Utilities Are Turning the pH Dial: Inside the Quiet Race to Strip Organics from Drinking Water

Natural organic matter (NOM, measured as dissolved organic carbon, DOC, or UV254 absorbance) is climbing in many sources, a byproduct of climate recovery and shifting vegetation [mdpi.com] [mdpi.com], and conventional treatment has struggled to keep pace [mdpi.com]. Regulators took note: the US EPA’s Stage 1 Disinfectants/Disinfection Byproduct Rule (DBPR) explicitly requires “enhanced coagulation” — lower pH, higher coagulant dose — to hit total organic carbon (TOC) removal targets [sciencedirect.com] [nepis.epa.gov].

Guidance now points plants to raw-water pH setpoints of roughly 5.5–7.0, depending on alkalinity, to maximize organic removal [nepis.epa.gov] [sciencedirect.com]. Miss those low-pH targets and the TOC goals can quickly slip out of reach [nepis.epa.gov] [mdpi.com]. The result: more plants worldwide are leaning on enhanced coagulation — often delivering ≳30–50% DOC/UV254 removal — as NOM and DBP pressures mount.

That operational shift has a hardware side. Accurate chemical addition for both coagulants and acids/alkalis is now central; many facilities specify high-precision dosing pumps for tight pH control while chasing point-of-diminishing-returns on TOC.

Regulatory targets and pH control

EPA’s playbook is direct: adjust to target pH (e.g., ~5.5–7.0 by alkalinity tier) and increase coagulant to the point of diminishing returns (PODR) in TOC removal [nepis.epa.gov], with the guidance explicitly listing “Target pH Under Step 2” values [nepis.epa.gov]. If raw alkalinity is high, EPA notes that operators may need to add coagulant until pH dips below target, then hold pH near ~5.3–5.7 with acid while pushing toward PODR [nepis.epa.gov]. Bench tests repeatedly confirm that pH is the dominant knob for NOM removal efficiency [sciencedirect.com].

Coagulant chemistry: alum, iron, polymers

Conventional aluminum sulfate (alum) and ferric salts (ferric chloride or sulfate) remain staples, but their NOM performance diverges. In dam-water jar tests, ferric chloride (FeCl₃) at ~1 mg/L delivered ~35% UV254 reduction with ~10% turbidity removal, while an equal alum dose managed ~11% UV254 removal [mdpi.com]. A study on hard, DOC‑rich Canadian water found FeCl₃ removed ~70% of DOC (at pH 6) versus ~57% for alum and ~36% for polyaluminum chloride (PACl) [researchgate.net]. Caveat: ferric programs can generate more sludge and introduce trace metals into residuals that must be managed [mdpi.com].

Pre‑hydrolyzed polymeric salts — notably PACl — change the equation. Industry literature ties PACl’s polynuclear Al species and higher charge density to faster, larger flocs and higher organics removal at equal Al dose; in comparisons, PACl outperformed both alum and FeCl₃ on UV254 removal [mdpi.com]. On filter backwash water, a polyaluminum–ferric chloride blend (PAFCl) at 13 mg/L reached ~50% DOC and ~50% UV254 removal, while FeCl₃ at 30 mg/L achieved ~44% DOC and ~47% UV254; both hit ~99.5% turbidity removal at optimum [springer.com] [springer.com]. In a screening of eight commercial PACl formulations, all variants beat alum on turbidity, with one local example showing that at 20 mg/L doses, alum left 0.49 NTU versus 0.22 NTU for PAC (TSS≈0 mg/L vs ≈1 mg/L) [researchgate.net] [ojs.unud.ac.id].

Plants weighing alum against PACl also face economics. One WTP reported alum at ~Rp 95/m³ treated vs PAC at ~Rp 215/m³; PAC delivered better turbidity but cost ~2.3× more on a volume basis [ojs.unud.ac.id]. That gap narrows when PACl is dosed lower: for the same NTU removal, industry sources report needing roughly 1–2× less aluminum (by mass) than alum, with smaller pH depression and lower residual Al in finished water [mordorintelligence.com] [nbinno.com] [nbinno.com]. Facilities moving to PACl often standardize on high‑purity PAC or specify custom basicity PAC to match source-water pH, and some deploy high‑basicity aluminum chlorohydrate (ACH) for difficult low‑pH sources.

Beyond PACl, new inorganic polymers and doped coagulants are emerging. Polyaluminum lanthanum silicate (PALS) posted 39% UV254 removal and ~DOC removal comparable to PAC at half the Al dose [mdpi.com]. Titanium (TiCl₄) and zirconium (ZrOCl₂) salts outperformed alum and iron in some studies, yielding denser sludge and sometimes reusable byproducts [mdpi.com]. These remain niche compared to commercial PACl/polyferric offerings. For conventional programs, utilities commonly procure bulk coagulants and, where needed, polymer aids.

