Heiner C. Peter B. Box Kisumu Kenya. Massoud A. Box Nairobi Kenya. Scientific bases for control of organic pollution in African inland waters by Massoud A.
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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 tremendous increase in discharge of a wide diversity of pollutants to receiving water bodies and has caused undesirable effects on the different components of the aquatic environment and on fisheries. Organic pollution of inland waters in Africa, in contrast to the situation in developed countries of the world, is often the result of extreme poverty and economic and social under-development.
According to Tolba , 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.
Dejoux et al.
Two are on the state of inland water pollution in eleven countries from eastern, western and central Africa Alabaster, ; Calamari, , reviewing the existing sources of water pollution, the scientific investigations on the subject and the legislation enforced in the different countries. Both documents confirmed the existence of pollution problems at various levels in the different countries.
The third report Biney et al. Due to population and industrial growth, inland waters rivers, lakes, etc. 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 Stirn, As stated by Dejoux et al.
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. 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 Stirn, ; Shuval, 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 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.
According to Dejoux et al. 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. 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. 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.
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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. The nutrients nitrogen N and phosphorus P have been identified as key factors in the eutrophication of inland water Vollen veider, They are measured in their various organic and inorganic forms e.
The amount of oxygen necessary for the oxidative decomposition of a material by micro-organisms is known as the biochemical oxygen demand BOD of the material.
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The ultimate carbonaceous BOD of a water or a liquid waste is the amount of oxygen necessary for micro-organisms to decompose the carbonaceous materials that are subject to microbial decomposition Warren, Domestic and industrial wastes often have BODs of several thousands of milligrams per litre; when inadequately diluted in receiving waters, these wastes can lead to severe oxygen depletion.
The chemical oxygen demand COD is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant under acidic conditions.
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COD is an important, rapidly measured parameter for surface and wastewater investigations and for the control of waste treatment plant efficiency. As oxidation occurs under forced conditions, it is a more complete process than biological oxidation. Solid materials finding their way into natural waters may have some undesirable effects while carried in suspension and can also have other undesirable effects after settling on the bottom. These effects include turbidity decreasing photosynthetic activity and oxygen depletion.
These solids are generally classified as total, suspended and dissolved. Total solids are measured as residue left in a container after evaporation and drying of a wastewater sample at a fixed temperature in an oven. Total solids include filtrable and non-filtrable residues, corresponding to dissolved and suspended matter respectively. Phosphorus is essential for the growth of autotrophic organisms, and in a number of cases is the nutrient that limits the primary productivity of a water body.
Phosphorus is in particulate, organic or dissolved form, in the majority of cases as orthophosphate, which is the form utilized by the organisms.
Organic phosphorus is released by enzymatic activity, while particulate phosphorus constitutes a reservoir; after precipitation to the bottom sediments, it can be released as soluble orthophosphate according to the redox conditions, determined mainly by the presence of oxygen. Dissolved nitrogen compounds include ammonia nitrogen, nitrite, nitrate and various organic compounds. In contrast to the phosphorus cycle, which involves a very important physico-chemical component, the nitrogen cycle is controlled mainly by microbiological processes.
Dissolved oxygen has a decisive influence on the cycle, determining the rates of the processes. Nitrate is an essential nutrient for many photosynthetic organisms and in some cases it constitutes the growth limiting factor. It is present only in small quantities in fresh water but it has a high concentration in effluents from treatment plants; it is frequently a contaminant of ground water.
Nitrite is an intermediate oxidation state of nitrogen, both in the reduction of nitrate and in the oxidation of ammonia. It has the highest toxicological significance for human health. Ammonia is naturally produced but its concentration is frequently increased by contamination resulting mainly from deamination of organic substances. It is highly toxic to aquatic animals, although it can be utilized directly as nutrient by several algal species. All the forms of nutrients can be analysed after various treatments of the samples by means of spectrophotometric methods.
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.
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For tropical countries some modifications are necessary and extrapolations would have to be made. Most of the following information is taken from a review by Vighi and Chiaudani The quantity of nutrients and organic matter discharged to surface waters from point sources can be estimated on the basis of some general assumptions without major difficulties; some practical problems, however, can arise with collection of the data needed.
A higher degree of uncertainty is connected with losses of nutrients in rivers before reaching eutrophic water bodies. According to different authors, a 50 percent loss can be assumed as a mean value. Domestic point loadings of phosphorus can be calculated by estimating both the metabolic contribution and the contribution from polyphosphates in synthetic detergents.
Detergent phosphorus is one of the main factors responsible for increased eutrophication since the fifties. As for the metabolic contribution, a phosphorus load of 0. In developed countries phosphorus contribution from detergents has decreased from 0. According to OECD, the contribution of phosphorus from industrial loadings is only one tenth of that from domestic loadings.
It is restricted to industries such as food processing, phosphoric acid production, etc. Although the phosphorus content of different industrial waste waters has been measured, a precise evaluation of phosphorus loadings is rather difficult to make and could only be arrived at by means of detailed surveys. More important is the industrial contribution of nitrogen compounds and organic matter. Table 1 shows the nitrogen contribution from some industrial activities Provini et al. As discussed in chapter 4, BOD loads can cause oxygen depletion and other deterioration of water quality.
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The BOD value for some industrial activities is also reported in inhabitant equivalents i. See Table 2 for conversion factors.