Comparing the Leopold Clari-DAF System to Upflow Contact Clarification

Transcription

1 JUST ADD WATER Comparing the Leopold Clari-DAF System to Upflow Contact Clarification James E. Farmerie Product Manager The F. B. Leopold Co., Inc. 227 South Division Street Zelienople, Pennsylvania In April 2004, a pilot plant study was conducted using raw water from a reservoir that is source water for a northeastern U.S. potable water plant. The goal of the pilot plant study was to evaluate the performance of the Leopold Clari-DAF (Dissolved Air Flotation) system vs. upflow contact clarification (CC) as well as mixed media filtration () vs. granular activated carbon filtration (). The treatment goals were: 1. Remove iron and manganese to levels below Secondary Standards (0.3 mg/l Fe, 0.05 mg/l Mn) 2. Remove objectionable taste and odors that occur seasonally 3. Remove low-density particulates, such as algae, and floc formed from the coagulation of waters that contain natural organic material (NOM) 4. Removal of NOM such as color and disinfection by-product (DBPs) precursors to minimize the production of DBPs in the distribution system 5. Handle turbidity spikes upward of 20 NTU The tests were completed during two seasons to depict winter and summer typical source water as the make-up for the pilot test apparatus. A summary of the source water quality data collected over one-year is presented in Table 1. Table 1 Summary of Source Water Quality Parameter Units Average Maximum Minimum Temperature C ph Alkalinity mg/l CaCO Turbidity NTU Apparent Color CU UV 254 nm TOC mg/l Iron mg/l Manganese mg/l HPC Bacteria CFU/100 ml Total Coliform no./100 ml

2 Historically, algae blooms resulted in taste and odor complaints. In the mid-1970s, a class of Cyanophyceae, Aphanizomenon (8,850 per ml), was detected, and in the early 2000s, Synedra, Navicula, Cyclotella, Crytomonas, and Chroccoccus were detected. The reservoir was susceptible to eutrophication, including algae blooms and nuisance aquatic vegetation, due to elevated levels of nitrogen and phosphorus. In the Clari-DAF system (DAF) process, flocculated particles are separated out of the water by floating them to the surface rather than settling them to the bottom of a basin. The process introduces micro-sized air bubbles through diffusers at the bottom of the contactor where they mix with coagulated solids and float the floc. The air bubbles are produced by recycling a portion of the effluent through a tank where air is introduced and the water saturated, then reduced to ambient pressure to create a pressurized flow. The floated sludge is removed from the top of the basin by a mechanical skimmer, while the clarified water is removed through laterals in the bottom of the basin. Contact clarification (CC) is an upflow clarifier where coagulation, flocculation, and clarification occur in one process. It passes chemically treated water through a mixing zone and then up through a sludge blanket, where the coarse media acts similar to a roughing filter. The floc particles attach themselves to the media and are settled and removed. The media is flushed on a regular basis using both air and water so that the solids can be removed to waste. The operating parameters for both processes are listed in Table 2. Table 2 Operating Parameters for Clari-DAF System and Contact Clarification Parameter Clari-DAF CC Flocculation - Design (minutes) 10 6* Flocculation - Peak (minutes) 7.5 5* - Design (gpm/ft²) Peak (gpm/ft²) 8 12 Recycle Rate (%) 11 N/A Saturation Pressure (psig) 80 N/A Filter - Design (gpm/ft²) 4 4 Filter - Peak (gpm/ft²) 5 5 * Detention time for upflow flocculation/clarification The Clari-DAF system solids were removed from the top of the unit by a skimmer at a rate of 1 ft/sec operating for 10 minutes and off for 50 minutes. The CC solids were removed from the unit by a wash cycle when the loss of head across the clarifier exceeded 8 ft. The wash cycle included 1 minute of air scour, 3 minutes of air scour/water (10 gpm/ft 2 ), 1 minute of media settling, and 7 minutes of solids to waste. 2

3 Both of the clarification processes were equipped with mixed media filters and granular activated carbon filters. The technical specifications are listed in Table 3. Table 3 Pilot Filter Media Specifications Parameter Filter 1 Filter 2 Description Mixed Media Granular Activated Carbon Media Type Anthracite Filtrasorb 8 x 30 Media Depth 18 in. 48 in. Effective Size mm mm Uniformity Coefficient Media Type Filter Sand Filter Sand Media Depth 9 in. 6 in. Effective Size mm mm Uniformity Coefficient <1.4 <1.4 Media Type Garnet N/A Media Depth 3 in. N/A Effective Size mm N/A Uniformity Coefficient <2.2 N/A Tests performed during both seasons indicated that potassium permanganate (KMnO 4 ), when fed prior to either the Clari-DAF system or CC clarification step, would oxidize both the iron and manganese to below the Secondary Standard. Iron levels were <0.1 mg/l and KMnO 4, when fed at 1.5 times theoretical for manganese, yielded values of 0.05 mg/l. pilot operation required a ph of at least 7.0, resulting in 0.03 mg/l of Mn in the effluent. In the pilot study, jar testing indicated that a polyaluminum chloride (PACl) coagulant was the product of choice and was used during both seasons, fed at a rate between 60 mg/l and 75 mg/l. The product that was fed had an aluminum content of 5.5%, a density of lb/gal, and a solution strength of 33.3%. In the winter, it was necessary to supplement the PACl with cationic polymer. In order to determine the ability of the processes to remove NOM, color and UV-254 were measured. Both measurements rely on the aromatic structure of the dissolved organic matter within the water for light absorption. Generally, the higher the absorption, the higher the NOM content of the water. The color removal data is listed in Table 4, while the UV-254 data is listed in Table 5. 3

