During the treatment of wastewater by a treatment plant, a solid residue remains which is called sludge and whose dryness can vary according to the case. The dryness is expressed as a mass percentage of dry matter. Thus a sludge with a dryness of 10% has a moisture content of 90%.
Depending on the type of water, these sludges are of residential origin (generally treated by municipalities), industrial sludges or sludges of agri-food origin.
This sludge, consisting mainly of organic and mineral matter, is also called bio-solid, and has a pasty or even solid appearance, especially if it is dewatered. Depending on its origin and its possible level of toxicity, this sludge may or may not fall into the category of Fertilizer Residual Materials (FRM).
Fertilizing residual materials
FRMs include, in addition to sludge from wastewater treatment, other residues that have fertilizing properties (ash, compost, urban and industrial residues that are safe for land application). Thus, soils are enriched with biodegradable nutrients that will be decomposed to enrich the substrate and optimize plant production. However, not all FRMs are spread on agricultural soils; they must meet certain requirements issued by the Ministry of the Environment. The alternatives are incineration and waste landfill. The original biosolids, being like manure and slurry, rich in phosphorus and nitrogen, are of interest for the cultivation of crops such as corn and soybeans.
In 2015, more than 1.3 million tonnes of FRMs were applied to Quebec soils, including 0.3 million tonnes of municipal biosolids. More than 70% of these FRMs were applied to agricultural land. Approximately 1,700 agricultural businesses used FRMs in 2015, 600 of which specifically used municipal biosolids. The total FRMs accounted for 3% of phosphorus inputs to agricultural soils, compared to 31% for mineral fertilizers and 66% for manure.
These biosolids were mainly applied to fertilize animal feed crops, as well as in forestry and for the revegetation of degraded sites. (1)
A sludge is characterized by its dryness, volatile dry matter (VSD) content and physical consistency.
Liquid sludge / dryness from 0 to 10 %.
o Pasty sludge / dryness from 10 to 25%.
o Solid sludge / dryness from 25 to 85 %.
o Dry sludge / dryness greater than 85 %.
These sludges generally have fertilizing virtues and are thus valorized by their spreading on agricultural soils, they can be used in biomethanization to produce biogas.
Solugen treats liquid manure, agricultural wastewater (e.g. water from the milking process) and also water that is part of an industrial process (water from breweries, cheese dairies, slaughterhouses, the food industry and other industries).
Contaminated water, once its solid fraction has been removed (which may or may not be recoverable, depending on the type of water and its organic and mineral content), is treated by means of an evaporation/distillation process involving steam compression and heat exchange to limit energy use.
The result of this treatment is :
- Pure water, which can be up to 97% of the initial volume,
- Concentrated organic matter in liquid form which can also be recovered, depending on the case, according to its fertilising potential.
- A solid bio-residue.