by
M.A.H. Saad, A.T. Amuzu, C. Biney, D. Calamari, A.M. Imevbore, H. Naeve and P.B.O. Ochumba
Over the last years, in many African countries a considerable population growth has taken place, accompanied by a steep increase in urbanization, industrial and agricultural land use. This has entailed a great increase in discharge of pollutants to receiving waters, causing undesirable effects on the aquatic environment and on fisheries.
Organic pollution of inland waters in Africa, in contrast to the situation in developed countries, is often the result of poverty and economic and social under-development. According to Tolba (1982), it is in these countries that the quality of water, and often the quantity, is lowest, sanitation and nutrition the worst and disease most prevalent.
Unfortunately, there are very few water quality studies for most African inland waters. In general, the available data come from scattered investigations which were carried out by individuals and by very few scientific projects concerned with African waters. Few reviews exist on the state of pollution of African inland waters, e.g. Dejoux et al. (1981), Alabaster (1981), Calamari (1985), Burgis and Symoens (1987), Dejoux (1988) and Davies and Gasse (1988). All documents confirm the existence of pollution problems at various levels in the different countries.
Due to population and industrial growth, inland waters (rivers, lakes, etc.) become often the recipient of organic matter in amounts exceeding their natural purification capacity, while in the past natural purification and dilution were usually sufficient.
Sewage and other effluents rich in decomposable organic material, cause primary organic pollution. Secondary organic pollution is defined as the surplus of organic matter, which is the sum of undecomposed organic material introduced into the water body with primary pollution and of the material resulting from an extremely increased bioproductivity within the polluted ecosystem itself (Štirn, 1973). As stated by Dejoux et al. (1981), organic wastes mineralize in the receiving water bodies and the resulting nutritive elements stimulate plant production, leading to eutrophication. In this situation, the biomass increases considerably and goes beyond the assimilation limit by herbivores. This secondary organic pollution is considerably greater than the primary organic load. The excessive production of organic matter leads to the build up of “sludge” and the mineralization process consumes all dissolved oxygen from the water column, which causes fish kills. Consequently, organic pollutants are called oxygen-demanding wastes. The relatively high temperatures in tropical countries accelerate this process.
Rain water transports soil to streams, rivers and lakes by erosion processes, including dissolved and particulate organic matter. Decomposition of this organic matter continues during transport and in the sediments, giving new soluble organic and inorganic matter. The quantities of organic matter transported, its characteristics and composition vary from one region to another. A man-made transport mode of organic material to natural receiving waters are sewage pipes. Man himself is unable to use all the energy stored in food and his wastes are often discharged into the water without treatment. It is well known that untreated sewage creates a public health danger, being a potential for epidemics of water-borne diseases, such as typhoid fever, and also causes a serious loss of the recreational value of the inland waters (Štirn, 1973; Shuval, 1986). The present paper, however, is dealing with organic load only since public health problems resulting from sewage require separate attention and a specific strategy of their own.
In addition to the ever-increasing urbanization, industry and development of agriculture and forestry contribute considerably to the organic loads, which pose a hazard for inland waters and fisheries. Accordingly, domestic sewage and organic industrial wastes, as well as wastes from agricultural and forestry products are considered as main sources of organic pollution of African waters. Alabaster (1981) pointed out that agriculture is being further developed in some African countries, leading to an extension of existing industries involved in the processing of plant and animal products and to an increase in the highly oxidizable discharges.
3.1 Municipal Waste Water
According to Dejoux et al. (1981), urban pollution is generally organic in origin and is very dependent on the size of the urban development, on the existence of the effluent treatment systems and on the waste disposal habits of the inhabitants.
The principal physical, chemical and biological characteristics of traditional sewage are known. Mixed sewerage systems of modern municipalities, however, do add increasing levels of organic and inorganic material, some of them toxic (e.g. heavy metals), to municipal effluents from small industries. The rise in municipal BOD wastes is related to industrial effects rather than to the great changes in the habits of the population. The increase in the phosphorous compounds transported by wastewater constitutes a major problem. Many African towns have open drain systems, which are flooded during the rainy season, leading to high organic discharge to the receiving waters over short periods of time.
3.2 Organic Industrial Wastes
In most developed countries, industries produce a larger load of organic wastes than municipalities. Wastes with high BOD loads are produced by textile industries, paper and pulp mills, rubber production and chemical industries. Metal industries and mining contribute to a lesser degree to organic loads.
