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Evaluation des dangers écologiques générés par les effluents liquides urbains sur l'écosystème de la baie de Port-au-Prince : Première approche méthodologique

( Télécharger le fichier original )
par Myrline Mompoint
Université de Quisqueya - Ingénieur Civil 2004
  

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Annexe

XXIX Inter-American Congress of Sanitary and Environmental Engineering, San Juan, Porto Rico, August 22-27, 2004

Communication orale: XIV-Emmanuel-Haití-1

ENVIRONMENTAL HAZARD ASSESSMENT OF UNTREATED URBAN WASTEWATER ON THE ECOSYSTEM OF PORT-AU-PRINCE BAY

Evens Emmanuel(*)1, Yves Perrodin2, Kettly Théléys1, Myrline Mompoint1, Jean-Marie Blanchard3

1Laboratoire de Qualité de l'Eau et de l'Environnement, Université Quisqueya, BP 796 Port-au-Prince, Haïti

2Laboratoire des Sciences de l'Environnement, École Nationale des Travaux Publics de l'État, Rue Maurice Audin 69518

Laboratoire d'Analyse Environnementale des Procédés et Systèmes Industriels, Institut des Sciences Appliquées de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne Cedex

SUMMARY

The bay of Port-au-Prince is served as disposal system for untreated wastewater coming from rain, residential areas, industries and manufacturing plants. The presence of contaminants into untreated urban wastewater pose a significant concern to biological equilibrium of the bay ecosystem. The aim of this study was (i) to implement an environmental hazard assessment framework of untreated urban wastewater, (ii) and to apply it on urban wastewater coming from an open channel of the combined sewer system of Port-au-Prince. COD and heavy metals have been considered by the main environmental stressors. The characterization of the environmental hazards of wastewater on the bay ecosystem, was studied by comparing the obtained results for the selected parameters with threshold values on effluents discharge. COD maximum concentration (1300 mg/L) was greater than the European threshold value for COD. In order to complete these first results, it is necessary to conduct a detailed environmental hazard assessment of the Port-au-Prince Urban Wastewater (PAPUW) on the bay by carrying out other physicochemical analysis and bioassays.

Keywords : urban wastewater, environmental hazards, COD, heavy metals

INTRODUCTION

The bay of Port-au-Prince represents a narrow zone of 15 km wide. All the watersheds around the city end at the bay, which becomes thus the natural receptacle of all rain waters. However, these waters carry along household wastes, sludge from pit latrine and sewage matters, which contribute largely to the pollution of the bay (Emmanuel et Azaël, 1998). The impact of urban wastewater on aquatic ecosystems is reported in the literature (Dyer, 2003). Indeed, the discharge of contaminants in natural water bodies pose a significant concern to water quality and to the health of aquatic organisms. The environmental hazards generated by wastewater on water quality is not merely due to the varied types of pollutants that impact these systems, but also to the many ways that pollutants can affect the health of aquatic organisms (Adams and Greeley, 2000).

In Port-au-Prince, the marine ecosystem is thus liable to suffer locally very serious damages caused by the direct discharge of noxious and toxic effluents. Untreated wastewater discharge into the bay puts in evidence three categories of risk: (i) human health risk due to fish morbidity, bacteriological contamination of shell-fish and beaches, (ii) ecotoxicological risk due to ecological modifications such as the sterilization of the sea bottoms, the decrease of the transparency by suspended matters, and the supply of supplementary nutriments, (iii) a risk of an economical unbalance (MTPTC, 1998). Indeed, in the developing countries fish represent the only source of protein for poorer families and fishing an important source of employment (Pollard and Simanowitz, 1997).

Among the three categories of risk, a preliminary ecological hazard assessment of chemical constituents of Port-au-Prince untreated wastewater on the bay could be undertaken by taking into account the adverse effects of increased biological oxygen demand (BOD), chemical oxygen demand (COD), and reduced dissolved oxygen (DO). Indeed, elevated levels of organic wastes from residential areas, industries, manufacturing plants cause increases in oxygen demand due to the concomitant increase in biological decomposition (Dyer et al., 2003). Constant discharge of chemical substances in aquatic ecosystems may also cause changes in biotic community structure and function, otherwise know as biotic integrity (Karr, 1991). The aim of this study was (i) to implement an environmental hazard assessment framework of untreated urban wastewater, (ii) and to apply it on urban wastewater coming from an open channel of the combined sewer system of Port-au-Prince.

EXPERIMENTAL SECTION

The contact of untreated wastewater with the elements of the aquatic ecosystems generates a danger which is linked to the existence of hazardous substances, i.e., which have the potentiality to exercise negative effects over the environment and the living species (Rivière, 1998). The notion of hazard is linked to the possibility of a chemical substance, of the fact of its intrinsic properties or its characteristics, to provoke harmful effects to human and the environment at determined conditions of exposures (Razafindradtandra et Seveque, 1998).

