Environmental ChemistryE6: Water Treatment

Pathogens- bacteria, protozoan, and some viruses. Source: dumped waste from boats, livestock waste, untreated sewage and septic tanks, and also storm drains. Some pesticides also contain pathogens to kill bugs.Organic Matter- Dirt, leaves, grass Source: Floods into water with rain from eroded land areas Nitrates & Phosphates- found in sewage. Source: direct leakage into bodies of water. They are also ingredients in fertilizers. Source: they flow into bodies of water after affected land is eroded by rain.

E.6.1: List the primary pollutants found in waste water and identify

their sources.

Heavy Metals- Mercury, lead, arsenic, cadmium, and many more.

Source: Run-off from urban areas, industrial waste, some occur naturally in the environment.

Pesticides/Insecticides- Used on crops to prevent destruction or

consumption by insects. Source: Run-off from farming areas.

PCBs- Chemicals used as coolants, plasticizers, in pesticides, etc. Source: Manufacturers’ disposal after use, released during use, unintentional leaks when they are moved, run-off from pesticides, etc.PCDDs- Dioxins and their multiple compounds. Source: Industrial production and accidental spill.

PRE/PRIMARY: As pretreatment, the

water is raked through with a screen

to remove large objects. Wastewater

is then pumped into a reservoir, so that the

sludge and heavy sediment (sludge of food, human waste and detergent) can settle, and the less

dense material (such as oil) can float to the

top. The sludge is removed to be

treated, and the oil is removed off top.

(PHYSICAL)

E.6.2: Outline the primary, secondary and tertiary stages of

waste water treatment, and state the substance that is

removed during each stage.

Secondary: The water is supplemented with oxygen, to increase the bacteria and protozoa populations which then break down dissolved organic material. This is called the “activated sludge process”. The water goes then goes through a secondary sedimentation process in which finer debris settles and is removed, or sand filtration is utilized. The removed sludge is also aerated, and broken down partially by bacteria. (BIOLOGICAL)

Tertiary/“Polishing”: Removal of toxic/heavy metals, nitrates, and phosphates by chemical or biological process (CHEMICAL/DISINFECTION)

Multi-stage Distillation:

E.6.3: Evaluate the process to obtain fresh water from sea water using multi-stage distillation and

reverse osmosis.Multi-stage distillation is the process of flash-steaming seawater in stages to procure drinkable water.The salt water in first heated in a brine chamber, then forced into a secondary chamber where the pressure is much lower.

This causes the water to rapidly boil and turn to steam. Only part of the water is flashed at a time, so the same process repeats itself several times to purify the entirety of the

water.

The steam is captured

above the reservoir in

tube heat exchangers and is now

drinkable water. The

brine is removed, and

any excess steam energy

produced is recycled

within the plant. This saves the

plant considerable

energy costs.

Reverse Osmosis:

It is the process of forcing a solution through a semi-permeable membrane, leaving the solute on one side and the solvent on the other. (It’s the opposite of the natural osmosis, in which solvent travels to areas with a high concentration of solute.)

First, the water is pretreated: screened for large particles, goes through cartridge

filtration which removes finer particles, injected with chlorine and other chemicals to kill bacteria, then injected with bisulfate to neutralize the chlorine which can harm

the membrane, and finally the water’s pH is adjusted. The water goes into a high

pressure pump which forces it through the membrane (wither spiral wound or hollow

fiber). After being forced through the membrane, the water is highly corrosive and lime is added to prevent corrosion and meet

potable water guidelines. Finally, to disinfect, the water is either exposed to UV

light or chlorine and ammonia are added.

Paper 3 Questions:1. Describe 3 of the main

pollutants found in water and their respective sources. (Addresses E.6.1)

2. Describe their effects on the environment. (Addresses E.6.1)

3. Describe the pre treatment and primary stage of water treatment. (Be able to do this for all three stages.) (Addresses E.6.2)

4. Describe the process of multi-stage desalinization. (Addresses E.6.3)

5. What is reverse osmosis? How is it used to purify salt water? (Addresses E.6.3)

Answers:

1. Nitrates/phosphates are referred to as “nutrients” because they stimulate plant growth. They are found in waste, and contaminate water from septic tank leakage, other sewage contaminating water, or from fertilizer run-off from eroded agricultural areas. Organic material is anything such as soil, silt, grass, leaves, etc. They wash into water after rain from overused land, eroding riverbanks, developing areas, or logging sites. PCBs are chemicals that have been used for almost 70 years in high-heat industrial processes; they have been found within the last 30 years to be dangerous and are usually banned in uses where they are openly exposed. However, their extensive use in the past still lingers in the environment. They come to contaminate the environment through any accidental industrial leaks and industrial waste, or if any product containing PCBs breaks or is exposed to the environment.

2. Nitrates/phosphates: cause overgrowth of underwater plant life; blocks waterways, keeps light from getting to deep water, and they also use up the dissolved oxygen other organisms need as they decompose.Organic material: when there is enough, it makes respiration difficult for sea life, makes it difficult for plant life to grow, and bacteria eventually take over, breaking down the material and using dissolved oxygen. This cause suffocation for other animals.PCBs: chemical properties allow it to linger for a long time; cause liver damage in both humans and animals that eat them, rashes, a decrease in immune system, cause a multitude of other systems, and have been linked to cancer.

3. As pretreatment, the water is raked through with a screen to remove large objects (screening process). For primary treatment Wastewater is then pumped into a reservoir, so that the sludge and heavy sediment (sludge of food, human waste and detergent) can settle (this is called sedimentation), and the less dense material (such as oil) can float to the top. The sludge is removed to be treated, the oil is removed off top, and the water goes on for further processing.

3. Multi stage distillation begins with a large amount of seawater. The seawater is heated then passes into a chamber with a very low pressure. The water flash-boils and part of it turns to steam. The steam (now pure water) is caught at the top of the chamber by heat-exchangers which cool it to water, and transferred out as drinkable water. The rest of the water is passed through several more low-pressure chambers and the process is repeated. At the final stage, only brine is left at the bottom of the chamber, and any steam produced that could not be captures is used a recycled energy.

4. Reverse osmosis is the process of forcing a solution through a semi permeable membrane, leaving the solute on one side and the solvent on the other. First, the water is pretreated: it is screened for large particles, goes through cartridge filtration which removes finer particles, injected with chlorine and other chemicals to kill bacteria, then injected with bisulfate to neutralize the chlorine which can harm the membrane, and finally the water’s pH is adjusted. The water goes into a high pressure pump which forces it through the membrane. After being forced through the membrane, the water is highly corrosive and lime is added to prevent corrosion and meet potable water guidelines. Finally, to disinfect, the water is either exposed to UV light or chlorine and ammonia are added.