In Africa, food processing is a major industry; plants are mainly located inland, and consequently the discharged wastes create pollution problems in the inland waters (rivers, streams and lakes). Typical examples for such industries are meat processing and dairy plants, sugar refineries, breweries, distilleries and palm-oil industries. The quantities and characteristics of wastes from these sources vary, and the pollution caused by them has to be calculated on a case-by-case basis, especially when the organic load is considered. In general, BOD loads are higher than those of ordinary sewage.
BOD, COD and suspended solids are the traditional parameters for measuring organic pollution. However, parameters such as dissolved oxygen (DO), hydrogen sulphide (H2S), pH, total dissolved solids (TDS) and nutrients are also important. The nutrients nitrogen (N) and phosphorus (P) have been identified as key factors in the eutrophication of inland water (Vollenweider, 1968). They are measured in their various organic and inorganic forms (e.g. NH3, NO2-, NO3-, PO43-).
The most relevant textbook on analytical methods is ‘Standard methods for the examination of water and waste water’ (APHA/AWWA/WPCF, 1985).
The assessment of nutrient loading and of the relative contribution of the different sources of nutrients to surface waters is of critical importance for the implementation of pollution control measures to prevent or reverse eutrophication.
Analyses provide a precise measurement of nutrient loads but they are very costly and time consuming and they fail to give adequate information regarding the contribution from different sources. The only possible approach to a large-scale evaluation is a theoretical estimate by means of quantification of the various sources, by collecting data on land use, population, agricultural and industrial activities and by applying appropriate and specific coefficients.
This method has been extensively used in developed temperate countries, and suitable coefficient are available. For tropical countries some modifications are necessary and extrapolations would have to be made.
The two major sources of organic matter and nutrients are domestic discharges and food processing industries. The increase in African population is exponential, especially in towns; the population of Bujumbura, for example, has tripled in twenty years. The populations of Abidjan and Lagos have increased by twenty times in forty years. Cairo, according to the last census, has more than 10 million inhabitants. Similar trends can be observed everywhere in Africa.
Despite the fact that in many situations the organic load has not been properly quantified, one should stress the relevance of food processing industry in all African countries in contributing to the BOD load, especially of big rivers. Dejoux (1988) gives the following examples: Wastes from fruit juice production and breweries are discharged to the Niger at Bamako, to the Maraohué in Côte d'Ivoire, to the Logone in Chad, as well as to the Abidjan and Lagos Lagoon and to the coastal lake Nokoué in Benin.
Wastes are seasonally discharged from sugar-cane processing at Ferkéssedougou to the Bandama River, at Banfora to the Comoé (Burkina Faso) and to the Sabi-Lundi River in Zimbabwe, from coffee processing at Bouaflé to the Bandama, and to River Nyando in Kenya. Palm oil factories, milk processing industries, sisal, etc., are other important sources for wastes with a high BOD.
Alabaster (1981) and Calamari (1985) have reviewed the state of aquatic pollution in Burundi, Cameroon, Côte d'Ivoire, Ghana, Kenya, Mali, Malawi, Nigeria, Sudan, Tanzania and Zambia. Both reviews reported on the research activities and publications in these countries. Some of this research deals with the organic wastes and their effects on the receiving inland waters.
In the following part, selected African experiences are reported, illustrating situations in which water bodies suffer from organic loads and referring to research conducted and effects observed.
6.1 Northern Africa
In North Africa, the delta lakes in Egypt are influenced by organic pollution to various degrees, depending mainly on the discharge rates and the dilution capacity. The depths of these water bodies (Nozha Hydrodrome, Lake Mariut, Lake Edku, Lake Brollus and Lake Manzalah) range from 1 to 1.5 m. Lake Mariut, which was once a highly productive lake, is now considered as the most polluted delta lake, severely affected by domestic sewage from the southern part of Alexandria, by industrial wastes from several factories constructed close to the northern side of the lake and by agricultural runoff. Next in order of pollution is the Nozha Hydrodrome. Because of their considerable size, Lake Edku, Lake Brollus and Lake Manzalah show a pollution gradient from regions further away from the direct effect of discharges to those near the waste outfalls.
