117 © IWA Publishing 2012 Water Quality Research Journal of Canada | 47.2 | 2012
Treatment of wet peat mining process water with acrotelm peat Susanne E. Walford, Peter F. Lee and Robert W. Mackereth
ABSTRACT
Mechanical dewatering of wet mined peat produced peat mining process water (PMPW) with low pH Susanne E. Walford (corresponding author) Peter F. Lee (5.49) and high total suspended solids (TSS 432 mg L 1), Al (1.39 mg L 1), Fe (4.36 mg L 1), Hg Department of Biology, Lakehead University, 1 1 1 1 (37.1 ng L ), MeHg (0.485 ng L ), Zn (55 μgL ), total nitrogen (TN 7.92 mg L ), total phosphorus Thunder Bay, μ 1 fi ON P7B 5E1 Canada (TP 303 gL ) and true colour (532 TCU). High removal ef ciencies were calculated for acrotelm peat E-mail: [email protected] fi – – lters (mesocosms) used to treat PMPW (TSS 45 83%; particulate organic carbon (POC) 47 89%; Robert W. Mackereth metals 53–100%; TN 85%; TP 81%), though final mesocosm leachate concentrations did not all Centre for Northern Forest Ecosystem Research, Ministry of Natural Resources, achieve Canadian Water Quality Guidelines. In one study, mesocosm leachate had higher colour and Lakehead University, Canada dissolved organic carbon (DOC) concentrations than PMPW. Our mesocosm studies suggest an initial reduction in PMPW solids be considered by industry before peatlands are used as treatment systems. Key words | acrotelm mesocosms, biofuels, peat mining, peat process waters, wastewater treatment, water quality
INTRODUCTION
Canada possesses approximately 30% of the world’s total peat and an extraction of peat from sites with coarse woody reserve, second only to Russia (NWWG ). Though Cana- debris and sites not amenable to drainage (Monenco ). dian peatlands contain an estimated 153 Gt of carbon Environmental impacts for wet peat mining in Ontario have (Tarnocai ), its use has been limited to horticultural pro- been extrapolated from dry harvesting research because of a ducts rather than for energy (Daigle & Gautreau-Daigle lack of current knowledge (Gleeson et al. ). ). Current energy needs could alter peat use in Canada One wet mining environmental concern is the fate of at any time, particularly in northern and isolated commu- wastewaters produced from dewatering peat. Solids, nutri- nities with abundant peat resources. Of recent interest are ents, metals, colour and pH in peat mining process water the peat deposits surrounding the ‘Ring of Fire’ chromite min- (PMPW) summarized by Monenco () would not meet eral deposit in northwestern Ontario (Sylvester ). current Canadian Water Quality Guidelines (CWQG). A Canadian company (Peat Resources Ltd) has devel- Industry has proposed treatment of PMPW by distributing oped a proprietary technique to wet mine and pelletize peat it onto intact peatlands adjacent to mining, where filtration for use as a combustible biofuel. Pellets from processing wet through acrotelm peat may improve PMPW quality. Such peat may be utilized as local energy, being amenable to com- on-site treatment would maintain the portability of current bustion in both small-scale generators (Obernberger ) wet mining technology, thus reducing transportation of and large thermal plants (OME ). Though wet mining wet peat, process waters and/or pellets. The proposed treat- peat for energy occurred in the former USSR for over 40 ment appeared plausible since previous studies highlight the years, the practice is quite uncommon (Tibbetts ). Com- efficiency and benefits of peat-based systems to remove pared with standard methods of dry harvesting peatlands, chemicals of concern from a variety of industrial and wet mining advantages include a longer processing season municipal waste streams (Viraraghavan ; Couillard
doi: 10.2166/wqrjc.2012.029
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, ; Bhatnagar & Minocha ) including runoff from peat production areas in Finland (Ihme et al. a, b; Heikkinen et al. ). To address knowledge gaps concerning water quality (pH, alkalinity, conductivity, metals (including methyl mer- cury, MeHg), nutrients, solids and organics) of PMPW produced from northwestern Ontario peat, experimental wet peat mining was conducted. The fen selected was ident- ified as possessing high value energy peat (DST ) and situated to meet regional energy needs (OME ). Acrotelm peat mesocosms were constructed to filter PMPW produced from the wet mined peat. The main objectives were to deter- mine whether acrotelm peat would significantly remove analytes of concern from dilutions of PMPW (treatments) compared with controls and to calculate the efficiency of acrotelm peat to remove analytes from PMPW.
