Step 4: Vertical sub-surface flow treatment SFS-v
The effluent is fed discontinuously from the top (15 cm from the surface) downwards through 4 PVC pipes with a diameter of 100 mm, perforated and positioned flat along the entire length of the tank. At the bottom of the tank is another collection pipe, perforated and open to the outside, which, together with the discontinuous arrival of effluent, oxygenates the substrate. It is therefore a predominantly aerobic system where the chemical processes involved are those of nitrification.
A substrate of gravel and sand of varying grain size is placed inside the 85 cm high tank, consisting of the bottom:
– non-woven safety fabric
– 25 cm of washed gravel with a diameter of 30-70 mm
– 15 cm of washed gravel with a diameter of 10-20 mm
– 20 cm of washed fine gravel with a diameter of 6 mm
– 10 cm of washed sand with a diameter of 0.2-0.5mm
– 15 cm voids
Bottom slope: 1%.
PVC aeration pipes perforated at the bottom with a diameter of 100 mm, 60 cm high and positioned 10 cm from the bottom of the tank,
The effluent is fed discontinuously into the four pipes parallel to the tank to create an aerobic-anaerobic situation ideal for denitrification-nitrification processes.
Tank size: SFS-h 4x30x0.75 m
The effluent is fed into the tank along its entire length from pipes (max. 100 cm apart) at 15 cm from the surface by means of a PVC pipe with a diameter of 100 mm perforated along its entire width.
The height of the effluent inside the tank, which is expected to be 20 cm, can be adjusted by the siphon in the outlet well, while the water inside the tank is 100 l/m².
Total m3 of wastewater in the tank 80 m² x 0.10 = 8.0 m³
Hydraulic residence time: TR= 1 day
Total residence time: 11 days
Step 5: Horizontal sub-surface flow treatment SFS-h
Same as step 3
Total effluent contained in tank 80m² x 0.2 = 16.0 m³
Hydraulic residence time: TR= 2 days
Total residence time: 13 days
Step 6: Surface flow treatment
Already at the outlet of the treatment in step 3, the effluent is odourless, and the further steps allow us to refine our purification with a surface flow. The water surface is exposed to the atmosphere with submerged aquatic plants (Elodea canadiensis which is a major producer of O²) and surface plants (water lentil or lemna minor) inside.
Tank height: 100 cm
Aquatic plants: Elodea canadiensis, algae and water lentil
Bottom slope: 0%.
Tank size SFS-h: 4 x 30 x 1 m
The height of the liquid inside the tank is adjustable from the siphon in the outlet shaft and is expected to be 90 cm, the wastewater content inside the tank is considered to be 833 l/m².
Total wastewater content in the tank 80m² x 0.83 = 66.0 m³
Already at this stage the breeding of cyprinids, frogs or other compatible species in the tanks is planned.
Hydraulic residence time: TR= 8 days
Total residence time: 21 days
Step 7: the lakes
At the end of our purification process, the last phase of refinement-safety is planned with two artificial lakes that allow us to considerably increase the residence time (time efficiency) and safety (in critical moments, we have an additional lung).
The lakes will be waterproofed with a suitable UV-resistant sheet.
Inside, it is planned to introduce cyprinids (carp, tench) or/and amphibians which, together with the production of biomass from mowing (phases 3-4-5) and algae (phase 6-7) should allow the company to pay the maintenance costs as well as being a marker of the efficiency of the system.
This vision of the purification plant (with integrated aquaculture) should lead to an oversizing of the phyto-purification plant in the final part of the surface tanks and the lake because it is convenient regardless.
Therefore, we do not try to minimise space and initial investment but, in line with natural logic, we give space and time to nature so that it can transform effluent into water.
The residence time of the first 6 phases is 21 days, for the final lakes we foresee an extension of about 10.000 m² with an average depth of 2 m and a containment capacity of 20.000 m³ for a residence time of about 208 days considering an input of 96 m³/d.
Residence time 229 days
Additional safety measures
If we consider the rainfall against and the evapotranspiration of the plants and the evaporation of the lake in favour, in 365 days we would have a decrease of liquid that would increase our residence time by 72 days.
Rainfall = 1300 mm/m²/year
Evapotranspiration-evaporation 1800 mm/m2/year
In practice, each m² of treatment surface would still evaporate 500 mm/m²/year, i.e. 13,840 m² x 0.5 m³ = 6920 m³ evaporated / 96 m³/d = 72 extra days of residence time that we do not take into account.
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