Evolution of Leachate Composition In a Calgary Landfill. Sean Buckles, M.Sc., P.Eng.

Transcription

1 Evolution of Leachate Composition In a Calgary Landfill Sean Buckles, M.Sc., P.Eng.

2 Background Landfill Construction and Operation Leachate Generation Typical Characteristics 2

3 Landfill Construction and Operation A well engineered landfill has containment systems and an engineered cover. Leachate (liquids in contact with waste, or generated from the decomposition of waste) is generated from the onset of waste placement. Operational practices (e.g.. exclusion of wet waste, surface water management) help to minimize leachate generation. Potential for exfiltration or leakage to the groundwater: Concerns for local water users and aquatic receptors. Typically greatest in non-engineered cells. 3

4 Leachate Generation Stages of waste decomposition commonly described in terms of biological breakdown of waste. Predictable phases in evolution, from aerobic to anaerobic. 4

5 Leachate Generation Range of composition and toxicity, depending on: Nature of waste (content and particle size). Moisture content and/or amount of precipitation. Temperature. Type of cover and composition. Time. Landfill design and operation (including degree of compaction). Decompositional stage of landfill/waste. Four common groups of pollutants (after Christensen 2001): Dissolved organic matter (COD/TOC, VFAs, methane, etc.). Inorganic macrocomponents (major ions). Heavy Metals. Xenobiotic organic compounds (aromatics, phenols, chlorinated aliphatics, etc). 5

6 Typical Leachate Characteristics NH4 ph BOD COD Cl Idealized Leachate Characteristics 6

7 The Project The Study Data Sets Data Evaluation 7

8 The Study Study was a self-directed Project, part of a part-time, distance-m.sc. Program with the University of Waterloo. Desktop study - analysis of a database of leachate chemistry data covering up to several decades. This study: Evaluated variations in leachate composition at a selected cross-section of waste cells at a landfill in Calgary. Focused the evaluation using a simplified set of characteristic parameters. Looked for evidence of decompositional phases seen in typical landfill leachates. 8

9 Data Sets Database of leachate chemistry with data up to 20 years old. Applied 3 levels of screening to the data: 1. Limited the number of landfill sites to one (an active facility with both active and inactive landfill cells). 2. Limited the number of sampling locations to 9 (leachate monitoring wells), with 2 additional manholes for comparison. Represented waste deposited over various eras, Limited the chemical parameters to 17: Indicators (ph, TDS, COD, BOD). Nitrogen parameters (TKN, NH 4, NO 3, NO 2 ). Cations (Na, K, Ca, Mg). Anions (Alkalinity, Cl, SO 4 ). Metals (Fe, Mn). Outlier analysis used to further screen data. 9

10 Data Evaluation Undertaken to evaluate how the characteristics at this landfill relate to what might be expected in an idealized leachate profile. Univariate analysis: Individual parameters over time. Ratios of parameters over time (e.g.. BOD:COD). Multivariate analysis using parameter groupings: Common leachate indicators. Ionic chemistry. Simplified redox chemistry. Focus on relative proportions of parameters, rather than absolute values. Evaluated parameters relative to time and each other (scatter plots). 10

11 The Outcome Leachate Types Redox Characteristics Component Relationships Comparison Against Predicted 11

12 Leachate Types Type groupings selected to correspond to stages in leachate evolution. Data typically related to an individual stage, or transition between stages. Type 1: Strong and acidic; early Stage 3 composition (anaerobic/initial methanogenic). Iron and sulphate dominant redox species. Little variation over time. Type 1 Type 3 Type 4 Type 2: Low strength; erratic, neutral to basic ph. Sulphate dominant, little to no ammonium. No obvious relation to predicted characteristics (possibly due to low strength). Type 3: Medium to high strength, decreasing over time and with rising ph. Iron and sulphate dominant, transitioning to ammonium dominant over time. Transitioning between Stage III (initial methanogenic) to Stage IV (stable methanogenic) over time. Type 4: Low strength; neutral ph and redox dominated by ammonium. Early Stage IV (stable methanogenic). 12

