Cross Connections - Study Notes

Cross Connections Objective: The purpose of this section is to define: • • • • cross connections backflow backpressure backsiphonage and to derive an equation to help predict these conditions. A cross connection is any physical connection between wastewater and potable water, with the potential for backflow. Backflow is any force pushing the wastewater towards potable water. In other words, a cross connection can result in contamination of drinking water, and it is a fundamental concern for environmental health professionals. In order to understand backflow, we start with an equation that should be familiar to all physics students: P = F/A where P = pressure, F = force, and A = area Pressure is typically measured in psi (pounds per square inch) with two scales: psi a (absolute scale) and psi g (gage scale) Psig is what we see on pressure gauges, and psia is the larger, absolute pressure in pounds per square inch. This may seem confusing at first, so consider two examples: 1. If we open a gauge at sea level (which releases the pressure), it may read zero, but we know that gravity is pushing down at about 14.7 pounds per square inch. Thus, psig = 0, but psia = 14.7 2. If a pump sucks all the air from a closed container (i.e., a perfect vacuum), the gauge will start reading negative numbers. Thus, we know that in a perfect vacuum at sea level, psia = 0 (by definition), but the gauge will read psig = -14.7 In each example, psig = (psia - 14.7). Using these units, we can now define backsiphonage as backflow that occurs in a negative psig (i.e., backflow caused by sucking liquids). In contrast, backpressure occurs in a positive psig (i.e., backflow caused by a higher pressure fluid forcing itself against a weaker pressure fluid). We can study backsiphonage by deriving an equation for the pressure of a water column at any height. Consider the pressure for one cubic foot of water. Since once cubic foot of water weighs 62.4 pounds, and this cube of water rests on an area that is 1 sq. foot, we can calculate the psi reading with the equation P = F/A: P = F/A = 62.4 lbs. / 1 sq. ft. = 62.4 lbs. / 144 sq. in. = .433 psi Next, for two cubic feet (one on top of the other), we double the weight resting on the same square foot: P = (62.4 lbs. / 1 sq. ft.) * 2 = .866 psi Finally, for any number of cubic feet: P = (62.4 lbs/sq. ft.) * H (where H is the height in feet) To generalize this equation for any water column (Pw): 1) divide 62.4 lbs. by 144 square inches (62.4 / 144 = .433), 2) add any other existing pressure in the system (P), and we then get the general equation: Pw = (.433*H) + P VARIABLES: P = pressure (psi) psig = lbs./square inch (gage) F = force (lbs.) water = 62.4 lbs./cubic ft. A = area (square inches) Pw = pressure in a water column (psi) psia = lbs./square inch (absolute) H = height of water column (in ft.) Modules D. Wastewater 2: WASTEWATER SOURCES Objective: This section analyzes wastewater in terms of its major sources. As discussed with drinking water, our knowledge about the sources of wastewater constitutes a major diagnostic tool that helps determine how we manage wastewater. This knowledge, combined with wastewater testing, can give Environmental Health Professionals a major role in this area. 1. Point sources of wastewater are discrete and identifiable sources that are divided into domestic and industrial sources. Domestic sources include residences and small businesses. Compared with industry, these are relatively small sources. As a result, a major issue in this category is the collection of wastewater (i.e., it is a major expense when added up for the entire community). Nationwide, about 80% of domestic wastewater is sent to sewerage systems, and about 20% to private systems (e.g., septic tanks). Industrial sources are relatively large sources that include such sub-categories as the chemical, pharmaceutical, oil, mining, and metal industries. Because of their size, these sources are generally easier to collect, but harder to treat (e.g., their chemical content can vary tremendously). Of course, collection is not always easy: a special sub-category includes the shipping industry, and shipping accidents can occur during transport (e.g., the Exxon Valdez). 2. Non-point sources are diffuse and generally occur from water runoff. Because they are spread over large areas, they tend to be more difficult to control, and in recent years they have gained greater attention from legislators. They are divided into agricultural, urban, and atmospheric sources. Agricultural sources include farms, which can contribute fertilizers, pesticides, soil erosion, and plant and animal wastes to water runoff. Collectively, they usually constitute the largest source of pollutants to water, and the erosion contributions are being worsened by the deforestation occurring in various parts of the world. Urban sources include the storm water systems that collect water from the gutters of streets in towns and cities. The true scope of the problem from urban sources is still not very well understood, but it is clearly a major contributor. Atmospheric sources include air pollution's contribution during precipitation (e.g., acid rain). We do not discuss it in detail here because we have already covered it in a previous section. Nevertheless, it is a classic example of the multi-media role of wastewater pollution. Modules D. Wastewater 3: Soils and Wells Objective: to analyze soils in terms of the content (elements), horizons, profiles, soil types, and biological components. This section also analyzes types of wells. 1. The earth's crust is composed of the following elements (given in the order of average % of soil content): Element % O2 46% Si 28% Al 8% Fe 5% Ca, Mg, Na, and K 11% all others about 1.5% 2. Horizons are horizontal layers of different soils. Examples of the different layers include: Horizon A: top soil: zone of maximum bioactivity Horizon B: sub soil: zone of eluviation (washing in) Horizon C: parent material: Horizon R: consolidated bedrock: 3. Profiles refer to different patterns of soil horizons. For example, the topsoil may be very thin or even missing in some soil profiles. 4. Soil types: soils are made of combinations of three different grains: sand: coarse grained (> .05 mm) silt: medium grained (.05 - .002 mm) clay: fine grained (< .002 mm) Finally, loam is a combination of all the above: 40% silt, 40% sand, 20% clay. 5. Humus refers to the organic material in soil 6. Biological components include such diverse components as: nematodes, fungi (molds), protozoans, algae, actinomycetes, and various bacteria. 7. Wells fall into at least three basic categories: dug wells: relatively shallow, hand excavated wells that are easily contaminated (diameter of 3-30 ft.). driven wells: constructed by driving a steel point into the ground (diameter of 1 to 4 inches). drilled wells: constructed by using various types of drills. This is generally the safest and most efficient of wells (diameter between 2-48 inches)