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Purifying sludge through oxygen-based digestion


May 3, 2013 – Dealing with sludge is one of the biggest issues of wastewater treatment.

“Sludge accounts for about 50 percent of the operating costs and for 65 percent of the environmental impact of waste water treatment, conditioning and management,” explains Jose Luis Bribián Fisac, head of sales at wastewater treatment company Bioazul, based in Málaga, Spain. “The company has just joined the EU co-funded WASTRED project, which aims to optimize waste reduction in the European meat and dairy industry.”

These sectors are two of the highest sludge generators within the food industry.

There are different techniques to reduce the volume of so-called activated sludge— composed, among others, of microorganisms — and to stabilize it. One solution promoted by the WASTERED project involves the use of LODOred, a new wastewater treatment additive, created by the company back in 2005. Lodo is the Spanish for sludge, and red stands for its reduction. Currently, the additive has been tested in around 50 wastewater treatments plants in Spain, Germany, Poland, Switzerland and Italy. Although it was intended for sewage sludge, the company is now trying to open up a new market in the meat and dairy industry.

What gives this additive its properties is that it is biodegradable. It can therefore be released onto activated sludge that undergoes an aerobic digestion — i.e. requiring the use of oxygen — to degrade its biological content. It works by encapsulating what are called flocs of bacteria. First, the gelatinous floc layer removes minerals and particulate from the wastewater, improving the sedimentation of the sludge.

Second, encapsulation increases the levels of degradation as the microorganisms’ metabolism are manipulated by specially designed enzymes and vitamins. Indeed, the additive generates additional energy to supply to the microbial degradation activity. This happens because the additive pushes the equilibrium towards a process called catabolism, whereby big molecules are digested and broken down. It therefore lowers bacterial duplication and, hence, sludge production.

Some experts, such as environmental consultant Tim Evans, based in Ashtead, UK, are “sceptical” of this solution.

“Aerobic treatment uses lots of energy,” he said. “Anaerobic systems would generate less sludge and use less energy.”

Bribián Fisac is aware of this issue.

“We propose LODOred only for the existing aerobic plants where we can reach 30 percent sludge reduction,” he said.

Other experts, such as Stephen Palmer, process efficiency technical leader at the British branch of the multinational environmental engineering and wastewater management firm MWH, located in Warrington, consider this solution as “credible.” Yet, “the view of sludge as a valueless waste is out of date,” he said. “Technologies to recover energy from sludge have become better.”

However, he believes that today “an aerobic treatment is a bad design option. An industrial waste has a very high calorific value. If it is concentrated, it might be more efficient to go for an anaerobic treatment and release biogas to produce electricity.”

Nevertheless, mainstream water treatment is not concentrated enough to sustain an anaerobic biomass, he believes, which makes the project solution an option in some cases.

Although the additive could prove suitable in specific circumstances, the saving in sludge processing may not be that significant. Indeed, going aerobic has an aeration cost attached.

“You may have less sludge to get rid of, but having to raise dissolved oxygen in the systems, you increase operating costs,” he said. “Even worse if the oxygen has to go through a jelly floc layer instead of just water.”

As a result, people may need to raise the oxygen injection rate to make it efficient.

“The net effect is that you are not saving as much as you think.”