STUDIES ON THE RECOVERY OF NUTRIENTS FROM URBAN SOLID WASTES

Abstract

The uncontrolled growth of population and industry in urban cities of Nigeria has been a threat to the socio-economic life and health of the communities. There is need for research into newer methods of solving the environmental health problems emanating from the increasing generation of solid waste coupled with indiscriminate disposal. Waste disposal with resource recovery may offer an attractive solution to the communities where shortage of resources is a perennial problem leading to closure of many viable programmes of economic development. This study examines the types of various wastes generated in an urban community and the feasibility of converting them into valuable manure and feedstuffs for the local agriculture and animal husbandry. Initially, different types of wastes generated in Ibadan, Nigeria were characterised. The wastes were from households with different socio-economic levels, public waste depots, markets, animal houses, zoo garden and catering houses. Analysis of the physical characteristics showed that there was a significant difference (p<0.05) in the rate of generation among the three socio-economic groups. The highest, 0.428 kg per capita per day was from the high socio-economic group, followed by 0.289 kg per capita from the middle and 0.132 kg per capita from the low socio-economic groups. A major portion of the waste generated in the high and middle socio-economic groups contained 66-90% decomposable matter. The presence of leaves which contributes to the bulkiness of refuse is attributed to the traditional food habits of the people. These habits are changing as reflected by the presence of plastics and polythene reported in this work. The chemical composition of the various wastes generated was (% dry weight): - total Kjeidahl nitrogen: domestic waste 1.1-2.6, food factories wastes 1.8-2.1, animal houses 2.0-3.8, cafeteria wastes 2.3-2,9, market wastes 1.5-3.3. - total phasphorus content: domestic wastes 1.6-3.3, food factories wastes 0.6-1.1, animal houses 1.7-6.5, cafeteria wastes 1.1-4.1 and market wastes 0.6-3.7. - carbon content: domestic wastes 40-50, food factories 29-47, animal houses 42-48, cafeteria wastes 44-49 and market wastes 28-47. -volatile substances domestic wastes 63.8-90, food factories wastes 52.2-84.8, animal houses 75.4-86.2, cafeteria waste 80-87 and markets 49.8-84.4. -water content: domestic wastes 55-57.6, food factories 17.2-63.9, animal houses 43-81.4, cafeteria wastes 44.2-89.5 and market wastes 47-91. After characterizing the wastes, more attention was paid to finding ways of increasing some of the nutrients in some of these wastes and utilizing them. One of the earliest practices in Nigeria was composting of various solid wastes. Subsequently this practice was abandoned as a result of the oil boom. Hence experiments were carried out to revitalize this process. Thus solid wastes were mixed with slaughter house wastes and a compost was prepared which promoted crop yield. The study showed that such compost with low heavy metal laid can still be used in Nigeria as a resource on farmland. Further experiments were performed by segregating cafeteria waste rich in organic matter, which is easily available but poses a serious environmental problem attracting many animals and vectors of disease. The nutrients from this waste were utilised for the growth of green alga, Chlorella sp, and aquatic macrophyte, Pistia stratiotes L. after appropriately diluting the waste suspensions and the leachates. The biomass of Chlorella increased over 25 fold in 25 days and the growth rate of Pistia showed over 10 fold increase in 70 days. However, with these systems, harvesting may be an additional factor to be considered for effective use. Experiments were therefore carried out using the aerobic and anaerobic stabilization of cafeteria waste and the leachates. The optimization of the process, time taken for stabilization, cell synthesis as evident from protein enrichment, changes in volatile organic matter and survival of possible pathogens were studied. There was an increase in the total Kjeldahl nitrogen of 34% by aerobic, and 26.4% by an stabilization. To enhance the stabilization process and increase the nutrient contents. The waste was pretreated for effective aerobic stabilization and also enriched with other nitrogen rich organic wastes such as brewery waste from Nigerian Breweries Limited and sewage sludge from University College Hospital, Ibadan. The pretreatment of waste that gave the best result was putrefaction for 7 days and stabilization in situ which was slightly acidic. The alternative pretreatment was steaming of the waste under high pressure before aerobic stabilization without adjusting the pH. Addition of brewery waste and sewage sludge to the stabilizing cafeteria waste increased its total Kjeldahl nitrogen by 41.4 and 37.6% respectively. These treatments also reduced the volumes of the waste by 58.3% and 63.3%, respectively which is desired in the waste management. The aerobically and anaprobically stabilized wastes were found to be acceptable to poultry and rats. Therefore further experiments were carried out on the weight gain, food efficiency, and egg production by laying hens. Poultry industry in Nigeria is badly suffering due to non-availability of feeds which are largely imported from abroad. There was a linear improvement in feed efficiency with supplementation of birds' diets with stabilized products of cafeteria waste enriched with brewery waste, and activated sludge. The highest weight gain was in the groups of birds fed with stabilized product of cafeteria waste and brewery waste, while the highest percentage production of eggs was from the group fed on stabilized product of cafeteria waste and sewage sludge. The solid wastes were also found to contain some in-organics and trace metals such as sodium, potassium, calcium, cadmium, chromium, cobalt, copper, iron, lead, magnesium; manganese, mercury, nickel and zinc. The levels was not high enough to affect the health of animals and the quality of their meat. The stabilized products were shown to be enriched with vitamins such as riboflavin and niacin, and pathogens such as Ascaris sp., Hookworm and Trichuris sp. were destroyed in the process. The results obtained are discussed in the light of the modern concept of integrated recovery system and suggestions are made of simple devices by which a less skilled person could convert his own waste into a useful product which he could feed to his own poultry. This study is of great value to countries like Nigeria, where vast quantities of valuable nutrients like carbon and nitrogen are otherwise destroyed or discarded into the environment.