t of water evaporated.

The company has built about 70 thermal drying facilities in the past 60 years.

“Thermal drying is a sensitive process that could be dangerous if not properly designed. After extensive research, in February 2012, Degrémont came up with two reliable and efficient processes, to work in conjunction with the Heliantis,” notes Luthuli.

The Evaporis LE (low energy) consumes 700 kWh to 750 kWh for each ton of water evaporated, compared with 600 kWh to 1 100 kWh for other processes.

The Evaporis LT (low temperature) is a single-stage band dryer that dries sludge using waste heat – low temperature calories that are generally lost, says Luthuli.

Edited by Tracy Hancock
Creamer Media Contributing Editor

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