Cement‑quarry washwater routinely tops 500–2,000 mg/L in solids and pushes pH toward 12.4 — a risky mix for receiving streams. A three‑stage train — settling, coagulation/flocculation, and pH neutralization — turns that effluent into reliable dust‑suppression water.
Quarrying and mineral processing leave a signature in water: coarse limestone and clay grains, cement fines, and a caustic bite. Left untreated, this effluent can be ecologically harmful (>500–2,000 mg/L solids is typical) (pmc.ncbi.nlm.nih.gov). The fix is not novel — it’s disciplined engineering: a settling pond or clarifier, chemical coagulation and flocculation, and a neutralization step before any discharge or reuse.
Done right, the system captures most solids, drives pH into the regulatory window (~6–9 is a common discharge band in Indonesia and elsewhere), and creates a stream fit for haul‑road sprays and stockpile mists (www.researchgate.net).
Primary solids removal: settling pond or clarifier
The front end is a large‑area settling pond or a deeper tank clarifier (a scraper‑equipped concrete basin). Many sites opt for an in‑ground pond sized for long retention — often 1–3 days — to let gravity peel off the heavier grit and sand (70–90% of solids by weight) while fine silt (<10–20 μm) tends to stay suspended (nepis.epa.gov) (nepis.epa.gov). An EPA survey even noted static ponds “have limited efficiency” for light clays and fines (nepis.epa.gov).
Still, without chemicals, operators can often see tens‑of‑mg/L suspended solids in the overflow. Empirical design guides point to surface overflow rates near ~0.5–1 m³/m²·h and roughly 1–2 day detention to clear coarse sediments. The trade‑off: footprint. Clarifying ponds “are generally large in area” and need periodic remobilization (“large land area… most have no overflow”) (nepis.epa.gov). Where space is tight, a tank‑based clarifier tightens geometry while preserving the same primary function.
Whether lagoon or tank, tracking sludge is non‑negotiable. Literature on aggregate plants reports solids build‑up of ~0.5–2% vol/day depending on loads (nepis.epa.gov). Chemical aids also matter: coagulants and flocculants can “speed up the settling rate… helping decrease pond size” (nepis.epa.gov).
Coagulation–flocculation for fine solids
To strip out the stubborn fines, plants layer in chemical coagulation (neutralizing particle charge) and flocculation (bridging particles into larger, heavier flocs). Cationic coagulants — alum, ferric chloride, or polyaluminum chloride (PAC) — neutralize the negative surface charge on clay‑like particles (nepis.epa.gov). Many operators dose a PAC coagulant via a metered dosing pump and follow with a high–molecular‑weight polymer flocculant, often cationic polyacrylamide, to “tie the floc particles together, making bigger and heavier flocs for faster settlement” (nepis.epa.gov). Coagulant choices mirror market offerings such as PAC and broader coagulants, paired with site‑tuned flocculants.
Dosage ranges in comparable quarry wastes are on the order of 5–50 mg/L for PAC/alum and 5–20 mg/L for polymer, best set by jar tests. In the lab, a blend of 5 g/m³ PAC and ~12 g/m³ cationic PAM (polyacrylamide) achieved excellent flocculation (pmc.ncbi.nlm.nih.gov). Jar testing is critical because overdosing can restabilize colloids — one study found excess alum actually reduced settling above the optimal dose (pmc.ncbi.nlm.nih.gov).
