Question

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 of 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 downstream 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 requirements?

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)"