Disposal Operations of Household Waste Generated 

        Historically, household waste has been disposed of by land filling. As communities became larger and more premises were built, usually at a higher density, particularly in urban areas, the area needed for the disposal of waste increased. Also, as society has developed, there have been significant changes in the composition of wastes collected from households, particularly with a change in the fuel used for heating purposes. This led to designated areas of land being set aside which became the local waste disposal site. In addition to decomposition, predators and fires on such sites reduced the volume of waste considerably. Nowadays, because of ever increasing volume of waste requiring disposal and an increasing need to protect the environment, sophisticated means of collection, transport, treatment and disposal need to be used. At the landfill site, the waste is deposited in layers in prepared cells and compacted to decrease its volume. It is then covered, at least daily, with a suitable soil like material to deter vermin, flies, birds and other scavengers but also to prevent injuries from sharps.

        Some biodegradations of the putrescible fraction in the household waste will have commenced before it was collected and will continue during its transportation. Its further processing by, for example, wet pulverization also will promote enhanced degradation. Some countries prohibit the addition of liquids to landfills for the purpose of accelerating degradation, being more concerned with the increased production of leachate resulting from such practices. Once in a landfill site the rate of degradation will increase rapidly, particularly in the presence of moisture. However, if the density of the waste is increased significantly to assist its handling and transportation, the ease with which moisture can gain access to the waste mass is decreased, which can result in a delay in the onset of degradation. Initially, the degradation is aerobic producing hydrogen and carbon dioxide as the principal by-products. As the oxygen in the mass of waste is used up, anaerobic conditions become established and the principal by-products are methane and carbon dioxide. Since methane is a highly flammable gas and in confined spaces can be explosive, special measures are needed to vent it from the landfill. At sites where the quantity of landfill gas produced is significant, harnessing it for use as a fuel is practised. It is possible to obtain usable gas quantities for several tens of years.

        At the same time as landfill gas is produced, other organic compounds are formed. Many of these are soluble in water and become dissolved in any surplus moisture in the landfill site to produce a liquid mixture termed leachate. Leachate can be highly polluting. Some countries strike a balance between high volumes of gas production and low pollution potential of leachate and the reverse to control the pollution by leachate. In any case it is necessary to prevent leachate migration away from a landfill site since it can continue to produce landfill gas away from a landfill site. Also, it is necessary to prevent it from contracting and mixing with ground and surface water.

        To ensure that waste deposited in a landfill site is more rapidly degraded it can be pulverized before landfilling. The process is usually carried out under wet conditions to reduce dust and, since the waste needs to be wet to promote maximum production of landfill gas, biodegradation occurs quickly after the waste has been landfilled.

        In parallel with the land filling of household waste, since many of its constituents are combustible, incineration is another option. Its attraction lies in the fact that large land areas are not removed from use for other purposes for an indefinite period of time, and surplus heat can be produced. Because household waste contains a large variety of materials, including those which are not combustible, plant used to incinerate such waste needs to be rugged and versatile to cope with a highly variable feedstock both in terms of waste composition and calorific value.

        Because the waste is not easy to feed to and through an incinerator it is usual practice to use furnaces based on either the chain or rocking grate principle or to a lesser extent a rotary kiln. To ensure high combustion efficiency the temperature range at which the furnace is operated and burns waste and the time during which the waste reaches and is maintained at furnace temperature and turbulence within the furnace chamber, all need to be strictly controlled, the so-called “3Ts Principle” – Temperature, Time and Turbulence exemplifies this requirement for good combustion.

        Waste delivered to an incinerator by a collection vehicle usually discharges its load into a large hopper from where the waste can then be removed by grab crane or bucket conveyer and fed to the incinerator furnace at a controlled rate. Ideally, the furnace should be operated on a continuous basis, thus ensuring that waste is not left in the hopper for an extended period of time. As indicated above, decomposition of the waste can take place in the hopper, which rapidly produced hydrogen, methane and carbon dioxide to give a gas concentration, which is hazardous. Also, it provides a suitable breeding ground for vermin and particularly flies, the eggs of which will in all probability have been laid in the waste before it was collected from a household.

        To meet increasingly more stringent limits on the concentration of gaseous and particulate emissions released to atmosphere from an incinerator, it is necessary to clean the off-gases before they are released into the atmosphere. At one time electrostatic precipitators were considered to provide sufficient removal of particulate matter in the gas stream.

        However, to deal with acidic constituents it is necessary to now use equipment that controls acid gas, such as dry lime injection prior to passing the gases through an electrostatic precipitator or wet (chemical) scrubbing. In addition to such control equipment, the height of the chimney from which the gases are released may need to be increased to aid their dispersion and ensure that ground level concentrations of constituents in the gases are environmentally acceptable

        An incinerator which is operated efficiently should produce a furnace ash (bottom ash), which contains only inorganic materials. However, in practice, it can be expected that also some organic carbonaceous material will be present at trace concentrations. Normally, the ash is landfilled at a site from which releases of leachate to ground and surface water are prevented. This is required because any water-soluble materials in the ash can be dissolved in leachate and could result in concentrations of pollutants in ground and surface water.

        In addition to solid wastes, household liquid waste is an environmental problem. Liquid waste disposed to sewer drains into surface water courses. This causes pollution of the aquatic environment with resulting health hazards. Therefore municipal wastewater must be collected and properly treated before discharging to surface water courses.