13 Leachate Characteristics - Indicators Indicators: TDS NH4 COD COD Cl Alk ph, TDS, BOD, COD NH 4 Na, Cl, Alkalinity Fe Assigned strength based on TDS: Low (<10,000mg/L) Med (10-20,000mg/L) High (>20,000 mg/l) 6 yr to 16 yr timelines 13

14 Leachate Characteristics Ionic Composition Ca Alk NH4 Cl SO4 Mg Major Ions: NH 4+, Na +, K +, Ca 2+, Mg 2+ NO 3- +NO 2-, Cl -, SO 4 2-, Alkalinity Chloride not a dominant player in these leachates (except well C1). Higher strength leachates more frequently out of balance. 14

15 Leachate Characteristics Simplified Redox Fe SO4 NH4 Simplified groupings from oxidized to reducing: NO 3- +NO 2 - Fe, Mn (not speciated) SO 4 2- NH 4 + Denitrification and Mn-reduction typically not evident. Fe, SO 4 and NH 4 play greatest role. 15

16 Leachate Characteristics NH 4 :SO 4 and COD:NH 4 Ratios NH 4 :SO 4 higher ratio indicating progressively reducing conditions. COD:NH 4 to indicate relative waste decomposition. 16

17 Component Relationships BOD:COD 17

18 Component Relationships - ph 18

19 Component Relationships TDS:BOD and Nitrogen Compound Ratios Multivariate analysis using scatter plots, excluding time as a ratio. TDS:BOD both common leachate indicator parameters. NO 3 +NO 2 to NH 4 to evaluate oxidized vs reduced forms of N. 19

20 Predicted Redox Characteristics Staged leachate breakdown plot simplified to area plot of basic redox parameters. Mn SO4 Fe NH4 20

21 Predicted Redox Characteristics SO4 Fe NH4 Fe SO4 NH4 21

22 Discussion and Conclusions 22

23 Discussion and Conclusions Study predicted that leachate would match a typical composition based on the phased decomposition of waste described in the literature. Most could be correlated: Leachates generally suggestive of wastes in relatively early stages of anaerobic decomposition; mostly initial methanogenic (Type 3), with some transitioned to stable methanogenic (Type 4). Several samples at very early stages of anaerobic decomposition (Type 1) and remained acidic. Notable since waste cells sampled all more than 20 years old, some more than 35 yrs. Samples most progressed along phases (Type 4) were from the oldest era cells, which also had, in general, the thinnest waste depths and greatest leachate thicknesses. Strongest, most acidic leachates from year old wastes, only in early stages of anaerobic degradation. 23

24 Discussion and Conclusions (continued) Overall parameter set considered useful and sufficient. Dissolved gaseous species (e.g.. H 2 S and CH 4 ) would be useful indicator compounds however typically not included in routine leachate analysis. Chloride often used as a conservative tracer in groundwater plumes, and commonly associated with MSW leachates Cl generally correlated well with leachate strength, but did not show strong trends; would be useful in evaluating potential dilution at a given location. 24

25 Discussion and Conclusions (continued) Results may be affected by: Mixing of leachates in wells (including long screen intervals). Fluctuating leachate levels. Areas with greater infiltration through the cap (e.g.. introduction of oxic waters). Mobilization of compounds from the waste under reducing conditions. Reactions between compounds in the leachate. All expected to play a role in the evolution of the Calgary leachates. May also result in overlapping redox zones where processes may be simultaneous. Waste observations often relatively little degradation in shallower wastes with wetter, more decomposed waste at depth. 25

26 Acknowledgements Jim Barker, University of Waterloo Gwen O Sullivan, Mount Royal University Tetra Tech Canada Inc. The City of Calgary, Waste & Recycling Services 26