2. The main objectives of wastewater treatment are
• (i) reducing water pollution and
• (ii) making use of the waste resulting from treatment processes
to ensure a more sustainable use of water resources.
3. 1 . Wastewater treatment plant
• A wastewater treatment plant (WWTP), receives and treats municipal
and industrial wastewater. That is, a WWTP treats water to remove
waste, grease and floating oils, sand and any coarse debris present in
the water; the treatments remove organic and inorganic materials
through settling processes, as well as biodegradable organic material
dissolved in the water.
4. 2 . How does a wastewater treatment plant work?
• Wastewater treatment plants use four stages of treatment:
• Preliminary treatment: it involves the separation of large solids (bottles, fabric,
plastics) found in the water using bar racks and screens.
• Primary treatment: physico-chemical treatments to sediment and precipitate
suspended solids and reduce the biochemical oxygen demand of the organic solids. In
addition, the treatment neutralises the water, eliminates volatile contaminants,
removes greases and oils, etc.
• Secondary treatment: biological treatments that reduce the amount of organic
matter in wastewater. They include aerobic processes which degrade organic material
in the presence of oxygen, as well as anaerobic processes which oxidise organic
matter without oxygen, followed by secondary settling.
• Tertiary treatment: advanced physical, chemical and biological processes, which
eliminate heavy metals, nitrogen, phosphorous, and pathogens. In some WWTPs
water undergoes further treatment to allow its reuse for purposes such as irrigation of
parks and green areas, street washing, or industrial uses.
5. 3 . What is sludge and how is
it treated?
• Finally, as a by-product of their treatment
processes, wastewater treatment plants produce sewage sludge.
Sewage sludge is a mud-like residue that may contain heavy
metals and pathogens such as viruses and bacteria. It also
contains valuable organic matter and nutrients such as nitrogen
and phosphorus, and can therefore be very useful as a fertilizer
or soil improver.
• The sludge is concentrated through dehydration and some of its
components are recovered (composting, gasification,
biomethanation). Four processes are involved: thickening,
digestion, dehydration, and tertiary treatment once again.
6. There are three main methods of sludge
disposal:
• Farmland: Sludge disposal on agricultural land is useful as a
fertilizer and soil conditioner.
• Landfill: Not all sludge is suitable for farmland. For example,
sludge containing toxic waste cannot be spread on fields.
Instead it is dumped in natural or artificial depressions in the
ground or in trenches, where it dries and decomposes slowly,
and covered with a layer of soil.
• Incineration: Sludge may be dried and incinerated, leaving an
inert ash. This is often the most expensive option.
•
7. Flow diagram of the stages of treatment in a sewage treatment plant (Source: The Open University)
https://smartwatermagazine.com/q-a/what-a-wwtp
8. How waste-to-energy plants work
• Waste-to-energy plants burn municipal solid waste (MSW), often called garbage
or trash, to produce steam in a boiler, and the steam is used to power an electric
generator turbine.
• MSW is a mixture of energy-rich materials such as paper, plastics, yard waste,
and products made from wood. For every 100 pounds of MSW in the United
States, about 85 pounds can be burned as fuel to generate electricity. Waste-to-
energy plants reduce 2,000 pounds of garbage to ash that weighs between 300
pounds and 600 pounds, and they reduce the volume of waste by about 87%.
• The most common waste-to-energy system in the United States is the mass-burn
system. In this system, unprocessed MSW is burned in a large incinerator with a
boiler and a generator to produce electricity (see illustration below). A less
common type of system processes MSW to remove uncombustible materials to
produce refuse-derived fuel (RDF).
•
9. • Refuse-derived fuel (RDF): A fuel produced by shredding
municipal solid waste (MSW). Noncombustible materials such
as glass and metals are generally removed prior to making
RDF. The residual material is sold as-is or compressed into
pellets, bricks, or logs. RDF processing facilities are typically
located near a source of MSW, while the RDF combustion
facility can be located elsewhere.
11. The process of generating electricity in a mass-
burn waste-to-energy plant has seven stages:
• Waste is dumped from garbage trucks into a large pit.
• A giant claw on a crane grabs waste and dumps it in a combustion
chamber.
• The waste (fuel) is burned, releasing heat.
• The heat turns water into steam in a boiler.
• The high-pressure steam turns the blades of a turbine generator to
produce electricity.
• An air-pollution control system removes pollutants from the combustion
gas before it is released through a smoke stack.
• Ash is collected from the boiler and the air pollution control system.
https://www.eia.gov/energyexplained/biomass/waste-to-
energy-in-depth.php
