potable reuse
refereed paper
Figure 2 shows the treatment train for the full-scale GAC-based IDWR plant that was used as the basis for costs and GHG calculations; major treatment processes for this plant include lime clarification, granular media filtration, GAC (operating in both adsorption and biological mode), chlorine disinfection, and dechlorination for advanced treatment. High-purity treated water mixes with surface runoff and is discharged to a drinking water reservoir. The treatment train for the second full-scale GAC-based IDWR plant is similar to the first, but uses ultrafiltration (UF) in lieu of pressure filtration and ozone before and after GAC. Note that for the GAC-based treatment approaches, a conventional drinking water treatment plant is located prior to potable water distribution, whereas some RObased facilities (particularly in California) only practice disinfection prior to potable water distribution.
Table 1. Design criteria for RO-based approach. Item
Value
Average FeCl3 Dose
40 mg/L
Rapid Mix G
1000 s-1
Flocculation Time
20-minutes
Clarifier Loading Rate
0.8 m/hr (0.32 gpm/sf)
Average Monochloramine Dose
5 mg/L
MF Design Flux
67 lmh (40 gfd)
Average Transmembrane Pressure
110 kPa (16 psi)
MF Mini-clean Frequency
Once every 3 days
MF CIP Frequency
Once per year
RO Design Flux
19 lmh (10.6 gfd)
RO Feed Pressure
150 m (213 psi)
RO Recovery
85%
RO CIP Frequency
Once every 3 months
Antiscalant Dose
5 mg/L
UVAOP EEo
0.29 kwh / 1000gal / 1-log NDMA
For clarity purposes, ancillary facilities such as chemical storage and feed, solids handling and intermediate pump stations are not shown but were included in the cost analysis. Major design criteria for all unit operations are shown in Tables 1 and 2. This design criteria, including chemical and power consumption, was collected from two full-scale operating plants (one GAC-based and one RO-based) to improve the accuracy of comparison. Some facilities, such as clarifiers, were updated from older type technology to a newer, more cost-competitive approach. For example, the conventional lime clarifiers included in the GAC plant were updated to high-rate solids contact clarifiers for cost comparison purposes.
UVAOP Average H2O2 Dose
6 mg/L
Average Final Chlorine Dose
3 mg/L
Average Finished Water Lime Dose
46 mg/L
Costs Capital and operations and maintenance (O&M) cost estimates for each treatment train were developed using an in-house proprietary cost-estimating tool for water and wastewater treatment plants. The tool was applied to generate capital costs (in US dollars) by using detailed quantity takeoffs and unit costs for material, which are based on an extensive database of constructed treatment plants. The costs are for a complete and fully operational plant with the necessary site development, electrical, computer, operations and maintenance buildings, and miscellaneous support infrastructure included in a typical plant. Standard percentages for items such as overheads and profit, contingency, engineering, and bonds and insurance, were then applied to generate a total capital cost estimate. Annual O&M costs include power, consumables and regular replacement for items with an expected life of less
Average Finished Water CO2 Dose
30 mg/L
Thickener Hydraulic Loading Rate
11 m/d (270 gpd/sf)
Thickener Solids Loading Rate
45 kg/d/m2 (9.2 gpd/sf)
Thickener Underflow % Solids
2%
Centrifuge Solids Loading
7,700 kg/d (17,000 lb/d)
Solids Disposal
On-site landfill
Table 2. Design criteria for GAC-based approach. Item
Value
Lime Dose
140 mg/L
Lime Type
Hydrated Lime
Lime Clarification Type
Solids Contact Clarifier
Clarifier Loading Rate
14.6 m/hr (6 gpm/sf)
CO2 Recarbonation Dose
18 mg/L
Filter Loading Rate
11.5 m/hr (4.7 gpm/sf)
Filter Media Depth
3 feet
GAC Influent PS TDH
25-feet
GAC Filter Loading Rate
16.1 m/hr (6.6 gpm/sf)
GAC Media Depth
6.1 m (20 feet)
GAC Regeneration Frequency
12.5% of media per year
GAC Regeneration
On-site
Average Chlorine Dose
2 mg/L
CCB Detention Time At Peak Flow
20 minutes
Average Sodium Bisulfite Dose
3 mg/L
Backwash Supply Tank
2.3 ML (610,000 gallons); four backwash events
Thickener Hydraulic Loading Rate
16 m/d (400 gpd/sf)
Thickener Solids Loading Rate
45 kg/m2/d (25 lb/d/sf)
Thickener Underflow % Solids
8%
Centrifuge Solids Loading
9,800 kg/d (21,600 lb/day)
Solids Disposal
On-site landfill
water
SEPTEMBER 2012 59