Denis ROSKWAS
ERASMUS student
France
The Beaurade waste water treatment
plan (Rennes – France)
Winter 2006-2007 Semester
Index
I-
What is waste water? ................................................................................................... 3
II-
The example of Rennes ............................................................................................... 4
The waste water net .......................................................................................................... 4
The Beaurade waste water plant ................................................................................... 5
Limited harmful effects for the neighborhood .............................................................. 5
The process step by step ............................................................................................... 5
References ............................................................................................................................ 10
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Introduction
Waste water treatment has become an obligation in our modern societies. The increase
of the human activities is always followed by an increasing of pollutant emissions. Moreover,
water stocks are not infinite. Due to the reject of waste water, the environment can be
severely damaged and also water stocks can be touched. Rivers have a natural capacity to
purify water, but this capacity takes it to much oxygen and is not without consequences for
the life in this one. When the rejects of waste water exceed the auto-capacity of the river to
purify it, the deterioration of the environment can be dramatic and durable with the death of
the flora and the fauna because of the lack of oxygen.
France has enough water stocks to satisfy the needs, but it is in their degradation that is the
risks. Due to the pollution, water stocks may not be able to be used for the production of
drinkable water with a cheap price. That’s why it is necessary to clean the waste water to
ensure keeping the good quality of our stocks.
In this report, we will discuss about the French rules and ways about waste water
treatment and we will focus on the case of the town of Rennes in Britain as an example.
I-
What is waste water?
We can distinguish three kind of waste water:
- Domestical waste water:
It comes from the private use of water. Essentially polluted by organic particles, they have to
origins: kitchens and bathrooms with detergents, grease, organic particles and solvents and
toilets with various nitrogenized organic particles and fecal germs.
Daily
-
pollution for a person who use 150 to 200l of water is estimated at:
70 to 90 grammes of floating particles
60 to 70 grammes of organic particles
15 to17 grammes of nitrogenized particles
4 grammes of phosphor
Billions of germs for 100ml.
- Industrial waste water
Its caracterics are very different from a factory to another. Added to the organic
particles, it can have solvents, heavy metals, some toxic products, hydrocarbons
etc…
Some of it should be pre-treated directly in the factories before their rejects in the
waste water net.
- Rain water
It can also be the cause of important pollution of rivers due to its contacts with
impurities in the air or on the floor.
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How to collect the waste water?
It is done by the evacuation of the waste water in the drains of the cleansing net.
The transportation is generally done by gravity, can also be done under pressure or
depressure.
It is very easy to quantify the rejects for the domestic or industrial waste water,
but it is very much harder for the rain water. Two kinds of nets have been developed
due to this fact:
- The common net is collecting all of the waste water in the same drains. This
solution combines two advantages, economical (one net to build and to
manage) and simplicity (no risk of connection mistakes), but you have also to
be aware of the brutal variations of flow due to the rain water in the
conception of the net that should be bigger than his normal capacity.
- The separated nets are collecting domestic waste water and rain water in two
different drains. This solution is better for a simple management of the flows
and for a better adaptation of the capacity of the waste water treatment units.
II-
The example of Rennes
The waste water net
Waste water treatment is an old preoccupation in Rennes. The first waste water net has
been developed in 1880. Since this date, the town has built a separated net for rain water
and domestic waste water. In 2003, the net was representing by 648km of drains : 155km
were common drains for domestic waste water and rain water, 214km were for domestic
waste water exclusively and 279km for rain water. This net is completed by 51 collecting
stations and 6 storage basin.
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The Beaurade waste water plant
The plant has a capacity of 360 000 people. Between 45000 and 80000m3 of waste
water is going through it every day before its release in the river.
The standards of this plant are higher than those that are said in the 1991 European directive
about nitrogen and phosphor.
The superiority of this plant is to have a two hours cycle instead of the 24 hours cycles asked
by the European standards. So it is possible to have a real time adaptation to the kind of
pollution that is going thru the plant.
Results after treatment (in 2003)
Annual average concentration
Output
23.4 mg/l
96.1%
Suspension particles (MES)
1.7 mg/l
99.3%
biochemical needs in oxygen (DBO5)
2.7 mg/l
98.9%
Nitrogen (NGL)
4.08 mg/l
93.4%
Phosphor (P)
0.58 mg/l
93.6%
Chemical pollution (DCO)
Needs according to the European norms:
Organical particles 79 %
Suspension particles 84 %
Nitragen : 45 %
Phosphor : 45 %
Limited harmful effects for the neighborhood
All of the equipments are soundproofed to respect the tranquillity of the neighbourhood
(50dB by night and 60dB by day).
Al of the bad smells are treated before their reject.
On the December 15th 2004, the plant has been certified ISO 14001 which recognize the
quality of the purification, the ability to manage the energy and water consumption, the
quality of the elimination of the rejects and the prevention of pollution and risks.
The process step by step
5
The waste water plant of Beaurade is dimensioned to treat 45000m3 of waste
water per day during dry days and 80000m3 during wet days.
The first step is to sort the gaps by stopping them with bar screens. These ones
are evacuated and burned.
To the oil
treatment
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The sand is sorted by sedimentation, the flow is slowed down and the sand is
falling down and evacuated.
To remove the oil, air is insufflating. Oil is floating on the surface and is picked up by
a scrapper.
Re-circulation
screw
With biological treatment it is possible to eliminate organic pollution, nitrogen and
phosphor. Here is the Véolia BIO-DENIPHO® that is employed. Oxygen is brought thanks to
the rotation of 44 brushes. Successive phases of nitrification (aeration) and de-nitrification
are made to eliminate nitrogen. Carbon is eliminated thanks to the bacterial flora created
thanks to the oxygen. These natural bacteria are consuming the nitrogen, the phosphor and
the organic wastes.
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Water is passing through 52m diameter basins sludge is falling down by sedimentation
and is aspired. A part of the sludge will be incinerated and the other one is used to feed the
biological treatment basins.
Clarified water still has some suspended particles, so it’s filtered with sand that is
retaining these. After this process it’s rejected in the river. After this last step, 93% of
nitrogen, 96% of phosphor and 99% of suspended particles have been eliminated.
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Sludge treatment:
The sludge is separated from water by means of filtration. Following this step, the sludge
wetted by centrifugation.
Drained sludge has to be dehydrated before drying and burning them. This step is done
by means of centrifugation.
This step is the last one of the complete procedure. The sludge is then used to fertilise
the soils by the agriculture.
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References
http://www.cieau.com/toutpubl/sommaire/texte/8/f84.htm website about the quality of water
http://www.ville-rennes.fr/ website of the town of Rennes
http://www.senat.fr/rap/l02-215-1/l02-215-161.html#toc622 National report about the quality
of water in France
« Usine de traitement des eaux de la petite Californie » Documentation about the waste
water treatment plant of de town of Nantes (France)
« Visite de l’unité de traitement de Beaurade » documentation about the waste water
treatment plant of Beaurade.
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