From wastewater to high quality slurry fertilizer

Slurry fertilizer
Slurry fertilizer, subproduct of wastewater treatment

The purification of water consists in extracting the polluting substance, with the aim of obtaining water in suitable conditions for consumption (purification) or, in the case of wastewater, to make it meet the current regulations for dumping. During this process, which occurs in wastewater treatment plants, slurries regulated by a specific legislation are also generated. These slurries, at the same time, may also be treated, which makes it possible for them to be reused in agricultural activities, as fertilizer or for composting.

Its use is regulated by the European Union through a directive that is more and more restrictive. Faced with the possibility of stricter laws, the industry is seeking alternatives compatible with the current uses, in other words, that will allow this by-product to continue to be commercially viable. But, it must be taken into account that the slurry from wastewater treatment can contain pathogenic germs and parasites that may be harmful to human beings, such as salmonella, Escherichia coli, roundworm, etc. For this reason it is imperative to treat the slurry before it is used.

Studies have been published that have shown that the incorporation of quicklime to these slurries eliminates these pathogens. The addition of lime to the slurry reduces odours and the level of pathogens by creating a high pH that is hostile to biological activity. The gases released during the anaerobic decomposition of the organic matter contain nitrogen and sulfur and are the main source of bad odors in slurry. When the lime is added, the microorganisms involved in the compound are strongly inhibited or destroyed in this strongly alkaline medium. The pathogens go through a similar process.

During the slurry treatment process using quicklime, it is necessary to maintain the pH above 12 for a minimum of 2 hours, to ensure the destruction of the pathogens and provide sufficient residual alkalinity to prevent the pH from dropping below 11. This allows sufficient time for the storage or regulation of the stabilized slurry. The amount of lime necessary to stabilize the slurry is determined by the time, the chemical composition and the concentration of solids. In general, it ranges from 6 to 51%. Taking into account that the primary slurry requires the least amount of lime and the activated slurry requires the most.

There are other methods of slurry treatment, such as aerobic and anaerobic digestion, but treating it with lime provides greater advantages when reused, as it gives us a greater volume of usable product in addition to providing acid soils with the necessary neutralization with no extra cost.

The high dose of lime also affects the physical and chemical characteristics of the slurry. These reactions reduce the nitrogen, which acts to limit the amount of slurry that can be applied to the land, it permits a greater amount of slurry per unit of surface, and at the same time improves its ability to lose moisture and the nature of the secondary fluids.
Another advantage of this system is that it can be a good alternative when a backup for another slurry treatment method is needed, as the system of stabilization using lime can be started and ended quickly. Therefore, it can supplement existing installations when the volume of slurry exceeds the design levels, it can replace incineration when there is shortage of fuel or when maintenance work is taking place.

To summarize, this slurry treatment method is more economical than other methods, it is also a safe and effective method for the final disposal of the slurry, avoiding risk to human health and environmental damage. Once the slurry has been treated and stabilized, it can be unloaded perfectly safely. It is ideal for agriculture, as its high lime content makes it the ideal high quality fertilizer for acidic soils, containing organic matter and fertilizers.