Modeling Surface Water Contamination

Transcription

1 Modeling Surface Water Contamination One of the resources required for an ecosystem to function is an available source of fresh water This is quite true for human settlements as well: If you examine the pattern of development, you will find a strong influence exerted by the availability of fresh water While fresh water is usually abundantly available in the locations where people tend to settle, there is also an unfortunate tendency for development to cause the contamination of that same fresh water source Surface water contamination through a variety of types of pollution results in reduced water quality

2 Types of Water Pollution We can classify types of pollution in a few different ways: Source We can distinguish between point source pollution, where the origin of the material can be identified and measures can be taken, and non-point source pollution, where there is no singular source of the pollution, usually because the material originates throughout the landscape Ecological Impacts We can group pollutants based on how they effect the ecosystem, e.g. pollutants that lower dissolved oxygen content vs. pollutants that mimic estrogenic compounds etc.

3 Types of Water Pollution General Types We can group pollutants based on the nature of the material (or energy) in question: 1. Heat (thermal) pollution As we heat the water (either by returning heated water to the system directly, or indirectly), the solubility of dissolved oxygen in the water is decreased 2. Silt & Sediment Erosion of soils from a variety of activities increases turbidity, which decreases light penetration, decreasing photosynthesis in aquatic plants, which lowers the dissolved oxygen content 3. Nutrients Through careless fertilizer use, increased N and P can boost algal growth, again blocking light penetration etc.

4 Types of Water Pollution 4. Organic Wastes When we add organic material to wastewater effluent, it can decompose in the water and reduce dissolved oxygen content 5. Microorganisms Also associated with organic material, these can transmit infectious disease 6. Acids Whether through acid precipitation or industrial effluent (including mining), by changing the ph of surface waters we can create conditions where some key organisms in an ecosystem cannot survive 7. Various chemicals Any number of toxic compounds, such as heavy metals, organochloranes etc., sources from pesticides and industrial discharges can be damaging to organisms

5 Dissolved Oxygen The concentration of dissolved oxygen (DO) in fresh water is a particularly important criterion of water quality, because most aquatic life depends on a certain amount of oxygen being present A few processes can reduce the DO content of water: 1. Dumping organic waste in the water provides organic carbon compounds in the presence of DO and various decomposers, thus organic waste + O 2 CO 2 + H various compounds 2. Aquatic plants photosynthesize and provide the DO, but excessive algal growth due to nutrients slows this 3. Some aquatic organisms use the DO (e.g. fish etc.)

6 Dissolved Oxygen We can model DO and CO 2 in an aquatic environment using the following schematic:

7 Henry s Law In order to model the dissolved oxygen content in an aquatic ecosystem, we first need to understand the physical law that determines how much dissolved oxygen the water can hold The amount of DO that water can hold is a function of Henry s constant and partial pressures, based on Henry s Law: C O2 = K O2 * P O2 where: C O2 is the concentration of DO in H 2 O (mg/l) P O2 K O2 is the partial pressure of O 2 at the atmosphere water boundary (atm) is Henry s Constant which varies inversely with temperature (mg/l*atm)

8 Henry s Law Henry s Constant can be determined for any gas-liquid interface through experiments We find that for water and oxygen, Henry s Constant becomes larger as the temperature drops, e.g. At 5 degrees Celsius, K O2 = 61.2 mg/l*atm At 20 degrees Celsius, K O2 = 40.2 mg/l*atm We can use Henry s Law to calculate the maximum amount of DO (called DO sat ) that water can contain at a given temperature and standard pressure (1 atm) Since O 2 comprises approximately 21% of the gases in the atmosphere, the partial pressure of oxygen in the atmosphere is thus 0.21 atm

9 Henry s Law C O2 = K O2 * P O2 At 5 degrees Celsius, using K O2 = 61.2 mg/l*atm: C O2 = 61.2 mg/l*atm * 0.21 atm = 12.9 mg/l At 20 degrees Celsius, using K O2 = 40.2 mg/l*atm: C O2 = 40.2 mg/l*atm * 0.21 atm = 8.4 mg/l You can see this phenomenon (of warmer water having a lower DO concentration) manifesting itself in the summer when fish come up to the surface on very hot days to gain access to the slightly more oxygen-rich water near the very surface: There is so little oxygen in the warm water that fish will take the risk of coming closer to surface

10 Biochemical Oxygen Demand From our previous listing of processes that diminish the DO in an aquatic system, we know that adding organic waste can reduce DO levels through the decomposition of that waste The amount of oxygen required to decompose a certain amount of waste is its biochemical oxygen demand (BOD), and like DO is measured in concentration units (such as mg/l) BOD is a useful way to describe the general water quality of a sample because it indirectly measures the amount of organic waste present in the water Ultimate BOD (BOD ult ) is used to describe the total amount of DO required to oxidize all the organic waste

