PAC Dosage for Sewage: Finding the Right Dose With a Jar Test
PAC dosage for sewage is not something you can copy from a table. The right dose depends on what the polyaluminium chloride is doing and on the water you are treating. PAC added at the end of the aeration tank for chemical phosphorus removal, PAC used to coagulate secondary effluent ahead of a filter, and PAC thrown into a packaged plant during an upset all need very different amounts. Inlet phosphorus, suspended solids, alkalinity and temperature move the number again. The dependable method is a jar test to find the smallest dose that meets your target, a conversion into a pump setting, and adjustment from effluent results. This guide is for operators of small municipal, campus and community wastewater plants.
PAC dosage for sewage starts with the purpose
Three common uses decide PAC dosage for sewage in domestic wastewater plants:
- Chemical phosphorus removal. When biological removal cannot meet the permit, PAC is added at the end of the biological stage or ahead of the clarifier, and aluminum precipitates phosphate. The dose follows how much phosphorus must be removed.
- Tertiary coagulation. Secondary effluent is coagulated before settling or filtration to cut solids and turbidity. Doses are usually lower and PAC is often paired with a polymer.
- Upset response in packaged plants. Dose is raised temporarily during a shock load. Bring it back down afterwards, or you waste chemical and make extra sludge.
The definition of "right" also changes: total phosphorus for removal duty, turbidity and floc settling for coagulation.
Converting PAC dosage for sewage into grams and pump flow
The conversion operators use most is simple:
1 mg/L equals 1 gram per cubic meter (per tonne) of water.
If the jar test gives an optimum of a certain mg/L, you need that many grams of PAC per cubic meter. Make sure you and the lab agree on whether the dose is expressed as commercial product or as alumina content.
For the pump:
- PAC demand (kg/h) = flow (m³/h) × dose (mg/L) ÷ 1000.
- Solution flow (L/h) = PAC demand ÷ solution strength (mass fraction; dilute solution density is close to 1).
Solution strength is usually set by the designer. Too strong and it gets viscous and crystallizes in lines; too weak and the day tank becomes huge.
Running a jar test
You need a few beakers and a stirrer:
- Fill several beakers with the same volume of sewage and add different PAC doses.
- Mix fast for a minute or two to disperse the chemical, then slowly for a few minutes to grow floc.
- Let them settle and compare floc size, settling speed and clarity, testing phosphorus or turbidity where needed.
- Take the lowest dose that meets the target as the starting set point.
Always test same-day samples. Stored samples change, and a test on old water gives a misleading PAC dosage for sewage. Repeat the test when the season changes or the influent shifts.
What changes PAC dosage for sewage
- Phosphorus: higher inlet phosphorus means a higher dose for removal duty.
- Solids and turbidity: more solids consume more coagulant but also help floc form.
- Alkalinity and pH: PAC hydrolysis consumes alkalinity, and if alkalinity runs short, pH falls and coagulation suffers.
- Temperature: cold water slows hydrolysis and gives small, loose floc, so winter usually needs more.
- Product quality: alumina content and basicity vary between suppliers, so retest after any product change.
Signs of underdosing and overdosing
Too little: fine floc that will not settle, cloudy supernatant, phosphorus or turbidity over the limit.
Too much: floc breaks up again, effluent turns milky, pH drops and sludge volume jumps, so chemical and sludge disposal costs both rise.
Both ends can fail the permit, which is why the best PAC dosage for sewage is the lowest one that works, not the highest.
Pump selection and setting
Once the PAC dosage for sewage is known, size the pump. Small plants usually run solenoid metering pumps; larger ones use mechanical diaphragm pumps. Choose PVC or PP heads with valve parts that tolerate the mildly acidic solution. Size for maximum flow and maximum dose with margin so the stroke does not sit below 30% for long.
Set stroke and speed from the calculated solution flow, then verify with a calibration column or graduated cylinder. PAC crystallizes, so flush the head and lines with water after any long stop. Wear gloves and goggles when batching.
Keep a log instead of trusting rules of thumb
Record flow, stroke, speed, day tank drawdown and effluent phosphorus or turbidity on every shift. Each day, back-calculate the actual dose from tank drawdown and compare it with the set point. A large gap points to a pump or line problem. After a month or two the log tells you the real PAC dosage for sewage at your plant across seasons and flows, which beats any number found online.
Conclusion
PAC dosage for sewage depends on purpose and water quality. Find the lowest effective dose with a jar test, convert it with the rule that 1 mg/L is 1 g per cubic meter, and trim it from effluent phosphorus or turbidity. More reading: typical PAC dosage ranges, signs of PAC and PAM overdose and PAC dosing pump selection. Equipment: solenoid metering pumps and dosing skids. Send flow, influent and effluent data and the feed point and contact our engineers.
FAQ
Liquid or solid PAC?
Liquid is easier for small plants. Solid is cheaper to ship and store at high usage but needs a dissolving tank and mixer.
Where should PAC be injected?
Coagulation needs rapid mixing, so inject ahead of a static mixer or at the most turbulent point of the rapid mix tank, never in a dead zone.
Can PAC and polymer be dosed together?
Usually PAC goes in first and polymer after a short mixing interval. Never blend them in one tank.
How much higher should PAC dosage for sewage be in winter?
There is no fixed ratio. Repeat the jar test in cold weather and go by the result.
The pump runs but effluent does not change. Why?
Confirm the pump is actually delivering with a calibration check, then check solution strength and whether the injection point is blocked, before changing the dose.