Under tuned dosing, coag/floc typically lifts total suspended solids (TSS, total suspended solids) removal from roughly 50% with settling alone to 80–95%. In a quarry‑slurry test, sequential dosing — 200 mg/L CaO, 5 mg/L PAC, 12 mg/L PAM — cut turbidity to ~97 NTU (Nephelometric Turbidity Units), meeting discharge limits; the raw feed registered pH ≈12.4 and TSS ≫500 mg/L (pmc.ncbi.nlm.nih.gov). With well‑tuned chemicals, >90% of 5–50 μm particles settle in a few hours (pmc.ncbi.nlm.nih.gov) (nepis.epa.gov), allowing a secondary clarifier to polish the stream. Captured sludge is then dried or dewatered on‑site or managed per solid‑waste rules.
pH neutralization system
Cement wash waters skew intensely alkaline — typical pH values exceed 11, with lab data showing raw washwater at ≈12.4 (www.mdpi.com). Most discharge regimes cap pH near neutral, and Indonesian guidance is roughly 6–9 (www.researchgate.net), with a case study noting pH as the lone parameter violating the Ministry of Environment standards (journal.ugm.ac.id). Neutralization is therefore mandatory.
Acid dosing — typically H₂SO₄ or HCl — consumes hydroxide directly. As a magnitude check, neutralizing 1 m³ of pH~12 cement water (roughly 800 mg/L Ca(OH)₂) may take on the order of 100 g H₂SO₄ (www.mdpi.com), commonly fed by a controlled dosing pump. Automatic pH systems with probes hold setpoints near 7–8; sulfuric acid is widely used, though studies flag chemical cost impacts (www.mdpi.com). Ancillary gear such as pH probes and mixers sits in the realm of wastewater ancillaries.
An alternative is CO₂ aeration or bicarbonate flow: bubbling CO₂ forms carbonic acid in situ, rapidly lowering pH and precipitating CaCO₃ — a “carbon mineralization” that can sequester CO₂ and yield calcite (www.mdpi.com). One treatment trial reported 84–99% Ca removal and full CO₂ capture via such carbonation (www.mdpi.com). In all cases, neutralization follows solids removal: clarified effluent passes through an acid or CO₂ reactor, bringing pH into limits; excessively high pH would otherwise be harmful to receiving waters (including killing aquatic life) and violate the typical ~6–9 discharge band (www.researchgate.net).
Reuse for dust suppression
Once solids and pH are controlled, most quarry water can be routed back on site. A high‑value outlet is dust control: treated water with TSS <100 mg/L and pH ~7–8 can be sprayed on haul roads, stockpiles, and crushing zones. Because no human or animal contact is involved, reuse here carries few drawbacks. Some operators pair reclaimed water with a hauling‑road dust suppressant as a stabilizing additive; dust‑control trials have shown water usage drops by tens of percent when stabilizers accompany recycle water.
Recycling is already familiar in the sector: concrete plants have found even high‑pH washwater (pH>11.5) suitable as concrete‑mixing water with no dilution (Tsimas et al., 2011) (www.researchgate.net). One cement producer cut water use by ~50% through effluent recycling (greencape.co.za). Typical results might allow >80% of daily water usage to be met by recycle, with overflow (if any) meeting limits for TSS, pH, and any metals before diversion to bowsers or fixed spray lines. The combined effect — settling, coagulation, and neutralization — often halves or better a site’s intake of outside water while ensuring compliant discharge (greencape.co.za).
Design notes and sources
The narrative above draws on mining/quarry wastewater studies and cement‑industry case reports: primary solids behavior and environmental risks (pmc.ncbi.nlm.nih.gov); EPA guidance on pond performance, coagulants/flocculants, and operational constraints (nepis.epa.gov) (nepis.epa.gov) (nepis.epa.gov) (nepis.epa.gov); coag/floc performance and dosing examples including 200 mg/L CaO + 5 mg/L PAC + 12 mg/L PAM to ~97 NTU and pH ≈12.4 feed (pmc.ncbi.nlm.nih.gov); alkaline chemistry and CO₂‑based neutralization with 84–99% Ca removal and full CO₂ capture (www.mdpi.com) (www.mdpi.com); Indonesian effluent criteria and operational experiences (journal.ugm.ac.id) (www.researchgate.net); and plant‑level recycling outcomes (~50% water savings) (greencape.co.za).
