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Posted By  On Nov 28, 2012 In Water In EuropeWater Technology & Innovation

Researchers at Aalto University in Finland created a method designed to reduce phosphorous in pulp mill wastewater.

The process uses iron sulphate, which is added to the wastewater prior to the biological wastewater treatmentprocess. The phosphorus dissolved into the effluent is precipitated simultaneously with the biomass in the treatment plant.

Simultaneous precipitation does not require additional wastewater treatment units, according to the researchers. This eliminates additional treatment and results in energysavings. Iron sulphate is also an inexpensive chemical, which makes the process more affordable.

Professor Olli Dahl, one of the researchers, explained:

From the viewpoint of comprehensive environmental protection and sustainable development, the best method is always the one that saves energy and minimizes the amount of waste. With the help of the studied simultaneous precipitation method, it is possible to completely avoid additional stages of wastewater treatment, reduce the amount of solid waste and save energy. Simultaneous precipitation produces hundreds of thousands of euros worth of savings in operating costs, as energy consumption and the need for additional chemicals is reduced.

The phosphorus precipitated using this process is removed with the sludge. Since sludge is typically burned in Finland, any phosphorus would be contained in the ash, which could be reused as fertilizer.

The researchers conducted initial testing in the university’s laboratories. “The results were so promising,” noted researchers, “iron precipitation was also successfully tested at the wastewater treatment section of a pulp mill.”

Additional pilot stage experiments were made at a pulp mill where they used iron sulfates during the secondary treatment of elemental chlorine-free kraft pulp mill wastewater.

The researchers say the best result they achieved was an 81% reduction in phosphorus when adding 10 milligrams per liter of iron sulphate into the wastewater during the activated sludge process. “No notable change in either chemical oxygen demand (COD) or adsorbable organic halogen (AOX) reduction nor in the properties of sludge settling were observed during this experiment,” they noted. “Furthermore, no problems in sludge treatment due to increased iron concentrations in the waste sludge were reported.”

The kraft process is a chemical process used to convert wood into wood pulp. Wood chips are mixed with a solution of sodium hydroxide and sodium sulfide that breaks apart the lignin and cellulose present in the wood pieces. This process can also be referred to as kraft pulping, or the sulfate process.

Creating paper requires a great deal of water, which results in the generation of a nearly equal amount of wastewater. Pulp mill effluent typically can contain (link is PDF) not only the unwanted lignin, but also substances such as alcohols, tannins, dyes, and heavy metals. The wastewater also has high biological oxygen demand and dissolved organic carbon as well. The effluent typically requires additional polishing, usually as a tertiary process, to meet local water discharge standards.

Finnish government officials are mandating reduced industrial phosphorus emissions. Several manufacturers are using aluminum in an additional post-treatment precipitation stage; however, the resulting sludge is difficult to process.

The researchers note that forestry-industry-generated wastewater typically contains less phosphorus than municipal wastewater and the iron added during their treatment process “remains within reasonable limits.”

The researchers’ paper, “Simultaneous precipitation of phosphorus in a kraft pulp mill wastewater treatment plant,” was published in the journal Water Science & Technology.

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noviembre 2012

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