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Value from sludge

Kemira has been very active in the sludge treatment area for more than 20 years, and its two recently developed, unique processes – KEMICOND and BION – have been instrumental in improving dewatering, cutting sludge volumes, and enhancing the potential for producing biogas. Kemira has also been a leader in using the ash from sludge incineration to create a forest fertiliser – ECOFOR.

To make it cost-effective to transport sludge to distant locations, and handle it efficiently there, sludge must be thoroughly dewatered.

Sludge – or the residual solids produced by wastewater treatment plants – is commonly considered as waste, despite the fact that it contains a number of valuable components. Converting sludge into useful products needs to address both the organic and inorganic components of sludge, if the maximum benefit is to be achieved.

The most commonly used application for sludge today is in energy generation, and the forest products industry, for example, burns large quantities as fuel.

Excellent dewatering performance

Traditionally, one of the easiest ways to reuse sludge components in a beneficial way has been to spread it on farmland as fertiliser, and the majority of sludge was recycled in this way in the past.

More recently, however, the use of wastewater sludge on farmland has fallen off dramatically, due to tougher limits on heavy metals and other toxic substances. This has seen the pressure on landfills grow considerably.

Since 2005, however, landfilling sludge with a high proportion of organic matter has been prohibited in some countries, such as Sweden. The easiest way to solve this problem is by incineration.

Given the fact that the cost of sludge management can account for as much as 50% of total wastewater treatment costs, because disposal is paid for on the basis of volume, and because higher taxes are being levied on sludge to an increasing degree, dewatering has become a key issue.

To make it cost-effective to transport sludge to potentially distant locations, and handle it efficiently there, sludge must be thoroughly dewatered – which is where the KEMICOND process comes in.

This new method is based on the fact that, under oxidation in acidic conditions, sludge changes shape and loses volume. KEMICOND offers both excellent dewatering performance and high throughput, and also eliminates unwelcome odours and pathogens.

One of the first locations to benefit from this new technology will be the Käppala wastewater treatment plant in Stockholm. The new dewatering unit based on the KEMICOND process under construction there is due to be commissioned in 2006.

Two recently developed Kemira processes have been instrumental in improving dewatering, cutting sludge volumes, and enhancing the potential for producing biogas.

The problem with conventional incineration

Even after dewatering, however, sludge contains up to 80% water on average, while its solid matter comprises approximately 70% organic and 30% inorganic matter.

The value of the organic component lies in energy generation, and municipal sludge is often burned in incineration plants. Incineration is costly, however, and destructive, as mineralisation destroys ammonium or deactivates phosphates, making them inaccessible for further recycling.

Digestion represents an alternative for converting the organic component of sludge into energy. Producing methane gas in the absence of oxygen in large digesting chambers has been identified by Kemira as a major opportunity for creating a new traffic fuel.

Mimicking nature’s own processes

Kemira’s new BION process opens up new opportunities for producing biogas competitively from sludge.

Originally developed to achieve better dewatering results, the digestion-based BION process can convert organic matter into energy, in the form of either methane or hydrogen. It works by mimicking nature’s own enzyme-based processes for degrading and digesting organic matter, by using enzymes to improve the digestibility of the organic matter in sludge and raise the methane conversion factor.

After digestion, the remaining inorganic fraction, in the form of phosphates, can be safely used as a fertiliser, as the organic toxins that may have been present before the process have been eliminated.

> Olof Norrlöw
(Published in High Technology Finland )