E6: Water Treatment (UNMODIFIED)

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Anthropogenic Sources:

Human waste

Fertilizers

Industrial Waste (70% = Untreated)

Interesting Facts:

If we continue this trend, by 2020, there will be 30 million environmental refugees in China

Primary Pollutants: Main Categories

Heavy Metals

Dioxins

Pesticides

PCBs

Nitrates

Organic Matter

Heav

Heavy Metals:

Cd2+

Zinc/ Mine

Ni-Cad Batteries

Yellow pigments

Pb2+

Lead pipes

Older paints

Automobile batteries

Leaded gasoline

Hg

Fungicides

Mercury Thermometers

Button batteries

Dioxins

What is it?

A product of incineration of wastes containing organochloro compounds

Pesticides:

DDT:

What is it?

A potent insecticide (banned, but is still in use by many countries)

Paraquat

What is it?

Herbicide used to control weed growth in crops

PCBs (Polychlorinated Biphenyls)

What is it?

Used as insulators in electrical transformers and capacitors (due to high electric resistance)

NO3-

Causes acid rain

Used as artificial fertilizers

Organic

Organic Matter

Raw sewage

Decomposition products

Agricultural runoffs

Why do that?

Sources of clean freshwater = rapidly depleted due to pollution and overuse

Arid regions lack access to freshwater (therefore resort to using seawater)

Why can't we use saltwater?

Seawater contains approximately 3.5% dissolved salt

Unable to drink and use for both agricultural and industrial processes (dehydration of cells and corrosion of equipment)

Meth

Method 1: Multistage Distillation

Process:

Incoming cold seawater is heated by steam under high pressure to >100˚C

Water evaporates under lower pressure and water vapor is cooled by incoming cold seawater to condense it --> heat released can preheat the seawater

Hot saltier water passed on to the next chamber

An even lower pressure in the next chamber allows water to evaporate at a lower temperature; followed by condensation

Process repeats

Process repeated again to the third chamber that has a further reduced pressure

Distillate collected and used as drinking

Pros:

Energy needs reduced when heat released during condensation heats incoming seawater

Desalination plants built near thermal power generating plants can use waste heat to heat system

Cons:

High energy requirements (use of fossil fuels for heating seawater generates GHGs)

Possibility of thermal pollution due to heated water

Corrosion in pipes due to hot salt water may introduce copper contaminants into the water outflow

Metho

Method 2: Reverse Osmosis

Background Info:

Osmosis refers to the flow of water from one solution to another across a semipermeable membrane

When 2 solutions are separated by a semipermeable membrane, water will naturally move from a solution with low solute concentration to a solution with a high solute concentration

Reverse Osmosis:

Using the osmosis theory, but have to reverse it to get desirable effects (i.e. move from a solution with a high solute concentration to a solution with low solute concentration)

Must use pressure to disrupt the natural flow of osmosis

Semipermeable membrane has microscopic pores that are only big enough for water molecules to pass through

To maximize the desalination process, the semipermeable membrane is coiled inside a cylindrical pressurized vessel

Pros:

Less energy is required to generate pressure than to heat water

Little thermal heating reduces production of GHGs

Cons:

Membranes must be cleaned to reduce build up of salts, and cleaning chemicals may enter the environment

Bacteria can breed on membranes

Expensive process

Aims of Wastewater Treatment:

Reduce hazardous materials

Reduce BOD

Kill microorganisms

3 Main Stages:

Primary Sewage Treatment: Removal of Solid materials and initial reduction of BOD (due to organic matter); Physical Treatment

Secondary Removal Sewage Treatment: Removal of oxygen demanding wastes; Biological methods

Tertiary Sewage Treatment: Removal of heavy metals, nitrates and organic compounds

Primary Water Treatment:

GOAL: To remove solid materials and reduce some of BOD

Process:

Filtration Screen (removal of large solids e.g. sticks, paper, plastics, etc.)

Grit Chamber (small rocks, stones and sand settle to the bottom)

Sedimentation chamber (Suspended solids settle out, flocculation* process increases settling out, sludge forms)

END DESTINATION OF WASTE: Landfill Sites

END RESULT OF PRIMARY TREATMENT:

Undrinkable

Removal of 30-40% of the BOD

*Flocculation:

Gelatinous precipitate attracts small suspended particles, forming large clubs ("flocs") that settle more easily

Secondary Water Treatment:

GOAL: To further reduce BOD via bacterial activity and aeration (bacteria effectively decompose organic materials in the presence of high oxygen levels)

Method: Activate Sludge Process

Waste water from primary treatment enters

Aeration tank contains aerobic bacteria + oxygen (therefore begin the aerobic decomposition of organic matter process)

Settling tank (with activated sludge containing bacteria returned to aeration bank)

Clean effluent to tertiary treatment

END RESULT:

Removal of >90% of BOD

Tertiary Water Treatment

GOAL: Removal of phosphates, heavy metals, nitrates and organic compounds

Method 1: Precipitation Reactions to remove phosphates and heavy metals

Addition of aluminum or calcium ions to remove phosphates [

Addition of sulfide ions to remove many heavy metals [

Method 2: Activated Carbon beds to Remove Organic Compounds

Organic compounds adhere to granules of carbon as water flows over the bed

Method 3: Ion Exchange to Remove Nitrates (since nitrates cannot be precipitated out of solutions)

Use anion exchange resin (resin exchanges with hydroxide ions since NO3 = easier to react with)

See illustration

Method 4: Removal of Nitrates biologically

Denitrifying bacteria in an anaerobic environment to convert NO3- into N2 and H2O

Primary Water Pollutants

Freshwater from Seawater

Waste Water Treatment