Effet de l'ozonation infralétale sur le potentiel zêta (ζ) des kystes d'Acanthamoeba en vue d'optimiser leur décantation

Effect of infralethal ozonation on Acanthamoeba cyst zeta (ζ) potential with a view to improve their decantation

¹ Laboratoire de Parasitologie
² Laboratoire de Pharmacie Galénique UFR des Sciences pharmaceutiques, boulevard H.-Becquerel, 14032 Caen Cédex - France

Résumé

Plus de la moitié des amibes libres constamment présentes dans les réseaux de distribution d'eau potable appartiennent au genre Acanthamoeba. Étant occasionnellement à l'origine de pathologie grave comme la kératite amibienne et pouvant aussi assurer la multiplication et le transport de bactéries comme les Legionella, ces amibes sont indésirables et doivent être éliminées.

Étant donné leur nombre dans les eaux brutes (1.10⁴ à 4. 10⁴/litre) et le fait qu'elles y sont à 85-91 % sous forme de kystes, le flux amibien reçu en permanence par une filière peut donc être assimilé à un système dispersé, caractérisé par un potentiel électrocinétique ou potentiel zêta (ζ) qui conditionne son aptitude à sédimenter.

C'est pourquoi ce potentiel a été mesuré avant et après ozonation infralétale (0,2 mg/0/1 pendant 2 minutes) sur les kystes de 4 souches d'Acanthamoeba.

Les mesures montrent que le potentiel ζ initial des kystes est faiblement négatif ce qui signe une tendance naturelle à une bonne sédimentation. L'ozonation pratiquée, réduit la valeur initiale de 50 % ce qui permet d'envisager une amélioration de la bonne sédimentation naturelle.

On peut donc attendre d'une filière pratiquant la préozonation, une élimination des kystes d'Acanthamoeba dès la floculation-décantation mais cela demande à être vérifié en station - pilote.

Abstract

Free-living amoebae and specially those belonging to the genus Acanthamoeba are constantly isolated from drinking water town mains. Because they are the causative agents of Acanthamaoeba keratitis, a severe corneal infection mainly diagnosed in soft contact lens wearers, and are known to be natural hosts for bacteria as Legionella, they are undesirable and have to be removed.

These amoebae are very numerous in raw waters (1.10⁴ to 4.10⁴ per liter in the water of the Oise river in France) where cysts are the predominating forms (85 - 91% of the total population) the size of which ranges from 10 to 15 µm.

So, treatment plants are fed with an amoebic flow consisting almost exclusively of cysts suspended in water with their interface organized according to the double-layer model and consequently characterized by an electrokinetic potential so called Zeta (ζ) potential.

Since ζ potential is an important parameter for the forecasting of a particle settling, the aim of the present study was to measure that of Acanthamoeba cysts before and after an infralethal ozonation (0.2 mg O³ per liter for 2 minutes).

The experiments were carried out on cysts of four Acanthamoeba strains, A. Castellanii a reference strain, A. sp. MR4 isolatef from drinking water town mains, A. sp. C1B1 and A. sp. C1B2, isolatef from the cornea of patients with Acanthamoeba keratitis.

Cysts were harvested from 3 weeks old cultures of amoebae at 30°C on agar medium bacterized with an E. coli suspension.

They were maintained for 2 minutes in contact with ozonated waters prepared, by bubbling of ozonized air, to contain a 0.2 mg/l initial dissolved ozone residual. At the end of the contact time, the controlled final residual was found never lower than 0.18 mg/l.

The measures of ζ potential were carried out on cysts suspended in KCl 10^-3M. The apparatus used for measures was the Zetaphoremetre II® by SEPHY - France. The displayed parameters during the measures were conductivity 150 µS/cm, pH 5,5, viscosity 0.8905 and temperature 25 °C.

The ζ potential values in mV, found before and after cyst ozonation, were respectively for A. Castellanii - 8.24 (± 0.91) - 4.46 (± 0.95); for A. sp. MR4 - 6.40 (± 0.56) and - 2.57 (± 1.0); for A. sp. C1B1 - 9.12 (± 1.21) and - 3.67 (± 1.52); and for A. sp. C1B2 -11.21 (± 1.64) and - 4.04 (± 2.0).

The initial ζ potential value of cysts is weakly negative indicating that they have a natural tendency to settle. The reduction of potential recorded after infra-lethal ozonation demontrates that natural settlement can be improved when preozonation is used in a treatment plant.

We think that Acanthamoeba cysts have to be removed at the floculation-decantation stage because numerous particles and microorganisms retained by sand filters are nutriments for amoebae, making them able to excyst, multiply under trophozoite forms and encyst. Moreover, because of the low values of their ζ potential, the new cysts resulting from this amoebic development in sand, could be easily released from the sand filters. Nevertheless that have to be verified in full scale treatment conditions.