Download (E6) Water Treatment

(E6)
Water Treatment
Sarah Black
Background:
• About ¾ of the Earth’s surface is covered by water.
• Water is a polar substance capable of hydrogen
bonding, which allows it to dissolve many chemicals.
– Thus, some toxic substances, bacteria, and viruses can be
carried by water.
• The purpose of sewage treatment is to remove these
hazardous materials, reduce the BOD (Biological
Oxygen Demand) of the sewage and kill microorganisms
prior to discharge.
• Depending on the availability of resources, three
separate levels of sewage treatment may occur.
– These all reduce the level of pollutants and the BOD, but the
tertiary treatment is the most effective (also the most expensive
to build and operate.)
Some Common Pollutants:
• Heavy Metals
– Cadmium  Rechargeable batteries, metal plating, pigments
– Copper  Household plumbing, copper mining, smelting
– Mercury  Batteries, mercury salts as fungicides, mercury cells
in chlor-alkali industry, discharge from pulp and paper mills
• Pesticides (include DDT, fungicides, and herbicides)
– Agricultural practics
• Dioxins
– From by-products of industrial processes such as waste
incineration, forest fires and burning fuels, manufacture of
chlorinated pesticides; used in Agent Orange as a defoliant in
Vietnam war
Some Common Pollutants (cont.):
• Polychlorobiphenyls (PCBs)
– Very stable compounds that were used widely as coolants and
lubricants in capacitors, transformers, and other electrical
equipment
• Organic Matter
– Waster treatment plants, decomposition of plants and animals,
oil spills, industrial waste
• Nitrates
– Agriculture: chemical nitrate fertilizers for farming; from acid rain
• Phosphates
– Detergents and chemical phosphate fertilizers
Primary Sewage Treatment
(Removes 30-40% of the BOD waste)
• 1st  passed through screens and traps which
filter out large objects such as trash and debris
and, from the surface, remove floating objects
including grease.
• 2nd  passed through settling tanks where
smaller heavier objects settle and can be
transferred to land fills.
• 3rd  passed through holding or sedimentation
tanks where it is allowed to settle and sludge is
removed from the bottom.
– The addition of certain chemicals can speed this
process up (called flocculation).
Secondary Sewage Treatment
(Removes about 90% of BOD waste)
• 1st  Air enriched with oxygen is bubbled ,
using large blowers, through sewage
mixed with bacteria-laden sludge (called
an activated sludge process).
– This allows aerobic bacteria to thoroughly mix
with the sewage in order to oxidize and break
down most of the organic matter.
• 2nd  passed through a sedimentation
tank where large quantities of biologically
active sludge collect.
Tertiary Sewage Treatment
• These processes involve specialized
chemical, biological, and/or physical
treatment to further purify the water.
• It can remove remaining organic materials,
nutrients, and substances not already
taken out.
• Examples of tertiary treatment include:
carbon bed, chemical precipitation, and
biological processes.
Carbon Bed Method:
• Uses activated carbon black.
– Which consists of tiny carbon granules with large
surface areas which have been treated and activated
by high temperatures.
– Has the ability to readily adsorb (the attraction of a
substance to the surface of a solid substance) organic
chemicals.
• Effective against many toxic organic materials
and charcoal filters are often used to further
purify tap water for drinking purposes.
Chemical Precipitation
• Basically, the precipitation of toxic heavy
metal ions as their sulfide salts (which
have low solubility in water).
• Carefully controlled amounts of hydrogen
sulfide gas are bubbled through a solution
containing heavy metal ions and the
corresponding sulfides can then be filtered
out.
Anaerobic Biological Process
• This is denitrification (achieved by
denitrifying bacteria), which turns the
nitrogen in nitrates back to atmospheric
nitrogen.
• This is a reduction process in which the
oxidation number of nitrogen is reduced
from +5 in the nitrate ion, NO3-, to 0 in N2.
Distillation
• Further methods of desalination (removal of
salts) to produce fresh water are multi-stage
distillation and reverse osmosis.
– Distillation allows the seperation of a volatile liquid
from non-volatile materials. The water vapor can then
be collected and separated as fresh water.
– Osmosis is the natural tendency of a solvent such as
water to move from a region of high solvent
concentration to one of lower solvent concentration
through a semi-permeable membrane.