A considerable part of the allochthonous organic matter in the form of sewage and the autochthonous organic substances resulting from the subsequent increase in bioproductivity, decomposes, consuming dissolved oxygen and causing deoxygenation of the water. The biota, particularly fish, may be asphyxiated. After depletion of dissolved oxygen, anaerobic decomposition of organic matter continues (the stagnating water turns septic) and decomposition gases are produced. Among them is the toxic hydrogen sulphide, which is recognized by its unpleasant smell. Suspended materials deposit on the bottom and thus blanket it. This affects the spawning of fish and reduces the numbers of bottom fauna, important for the food chain. In Lake Mariut, fisheries have been affected since about 30 years. Some fish species have decreased in number or disappeared. Tilapia species, relatively unaffected by pollution, now account for about 80% of the fish production in this lake.
Organic pollution of Lake Mariut and its effects have been widely studied by Saad (1972, 1972a, 1973, 1974, 1980, 1985a), Saad et al. (1984), Wahby and Abdel-Mouniem (1979) and Wahby et al. (1978). Sediments in different delta lakes were analyzed by Saad (1978, 1979, 1980a, 1980b). Saad (1978a, 1985b) also describes the dissolved organic matter content of Lake Edku. Changes in the blood of fish due to pollution were found by Saad et al. (1973).
The Nile in Egypt, being the lower part of the river, contains considerable concentrations of organic matter resulting from allochthonous supply (mainly domestic wastes) and autochthonous production (Saad, 1980c; Saad and Abbas, 1985). Levels of organic matter in Aswan Lake are not so high as to cause pollution in this second largest man-made lake in the world. Obviously, part of the organic matter in this lake originates from the inflowing Nile water, which receives this matter mainly from discharges of the countries upstream.
The Lake of Tunis suffers much from organic pollution (Štirn, 1967, 1968, 1972, 1973). In certain areas dissolved oxygen is depleted and hydrogen sulphide is produced as a result of anaerobic decomposition of organic matter; the unpleasant odour of this gas can be smelled near these septic zones.
6.2 West and Central Africa
Of the total population of Côte d'Ivoire, estimated at 7.9 million in mid 1979, about 1.5 million are concentrated in and around Abidjan. Of these, only about 350 000 are serviced by the sewage system discharging into Ebrié Lagoon. Pagès and Citeau (1978) analyzed the concentrations of faecal coliforms in the central part of the lagoon over a year cycle and found several heavily contaminated areas. Environmental degradation is also reflected in low concentrations or even absence of oxygen at the bottom, i.e. in Coccody, Marcory and Bietry bays and in subsequent changes in the benthos fauna. In fact the benthic populations in the bays are now dominated by certain species of oligochaetes, considered as indicators of heavy pollution. The area suffers also from industrial discharges from light industry.
In Ghana, Biney (1982) classified all areas of the country according to the BOD level into three categories: “unpolluted and recovering from pollution” (< 4 mg/l), “doubtful and poor quality” (4–12 mg/l) and ‘grossly polluted’ (> 12 mg/l). Of the 16 lagoons investigated by Biney (1982), 12 were found polluted in varying degrees; grossly polluted are Korle (Accra) and Chemu (near Tema), serving as receptacles of industrial and domestic wastes. Lake Barekese, a man-made lake in the Ashanti region of Ghana which is used as a water supply reservoir, has been extensively studied (Amuzu, 1973, 1975).
In Mali most of the activities depend on the Niger and its tributaries. According to the information obtained by Calamari (1985), pollution problems are not too critical and could easily be managed; most of the wastes consist of oxidizable matter.
In Cameroon, several mass mortalities were recorded in partially-managed river areas near Bafussam, caused by oxygen depletion due to the large organic load (Calamari, 1985).
Except for a few regions, in Nigeria urban areas do not have any central sewerage system or sanitary excreta disposal system. The waste water from most parts of more than 186 urban centres is carried in open drains into streams and rivers, a characteristic feature of many developing countries (Sridhar et al., 1981). According to Calamari (1985), analytical data on the lagoon of Lagos (the largest town in Nigeria) do not exist. However, this lagoon, once very productive in fish, is now considered as a bad place for fishing (Adeyanju, 1979). Ekundayo (1977) reported that the eutrophication of Lagos Lagoon was due primarily to extensive pollution by large quantities of industrial and domestic wastes. In Ibadan, the chemical and microbiological characteristics of the waste water flowing in open drains has been investigated by Sridhar et al. (1981). These open drains, carrying various pollutants, contribute to the pollution of streams, since they travel short distances and consequently offer only limited self-purification of the waste water. In densely populated areas the waste water showed higher values of turbidity, total and suspended solids, oxidizable organic matter, BOD and ammonia nitrogen, with negligible concentrations of dissolved oxygen. The waste water finally enters three major streams, which could be considered open sewers, with water colour ranging from greyish to black, devoid of fish. The water quality in one of these streams can be compared to that of partly treated waste water, turbid and with considerable amounts of total and suspended solids, oxidizable organic matter, BOD and ammonia nitrogen (Sridhar et al., 1981).
