Heap Leach Bleed Pretreatment Before MBR: 2026 Process Guide

Why Heap Leach Bleed Is a Special Case for MBR Pretreatment

Heap leach bleed (also called Pregnant Leach Solution, or PLS, after precious-metal recovery) is not a generic industrial wastewater stream, and it cannot be fed to a submerged PVDF membrane bioreactor on a conventional headworks specification. Typical operating envelopes run pH 9–11, free WAD (weak-acid dissociable) cyanide 5–500 mg/L, total suspended solids 50–2,000 mg/L, sulfate 500–5,000 mg/L, and conductivity equivalent to 2–15 g/L NaCl. MBRs deliver <1 μm effluent and high MLSS, which is exactly why the same biology that produces clean permeate also makes the membranes intolerant of free cyanide, divalent scale-formers (Ca²⁺, Mg²⁺, Fe²⁺/³⁺), and colloidal clays that bleed through conventional clarifiers.

The PMC 2023 review on industrial MBRs draws the line explicitly: once influent salinity exceeds 10 g/L NaCl, conductivity must be cut by more than 80% upstream of the MBR or flux declines and salt stress on the biomass become uncontrollable. That threshold, plus the three failure modes unique to heap leach bleed (WAD cyanide lysis of biomass, divalent-cation scaling of the PVDF, and colloidal-clay fouling), is what forces a five-stage pretreatment train rather than a screen and equalization basin. The rest of this article specifies each stage with commissioning-ready numbers.

Stage 1 — Coarse Screening and Grit Removal

The first unit operation must remove coarse ore fragments and pump-damaging debris before the chemistry train starts. Specify a rotary mechanical bar screen with 3–6 mm aperture on stainless steel rake teeth as the minimum headworks protection; 6 mm is acceptable on flows above 200 m³/h, but drop to 3 mm when the heap leach pad has visible clay fines or when downstream is a positive-displacement pump.

Downstream of screening, a grit chamber sized at 1 m³ per m³/h of peak flow with 60-second detention drops sand and ore particles above ~200 μm that would otherwise abrade the flat-sheet PVDF surface. Equalization follows, with 8–24 hours of HRT to dampen the diurnal pumping cycles typical of heap-leach operations (pregnant solution often flows in 12–18 hour batches tied to irrigation cycles on the pad). Without equalization, the downstream chlorine dose on Stage 2 will hunt continuously and either over-oxidize or under-treat.

Stage 2 — Cyanide Destruction

Free WAD cyanide is the single most damaging influent parameter for an MBR; it lyses heterotrophic biomass and strips nitrification. The baseline chemistry for heap leach bleed is alkaline chlorination: dose Cl₂ or NaOCl to ORP >+650 mV at pH >10, hold 30–60 minutes in a baffled reaction tank, and target free WAD CN <0.5 mg/L at the discharge of the contact basin. Stage-by-stage chlorination (maintaining pH 10–11 throughout) prevents the formation of cyanogen chloride gas that occurs at low pH.

For sites where chlorinated byproducts (total residual chlorine, AOX) are restricted in the discharge permit, switch to H₂O₂ + Caro's acid (H₂SO₅) at a stoichiometric ratio of roughly 2–4 g H₂O₂ per g CN destroyed, holding pH 9–10 for 45–90 minutes. Caro's acid gives faster kinetics than peroxide alone and leaves no halogenated residue. The SO₂/air INCO process is acceptable for very large flows (above ~1,000 m³/h) but requires long retention basins and a downstream aeration step, which makes it less MBR-friendly than alkaline chlorination at the typical mid-sized heap-leach flow range.

One hard rule: any residual H₂O₂ leaving the cyanide contactor must be quenched with catalase or granular activated carbon before the MBR. Even 5–10 mg/L of residual peroxide will lyse floc-forming bacteria and crash MLSS within 24 hours. Confirm with a peroxide test strip at the MBR feed well during commissioning.

Stage 3 — pH Adjustment, Temperature Control, and Alkalinity Buffering

The biomass and membrane chemistry both demand a narrow window at the MBR inlet: pH 6.5–8.0, temperature 15–35 °C, and alkalinity ≥100 mg/L as CaCO₃. Use two-stage acid neutralization to avoid localized low-pH pockets that would hydrolyze the PVDF. Step one drops pH from 10–11 to ~9 with H₂SO₄ (sulfuric is preferred over HCl because the sulfate load is already high and chloride stress on the biomass must be limited); step two trims pH to 6.5–8.0 with CO₂ sparging or a weak acid. A single-stage acid dump tends to overshoot and creates pH dead-zones near the injection point.

Temperature conditioning is site-specific. Heap leach bleed in arid summer operations routinely arrives at 35–40 °C, which is at the upper edge for mesophilic MBR biomass; install a cooling tower or plate heat exchanger ahead of the equalization basin. Winter operations on heap pads in cold climates can drop to 5–10 °C, which starves nitrification and slows COD removal — install a heat exchanger fed from waste heat or a small boiler. Throughout, alkalinity must hold ≥100 mg/L as CaCO₃ to support nitrification and buffer the CO₂-driven pH crash that otherwise occurs in the aeration tank.

Dosing accuracy on this stage is non-negotiable. A PLC-controlled chemical dosing system feeding acid, caustic, and anti-scalant on trim loops with redundant pH and conductivity probes keeps the window inside ±0.2 pH units and ±2 °C.