Everywhere in the world, the precautionary principle predominates in the assessment of wastewater discharges and effluents, i.e. the reduction of specific pollutants or substances in the framework of emission policies (Kinnersley, 1990). European Commission Directive 98/15/EEC (1998) proposes a wastewater pollutant emission limit for all the member states of the European Union. Since, Haiti does not have regulation on emission limit of pollutants in wastewater, French legislation has been considered in this study to assess, in absence of treatment, the generated hazard of PAPUW on the bay ecosystem.

Urban wastewater (Streck and Richter, 1997), and rain waters (Valiron et Tabuchi, 1992; Lassabatère, 2002) are loaded of different pollutants (anions, cations, heavy metals, organic pollutants, etc.). In these kind of mixtures, heavy metals are presented under dissolved form (free cations or complexing) and under particle form, i.e., linked to suspended particles (Artières, 1987).

DO values could provide information on man-induced stressors, which the aquatic organisms would experience when a natural aquatic ecosystem receives untreated urban wastewater. Indeed, the biodegradation of organic matters requires important concentrations in DO. Concentrations of DO in urban wastewater less than 5 mgO2/L, critical concentration to the survival of living things in the natural ecosystem, could thus attribute to the presence of organic or inorganic oxidizable substances, which may have harmful effects on fish and invertebrate communities (USEPA, 1986). In absence of information on the different chemical substance of the wastewater, the COD parameter is sometimes used to characterize in global manner the concentrations of organic pollutants. Its measure corresponds to an estimation of presented oxidizable matters in wastewater, whatever their origin organic or inorganic (Rodier et al., 1996). COD can also provide information on the presence of organic substances which can not be oxidized by aerobic biological process (U.S. EPA, 1993). Among the main present heavy metals Cd, Pb, Hg are highly toxic and have a bioaccumulation tendency (Förstner and Wittman, 1979 ; Nriagu,1987).

The conceptual framework for primary hazard assessment of PAPUW (figure 1), is based on a characterization of the hospital effluents in function of their chemical composition (measurement of global parameters and mineral and organic pollutants). The main selected parameters for the primary hazard assessment of urban wastewater were: COD and heavy metals (arsenic, cadmium, chromium, copper, nickel, lead and zinc).

FIGURE 1 : Conceptual framework for environmental hazard assessment of PAPUW

The obtained results for the chemical characterization (CC) of PAPUW have been compared with threshold values (TV) which were established in regulations on effluents discharge. The framework is based on the measurements of COD and heavy metals. French legislation (MATE, 1998) fixes the following threshold value for the other selected parameters: COD (125 mg/L); Cd (0.2 mg/L), Zn (2 mg/L), Cr, Ni et Pb (0.5 mg/L). For any COD and heavy metals concentration greater than the fixed threshold value or any ratio Pc/Vt > 1 (Pc: pollutant concentration; Vt: threshold values), the framework recommends the estimation of the biodegradability index of the wastewater.

MATERIALS AND METHODS

Sampling

Bois de Chêne channel (figure 2), the biggest collector of the rain sewage system of Port-au-Prince has been retained for the campaigns of sampling. 3 samples have been collected on 8 points predefined of the channel from 8th to 18th March 2004. Samples for metal measurements were collected only on the 8th point. Water samples were collected by means of a telescopic perch in a 1-L glass flask. All water samples were kept at 4°C and transported to the laboratory in less than 3 hours.

FIGURE 2: The study area including the sampling points

Physicochemical analysis

An oxygen sensor WTW Cellox 325 was used in the determination of dissolved oxygen. COD was carried out on diluted and filtered samples at 0.45 um and measured by the potassium dichromate method using a HACH 2010 spectrophotometer and the test procedure provided by the supplier.

Heavy metals have been determined according to ISO 11 885 protocol on filtered sample (0.45 um) and acidified using nitric acid (pH<2) and using ICP-AES (Inductively Coupled Plasma-Atom Emission Spectroscopy).

pH: For the measurement of this parameter a pH-meter WTW pH 340 ION was used. This instrument has 2 electrodes: an electrode of reference, metal type and an electrode (specific to the measurement of the pH) out of glass. Conductivity was measured directly on the sites of study using a multipurpose potentiometer WTW - LF 330 provided with specific electrodes.

The method of Mohr was used for proportioning chlorides with silver nitrate and potassium chromate. In the presence of silver nitrate, the ions Cl are mobilized to form cerargyrite. When all the ions chlorides precipitated under AgCl form, silver nitrate reacts with chromate of potassium and a red precipitate brick appears. Knowing the concentration of the solution of AgNO3(Co = 10-2 M) in 100 ml of solution (E = 100 ml), volume necessary to arrive at equivalence (Ve), the concentration of the ions Cl in the solution is given by the formula: [ Cl - ] = Co * Ve/E

RESULTS AND DISCUSSIONS

The average of physical and chemical characteristics of the wastewater samples collected from P1 to P8 are shown in tables 1, 2 and 3.