The effects of sewage pollution on the distribution and abundance of some organisms, including insects, algae and crustaceans have been studied (Oladimeji and Wade, 1984). In the anaerobic zones of the study area only a few tolerant invertebrate species such as Eristalis and Psychoda were found. The number of organisms increased as conditions such as dissolved oxygen concentration and electrical conductivity improved. Fishes such as Epiplatys sp. and Barilius niloticus were abundant in areas with high dissolved oxygen concentrations.
According to Beecroft et al. (1987) and Awanda (1987), the bulk of the organic load discharged into the Kaduna River comes from the breweries (NBL and IBBI). Waste water discharge from these sources originates from liquors extracted from grains and yeast and have the characteristic smell of malt. They are slightly acidic (pH 5–6), with high particulate and soluble organic content. Effluents from the textile industries have been shown to contain fibres, toxic organic chemicals and heavy metals. The phytoplankton density is lower in points of the Kaduna receiving heavy loads of organic pollutants. The lower number of species of flora and fauna below the Kakuri drain, which carries the effluents from the textile mills and breweries to the river, are attributable to organic pollution. Awanda (1987) noted that the higher density of certain dominant groups of chironomid (midge) larvae and nematodes at some of these heavily polluted points may be an indication that only these species, tolerant of low dissolved concentrations, are able to survive.
6.3 Eastern Africa
East African inland water systems have undergone successive changes since the mid-fifties due to intensive selective fisheries, modification of the drainage area, invasion by introduced species and the increasing physico-chemical changes in the environment. In Kenya, organic pollution and eutrophication of inland waters is to be attributed to the increase in population, urban and industrial activities and agricultural production (Kallquist and Meadows, 1977; Meadows, 1980; Alabaster, 1981; Ochumba, 1985). Indicative of the deterioration of water quality in Lake Victoria are algal blooms and fish mortality (Ochumba, 1987; Ochumba and Kibaara, 1989) and species reduction (Okemwa, 1984; Barel et al., 1985). The rift valley lakes (Baringo, Turkana, Naivasha, Nakuru and Elementaita) are threatened by receding lake levels due to drought and by industrial and riparian agriculture (Meadows, 1978; Harper, 1984).
The potential problems of organic pollution in Tanzania's inland waters are described by Ngoile et al. (1978), Alkbrant (1979), and McAuslan (1980); main threats to Lake Tanganyika are riparian agriculture and oil prospecting. In Burundi, the main concerns are the discharge of sewage and industrial wastes to Lake Tanganyika (REGIDESO, 1980) and agricultural inputs to the Rizizi River (Autrique, 1977). Internal overturns in the rift valley lakes in Uganda and Ethiopia have been associated with fish kills and algal blooms (Burgis, 1978; Belay and Wood, 1984) due to nutrient enrichment and oxygen depletion. Magasa (1978) concluded that organic pollution in Malawi is centred around Blantyre City and the Shire River, which receives agriculture-based wastes. In Zambia, Mumba et al. (1978) have indicated that water pollution is mainly a problem with the Kafue River due to industrial and urban development; fish kills in the river (Kaoma and Salter, 1979) were due to excessive levels of nitrogen compounds.
6.4 Southern Africa
Marshall reported limnological data for 33 Zimbabwean lakes and used them to predict their fish yields. Except for eutrophic lakes such as the Mcllwaine, phosphorus was found to be the major limiting nutrient.
Since the early sixties, several algal blooms occurred in Lake Mcllwaine due to nutrient enrichment through urban discharges from the town of Harare. Measures had been taken to reduce eutrophication, e.g. some of the effluents were used to fertilize the farmland around the town, particularly during the rainy season. However, such measures were considered insufficient (Wells, 1975).