Stage 4 — Suspended-Solids and Colloidal-Fines Polishing

TSS to the MBR must be <50 mg/L; <20 mg/L is preferred to extend CIP intervals. This stage is where most heap-bleed-MBR trains fail in practice, because colloidal clays (kaolinite, montmorillonite, illite) from the leach pad carry over at 10–100 μm and blind conventional sediment basins.

For flows of 4–300 m³/h, especially when the feed carries emulsified oils, flocculant residue, or hydrophobic colloids, specify a ZSQ dissolved air flotation system operating at surface loading 5–20 m/h with 4–6 bar saturation pressure. DAF outperforms sedimentation on this stream because the buoyant force lifts sub-100 μm particles that would settle too slowly in a lamella unit. For higher flows (above ~300 m³/h) or where chemical budgets are tight, a lamella clarifier with sludge recirculation delivers comparable TSS at lower operating cost, and sludge recycle to the flocculation zone cuts coagulant dose by up to 30%.

Polymer conditioning is required ahead of either clarifier. Jar-test on site, then dose 0.5–5 mg/L of anionic or cationic polyacrylamide (selection depends on the zeta potential of the clay fraction; cationic works for the predominantly negative surface charge of most heap-leach colloids at pH 7). Without polymer, the clarifier effluent will routinely run 80–150 mg/L TSS regardless of coagulant dose, and the MBR will foul on a 7–14 day cycle instead of 30–90 days.

Stage 5 — Fine Strainers and Membrane Protection

The last physical barrier is a two-step mechanical guard: a 50–100 μm self-cleaning automatic strainer immediately downstream of the clarifier, then a 10–20 μm bag or cartridge polisher as a sacrificial prefilter to the MBR. The strainer handles volumetric slugs of fine solids (filter media breakthrough, clarifier upsets), and the bag polisher catches what the strainer misses.

The failure mode this stage prevents is twofold on a DF series PVDF flat sheet membrane module (0.1 μm nominal pore, 80–225 m² configurations with integrated aeration box). A single stray shard above 50 μm can tear the membrane face, forcing a module replacement that runs into the tens of thousands of dollars; a sustained 20–50 μm overload drives irreversible pore-plugging that no CIP recovers. The sacrificial bag is changed weekly during normal operation and is the cheapest insurance on the train.

Pretreatment-to-MBR Process Flow and Parameter Summary

The full train in flow order: Screen (3–6 mm) → Equalize (HRT 8–24 h) → CN Destruction (Cl₂/H₂O₂ to <0.5 mg/L WAD CN) → pH/T/Alkalinity trim (pH 6.5–8.0, 15–35 °C, alk ≥100 mg/L CaCO₃) → DAF or Lamella (TSS <50 mg/L) → 50–100 μm self-cleaning strainer → 10–20 μm bag polisher → integrated MBR membrane bioreactor system.

Acceptance Criteria and Commissioning Checklist

Before the MBR is fed live bleed, the operator must pull and pass six 24-hour composite samples from the bag-polisher outlet: free WAD CN <0.5 mg/L, TSS <20 mg/L, pH 6.5–8.0, temperature 15–35 °C, conductivity <2 g/L NaCl equivalent, and total hardness <250 mg/L as CaCO₃. Run the pretreatment train at 100% design flow for seven days on clean water, then three days on live bleed at 50% flow, and only then ramp to full design flow.

During the clean-water phase, establish a 24-hour flux baseline (LMH at a fixed TMP and aeration rate) on the MBR. That baseline is the reference point for the rest of the campaign: any subsequent flux decline can be attributed to upstream drift, not membrane defect, and the CIP interval can be adjusted against it.

Frequently Asked Questions

Can heap leach bleed be sent straight to an MBR?

No. Free WAD cyanide above ~1 mg/L lyses MBR biomass, TSS above 50 mg/L fouls the PVDF membrane on a 7–14 day cycle, and conductivity above 2 g/L NaCl equivalent pushes the system past the operating envelope defined in the PMC 2023 review. A five-stage pretreatment train is mandatory.

What is the best cyanide destruction method ahead of an MBR?

Alkaline chlorination (Cl₂ or NaOCl) at pH >10, ORP >+650 mV, with a 30–60 minute contact time, is the default because of reagent availability and fast kinetics. H₂O₂ + Caro's acid is the preferred alternative where chlorinated byproducts are restricted in the discharge permit, at a stoichiometric dose of 2–4 g H₂O₂ per g CN destroyed.

What TSS can an MBR tolerate from heap leach feed?

<50 mg/L is the maximum, and <20 mg/L is strongly preferred to extend CIP intervals. Above 50 mg/L, colloidal clays and ore fines build a surface cake on the PVDF that chemical cleaning does not fully remove, and flux drops within 7–14 days.

How is high sulfate from heap leach bleed handled ahead of an MBR?

Sulfate reduction is not done inside the MBR. The standard approach is a dedicated biological sulfate-reduction stage (using an upflow anaerobic sludge blanket or ethanol-fed sulfidogenic reactor) upstream of the train, or an RO split that bypasses a portion of the bleed around the MBR. Either way, the MBR itself should see <2 g/L NaCl-equivalent conductivity per the PMC 2023 threshold.

How often will the MBR membrane need CIP with heap-bleed feed?

30–90 days when the five-stage train meets all the targets in this article, 7–14 days when TSS, hardness, or CN control slip. Track CIP interval against the clean-water flux baseline established at commissioning to distinguish upstream drift from membrane defect.

Further Reading

• copper concentrator water pretreatment guide
• belt filter press cost guide for mining wastewater
• MBR configuration for sanitary sewage