Dispersion et disparition des bactéries conformes dans la baie de Thessaloniki Dispersion and disappearance of coliform bacteria in the bay of Thessaloniki

Résumé

Dans les baies presque fermées de la Méditerranée, le taux de renouvellement et le pouvoir dispersif du milieu marin sont faibles. Ceci amène à des concentrations souvent élevées de bactéries califormes à la sortie des stations d'épuration, même munies de filières biologiques. L'élevage de coquillages dans ces baies constitue une contrainte supplémentaire aux concentrations maximales de bactéries permises selon les normes de la CEE.

Le problème a été posé pour le fonctionnement de la station d'épuration de la ville de Thessaloniki. Le but de cette étude a été l'analyse quantitative de la dispersion et de la mortalité des bactéries coliformes. Pendant le premier semestre 1990, plusieurs campagnes de mesures ont permis l'échantillonnage et la détermination des concentrations de bactéries coliformes à quatre stations, situées au voisinage d'une source de contamination bactérienne. Parallèlement un modèle mathématique basé sur la simulation de la marche au hasard a été mis au point.

Après étalonnage, ce modèle a servi comme outil pour simuler les impacts, sur les sites de conchyliculture, des eaux usées de la ville.

Mots-clés:

• Dispersion,
• mortalité,
• bactéries coliformes,
• modélisation mathématique,
• marche au hasard,
• prévision,
• contamination,
• coquillages

Abstract

In this paper, the contamination of coastal waters by coliform bacteria is considered. The problem is studied in the bay of Thessaloniki (N. Greece) using sampling, laboratory analysis and computerized mathematical modelling. The case study is typical for semi-enclosed bays in the Mediterranean sea. The water renewal and dispersion capacity of the sea are very low and high concentrations of coliform bacteria can be expected. This is the case when modern technology (biological treatment) is used in the sewage treatment stations and chlorination or other types of disinfection are kept low in order to avoid the formation of THM (Tri-Halo-Methanes). The problem of meeting the water quality standards is more difficult, when shellfish growing waters are to be protected.

Impacts to marine environment from sewage and industrial effluents depend on 1) the degree of wastewater treatment, 2) the location of the disposal site and 3) the receiving capacity of coastal waters. The later means the maximum quantity of pollutants that a given area can receive without adversing effects (e. g. eutrophication, change of colour, odours). In the marine environment the receiving capacity is very difficult to assess, as it varies with very irregular way both in space and time. For coliform bacteria, the receiving capacity of a coastal area depends on the dispersive characteristics of the bay and the morality conditions of the bacteries. These are studied here for the case of the bay of Thessaloniki.

The bay of Thessaloniki is located in the NW Aegean sea (Eastern Mediterranean). It is a shallow, semi-enclosed basin communicating with the open sea from the south boundary only. The northern part of the bay has a total area of 300 Km² and a maximum depth of 30 m. it is actually heavily polluted by untreated sewage coming from the city of Thessaloniki. Pollutant loads are estimated as 150 000 m³/d of sewage and 60 000 m³/d of industrial effluents.

The sewage treatment station of the city has started now operating. The provisional disposal site is located in the west toast, three Km from Paliomana, where marine farms for mainly mussels and other shelffishes have been developed. In view of the economic importance of these activities and the need to protect the public health, it is important to assess the impacts from wastewaters to the coastal area.

The main objective of the study is the quantitative evaluation of the dispersion and mortality conditions of coliform bacteria in the bay of Thessaloniki. The ain of the project is the choice of the disposal site of the sewage after treatment, by predicting the water impacts from sewage discharges. During the tire semester of 1990 (February-July 90) sampling and laboratory analyses of E. coli concentrations have been made every 15 days in 4 stations. Some of the samples have been taken during the night. Statistical analyses of the results gave the distribution in space of the max, min, median and C80 values of E. coli (C₈₀ is the concentration which is not exceeded for 80 % cases).

The tale of bacteries in the marine environment is described by the convective-dispersive equation, including the decay term. A linear dependance between bacterial morality and bacterial concentration is assumed. Over the years the advective dispersive equation has been extensively investigated and numerically approximated by numerous methods. Finite differences and finite elements have been used and produced stable numerical results. However, significant errors are introduced in ail these numerical simulations. These are due to the fact that only a limited number of terms in the Taylor series expansions are taken into account. Explicit algorithms suffer from the so-called numerical diffusion. This is an artificial diffusion related to the truncation errors. It is superimposed on the physical diffusion and leads to an excessive attenuation of the input signals. Implicit finite difference algorithms introduce trading effects because the initial signals are propagated at velocities that differ from the physical ones. It seems that particle methods based on random walks are more flexible and easy to use and lead to relatively accurate results.

A random walk computerized mathematical algorithm is developed to simulate the dispersion and mortality of coliform bacteria in the bay. By use of a large number of particles (10³ - 10⁴) which move with the current velocities and by random dispacements following a Gaussian distribution, the contour lines of equal concentrations are obtained. The couple of values for the dispersion coefficient D and the mortality time T₉₀, which simulate better the space distribution of C₈₀ values is : D = 4 m²/s, T₉₀ = 5 h.

The same value of the dispersion coefficient has been independently found by tracking flotting drogues in similar wind conditions (moderate wind). It is concluded that the above values of dispersion and bacterial mortality reflect the characteristic conditions of the bay and can be used to predict the impacts from sewage discharges.

Keywords:

• Coastal waters,
• dispersion,
• coliform bacteria,
• mortality,
• mathematical modelling,
• random walk,
• simultation