Raw Water Coagulation With PAC: Adjusting the Dose When River Turbidity Changes

Raw water coagulation on a river source is a moving target. After a stretch of dry weather the river runs fairly clear. One heavy storm upstream and turbidity can climb many times over within hours. In winter the water turns cold and clear, and floc refuses to grow. For a small plant drawing from a river, the real question is not how much PAC to add but how to follow turbidity as it changes. The practical method is to run jar tests whenever conditions shift and build your own chart of raw water turbidity against optimum dose, then dose in proportion to plant flow and correct the dose rate from raw and settled water turbidity. This guide is for operators of small town and village water plants and industrial river intakes.

Why raw water coagulation needs a moving PAC dose

In raw water coagulation, PAC destabilizes fine suspended particles so they gather into floc and settle. More particles mean more charge to neutralize, so the dose generally rises with turbidity, but not in straight proportion.

• At high turbidity there are many particles and many collisions, so floc forms readily and the dose per unit of turbidity is relatively low.
• At low turbidity particles rarely meet, floc stays small and light, and the dose is often higher than expected or needs a coagulant aid.
• In cold water PAC hydrolyzes slowly and the water is more viscous, so floc settles more slowly.

The same river therefore has a different optimum in the flood season, in normal flow and in the cold low-flow season.

Build a turbidity and dose chart for your plant

This is the foundation of good raw water coagulation, and it is worth the effort:

1. Whenever raw turbidity changes noticeably, run a jar test with several doses.
2. Reproduce the plant's real mixing, flocculation and settling times in the test.
3. Measure settled supernatant turbidity and pick the lowest dose that meets the target.
4. Record raw turbidity, water temperature and the optimum dose as one line. Over time the lines become your chart.

With the chart in hand, the operator on shift can read raw turbidity and know roughly where to set the dose, then confirm with a jar test. That is far steadier than adjusting by feel.

Control options for raw water coagulation

Small plants usually choose one of three schemes, in rising order of complexity:

• Manual fixed feed. The operator sets pump output and changes it when flow or turbidity changes. Simple, but someone must watch closely.
• Flow pacing. The raw water flowmeter sends 4–20 mA or pulses to the metering pump, so feed follows flow automatically. The operator sets the dose in mg/L from turbidity.
• Flow pacing with turbidity feed-forward. An online turbidimeter corrects the dose rate automatically using the chart.

Flow pacing is enough for raw water coagulation at most small plants, as long as operators change the dose promptly when raw turbidity moves. Feed-forward needs well-maintained instruments; a blocked turbidimeter sample line drags the whole control loop with it.

Raw water coagulation during a flood

Storm runoff is when effluent is most likely to go out of compliance:

• Watch raw turbidity and raise the dose as soon as it climbs. Do not wait for settled water to turn cloudy, because clarifier detention adds a delay.
• After increasing the dose, check the flocculator. Large, dense floc and clear supernatant mean you have it right.
• At very high turbidity a polymer aid may help, but test it first rather than adding it blindly.
• Keep up with desludging. A high sludge blanket spoils effluent no matter how much coagulant goes in.

Handling cold, clear water

In winter raw water coagulation runs into a different problem: water is clear but cold, floc is fine and will not settle, and settled turbidity actually rises.

• Raise the dose slightly or add a coagulant aid, guided by jar tests.
• Check that rapid mixing is strong and the injection point is in the most turbulent zone.
• If flocculation time is too short, reduce plant throughput.

Size the metering pumps for the full range

For raw water coagulation, size the top end on maximum plant flow times the highest flood-season dose, and check that minimum flow times the lowest normal dose still sits inside the pump's useful adjustment range. If one pump cannot cover both, use a large and a small pump, or a variable speed metering pump.

PAC is mildly corrosive and crystallizes, so choose PVC or PP heads, flush lines regularly and rinse the head with clean water before a long stop. Wear gloves and goggles when batching.

Conclusion

Raw water coagulation on a river source has to follow turbidity. Build a chart of turbidity against optimum dose from jar tests, pace PAC to flow and correct the dose rate from turbidity, dose early in floods and keep desludging, and add dose or a coagulant aid in cold, clear water. Make sure the pumps cover everything from low flow to flood. Further reading: typical PAC dosage ranges, online turbidity monitoring and PAC and PAM dosing sequence. Equipment: metering pumps and dosing skids. Send plant flow and raw water turbidity range and contact our engineers.

FAQ

Is PAC still needed when raw turbidity is very low?

Usually yes. Low-turbidity water is harder to settle, so raw water coagulation still needs PAC, with the dose and method set by testing.

How long before a dose change shows up?

The water must pass through mixing, flocculation and settling, so the delay depends on clarifier detention. Do not change the dose again before the first change has had time to show.

Can I trust the online turbidimeter?

Clean and calibrate it regularly and keep the sample line clear. Check against a portable meter when readings look odd.

The PAC tank runs short during floods. What can I do?

Stock up before the wet season. A slightly stronger solution helps, but not too strong, or it becomes viscous and crystallizes.

One pump cannot cover the range. What are the options?

Use a large and a small pump for different conditions, or a variable speed metering pump with wider turndown.