A review of the more recent situation of this lake is given by Thornton (1981) with a self explanatory title, ‘Lake Mcllwaine: an ecological disaster averted’. He describes the recovery of the lake after severe measures had been taken to reduce the total load of nutrients. In the same area, Harare region, Thornton and Nduku (1982) showed that drainage waters from heavily urbanized areas had nutrient contents two to twenty times higher than waters from forest and savannah areas.
The above review of effects of organic pollution on inland waters evidences the need for control of this type of pollution, which is best achieved by control at its source. As many sources of organic pollution are at the same time generating other pollutants, their control resolves a number of pollution problems.
Although organic matter is the most important source of African inland water pollution, discharges of limited quantities of organic pollutants are unlikely to have harmful effects on large lakes or even small water bodies. In fact, each environment has the capacity to accommodate limited, quantifiable loads of pollutants, defined as Environmental Capacity (GESAMP, 1986). Furthermore, large amounts of organic wastes can be controlled by the introduction of more economical technologies which may even facilitate constructive use of the nutrients. However, not only primary consequences but also processes of secondary organic pollution have to be kept under control as they may lead to irreversible damage of the ecosystems.
Legal, administrative and technical measures are necessary to reduce or eliminate the undesirable effects of organic loads, e.g. unacceptable physical, chemical and biological changes in the receiving inland waters. Consequently, multi-disciplinary team work is needed for water pollution control.
At national level control of aquatic pollution is achieved by:
formulation of national policy for pollution control;
enactment of appropriate legislation, and
establishment of appropriate institutional arrangements (administrative and technical) for monitoring, implementing and regulating pollution control.
Alabaster (1981) and Calamari (1985) reviewed the existing situation in eleven countries in Eastern, Western and Central Africa. According to Alabaster (1981), water pollution control legislation has evolved rapidly in several countries but is still under review in others. In a few cases environmental legislation is already being used effectively for pollution control.
It is hoped that each African country will achieve its own environmental legislation in the near future, taking into consideration the experience of developed countries in order to avoid the mistakes made there. It is important that such legislation includes requirements for environmental impact assessment, a process for incorporating environmental considerations into all development activities already at the planning stage. Also, legislation must provide mechanisms for the enforcement of aquatic pollution control laws, ideally with the support of local and provincial offices.
Monitoring systems have to be established to check on the health of the aquatic environments and the effects of large loads of organic wastes on the biota, especially the commercially important species. In African countries, the steps indicated below are proposed for a monitoring programme.
A survey should be carried out to identify the principal pollution sources. According to Calamari (1985), a register of point sources has been completed or is at an advance stage of preparation in the five countries he visited in West and Central Africa. For each water body, calculations are made of the BOD and other water quality characteristics of the organic wastes, discharged untreated or from municipal and industrial treatment plants. The organic loads of the receiving waters should also be investigated, taking into consideration seasonal changes of runoff and biological activities.
A map of the receiving waters based on the survey is then prepared, showing for each water body the organic pollution load in BOD and its different sources, as well as the number of treatment plants for domestic and industrial wastes. On the basis of this map, certain areas in which organic pollution is serious can be selected. Special attention should be given to these areas, and the different compartments (water, sediments and organisms) should be monitored over a suitable period (one to two years).
Biological monitoring uses living organisms as sensors for environmental quality; species composition and diversity, as well as population density, normally decrease with lowering of the water quality. Methodology comprises sample collection and processing, identification and counting of aquatic organisms as well as biomass measurements. Since sample collection and observations are performed in the field and analyses are carried out in the laboratory, research stations are needed, preferably near the polluted areas.
The results obtained from the selected heavily polluted areas are useful for the design of a continuous monitoring programme, which could cover all polluted inland waters in a country. Such large-scale monitoring activities necessitate a number of suitable laboratories in well-equipped research institutes and fishery departments, as well as a sufficient number of trained technical and scientific personnel. In some African countries water pollution laboratories do exist, but with insufficient means. Therefore, improvement of the equipment and structure of the existing laboratories, as well as the establishment of new laboratories, is highly recommended.
Pollution from organic matter and nutrients is of considerable importance in African inland waters and fisheries. Remedial actions should be undertaken.
A lot of information is still missing, and specific research programmes at national and regional level are needed; this entails the need for a network of adequately equipped laboratories.
Exchange of data and experience between African countries concerning organic pollution and its control is necessary, in particular on self-purification processes under African climatic conditions and on waste treatment processes for domestic and industrial organic wastes appropriate for Africa.
An increase in the number of experts on the subject and intensification of training and education are called for.