Figure 1 | Schematic of a peat mesocosm. MATERIALS AND METHODS Initial chemical analysis of mesocosm leachate found fi Mesocosm construction signi cant differences (analysis of variance (ANOVA), p 0.05) among mesocosm groups. Because an initial difference fi During the summer of 2008, hummock peat cores were cut by would affect the nal interpretation of data, an equilibrium handsaw to precisely fit 25 L plastic buckets (the mesocosms) period (approx. 60 d) consisting of dilution water appli- from an undisturbed area of the fen near the wet mined site cations and drainage of leachate via the drainage valves (40W 57033″ N; 90W 6020″ W). Careful handling prevented was required. Mesocosm DWT before and during studies peat compaction and destruction of vegetation. Hummock was maintained (Table 1). vegetation was typical for northwestern Ontario open poor fens, consisting of mainly Sphagnum sp. (e.g. S. fuscum, EXPERIMENTAL DESIGN S. magellanicum) interspersed with sedges and various eri- caceous plants. Mean bulk density of acrotelm peat from Overview this 1,080 ha fen was 0.09 ± 0.04 g cm 1 (n ¼ 263; DST ). Each mesocosm (Figure 1) was fitted with a vertical piezometer. The piezometer had inlet holes over a 15 cm First, PMPW was extracted from wet mined peat and its length, wrapped in 500 μm Nitex. Inlets were nearest the water quality (pH, alkalinity, conductivity, redox, metals base of the bucket. The piezometers were later fitted with Table 1 | Treatment group depth to water table (DWT) for Study 1 treatment group meso- cosms (mean ± SD, n ¼ 43) over pre-treatment to post-treatment time period. bottom valves protruding through the base of the bucket Dilution water used to dilute peat mining process water (PMPW) to specified to facilitate sampling of mesocosm leachate. A sufficient percentages number of mesocosms were constructed to provide replica- Treatment group DWT (cm) tion in both studies. Mesocosms were housed in a T0 (0% PMPW) 20.5 ± 3.4 greenhouse and watered as required with dilution water to T1 (100% PMPW) 20.4 ± 3.0 prevent desiccation. Depth to water table (DWT) was T2 (50% PMPW) 20.9 ± 2.5 measured via the piezometer and defined as top of peat sur- T3 (33% PMPW) 19.2 ± 3.1 face to top of water surface.
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(including MeHg), nutrients, solids and organics) compared cubic metre of acrotelm (hummock) peat. This was a best with current CWQG. Then, PMPW was applied to meso- estimate of what may occur during an actual mining oper- cosms as pulse impacts in two studies. Pulse impacts were ation, yet remained within the physical constraint imposed chosen as a best approximation to what would occur by mesocosm volume capacity. under ‘real world’ conditions (personal correspondence, The procedure on days that pulse impacts were applied Peat Resources Ltd). In Study 1, pulses of diluted PMPW to mesocosm treatment groups (days 1, 2, 3, 4, 7, 8, 9, 10, 11) (treatments) were applied to mesocosms to determine was as follows. First, DWT and peat height were measured. whether mean concentrations of analytes in mesocosm lea- Then, 4 L of diluted PMPW or control water was applied to chate would be significantly different from controls after two each mesocosm surface with a watering can (the pulse). weeks of exposure (ANOVA, p 0.05). Mesocosm leachate Finally, sufficient mesocosm leachate was drained within concentrations were also compared with CWQG. Solids an hour of application to restore DWT (approx. 20–30 cm were qualitatively observed in mesocosm leachate after below peat surface). Drainage of leachate was assumed to each pulse was applied. Therefore, Study 2 was conducted mimic natural water table movement and was similar to to quantify the relative amount of solids and organics eluting changes in water tables observed by Heikurainen et al. after each pulse of 100% PMPW was applied to a mesocosm () for laboratory additions to woody sedge-Sphagnum in rapid succession. Removal efficiencies of analytes were profiles. calculated for both studies. Specific details for each step Mesocosm leachate was sampled for chemical analysis follow. only on day 0 (pre-exposure) and day 14 (post-exposure) owing to the cost of screening numerous analytes in this Process water extraction initial study. Therefore, leachate water quality on day 14 reflects the ability of the mesocosms to retain analytes of Catotelm peat was wet mined from the site in spring 2008 interest in only the final application of PMPW, but after pro- with a backhoe excavator. Wet peat was mechanically dewa- longed exposure to PMPW (40 L over 14 days). Analytes tered hydraulically with a custom squeezer (30 cm × 18 cm measured were pH, redox, conductivity, alkalinity, solids area, 1.2 mm screen) producing a 900 L batch of PMPW (TSS, POC), dissolved organics (DOC, colour), anions, that was stored protected from light. Pond water pumps cations, metals (and MeHg) and nutrients (TN, TP, were used to homogenize PMPW when aliquots were ammonia). required. Chemical analysis of PMPW occurred during active dewatering and during both mesocosm studies. Mesocosm Study 2