11 Modeling Surface Water Contamination A municipal wastewater treatment facility is planning to locate upstream on a popular fishing river. The wastewater facility will continuously discharge wastewater into the river. The effluent will have have high biochemical oxygen demand (BOD) and low dissolved oxygen (DO) concentrations We want to determine the impact of the effluent on the DO concentration of the river (and ultimately the fish population in the river) We also want to evaluate the potential impact of corrective measures the facility can take to address potential DO problems

12 Flows in this System Objective: Predict DO levels in the river when organic waste is added from the treatment plant What pieces of information do we need to do this? We need all the inflows and outflows of O 2 : 1. DO of river water before organic waste is added 2. DO of effluent containing organic waste 3. BOD of river water before organic waste is added 4. BOD of effluent containing organic waste 5. Volume of river flow before effluent is added 6. Volume of effluent flow 7. The DO sat level of the river before mixing with the effluent

13 Processes and Model Structure We will model the resulting DO concentration in the river using a two-step approach: 1. We will establish initial values of DO and BOD immediately after the river water and effluent are mixed using a mass-balance approach 2. We will predict the expected downstream DO levels, taking into account some processes that will occur as the water moves downstream that change the DO: Reoxygenation (or aeration) will absorb O 2 from the atmosphere as the water moves downstream Deoxygenation will consume DO in the river through the consumption of the organic waste

14 Initial Stage: River and Effluent Mixing We will begin by assuming that the river water and effluent mix uniformly throughout the original body of water (this is the uniform mixing assumption) The total mass of the the DO after mixing is equal to the sum of the masses of the DO in the river and in the effluent (i.e. there is conservation of mass, making the mass-balance assumption a reasonable one) We can calculate the DO of the mixed water using a mass-balance equation: DO s V s + DO p V p = DO a (V s + V p ) DO a = (DO s V s + DO p V p ) / (V s + V p )

15 Initial Stage: River and Effluent Mixing DO a = (DO s V s + DO p V p ) / (V s + V p ) where: DO a is the DO conc. in the mixed water (mg/l) DO s is the DO conc. in the river water (mg/l) V s is the volume of river water before mixing (L) DO p is the dissolved oxygen concentration in the effluent (mg/l) is the volume of effluent (L) V p For flowing water, we can use flow rates (Q in L/sec) instead of volumes, yielding: DO a = (DO s Q s + DO p Q p ) / (Q s + Q p ) We can apply the very same mass-balance principles to calculate the BOD values immediately after mixing the river water and the effluent

16 Downstream DO Levels - Outflows As the mixed water, containing newly added organic waste from the effluent, flows downstream, decomposition of the waste will begin to occur rapidly The more organic waste was added, the bigger the BOD, and the easier it will be for microbes in the water to find organic waste to decompose and decrease the BOD, which we can model using: BOD out (t) = k 1 BOD(t) or in STELLA terms, a typical exponential model structure: k 1 is known as the deoxygenation coefficient, and has units of inverse time, e.g. days -1

17 Downstream DO Levels - Outflows As the waste is consumed and the BOD is reduced, the DO is reduced at the same rate: DO out (t) = k 1 BOD(t) or in STELLA terms, we must define the outflow from the DO reservoir to be identical to that from the BOD reservoir, using this model structure:

18 Downstream DO Levels - Inflows While DO is being consumed through organic waste being decomposed, it is also added back to the water through the reoxygenation process The rate at which reoxygenation occurs is a f(x) of how much more oxygen the water could theoretically hold, i.e. how far from DO sat is the water, or what is the water s oxygen deficit (DO sat DO) If the oxygen deficit is large, the water can rapidly absorb oxygen from the atmosphere, whereas if it is small, atmospheric oxygen will be absorbed slowly, according to the following equation: DO in (t) = k 2 [DO sat (t) DO(t)]

19 Downstream DO Levels - Inflows DO in (t) = k 2 [DO sat (t) DO(t)] k 2 is the reoxygenation coefficient, also measured in inverse time units (e.g. days -1 ), and varies according to surface water exposure to the atmosphere (e.g. whitewater vs. stagnant water), and does not account for photosynthesis In STELLA terms:

20 Downstream DO Levels - Inflows We can also take into account that BOD is increased naturally in the river when organic matter flows into the system, which we will model as occurring at a constant rate: In STELLA terms: BOD in (t) = A