Question

The town of Prettybrooks (population 65,000) is to build a new sewage treatment works. The Environment Agency requires an environmental impact assessment to be carried out for the project. The water quality targets to be met in the receiving watercourse, 18km downstream of the proposed sewage outfall are set as follows:

(i) Dissolved oxygen (DO) not less than 6 mg/I, when the flow of the receiving water course, the river Little Lousy, upstream of the proposed plant is 12 m'/s.

(ii) BODs less than 6 mg/I within 0.5 days

Water consumption in Prettybrooks is 250 litre / head / day on average and the estimated water losses of the system in the summer months (evaporation, gardening losses, leakage of the sewer system, etc) are 2096. Estimated raw sewage strength BODs is 550 mg O₂/l.

Estimated raw sewage DO concentration is 1.5 mg/l. Assume a de-oxygenation rate constant, K₁ of 0.37 day^-1 at 20°C (to base e).

The Environment Agency has supplied the following background data for the river Little Lousy upstream of the proposed discharge point:
- BOD_s concentration: 6 mg/l
- DO: 7 mg O₂/l
- River velocity = 0.4 m/s
- Mean depth of stream: 1.3 m
- The mean river temperature is 19 °C

Calculate:

(i) if raw sewage is discharged with no treatment, the critical time corresponding to the critical deficit, and the distance downstream of the discharge point at which it occurs.

(ii) the critical DO deficit

(iii) The time and distance for which the BOD criteria are violated if raw sewage is discharged with no treatment.

(iv) The amount of BOD to be removed at the proposed 51W in order that the EA requirements are met.

(v) The revised critical time and deficit associated with item (iv).