Monitoring of organic load in a tropical urban river basin (Cameroon) by means of BOD and oxydability measurements

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Abstract

The tenor in organic matter has been assessed in an urban aquatic ecosystem the Mfoundi River Basin of Cameroon.The double technique of the Biochemical Oxygen Demand (BOD5) and the Oxydability test have been employed in this assessment. The results obtained indicate a high content of organic matter in the watershed with the highest values of BOD 297.5 mg O2 dm–3 and Oxydability 50.255 mg O2 dm–3 observed at the Akeu stream. The spearman correlation coefficient between BOD5 and Oxydability for the seasonal distribution is 0.22 which is positive but not statistically significant due to a high variability of biodegradable and non-biodegradable organic matter present in the river basin. The same value calculated between these organic tests with respect to the sampling sites is negative (–0.15) due probably to their abiotic and biotic heterogeneity. The results reveal a high tenor in organic matter in the hydro-system during the dry season, with a relative reduction during the rainy season, a situation that exposes the population of Yaoundé and its environs to acute pollution which can lead to numerous water related health hazards.

Introduction

One of the most important reason for a proper management of water resources is that water, as a transportation medium for many pathogens can cause serious health hazard (Karanis 2006). Especially the increasing world population makes the threat connected with polluted water more and more serious (Robertson, Gjerde 2006).

The infiltration of faecal matter into water, through the leakages in sewage systems and the inappropriate disposal of waste material bring contamination of waters with bacteria, protozoa and worms. These infections can be transmitted by the direct and indirect use of water for domestic and industrial purposes (Zhang et al. 2009).

Organic matter can occur in water in suspension, colloidal or true solution form (Chabbi, Rurpel 2000) and its determination method depends on its nature. The particulate organic load may comprise of a wide variety of life forms such as viruses, bacteriae, amoebae, algae, spores and micro-plankton of vegetable and animal origin. The worst examples are the contamination of aquatic ecosystem with non biodegradable products. The presence of lubricating oils, organic solvents, detergents, fertilizers and pesticides may render the watercourse inappropriate for exploitation in irrigation or as a source for potable water production as they are responsible for sanitary risks and have been known to have adsorption and sorption properties for other toxic substances as presented by Dontsova et al. 2009. This level of pollution transforms the aquatic ecosystem from oligosaprobic (low pollution) to polysaprobic (very high pollution).

This organic load can be in the form of Dissolved matter which adsorbs light in the 400 to 800 nm range, in the presence of suspended colloidal and particulate matter that scatters light and creates the appearance of colour in water. Various compounds in raw water may be transformed into unwanted taste and odour during the various processes of biochemical breakdown of these substances. The Oxidation of the organisms in water can result in Biodegradable Organic Carbon (BDOC) which is measured as Assimilable Organic Carbon (AOC). Biodegradation is therefore the ability for an object to break down once disposed of in a natural environment. There is a varied kinetics of decomposition, releasing intermediary products which can be toxic. Some of these substances can occur in the aquatic matrices in relatively low concentrations, thereby indicating the complexity of environmental samples (Arditsoglou, Voutsa 2008) or in higher concentrations (Shah et al. 2008; Ait-Benichou et al. 2008). If this BDOC is not removed or the effl uent sufficiently chlorinated to the organic demand, biological organic re-growth can occur in the distribution system which can cause adverse effects on the taste, odour and colour of potable water (Jago, Easten 1991). One component of the biodegradable organic matter is the dissolved organic carbon (DOC) which constitutes an important parameter for organic pollution of waters and waste waters, which is not often studied and which is yet to be regulated by directives. It may contain humic substances, hydrophobic bases, hydrophobic neutrals, hydrophilic acids, hydrophilic bases and hydrophilic neutrals (Katsoviannis, Samura 2007). According to Lin and Jing (2001), the oxidation-reduction potential of the ecosystem could infl uence the dynamics of the Carbon cycle in the presence of other factors such as pH, alkalinity and water hardness.

The Chemical Oxygen Demand (COD) value can be determined as a measure of the oxygen required to convert organic matter into water and CO2 under given conditions. This test is supplemented by the calculation of Oxydability. The Total Organic Content implies the direct determination of the amount of organics through the measurement of Carbon molecules generated during organic matter oxidation. These purely chemical tests are much quicker to carryout but they do not measure the amount of biodegradable matter which is usually the central interest because of the effects of its decomposition on the outcome of the water quality in the ecosystem as demonstrated by Szlachta, Adamski, 2009. The Biochemical Oxygen Demand (BOD) characterises more precisely water and the biodegradable organic quality of the hydrosystem.

