Estimation of Mercury Bioavailability in Wastewater Treatment Plant Effluents. Robert P. Mason J. David Dean

Transcription

1 Estimation of Mercury Bioavailability in Wastewater Treatment Plant Effluents Robert P. Mason J. David Dean Feb 5, 2009, SFEI Meeting, Oakland, CA ESTIMATION OF MERCURY BIOACCUMULATION POTENTIAL FROM WASTEWATER TREATMENT PLANTS IN RECEIVING WATERS (WERF Project 05-WEM-1CO) Co-Funded by BACWA ArcTellus Watershed Science & Simulation

2 Purpose and Background Assess the bioaccumulation potential of mercury in various sources and receiving waters Primary question: Is Hg from Waste Water Treatment Plants (WWTP s) more or less bioavailable than Hg from other sources?

3 Key Phase 1 Objectives Define bioavailability and identify enhancers/mitigators of bioavailability and profile various sources Develop and apply an effluent ranking procedure Perform a data/literature review of the effects of wastewater treatment on Hg removal/bioavailability Key Phase 2 Objectives Develop a screening procedure for evaluating effluent Hg bioavailability in various receiving water types Develop a detailed assessment procedure and guidance document Perform a study of Hg bioavailability in WWTP effluents

4 Bioavailability Defined Bioavailable mercury includes any inorganic Hg and Methyl Hg, which are readily transported across biological membranes, and subsequently accumulated, and/or biotransformed to a more toxic and bioaccumulative species. Potentially bioavailable mercury includes any form that can be converted into bioavailable mercury in a scientifically reasonable time span. All MeHg in water is considered to be bioavailable

5 Bioavailability Enhancers/Mitigators: Key Factors Total Hg concentration (+ve response) MeHg concentration and methylation potential TSS concentration Dissolved/solid phase partitioning Mercury speciation DOM (DOC) / sulfide interactions Biogeochemistry of receiving waters Dilution / mixing

6 Bioavailability Estimation: Fraction Bioavailable Hg (fb-hg) fb-hg = (MeHg + R-Hg)/T-Hg MeHg is total aqueous MeHg (unfiltered) R-Hg is reactive Hg (filtered) (assumed to be represent the bioavailable Hg(II) (IB-Hg)) T-Hg is the total mercury (unfiltered) Since R-Hg in effluents and receiving waters is rarely known, R-Hg is approximated by estimating inorganic bioavailable mercury, IB-Hg.

7 Bioavailability Estimation: Estimation of IB-Hg Important Reactions Hg 2+ + nx - = HgX n 2-n (X = Cl, OH) various Hg 2+ + HS - = HgS + H + logk HgS = 26.0 Hg RSH = Hg(RS) 2 + 2H + logk NOM = 22.1 Hg 2+ + HS - + RSH + H 2 O = S-Hg-RSH + H + logk com = 34.7 In the absence of sulfide, inorganic complexes and organic complexes dominate, but inorganic complexes are the only likely bioavailable (reactive) species. With sulfide present, given similar or higher concentrations of HS - than binding sites on NOM (RSH), direct binding of Hg to NOM is not important, and sulfide speciation dominates. The interaction of Hg, HS and RSH forms non-bioavailable Hg species.

8 Reactive (Bioavailable) Mercury Estimation: Concentration estimate based on dissolved Hg complexation by DOC (assuming low sulfide levels) Dissolved Hg(II) content can be estimated from measurement, or using K D values 100 Bioavailability in presence of sulfide %Bioavailable 10 ph 7 ph 6 ph 5.5 ph 5 % IB-Hg = * exp[-1.48 * DOC] DOC (mg/l) Miller et al. (07), Lamborg et al. (03, 04), Hsu-Kim and Sedlak (03; 05) and others

9 Bioavailability Estimation: Source Comparisons Water Type/Source Range of estimated bioavailable Hg fraction in the literature Atmospheric Deposition 0.2 to > 0.9 Mining Runoff < 0.1 Impacted Sediment Porewater 0.15 to 0.45 Unimpacted Sediment Porewater 0.2 to 0.45 Urban Runoff < 0.1 Non-urban Runoff 0.2 to 0.3 Wastewater Treatment Plant Effluent < 0.2 (literature) , Av 0.21 (this study)

