Chapter 14 Natural Organic Matter Characterization and Treatability by Biological Activated Carbon Filtration Croton Reservoir Case Study 1
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C. M. Klevens , M. R. Collins , R. Negm , M. F. Farrar , G. P. Fulton , and R. Mastronardi 5
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Department of Civil Engineering, College of Engineering and Physical Sciences, Kingsbury Hall, and Department of Microbiology, College of Life Science and Agriculture, Rudman Hall, University of New Hampshire, Durham, NH 03824 Metcalf & Eddy of New York, Inc., 603 42nd Street, New York, NY 10165 Hazen & Sawyer, P.C., 730 Broadway, New York, NY 10003 New York City Department of Environmental Protection, Jerome Park Demonstration Plant, Bronx, NY 10468 2
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Raw waterfiltrationof the Croton Reservoir water by GAC (10 min EBCT) was compared to treatment by ozone/biological activated carbon (BAC) filtration (10 min and 5 min EBCT) which may precede the diatomaceous earth filters proposed for this water supply for New York City. Average results showed 44-57% reduction in trihalomethanes and haloacetic acids, 28-30% reduction in chlorine demand and 21-23% reduction in TOC after BAC treatment compared to removals of 14%, 8% and 14%, respectively, achieved by GACfiltrationalone. The removal of BDOC was strongly related to the removal of the 'fast' biodegradable fraction. Substrate in BAC treated effluents was comprised of more slowly biodegradable compounds than were present in the raw water. The changes in dissolved organic carbon characteristics from ozonation (i.e., to more hydrophilic, smaller, less reactive compounds) were confirmed using standard NOM fractionation techniques. In general, biomass levels were heterogeneously distributed with the highest levels at the filter surface. In 1971 the City of New York instituted a research program specifically directed to its Croton Reservoir supply, in anticipation of the eventual need to provide some treatment beyond chlorination for this watershed. Metcalf & Eddy of Ν. Y. and Hazen & Sawyer P.C. were engaged at that time, in joint venture, to conduct the corresponding investigations (Principe et al. 1994). Croton is the oldest and smallest 6
Corresponding author 0097-6156/96/0649-0211$20.00/0 © 1996 American Chemical Society In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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of three reservoir systems serving New York City; it was placed in service in 1852 and provides roughly 10% or 140-240 mgd water yield compared to 470-600 mgd (40%)fromthe Catskills system and 580-750 mgd (50%) from the Delaware system (Bunch and Kerr 1995). Construction of a new, low profile, 450 mgd filtration facility for treatment of Croton Reservoir source water has been proposed (Principe et al. 1994). Commitment to providing filtration of the Croton supply was formalized in a report in 1979 wherein the Consultants recommended ozone and diatamaceous earth (DE)filtrationto provide water quality comparable to rapid sandfiltration,at a lower cost (Principe et al. 1994). Operating experience with DEfiltrationshowed that only particulate matter was removed, and treated effluent contained essentially the same levels of dissolved organic carbon (DOC) as the raw water (Mastronardi et al. 1993). Since ozone at the low doses applied (0.5-1.5 mg/L or «0.5 mg0 /mg DOC) accomplished little mineralization of DOC, many of the large molecules of NOM were converted to more readily biodegradable dissolved organic carbon (BDOC). The potential for problems with regrowth in the water distribution system was of growing concern. Thus an additional recommendation resulting from the Demonstration Plant experience was to incorporate granular activated carbon (GAC) filter-adsorbers between ozone and DE, whereby additional DOC and BDOC could be removed, along with reductions of chlorine demand and disinfection by-product (DBP) formation. Biofilm development is promoted in the adsorber by the increased biodegradability of ozonated organics which, together with the GAC surface for bacterial growth, results in biological "activation" of the carbon. The combination of ozone followed by activated carbon is commonly referred to as biological activated carbon (BAC). Research questions to