Mesocosm Study 1 Study 2 was much shorter (<3 h) and quantified the relative amount of solids and organics eluting in mesocosm leachate Study 1 was a 14 day study to determine whether groups of as successive pulses of 100% PMPW were applied. Three mesocosms receiving different concentrations of PMPW replicates were conducted on separate days. Each batch of would differ in leachate water quality. Mesocosms (n ¼ 6) 100% PMPW used for each replicate was analysed. After were randomly assigned to four treatment groups: T0 con- chemically stable leachate concentrations were attained trol (100% dilution water), T1 100% PMPW, T2 50% with dilution water additions, 100% PMPW was applied. PMPW (50% dilution water) and T3 33% PMPW (67% The total volume of 100% PMPW used was 40 L. dilution water). Dilutions of PMPW were prepared by For each replicate, mesocosm leachate was first sampled mixing appropriate volumes of PMPW and dilution water (I). Next, three 2-L pulses of dilution water were applied to in batches as required. Treatment groups received 4 L the mesocosm surface followed by immediate leachate pulse impacts of treatment water. sampling after each pulse (D). Finally, 20 2-L pulses of Pulse impacts were applied on 9 days over a 14 day 100% PMPW were applied with immediate leachate period. Each pulse was equivalent to 138 L applied per sampling after each pulse (P). On each leachate sample,
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total solids (TS), total suspended solids (TSS), total dissolved analytes (Table 2). Dilution water was dechlorinated Lake solids (TDS), dissolved organic carbon (DOC), particulate Superior municipal water (Table 2) and was used to main- organic carbon (POC) and colour were measured. Metals tain mesocosm DWT before studies and dilute PMPW for and nutrients were determined on initial leachate samples Study 1 treatments. of the third replicate, but trends for those analytes appeared to be similar to Study 1 and were not included. Sampling and analytical procedures
Site reference waters and dilution water Mesocosm leachate was sampled from bottom drains fitted to piezometers (Figure 1). Analytical chemistry was con- Water chemistry from two reference sites (surface water and ducted at the Lakehead University Environmental peatland porewater) within 300 m of the mining operation Laboratory (LUEL), with accreditation (pH, alkalinity, con- was available from a coincident research project. These ductivity, total nitrogen (TN), total phosphorus (TP), TSS), data provided ambient background concentrations of and demonstrated proficiency (anions, cations, metals,
Table 2 | Analytes in reference waters (2008) and dilution water (mean ± SD)
Analyte (units) Surface watera Peat porewaterb Dilution waterc
pH 6.04 ± 0.32 5.76 ± 0.05 7.45 1 Alkalinity (mg L as CaCO3) 12.5 ± 9.97 18.1 ± 2.1 45.3 Conductivity (μScm 1) 30.7 ± 18.9 41.5 ± 4.6 108 TSS (mg L 1) 2.1 ± 1.9 4.6 ± 2.7 <2.0 True colour (TCU) 119 ± 39.9 138 ± 27.7 1.0 DOC (mg L 1) 13.2 ± 5.1 12.1 ± 4.9 2.2 POC (mg L 1) 11.4 ± 5.1 14.8 ± 5.2 <1.0 Redox (mV) 225 ± 45 159 ± 53.1 682 Reduced Fe (mg L 1) 0.731 ± 0.573 2.18 ± 0.52 <0.5 Chloride (mg L 1) 0.23 ± 0.20 <0.05 3.42 1 Sulfate (mg L as SO4) 0.13 ± 0.18 <0.05 3.36 Al (μgL 1)49± 19 39 ± 10 8 Ba (μgL 1)6± 47± 110 Ca (mg L 1) 3.98 ± 2.76 4.66 ± 0.69 14.2 Fe (mg L 1) 0.851 ± 0.812 2.72 ± 0.57 0.003 Hg (ng L 1) 2.49 ± 0.83 2.33 ± 1.88 <0.50 MeHg (ng L 1as Hg) 0.083 ± 0.035 0.065 ± 0.075 <0.030 K (mg L 1) 0.12 ± 0.11 <0.10 0.56 Mg (mg L 1) 1.35 ± 0.86 1.55 ± 0.21 2.84 Mn (μgL 1)37± 49 97 ± 17 <1 Na (mg L 1) 0.06 ± 0.24 0.70 ± 0.06 3.26 S (mg L 1) 0.31 ± 0.80 0.33 ± 0.49 1.32 Zn (mg L 1)38± 14 33 ± 10 32 TN (μgL 1) 0.454 ± 0.174 0.668 ± 0.053 0.168 TP (μgL 1)9± 912± 16 46
aSampled from a pre-existing drainage ditch (ca. 1940s) receiving upland flow from study fen (n ¼ 40 except redox, reduced Fe, Hg, MeHg n ¼ 27). bSampled via piezometer 50 cm below the peat surface and upfield from the mined site (n ¼ 9 except previous analytes n ¼ 8). cn ¼ 2.