10 Screening Procedure: Assessing Net Hg Methylation Used to estimate the impact of mixing effluent with receiving waters when there is insufficient detailed data Developed from USGS database on >100 US rivers for fresh waters Developed from data on six estuaries and ocean waters Developed a relationship between IB-Hg and MeHg and rate constants for the net processes (includes production and transport) of methylation & demethylation

11 Relationship Between Calculated Ratio of Methylmercury to Bioavailable Inorganic Mercury and Estimated Net Methylmercury Production Constant IB-Hg determined using the relationship with DOC Demethylation rate in water a function of DOC (color) and water depth 10 1 [MeHg]/[Hg IB ] Fresh LIS Hudson Chesap SFBay k meth (per day)

12 Screening Procedure: Assessing Net Hg Methylation At steady-state k meth / k demeth = [MeHg]/[IB-Hg] % MeHg <2% 2 to 3% 3 to 5% 5 to 10% >10% k meth /k demeth DOC ph < 2 mg/l 2 to 7 mg/l > 7 mg/l < to > For any water body: k demeth in water column is determined by equations related to DOC and water depth. If MeHg content known, then the overall k meth can be estimated if total Hg, DOC, TSS etc are known

13 Screening Assessment: Key Assumptions Reactive Hg is equivalent to inorg. bioavailable Hg (IB-Hg). Total bioavailable mercury in mixtures can be derived using the flow ratio of effluent to receiving water (conservative mixing). No other changes occurs on mixing. There are well-defined relationships between DOC and R- Hg in effluents and receiving waters. On mixing, relative amounts of MeHg and IB-Hg change depending upon the methylating potential of the receiving water compared to the source/effluent. The ratio MeHg/IB-Hg at steady-state is essentially equal to the ratio of the methylation and demethylation constants. Estimates of bioavailable Hg (MeHg and R-Hg or IB-Hg) in water are good indicators of bioaccumulation potential.

14 Screening Assessment: Bioavailability Tool Inputs FRESHWATER BIOAVAILABILITY COMPUTATIONS Evaluate Bioavailability of a Single Effluent Effluent #1 Effluent #2 Mix Effluent #1 With Receiving Water Inputs Effluent #1 Inputs Receiving Water T-Hg (ng/l) = 2.4 T-Hg (ng/l) = 1.57 MeHg (ng/l) = MeHg (ng/l) = R-Hg (ng/l) = R-Hg (ng/l) = DOC (mg/l) = 10.7 Optional DOC (mg/l) = 14 Optional TSS (mg/l) = 4 Optional TSS (mg/l) = 7.3 Optional A log K D = 5.32 Optional log K D = 5.1 Optional ph 7 Optional Flow rate or Volume (Effluent #1) Flow rate or Volume (million gallons or MGD) Optional Flow Rate or Volume (Receiving Water) %MeHg = 1.50 Mixing Ratio (Fraction of effluent to receiving water) = Dissolved Inorganic Hg (ng/l) = Water Depth (m) = 2 Dissolved Reactive Hg (%) = ph = 7.6 Optional C Outputs Reactive Hg (% of Total) = Far Field Flag, 0 - Use MeHg/T-Hg, 1 - Use ph, DOC, 2 Use MeHg/R-Hg 1 Bioavailable Inorganic Hg (% of Bioavailable Hg) = Outputs Near Field Far Field Methylmercury (% of Bioavailable Hg) = T-Hg concentration (ng/l) = 1.99 Bioavailable Hg (%) = MeHg concentration (ng/l) = Bioavailable Hg Concentration (ng/l) = 0.33 MeHg % = Bioavailable Hg Flux (ug or ug/day) DOC concentation (mg/l) = Compare Bioavailability of Two Effluents ph = 7.20 Inputs Effluent #2 Dissolved Inorganic Hg of Receiving Water (ng/l) = 0.82 B T-Hg (ng/l) = Dissolved Reactive Hg of Receiving Water (ng/l) = 0.02 MeHg (ng/l) = Mixed Reactive Hg (ng/l) = R-Hg (ng/l) = Reactive Hg (% of Total) = DOC (mg/l) = Optional Total Bioavailable Hg (ng/l) = 0.34 TSS (mg/l) = Optional Bioavailable Inorganic Hg (% of Bioavailable Hg) = log KD = Optional Methylmercury (% of Biaovailable Hg) = Flow rate or Volume (million gallons or MGD) Optional WWTP Bioavailable Hg Mass Flux (% of Total Flux) 49.2