be addressed in this research program were to evaluate BAC treatment of NOM in ozonated Croton Reservoir water. Specific objectives for evaluation of the pilot treatment facility were to: • evaluate seasonal variability on raw water NOM characteristics including relative biodegradability, hydrophilic/hydrophobic content and apparent molecular weight (AMW) distributions. • analyze and evaluate treatment performance with respect to seasonal variability and removal of total and dissolved organic carbon, UV254 absorbance, chlorine demand, DBP formation with respect to THM and HAA byproducts, and BDOC stability, • evaluate NOM treatability by characterization of NOM fractions after GAC, ozone, and ozone/BAC, and to • quantify levels of biomass and bioactivity with depth in mature, steady state GAC and BAC biofilters. 3
Methodology Pilot Facility Description. The Consultants constructed a 38 L/min pilot facility at the Croton Reservoir Intake Gatehouse Facility which began continuous operation in Sept 1993. The pilot plant included initially two parallel trains: a control 'GAC adsorber alongside a preozonated or 'BAC adsorber, each maintained at a flowrate In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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of 9 L/min (7.3 m/h) to attain 10 min empty bed contact time (EBCT). In May 1994, a second BAC adsorber was installed to evaluate performance with a higher flowrate, 18 L/min (15 m/h) or 5 min EBCT (BAC5). The plant continued continuous operation until Feb 1995, except for regular shutdowns for backwashing and periodic repairs. The present monitoring program was initiated in Jan 1994 after the 10 min GAC and BAC filters had been exhausted with respect to adsorption, and biodégradation was the primary mode of treatment. However, the initial maturation period for the 5 min BACfilterwas included in this program, therefore, water quality parameters have been corrected to exclude the initial 30 days (May - June 1994) of operation of this column where organic carbon removals were higher due to adsorption. Raw water influent to the pilot facility was bled from the main Croton aqueduct intake prior to chlorination. A 1 kg/day air feed ozone generator (Griffin Technics, Lodi, NJ) was used to apply 1.5 ± 0.5 mg0 /L or roughly 0.5 mg0 /mg DOC. Ozone bubbles were dissipated through a disk diffuser at the bottom of a 5.5m χ 10cm diameter countercurrentflowcolumn. GAC media Type BPL 4x10 (Calgon Carbon Corp., Pittsburgh, PA), as selected by The Joint Venture, was packed to a bed height of 1.2 m in each of the three adsorbers. This media type was selected to allow high surface area (1100 m/g) with low headloss (< 1cm H 0 at 18 m/h, 3.7 mm mean particle diameter). Thefiltersbeds were housed in 30 cm diameter Schedule 40 PVC pipe of 5.5 m height, to allow for bed expansion of roughly 50% during backwash. Columns were equipped with side sampling ports spaced at 30 cm distances along the height of the bed for extracting water and media samples, with additional ports every 5 cm near the top. Water level above thefilterbeds was allowed to accumulate to approximately 1.2 m head before backwashing, but backwashing was conducted weekly whether or not terminal headloss had been reached. Sampling was usually conducted several days later, to allow reacclimation of the columns following this disturbance. Influent raw water was diverted for use as backwash, with waste solids returned to the reservoir. Somefilterrun lengths were cut short (1 day) due to problems with air binding (Farrar 1994). Excessive headloss caused by air bubbles entrenched within the filters was experienced when the temperature difference between the raw water and adsorbers was significant, as in the spring season. This temperature differential caused dissolved gases, i.e., oxygen, to come out of solution upon contacting the warm media. 3
