WATER TREATMENT PLANT PROCESS COSTS Information: (Please see the attached image depicting the schematics + Text) The results obtained from the four operating modes showed that the hardness decreased by 4.7% when both processes were in operation (i.e., sludge recirculation and aeration). When only sludge was recirculated, the reduction was 3.8%. There was no reduction due to aeration only, or when there was neither aeration nor recirculation. For turbidity, the reduction was 28% when both recirculation and aeration were used. The reduction was 18% when neither aeration nor recirculation was used. The reduction was also 18% when aeration alone was used, which means that aeration alone was of no benefit for turbidity reduction. With sludge recirculation alone, the turbidity reduction was only 6%, meaning that sludge recirculation alone actually resulted in an increase in turbidity-the difference between 18% and 6%. Since aeration and sludge recirculation did cause readily identifiable effects on treated water quality (some good and others bad), the cost-effectiveness of each process for turbidity and hardness reduction was investigated. The calculations are based on the following data: - Aerator motor = 40 hp - Aerator motor Efficiency = 90% - Recirculation pump efficiency = 90% - Electricty cost = 9 cents ¢/kWh (previous analysis) - Lime cost = 7.9 ¢/kg - Lime Required = 0.62 mg/L per mg/L hardness - Coagulant Cost = 16.5 ¢/kg - Days/ month = 30.5 Aeration cost: 40 hp x 0.75 kW/hp × 0.09 $/kWh × 24h /day /0.90 = $72 per day or $2196 per month Sludge recirculation cost: 5 hp x 0.75 kW/hp × 0.09 $/kWh × 24h /day /0.90 = $9 per day or $275 per month The estimates appear in columns 1 and 2 of the cost summary in Table 13-1. (See 2nd attached image) Costs associated with turbidity and hardness removal are a function of the chemical dosage required and the water flow rate. The calculations below are based on a design flow of 53 m^3/min. (Please see attached image with chart + Text) When sludge recirculation was discontinued, there was no hardness reduction through the clarifier, so that the extra lime cost would be $1380 per month. Obviously, the optimum condition is represented by "sludge recirculation only." This condition would result in a net savings of $1089 per month, compared to a net savings of $622 per month when both processes are discontinued and a net cost of $1574 per month for aeration only. Case Study Exercises 3. If the cost of lime were to increase by 50%, would the cost difference between the best alternative and second-best alternative increase, decrease, or remain the same? 4. If the efficiency of the sludge recirculation pump were reduced from 90% to 70%, would the net savings difference between alternatives 3 and 4 increase, decrease, or stay the same? 5. If hardness removal were to be discontinued at the treatment plant, which alternative would be the most cost- effective?
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WATER TREATMENT PLANT PROCESS COSTS
Information: (Please see the attached image depicting the schematics + Text)
The results obtained from the four operating modes showed that the hardness decreased by 4.7% when both processes were in operation (i.e., sludge recirculation and aeration). When only sludge was recirculated, the reduction was 3.8%. There was no reduction due to aeration only, or when there was neither aeration nor recirculation. For turbidity, the reduction was 28% when both recirculation and aeration were used. The reduction was 18% when neither aeration nor recirculation was used. The reduction was also 18% when aeration alone was used, which means that aeration alone was of no benefit for turbidity reduction. With sludge recirculation alone, the turbidity reduction was only 6%, meaning that sludge recirculation alone actually resulted in an increase in turbidity-the difference between 18% and 6%. Since aeration and sludge recirculation did cause readily identifiable effects on treated water quality (some good and others bad), the cost-effectiveness of each process for turbidity and hardness reduction was investigated. The calculations are based on the following data:
- Aerator motor = 40 hp
- Aerator motor Efficiency = 90%
- Recirculation pump efficiency = 90%
- Electricty cost = 9 cents ¢/kWh (previous analysis)
- Lime cost = 7.9 ¢/kg
- Lime Required = 0.62 mg/L per mg/L hardness
- Coagulant Cost = 16.5 ¢/kg
- Days/ month = 30.5
Aeration cost:
40 hp x 0.75 kW/hp × 0.09 $/kWh × 24h /day /0.90 = $72 per day or $2196 per month
Sludge recirculation cost:
5 hp x 0.75 kW/hp × 0.09 $/kWh × 24h /day /0.90 = $9 per day or $275 per month
The estimates appear in columns 1 and 2 of the cost summary in Table 13-1. (See 2nd attached image) Costs associated with turbidity and hardness removal are a function of the chemical dosage required and the water flow rate. The calculations below are based on a design flow of 53 m^3/min.
(Please see attached image with chart + Text)
When sludge recirculation was discontinued, there was no hardness reduction through the clarifier, so that the extra lime cost would be $1380 per month. Obviously, the optimum condition is represented by "sludge recirculation only." This condition would result in a net savings of $1089 per month, compared to a net savings of $622 per month when both processes are discontinued and a net cost of $1574 per month for aeration only.
Case Study Exercises
3. If the cost of lime were to increase by 50%, would the cost difference between the best alternative and second-best alternative increase, decrease, or remain the same?
4. If the efficiency of the sludge recirculation pump were reduced from 90% to 70%, would the net savings difference between alternatives 3 and 4 increase, decrease, or stay the same?
5. If hardness removal were to be discontinued at the treatment plant, which alternative would be the most cost- effective?
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6. If the cost of electricity decreased to 8 ¢/kWh, which alternative would be the most cost-effective?
7. At what electricity cost would the following alternatives just break even? (a) Alternatives 1 and 2, (b) alternatives 1 and 3, (c) alternatives 1 and 4.
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