TABLE 1 : Physical and chemical characteristics of the samples

Sampling points

pH

 

Conductivity

(uS/cm)

Chlorides

(mg/L)

DO

(mg/L)

P1

7.88

7.87 - 7.90

1291.33

890 - 1864

383

149 - 407

3.49

3.21 - 3.90

P2

7.74

7.72 - 7.77

1588.66

1300 - 1736

334.33

258 - 373

3.52

2.93 - 3.83

P3

7.62

7.57 - 7.68

1542.66

1170 - 1998

322

223 - 443

3.8

3.42 - 4.54

P4

7.65

7.58 - 7.72

1441

1223 - 1590

294.66

237 - 334

3.11

2.62 - 3.46

P5

7.67

7.63 - 7.71

1866,66

1580 - 2260

408

332 - 512

2.77

1.75 - 3.37

P6

7.66

7.60 - 7.72

1850

1540 - 2290

403.33

321 - 520

2.64

1.36 - 3.41

P7

7.55

7.35 - 7.72

1870.33

1640 - 2231

409

348 - 505

2.51

1.35 - 3.03

P8

7.41

7.34 - 7.54

1810.33

1560 - 2231

393.33

327 - 505

2.85

2.02 - 3.37

pH was always in an alkaline range (7.34-7.90) in all the samples, with a variation lower than 1 pH. Temperature of effluents during sampling time was ranged from 28 to 33 °C. The variation on the conductivity (890-2290uS/cm) indicates an important mineralization level of the samples. Conductivity results confirm the presence of anions and cations, the main constituents of urban wastewater. Since the samples' salinity, expressed by conductivity and chlorides result, are lower than the sea salinity, it seems evident to mention that samples salinity could be due to metallic salt, which are toxic to aquatic organisms.

The obtained values for DO ranged from 1.35 to 4.05 mgO2/L, which are lower than 5mgO2/L. DO concentrations increase gradually from P1 to peak at P3, and decrease from P4 to P7. The measurement of DO downstream or P8 showed a slight increase. The low DO concentrations of the samples could be the consequence of biological degradative reactions of the organic matters containing in the channel.

Arsenic concentrations were lower than the equipment detection limit (table 2). The same observation was done for sample 1 and 2 of Cd. Concentrations of all other metals analysed (Cr, Ni, Pb, Zn) were lower than the threshold value. Since metals measurements have been carried out on filtered samples, and since the channel contains an important volume of sediments it seems that heavy metals could exist under particle form. To have, a better understanding of heavy metals effects contained in PAPUW, it would be necessary to determine these elements under both dissolved and particle forms.

TABLE 2: Results of heavy metals analysis

Parameters

(mg/L)

Detection limit

Sample 1

Sample 2

Sample 3

Threshold values (MATE, 1998)

As

0,0093

<DL

<DL

<DL

?

Cd

0,0046

<DL

<DL

0,011

0,2

Cr

0,0017

0,024

0,028

<DL

0,5

Ni

0,0017

0,007

0,006

0,005

0,5

Pb

0,0033

0,015

0,012

<DL

0,5

Zn

0,00033

0,085

0,229

0,056

2

In the effluent samples, COD concentrations ranged from 800 to 1300 mg/L (table 3), exceeding the discharge standards given by selected legislation 125 mg/L (MATE, 1998). This could be attributed to the presence of organic substances. COD concentrations are also greater than the values proposed by Metcalf and Eddy (1991) for domestic wastewater.

TABLE 3: Results of COD analysis

Sampling

points

P1

P2

P3

P4

P5

P6

P7

P8

Threshold values (MATE, 1998)

COD (mg/L)

1155

1128

1073

1268

945

933

883

946

125

The different results obtained from the application of the framework showed that PAPUW generate an effluent very hazardous for the aquatic organisms of the bay ecosystem. The approach showed that a preliminary and quick assessment of urban wastewater on natural ecosystem could be done with low cost and short time. However, it must be implemented by the realization of other physicochemical analysis and bioassays particularly on the first organization level of the marine food chain.

CONCLUSION

The aim of this study was (i) to implement an environmental hazard assessment framework of untreated urban wastewater, (ii) and to apply it on urban wastewater coming from an open channel of the combined sewer system of Port-au-Prince. The first results obtained from the framework application showed that discharge of PAPUW in the bay could have important negative effects on aquatic organisms. In the future, it will be necessary to confirm these first results by the realization of other physicochemical analysis and bioassays on these effluents.

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* Corresponding author. Tel : (509) 221 4330; Fax: (509) 221 4211 e-mail : evemm1@yahoo.fr

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