15 Bioavailability Tool: Example 1, Compare Bioavailability of Two Effluents Evaluate Bioavailability of a Single Effluent Effluent #1 Effluent #2 Inputs Effluent #1 T-Hg (ng/l) = 3.83 MeHg (ng/l) = 0.23 R-Hg (ng/l) = Outputs DOC (mg/l) = TSS (mg/l) = log K D = Optional Optional ph Optional Flow rate or Volume (million gallons or MGD) Optional %MeHg = Dissolved Inorganic Hg (ng/l) = Dissolved Reactive Hg (%) = Reactive Hg (% of Total) = Bioavailable Inorganic Hg (% of Bioavailable Hg) = Methylmercury (% of Bioavailable Hg) = Bioavailable Hg (%) = Bioavailable Hg Concentration (ng/l) = Bioavailable Hg Flux (ug or ug/day) Compare Bioavailability of Two Effluents Inputs Effluent #2 T-Hg (ng/l) = 13.4 MeHg (ng/l) = R-Hg (ng/l) = DOC (mg/l) = TSS (mg/l) = log KD = Flow rate or Volume (million gallons or MGD) Optional Optional Optional

16 Outputs Bioavailability Tool: Example 2, Compare Bioavailability of an Effluent Mixed with a Receiving Water Lacking R-Hg Data FRESHWATER BIOAVAILABILITY COMPUTATIONS Evaluate Bioavailability of a Single Effluent Effluent #1 Effluent #2 Mix Effluent #1 With Receiving Water Inputs Effluent #1 Inputs Receiving Water T-Hg (ng/l) = 2.4 T-Hg (ng/l) = 1.57 MeHg (ng/l) = MeHg (ng/l) = R-Hg (ng/l) = R-Hg (ng/l) = DOC (mg/l) = 10.7 DOC (mg/l) = 14 TSS (mg/l) = 4 Optional TSS (mg/l) = 7.3 Optional log K D = 5.32 Optional log K D = 5.1 Optional ph 7 Optional Flow rate or Volume (Effluent #1) Flow rate or Volume (million gallons or MGD) Optional Flow Rate or Volume (Receiving Water) %MeHg = 1.50 Mixing Ratio (Fraction of effluent to receiving water) = Dissolved Inorganic Hg (ng/l) = Water Depth (m) = 2 Dissolved Reactive Hg (%) = ph = 7.6 Optional Reactive Hg (% of Total) = Far Field Flag, 0 - Use MeHg/T-Hg, 1 - Use ph, DOC, 2 Use MeHg/R-Hg 1 Bioavailable Inorganic Hg (% of Bioavailable Hg) = Outputs Near Field Far Field Methylmercury (% of Bioavailable Hg) = T-Hg concentration (ng/l) = 1.99 Bioavailable Hg (%) = MeHg concentration (ng/l) = Bioavailable Hg Concentration (ng/l) = 0.33 MeHg % = Bioavailable Hg Flux (ug or ug/day) DOC concentation (mg/l) = Compare Bioavailability of Two Effluents ph = 7.20 Inputs Effluent #2 Dissolved Inorganic Hg of Receiving Water (ng/l) = 0.82 T-Hg (ng/l) = Dissolved Reactive Hg of Receiving Water (ng/l) = 0.02 MeHg (ng/l) = Mixed Reactive Hg (ng/l) = R-Hg (ng/l) = Reactive Hg (% of Total) = DOC (mg/l) = Total Bioavailable Hg (ng/l) = 0.34 TSS (mg/l) = Optional Bioavailable Inorganic Hg (% of Bioavailable Hg) = log KD = Optional Methylmercury (% of Biaovailable Hg) = Flow rate or Volume (million gallons or MGD) Optional WWTP Bioavailable Hg Mass Flux (% of Total Flux) 49.2

17 Detailed Bioavailability Assessment An entire chapter section (70 pages) is devoted to this topic Should be used if a more definitive answer is required or if any of the key assumptions are questioned Information on what type, where, when, and how many samples to collect to achieve desired certainty in results

18 WWTP Reactive Hg Study Sampled 7 WWTPs from around the US Weekly for 10 weeks, late June through August, 2008 Mercury species measured: Unfiltered total Hg Filtered total Hg Methyl Hg Reactive Hg Ancillary parameters (DOC, ph, sulfate, Cl, TSS) Analytical work by Frontier Geosciences, Inc. and Test America, Dr. Amy Dahl, FG Project Manager

