Refuse and waste disposal

Bacteria, fungi and other microbes convert organic waste into biogas, either aerobically or anaerobically. The mechanism deserves a name. In the absence of oxygen, four groups of bacteria work in sequence: hydrolytic bacteria first break polymers such as cellulose, starch and protein into sugars and amino acids; acidogenic bacteria then ferment those into volatile fatty acids, alcohols and hydrogen; acetogenic bacteria convert those products into acetic acid, hydrogen and carbon dioxide; and finally methanogenic archaea — strict anaerobes that are poisoned by oxygen — convert acetic acid and hydrogen into methane. The product is a gas mixture of roughly 50–70 per cent methane and 30–50 per cent carbon dioxide, which is why it burns and why it must be captured rather than vented. The same biology runs in an unmanaged landfill, where the waste is simply buried without a digester: methane and carbon dioxide are generated underground over years, and methane has a global warming potential many times that of carbon dioxide. That is the real reason a closed landfill cannot be built on. It continues to settle as the mass compacts, the trapped gas can migrate into nearby buildings, and rainwater percolating through the waste dissolves organic acids and heavy metals to form leachate, a dark liquid that can contaminate groundwater if the base is not sealed. A controlled landfill therefore needs a clay or synthetic liner, a leachate collection system and gas wells, and the site must be monitored long after the last truck leaves.

Incineration works because high temperature drives the oxidation of organic matter into carbon dioxide, water vapour and ash. The headline figure — a volume reduction of about 90 per cent — follows from the physics: what is removed leaves as gas, so only the incombustible fraction remains as bottom ash. Roughly a quarter to a third of the ash mass is fly ash, the fine particulate fraction carried with the exhaust gases; it concentrates heavy metals and other toxic compounds and is classified as hazardous waste, so it cannot simply be tipped with the rest. This is exactly why exhaust gases may be toxic and must be treated before release. A modern plant passes the gas through cyclones and electrostatic precipitators to remove particulates, then through scrubbers that neutralise acid gases such as hydrogen chloride and sulphur dioxide, and often through activated carbon or a catalytic stage to capture dioxins and heavy metals. Waste heat is recovered to raise steam that drives a turbine, which is the electricity generation described here. What combustion cannot do is deal with carbon. Every organic molecule in the waste becomes carbon dioxide, and incomplete combustion adds carbon monoxide, which is why carbon dioxide and carbon monoxide are the environmental objection. Compaction has a smaller but related limitation. Pressing cans and bottles into blocks prevents the oxidation of metals, because a compacted bale has far less exposed surface, and it reduces the airspace needed and eases transport and positioning. But compaction changes only the density of the waste, never its identity. A bale of plastic bottles is still plastic; without a recycler who will melt it, the bale is merely a slow-release landfill. Compaction therefore belongs upstream of a recycling industry, and it fails as a disposal strategy whenever that industry is missing, which is common in low-income settings.

Composting and vermicomposting are the only methods in this section that pay the soil back. The agricultural claim — compost replenishes nutrients, increases water retention and is the best alternative to chemical fertilizers — has its reason in the chemistry of humus. Humus is the stable, dark organic fraction that remains after microbial decay; its negatively charged surfaces hold positively charged nutrient ions such as ammonium, potassium, calcium and magnesium, so nutrients are retained in the root zone instead of being leached away by rain. Its porous structure holds water in the spaces between particles, which raises the water-holding capacity of sandy soils in particular and improves the crumb structure of heavy clay. Vermicomposting goes further because the earthworm gut is itself a bioreactor: worms fragment the waste, increase its surface area for microbial attack, and their casts are richer in available nitrogen, phosphorus and potassium and carry a denser, more diverse microbial community than the material they ate. The practical rules follow from the biology: keep the heap moist but not waterlogged, turn it to supply oxygen so the process stays aerobic, and keep the carbon-to-nitrogen balance reasonable by mixing dry leaves or straw with wet kitchen scraps. If a heap becomes anaerobic it turns sour, releases methane and ammonia, and loses the nitrogen the farmer wanted to keep. For the nurse, the same comparison becomes a bedside duty. Household refuse streams are one question; health-facility waste is another, and the principles transfer directly. General waste that carries no infectious risk goes into the ordinary municipal stream. Infectious and pathological waste must be destroyed or contained by a method that kills pathogens — incineration, autoclaving or another validated treatment — and never placed in the general refuse, because once it is in a shared bin the hazard is out of the nurse’s control. Sharps go into a puncture-resistant, closable container at the point of use, so that the person who handles the waste later cannot be injured. This segregation step is the health-care equivalent of the sorting step that makes compaction and recycling possible, and it is the nurse’s responsibility at the moment the waste is created. Beyond the facility, the nurse is often the person who explains to a family why burning mixed plastic in the yard is not the same as incineration in a controlled plant: an open fire runs cool and oxygen-poor, so it produces carbon monoxide, fine particulates and toxic compounds without the exhaust treatment, and it leaves the household breathing the result.