CPCB Source Document
Environment (Protection) Rules 1986 — General Effluent Standards; CPCB Design Manual for Effluent Treatment Plants
Authority: CPCB under Environment (Protection) Act 1986 · Applicable to all industrial ETPs including dairy, food, paper, and petroleum sectors
View effluent standards on cpcb.nic.in ↗CPCB website links may change — search "DAF ETP design" on cpcb.nic.in if the link is broken.
What is DAF and Where is it Used in Industrial ETP
Dissolved Air Flotation (DAF) is a physical-chemical separation process that removes suspended solids, colloidal particles, oils, greases, and fine fibres from wastewater by attaching microscopic air bubbles to the particles, causing the particle-bubble aggregates to float to the water surface where they are mechanically skimmed off as a concentrated sludge (called "float").
In the context of industrial ETPs in India, DAF units serve as either primary treatment (replacing or supplementing conventional primary clarifiers for wastewater with high oil/grease or fine TSS content) or as a tertiary polishing step after biological treatment to meet stringent CPCB TSS discharge limits. DAF is particularly effective for wastewater streams where conventional gravity sedimentation is insufficient due to the low density or colloidal nature of the suspended matter.
- Oil and grease removal: DAF can reduce oil and grease from 200–1,000 mg/L in raw food or dairy wastewater to below 10 mg/L (CPCB limit) in a single stage with appropriate chemical dosing.
- Fine TSS capture: Particles too small or too light to settle by gravity — including algae, activated sludge fines, and paper fibres — are efficiently captured by flotation.
- Compact footprint: DAF achieves separation in a fraction of the retention time required for gravity clarification, making it suitable for sites with limited land availability.
- Sludge concentration: DAF float has 2–6% solids content, significantly more concentrated than primary clarifier underflow, reducing downstream dewatering requirements.
CPCB Regulatory Context for DAF in Industrial ETP
CPCB prescribes outcome-based discharge standards rather than mandating specific treatment technologies. For industries with high oil and grease content in their wastewater — dairy, edible oil refining, fish processing, petroleum refining — the CPCB limit of ≤ 10 mg/L oil and grease for inland surface water discharge practically necessitates a DAF unit or equivalent physical-chemical treatment upstream of biological treatment.
The CPCB Consent to Establish (CTE) process requires industries to document their proposed ETP process flowsheet. For wastewater with emulsified oils, high FOG (fats, oils, grease), or fine suspended solids, DAF is well-accepted by pollution control boards as the appropriate technology choice, and SPCBs may specifically ask about DAF design parameters during CTE review for industries in the above sectors.
DAF Design Parameters at a Glance
The following table summarises key DAF design parameters for industrial wastewater applications in India:
| Parameter | Typical Design Range | Notes |
|---|---|---|
| Hydraulic surface loading rate | 3–6 m³/m²·hr | Up to 10–15 m³/m²·hr for high-rate DAF |
| Air-to-solids (A:S) ratio | 0.005–0.060 mL/mg | Depends on TSS and bubble size |
| Recycle ratio | 15–50% | 25–35% most common for food/dairy |
| Saturation pressure | 4–6 bar (gauge) | Higher pressure = more dissolved air |
| HRT (contact zone) | 10–20 minutes | Including separation zone |
| Coagulant dose (alum) | 50–200 mg/L | Adjust based on jar test |
| Flocculant dose (PAM) | 0.5–3 mg/L | Cationic for negatively charged particles |
| Float solids content | 2–6% TS | Higher than primary clarifier sludge |
| Effluent TSS (typical) | 10–30 mg/L | After chemical dosing optimisation |
Hydraulic Loading Rate and Tank Sizing
The hydraulic surface loading rate governs the plan area of the DAF flotation tank. At the standard design rate of 3–6 m³/m²·hr, a DAF unit treating 100 m³/hr of industrial wastewater requires 17–33 m² of tank plan area. Rectangular DAF tanks are most common in Indian industrial installations, with length-to-width ratios of 3:1 to 6:1 to promote plug flow and prevent short-circuiting.