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DOC) through the Canadian Association of Laboratory reported as true colour units (TCU). Chloride (Cl ), nitrate
Accreditation. Further proficiency was demonstrated (NO3-N), nitrite (NO2-N), sulfate (as SO4) and total
through round robin studies (National Water Research Insti- ammonia (NH3-N) were determined on filtered samples tute; above analytes, true colour, Hg). Analyses were in (0.45 μm) by ion chromatography (Dionex DX-120). Auto- accordance with LUEL standard operating procedures mated flow injection and colorimetric instrumentation þþ which included the use of blanks, duplicates and quality (Skalar Sans , Netherlands) was used for the following: control samples. Mesocosm leachate sample duplicates DOC was determined after online filtration and acidifica-
with concentrations greater than 10× the method detection tion, releasing CO2 gas that passed through a membrane limit (DL) had 15% relative deviation, exceptions being into weakly buffered alkaline solution with phenolphtha- TN 33%, POC 51% and Zn 31%. lein indicator for detection and quantification; TP was Mesocosm leachate, dilution water and site water were determined via phosphor-molybdic acid complex after all analysed following similar methodology. Briefly, Hg fuming acid digestion with a sulfuric acid/potassium sul- and MeHg were determined without filtration, where fate/mercuric oxide solution; TN was determined by samples were preserved with HCl (pH <2, Fisher Omni- online digestion with potassium peroxodisulfate/sodium Trace) in amber glass bottles before analysis (<30 d) with hydroxide solution and heating, UV radiation with a atomic fluorescence spectrophotometry (Brooks-Rand borax buffer and subsequent nitrate quantification with Model III) after pretreatment and purge and trap techniques the Griess reaction after reduction by a cadmium copper based on Methods 1631 (USEPA ) and 1630 (USEPA reductant. ), respectively. Total extractable metal analysis was con- ducted on samples stored in high-density polyethylene Statistical procedures (HDPE) bottles preserved with HCl (pH < 2, Fisher Trace- Metal, <30 d), then digested and concentrated by Analytes were removed from the dataset when >75% of data
microwave oven after the addition of HNO3 (Fisher Trace- points were censored by LUEL as below DL, otherwise DL Metal) and analysed by inductively coupled plasma atomic values were set equal to DL/2 prior to statistical analysis. emission spectroscopy (ICP-AES, Varian VistaPro). In For Study 1, this was applied only when both pre- and some cases, analysis required filtration of samples post- post-treatment group leachate met the criterion. Statistics digestion to prevent ICP-AES damage. Original preserved were conducted with R (R Development Core Team ). samples were used to determine reduced Fe by the phenan- Results are presented as mean ± standard deviation throline colorimetric method (Varian Cary 50). (SD), unless stated. Study 1 treatment groups for each The following general chemistries were conducted on analyte were compared using one-way ANOVA and Dun- unpreserved samples collected in HDPE bottles. The TS nett’s test. were determined gravimetrically after drying known The 100% PMPW possessed high SD for most analytes volumes at 180W C. The TSS were determined gravimetri- (Table 3), thus median results were included and used for cally on solids retained by 0.45 μm filters dried at 105 W C. efficiency calculations in Study 1. Note when PMPW The POC was determined on solids retained by 0.7 μm fil- median was lower than the mean, a more conservative esti- ters, by difference, after drying (105 W C) then ashing mate of removal efficiency was calculated (Equation (1)). (575 W C). Conductivity was determined by calibrated elec- Study 2 100% PMPW means and medians were similar to trode. Redox potential was determined by probe, verified each other and to Study 1 medians. Therefore, means were with Zobell’s solution. The pH was determined potentio- used for removal efficiency calculations in Study 2. metrically with calibrated electrode prior to alkalinity determined by autotitration (Mettler) to pH 4.5 with 0.02 Efficiency calculation
