Introduction

Effectual handling of solid wastes in a finite world (like ours) that is continually  being exposed to threats from pollution results can only be guaranteed via an sufficient understanding of what solid wastes are, their types, sources, composition and the uses to which they might be put. Perhaps the hardest task facing an environmental scientist is to expect the composition of solid wastes that will be gathered now and in the future. The difficulty is complicated since of the heterogeneous nature of waste substances and the fact that unpredictable externalities can influence the long-term abundance of the individual waste components.

Solid wastes

Solid wastes are all the wastes arising from being and animal activities that are generally solid and abandoned as useless or unwanted. The term 'solid wastes' encompasses the heterogeneous mass of throw-always from residences and commercial activities in addition to the more homogeneous accumulations of a single industrial activity. Usually, solid wastes are scrap substances or other unwanted surplus substance or any these substance that to be disposed of as being broken, worn out, contaminated or otherwise spoiled.

Types and Sources of Solid Wastes

Solid wastes are frequently classified according to the source from that they are produced. Therefore, we have municipal waste, industrial wastes, agricultural wastes and hazardous wastes. Municipal Wastes are wastes that are related to land utilize and zoning.

The hardest source to deal through is open areas because; in these locations the generation of wastes is in a diffuse process. Such are mostly wastes from residential and commercial sources such as food wastes, rubbish, ashes, demolition and construction wastes, special waste, and occasionally, hazardous wastes. Industrial Wastes are those wastes arising from industrial activities and typically these comprise rubbish, ashes, demolition and construction wastes and special wastes.

Agricultural Wastes are wastes arising from the by-products of agricultural produce. Hazardous Wastes are wastes that pose a substantial danger instantly or over a period of time to human, animal or plant life. A waste is classified as hazardous if it exhibits any of the subsequent traits: (i) ignitability, (ii) corrosivity, (iii) reactivity and (iv) toxicity.

In the past, dangerous wastes were frequently grouped into the following categories: (i) radioactive substances, (ii) chemicals, (iii) biological wastes, (iv) flammable wastes and (v) explosives. The principal sources of hazardous biological wastes are hospital and biological research facilities.

Physical Composition of Solid Wastes

Individual components: Typical components of most municipal solid wastes are food wastes,  paper, cardboard, plastics, textiles, rubber, leather, garden trimmings, wood, miscellaneous organics, glass, tin cans, ferrous metals, non-ferrous metals, ashes, bricks, and so on. Such are the eagerly identifiable ones consistent through component categories of solid wastes.

Particle size: The size of the component materials in solid wastes is of significance in the recovery of materials particularly through mechanical means such as trommel screens and magnetic separators. The element size distribution of solid wastes is these that those from 0.05 - 0.25m (mesh size) are often much more than those of 0.25 - 0.5m and above.

Moisture content: The moisture content of solid wastes is generally stated as the mass of moisture per unit mass of wet or dry substance. That is,

Moisture content = (a* b)/a x100    

Where a = initial mass of sample as delivered

 and b = mass of sample after drying.

To attain the dry mass, the solid-waste substance is dried in an oven at 77⁰C for 24 hours. This temperature and time is utilized to dehydrate the substance totally and to limit the vaporization of volatile substances.

Chemical Composition of Solid Wastes

Information on the chemical composition of solid wastes is significant in evaluating alternative processing and energy recovery options. If solid wastes are to be utilized as fuel, the 4 most significant properties to be identified are:

a. Proximate analysis

- Moisture (loss at 105⁰c for 1 hour)

- Volatile matter (additional loss on ignition at 950⁰c)

- Ash (residue after burring)

- Fixed carbon (remainder)

b. Fusing point of ash

c. Ultimate analysis = percent of C, H, O, N, S and ash.

d. Heating value (energy value)

Changes in Solid Waste Composition

The composition of solid wastes in Nigerian urban centres is computed via a number of factors: type of settlers (low income earners, medium income earners or high income earners), part of the city; period of the year and technological advancement. For instance, the percentage of leaves utilized for wrapping foodstuff has been reducing gradually since mid 1980s while that of employed polythene bags is rising. Similarly, the volume of papers and cans generated as part of solid wastes in an elite settlement is probable going to be greater than that produced in a village as a whole.

The replacement of leaves and papers for 'packaging' food products through polythene bags or   plastics has its attendant ecological difficulties. While leaves and papers decompose totally inside two or three months of contact by soil, polythene bags take infinitely long period of time to decay. Decline dumps where these non-biodegradable matters are abundant accumulate into long-lasting huge heaps in most Nigerian cities. Separately from constituting eye sores, they block streets, disrupt smooth flow of rivers and cause floods, create serious stench and constitute tremendous health hazards. The non-free flowing or sticky nature of solid wastes in a reject dump provides increase to their amassing on several habitable parts of the soil surface therefore impairing the productive capacity of soils. Groundwater are frequently contaminated (through seepage), through leachate arising from solid wastes dumped on the ground.

Solid wastes management

Solid waste management means the collection, transportation, storage, disposal, treatment, and recycling of waste as well as the care of the disposal site.

Collection, Transportation and Storage of Solid Wastes

In most houses and market stalls in Nigeria, metal or plastic dustbins are utilized to gather and temporarily store solid wastes. If the houses are simply reachable to vehicles, door-to-door collection would be done through merely vacating of the dustbins into the vehicles. But in cases where the accessibility is bounded, tricycles through carriers are utilized for the similar purpose. Otherwise, household solid wastes are deposited into solid wastes depots situated inside the community in a place accessible to vehicles. From the depots, sanitary inspectors transport the solid waste in their vehicles to sites where the wastes are further treated. Refuse collection and transportation take up to 75percent of the total expenses for waste disposal in countries as Nigeria.

