Modern tailings storage facilities lean on composite geomembrane liners, drains, and toe sumps to trap leakage—and a conventional chemical‑precipitation plus clarification train to make the water reusable. Studies report 65–75% water recovery in conventional dams, ~80% with thickened discharge, and ~90% in dry‑stacked systems (www.mdpi.com).
Coal‑tailings TSFs (tailings storage facilities) leak at the margins. The industry’s answer is a multi‑barrier system: engineered liners to block flow, drains and sumps to recover what escapes, and a treatment train—neutralization, precipitation, and clarification—to polish the water before discharge or reuse.
The approach is not optional. Modeling shows even a few defects can erase liner benefits, while “zero discharge” policies now push operators to capture and pump back essentially 100% of seepage (www.mdpi.com).
Liner system types and materials
Coal‑tailings TSFs commonly use engineered basal liners to block seepage. In practice the most prevalent designs are single‑composite liners (prepared subgrade plus low‑permeability soil or GCL, i.e., geosynthetic clay liner, topped by a geomembrane and drainage) and double‑composite liners (an additional geomembrane/leak‑drain beneath) (www.mdpi.com).
Räsänen et al. (2021) report that ~80–90% of modern TSF base designs worldwide employ a clay‑on‑geomembrane composite, with many adding a secondary geomembrane plus leak‑collection layer (www.mdpi.com) (www.mdpi.com).
Geomembrane materials—typically HDPE (high‑density polyethylene) or LLDPE (linear low‑density polyethylene) sheets—are favored for their chemical resistance and weld strength. (HDPE is broadly inert to tailings chemistry, but can be stress‑crack sensitive and requires thicker (≥1.5–2.0 mm) and judicious installation (www.mdpi.com) (www.mdpi.com).) Alternative liners such as PVC/EPDM or bituminous geomembranes can be used where flexibility or low‑temperature performance is needed, albeit at the cost of lower chemical durability, so material choice is driven by tailings pH (acidity/alkalinity), sulfates, hydraulic head (driving pressure), and climate (www.mdpi.com) (www.mdpi.com).
For instance, thick‑bentonite/LG‑lined systems achieve K≈10⁻⁹–10⁻¹⁰ m/s (hydraulic conductivity, a measure of permeability) but their conductivity can degrade if high‑sulfate tailings breach the liner (www.mdpi.com). Industry guidelines typically recommend geomembrane and GCL thicknesses to match site loads; a rule‑of‑thumb is ≥1.5 mm for static HDPE liners (≥2.0 mm where settlement or dynamic loads are expected) (www.mdpi.com).
Protective cushions (geotextile layers) are often used under the membrane in landfill practice, but tailings impoundments sometimes omit these to cut cost (www.mdpi.com)—a trade‑off that heightens puncture risk and therefore underscores the need for rigorous construction quality assurance (CQA) and testing. Indeed, even a few fabrication or installation defects can dramatically increase leakage: modeling by Brachman et al. shows that an HDPE liner only markedly reduces seepage if defect rates are kept extremely low through strict CQA (www.mdpi.com).
In short, engineers must select liner types and thicknesses to suit the tailings’ chemistry, high hydraulic heads, and temperature regime, and enforce stringent QA during construction to meet target barrier conductivities on the order of 10⁻⁹ m/s or lower.
Drainage, collection, and recovery sumps
Even with liners, some residual seepage usually escapes. A key design element is a drainage and collection system beneath the liner: a graded coarse‑drain layer (sand/gravel or geocomposite) placed directly below the geomembrane and above the GCL/clay to capture infiltrating fluids, pitched toward perforated collector pipes along the dam toe. Collected seepage flows by gravity into lined recovery sumps downstream of the dam—essentially excavated pits lined with geomembrane that serve as sealed catchment basins (www.mdpi.com).
Cacciuttolo et al. (2023) describe a typical toe sump excavated below the dam, lined with the same geomembrane, where filtrate accumulates before pumping; sump pumps (often low‑lift centrifugal units) then recirculate the water back to the surface tailings pond or process plant (www.mdpi.com). In practice nearly all seepage can be recovered this way: filtrate collected in the sump is “driven through pumping” back to the mill water pond for reuse (www.mdpi.com). Recirculation both conserves water and avoids discharge of contaminated water.
Cutoff barriers and pond hydraulics
Additional containment features are used where geology or water tables demand it. For high‑gradient sites, cutoff barriers (grout curtains, slurry walls, or deep trenches) are installed along the upstream toe to block underflow, backfilled with low‑permeability material (bentonite, concrete) and integrated with the liner. A common design is an armored geomembrane installed on the upstream dam face, anchored at its bottom in a deep cutoff trench or grout curtain (www.mdpi.com).