NH2SO4. True colour was determined on filtered samples (0.45 μm) using a spectrophotometer (Varian Cary 50; The efficiency of mesocosms to remove analytes from 456 nm), calibrated with platinum-cobalt standards and 100% PMPW was calculated in cases where PMPW
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Table 3 | Chemistry of peat mining process water (PMPW) by mechanical dewatering of northwestern Ontario fen peat and means reported for: amechanical dewatering pressate waters with no pretreatment (Washburn & Gillis 1983); b1mechanical dewatering with low severity heat pretreatment (ORF 1984); and b2mechanical dewatering with rotary press (ORF 1984); cCanadian Water Quality Guidelines (CCME 2007); dCCME 2001; eCCME 2004; fCCME 2002. Dashes indicate no data available
This paper
Analyte (units) Mean ± SD (n) Median Washburn & Gillis (1983)a ORF (1984)b1 ORF (1984)b2 CWQGc
pH 5.49 ± 0.40 (5) 5.55 — 4.74 4.30 6.5–9.0 1 Alkalinity (mg L as CaCO3) 9.14 ± 4.90 (5) 7.70 ———— Conductivity (μScm 1) 35.6 ± 4.5 (8) 37.0 ———— True colour (TCU) 532 ± 328 (8) 447 — 280 320 Not > ref. sited DOC (mg L 1) 20.1 ± 2.0 (7) 20.3 136 ——— TOC (mg L 1) ——— 230 7,200 — POC (mg L 1) 407 ± 333 (8) 335 ———— Redox (mV) 399 ± 128 (3) 395 ———— Reduced Fe (mg L 1) 2.42 ± 1.15 (3) 2.06 ———— Chloride (mg L 1) 0.71 ± 0.32 (5) 0.75 — <1 <1 — Nitrate (mg L 1 as N) 0.554 ± 0.581 (5) 0.330 — <1 <1 2.93 1 Sulfate (mg L as SO4) 2.82 ± 1.45 (5) 2.45 — 8.6 29 — Ammonia (mg L 1 as N) <0.010 (2) <0.010 — 6 <1 0.019 Al (mg L 1) 1.39 ± 0.80 (5) 1.38 4.72 <0.5 0.08 0.005 Ba (μgL 1)49± 19 (5) 56 21,000 ——— Ca (mg L 1) 10.76 ± 4.53 (5) 11.44 — 18 11 — Cr (μgL 1)15± 27 (5) 4 52 <30 10 Cr(III) 8.9 Cr(IV) 1.0 Cu (μgL 1)6± 3 (5) 6 <20 <30 <10 2 (soft water) Fe (mg L 1) 4.36 ± 2.23 (5) 4.89 2.03 1.6 1 0.300 Hg (ng L 1) 37.1 ± 16.6 (5) 27.7 <1,000 300 <100 26.0 MeHg (ng L 1 as Hg) 0.485 ± 0.368 0.498 ———4 K (mg L 1) 0.90 ± 0.94 0.51 0.75 1.5 2.0 — Mg (mg L 1) 1.45 ± 0.37 (5) 1.57 — 1.6 3.2 — Mn (μgL 1) 109 ± 66 (5) 87 45 80 100 — Na (mg L 1) 0.791 ± 0.054 (5) 0.800 — <1 8.8 — Ni (μgL 1)9± 7 (5) 7 <4402025 S (mg L 1) 1.80 ± 0.585 (5) 1.86 ———— Zn (μgL 1)55± 7 (5) 57 24 60 120 30 TN (mg L 1) 7.92 ± 6.25 (5) 5.30 38.1 18 207 — TP (μgL 1) 303 ± 212 315 1,000 — 4,600 10–20e TSS (mg L 1) 432 ± 381 (10) 307 — 19 14,300 ref. site þ 5f TS (mg L 1) 303 ± 95.7 (6) 289 — 387 14,400 Seef
concentration was greater than leachate concentration For Study 1, CP was the median concentration of
after 100% PMPW pulses were applied. The percentage PMPW (Table 3), CF was the mean leachate concentration removal efficiency (E%) was calculated as on day 14 for treatment group T1 (100% PMPW, Figure 2)
and CC was the mean concentration of all 24 mesocosms
E% ¼ (CP (CF CC))=CP × 100 (1) before the study commenced. The value of CC corrects
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Figure 2 | Study 1 mesocosm leachate concentration (n ¼ 6) pre-exposure (hollow bars) and post-exposure (solid bars) to peat mining process water (PMPW) treatments over 14 d (mean ± SD). Treatment groups were: T0 control (100% dilution water); T1 100% PMPW; T2 50% PMPW (50% dilution water); T3 33% PMPW (67% dilution water). Solid line: PMPW median concentration (note y-axis breaks for TSS, Al and TN). Dashed line: Canadian Water Quality Guidelines, if applicable (CCME 2007). Dotted line: mean concentration of reference surface water. ANOVA (F values) for post-exposure means with asterisks indicating significant differences compared with T0 (Dunnett’s test): ***p 0.001, **0.001 < p 0.01, *0.01 < p 0.05.
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for an analyte concentration in mesocosm leachate prior to several analytes after pulses of diluted PMPW were applied pulse impacts. If the equilibrium period with dilution water over a two week period (Figure 2). Though significant
resulted in CC being greater than CP and CF, a removal effi- (p < 0.001), Na and chloride were higher in dilution ciency could not be calculated (alkalinity, pH, conductivity, water (Table 2)thanPMPW(Table 3) and not considered K, Mg, ammonia, Fe, reduced Fe, Mn and MeHg). In the further.