The Mfoundi River Basin of Yaounde Cameroon is the main watershed of the cosmopolitan city of Yaounde. This River System is exposed to all sorts of organic releases from residential quarters, hotels, hospitals, administrative blocks and industries. The notion of purification stations is arbitrary and thus sewage is deposited in to the streams without prior treatment. Additionally, there are practices of solid waste (garbage) deposition into streams, thereby exposition of the population to acute waterborne diseases is strong. Taking into consideration the fact that the ideal amount of nutrients in waste waters is normally considered to be the ratio of C:N:P = 100:5:1 with a COD:TOC ratio presumed to be around 3, the organic load in a given aquatic ecosystem is presumed to reveal on the pollution status. Generally the COD is about 1.5 to 2 times the value of BOD for household domestic wastewater.

This drainage system is the main source of water that is being exploited as potable water for the population of Yaounde and its environs. With the hazardous use of the streams as a transport medium for wastes, the qualification and quantification of the biodegradable and non biodegradable contents of the streams becomes an impending problem for the water industry in line with the research of Muller et al. 2008; Smith, Algabany 2009.

Our research was aimed at assessing the content of the organic load in the different streams of the Mfoundi River Basin, by the application of the Biochemical Oxygen Demands (BOD) and the Oxidability test. The values measured will enable a verification of the compatibility of these tenors to the norms of the World Health Organisation (WHO) for streams quality, determine the applicability of this system as a source for the exploitation of potable water in the region and to associate the data recorded to the health risk linked to the presence of organic pollutants in the environment. The association of the data obtained to the meteorological conditions of a tropical region will lead to the prediction of the likely effects of rainfall and temperature on the specific model of organic load as identified by Servais et al. 1995; Koh, Chris 2001. This assessment is a contribution to the identification for practical tools for quantifying and assessing the degree of relative sustainability of water pollution systems based on risk-based indicators, including reliability, resilience and vulnerability which is vital for environmental protection and sustainable management (Sarang et al. 2008). Chemometric assessments confirm the classification, the characterisation and the forecast of the consequent effects of irrational exploitation of the hydrographical basin that is exposed to all sorts of urban usage (Kowalkowski et al. 2008). The decomposition of biodegradable substances produces by-products which can be harmful to the aquatic health through their bioaccumulation in the various organisms that are found in the same food chain and they are also used as natural culture mediums for pathogenic and non pathogenic micro-organisms that are present in the ecosystems. This study is therefore a prelude to the understanding of the interactions between materials, micro-organisms and the possible biochemical mechanisms involved in the oxidations and reductions of natural and synthetic compounds that are present in the aquatic medium. Evaluating the ecotoxicological outcome of biodegradable organic matter in an aquatic ecosystem is a reliable way to precise on pollution and its hazard assessment which dictates the vulnerability of the environment to human and natural activities and the necessary sanitation procedures required for its surveillance and restoration from upstream to downstream. This study will guide us to test the hypothesis whether urbanisation, seasonality and enviro-dynamical features play a crucial role in the intensity of dissolved and particulate organic intake in aquatic systems.

Section snippets

Materials and methods

The tenor in biodegradable organic matter was evaluated in 12 streams and one lake of the Mfoundi River Basin (Fig. 1). These are Akeu, Biyeme, Ebogo, Ewoue, Mfoundi, Mingoa, Mitsomo, Nkie, Ntem, Odza, Olezoa Tongolo streams and the Municipal Lake of Yaounde. This watershed is located in the city of Yaounde which is the political capital of Cameroon. It is situated between latitude 3°53’ N and longitude 11°32 E. It has a surface area of 388 km2 and is positioned at an altitude of 700 m to 800 m

Results

The results obtained for the measurement of the tenor in organic matter in the Mfoundi river basin is presented in Table I and II. The analyses of the Biochemical Oxygen Demand (BOD5) are represented in Table I, while those for the Oxydability test are represented in Table II.

In the Akeu stream which receives effluent waste from a brewery plant, the lowest BOD5value of 30 mg O2 dm–3 was recorded in May, while the highest value of 297.5 mg O2 dm–3 was recorded in March. In Biyeme, Ebogo, Ewoue,

Discussion

The biodegradable organic matter content of pollution in the Mfoundi River Basin is represented by the high values of the BOD and the Oxydability test (Tables I and II). This reduced carbon charge is one of the main factors affecting the hydrochemical regime of natural water resources which are being exploited in developing countries. The pollution of this river basin can be likened to that of the Sebou water surfaces in Morocco, which presents a great deterioration in its quality from its

Conclusion

The observed values of Biochemical Oxygen Demand and the Oxydability indicate a very high content of organic matter in the streams of Mfoundi River Basin, with values increasing from their upper to lower reach in most of the streams except those at the periphery of the town such as Odza and Nkie were the unequal distribution of the profile in organic matter is not remarkable. The profile of these organic load is implicated in the pollution trend in the streams which varies from oligosaprobic to

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