Download
1 / 42
420 likes | 527 Vues
WATER & THE CITY. Keeps us alive Moderates climate Sculpts the land Removes and dilutes wastes and pollutants Recycled by the hydrological cycle Humans are made up of over 60% water It is a poorly managed resource Water rich country: Canada (0.5% population with 1/5 of freshwater)
E N D
Keeps us alive Moderates climate Sculpts the land Removes and dilutes wastes and pollutants Recycled by the hydrological cycle Humans are made up of over 60% water It is a poorly managed resource Water rich country: Canada (0.5% population with 1/5 of freshwater) Water poor country: China (1/5 population with 7% of freshwater) All About Water
Hydrological cycle • Powered by solar energy, water moves through different stages and around the earth • Evaporation from ocean----condensation in the atmosphere-----precipitation over land---runoff and ground water recharge----back to ocean Surface runoff: precipitation that is not infiltrated into the ground or evaporated back to the atmosphere. 2/3 of annual runoff is lost by seasonal floods and 1/3 is reliable runoff that we can count on as a stable source of fresh water
Watershed (or drainage basin): the region from which surface water drains into a river, lake, wetland, or other body of water.
Groundwater: water stored in the pores, fractures,, crevices, and other places in soil and rock (one of the most important source of fresh water) Structure of underground Zone of saturation: the spaces in rocks are completely filled with water. Water table: top of the zone of saturation Aquifers: porous, water-saturated layers of sand, gravel, or bedrock through which groundwater flows. (store water and provide the underground conduit for groundwater flow) Natural recharge: aquifers are replenished naturally by precipitation that percolates downward through soil and rock (very slow, 3 feet per year and no more than 1 foot per day). Some aquifers get very little recharge and are nonrenewable resource at human time scale. For example, in desert climate zone, ground water was formed 15,000-30,00 years ago).
Water usages for human: 70% for irrigation (produce 40% of the world’s food), 20% in industry and residential use is 10% Water distribution in United States: Ample precipitation in the east and too little in the west. Problems in the east: flooding, occasional urban shortages,and pollution. Problems in the west: high evaporation and recurring prolonged drought. • In US, the average total water supply is 1.2 trillion gallon per day (discharge of streams), total water used in 1960 was 22% of total supply, the consumption of water is about 5% of the total supply (not returned after use)
Urbanization and water cycle Water input: Natural means: rivers, springs, precipitation, lakes Engineering works: canals, wells, pipes, pumps,etc. Once in the city, hydrological cycle works again (evaporation, infiltration, consumption, runoff) Two types of usage • Consumptive: remove water from the system and depletes the outflow (5% total supply) (beverage industry, evaporative cooler, etc.) • Nonconsumptive uses: water is returned to the system undamaged theoretically (recreation, navigation, cooling; generally results in modification in chemical and thermal modification
Ways to provide more fresh water • Build dams and reservoirs to store runoff • Bring in surface water from another area • Withdraw groundwater and convert salt water to fresh water (desalination) • Reduce water waster and import food to reduce water use in growing crops and raising livestock
Problems with Groundwater withdrawal: • land subsides (80% is related to underground water usage) .(San Joaquin Valley southwest of Mendota, CA; San Clarita Valley during Jan 4 to Aug 2, 1997) http://water.usgs.gov/ogw/subsidence.html
(2) salt water intrusion: when fresh water is withdrawn at a faster rate than it can be replenished, a draw down of the water table occurs with a resulting decrease in the overall hydrostatic pressure, salt water from the ocean intrudes into the fresh water aquifer. http://www.elmhurst.edu/~chm/onlcourse/chm110/outlines/saltintrusion.html