Waste Disposal and Treatment

Solid waste disposal terms to the last placement or discharge or deposit of waste into a given environment in such a way that it causes little or no harm to the environment. In Nigeria, waste disposal techniques have been unsuccessful and unsatisfactory. Wastes are often dumped indiscriminately along the streets and open spaces; passengers in transit throw wastes at will on the roads and open gutters are carelessly filled up with wastes. Where a depot is used, sanitary inspectors often delay waste collection until when the depots have been over filled.  

The mostly used disposal methods are open dumping, sanitary land filling, compositing, incineration and recycling.

-Open dumping:

Open dumping is commonly practiced in many cities in the third world. The refuse is simply disposed of into the nearest open space on land or surface water without any environmental consideration. This method isn't environmentally sound or safe because of so many hazards associated with it.

Open dumping encourages the spread of diseases by harboring the flies and vermin acting as agents of transmission. Some of the communicable diseases connected through dump sites are fly-borne, rodent-borne and mosquito-borne. At times, open dumping is related by burning of waste, which can cause fire accident and air pollution. Open dumping can reason contamination of surface and ground waters.

-Sanitary Landfill:

Landfill is one of the most environmentally sound techniques of solid waste disposal accepted via many countries of the expanded world. Sanitary landfill operations engage depositing solid wastes in natural or man-made depressions or trenches, compacting them into smallest practical volume and covering them through compacted earth or other substance. The microbial degradation reaction that happens in the landfill generates methane gas that can be gathered and utilized as a source of energy.

Modern landfill sites require be locating in impermeable starter or sealing through an appropriate membrane before the wastes are deposited and should have a system for managing the leachate to stop groundwater surface water pollution.

-Composting:

Composting engages shredding and separating the putrescible fraction of municipal waste, mixing it through other organic substances and permitting microbial decomposition to occur. Humus eroded from topical soils can be swapped via compost that has good moisture retaining capacity.

It isn't as simply washed away as chemical fertilizer. Therefore, compost assists to develop soil structure and control soil erosion since it is rich in phosphorus, potassium, nitrogen, carbon and Sulphure. Nevertheless, compost might enclose high levels of hazardous (heavy) metals that may source soil surface or ground water pollution.

-Incineration:

Incineration entails passing the waste through a chamber at a temperature of just about 1200⁰C in an adequate supply of air or oxygen. In the procedure, all the organic materials (carbohydrates, lipids and proteins as well as plastics, fibres and woods) will be oxidised to volatile compounds such as CO₂, H₂O, SO₂, NO₂ etc. The residue after the incineration has a volume of about one-tenth of the original solid waste. Therefore, incineration is only a system of waste reduction because the residual ash requires to be disposed of watchfully afterward in a landfill.

Advantages of incineration are:

1. The heat produced can be harnessed for electricity creation and hearting of the neighborhoods.

2. Incineration decreases the need for extra landfill sites.

3. It can dispose of up to 99.999percent organic wastes. As well as chlorinated organic xemobiotics of properly carried out.

4. Pathogens in the waste are efficiently removed.

5. The ash residue might be recycled and utilized in the manufacture of cement, as aggregate in concrete, structural components in road beds and in asphalt road surfacing mixes.

Disadvantages of incineration include:

1. Incineration is comparatively expensive.

2. There is a hazard of extremely toxic pollutants these as polychlorodibenzo-dioxins (PCDDS) and polychlorodibenzonfurans (PCDFs) being synthesized and emitted into the atmospheres.

3. Toxic heavy metals in fly ash from incinerators might constitute direct and detrimental results in the instant environment as they stress up other environmental matrices these as soil, water and the biota.

4. Incineration metals and metalloids these as Se, Hg, As, Sb and Pb might be transported in the air or enriched on gaseous particulates and travel long distances before they are placed on and or surface of crops where they might be absorbed by the roots to the foliage into the plant. For instance, it has been shown that about 90percent of the total plant uptake of Pb is due to deposition from the atmosphere rather than transport from the soil. This implies that atmospheric deposition of metals could pose a significant source of heavy metal inputs to the food chain where background soil levels are even comparatively low.

-Recycling: 

Recycling encompasses the full range of waste resources recovery and reuse techniques. This doesn't only build the waste substance harmless, but transfers it to another or original helpful material. The practice of recycling solid waste is an ancient one through modern improvements.

Metal implements were melted down and recast in prehistoric civilizations. In the year 1972, in Britain, metal scrap was worth 1,250 million pounds. In that year, 60per cent of Pb, 36per cent of Cu and 32per cent of paper utilized in Britain came from scrap. In Nigeria too, lately more and more people are getting engaged in scavenging business of metals (steel and aluminum particularly), plastics, bottles and so on and so forth. In Cuba, a technology has been expanded for collecting, selecting and preparing urban wastes to feed pigs and poultry. The arranged wastes create between 18 and 22per cent protein. From recycling point of analysis, solid wastes are expensive resources in the wrong places.

Waste products, when efficiently managed, can produce biogas (for instance methane),   fertilizer, textile substances, paper, crude oil (via thermal depolymerization) and metals (for instance aluminum).

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