Jar-test for PAC — Step-by-Step Guide for Operations Engineers | Hóa Chất Lộc Thiên

Jar-test for PAC — Step-by-Step Guide for Operations Engineers

TL;DR: Jar-test is the standard method (ASTM D2035) for determining the optimal PAC dosage for each specific water source. 6 steps: sample preparation → prepare PAC stock → gradient dosing → rapid mix 200 rpm/60s → slow mix 40 rpm/20 min → settle 30 min → read results. Common mistakes: wrong stock concentration, insufficient rapid mix time, ignoring pH, selecting dose by largest floc instead of lowest turbidity.

Purpose of Jar-test for PAC

Jar-test simulates the coagulation — sedimentation process in the laboratory at small scale (1 L/beaker), allowing comparison of multiple PAC dosages on the same raw water sample. Jar-test results are the sole basis for establishing operational dosage at the actual treatment plant — they cannot be replaced by theoretical calculations or pure experience.

For PAC, jar-test helps determine:

• Optimal dose — the lowest PAC concentration achieving target turbidity
• Charge reversal point — the dose at which turbidity begins to rise again
• Effect of pH and temperature on coagulation efficiency

6 Steps of Jar-test for PAC — Standard Procedure

Step 1 — Preparation

Equipment:

• 6-paddle gang stirrer (6 positions) — with adjustable rpm
• 6 glass beakers, 1,000 mL (square jars preferred — more accurate simulation of actual mixing zone)
• pH test kit, turbidity meter (or handheld turbidimeter)
• Pipette 1–10 mL, analytical balance (± 0.01 g)
• 100 mL graduated cylinder, volumetric flask

Water sample:

• Collect at least 8 L of raw water at the coagulant inlet point
• Collect at mid-depth (not surface, not bottom)
• Measure and record baseline parameters: pH, turbidity (NTU), temperature (°C), alkalinity (mg/L CaCO₃)
• Conduct jar-test within 2 hours of sampling — water properties change over time

Step 2 — Prepare PAC Stock Solution

Formula for PAC 1% w/v stock (10,000 mg/L):

Weigh 10.0 g PAC powder (pure, undiluted)
Dissolve in ≈ 800 mL distilled water
Stir until completely dissolved
Make up to 1,000 mL

Note: PAC stock solution is only usable within 24 hours — PAC continues hydrolyzing in solution, old stock yields inconsistent results.

Calculating dose from stock:

C₁V₁ = C₂V₂ formula:

Example: need 30 mg/L dose in 1 L sample, using 10,000 mg/L stock:

• V₁ = (30 mg/L × 1,000 mL) ÷ 10,000 mg/L = 3.0 mL stock

Step 3 — PAC Dosing (Gradient Dosing)

Set up 6 beakers with evenly spaced dosage range covering the expected optimal region.

Reference dosage table for PAC 30% (powder) — common concentrations:

For PAC 10% (Al₂O₃ ~10%, typically liquid or yellow PAC): Multiply dose × 2.5 from the table above. For PAC 17% (Al₂O₃ ~17%, common yellow PAC): Multiply dose × 1.8 from PAC 30% table. For PAC 31% (Al₂O₃ ≥ 31%, white Indian PAC): Use standard dose as per table.

Rule: the lower the Al₂O₃ content, the higher the dose. Always record PAC content in jar-test report.

Step 4 — Rapid Mix (Coagulation)

• Speed: 200–250 rpm (equivalent to G = 200–300 s⁻¹)
• Time: 60 seconds
• Procedure: Add PAC stock to each beaker simultaneously (within 30 seconds), start mixing immediately

Purpose: evenly disperse PAC throughout the entire water volume, allowing polynuclear aluminum species to hydrolyze and neutralize colloid charge before localized precipitation.

Note for cold water (< 8°C): Extend rapid mix to 90–100 seconds — PAC hydrolysis slows at low temperature.

Step 5 — Slow Mix (Flocculation)

• Speed: 30–40 rpm (equivalent to G = 20–40 s⁻¹)
• Time: 15–20 minutes

Purpose: allow micro-flocs to collide and bind into large floc masses. Do not increase mixing speed at this stage — high shear breaks newly formed flocs.

