Question 16 of 19: Cost-Effective Wastewater Treatment for a Seasonal Production Industry

Question 16: Cost-Effective Wastewater Treatment for a Seasonal Production Industry (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A seasonal wastewater generator has one structural disadvantage a year-round industry does not: any dedicated on-site treatment plant sized for the peak season sits idle (and loses biomass acclimation) for the rest of the year, which is exactly the kind of inefficiency that discharge to an existing, continuously-operated municipal plant avoids. The assignment is therefore to quantify that trade-off explicitly rather than assume either option.

1. Characterize the wastewater fully. Determine flow rate and its seasonal profile, pollutant strength (BOD₅, COD, TSS, oil/grease, pH), and how sharply the season starts and ends, since the degree of seasonality drives the whole comparison.
2. Assess the municipal plant's spare capacity. Obtain the municipal plant's hydraulic and organic design capacity versus its current loading, and specifically check whether it has headroom during the industry's own peak season (which may or may not coincide with the municipality's own peak).
3. Review the municipal sewer-use bylaw. Identify pretreatment requirements, prohibited discharges, and the surcharge rate structure (typically based on BOD/TSS concentration above a domestic-strength baseline) that would apply to the industry's discharge.
4. Size and cost Option A — discharge to the municipal system. Estimate any needed pretreatment (equalization for the seasonal peak, oil/grease removal, pH adjustment, screening) plus the ongoing surcharge cost, and compare against Option A's low capital cost.
5. Size and cost Option B — dedicated on-site treatment. Estimate the capital cost of a plant sized for peak-season flow/load, and the operating cost including the productivity loss from re-acclimating biomass (or re-starting a physical-chemical process) each season, plus off-season care-and-maintenance cost for an idle facility.
6. Evaluate equalization as a hybrid measure. Consider an equalization/storage basin that lets the industry discharge the seasonal peak to the municipal system in smaller, more uniform daily increments rather than as a raw seasonal spike, reducing both surcharge risk and the municipal plant's own peak loading.
7. Life-cycle cost comparison. Compare the present worth of capital plus O&M for both options over a common design horizon (e.g. 20 years), explicitly capturing the seasonal under-utilization penalty for Option B.
8. Negotiate terms with the municipality. If Option A is favoured, negotiate a formal discharge/surcharge agreement (rate, monitoring requirements, any capacity reservation) with the municipality before finalizing the recommendation.
9. Confirm regulatory approvals. Whichever option is chosen, confirm the required permits (municipal discharge permit, or a direct-discharge environmental permit for an on-site plant) are achievable within the project timeline.
10. Recommend and document. For a genuinely seasonal operation, discharge to the municipal system with pretreatment and equalization is typically the most cost-effective outcome (it avoids the capital cost and off-season inefficiency of a dedicated plant), unless the municipal plant lacks capacity or the surcharge cost structure makes on-site treatment cheaper over the life-cycle horizon — the final recommendation should state which condition applied and show the comparative cost basis.