From rainy-season spikes to day‑to‑day variability, textile plants depend on a pretreatment step that can strip 80–99% of turbidity before filtration. The playbook: jar testing to lock in the right coagulant–polymer pair, a clarifier to settle the heavy flocs, and a filter to polish to sub‑NTU.
Coagulation–flocculation—adding chemicals to destabilize colloids and then gently growing them into settleable “flocs”—is standard for cleaning up surface water or shallow groundwater before downstream treatment. In practice, inorganic coagulants such as aluminum sulfate (alum), ferric chloride/ferric sulfate, or polyaluminum chloride (PAC) are paired with high‑molecular‑weight polyelectrolytes to bridge flocs. Industry handbooks cite alum and ferric chloride (effective across pH 5–11) as staples (Water Technologies), while cationic polymers such as poly‑DADMAC and polyamines are often used as primary coagulants or floc aids (Water Technologies).
Performance is not hypothetical. Lab and plant data show properly selected coagulants routinely remove 80–99% of turbidity; one PAC trial (~8 mg/L at pH 8) achieved ~97.7% turbidity removal (Mazloomi et al. 2018). Combined with filtration, operators meet drinking‑water targets—WHO calls for <1 NTU (nephelometric turbidity units), and in practice many aim for <0.2 NTU (WaterTech). Indonesia’s PDAM utilities likewise depend on coagulation–flocculation to hit treated water turbidity around ~1 NTU even when rivers jump into the hundreds of NTU during storms.
Chemicals, pH windows, and floc aids
The chemistry is tunable. Coagulants (alum, ferric salts, PAC) destabilize particles; cationic or anionic polyacrylamide polymers bridge microflocs into larger, faster‑settling aggregates. Coagulant efficacy is pH‑dependent—one study showed PAC peaking near pH 8 (Mazloomi et al. 2018), while alum is often optimal around pH ~6–7 (Water Technologies). Operators source these as standard coagulants and pair them with polymer flocculants when “pin floc” suggests bridging is needed. For PAC specifically, procurement as polyaluminum chloride is common in industrial raw water trains.
Jar testing protocol (bench‑scale simulation)
Jar testing—a parallel set of beakers mixed under controlled energy—has a simple goal: find the lowest dose that achieves the largest, stable floc and the lowest residual turbidity. Recommended steps include collecting a representative raw sample, filling six or more 1 L jars, and rapidly dispersing graded coagulant doses (e.g., 10–100 mg/L for alum or PAC) at ~100–200 rpm for ~1–2 minutes to simulate coagulant mixing (WaterTech; NESC; SSWM).
Immediately after the rapid mix, add a polymer dose—often tested at 0.5, 1, 2, 5, and 10 mg/L—and reduce stirring to ~20–40 rpm for ~10–15 minutes to form flocs (WaterTech). Stop mixing and let the jars settle for 10–30 minutes. Then measure the supernatant turbidity in each jar using a calibrated turbidimeter (WaterTech), plot residual turbidity versus dose, and identify the optimum as the lowest dose that meets the target (commonly <1 NTU and, for high‑purity goals, <0.2 NTU; WaterTech). If raw water pH sits outside the sweet spot, repeat with pH adjustments (e.g., test at pH 6, 7, 8), since coagulants are pH‑dependent (Mazloomi et al. 2018; Water Technologies).
Jar‑test data scale directly to plant dosing. If 20 mg/L alum plus 2 mg/L anionic polymer delivers >95% turbidity removal in the lab, operators calibrate dosing pumps accordingly and track performance with in‑line turbidimeters. Guides emphasize frequent jar testing because raw water shifts with seasons and storms (“raw water samples should be taken regularly, and tested with a range of coagulant concentrations to determine the optimum dose”; SSWM; see also NESC on “simulates the full‑scale process” and detailed steps here).
Clarification and filter polishing
Post‑flocculation, heavy flocs are removed by sedimentation in clarifiers. Typical basins (often 2–4 m deep) provide ~2–3 hours of residence time, with lamella or horizontal‑flow designs widely used. After optimum coagulation/flocculation, gravity settling removes most solids (Water Technologies), and well‑run basins can deliver effluent turbidity of only a few NTU—routinely <5 NTU and often <1–2 NTU in practice (Mazloomi et al. 2018; WaterTech). Plants often select a conventional clarifier or a compact lamella settler to reduce footprint.
The clarifier effluent typically passes through a gravity media filter with dual layers—anthracite over sand—to capture fine flocs. Polymers are especially useful ahead of filtration: they build flocs large enough that the bed captures them by straining instead of deep penetration, extending run length. Industry practice shows coag–floc plus media filtration achieves <0.5 NTU consistently; in drinking applications, operators often hold <0.2 NTU (WaterTech on sand filtration mechanisms and targets). For filter media selection, dual‑media beds commonly use anthracite over sand/silica.
