Complete Chemical Program for a Municipal WWTP

Treatment Stage Overview

Stage	Chemical	Function	Typical Dose
Primary	PAC 30%	Coagulation — TSS/BOD removal	30–80 ppm
Primary	Anionic PAM (medium MW)	Flocculation — settle coagulated flocs	0.5–2 ppm
Secondary	Sodium hydroxide or soda ash	pH adjustment for biological process	As needed
Tertiary	PFS or FeCl₃	Chemical phosphorus removal	10–30 ppm as Fe
Disinfection	SDIC or NaOCl	Pathogen kill before discharge	2–5 ppm Cl₂
Sludge	CPAM (60–80% charge)	Sludge dewatering conditioning	2–5 kg/ton dry solids

Primary Treatment

Objective: Remove 50–70% of TSS and 30–40% of BOD through coagulation-flocculation-sedimentation.

Sequence: PAC dosing → flash mix (1 min, 200 rpm) → anionic PAM → slow mix (15 min, 40 rpm) → settling basin (2–4 hr HRT)

Key control: Optimize PAC dose by jar test weekly. Seasonal adjustment needed — dose increases in cold weather.

Phosphorus Removal

To meet TP < 0.5 mg/L discharge limits, chemical P removal is typically needed in addition to biological uptake:

• Dosing point: Before secondary clarifier or in tertiary treatment
• Chemical: FeCl₃ or PFS at Fe:P molar ratio of 1.5–2.5:1
• pH control: Maintain pH 6.5–7.5 for optimal precipitation

Sludge Dewatering

The largest chemical cost in most WWTPs. CPAM selection is critical:

1. Collect sludge sample
2. Test 3–4 CPAM grades (low/medium/high charge) in beaker test
3. Measure CST (capillary suction time) — lower = better dewaterability
4. Select grade with lowest CST and best cake appearance
5. Typical dose: 2–5 kg polymer per ton dry solids

Annual Chemical Cost Estimate

For a 50,000 m³/day municipal WWTP:

Chemical	Annual Cost (approx.)
PAC	$150,000–250,000
Anionic PAM	$20,000–40,000
CPAM (sludge)	$80,000–150,000
FeCl₃ (P removal)	$50,000–100,000
Disinfectant	$15,000–30,000
Total	$315,000–570,000