POLLUTION AND WATER QUALITY

Answer both questions. Any graphs or charts should be produced using software packages, but the method of working for each question must be shown.

Question 1

The village of pongelton is in the valley of the River Pong. The mean flow of the river is 3.0 m³/s. The population of Pongelton is 6,000. The daily average water consumption is 250 litre/ head/day. Water utilisation (i.e. the water spent for watering gardens, watering animals, etc) is 20%.

Sewers are built in the village and the construction of a sewage treatment works is planned. The local government also wishes to construct a small recreational reservoir downstream & the site of the planned effluent discharge. The Environment Agency requires an assurance that the aquatic environment of the planned reservoir will not be impaired by the discharge of the final effluent from the sewage treatment works. Thus, the Environment Agency will require the sewage treatment works to be designed for phosphorus removal, in order to avoid eutrophication downstrearn of the discharge. To achieve this, the concentration of phosphorus in the inflow to the planned reservoir must be less than 40 mg/m³. The average phosphorus concentration of household sewage is 10 mg/l and the phosphorus concentration of the unpolluted river, upstream of Pongelton is 10 mg/m³.

You are the designer and must address the following issues:

1. If no treatment facility is provided, what will the phosphorus concentration of the river be downstream of the sewage discharge?

2. Does this value meet the EA's quality requirement?

3. If not, in percentage terms, what degree of treatment should be provided for phosphorus removal. Express your answer in percentage terms (i.e. percentage of phosphorus to be removed).

POLLUTION AND WATER QUALITY

Question 2

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, 12km downstream of the proposed the  sewage outfall are set as follows:

i) Dissolved oxygen (DO) not less than 6 mg/l, when the flow of the receiving water course, the river Little Lousy, upstream of the proposed Mantis 12 m³/s.

ii) BOD₅ less than 6 mg/l 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 20%. Estimated raw sewage strength BOD₅ 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^oC (to base e).

The Environment Agency has supplied the following background data for the river Little Lousy upstrearrm of the proposed discharge point:

• BOD₅ concentration: 6 mg/I
• DO: 7 mg O₂/1
• River velocity= 0.4 m/s
• Mean depth of stream: 1.3 m
• The mean river temperature is 19 ^oC

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

POLLUTION AND WATER QUALITY

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

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

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