4 Table 4 Color Removal Value % Removal Value % Removal Raw Clari-DAF Effluent CC Effluent Table 5 UV Removal Average % Reduction Clari-DAF CC The UV removal was slightly less for the CC system during higher loading rates, but showed no effects on the Clari-DAF system even with increased loading rates. NOM content can also be measured by total organic carbon (TOC) readings in potable drinking water. An independent lab measured the TOC concentration once for collected samples of raw water, clarified water, and filtered effluent water. The average percentage removals are presented in Table 6. Table 6 Total Organic Carbon Removal Average % Reduction Clari-DAF Clari-DAF Clari-DAF CC CC CC This data indicates that the Leopold Clari-DAF system performed better than CC, and that granular activated carbon filters performed better than the mixed media filters. The Clari- DAF system showed no reduction in TOC with increased loading rates, but the TOC removal in the CC system was significantly decreased with higher loading. 4

5 In order to measure the effect of the process units on the disinfection by-products (DBPs) that may react with the chlorine and NOM, both total trihalomethane (TTHM) and haloacetic acid (HAA5) were measured and listed in Table 7. Table 7 Disinfection By-Products TTHM (µg/l) HAA5 (µg/l) Clari-DAF Clari-DAF CC CC As expected, the GAC media filters were significantly more effective at removing DBP precursors than the mixed media filters. Turbidity removal data is listed in Table 8. Raw water turbidities were greater than 8 NTU during the winter season, and less than 1.0 NTU during the summer period. Table 8 Turbidity Removal Raw Water NTU = 8.5 Clarified % Removal Filtered % Removal Clari-DAF NTU NTU 99.5 Clari-DAF NTU NTU 99.5 CC NTU NTU 98.8 CC NTU NTU 97.6 Raw Water NTU = 0.71 Clarified % Removal Filtered % Removal Clari-DAF NTU NTU 95.6 Clari-DAF NTU NTU 95.9 CC NTU N/A NTU 94.8 CC NTU N/A NTU

6 Only the Clari-DAF system produced effluent turbidities less than 0.5 NTU during the winter season, but only 21% of the time. Clari-DAF system turbidities were less than 1 NTU for 71% of the time, while CC turbidities were <1 NTU only 1% of the time. In the summer, The Clari-DAF system achieved turbidities <0.5 NTU 99% of the time, while CC was only able to produce turbidities <0.5 NTU 17% of the time. In addition, Clari-DAF system turbidities were not affected by increased loading rates. Filter run times were calculated as either the time to reach terminal head loss (8 ft) or turbidity breakthrough (0.3 NTU). The average filter run times for each pilot unit is presented in Table 9. Table 9 Average Filter Run Times per Pilot Unit 4 gpm/ft 2 24 hr 12 hr 11 hr 7 hr 5 gpm/ft 2 16 hr 7 hr 6 hr 2 hr 4 gpm/ft 2 47 hr 25 hr 9 hr 5 hr* 5 gpm/ft 2 29 hr 12 hr 6 hr 9 hr* *Backwash due to turbidity break through, all others due to terminal head loss Filter run times were longer for Clari-DAF system effluent than for CC effluent. The data in previous sections shows the Clari-DAF system process as more effective in removing turbidity and particulate matter, resulting in lower solids loading to the filters and therefore longer filter runs. Water production is a ratio of total volume of treated water produced for distribution divided by the total volume of raw water processed. Each process consumes treated water for cleaning and backwash purposes. The net water yield for each treatment is listed in Table 10. Table 10 Net Water Yield 4 gpm/ft 2 98% 96.6% 92.2% 88.2% 5 gpm/ft % 94.8% 90.0% 84.7% 4 gpm/ft % 98.7% 87.0% 92.6% 5 gpm/ft % 97.0% 93.5% 96.1% 6

7 The data shows that the Clari-DAF system was the most efficient treatment process in terms of net water yield (water produced vs. water sent to waste). In general, the mixed media filters had higher net water yields than the GAC filters. The comparison of data included in this paper indicates the advantages of using the Leopold Clari-DAF system to the contact clarification process. The performance and overall operating cost dictate that the Clari-DAF system would be the best choice of clarification technology. 2006, All Rights Reserved. 7