Introduction

Way back, when I was in grad school, the "solution to pollution was dilution". Although this phrase had a nice ring to it, there were many problems that cropped up. None-the-less there were many aspects of sewer treatment and industrial combustion waste that were treated by dispersing pollutants into the water or air. Strategies for dispersion used technologies that would be able to dump high quantities of waste at the lowest concentrations near by. These included taller smokestacks and pumps and diffusers for water-borne wastes. Now we recognize that some of these efforts were damaging and have taken up a new slogan "reduce, reuse, recycle". Hopefully, this new direction will reduce the initial amount of pollution generated.

Even if humans "reduce, reuse and recycle" there will still be many pollutants that need to be tracked. This exercise illustrates a mass balance approach for following a potential pollutant as it is diluted out of a reservoir of water. It does this by keeping track of the flow of water, the concentration and flow of pollutant, and how the two flows (water and pollutant) are related. A similar approach could be used to follow other environmental systems, such as:

• the flow of nutrients and energy in food to an animal and the loss of heat
• the concentration of a drug in a patient following a particular dose
• the production of air-borne mercury as a consequence of burning coal
• carbon-monoxide pollution as a function of car traffic

In each of these cases there is a major carrier flow which is tied to the transfer of another element, molecule or heat to or from the environment.

You will be starting with a simple example of a reservoir of water that contains a low concentration of pollutant. As clean water flows in, polluted water flows out. The water coming in has no pollutant and the concentration of the pollutant in the water flowing out is equal to the concentration of pollutant in the reservoir and is constantly decreasing.

Description of Exercise

This exercise has four parts:

part 1: week four in lab - use a spectrophotometer and a standard curve to measure the level of dye in water

part 2: week five in lab- measure the change in concentration of dye in the reservoir with time for a set inflow and starting volume

part 3: week seven on computers - create a STELLA model for the dilution

part 4: written assignment - compare measured to modeled

 

Part 1: week four in lab - make dilutions of dye in water and measure with a spectrophotometer

You will need:

• known concentration of dye in water, the "standard"
• tubes and pipettes to make dilutions
• spectrophotometer and tubes for the sample
• unknown solution

Step 1: Create a standard curve using single shot dilutions

• such as 100 uL dye standard into 10 mL of water or 50 uL dye standard into 10 mL of water
• measure each of these in the spectrophotometer at the given wavelength
• use appropriate combinations of water volume and dye standard to span the range of absorbance from 0 to approximately 0.200 abs. units.

Step 2: Create serial dilutions

• 100 mL of standard, 20 mL standard + 80 mL water, 20 mL of previous solution + 80mL of water, continue on
• measure each of these as above
• be sure to focus on the same region of the dye concentrations

Step 3: obtain an unknown dilution of the standard (unknown to you) and measure it

• calculate the concentration of dye from the absorbance using
  • both curves from steps 1 & 2
  • looking at the graphs
  • the equations you fit to the points in each

Part 2: week five in lab - measure the heights and flow rates in a multiple cup system

You will need:

• same materials as for part 1 for sampling the dye concentration over a period of time
• a constant flow set up (bottle with constant head)
• a receiving reservoir (beaker or some other container)

Get the water flowing in your constant head apparatus

Put water and dye in your reservoir make an initial measurement.

Start letting water pour in and continue to make measurements at your "best-guess set time interval"*. If things are going too slow, you can modify your time interval. If you measurement period is too long, and you missed the good part, you'll have to do the experiment over again.

* How to get a "best-guess set time interval".

Divide the flow by the volume of the solution to get an estimate of the dilution per second.

For example, if the flow is 20 mL per second and the volume is 1000 mL then the dilution per second would be 20/1000 or 0.02 or 2% per second. Round off to the nearest percent.

Then calculate how long you'll have to wait to get a change in the solution of between 10 to 20%. In the case abovefor a 20% change it would only be 10 seconds. For slower flows it might be minutes.

 

Part 3: third week on computers - create a STELLA model for dilution of pollution exercise

The STELLA diagram should look something like this.

The equations look something like this, but yours will have to be modified to match your own measurements.

Part 4: written assignment - compare measured to modeled (due week 7)

Each student will submit a separate report. The report will contain four parts:

1. Introduction: A description of the overall assignment and its practical application to “real world” issues.
   
2. Methods:

• How you collected the data and created your standard curve
• A description of the STELLA model

3. Results:

• the Excel data and graphs (in a neat and presentable manner) from the measurement of the dilution system
• example output from the STELLA model

4. Discussion/Conclusion: A comparison and evaluation of how closely the model described your direct observations and reasons why they might be different. Suggestions how what might have lead to errors and what you might do differently if you were to do this again.  Try to relate your results back to the real world application that you brought up in your introduction.