Objective

The objective of this project is to design, within given constraints, a prototype water treatment system that can be scaled up to a full-scale water treatment system. The effectiveness of the water treatment system design will be evaluated by the yearly operational and maintenance costs and effluent quality standards. Each prototype system will be scaled-up to handle a flowrate of 20 million gallons per day (MGD).

Design Constraints

The prototype treatment system must meet the following criteria:

• the flowrate into the treatment system may be adjusted

• the coagulant dosage may be adjusted

• the coagulation and flocculation units may not be adjusted

• the size of the sedimentation tank may be adjusted

• the filter material height must be less than 8 in.

• the filter materials are limited to anthracite and/or filter sand

• the filter diameter is 3.5 in.

• the water height above the filter material must be maintained at 6 in.

• the maximum filter run is 60 minutes

• the effluent must have an average turbidity of less than 2 NTU

The full-scale treatment system will have the following characteristics: 

• each coagulation and flocculation unit can handle 5 MGD and cost $25,000/unit

• ferric chloride costs $1/kg

• each sedimentation unit has a volume of 75,000 gallons and costs $35,000/unit

• each filters unit has a loading area of 1,000 ft.² and costs $45,000/unit

• Filter material costs are: anthracite costs $9.50/ft.³ and filter sand $5.90/ft.³

• the filter media is replaced every five years

• The overall size of the treatment system should be increased by 20% or a safety factor SF of 1.2 to handle backwashing and cleaning of the treatment system.

If either the pressure head or the turbidity criteria are violated before the 60-minute filter run is complete, then the time when the effluent water quality exceeds these criteria is the filter run time (less than 60 minutes).

Water Treatment System Annual Cost

The cost of the water treatment system will be estimated as follows:

Total Cost = Cost_CF + Cost_S + Cost_F + Cost_FM

where:

Cost_CF are the cost of the full-scale coagulation and flocculation units;
Cost_S are the costs of the full-scale sedimentation units;
Cost_F are the costs of the full-scale filters; and
Cost_FM are the costs of the filter materials

Coagulation and Flocculation Cost - Cost_CF

The weight of coagulant wt_c required per gallon of treated water is estimated as:

The number of coagulation and flocculation units NCF required is:

The yearly cost to operate the full-scaled coagulation and flocculation system Cost_CF is estimated as:

Sedimentation Cost - Cost_S

1. Compute the prototype sediment tank retention time t_p:

where n_t is the number of chambers active in the prototype sedimentation tank (1, 2, 3, or 4), and the volume of each chamber in the tank is approximately 1.56 gallons.

 

2. The full-scale treatment flowrate Q_ST per sediment tank is:

3. The effective flowrate Q_SE in a sediment tank is:

4. The number of full-scaled sedimentation tanks NS required to handle the daily volume is estimated as:

5. The yearly cost to operate the full-scale sedimentation system Cost_S is estimated as follows:

 

Filtration Cost - Cost_F  

1. Convert the average flowrate through the prototype filter (the 3.5 in. diameter prototype filter has an area of 0.0668 ft.²) into a prototype filter loading rate Q_F (gpm/ft.²):

2. The full-scale treatment flowrate Q_FT is:

3. Considering that each filter is inoperable during backwashing, the effective flowrate Q_FE is:

4. The number of full-scale filters NF required to handle the daily volume is estimated as:

5. The yearly cost for the full-scale filters Cost_F is:

 

Filter Media Cost - Cost_FM

The cost per year for anthracite Cost_FMa is estimated as:

where the area of the full-scale filter is 1,000 ft.², and the factor NF accounts for the replacement of the filter media each year. The value of 5 in the denominator indicates that the media is replaced every five years.

The cost per year for filter sand Cost_FMs is estimated as:

Therefore, Cost_FM = Cost_FMa + Cost_FMs

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