PAC Coagulant Guide: Dosing, Grades, Jar Tests & Troubleshooting

What PAC coagulant solves best

PAC coagulant is widely used for surface water, industrial water, and many wastewaters because it forms strong floc quickly and performs across a broader pH range than traditional alum in many real-world conditions.

Typical targets PAC handles well

• High turbidity events (storm runoff, river spikes) where rapid clarification is needed
• Color and natural organic matter reduction when optimized with pH and (optionally) polymer aid
• Cold water conditions where floc formation often slows for other coagulants
• Systems aiming to reduce sludge volume and improve settleability compared with some conventional programs

A common operational success criterion is stable, well-formed floc within 1–3 minutes of rapid mix, followed by firm settling and low carryover into filtration.

How PAC coagulant works in practical terms

PAC coagulant is a pre-hydrolyzed aluminum coagulant. In water, it rapidly forms positively charged hydrolysis species that neutralize negatively charged colloids (clays, organics), allowing them to collide, attach, and form floc. Because it is partially “pre-built,” PAC often needs less alkalinity adjustment than some alternatives, but pH control is still important.

What you should see when it is working

• Microfloc appears quickly in rapid mix (water looks “snowy” rather than uniformly cloudy)
• Floc grows in slow mix without breaking into pin floc
• Settling produces a clear supernatant and a compact blanket

If you see “cottony” floc that never densifies, or persistent haze after settling, the issue is usually dose, mixing energy, pH/alkalinity, or polymer timing—not the concept of PAC itself.

Choosing the right PAC grade

PAC coagulant products differ by aluminum concentration, basicity (degree of pre-hydrolysis), and impurity controls. Selection should match your water type, compliance requirements, and feed system constraints.

A practical buying checklist

• Specify dose basis in contracts: mg/L as product vs mg/L as Al (avoid pricing misunderstandings)
• Request a current certificate of analysis (CoA) and confirm consistent basicity and aluminum concentration
• Confirm storage temperature limits and compatible tank/piping materials before delivery

Jar testing PAC coagulant the right way

Jar testing is the fastest way to lock in the correct PAC coagulant dose, pH, and polymer strategy for your exact water. The goal is not “biggest floc,” but lowest settled turbidity and stable filtration performance at the lowest reliable chemical dose.

Recommended jar test sequence

1. Measure raw water pH, alkalinity, turbidity, and temperature (these drive repeatability).
2. Prepare PAC feed so dose calculations are simple (for example, a 1,000 mg/L as product working solution).
3. Rapid mix: 30–60 seconds at high speed immediately after PAC addition (timing matters more than exact RPM).
4. Slow mix: 10–20 minutes at gentle speed to grow floc without shearing.
5. Settle: 10–20 minutes, then measure supernatant turbidity and visually check carryover.
6. If using polymer aid, add it after PAC during early slow mix and test at least 2–3 polymer doses (too much polymer can create haze).

Example: simple dose math you can reuse

If you make a 1,000 mg/L PAC working solution and each jar contains 1.0 L, then adding 10 mL equals a 10 mg/L jar dose. This makes a 10–60 mg/L screening test straightforward (10, 20, 30, 40, 50, 60 mL additions).

In many surface waters, you will often observe a “sweet spot” where turbidity drops sharply and then plateaus; choose the first dose on the plateau to protect cost and minimize residual carryover risk.

Field dosing and control points that matter most

Once the jar test identifies a target dose, stable performance depends on controlling a small set of high-impact variables: mixing, pH/alkalinity, and real-time raw water changes.

Mixing and injection location

• Inject PAC coagulant where you have immediate, intense turbulence (static mixer, flash mix, or high-energy zone).
• Avoid long, low-turbulence feed lines that let PAC “react in the pipe” before contacting colloids.

pH and alkalinity targets

A reliable operating band for many waters is pH 6.0–8.0, with optimization commonly near neutral. If alkalinity is low, pH can sag during coagulation and floc quality may degrade; use your jar test to decide whether a small alkalinity supplement (e.g., lime, soda ash) improves clarity and stability.

A practical control strategy

• Use raw water turbidity and temperature as leading indicators for dose adjustment during rapid changes.
• Track settled water turbidity (or clarifier effluent) as the primary feedback signal.
• When filters are present, include differential pressure or run time as a downstream confirmation of “good coagulation.”

Troubleshooting PAC coagulant: symptoms and fixes

Most PAC coagulant issues present as carryover haze, weak settling, or filter problems. The fastest way to diagnose is to match the symptom to the most likely control lever and adjust one variable at a time.

Common symptoms and what to try first

• Persistent haze after settling: reduce polymer (if used), confirm rapid mix energy, and check pH drift during coagulation.
• Pin floc carryover: lower PAC dose slightly or increase slow-mix time; excessive shear can also break floc into fines.
• Floc won’t form in cold water: trial a modest PAC dose increase and/or a higher-basicity grade; verify mixing is not underpowered.
• Filter run times shorten: confirm clarification is truly minimizing fines; optimize coagulation to reduce particle breakthrough rather than “making bigger floc” alone.

When to re-jar test immediately

Re-run a quick jar test if raw water turbidity changes by 2–3×, temperature shifts materially, or a new upstream source/blend is introduced. These changes often move the optimal PAC coagulant dose enough that “small tweaks” become unreliable.

Residuals, sludge, and downstream impacts

PAC coagulant can reduce sludge volume and improve dewaterability in many systems, but overfeeding can increase fines and elevate residual aluminum risk. Managing residuals is primarily about avoiding overdose and keeping coagulation in the correct pH window.

Practical safeguards

• Use the lowest dose that consistently meets settled turbidity targets, especially before filtration.
• Keep pH stable; drifting too low or too high can increase soluble aluminum species and reduce performance.
• If you operate filters, monitor post-filter turbidity trends after any PAC change; filtration is often the first place poor coagulation appears.

As an operational rule, good clarification should simplify filtration. If clarification looks acceptable but filters worsen, the coagulation program is likely creating fines or destabilizing floc at the wrong point in the process.

Handling and storage essentials

PAC coagulant is acidic and can be corrosive to incompatible materials. Safe, stable performance depends on using proper materials of construction and preventing contamination or freezing/overheating depending on the product.

Operational best practices

• Keep storage tanks sealed to limit contamination; mix gently if recommended by the supplier to maintain uniformity.
• Use compatible piping and pumps (confirm with supplier documentation); avoid unverified metals and elastomers.
• If you dilute, use clean water and consistent dilution ratios; label working solutions with date/time to avoid confusion.

Bottom line: the fastest way to succeed with PAC coagulant

Start with a jar test, control pH, and dose PAC coagulant at the lowest stable point on the performance plateau. In day-to-day operation, mixing quality and raw water shifts drive outcomes more than fine differences between grades.

If you implement only three actions, make them these: rapid mix immediately at injection, operate near pH 6.0–8.0, and re-jar test when the source water changes materially. This combination delivers dependable clarification, minimizes carryover, and keeps chemical cost predictable.