Design trade‑offs are straightforward: clarifiers handle variable flows but need land; high‑rate filters demand precise dosing and regular backwashing. In tight footprints or where solids float (e.g., algae), plants opt for in‑line coagulation/clarification or dissolved air flotation (DAF), relying on the same jar‑tested chemistry; DAF remains a standard option (dissolved air flotation) in high‑turbidity or variable conditions.
When turbidity soars: roughing filters upstream
At raw water spikes above ~500–1000 NTU during floods, sedimentation alone can struggle. Evidence from Hashimoto et al. (2019) shows three consecutive roughing tanks (~90 cm depth each) removed ~97–99% of extreme turbidity—dropping ≈1100 NTU to an average of 13 NTU and delivering final values under 10 NTU (WPT study; follow‑up). Even a single ~90 cm stage achieved ~91–95% removal, bringing ≈1100 NTU down to ~50–100 NTU (WPT study). Roughing filters (e.g., coarse anthracite gravel) are therefore used upstream of rapid filters to protect media beds.
Mixing energy and scale‑up parameters
Bench results only translate if full‑scale hydraulics are similar. Rapid mixing should fully disperse the coagulant; in practice, G‑values (a measure of velocity gradient) are ~500–1000 s⁻¹ for rapid mix, followed by flocculation at G ≈30–70 s⁻¹. The sequence—rapid mix then gentle flocculation—must be preserved, and flocculators should avoid shear that breaks flocs (“gentle mixing…without breaking the flocs”; SSWM). Operators then select the lowest doses that repeatedly form flocs large enough to settle to the target clarity.
Troubleshooting the clarification stage
Patterns diagnose problems quickly. Excessive effluent turbidity usually points to under‑dosing or floc breakup—re‑run jar tests, adjust coagulant/polymer or pH, and check mixing intensity (SSWM). Beware the other extreme: over‑dosing can inflate sludge volume without improving clarity (Lautan Air Indonesia).
Sludge carry‑over signals hydraulic overload or poor inlet hydraulics—if detention falls below roughly the minimum needed, even good flocs will be swept out (Lautan Air Indonesia). Inlet baffles or diffusers help; one guidance notes that fixing short‑circuiting and inlet distribution can boost efficiency by ~20–40% (Porvoo Clean‑Tech).
When sludge accumulates, confirm mechanisms: over‑dosing and solids spikes raise sludge yield; broken scrapers or pumps let it build unchecked (causes and remedies summarized by Lautan Air Indonesia, see also raw‑water changes, here, and mechanical failures, here).
pH/alkalinity imbalances often undermine coagulation. Alum dosing consumes alkalinity and can depress pH; if it falls below ~6, aluminum hydroxide flocs begin to dissolve, so pre‑adjust to the jar‑tested range (~6–7 for alum) (Water Technologies). Cold water slows floc formation and warm seasons bring algae that interfere with settling; plants maintain seasonal “winter” and “summer” jar‑test regimes. Proactive adjustments for temperature and algae can improve year‑round clarity by up to ~30% (Porvoo Clean‑Tech; additional note).
The fix is often data‑driven and incremental: historical jar‑test curves and live turbidity logs guide small changes. If turbidity creeps from 1 to 2 NTU after a storm, increasing the primary coagulant by ~10%—as indicated by the jar‑test slope—often restores clarity. Optimization via regular jar testing “is the key to running the plant more efficiently,” with empirical tuning commonly saving 10–20% on chemical use while improving effluent quality (Porvoo Clean‑Tech; see also SSWM).
Clarification issues: quick checklist
- High effluent turbidity: Re‑run jar tests; adjust coagulant/polymer doses or pH; reduce flocculation shear if flocs are breaking (SSWM; pH/dose sensitivity also supported by Mazloomi et al. 2018).
- Overhead sludge carry‑over: Reduce flow to increase detention; fix inlet baffles; consider polymer addition; avoid overloading overflow launders.
- Excess sludge buildup: Verify scraper operation; reduce coagulant if overfeeding; increase sludge wasting frequency (Lautan Air Indonesia; mechanisms).
- Short‑circuiting: Inspect diffusers and baffles; add or repair baffles; compact settlers help when footprint is limited.
- Variable raw water quality: Increase jar‑test frequency during rainy season; be ready to pulse‑feed coagulant when turbidity spikes arrive.
Bottom line: coagulation–flocculation followed by clarification and media filtration is simple and cost‑effective for diverse contaminants (SSWM; operational notes and design guidance in Water Technologies). The trade‑off is chemical consumption and residual sludge handling, but with jar‑tested dosing and steady monitoring, plants consistently meet WHO’s <1 NTU—and often drive below 0.2 NTU (WaterTech).
