Question 2 of 19: Municipal Wastewater Characteristics With and Without Industrial Contributions

Question 2: Municipal Wastewater Characteristics With and Without Industrial Contributions (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.

Given. Municipal population, per-capita flow and the domestic per-capita BOD₅/SS generation rates, plus production rates for a cannery (seasonal) and a textile mill (daily).

Find. (a) municipal BOD₅ and SS mass loading with the two industries served, (b) the same without them (domestic-only), and (c) the population equivalent of the cannery on a BOD₅ basis.

Approach. Convert the domestic per-capita rates to a total domestic mass load, convert each industry's production rate to a daily basis and apply a typical published unit BOD₅/SS loading factor for that industry type, sum for "with industries," and use the domestic per-capita rate again to express the cannery's own load as a population equivalent.

1. Domestic BOD₅ and SS load. $$BOD_{dom} = 9{,}000 \times 76\ \text{g/cap}\cdot\text{day} = 684{,}000\ \text{g/day} = 684\ \text{kg/day}$$ $$SS_{dom} = 9{,}000 \times 90\ \text{g/cap}\cdot\text{day} = 810{,}000\ \text{g/day} = 810\ \text{kg/day}$$
2. Cannery daily production and load. A 7-month season is taken as $7/12 \times 365 \approx 213$ days of continuous operation: $$\dot{m}_{cannery} = \dfrac{4{,}000\ \text{tonnes}}{213\ \text{days}} \approx 18.78\ \text{tonnes/day}$$ Applying the assumed unit factors: $$BOD_{cannery} = 18.78 \times 20 \approx 375.7\ \text{kg/day}, \qquad SS_{cannery} = 18.78 \times 15 \approx 281.8\ \text{kg/day}$$
3. Textile mill load. $2{,}000\ \text{kg/day} = 2.0$ tonnes/day of cotton goods: $$BOD_{textile} = 2.0 \times 30 = 60\ \text{kg/day}, \qquad SS_{textile} = 2.0 \times 20 = 40\ \text{kg/day}$$
4. (a) Combined load with industries served. $$BOD_{with} = 684 + 375.7 + 60 \approx \boxed{1{,}120\ \text{kg/day}}$$ $$SS_{with} = 810 + 281.8 + 40 \approx \boxed{1{,}132\ \text{kg/day}}$$
5. (b) Load without the industries (domestic only). $$\boxed{BOD_{without} = 684\ \text{kg/day}}, \qquad \boxed{SS_{without} = 810\ \text{kg/day}}$$ Serving the two industries roughly doubles the municipal BOD₅ load and adds about 40% more SS load compared with the domestic-only system — a material increase a treatment plant sized on population alone would not accommodate.
6. (c) Population equivalent of the cannery. PE expresses the cannery's own BOD₅ load in terms of an equivalent number of people, using the same domestic per-capita rate given in the problem: $$PE = \dfrac{BOD_{cannery}}{76\ \text{g/cap}\cdot\text{day}} = \dfrac{375{,}700\ \text{g/day}}{76\ \text{g/cap}\cdot\text{day}}$$ $$\boxed{PE_{cannery} \approx 4{,}940\ \text{people}}$$ That is, the seasonal cannery alone contributes an organic load equivalent to a town of nearly 5,000 additional people — more than half the municipality's own population — even though it operates only part of the year.