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Aerated ponds

In terms of municipal water treatment, there are various methodologies and technologies applied depending on the circumstances (volumes to be treated, type of municipality, available budgets).

Among these solutions is the so-called lagooning method, a process that is characterised by its relative simplicity of implementation.  In terms of lagooning, different options are possible: continuous discharge, periodic emptying, or total retention.

Lagoons can be aerobic, anaerobic, or a combination of both processes in different areas. Facultative aerated lagoons are common in Quebec for the treatment of domestic wastewater. To treat this wastewater, the methodology relies on the action of bacteria to assimilate organic matter to an acceptable level.

Aerated ponds are equipped with air injection and mixing systems to maintain sufficient dissolved oxygen and ensure that most of the solids remain in suspension. However, the ponds are in a partially mixed condition, i.e. the mixing energy is insufficient to prevent deposition. Only part of the solids are kept in suspension. Some of the suspended solids settle to the bottom of the ponds, where they enter into anaerobic digestion. The conventional configuration is that of several ponds on the same site that successively contribute to the treatment of this wastewater.

Accumulation, sludge extraction

Gradually, despite the aeration process, an accumulation of sludge occurs on the sides and mainly at the bottom of the pond. To ensure a sustainable operation of the pond, it is therefore important to set up a sludge volume monitoring procedure. To do this, municipalities use different methods to measure the level of sludge accumulation and, depending on the extent of this accumulation, implement a sludge extraction and treatment operation to prevent alteration of the effluent (treated water). The sludge accumulated at the bottom of the lagoons has an impact on wastewater treatment since it reduces the liquid volume available in the lagoons. An increase in the concentration of ammonia nitrogen is noticed  as well in ponds where there is a large accumulation of sludge . As a general rule the level of accumulation should generally not reach or exceed 15% of the volume of the pond.

The majority of companies involved in sludge removal operations conclude that it is not possible to retrieve all the sludge. Therefore, the measurement of accumulation is important and, depending on the accumulation parameters, an extraction infrastructure should be set up with a frequency appropriate to the situation.

The quantity of sludge extracted is determined in dry tonnes. The extraction volume therefore varies according to the dryness of the sludge (percentage of dry matter). Take a volume of 1000m3 for example of sludge with a dryness of 5%, the counter-value in dry tons will be 50 dry tons.

The sludge is extracted by pumping or dredging using different types of dredgers depending on the size of the ponds to be treated. The sludge is generally filtered by screening to isolate the macroelements it contains and then pumped.

The dewatered sludge may be directly available for land application to nearby agricultural plots to reduce transportation costs, or it may be treated using a variety of technologies:

-Drying process

-Mechanical dewatering

-Thickening process

Sludge characterization

Pond sludge is of interest for agriculture because of its proven fertilizing qualities. The distribution of nutrients varies between ponds, the frequency of extraction and various other parameters. In general, pond sludge is characterized by the following elements:

Major Nutrients  

Minor Nutrients

 

Nitrogen (N) Boron (B)
Phosphorus (P) Copper (Cu)
Potassium (K) Iron (Fe)
Sulphur (S) Chlorine (Cl)
Calcium (Ca) Manganese (Mn)
Mangesium (Mg) Molybdenum (Mo) Zinc (Zn)

Solugen’s contribution

Solugen optimizes the sludge treatment process in aerated ponds. In general, municipalities only partially treat their pond sludge. Solugen offers a solution that not only allows the sludge to be recycled as FRM (Fertilizing Residual Materials), but also, in the interest of eco-responsibility, to completely purify the residual water from the liquid-solid separation.

By pumping the sludge, which gradually accumulates in the aerated lagoon, the sludge does not pile up at the bottom of the water body. Solugen thus reduces its compaction, which makes the treatment, and in particular the dewatering process as it is usually carried out, more difficult. In addition, this treatment process contributes to better water quality in the aerated ponds and optimizes its purification process. Avoiding the extraction of sludge by dredging not only improves the handling of recyclable materials, but also leads to a better environmental management of the operation. In addition, pumping  mostly eliminates the greenhouse gases emitted during the transport of the dredger and during its operation.  Finally, pumping reduces the risk of dredging damage to the aeration systems in the lagoon facilities.

Sources:

http://collections.banq.qc.ca/ark:/52327/bs1988851

https://savoirs.usherbrooke.ca/handle/11143/15397

http://www.environnement.gouv.qc.ca/eau/eaux-usees/domestique/Chap6.pdf

http://www.environnement.gouv.qc.ca/matieres/articles/caract_boues1.pdfhttps://www.reseau-environnement.com/wp-content/uploads/2014/04/M%C3%A9moire_Guide_boues_2014.pdf

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