With a view to the circular economy, some types of usable sewage sludge have been identified, but disposal remains complex

It seems to be a struggle already seen in other sectors between eco-friendly and industrial-derived products that has affected the population so much and a little less politics. As with plastic, glass, wood, paper and metals, there is an under-track competition between “virgin” product and reuse product.

The sewage mud is another example of the complicated regulation that governs the waste market with respect to the needs of the operators in the sector.

There are, in some areas, absolute prohibitions in the use of these treated sludge and freedom of use in others, also thanks to legislation that in some countries is still of the last century.

But what is sewage mud?

The so-called black water of sewage that flows into the city’s sewage treatment, is treated mechanically, biologically and chemically in order to make the resulting fluid suitable for re-entry into the wild without creating alterations in the ecosystem.

These operations create a processing waste that is composed of a mud containing largely biodegradable organic and inorganic parts.

Sludge is divided into primary and secondary sludge. The primary is mainly made up of:

• Organics, such as cellulose sugars lipids and proteins, which are easily biodegradable
• Inorganic,such as sand metal oxides and carbonates
• Organics not easily biodegradable,such as fibers the gums and seeds

Secondary sludge consists mainly of:

• Suspended solids that have not been held back by primary sedimentation
• Solids produced directly by the plant,such as substances that are not attacked by bacteria and biodegradable dissolved solids that are attacked by bacteria.

Without going into specifics of the chemical differences of primary and secondary sludge and their different treatment in a sewage treatment plant we can say that secondary sludge is the most nutrient-rich,such as nitrogen and phosphorus than the primary, therefore more suitable for use in agriculture.

Primary ones, on the other hand, have a higher calorific power than biological secondarys and therefore more suitable for disposal by incineration.

In fact, because of the difficulties that environmental regulations are putting in place, a substantial fraction of sludge, which could be used in agriculture, is accumulating in the warehouses as they do not find a commercial outlet.

If we consider that the production of sewage sludge never stops, as black water flows every day into the purifiers, the riddle of where to place them increases more and more every day.

The problem is not only for sewage treatment plants, but also involves farmers who are forced to use chemical fertilizers when nature gives us the same substances that the earth needs in the form of treated sewage.

The methods to deal with this emergency sees the reaction of operators divided between passive and active. By passive we mean the technical solutions that aim, through methods of managing the sewage cycle, to reduce the amount of resulting mud.

Among the active ones we find proposals to turn mud into “defecation chalk” obtaining a product that is no longer to be considered waste, but as an additive that can be used in agriculture as a corrective to fertilizer recipes.

Another project is the “accelerated mud carbonization” by permaning sludge in a high-pressure (18 bar) and high-temperature (190) autoclave. This results in a transformation of sludge into a product called“biocarbon”.

An additional line of disposal is to mix the sewage sludge, through a dehydrated sludge injection plant, to the waste burning processes, creating a co-combustion that would use a percentage of sludge between 7 and 8 compared to the waste entered.

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