19 Dissolved Hg Fraction and Relationship Among Bioavailable Mercury Species in Seven Wastewater Effluents A B C D E F G % Dissolved %MeHg was low for most WWTPs The %RHg was higher than the %MeHg in nearly all cases BHg ranged from 5-35% Diss Hg was a relatively high fraction of the total Hg (compared to typical receiving waters) A B C D E F G Facility %MeHg %R-Hg %B-Hg

20 Relationship Between Cl and %R-Hg %R-Hg y = ln(x) R² = Indicative of the influence of chloride on Hg complexation Chloride mg/l Relationship for DO and %MeHg %MeHg y = x R² = DO mg/l

21 Relationship between DOC and Reactive Hg as a Percent of Dissolved Mercury %(R-Hg/D-Hg) DOC mg/l DOC v %(R-Hg/D-Hg) Theoretical DOC Function WWTP DOC Function The relationship indicates that WWTP DOC binds Hg less strongly than freshwater organic matter

22 Calculated Binding Constants for Hg to Effluent Organic Matter for Each Facility Assuming # binding sites/mol DOC found by Lamborg et al. (04) Facility f(r-hg) / Diss. Hg DOC (mg/l) logk A B C D E F G Values across the various facilities is relatively constant

23 Reactive Hg Study Conclusions Average bioavailable mercury in seven advanced WWTP effluents was 21% of total mercury, among the lowest of several sources investigated. A relatively small fraction of this was MeHg Given Hg levels in suspended sediments in these WWTP effluents (average 0.46 mg/kg), and low TSS (average 5.1 mg/l) discharged, sediment impacts due to deposition should be minimal Reactive Hg in WWTP effluents is a function of DOC levels but was higher than anticipated based on computations for natural waters This approach is reasonable for examining the issue of bioavailability of mercury in complex environmental situations

24 Further Conclusions From the Reactive Hg Study (seven plants studied over ten weeks): Average T-Hg in effluent ranged from 1.78 ng/l to 13.3 ng/l, overall mean was 5.3 ng/l Five WWTPs had average effluent T-Hg below 5 ng/l Two WWTPs had average effluent T-Hg below 2 ng/l Treatment train of the plant with the lowest effluent Hg consists of screening and grit removal, primary sedimentation, [activated sludge] secondary treatment with biological nutrient removal, secondary clarification, filtration, disinfection, and dechlorination

25 Key Project Findings Compared to non-point sources of mercury, WWTPs are among the lowest in terms of Hg bioavailability (average 21%, typically < 30%); therefore, there should be no special concern for WWTPs versus other sources WWTPs are very effective at removing mercury from wastewater (typically > 95% removal) On average, Reactive Hg in WWTP effluent (15.7%) appears to be typically higher than %MeHg (5.1%).

26 Key Project Findings Advanced WWTP effluent also appears to be higher in %Reactive Hg than natural waters; therefore, in stronger methylating receiving water environments, net MeHg may ultimately be produced upon mixing. Many receiving waters are likely to have higher DOC concentrations, however, and there may be a decrease in %Reactive Hg upon mixing. The actual outcome will be dependent on the specifics of the particular environment.

27 Project Accomplishments Developed a working definition of bioavailable Hg Profiled and ranked WWTPs and other sources wrt bioavailable Hg Developed a Bioavailability Tool which implements a screening procedure for estimating bioavailable Hg and net methylation in fresh, estuarine and marine waters Developed detailed guidance for studying bioavailable Hg in effluents and receiving waters Conducted a reactive Hg study in 7 advanced WWTP effluents Status Phase I and Phase II reports completed Publication early 2009

28 Special Thanks Dr. Amy Dahl, Frontier Geosciences, Inc. Dr. Helen Hsu-Kim, Duke University Patricia McGovern, Patricia McGovern Engineers Tim Tuominen, Western Lake Superior Sanitation District Beverly Van Buuren, Jennifer Parker, Eric Von Der Geest, Van Buuren Consulting Dr. James Wiener, University of Wisconsin, LaCrosse

29 Additional Questions? Dr. Robert Mason Co-PI; (860) Mr. David Dean Co-PI; (864) Jane M. Casteline Program Manager; (703) ArcTellus Watershed Science & Simulation