Municipal wastewater treatment with DAF
Most African municipal works are hydraulically overloaded, short of land, and on an unreliable grid. High-rate flotation addresses all three at once, which is why it keeps appearing in uprating projects where a new clarifier is simply not buildable.
High-rate dissolved air flotation is used in municipal treatment for four duties: primary clarification in place of a settlement tank, uprating an overloaded works without new civils, algae and colour removal from surface water for potable supply, and thickening waste activated sludge from 0.5–1 % to 4–6 % dry solids. On raw sewage it removes 70–90 % of suspended solids and 40–60 % of BOD at surface loading rates of 15–30 m/h — roughly twenty times a conventional primary settlement tank — using 30–120 mg/L of ferric chloride or alum plus 0.5–2 mg/L of anionic polymer.
Four municipal duties
1. Primary clarification
Chemically enhanced flotation replaces a primary settlement tank at a fraction of the footprint. A conventional PST runs at 0.8–1.5 m/h and removes 50–65 % TSS and 25–35 % BOD. A high-rate DAF at 20 m/h removes 70–90 % TSS and 40–60 % BOD in roughly one-fifteenth to one-twentieth of the plan area, and it removes phosphorus as a bonus of the iron or aluminium dose.
2. Uprating an overloaded works
This is the most common African application. A works designed for 8,000 m³/d is receiving 14,000 m³/d because the catchment grew. Adding a high-rate DAF ahead of the existing biology cuts the organic load reaching the aeration basin by 40–60 % and the solids load by 70–90 %, which effectively restores the works to design capacity without new tanks, new blowers or new land.
3. Algae and colour removal from surface water
Algae are buoyant, which is precisely why sedimentation struggles with them and flotation excels. On eutrophic reservoir water, DAF removes 90–99 % of algal cells and 80–95 % of associated turbidity, producing water at below 1 NTU into the filters. Where a works suffers seasonal blooms that blind the sand filters, DAF ahead of filtration typically extends filter runs by a factor of three to five.
4. Sludge thickening (DAFT)
Waste activated sludge at 0.5–1.0 % dry solids is expensive to move and slow to digest. Flotation thickening lifts it to 4–6 % — a five-fold volume reduction — at a solids loading rate of 2–5 kg/m²·h, using only 1–3 mg/L of polymer. It handles filamentous, bulking sludge that a gravity thickener cannot.
Performance against a conventional primary tank
| Parameter | Primary settlement tank | High-rate DAF |
|---|---|---|
| Surface loading rate | 0.8 – 1.5 m/h | 15 – 30 m/h |
| Plan area for 10,000 m³/d | ~350 – 500 m² | ~15 – 30 m² |
| TSS removal | 50 – 65 % | 70 – 90 % |
| BOD₅ removal | 25 – 35 % | 40 – 60 % |
| Total phosphorus removal | 10 – 20 % | 60 – 90 % |
| Sludge dry solids | 1 – 3 % | 3 – 6 % |
| Chemical demand | None | 30 – 120 mg/L coagulant |
| Energy | ~0.005 kWh/m³ | 0.05 – 0.12 kWh/m³ |
| Recovery after power loss | Immediate (passive) | 5 – 10 minutes |
| Civil works | Extensive | Slab only for packaged units |
The load-shedding argument
This matters more in Africa than anywhere. A DAF is a physical process with no biomass to protect. After a six-hour outage it returns to specification within five to ten minutes of the saturator repressurising. An activated sludge plant that loses aeration for a full day can take one to three weeks to rebuild nitrifying biomass, during which the works is out of consent and the receiving water takes the load.
On a grid with frequent interruptions, placing a DAF ahead of the biology gives two protections: it strips most of the load so the biology has less work to do, and it provides a compliant physical-chemical treatment path that keeps functioning on a modest generator when the biological stage cannot.
Worked sizing example: 12,000 m³/day works uprating
- Design flow. 12,000 m³/d average = 500 m³/h; peak dry weather factor 1.8 = 900 m³/h design.
- Load. Raw TSS 320 mg/L, BOD 280 mg/L, total P 8 mg/L. Solids at peak: 900 × 0.320 = 288 kg/h.
- Flotation area. At 22 m/h high-rate: 900 ÷ 22 = 41 m². Two cells of 21 m² each, for redundancy and turndown.
- Coagulant. Ferric chloride at 70 mg/L for combined solids and phosphorus removal: 63 kg/h as product, generating roughly 30 kg/h of additional hydroxide floc.
- Air demand. A/S of 0.012 on a municipal stream: (288 + 30) × 0.012 = 3.8 kg air/h.
- Recycle flow. At 5 bar and 25 °C, usable dissolved air ≈ 103 mg/L. Recycle = 3.8 ÷ 0.103 = 37 m³/h, a recycle ratio of just 4 % — municipal streams are cheap to float compared with industrial ones.
- Outcome. TSS to biology falls from 320 to roughly 50 mg/L, BOD from 280 to roughly 130 mg/L, total P from 8 to under 2 mg/L. The aeration basin, previously at 175 % of design load, returns to approximately 80 %.
- Sludge. Roughly 250 kg/h dry at 4 % ≈ 6.3 m³/h, thickened enough to go straight to dewatering without a separate thickener.
Frequently asked questions
Can DAF replace primary settlement tanks?
Yes, and it outperforms them: 70–90 % TSS removal against 50–65 %, 40–60 % BOD against 25–35 %, and 60–90 % phosphorus removal against almost none, in roughly one-twentieth of the plan area. The cost is 30–120 mg/L of coagulant and 0.05–0.12 kWh/m³ of electricity. Where land is cheap and chemicals are hard to source, settlement is still the more robust choice.
What surface loading rate should a municipal DAF use?
15–30 m/h for high-rate units on raw or settled sewage, with 20–22 m/h a common design point. Conventional-rate designs at 5–12 m/h are used where influent quality is highly variable or where operator attention will be intermittent. Sludge thickening duty is sized on solids loading rather than hydraulic loading: 2–5 kg dry solids per m² per hour.
Is DAF good for removing algae from drinking water?
It is the process of choice. Algal cells are buoyant, which defeats sedimentation and suits flotation. On eutrophic reservoir water a DAF removes 90–99 % of algal cells and 80–95 % of turbidity, delivering below 1 NTU to the filters and typically extending filter runs three- to five-fold during bloom season. It also removes the cells intact, which matters for cyanobacteria, since lysing them releases toxins.
How much does municipal DAF cost to run?
Energy is 0.05–0.12 kWh per m³ treated, dominated by the recycle pump and air compressor. Chemicals at 70 mg/L ferric and 1 mg/L polymer are typically USD 0.03–0.08 per m³ depending on local delivered chemical prices. Against that, set the avoided cost of new civil structures, the land not purchased, and the phosphorus removal that would otherwise require a separate dosing stage.