Pollutant removal mechanisms
Once the organisms present in the treatment plant have implemented the biological processes outlined in the previous article, each pollutant is removed through specific mechanisms.
Suspended solids: mechanical separation, sedimentation and filtration (steps 1-2-3).
Organic matter (BOD): aerobic bacterial degradation (mainly step 4) and anaerobic degradation (mainly steps 3-5-6-7).
Nitrogen: ammonification – nitrification – denitrification, root and substrate uptake (adsorption) (steps 3-4-5-6-7).
Phosphorus: adsorption, root uptake, precipitation (steps 3-4-5-6-7).
Pathogenic microorganisms: separation and sedimentation (stages 1-2), filtration, predation, adsorption, natural death and by stress aerobic and anaerobic environment and by action of antibiotics emitted by roots (steps 3-4-5-6-7).
This is the base of phytodepuration, a natural process that slowly transforms substances (which we could call organic pollutants) into nutrients for the ground and plants.
In practice, the treatment of livestock effluents with natural Phyto-purification plants is accomplished by creating beds of gravel, stones, etc. that multiply the surface area on which enzymes and bacteria can work.
Let’s see how the various phases are developed from farm to the end of treatment.
Step 1: Liquid-solid separation
Fresh zootechnical effluents stay on the farm in the under-grid and collection pits and are then conveyed through sumps and channels to a high efficiency separator that extracts from 5% to 10% of dry matter consisting of hair, solid part and suspended solids.
From the moment the slurry is created until it arrives at the separator, the residence time in the shed pits, collection sumps and connecting pipes can be estimated at approximately 7 days.
Hydraulic residence time TR= 7 days
Sewage to be treated at the outlet: 95 m3/d
Step 2: collection in tank with decantation
Once the liquid-solid separation has taken place, the liquid part is put into an existing concrete tank with collection and decantation function.
The liquid part will enter from one side at the bottom and will be taken in the upper part from the opposite side, in order to decant on the bottom any small solid parts not separated. Stainless steel separation baffles are provided
The concrete tank measures 8x4x3 m, for a total of 96 m3
Hydraulic residence time: TR= 1 day
Total residence time: 8 days
From the collection and decantation tank starts a PVC line that brings the liquid to the purification tanks, in this project dimensioned in 12 lines of 8m3/d each.
We move on to the composition of one of the 12 lines of 8m3/d.
Step 3: SFS-h horizontal sub-surface flow treatment
The horizontal sub-surface flow treatment is biological and consists of a tank with an impermeable sheet. The plants planted on the bed of Phyto depuration have the natural ability to transport oxygen and lead it, through the stem, to the rhizomes. Already after a few months from the start-up of the plant, their surface will be covered with a bacterial film of microorganisms that are the real responsible of the purification process. The medium (sand/gravel filling) will be able to oxygenate naturally and the bacterial film will also extend onto the substrate itself.
It is therefore a mixed treatment that works in anaerobiosis where the prevailing chemical processes in this phase are those of denitrification.
A substrate of gravel and sand of variable grain size is placed inside the 85 cm high tank, consisting of:
- non-woven safety fabric
- 60 cm of washed gravel with diameter 3-12 mm
- 10 cm of washed sand with diameter 0.2-0.5 mm
- 15 cm empty
Slope of the bottom: 1%
Plants used: among the known plants suitable for use, we opted for the marsh reed Phragmites australis or Typha latifolia, elophytes that have an excellent yield in biomass and that from an ecological point of view, are widely spread in Piedmont, site of the farm considered.
Tank size: SFS-h 4 x 20 x 0.85 m
At the beginning and at the bottom, a band of 1 m with large-sized material (washed stones) is devised in order to avoid the creation of preferential lanes for the wastewater with a consequent decrease of the purifying efficiency.
The effluent is fed into the tank through a band of stones at 15 cm from the surface by means of a PVC pipe 110 mm in diameter, perforated across the whole width.
The height of the effluent inside the tank, devised in 40 cm, is adjustable from the siphon in the outlet well while the effluent inside the tank is considered 200 lt/m2
Total m3 of wastewater contained in the tank 80m2 x 0.2 = 16.0 m3
Hydraulic residence time: TR= 2 days
Total residence time: 10 days
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Phytodepuration is the eco-friendly solution for wastewater filtration. Find out how it works and how to implement it.