case of Zn, CC was greater than CF but less than CP, result- A clear decrease in leachate concentration occurred ing in an efficiency of over 100%. It appeared 100% of Zn when PMPW was diluted (Figure 2). Pearson correlation was retained by mesocosm peat. coefficients (r) were highly significant (p 0.001) between
For Study 2, Cp was the mean concentration of 100% TSS and Al (0.923), DOC (0.878), TN (0.865), Hg (0.834),
PMPW used for that replicate, CF was the mean concen- true colour (0.787), POC (0.761) and TP (0.741) and also sig- tration of the last 10 mesocosm leachates sampled (i.e. nificant for MeHg (r ¼ 0.546, p ¼ 0.006), for post-treatment
steady state) and CC was the mean concentration of leachate mesocosm leachate concentrations (n ¼ 24). sampled after each application of dilution water. Initial Study 1 mesocosm peat depth was 44.0 ± 3.3 cm and peat volume was 0.029 ± 0.002 m3. Mean change in peat height before the study period to after the study RESULTS period ranged from an increase of 1.5 cm to a decrease of
3.4 cm (n ¼ 24) and was not significant (F3,20 ¼ 1.35, p ¼ Process water quality 0.286). Deleterious effects to acrotelm vegetation were not qualitatively evident after 14 d exposure to PMPW. For extracted PMPW, mean pH was below CWQG and mean Al, Fe, Hg, Zn, TP and TSS concentrations exceeded CWQG Peat filtration capacity (Table 3). True colour in PMPW was higher than a CWQG based on reference site true colour (i.e. 119 TCU). Laboratory Though solids were evident in Study 1 mesocosm leachate, DLs were too high to assess Cu and Pb (2 μgL 1 and 5 μgL 1, Study 2 showed that 100% breakthrough of TSS, POC and respectively). Other analytes below DL (DL in parentheses) TS did not occur when 40 L of 100% PMPW was applied were nitrite as N (10 μgL 1), As (5 μgL 1), Be (2 μgL 1), in successive 2-L pulses over a brief time (<3h)(Figure 3). Cd (1 μgL 1), Co (10 μgL 1), Ti (10 μgL 1) and V (6 μgL 1). Mean TSS and true colour in PMPW for replicate appli- For PMPW produced elsewhere (Table 3), the following cations ranged from 99.4 to 344 mg L 1 and 202 to 306 observations were noted (ORF ): (1) TSS methodology TCU, respectively, exceeding CWQG. As in Study 1, Study used 1.5 μm pore size filters, larger than 0.45 μm used here; 2 showed a leaching of organic analytes (Figure 3). True (2) TN data were reported as total Kjeldahl nitrogen (TKN) colour in mesocosm leachate replicates was 81.7%, 35.1% and would not include inorganic N as our TN data; (3) for and 30.6% higher than PMPW. For DOC, only one replicate low severity heat processing, small batches of well decomposed showed leaching (Figure 3), being 18.5% higher in leachate raw peat (approx. 300 g) were diluted with distilled water, than PMPW. heated to 170 W C, then cooled before ‘processing’ in Buchner funnels (Whatman #1 filters) which would remove particulate Removal efficiencies matter; and (4) the rotary mechanical press was novel (17 cm diameter) and best suited for dewatering fibrous, less humified Removal efficiencies for solids in Study 2 were lower than peat rather than catotelm peat as dewatered by us. those in Study 1 (Table 4). A removal efficiency was not calculated for analytes that leached from mesocosms Differences in mesocosm leachate concentrations (DOC and colour) nor analytes with pre-exposure leachate concentrations greater than PMPW (alkalinity, pH, con- In Study 1, significant differences among mesocosm treat- ductivity, K, Mg, ammonia, Fe, reduced Fe, Mn, MeHg, ment group leachate concentrations were found for Na and chloride).
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Figure 3 | Study 2 relative difference in leachate concentration (mesocosm leachate/mean PMPW) as 2-L pulse impacts were applied to mesocosms (<3 h). Pulse impacts indicated on x-axis as: I initial leachate; D leachate after 100% dilution water; P leachate after 100% PMPW. Dashed line is mean of replicates.
DISCUSSION Whether low concentrations were a consequence of peat being ‘pristine’ at this site or a consequence of dewatering Process water quality methodology remains unclear. Though ORF () suggested caution when comparing various dewatering tech- Low concentrations of most analytes were present in PMPW nologies on various scales and with various raw peat we extracted when compared with other studies (Table 3). sources, our data appears to represent some minimum
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Table 4 | Removal efficiency (%) of analytes from 100% PMPW by peat mesocosms a potential environmental problem for mesotrophic lakes (Equation (1); dashes, not analysed) and rivers (CCME ; assumes reference site as meso-
Study 2 trophic as per TP in Table 2). Higher TP reported in other Analyte Study 1 Replicate A Replicate B Replicate C PMPW (Table 3) further suggests eutrophication of receiv- Al 93.2 ———ing water can result if PMPW were directly discharged. Ba 91.1 ———Transport of nitrogen and phosphorus species by organic Ca 91.6 ———and particulate matter as a consequence of traditional dry S 87.4 ———peat harvesting has been described elsewhere (Heikkinen Hg 52.9 ———; Kløve , ). Zn 100 ——— The release of organic constituents via PMPW (POC, Sulfate 76.9 ———DOC, colour; Table 3) would likely alter the spectral quality TN 84.8 ———of receiving water since concentrations were higher than TP 80.8 ———those in reference waters (Table 2). Water colour has been TSS 82.9 54.9 60.8 45.3 correlated to primary production, with any significant POC 88.8 49.4 73.7 47.2 change in spectral quality from anthropogenic disturbances TS — 55.7 65.8 43.4 causing concern (CCME ). Pastor et al.