Desalination: removing dissolved salts from ocean or blackish water in aquifers or lakes (slightly salty water) Methods: • Distillation: heating salt water until it evaporates, leaves behind slats in solid form and condenses as fresh water • Reverse osmosis: pumping salt water at high pressure through a thin membrane with pores that allows water molecules, but not dissolved salts to pass through Problems: • High costs for energy usage • produces large quantities of briny wastewater that contains lots of salt and other minerals (no where to disposal of without causing environmental problems). Better solution: 1. reduce waste water: 2/3 of water is wasted, can be reduced to 15% (evaporation, leaks, and other losses) 2. Water recycle
Floodplain Features Figure 11.21
Flood: water in a stream overflows its normal channel and spills into the adjacent area (floodplain) Benefits: provide productive farmland, recharge groundwater, and help refill wetland. Causes in Increased floods in City • Increases in percentage of impervious surfaces (reduce infiltration and increase surface runoff) • Paving, straightening, or “improving” stream channels (reduces the time lag between rainfall and runoff) • Landscaping and subdivision of the land into building sites (shorten distance in water flow and thus time lag) • Filling in and human settlement of flood plains (reduces space for storing flood water)
Urban Flooding Figure 11.28
Water pollution: any chemical, biological, or physical change in water quality that has a harmful effect on living organisms or makes water unsuitable for desired uses. • infectious agents/cause diseases ; example: bacteria, viruses, parasites; sources: human and animal wastes • oxygen-demanding wastes/deplete dissolved oxygen needed by aquatic species ; Example : biodegradable animal wastes and plant debris ; sources : sewage, animal feedlots, food processing facilities, pulp mills • Plant nutrients/cause excessive growth of algae and other species; example: nitrates (NO3-) and phosphates (PO43- ); source: sewage, animal wastes, inorganic fertilizers • organic chemicals/toxins to aquatic systems; example: oil, gasoline, plastics, pesticides, cleaning solvents; source: industry, farms, households • inorganic chemicals/toxins to aquatic systems; example: acids, salts, metal compounds; sources: industry, households, surface runoff • Sediments/disrupt photosynthesis, food webs, other processes; example: soil, silt; source: land erosion • thermal/make some species vulnerable to disease; heat; sources: electric power and industrial plants.
On June 22, 1969, an oil slick and debris in the Cuyahoga River caught fire in Cleveland, Ohio, drawing national attention to environmental problems in Ohio and elsewhere in the United States.
Impact on water quality and temperature on streams • average nitrates content is 14 times greater; detergent content is 9 to 18 times greater, amount of dissolved solids is 3-4 times greater in the urbanized Nassau County stream, New York; • Urban streams are much warmer in summer (5°-8°C and up to 10°C) and somewhat cooler (1.3°C to 5°C) in winter than their rural counterparts due to increases in storm runoff, decreases in groundwater recharge, and removal of vegetation around the banks. • Eutrophication: natural nutrient enrichment of lakes, mostly from runoff of plant nutrients such as nitrates and phosphates from surrounding land. • Cultural eutrophication: human activities greatly accelerate the input of plant nutrients to a lake (near urban or agricultural area; farmland, animal feedlots, urban areas, mining sites, treated and untreated municipal sewage, atmosphere). Promote growth of dense growth or bloom of organisms. These plant life reduce lake productivity by decreasing the solar energy input needed by phytoplankton that support fish. When algae die, the bacteria depletes dissolved oxygen in the surface layer of water near the shore and in the bottom layer. Eventually produce gaseous decomposition products such as smelly, highly toxic hydrogen sulfide and flammable methane.