During the 20 minutes, observe and record:

• Time of first floc appearance (at which minute)
• Floc size at end of stage (mm) — visual estimation
• Floc density (sparse/moderate/dense)

Step 6 — Settling and Reading Results

• Stop mixing completely
• Allow quiescent settling for 30 minutes
• Do not move or vibrate beakers

Post-settling record (sample table):

How to select optimal dose:

• The optimal dose is the beaker with the lowest turbidity — not the beaker with the largest floc
• If both the lowest and a higher dose give low turbidity, choose the lower dose to save chemicals
• The charge reversal point (sudden turbidity increase) defines the maximum dose threshold — actual operation should use a dose 10–15% below this point

Calculating PAC Dose for Full-Scale Operation from Jar-test

Conversion formula from jar-test to dosing pump:

Pump dose (L/h) = [Jar-test dose (mg/L) × Flow rate (m³/h) × 1,000] ÷ [PAC solution concentration (mg/L)]

Example: Jar-test gives optimal dose of 35 mg/L PAC, plant flow 200 m³/h, using liquid PAC 10% (~100,000 mg/L):

• Pump dose = (35 × 200 × 1,000) ÷ 100,000 = 70 L/h

Add a 5–10% safety factor for differences between lab and actual conditions (non-ideal mixing, influent water quality fluctuations).

4 Common Mistakes in Jar-test for PAC

Mistake 1 — Incorrect PAC stock concentration or stock older than 24 hours

Non-standard 1% stock leads to dosage deviation across the entire test range. PAC stock > 24 hours undergoes further hydrolysis, effective concentration decreases — jar-test results are not reproducible.

Solution: Weigh accurately to ± 0.01 g. Prepare fresh stock for each jar-test session. Record stock concentration on the test sheet.

Mistake 2 — Insufficient rapid mix time or speed

Rapid mix < 30 seconds or < 150 rpm prevents PAC from dispersing properly, creating local high-concentration zones — flocs form unevenly, are 2–3× smaller and settle slower.

Solution: Always maintain rapid mix ≥ 60 seconds at 200–250 rpm. Verify actual rpm with a tachometer periodically.

Mistake 3 — Not measuring and adjusting pH before jar-test

As described, PAC completely loses effectiveness at pH > 8.5. If pH is not adjusted to 7.0–7.5 before running the jar-test, all dosage levels will yield similar high turbidity — leading to the false conclusion of “poor PAC quality”.

Solution: Always record baseline pH and post-adjustment pH. For water with pH > 8.0, run an additional jar-test series at different pH values (pH 6.5 / 7.0 / 7.5 / 8.0) at the same PAC dose.

Mistake 4 — Selecting dose based on largest floc instead of lowest turbidity

The largest floc typically appears at the charge reversal zone — flocs look large and settle fast, but post-settling water remains turbid due to re-stabilized fine particles. This is a common error among new engineers.

Solution: Always measure turbidity (NTU) with a turbidity meter instead of visual assessment. Only select the optimal dose when confirmed by the turbidity meter.

Frequently Asked Questions

How does jar-test for PAC differ from jar-test for alum?

PAC requires a narrower dosage gradient (5–10 mg/L instead of 10–20 mg/L) because PAC is 3–5× stronger. PAC is also less pH-sensitive, so alkalinity adjustment is not needed as with alum.

How often should jar-test be repeated?

At minimum each season change (4 times/year). Immediately when the water source changes (after heavy rain, flooding, abnormal discharges).

Is a separate jar-test needed for PAC powder vs. liquid?

Yes. Liquid PAC typically has different Al₂O₃ content (10–17%) compared to powder PAC (≥ 30%). Always calculate dosage based on actual Al₂O₃ content as stated on the COA.

Does jar-test for PAC require PAM (coagulant aid)?

Not mandatory. Run jar-test with PAC alone first to find the optimal coagulant dose, then run a PAC + PAM series to optimize the combination if faster settling is needed.

Does a sample taken from the actual coagulation tank affect results?

Take the sample from the coagulation tank inlet (raw water), not from after the mixing chamber. Water that has already passed through chemical mixing is not representative of the actual water source being evaluated.

🛒 Need PAC 31% with certified COA for jar-test? Get a quote for PAC Poly Aluminium Chloride 31% — test samples, batch COA/MSDS, delivered to industrial parks nationwide. Hotline 0979 891 929.

This article is part of the water treatment technical series by Hóa Chất Lộc Thiên — a large-scale industrial chemical supplier, direct importer, 6 warehouses across 3 regions, nationwide industrial park delivery. All technical data based on PAC with Al₂O₃ content ≥ 30% (equivalent to batch COA).

Quality control: QC Phan Cẩm Thủy tests and Võ Thị Như Hòa approves each PAC batch. Batch COA, certified ZDHC MRSL Level 1 (TÜV Rheinland).