Urban Waste Management: general topics

Waste as a resource

In the last few centuries, in order to satisfy their own needs, some of which were real and others fictitious, humans have taken, used and eventually destroyed natural resources in an uncontrolled fashion only returning them to the planet as a waste product.

Only over the last few decades have we realised that waste is the cause of the following kinds of problems:

  • Environmental: to this day most waste products simply end up in a landfill. The demands and new needs that modern lifestyle exert on our well-being have exponentially increased the production of waste products, to the extent that without intervening, our planet is destined to become one massive dump. This has severe environmental repercussions and removes land, not only from the ecosystem, but also from other productive alternatives.
  • Economic: natural resources are not infinite and if we keep on using and destroying them they will end up running out. Furthermore disposal and transport costs increase due to the large amount of packing (plastic containers et al) destined for this purpose: 40% in weight and 50% of the total volume of waste products.
  • Social: the issue of waste products concerns all of society; those involved in producing, distributing, using and consuming goods from which waste products originate.

Nevertheless, waste products are also a great resource and should be treated as such. Some of the raw materials used to produce the goods from which the waste products we now want to rid ourselves of, can be salvaged, and the advantages of that are manifold:

  • Environmental: reducing the volume of waste products that are tossed out involves prolonging the average life of existing landfills, thus reducing the need for new facilities. Reducing the consumption of raw material is vital for the ecological balance of the planet.
  • Economic: using resources that would otherwise be destroyed, preserving natural resources and using waste materials for energy purposes. Savings obtained by reducing quantities to be burned off and recovering raw materials.
  • Social: new work opportunities. Collecting, selecting, recycling waste products fall under the umbrella of new business projects with a high technological focus that represent one of the new frontiers for system productions.

Only by separating waste products can riches be recovered

Mixed waste products are, naturally, more difficult to treat than those that are sorted. Three main paths can be followed:

  • Mechanical biological treatments, that is separating and partially recovering materials, bio stabilisation and taking them to a landfill;
  • Thermal treatments, that is incinerating as is or downstream separation and producing RDF and taking it to a landfill;
  • Directly taking it to a landfill (much used nowadays but, surely, to be avoided).

Yet, it is clear that the inevitable by-products from the aforementioned processes will of course be waste products. The purpose of mechanical biological treatment of mixed waste products or residues (i.e. waste that remains after sorting) is to recover even more recyclable materials, reduce the volume of material that will eventually be burned off and to stabilise the waste products in such a way that gases formed from decomposition and leachate are minimised. Both recyclable materials and biogas, that is to say methane, can generally be recovered from such processes.

The main kind of cold treatment is mechanical biological treatment (MBT). This separates the organic components from recyclable materials, thus further reducing the use of landfills and kilns. All of this is achieved while considerably lower polluting emissions than in power plants. In fact, it treats unsorted waste downstream from the mixed collection, increasing the materials that are recovered. MBT can also be used to produce RDF (Refuse Derived Fuel). In that case only moisture and non-combustible materials (glass, metals) should be removed while paper and plastic are packaged in "eco-bales" to be incinerated; thus mechanical biological treatment can be espoused with thermal one.

Difficulty using sorted collection

Burning refuse is not the counterpoint or the alternative to the practice of sorted collection for the purposes of recycling per se, but rather forms one of the links in the chain of waste disposal. For technical and economic reasons, nowadays the tendency is to create ever bigger kilns at the expense of sorted collection that struggles to find widespread and extensive use.

Limitations to grill thermovalidators

These incinerators possess a huge furnace, with metal grills usually with little steps made up of parallel drums or bars. The grill can be mobile or fixed and in different areas different temperatures are reached that allow gradual heating. The ash produced is gathered and cooled down in vats of water. Currently, plants that have mobile grills are the most used for urban waste since, thanks to the movement of waste inside the combustion chamber; a better combustion accrual can be achieved. Part of the air needed for primary combustion is supplied by the lower part of the grill and this flow is also used to cool down the grill itself. Cooling down is important for maintaining the mechanical features of the grill, and most mobile grills also exploit the cooling down mechanism using an internal flow of water.

It is nevertheless worth noting that a series of technical challenges are associated with grills, amongst which dust deposits should be highlighted, along with needing a certain amount of scheduled periodic maintenance. Moreover incinerators with a fixed grill, indiscriminately burning the entire black bag, are characterised by reduced efficiency. In fact, since part of the heat is used in the endothermic reaction of metal fusion, the useful heat obtained is usually of lower value.

Increases in landfills

The main challenge with landfills is the leachate and gas emissions, that are often unpleasant-smelling, that are produced due to the decomposition of the organic fraction. Both problems can be overcome by removing the organic fraction through sorted collection or pre-treating the refuse with mechanical biological treatment beforehand, which would, besides, also reduce the volumes to be burned off. The landfill can be used to dispose of all residues from the integrated system of waste management with a minim impact on the environment.

Danger of Atmospheric Pollution

Gases emitted by wind fields spread pollutants into the atmosphere, transporting VOCs (volatile organic compounds), CH4 (methane), SO and SO2 (sulphur oxides), NO and NO2 (nitrogen oxides,) HC (hydrocarbonates).

Danger of Water Pollution: risk factors
The natural porosity and permeability of the land is the very reason why it is vulnerable. Leachate is a liquid that mostly originates from infiltrating water from the mass of refuse or decomposing waste and can contain various dangerous pollutants, amongst which heavy metals may be present. A tiny drop is all that is needed to contaminate the surroundings. Leachate has to, by law, be captured and appropriately treated in the landfill or transported to ad hoc plants or to urban waste water treatment plants. The system of capture consists of a series of fissured tubes immersed into a layer of drained gravel just above the impermeable layer. Tailor-made plants only serve to reduce the risk and kind of vulnerability of the site.

Danger of Land Pollution:
The presence of leachate can also pollute land, besides ground water. Loading, transporting and disposing of waste all run a real danger of contaminating the soil. These potential infiltrations can spread widely due to migration factors beneath the surface level. Furthermore, normal ventilation is enough to transport odours and potentially damaging substances quite some distance from the landfill site. The growing demands on waste storage have involved using ever-larger areas, removing resources from industrial, agricultural and building industries. Moreover, and more often than not, locations that were initially identified as landfill sites since they were outside urban areas have since been swallowed up by the outskirts of cities, thereby posing considerable problems.