Development of Drainage and sewage disposal system • Ancient Rome constructed drainage system to drain storm water runoff (to rivers and estuary) only. (“It is not uncommon to threw wastes out of the window onto street and to the people passing by”). (Privies (outdoor toilets) were used, but their contents frequently oozed through the basement walls of the occupied apartments beside them. In other places, wastes were simply heaped on the ground or placed in pits in the centers of courtyards; some courtyards were covered with filth up to the doorways of the houses.) • In nineteen century, combined sewers were used. (Organic waste load decomposed after entering the water body, exhausted dissolved oxygen, and produced anaerobic conditions that accompanied by the offensive orders of hydrogen sulfide and other putrid matter) • During mid-nineteenth century, a period known as the Great Sanitary Awakening, Sir Edwin Chadwick led a crusade to build sewage treatment facilities: construction of interceptor sewers that could drain to sewage treatment plants. • In 1964, many cities in US have combine sewages
Waste water treatment Primary waste treatment consists of removing the suspended solid materials by screening, sedimentation, and floatation; requires 103 hours; the solid materials form a sludge that is treated in digesters where the organic material undergoes anaerobic decomposition; outputs: • a liquid effluent that still contains more than half of the dissolved and suspended organic material from the raw waste; and • the solid waste from the sludge digesters. In Boston, the primary effluent is chlorinated to kill the pathogenic bacteria and then released to the harbor through a system of offshore outfall pipes. Sludge disposal is a relatively expensive part of waste treatment. So, sometimes, it is dumped into the harbor.
1. Preliminary treatment: Upon arrival via the sewer system, the wastewater is sent through a bar screen, which removes large solid objects such as sticks and rags. Leaving the bar screen, the wastewater flow is slowed down entering the grit tank. This allows sand, gravel, and other heavy material that was small enough not to be caught by the bar screen to settle to the bottom. All the collected debris from the grit tank and bar screen is disposed of at a sanitary landfill or recycled. • 2. Primary treatment: the physical separation of solids and greases from the wastewater. The screened wastewater flows into a primary settling tank where it is held for several hours. This allows solid particles to settle to the bottom of the tank and oils and greases to float to the top. • 3. Secondary treatment: a biological treatment process that removes dissolved organic material from wastewater. The partially treated wastewater from the settling tank flows by gravity into an aeration tank. Here it is mixed with solids containing micro-organisms that use oxygen to consume the remaining organic matter in the wastewater as their food supply. The aeration tank uses air bubbles to provide the mixing and the oxygen, both of which are needed for the micro-organisms to multiply. From here the liquid mixture, composed of solids with micro-organisms and water, is sent to the final clarifier. Here the solids settle to the bottom where some of the material is sent to the solids handling process, and some is re-circulated to replenish the population of micro-organisms in the aeration tank to treat incoming wastewater.
4. Final treatment: Wastewater that remains is disinfected to kill harmful micro-organisms before being released into receiving waters. Although there are many methods available to kill these micro-organisms, chlorine and ultraviolet disinfection are the most widely used. • De-chlorination occurs in the final wastewater treatment step. A solution of sodium bisulfite is added to the chlorinated effluent to remove residual chlorine. • Following disinfection and de-chlorination, the treated wastewater (now called final effluent) can be returned to the receiving waters from which it came. The flow is conveyed to an outfall and discharged through a series of diffusers into a surface water body or stream. • 5. Solids processing • Primary solids from the primary settling tank and secondary solids from the clarifier are sent to the digester. During this process, micro-organisms use the organic material present in the solids as a food source and convert it to by-products such as methane gas and water. Digestion results in a 90% reduction in pathogens and the production of a wetsoil-like material called “biosolids” that contain 95-97% water. To remove some of this water and reduce the volume, mechanical equipment such as filter presses or centrifuges are used to squeeze water from the biosolids. The biosolids are then sent to landfills, incinerated, or beneficially used as a fertilizer or soil amendment.
More complete waste water treatment is available now consists of 4 steps http://www.regulatorystaff.sc.gov/ORSContent.asp?pageID=654
Anaerobic digestion component of Lübeckmechanical biological treatment plant in Germany, 2007
Ground water pollution (Super Fund) Is difficult and expensive to cleanup: 1. Contaminants are not diluted and dispersed effectively 2. Low concentration of dissolved oxygen and smaller populations of decomposing bacteria, and cold temperature, slow down chemical reactions that decomposes wastes. 45% of municipal groundwater in US has been contaminated by one or more chemicals; 76,000 underground tanks storing gasoline, diesel fuel, home heating oil, and toxic solvents were leaking their content into groundwater in the U.S in 2002. Prevention is the most effective way
Coastal Pollution • 40% world population live on or within 100KM (62miles) of the coast and 14 of the world 15 largest metropolitan areas (10 million or above) are near coastal waters. • 1/4 of people using coastal beaches in the US develop ear infections, sore throats, eye irritations, respiratory disease, or gastrointestinal disease • Runoffs of sewage and agricultural wastes into coastal waters introduce large quantities of nitrate and phosphate plant nutrients, cause harmful algae growth (red, brown, or green toxic tides). They release waterborne and airborne toxins that damage fisheries, kill some fish-eating birds, poison seafood.
