28-12-2012, 05:14 PM
Solid waste management
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Introduction to solid waste management
Solid waste is the unwanted or useless solid materials generated from combined
residential, industrial and commercial activities in a given area. It may be categorised
according to its origin (domestic, industrial, commercial, construction or institutional);
according to its contents (organic material, glass, metal, plastic paper etc); or according to
hazard potential (toxic, non-toxin, flammable, radioactive, infectious etc).
Management of solid waste reduces or eliminates adverse impacts on the environment
and human health and supports economic development and improved quality of life. A
number of processes are involved in effectively managing waste for a municipality.
These include monitoring, collection, transport, processing, recycling and disposal.
Reduce, Reuse, Recycle
Methods of waste reduction, waste reuse and recycling are the preferred options when
managing waste. There are many environmental benefits that can be derived from the
use of these methods. They reduce or prevent green house gas emissions, reduce the
release of pollutants, conserve resources, save energy and reduce the demand for waste
treatment technology and landfill space. Therefore it is advisable that these methods be
adopted and incorporated as part of the waste management plan.
Waste reduction and reuse
Waste reduction and reuse of products are both methods of waste prevention. They
eliminate the production of waste at the source of usual generation and reduce the
demands for large scale treatment and disposal facilities. Methods of waste reduction
include manufacturing products with less packaging, encouraging customers to bring
their own reusable bags for packaging, encouraging the public to choose reusable
products such as cloth napkins and reusable plastic and glass containers, backyard
composting and sharing and donating any unwanted items rather than discarding them.
All of the methods of waste prevention mentioned require public participation. In order to
get the public onboard, training and educational programmes need to be undertaken to
educate the public about their role in the process. Also the government may need to
regulate the types and amount of packaging used by manufacturers and make the reuse of
shopping bags mandatory.
Recycling
Recycling refers to the removal of items from the waste stream to be used as raw
materials in the manufacture of new products. Thus from this definition recycling occurs
in three phases: first the waste is sorted and recyclables collected, the recyclables are
used to create raw materials. These raw materials are then used in the production of new
products.
The sorting of recyclables may be done at
the source (i.e. within the household or
office) for selective collection by the
municipality or to be dropped off by the
waste producer at a recycling centres. The
pre-sorting at the source requires public
participation which may not be
forthcoming if there are no benefits to be
derived. Also a system of selective
collection by the government can be
costly. It would require more frequent
circulation of trucks within a
neighbourhood or the importation of more
vehicles to facilitate the collection.
Another option is to mix the recyclables with the general waste stream for collection and
then sorting and recovery of the recyclable materials can be performed by the
municipality at a suitable site. The sorting by the municipality has the advantage of
eliminating the dependence on the public and ensuring that the recycling does occur. The
disadvantage however, is that the value of the recyclable materials is reduced since being
mixed in and compacted with other garbage can have adverse effects on the quality of the
recyclable material.
Waste Collection
Waste from our homes is generally collected by our local authorities through regular
waste collection, or by special collections for recycling. Within hot climates such as that
of the Caribbean the waste should be collected at least twice a week to control fly
breeding, and the harbouring of other pests in the community. Other factors to consider
when deciding on frequency of collection are the odours caused by decomposition and
the accumulated quantities. Descriptions of the main types of collection systems are
given in the table below.
Treatment & Disposal
Waste treatment techniques seek to transform the waste into a form that is more
manageable, reduce the volume or reduce the toxicity of the waste thus making the waste
easier to dispose of. Treatment methods are selected based on the composition, quantity,
and form of the waste material. Some waste treatment methods being used today include
subjecting the waste to extremely high temperatures, dumping on land or land filling and
use of biological processes to treat the waste. It should be noted that treatment and
disposal options are chosen as a last resort to the previously mentioned management
strategies reducing, reusing and recycling of waste (figure 1).
Biological waste treatment
Composting
Composting is the controlled aerobic decomposition of organic matter by the action of
micro organisms and small invertebrates. There are a number of composting techniques
being used today. These include: in vessel composting, windrow composting,
vermicomposting and static pile composting. The process is controlled by making the
environmental conditions optimum for the waste decomposers to thrive. The rate of
compost formation is controlled by the composition and constituents of the materials i.e.
their Carbon/Nitrogen (C/N) ratio, the temperature, the moisture content and the amount
of air.
The C/N ratio is very important for the process to be efficient. The micro organisms
require carbon as an energy source and nitrogen for the synthesis of some proteins. If the
correct C/N ration is not achieved, then application of the compost with either a high or
low C/N ratio can have adverse effects on both the soil and the plants. A high C/N ratio
can be corrected by dehydrated mud and a low ratio corrected by adding cellulose.
Moisture content greatly influences the composting process. The microbes need the
moisture to perform their metabolic functions. If the waste becomes too dry the
composting is not favoured. If however there is too much moisture then it is possible that
it may displace the air in the compost heap depriving the organisms of oxygen and
drowning them.
A high temperature is desirable for the elimination of pathogenic organisms. However, if
temperatures are too high, above 75oC then the organisms necessary to complete the
composting process are destroyed. Optimum temperatures for the process are in the range
of 50-60oC with the ideal being 60oC.