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Sewage sludge as valuable contribution as a "soil improver” or “remediator”.

The management of sewer networks involves the treatment of millions of tons of waste each year. Recent institutional reports in Italy (published by the Higher Institute for Environmental Protection and Research – ISPRA) estimate waste from sewer network management to be about 4 million tons each year, most of which is attributable to the biological sludge produced by the sewage treatment process.

Depending on its chemical composition and the presence of pollutants, this sludge must be disposed of in landfills, aggravating the already critical conditions of the waste disposal system. The possibility of applying treatments that allow its reuse form part of the virtuous circular economy framework, which aims to fully exploit and recycle products otherwise considered waste.

Sewage sludge can make a valuable contribution as a "soil improver” or “remediator”, and can contribute to plant nutrition thanks to its organic component and the content of suitable amounts of nitrogen and phosphorus, to which potassium, calcium and sulphur can be added. 

In collaboration with ALAN, a company specialised in treatments for the reuse of sewage sludge and digestates, SOL has developed and patented a process for the production of an agricultural corrective called "Calcium carbonate of defecation” produced from biological sludge and digestates.

The processing to obtain this remediator initially involves the addition of limestone to the sludge. The biomaterial thus obtained is then treated with carbon dioxide to produce the calcium carbonate. The innovation contributed by SOL concerns the way the CO2 is infused in the sludge. In fact, solid carbon dioxide is used in the form of crystals, made using “snowblowers” consisting of special nozzles, which use liquid CO2 taken directly from the storage tanks. The use of carbon dioxide allows to avoid the use of mineral acids, such as sulphuric acid, which generates high amounts of sulphur compounds.

The use of carbon dioxide brings many advantages: it creates products that are completely compatible with the environment (such as carbonates), it reduces maintenance costs, and in particular mitigates the bad odours when spreading the soil improver on agricultural land.