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Sampling and Analysis Program. Sampling and analysis efforts were conducted between 5 Jan 94 and 6 Feb 95. Water and media samples were collected from the pilot plant for analysis of various parameters as summarized in Table I. Biweekly or monthly aqueous monitoring samples were collected from five sampling locations, the raw water, the GAC column effluent, the ozonated water, the BAC 10 effluent, and the BAC 5 effluent. Samples were shipped overnight in ice-packed coolers to the University of New Hampshire, where they were refrigerated at 4°C and analyzed for parameters including total and dissolved organic carbon (TOC/DOC). UV-absorbance at 254nm wavelenght (UV^), chlorine demand, total trihalomethanes - simulated distribution system test (TTHMSDS), and In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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Table I, Sampling and Analyses Summary Sampling WATER ANALYSIS Frequency
Preservation, Max. Holding
Method
Reference
Nonpurgeable Total Organic Carbon (TOC)
Biweekly
2 wks, H3P04 4°C
UV-promoted persulfate oxidation
Std Methods 1992
Dissolved Org. Carbon (DOC)
Seasonal
20 hours
0.7 urn GF/F Whatman
UV-absorbance @ 254nm
Seasonal
2 wks, 4°C
Spectroscopy
Chlorine Demand Simulated Distrib System
Monthly
2 wks, 4°C headspace free
Colori metric DPD Pillows
Trihalomethanes*
Monthly
2 wks, 4°C headspace free
Liquid-liquid Extraction GC-EDC
Haloacetic Acids (6)"
Seasonal
2 wks, 4°C headfree, NH C1
Method 552-GC
Biodegradable Organic Carbon (BDOC)
Monthly
2 wks, 4°C
Recirculating Sand Biofilters
Hydrophilic/Hydrophobic Separations on XAD8
Seasonal
2 wks, 4°C
Resin adsorption and ion exchange
Leenheer 1981
Apparent Molecular Weight
Seasonal
2 wks, 4°C
Ultrafiltration
Collins et al. 1986
Iron, Magnesium, Calcium, Manganese cations
Seasonal
1 month, 4°C HN0
ICP Emiss. Spectroscopy
Std Methods 1992
Id, 4°C
Pyrophosphate, pH7
Balkwill 1985
103°C dried 24 hr
Std Meth 1992
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Mogren ei 1990
GAC/BAC MEDIA Bacteria Extraction
Summer, Fall
Dry Weight Determination
Summer, Fall
Id, 4°C
Biomass
Summer, Fall Summer, Fall
Immediate
Protein Assay
Summer, Fall Summer, Fall
Immediate
Dodecyl Sulfate/NaOH Bicinchroninic Acid
Chesbro et al. 1994
DAPI Direct Counts
Summer, Fall
5d,4°C
Epifluorescent Microscopy
Std Meth 1992
HPC on R2A Agar
Summer, Fall
Immediate
5.10d Incubation, 15°C
Std Meth 1992
Phospholipid Analyses (University of Cincinnati)
Summer, Fall
d, 4°C
Attached organisms extract w/CHC 13/CH30H
Wang 1995
Activity - Incorporation of Phosphate in Lipids
Fall
Immediate
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Chesbro et al. Formaldehyde 1994 fixation/Vacuum Dessication
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Phosphate labeling extract
Chesbro et al. 1994
W/CHCI3/CH3OH a
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BDOC Removal Rate Comparisons with (a) DOC Removal Rate and (b) Fast BDOC Removal Rate
In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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Langlais et al. (1991) reviewed studies by Billen et al. (1989) where the more slowly hydrolyzable BOC was virtually unchanged through biofiltration treatment, reporting that much longer contact times (beyond 15 min) would be required to reduce this fraction. Proposed treatment by DE filtration after BAC for Croton is not likely to affect BDOC concentrations further (Spencer and Collins 1995). Another approach to emphasize the treatability differences between the fast and slow BOC fractions can be demonstrated using the relationships explored by Huck (Mitton et al. 1993, Huck et al. 1994). Parameter removal rates can be calculated from the influent and effluent concentrations for the biofilter and the EBCT as follows: •η χ (mg/L/h) / ft Removalι τ> Rate = Influent - Effluent Concentrations \(mg/L) & s EBCT(h)
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As shown in Figure 8a, the DOC removal rate calculated for the biofilters was correlated to the BDOC removal rate although there was significant scatter around the line-of-equality, especially for the BAC5 filter. A more distinct relationship developed between the fast BDOC fraction removal rate and the total BDOC removal rate as noted in Figure 8b. The excellent agreement between the regressed slope with the line-of-equality confirmed the importance of the fast BOC fraction as being the most susceptible to removal by biofiltration. Treatability of NOM Fractions. AMW distributions based on DOC through the various treatment units, averaged for the four seasons, are summarized in Figure 9a. Data for UV absorbance are not shown. Error bars depict the pooled analytical standard deviation for seasonal samples. Ozone/BAC treatment removed principally small (