Quality of drinking water • In developed countries, surface water is stored in reservoir for several days to improve clarity and taste by increasing dissolved oxygen content and allowing suspended matter to settle; then pumped to a purification plant and treated to meet government drinking water standards. • In US bottle water is 240 times to 10,000 times more expensive than tap water (about ¼ of its tap water, but 1/3 of bacteria contamination). Home water treatment systems are not worth the expense and maintenance hassles (unless contamination is identified).
Drinking water contaminants 1. arsenic: a semi-metal element in the periodic table, it is odorless and tasteless. It occurs naturally in the environment and as a by-product of some agricultural and industrial activities; enter through ground or as runoff into surface water sources; Health impact: cancer of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate. The new standard is 10ppb in drinking water. Shot term effects occur within hours of days of exposure, long term over many years. 2. Lead and copper: Health impact: stomach stress and brain damage, especially for children under 6 years old. And stomach stress. Enter water through pipes. If lead concentrations exceed an action level of 15 ppb or copper concentrations exceed an action level of 1.3 ppm in more than 10% of customer taps sampled, the system must undertake a number of additional actions to control corrosion.
3. Microbial and disinfection byproducts Microbial pathogens include a few types of bacteria, viruses, protozoa, and other organisms. Some pathogens are often found in water, frequently as a result of: • Fecal matter from sewage discharges;(2) Leaking septic tanks; (3) Runoff from animal feedlots into bodies of water Health impact: epidemics such as typhoid and cholera 4. MTBE (methyl-t-butyl ether): MTBE is a member of a group of chemicals commonly known as fuel oxygenates. Oxygenates are added to fuel to increase its oxygen content. MTBE is used in gasoline throughout the United States to reduce carbon monoxide and ozone levels caused by auto emissions. Releases of MTBE to ground and surface water can occur through leaking underground storage tanks and pipelines, spills, emissions from marine engines into lakes and reservoirs, and to some extent from air deposition.
5. Radionuclides: A nuclide is a general term applicable to all atomic forms of an element. Nuclides are characterized by the number of protons and neutrons in the nucleus, as well as by the amount of energy contained within the atom. A radionuclide is an unstable form of a nuclide. They may occur naturally, but can also be artificially produced. (1) Combined radium-226/-228; Naturally occurs in some drinking water sources. Some people who drink water containing radium –226 or -228 in excess of the MCL over many years may have an increased risk of getting cancer. (2) (Adjusted) Gross Alpha; Naturally occurs in some drinking water sources. Some people who drink water containing alpha emitters in excess of the MCL over many years may have an increased risk of getting cancer. (3) Beta Particle and Photon Radioactivity; May occur due to contamination from facilities using or producing radioactive materials; Some people who drink water containing beta and photon emitters in excess of the MCL over many years may have an increased risk of getting cancer. (4) Uranium; Naturally occurs in some drinking water sources. Exposure to uranium in drinking water may result in toxic effects to the kidney.
6. Radon: Radon is a naturally-occurring radioactive gas that may cause cancer, and may be found in drinking water and indoor air. Some people who are exposed to radon in drinking water may have increased risk of getting cancer over the course of their lifetime, especially lung cancer. Radon in soil under homes is the biggest source of radon in indoor air, and presents a greater risk of lung cancer than radon in drinking water. Good drinking water: Low BOD (biological oxygen deman) Low coliform organisms Low sediment load
