7.1 Cost
of Unitary Processes
7.2 Cost of
Operation and Maintenance
7.3 Capital Costs
7.4 Estimation of
Total Costs
After the preliminary selection of which process to apply, economic considerations are among the most important parameters that influence the final decision as to which process should be chosen for wastewater treatment.
To develop cost estimates, the data from the wastewater characterization should be available together with the design parameters for alternative processes and the cost correlations for these. The costs related to these alternative processes and information on the quality of effluent should also be obtained prior to the elaboration of cost estimates before discharge in compliance with local regulations.
Table 7.1 lists the construction costs for the most common unitary processes of wastewater treatment. These were developed for municipal sewage treatment and may not be totally applicable for very small wastewater treatment plants but are useful for preliminary estimation and comparison among alternatives.
Table 7.1. Construction costs for selected unitary operations of wastewater treatment adapted from EPA (1978)
Liquid stream |
Correlation |
Preliminary treatment |
C = 5.79 x 104 x Q1.17 |
Flow equalization |
C = 1.09 x 105 x Q0.49 |
Primary Sedimentation |
C = 1.09 x 105 x Q1.04 |
Activated Sludge |
C = 2.27 x 105 x Q0.17 |
Rotating Biological Contactor |
C = 3.19 x 105 x Q0.92 |
Chemical Addition |
C = 2.36 x 104 x Q1.68 |
Stabilization Pond |
C = 9.05 x 105 x Q1.27 |
Aerated Lagoon |
C = 3.35 x 105 x Q1.13 |
Chlorination |
C = 5.27 x 104 x Q0.97 |
Solids stream |
Correlation |
Sludge Handling |
C = 4.26 x 104 x Q1.36 |
Aerobic Digestion |
C = 1.47 x 105 x Q1.14 |
Anaerobic Digestion |
C = 1.12 x 105 x Q1.12 |
Incineration |
C = 8.77 x 104 x Q1.33 |
The main factors that influence the costs of operation and maintenance are: energy costs (power demand); labour costs (winch should include the personnel for operation, maintenance and administrative services); materials costs; cost of chemicals (sometimes included in the previous item); and cost of transportation of sludges for final disposal and discharge of treated wastewater.
The relative importance of these items vanes significantly depending on the location, the quality of the effluent discharged and on the specific characteristics of the wastewater being treated.
These comprise mainly the unit construction costs, the land costs, the cost of the treatment units, and the cost of engineering, administration and contingencies.
The location should be carefully evaluated in each case because it affects the capital costs more than the operating costs.
The cost of equipment may also be a significant portion of the capital costs in more automated installations.
A recent compilation of capital costs for several processes of physical treatment has been presented by Wright and Woods (1993), where capital cost correlations are given for oil-water separators, equalization basins, primary clarifiers, secondary clarifiers, a reverse osmosis unit, ultrafiltration units, gravity filters and microscreens.
For small wastewater treatment plants, an initial estimate of the total cost can be obtained from the cost of a similar plant with a different capacity, a relationship derived from costs relationships in chemical industries. The cost of plants of different sizes is related to the ratio of their capacity raised to the 0. 6 power:
where
Capital cost 1, 2 = capital costs of plants 1 and 2
Capacity 1, 2 = capacity of plants 1 and 2
The operation and maintenance costs can be estimated by a similar formula:
where:
O.M. 1, 2 11 2 = operation and maintenance costs of plants 1 and 2
Capacity 1, 2 = capacity of plants 1 and 2
An alternative procedure for the development of cost models for wastewater treatment systems includes the preparation of kinetic models for the possible treatment alternatives, in terms of area and flow rates at various treatment efficiencies, followed by the computation of mechanical and electrical equipment, as well as the operation and maintenance costs as a function of the flow rates (Ulatam, 1991).
The models so developed can be used to select the most appropriate treatment process.
