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ENGR 102 – Thermal load reduction assignment (to be completed individually)

Screening-level design to meet temperature credit

Given:  See the spreadsheet developed by consultants at CH2MHill (on Canvas, Temperature Credit Calculator_Simplified.xlsm)

Problem statement: You have been hired by a local city council to help them evaluate their options for meeting a 18 Mkcal/day reduction in thermal load (i.e. thermal credits generated) each month (not averaged annually) to the Willamette River. The city’s wastewater treatment facility is currently releasing 2 million gallons per day (MGD) of treated, warm effluent into the Willamette River. Your job is to help them understand whether they’ll be able to achieve this target with a constructed wetland and to make recommendations regarding the size and amount diverted into the wetland. Keep in mind that land is expensive, so your design should minimize wetland area while achieving the thermal credits objective.

Required:

a) Determine what wetland area and effluent discharge rate will allow the city to achieve the required thermal reduction (measured as thermal credits).

b) Conduct a sensitivity analysis (described below) to identify which design parameters are most influential in the wetland’s treatment efficiency.

Design criteria:

a) Objective:

a. 18 Mkcal/day reduction in thermal load (i.e. thermal credits generated) each month

b) Constraints:

a. Minimize wetland area – Be strategic! Which month is going to be most difficult at meeting your thermal load reduction requirements? Set the wetland area based on this month.

b. Effluent discharge rate is 2 MGD or less

c. HRT (hydraulic retention time) should be between 1 - 5 days

Approach:

1. Read the “readme” worksheet plus the text boxes within Worksheet 4. You will need to read and understand this content to do well on the assignment.

2. Do not change the effluent temperature. This is the temperature of the wastewater coming into the wetland.

3. For practical reasons, use only a single value for wetland area across all months. Changing the size of a wetland on a monthly basis is not practically feasible because it would involve opening and closing different areas, which creates problems for the plants and soils in those areas. Instead, the flow rate diverted (in MGD) can vary on a monthly basis.

4. Sensitivity analysis: investigate the effects of two input variables:

a. Volume of flow diverted into wetland (Q)

b. Wetland area (A)

on the response variable:

a. of credits generated

Please follow the approach outlined below.

I. Independently vary each input variables (from original value) in a range of +/-10%, +/-25%, and +/-50%. Change values for all months simultaneously. That is, increase Q by +/-10% for all months before moving on to +/- 25% and +/- 50%. Then return Q to the original value and begin modifying wetland area.

II. Make a table of response variables at the annual scale- changes in the  thermal credits calculated as the sum across the entire year for each change in input parameter (Q and A).

III. Plot % change in each input variables (x-axis) against % change in the two response variables (y-axis). % change = (initial value – changed value) / initial value.

IV. Synthesis:  How does thermal credits change with change in flow diverted and wetland size? Which input parameter is more important in determining the thermal credits? You can see this from looking at the steepness of the curves in your plots.

5. Wetland design: What wetland size, for a maximum of 2MGD, is required to meet the thermal load reduction target for each month? Make sure that the HRT is between 1- 5 days. Please note that it is okay if it does exceed the HRT for the sensitivity analysis.

Engineering Memo:

· State the objective of the study

· Explain how the model was set up and any stated assumptions and limitations. Explain what the units of M kcal/day represent. You will not receive credit for simply spelling out the acronym.

· Describe your analytical approach

· Include a table of the wetland design (area, flow, thermal credits) across the different months.

· Include the plot that reflect the sensitivity of thermal credits to 1) wetland area, and 2) discharge diverted into the wetland.

· State your key findings

o Recommendation on the size and discharge rate for the design wetland

o Discuss how the reduction in thermal load varies across the months, and why.

o Explicitly state to which of the input parameters (area or discharge) the model output is most sensitive.

o Discuss the sensitivity analysis in a way that helps the city council think about the most effective way of achieving further temperature credits.

· Conclude with some other factors that are relevant to the design of the wetland that are not considered in your analysis.

You should submit your summary of your results as a PDF in a 2-page engineering memo format (see example format here: http://www.writing.engr.psu.edu/workbooks/memo_format.html). It should include one page of a cover letter summarizing the problem statement and your key findings, and one page of attachments that includes your figures.