2. Water. Chapter 2. Water Hydrological cycle

Transcription

1 2. Water 2.1. Hydrological cycle Every year, 456 x m 3 of water evaporates from the ocean, which corresponds to the removal of the upper 1.27 m of water from the ocean surface, while 62 x m 3 of water is removed from land areas by a combination of evaporation and transpiration. Annual precipitation is 410 x m 3 over the oceans and 108 x m 3 over land, causing a net transfer of 46 x m 3 of water from the oceans via the atmosphere and the return of the same amount of water from the land to the ocean by surface and subsurface runoff [1, page 355]. The total amount of water on earth is estimated to be x 10 9 km 3, of which only 3.5 x 10 7 km 3 (2.5%) is in the form of freshwater [1, page 358]. The amounts in various forms and their respective residence times are estimated to be: Location Amount Fraction Residence time (km 3 ) Oceans 1,338,000, % 3,200 years Polar ice caps, glaciers, permanent snow 24,064, % 20 to 20,000 years Groundwater 23,400, % 100 to 10,000 years Ground ice and permafrost 300, x 10 4 Freshwater lakes 91, x to 100 years Saline lakes 85, x 10 5 Soil moisture 16, x to 2 months Atmospheric water vapor 12, x days Rivers and swamps 2, x to 6 months Living biomass 1, x weeks in human body Sources: [1], [2], [3] 2.2. Energy for water Energy is required for the distribution of municipal drinking water, specifically for its extraction in nature and conveyance to the treatment plant, the various treatment processes, and the pumping through the often crusty pipe network of the municipality. If the water source is from an underground aquifer, the pumping energy requirement increases with depth: 537 kwh per million gallons (461,000 m 3 ) from a depth of 120 ft (36.6 m), 896 kwh per million gallons from a depth of 200 ft (122 m), which corresponds to about 70% pumping efficiency [4, page 11]. The other energy requirements are tabulated below, with the higher numbers being typical for California where water procurement and treatment are more costly. 13

2 Step Sources & Conveyance Treatment Distribution TOTAL Source: [4 page 32, 5 page 2] Energy required (kwh/10 6 gallons) 6, , ,000 1, ,200 7, ,200 Energy required (kwh/1000 m 3 ) 1, , , , ,220 Energy is required to extract freshwater from seawater. For a typical salinity of 3.45% salt and at temperature of 25 o C, the minimum energy required is 0.86 kwh per m 3 of freshwater produced [6]. In practice, the amount of energy required far exceeds this minimum and varies depending on the desalination process. Multi Stage Flash Distillation (MSF) 50,000 70,000 MED TVC combination Multi Effect Distillation (MED) Mechanical Vapor Compression (MVC) Reverse Osmosis (RO) Capacity 10,000 5,000 (m 3 /day) 35,000 15, ,500 24,000 Electricity consumption (kwh/m 3 ) Thermal energy consumption None None (kwh/m 3 ) Total energy consumption (kwh/m 3 ) Source: [6], crediting Wangnick Consulting (2010) 2.3. Water consumption Water withdrawals for various activities in the United States during 2005 amounted to the following: Amount used Fraction of Activity (10 6 L/day) (10 6 gal/day) freshwater Freshwater 541, ,000 41% Power Seawater 219,000 58,000 plants All 760, ,000 Irrigation 484, ,000 37% 14

3 Public supply 167,100 44, % Industrial uses 68,800 18, % Aquaculture 33,200 8, % Mining 15,200 4, % Livestock 8,100 2, % Other withdrawals 2, < 1% Freshwater 1,320, , % Totals Seawater 230,600 61,000 All 1,550, ,000 Source: [7 page 1] The freshwater footprint per person in the United States (total consumption above divided by population of million in 2005) is about 1,200 gallons/day (4,500 L/day). Domestic consumption of water in the United States, excluding outdoor use (which varies greatly with location and residence type) is estimated to break down as follows: Activity Amount used per person (L/day) (gal/day) Fraction Toilets % Clothes washing % Showers % Faucets % Leaks % Baths % Dishwashing % Other domestic uses % Total % Source: [1 page 365] Humans also consume water indirectly, some of which is due to food production as the table below indicates. Food Amount of water used in production L/kg gallons/lb Beef 45,510 5,214 Pork 13,600 1,630 Chicken 6, Apples Carrots Wheat Potatoes Tomatoes Lettuce Source: [8] 15