() showed that DOC exerts significant control over productivity, bio- geochemical cycles and the attenuation of visible and UV concentrations that can be expected from dewatering peat radiation in downstream ecosystems. by mechanical means. Ultimately, process water obtained The pH of PMPW here (Table 3), coupled with a low from any mechanical dewatering of peat will depend on buffering capacity of reference waters (alkalinity; Table 2), peat and peat porewater chemistry in addition to the specific suggests untreated PMPW would increase receiving water dewatering processes employed (Monenco ). However, acidity. As expected, fen peat produced PMPW with a any discharge of PMPW with analytes outside values set higher pH than bog peats dewatered elsewhere (Washburn & by Canadian regulators will degrade receiving water quality. Gillis ; ORF ). Water quality of lakes adjacent to Dewatering of wet mined peat produced effluents with natural peatlands are already influenced by surrounding substantial quantities of TS, TSS and POC (Table 3). Particu- ecosystems, especially with respect to acidity (Keskitalo & late matter discharged from dry peat harvesting sites has Eloranta ). Therefore, reference water pH should be been commonly reported (Sallantaus ; Winkler & considered if it lies outside CWQG when assessing environ- DeWitt ; Shotyk ; Ouellette et al. ; Pavey mental impacts to receiving waters caused by PMPW et al. ) and such particulate matter likely caused an discharge. However, in our case reference water pH was alteration in downstream benthic invertebrate and fish com- still higher than PMPW (Figure 2). munities (Laine & Heikkinen ). Solids discharge thus Some metals would increase if PMPW were directly dis- appears a legitimate environmental concern for wet peat charged (Table 3). It was not surprising that Al, Fe, Hg and mining should PMPW be released to streams, rivers and Zn exceeded CWQG since reference waters also had elevated lakes untreated. concentrations of those metals (Table 2). Metals were likely The TN in our PMPW was lower than others (Table 3), bound to particulate matter, as reviewed in Winkler & yet higher than reference site waters (Table 2). In our case, DeWitt (). Elevated Hg concentrations of a New Brunswick TN was not evident as any species with a specific numeric peat harvesting operation were associated with sediments con- guideline (i.e. ammonia, nitrate, nitrite), whereas ammonia taining a higher percentage of peat particles (Surette et al. ). was detected in heat treated pressate waters (Table 3). It Therefore, an increase of Hg species to receiving waters pre- was hypothesized that TN in our PMPW remained com- dicted by Gleeson et al.() seems reasonable though plexed to organic and/or particulate matter. The TP in our particulate bound Hg from peatlands was not bioavailable to PMPW was 15 to 30 times the trigger range that identifies benthic organisms (DiGiulio & Ryan ; Surette et al. ).
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However, the MeHg concentration in our PMPW was five humified peat removed 77–98% of Zn from sulfide mine tail- times the reference site (Figure 2). Although MeHg in PMPW ings and 46–56% from landfill leachate. In batch was less than current CWQG, those guidelines admittedly experiments, Viraraghavan & Kapoor () found Hg was may not be protective of aquatic life owing to food web trophic reduced by 71.6% when wastewater was spiked to 1 mg transfer (CCME ). We are currently investigating the bioa- Hg L 1 and treated with peat. In field trials, Toth () cal- vailability of MeHg from our wet mined site. culated 99.2 to 99.4% of TP in sewage sludge as retained by Our data presented here and conclusions by Monenco fen soils. Kangsepp & Mathiasson () found vertical flow () mostly agree with Gleeson et al.() who postu- peat filters reduced TP by 58 and 63% in municipal waste- lated the potential cumulative impacts of fuel peat mining water, while full-scale peat and organic biofilters used to in Ontario to include an increase of metals, nutrients, acidity treat metal recycling landfill leachate removed 37 to 73% and solids being released to the environment. Therefore, of various metals. Reductions of TN (25%), DOC (30%) some treatment of PMPW seems required before it is dis- and suspended solids (38%) were also found (Kangsepp & charged. Results from mesocosm studies were used to Mathiasson ). However, most studies failed to comment assess the suitability of peatlands as primary treatment sys- on the suitability of the peat-treated wastewater for sub- tems for PMPW. sequent release to the environment. High removal efficiencies for our studies (Table 4) did Mesocosm leachate quality not translate into leachate water quality that would meet CWQG for TSS, true colour, pH, TP, Al, Fe and Zn Acrotelm peat hummocks in mesocosms were quite efficient (Figure 2). As stated, receiving water quality should be con- at removing large concentrations of analytes (Table 4), sidered for analytes such as pH, Al, Fe and Zn. Furthermore, although organic constituents sometimes increased in con- dilution effects in the field, as surmised by Dubuc et al. centration (Figures 2 and 3). The first research on a () and that likely occurred in Toth (), may similarly peatland used as a primary treatment method was for occur in peatlands filtering PMPW, especially fens. Study 1 sewage discharge from a work camp near James Bay, QC showed a simple dilution of PMPW by 33% reduced analyte (Dubuc et al. ). Average reduction percentages greater concentrations in leachate below CWQG and leachate con- than 90% for Ca, Mg, TP and TN and of 70.6% for total centrations were not significantly different from controls, carbon were reported, without evidence of organic leaching with the exception of TN (Figure 2). (Dubuc et al. ). However, high removal efficiencies of As a cautionary note, McLellan & Rock () noted solids in the sewage by peatlands were likely the result of metals desorbed from spent peat columns that were used initial settling in septic tanks (Dubuc et al. ). A further to filter landfill leachates when distilled water was added. dilution of wastewater analytes along the 1.5 km treatment Therefore, any desorption of analytes by potential dilution peatland by groundwater flows could also not be discounted waters should be explored during future research. Elution (Dubuc et al. ). Such initial solids removal and dilution waters characteristic of peatland ecosystems (e.g. rainwater, are likewise recommended here for PMPW before treatment groundwater, peat porewater, snowmelt) would be prefer- with peat. Peat filters and settling pond designs for runoff able to distilled water. from peat ditching and dry harvesting have been developed Colour clearly leached from mesocosm peat in Study 1 and optimized for peat production areas in Finland (Ihme (Figure 2) and Study 2 (Figure 3), while results for DOC et al. b; Kløve ). were less consistent. Kalmykova et al.