Le manganèse est un élément minéral fréquemment rencontré dans différents types d'eau, surtout souterraines. Dans le cas des eaux de surface, le manganèse peut se trouver lors de pollutions accidentelles où lorsque la rivière reçoit des eaux d'un barrage réservoir en fin de vidange.
Des essais de traitement d'une pollution par du manganèse par filtration lente ont été effectués sur un pilote.
Les essais ont porté sur différentes concentrations de manganèse pendant des périodes de courte durée. La filtration lente semble très efficace pour l'élimination de ce composé, mais il faut noter que la nitrification biologique est altérée par la présence de fortes concentrations de manganèse.
Manganese in water - An overview of biological treatment
Manganese is a mineral element which is often found in differing types of water. The manganese content is rather consistent, especially in underground waters. In surface waters, manganese can only be found as a result of an occidental pollution or in rivers supplied with water from a dam whose tank has been sewed.
This metal is not dangerous for humans but it can create problems in drinking water as it becomes progressively oxidized and thus precipitates in the water supply. This oxidation can produce direct effects like spots on linen or sanitary appliances, but also indirect effects, like important bacterial developments, as well as bacteria protection against biocides used as bactericidal or bacteriostatic agents.
This study aims at :
1) Underlining the possible effects of manganese in case of an accidental pollution.
2) Underlining how much time the system takes to react to an increased manganese content, given that ferric salts which permanently incluse manganese (500 to 600 ppm in the 40 % commercial solution) are used in the coagulation phase.
Amongst several methods to eliminate the manganese, the biological oxidation seems to be the most appropriate to this occasional pollution problem. Indeed, it filters are continually contaminated by manganese, the bacteria is likely to oxidize the divalent manganese, resulting in an immediate ceasing of the pollution, without human intervention. This is what the operation aims at.
The existing data regarding the biological elimination of manganese in underground waters shows it to be consistent. There is no such data on manganese elimination by slow sand filtration, in surface waters, in case of a short accidental pollution. As far as slow sand filtration is concerned, the available results only represent waters with consistent manganese contents.
This study has been carried out with a 50 m3/h pilote system, which was located in Paris surroundings. It was supplied with water from the Marne river and combined physicochemical and biological treatments. The physicochemical phase consisted of a coagulation-flocculation-decantation reaction in a pulsator, with ferric chloride including 500 to 600 ppm divalent manganese. Then, it was quickly filtered through sand, at a flow rate of 5 m/h. The water was subsequently poured into a storage tank in which it remained for around two hours. Then, it was filtered through slow sand filters, which biologically treated it. The filters contained 1 m high of sand and the filtration rate was 1 m/h, which allowed the development of a biological membrane on their surfaces.
Simulated manganese pollution tests have been realised as follows :
- water with two different manganese contents have been tested (0,5 and 1 mg/l),
- ammonium Ions have been simultaneously added to show a possible inhibition of nitrification.
Each test lasted around two weeks and time was spent as follows :
- polluting agents were continuously added during 3 days (which is the maximum duration of an accidental pollution).
These two tests gave the following results :
1st test (0,5 manganese/l) :
- 35 % of the manganese was eliminated during the coagulation-decantation-quick filtration phase.
- 100 % of the manganese was held bock by the slow sand filtration.
- An important amount of nitrites appeared in water at the end of the treatment, which suggested an inhibition of the nitrification, or a competition between manganese and ammoniacal oxidation.
- After the manganese pollution was stopped, no release of manganese and no inhibition of the nitrification were noticed.
2nd test (1 mg manganese/l) :
- 25 % of the manganese was eliminated during the coagulation-decantation-quick filtration phase.
- 97 % of the manganese was eliminated by slow sand filtration.
- In this case, the ammonium and nitrites were eliminated by the biological filtration, which suggested a readaptation of the bacterial flora to this kind of pollution.
The result showed chat the biological chain adapts itself very quickly to a rapid increase in water manganese content (no more than 2 hours). Indeed, the biological membrane, which was already contaminated by a permanent accumulation of manganese, reacted very quickly. The first test shows that nitrites appear and confirms the results that other searchers had already obtained before, in there is competition between nitrobacteria and the bacteria which oxidize manganese.
A less efficient elimination of organic matter also shows competition during the biodegradation phase.
In the second series of tests, a modification of the bacterial flora, due to the first manganese simulated pollution tests, is noticed.
These encouraging results allowed us to propose an original system for water treatment, based on a physicochemical treatment and biological slow treatments and, afterwards, a refining phase with combined ozone and granular actiated carbon.
In addition to its well-known properties, ozone allows to avoid the possible appearance of nitrites in water.