4 For water usage in the processing of materials, the reader is referred to the individual materials listed in Chapter 1. In larger buildings where cooling towers are used to provide central air conditioning, the rate of water consumption by evaporation is 3 gallons of water per minute for each ton of refrigeration provided [9], which amounts to gallons of water per BTU of heat removed. In metric units, this amounts to 54 L of water per MJ of heat removed. For the average commercial building relying on cooling towers to provide 500 tons of refrigeration, the water consumption is 1,500 gallons of water per minute = 900,000 gallons for a 10 hour day, which is equivalent to 13,000 occupants each consuming 69.3 gallons per day (table above). Put another way, the refrigeration of the average commercial building by means of cooling towers consumes in a single 10 hour day as much water as one person consumes in 13,000 days = 36 years Wastewater In the United States, the domestic wastewater generation is about 120 gallons per person per day (= 450 L/day), with a spread of 50 to 250 gallons per person per day [10, page 314]. Typical wastewater flowrates from various sources are tabulated below, in liters per day per specified unit. Source Flow (L/day) Typical Range Unit Airport passenger High rise person Apartment Low rise person Resort person Automobile service station vehicle served employee Bar customer employee Cabin, resort person Cafeteria customer Campground (with facilities) person Cocktail lounge seat Coffee shop customer Cottage (summer use) person Country club member present employee Day camp no meals person 16

5 Department store toilet room Dining hall meal served Dormitory, bunkhouse person Hospital medical bed employee Hospital mental bed employee Hotel guest employee Typical home person House Better home person Luxury home person Older home person Industrial building (toilets only) employee Laundry facility (self service) laundry machine wash Motel with kitchen unit without kitchen unit Office building employee Prison inmate employee Rest home resident Restaurant meal served with cafeteria or gym student School with cafeteria student with cafeteria and gym student boarding student Shopping center employee parking space Store customer Swimming pool customer Theater seat Trailer park person Visitor center visitor Source: [11] Sources [1] Mihelcic, J. R., and J. B. Zimmerman, 2010: Environmental Engineering Fundamentals, Sustainability, Design. John Wiley & Sons, 695 pages. [2] U.S. Geological Survey <ga.water.usgs.gov/edu/earthwherewater.html> 17

6 [3] PhysicalGeography.net < [4] Cohen, R., B. Nelson and G. Wolff, 2004: Energy Down the Drain The Hidden Costs of California's Water Supply, National Resources Defense Council, Pacific Institute, Oakland, California, August < [5] U.S. Environmental Protection Agency, Energy Efficiency in Water and Wastewater Facilities, 2013 < 08/documents/wastewater guide.pdf> [6] DESWARE Encyclopedia of Desalination and Water Resources < [7] Kenny, J. F., N. L. Barber, S. S. Hutson, K. S. Linsey, J. K. Lovelace and M. A. Maupin, 2009: Estimated Use of Water in the United States in 2005, U.S. Geological Survey, Circular <pubs.usgs.gov/circ/1344/pdf/c1344.pdf> [8] Robbins, J., 2010: The Food Revolution, Conari Press, 10 th ed., 480 pages. For more accessible quotes from this book, see Vegetarianism and the Environment at <michaelbluejay.com/veg/environment.html> [9] Conservation Mechanical Systems, Inc.: Water Use in Cooling Towers < content/siteimages/water use in Cooling Towers.pdf> [6] Hammer, M. J. and M. J. Hammer, Jr., 2001: Water and Wastewater Technology, Prentice Hall, 4 th ed., 636 pages. [7] Tchobanoglous, G., F.L. Burton, H. D. Stensel, 2002: Wastewater Engineering Treatment and Reuse (Metcalf and Eddy), McGraw Hill, 1848 pages. Table adaptation by Timothy G. Ellis, Department of Civil, Construction and Environmental Engineering, Iowa State University, as posted at < / E TXT 05.aspx#4._Wastewater_Quantities_> 18