Jar tests, PODR, and removal metrics

Bench and pilot jar tests quantify DOC (or TOC) reduction and the UV254 surrogate. Examples span water types: FeCl₃ at 0.94 mg/L delivered 35% UV254 removal (alum ~11%) in dam water, with PACl exceeding those results in the same comparison [mdpi.com]; in Alberta surface water, Fe salts reached ~70% DOC removal at pH 6 versus alum at 57% and PACl at 36% [researchgate.net] — with performance dropping outside that pH optimum [researchgate.net]. In Iranian backwash water, PAFC at 13 mg/L hit ~50% DOC/UV254 versus FeCl₃ at 30 mg/L achieving ~44% DOC and ~47% UV254 [springer.com] [springer.com]. A Jakarta‑area plant (WTP Ciapus) needed ~20 ppm in both cases for clear water, with alum producing 0.49 NTU (TSS≈0 mg/L) and PAC 0.22 NTU (TSS≈1 mg/L) [ojs.unud.ac.id].

EPA formalized how to set “point of diminishing returns” in these jar tests — the coagulant dose beyond which additional chemical yields negligible NOM removal — and then to benchmark compliance at or before that point [nepis.epa.gov]. Many utilities route settled floc to a clarifier as part of the standard coagulation–flocculation–sedimentation train before filtration.

Operating pH: the primary control knob

Removal vs. pH typically traces a U‑curve. In Swedish jar tests, alum (ALG) showed a UV254 minimum near pH ~6.1, while ferric chloride (PIX‑311) peaked around pH ~5.1 — echoing field practice: Al‑based coagulants perform best near pH ~6 and Fe‑based near pH ~5 [mdpi.com] [mdpi.com] [mdpi.com]. PACl blends often work well over pH ~5.5–6.5, with higher‑basicity PAC favored at neutral‑to‑basic pH and lower‑basicity PAC at slightly acidic pH for more in‑situ polymerization [sciencedirect.com]. Drop pH below optimum (e.g., <5) or fail to lower it enough, and NOM removal can fall sharply [researchgate.net] [sciencedirect.com]. EPA’s target tables anchor acidic setpoints — e.g., 5.5 for low alkalinity and 6.0 for moderate alkalinity waters — to maximize removal [nepis.epa.gov].

Dialing in pH yields tangible chemical savings. In one enhanced coagulation test, about 50% less alum was needed to meet a TOC goal after shifting pH from ~7 to ~5.5 [nepis.epa.gov] [mdpi.com]. In alkaline windows (pH ~7.3–7.9), adding calcium chloride (CaCl₂) can boost charge neutralization; one study reported that 10–20 g/L CaCl₂ nearly doubled organic removal efficiency at those pH values [mdpi.com]. pH control is operationally central; many plants pair acid feed with accurate dosing equipment and, where mixing strength is low, a polymer aid from the plant’s flocculants program.

PACl’s broader effective pH window (roughly pH 5–9) and smaller pH drop can further smooth operations and reduce alkali demand for stabilization [nbinno.com]. High‑basicity PACl can be specified directly via custom basicity, while difficult low‑pH sources often justify very high‑basicity ACH.

Plant outcomes and market signals

Operators report practical gains: in a Lao WTP study, optimized coagulation reduced THM precursors (TOC and UV) by tens of percent, delaying disinfection by‑product formation. In pilot work, raising coagulant by ~50% and dropping pH from ~7.5 to ~5.5 often doubled DOC removal. Where budgets allow, hybrid strategies are surfacing; for example, high‑basicity PACl paired with a polymeric aid (e.g., chitosan) achieved ~63% DOC and ~82% UV removal in low‑turbidity water [mdpi.com]. For turbidity‑focused optimization, one AlcoPAC program jar‑tested to an optimal PAC dose of ~22–28 mg/L near pH ~7.5–8.0 to minimize settled turbidity — underscoring that dose and pH co‑optimize in practice [researchgate.net].

Market data mirror the shift. The polyaluminum chloride market is growing at ~6% CAGR through 2030, as large plants favor lower‑dose, higher‑efficiency coagulants [mordorintelligence.com]; Asia‑Pacific demand is surging on water reuse and mineral industries [mordorintelligence.com]. Indonesian utilities are piloting PACl and hybrids to tackle NOM‑rich rivers, with one Jakarta‑area unit showing PAC cut residual turbidity by more than half versus alum at equal dose (0.49 NTU vs 0.22 NTU at ~20 ppm) [ojs.unud.ac.id]. Procurement teams increasingly bundle coagulants with service packages for jar testing and on‑site pH optimization.

The rule of thumb emerging from both bench and full‑scale work: operate at the pH that gives the lowest UV254 in jar tests and dose just past PODR. Combined with polymeric coagulants — PACl, polyferrics, and related chemistries — and careful pH control (often ~5.5–6.5), plants consistently strip more DOC/UV254 than with alum or FeCl₃ alone [mdpi.com] [springer.com]. Those gains translate directly into lower disinfection by‑product formation and safer finished water.