Studies in Chile/Peru show that cutoff walls combined with geomembranes “provide the dam with a continuous impervious layer running all along its upstream face, from the bottom cutoff trench” (www.mdpi.com). Locating the supernatant pond away from the dam crest (on an upstream bench) reduces hydraulic head on the liner, thereby limiting absolute seepage rates (www.mdpi.com).
Chemical precipitation and clarification
Collected seepage (“process water” or tailings pond decant) often contains elevated sulfate, dissolved metals, and suspended solids. Before reuse or discharge it generally must be treated by conventional mining wastewater methods, chiefly neutralization, precipitation, and clarification (projects.itrcweb.org). A typical flowsheet is: pH adjustment/precipitation—add alkaline reagents to raise pH (e.g., lime or sodium hydroxide) and/or sulfide reagents to precipitate metal sulfides; coagulation/flocculation—add coagulants (e.g., aluminum sulfate or iron chloride) or polymers; settling/clarification; and final filtration/polishing (e.g., cloth filters, sand filters) (projects.itrcweb.org).
Chemical precipitation separates contaminants in a clarifier (projects.itrcweb.org), and reagents are chosen based on target metals—lime/Ca(OH)₂ or Mg(OH)₂ for Fe/Al, sulfides like NaHS for Cu/Pb/Zn, etc. (projects.itrcweb.org). A case study of an acid‑mine drainage plant achieved >99% removal of Zn (from 123,000 µg/L to <4 µg/L) and >90% removal of Cd by adding sulfide reagents and settling (projects.itrcweb.org). In practice, heavy metals like Cu, Ni, Pb, and Zn can routinely be removed to sub‑ppm levels by this method (projects.itrcweb.org).
After precipitation, the sludge is decanted and often dewatered via thickeners or filter presses, while the supernatant is recycled or discharged. A typical clarifier ensures low TSS (total suspended solids) of <10–20 mg/L in the effluent (projects.itrcweb.org). Coagulant selection and polymer make‑up are central here; operators typically deploy coagulants and flocculants during the solids‑liquid separation step. For polishing, many plants rely on sand‑media filtration; “sand filters” in this context commonly means dual‑media beds such as sand/silica filtration.
Compliance targets and reuse
Ultimately, treated water may be recycled into the mill or, if discharge is necessary, must meet regulatory limits (e.g., per Indonesian standards or EU mining directives). In Indonesia, effluent standards (Government Reg. No. 82/2001 and Permen LH guidelines) typically limit heavy metals to the low mg/L range; the above treatments are designed to achieve those targets. Residual parameters (pH, TSS, oil/grease, etc.) are polished as needed by sedimentation and filtration. In many modern operations, essentially all recovered seepage is returned to the circuit, resulting in minimal net discharge.
Recovery rates and the zero‑discharge trend
Recent studies validate the effectiveness of these measures. Well‑operated conventional tailings ponds (with full drainage and pumping) recover on the order of 65–75% of the ponded water (www.mdpi.com), significantly reducing losses. Upgraded tailings management—e.g., thickened discharge—can reach ~80% water recovery (www.mdpi.com), while fully dewatered (dry‑stacked) tailings have achieved ~90% recovery (www.mdpi.com).
Higher recovery directly translates to smaller pond area and lower seepage. Conversely, failures to install robust liners or drains have resulted in serious contamination incidents, so the industry trend is toward composite geomembrane liners plus comprehensive seepage capture. Guidelines (ICMM/PRI, national regulations) now strongly favor “zero discharge” approaches, which in practice means 100% of seepage is collected and treated or reused.
Multi‑barrier summary and outcomes
Best‑practice seepage control for coal tailings relies on a multi‑barrier approach: synthetic (or clay) liners to reduce permeability (www.mdpi.com) (www.mdpi.com), integrated drainage and sump systems to recover what does leak (www.mdpi.com) (www.mdpi.com), and a full water‑treatment train (chemical precipitation plus clarification) on the return flow (projects.itrcweb.org). This combination—backed by stringent QA/QC—yields predictable outcomes: low hydraulic conductivity sites and >90% contaminant removal in the processed seepage (www.mdpi.com) (projects.itrcweb.org). Such measures turn a potential environmental liability into a controllable stream, a critical factor for safe, compliant coal‑mining operations.
Sources: Authoritative reviews and case studies of tailings liners, drainage and water treatment (www.mdpi.com) (www.mdpi.com) (www.mdpi.com) (projects.itrcweb.org) (projects.itrcweb.org) were used, along with industry guidance (e.g., EPA/ITRC) and regional examples. Full citations appear below.
