E6: Water Treatment (UNMODIFIED)

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

DioxinsWhat 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)ParaquatWhat 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 DistillationProcess: 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 OsmosisBackground 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 BODProcess: 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 SitesEND 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 TreatmentGOAL: Removal of phosphates, heavy metals, nitrates and organic compoundsMethod 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 CompoundsOrganic compounds adhere to granules of carbon as water flows over the bedMethod 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 biologicallyDenitrifying bacteria in an anaerobic environment to convert NO3- into N2 and H2O

Primary Water Pollutants

Freshwater from Seawater

Waste Water Treatment