() also noted a Efficiencies in our studies without pre-treatment leaching of DOC from peat filters receiving various efflu- (Table 4) were consistent with previous research employing ents, with DOC leaching associated with higher metal peat to treat other wastewater types. Raw peat filters concentrations in eluate. We hypothesize that exposure of removed 56–72% TSS, 8–12% TN, 11–9% TP and 8–23% peat mesocosms to PMPW increased the humification of Fe from a Finnish peat mining area (Ihme et al. b). In peat and released organic constituents. Losses of carbon to column experiments, Ringqvist et al.() found poorly the atmosphere from constructed wetlands receiving peat
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mining runoff waters were measured by Liikanen et al. POC 47–89%), were still present in mesocosm leachate at (), an indication of peat degradation. levels likely to be detrimental to aquatic life. Furthermore, Peat decomposition may further explain the higher organic constituents (true colour, DOC) were observed to removal efficiencies of solids in Study 1 than in Study 2, increase in mesocosm leachate; above concentrations in as well as the increased leaching of DOC in Study 1 than PMPW and exceeding CWQG. High removal efficiencies in Study 2. The necessary mixing of PMPW and dilution for nutrients (TN 84.4%; TP 80.8%) were measured, but water for homogenization produced aerobic conditions eutrophication of receiving water remains a concern. High (Tables 2 and 3) that would decompose peat in mesocosms percentages of metals were also removed by peat meso- during exposure to treatment waters. Any decomposition of cosms and based on CWQG and reference site hummock peat increases its amorphous nature, thus redu- concentrations, metals in leachate do not appear to pose a cing the size of pore spaces within the peat matrix and threat to aquatic life. A possible exception is MeHg. Field enhancing its effectiveness as a sorbent (Couillard ). trials are required to verify mesocosm results. Peat mesocosms were exposed to PMPW for a longer Wetlands have traditionally been employed as tertiary period in Study 1 (14 days) than in Study 2 (<3 h), increas- and not primary filtration systems. Although it is anticipated ing the decomposition potential of Study 1 mesocosm peat that constructed peatlands or the use of peat filters could more than in Study 2 peat. Changes in physical peat proper- improve PMPW quality by reducing concentrations of ties (e.g. bulk density, pore structure, peat chemistry) solids, nutrients and metals, some primary treatment of exposed to PMPW should be included in future work. PMPW to remove solids seems required. Potential field trials in northwestern Ontario must con- sider loading volumes and rates of PMPW. Finnish field studies showed peatlands used as filters for peat production ACKNOWLEDGEMENTS runoff performed less efficiently when subjected to higher loads (Ihme et al. a, b). Solids, organic matter and nutri- This research was funded by the Ontario Centres of ents were also leached under higher load. Peatland Excellence as part of the Atikokan Bio-Energy Research hydrology must also be considered. Mesocosms in this Centre. Peat Resources Ltd was an industrial partner, study could only utilize vertical flow, whereas horizontal providing in-kind support. All analytical work was flow would occur in a fen. Furthermore, research with conducted by the Lakehead University Environmental stable isotopes has revealed active flow areas for wastewater Laboratory and their assistance is gratefully acknowledged. purification in peatlands (Ronkanen & Kløve )in addition to short-circuiting and dead zones that act to limit removal efficiencies (Ronkanen & Kløve ). Finally, field trials in northwestern Ontario should include par- REFERENCES ameters to monitor peat degradation and peatland ecology. Bhatnagar, A. & Minocha, A. K. Conventional and non- conventional adsorbents for removal of pollutants from water: a review. Indian J. Chem. Technol. 13 (3), CONCLUSIONS 203–217. CCME (Canadian Council of Ministers of the Environment) Process water produced by mechanically dewatering peat Canadian water quality guidelines for the protection of aquatic life: colour. In Canadian Environmental Quality should not be directly discharged to water bodies according Guidelines, 1999. Canadian Council of Ministers of the to current CWQG. This research provided some insight into Environment, Environment Canada, Winnipeg. whether PMPW quality would be sufficiently improved by CCME Canadian water quality guidelines for the protection of aquatic life: total particulate matter. In Canadian filtration though intact acrotelm hummocks. Mesocosm Environmental Quality Guidelines, 1999. Canadian Council studies showed that high levels of particulate matter present of Ministers of the Environment, Environment Canada, in PMPW, though removed in high quantities (TSS 45–83%; Winnipeg.
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First received 6 September 2011; accepted in revised form 15 May 2012
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