Project Grade 11 Engineering & Design
Ledgemont Wastewater Plant: A Phosphorus Removal Upgrade
A grade 11 engineering project where teams design a phosphorus removal upgrade for a fictional wastewater plant, sizing a chemical feed system within a budget.
Objectives
- Students will calculate the daily phosphorus load a plant must remove to meet a permit limit.
- Students will size a chemical feed and storage system and test it against a footprint and a budget.
- Students will justify one treatment recommendation in a written memo to a utility board.
Vocabulary
Key words: total phosphorus, effluent permit limit, chemical precipitation, biological phosphorus removal, pounds formula
Teacher overview
Phosphorus is a nutrient that plants need, yet too much of it in a river feeds algae blooms that strip oxygen from the water. Secondary treatment rules set limits on BOD and TSS, and many states add a separate phosphorus limit to a plant's discharge permit when the receiving stream is already stressed. In this five-session project, teams design a phosphorus removal upgrade for the fictional Ledgemont wastewater plant, size the chemical feed and storage it would need, and defend the choice in a memo and a short presentation to a fictional utility board. The order of the work matters in this project, because teams need the load calculation from Milestone 1 in hand before they size the feed system and compare the two technologies, and the $250,000 capital budget is what separates the two options.
Project Brief
Driving question: How should the fictional Ledgemont wastewater plant cut its phosphorus discharge to meet a new permit limit, and can the upgrade fit the district's building and budget?
Scenario. The fictional Ledgemont Sanitary District operates a wastewater plant that discharges to a small creek. The state has written a new phosphorus limit into the plant's permit. The district board has asked your engineering team for a recommendation and a sized design.
Plant and permit values (all fictional, set for this project):
- Plant flow: 4.0 MGD, steady year round.
- Influent total phosphorus: 6.0 mg/L.
- New permit limit for total phosphorus: 1.0 mg/L as a monthly average.
- Ferric chloride solution: 38 percent ferric chloride by weight, weighing 12.4 pounds per gallon.
- Ferric chloride dose the district's pilot test found effective: 45 mg/L.
- Delivered price of the solution: $0.85 per gallon.
- Estimator's prices for the chemical feed system: $38,000 for the storage tank and its spill containment, $22,000 for each metering pump with two pumps required so one can stand by, and $84,000 for piping, electrical work, and installation labor.
Constraints your design must meet:
- The finished design must hold the effluent at or below the 1.0 mg/L permit limit.
- Chemical storage must cover 14 days of feed, so a delivery truck delay cannot cause a violation.
- The storage tank and feed pumps must fit inside the plant's existing chemical building, which has 40 feet by 25 feet of open floor.
- Capital cost must stay at or below $250,000.
Formulas and constants you may use: pounds per day = concentration in mg/L times flow in MGD times 8.34; removal efficiency in percent = (influent minus effluent) divided by influent, times 100; cylinder volume in cubic feet = 0.785 times diameter squared times depth; 1 cubic foot holds 7.48 gallons.
Final deliverable. Each team turns in a two-page design memo addressed to the fictional Ledgemont Sanitary District board, a labeled process flow diagram marking the chemical dose point, a sizing table, and a five-minute presentation of the recommendation.
Milestones
- Session 1, define the load. Calculate the influent phosphorus load in pounds per day, the load the 1.0 mg/L permit allows, and the pounds per day the upgrade must remove. Then calculate the removal efficiency the permit demands. Output: a signed load calculation sheet showing all four values with units.
- Session 2, decompose the plant and pick a dose point. Draw the plant as a series of unit processes, from screening through primary settling, aeration, secondary clarification, and disinfection. Mark where ferric chloride would be added and write two sentences on why that point works better than the others. Output: a labeled process flow diagram.
- Session 3, size the feed and the storage. Calculate the ferric chloride feed rate in pounds per day at the 45 mg/L dose, convert it to gallons of solution per day, and find the tank volume that holds 14 days of supply. Choose a tank diameter, calculate the depth it needs, and show that the tank fits the 40-foot by 25-foot floor. Output: a sizing table with every conversion shown.
- Session 4, cost the options and compare. Add up the estimator's prices to get the capital cost of the chemical system, and check it against the $250,000 budget. Price the annual chemical purchase at $0.85 per gallon of solution. Compare the chemical design against a biological phosphorus removal option, which the fictional district estimates at $410,000 to build but under $6,000 a year to run. Build a trade-off table scoring both options on capital cost, annual cost, sludge produced, and operator attention. Output: a completed trade-off table with a recommendation circled.
- Session 5, write and present. Write the two-page memo and present it to the class acting as the fictional district board. The memo must name the recommended option, show the sized equipment, and state one weakness of the recommendation honestly. Output: the memo, the diagram, the sizing table, and the presentation.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Load and removal calculation | Loads are missing or carry no units | Loads are calculated but the required removal or the efficiency is wrong | All four load values and the removal efficiency are correct with units shown | Values are correct, units are shown at every step, and the team states how a wet-weather flow increase would change the load |
| Feed and storage sizing | Feed rate is missing or uses the wrong formula | Feed rate is found but the solution volume or the 14-day storage is not completed | Feed rate, daily solution volume, tank volume, and tank dimensions are correct and fit the building floor | Sizing is correct and the team explains why the chosen tank diameter is a better fit for the room than an alternative they tested |
| Meeting the stated constraints | The design misses the permit limit, the footprint, or the budget without comment | The design meets some constraints, and the others are not checked | The memo shows the design meets the permit limit, the 14-day storage, the footprint, and the $250,000 budget | All constraints are met and the team quantifies the margin it holds on cost and on storage days |
| Trade-off analysis and memo | No comparison is made between the two options | The two options are described but not scored on stated criteria | The trade-off table scores both options on all four criteria, and the memo defends one choice with the capital and annual costs it calculated | The memo defends the choice with numbers, states plainly that the biological option cannot meet the capital budget, names one honest weakness of the recommendation, and proposes how the district could monitor it |
Differentiation
Support
Give these teams the Milestone 1 load calculation already worked, so their five sessions center on the sizing and the trade-off analysis rather than on the opening arithmetic. Provide a partly filled sizing table with the conversion factors printed in each column header, and a process flow diagram with the unit processes drawn but unlabeled.
Extension
Ask teams to recalculate the ferric chloride feed rate for a fictional wet-weather day when plant flow rises to 6.5 MGD while the influent phosphorus concentration falls to 4.0 mg/L, then decide whether the 14-day storage tank they sized still holds enough chemical. Have them state, in one paragraph, whether a plant should size chemical storage on average flow or peak flow.
ELL supports
Pre-teach "influent," "effluent," "dose," and "permit limit" with a labeled cross-section of the fictional plant showing water entering on the left and leaving on the right. Provide a memo template with headings and sentence frames, such as "We recommend ___ because it removes ___ pounds per day of phosphorus at a capital cost of ___." Allow the trade-off table to be scored with symbols before the team writes the sentences.
Extensions
Teams that finish early can research how the fictional district might recover phosphorus rather than discard it, since phosphorus captured from wastewater can be processed into fertilizer. Ask them to add one paragraph to the memo describing what recovery would require and why a small district might still choose disposal.