The contact zone (inlet section where pressurised recycle is introduced) must be designed to allow gentle mixing of the recycle stream with the influent — turbulent mixing at the inlet can break up flocs before they have been captured by air bubbles, significantly reducing removal efficiency. A typical contact zone retention time of 1–3 minutes is maintained at a low upward velocity to promote bubble-particle contact.
- Separation zone depth: The separation zone (main flotation tank) should have a minimum water depth of 1.5–2.5 m. Deeper tanks can accommodate sludge blankets without hydraulic disturbance; shallower tanks risk float layer re-entrainment.
- Skimmer design: Chain-and-flight skimmers or rotating beach skimmers should be specified with adjustable weir height to maintain float layer thickness of 5–15 cm without excessive water carryover into the float sludge.
- Sludge removal: Settled solids in the DAF tank floor (for particles that do not float) are removed by a bottom scraper system into a hopper and pumped to sludge handling. Bottom sludge accumulation is a common maintenance issue in DAF units treating wastewater with high inorganic TSS.
Pressurisation System: Recycle Ratio and Saturation Pressure
The pressurisation system consists of a recycle pump, a saturation tank (or inline contact pipe), and a pressure control valve. Clarified effluent from the DAF outlet is recycled, pressurised to 4–6 bar by the recycle pump, and saturated with compressed air in the saturation tank. When the pressurised recycle is released to atmospheric pressure through the inlet nozzles in the contact zone, dissolved air nucleates out of solution as millions of fine bubbles (20–100 micron diameter).
- Saturation efficiency: Well-designed saturation tanks achieve 80–90% saturation efficiency (80–90% of theoretical air dissolution at the operating pressure). Poorly designed systems may achieve only 50–60%, requiring a higher recycle ratio to compensate.
- Bubble size: Smaller bubbles (20–40 micron) are more effective at attaching to fine particles than larger bubbles (100+ micron). Nozzle design and recycle flow distribution affect bubble size distribution in the contact zone.
- Energy consumption: Pressurisation is the main energy consumer in a DAF system. At a recycle ratio of 30% and saturation pressure of 5 bar, the recycle pump consumes approximately 1–2 kWh per 10 m³ of wastewater treated — comparable to secondary biological treatment energy consumption in a well-optimised ETP.
Chemical Dosing for DAF Optimisation
Chemical dosing is essential for effective DAF performance with industrial wastewater. Without chemical coagulation and flocculation, DAF efficiency for colloidal and emulsified contaminants is poor, particularly for dairy, food, and oily wastewater where the contaminants are stabilised as emulsions or colloids.
- Coagulant selection: Polyaluminium chloride (PAC) is preferred over alum for its faster hydrolysis and performance across a wider pH range (pH 5.5–8.5). For wastewater with high colour or organic content (brewery, winery, fruit juice), ferric chloride or ferric sulphate provides better COD and colour removal alongside TSS removal.
- Flocculant selection: Cationic polyacrylamide (PAM) is effective for most food and dairy wastewater applications. Anionic PAM may be more effective for mineral-based suspended solids (paper mill effluent, mineral processing wastewater). Molecular weight and charge density should be optimised through jar testing.
- pH adjustment: For emulsified oil separation, pH reduction to 4.5–5.5 before coagulant addition breaks the electrical double layer stabilising oil droplets. For biological floc concentration (tertiary DAF after biological treatment), pH is typically maintained near neutral.
- Jar testing protocol: Before commissioning a DAF, coagulant and flocculant doses must be optimised through bench-scale jar testing using the actual wastewater. Jar test protocols should replicate the mixing energy and sequence used in the full-scale plant — rapid mix for 1–2 minutes at 100–200 rpm for coagulation, then slow mix for 10–15 minutes at 20–40 rpm for flocculation.
DAF Applications by Industry Sector
DAF is used across a range of industries in India with oil, grease, FOG, or fine TSS removal requirements:
- Dairy and milk processing: Fat, protein, and casein removal from milk wash water and CIP (clean-in-place) drains. DAF with chemical dosing can reduce BOD load on downstream biological treatment by 40–60% by removing floating fats and suspended protein.
- Food processing: Edible oil refineries, fish processing, poultry slaughterhouses, and vegetable processing all generate high-FOG wastewater requiring DAF for primary treatment.
- Paper and pulp mills: Fibre fines and fillers from paper machine white water recovery — DAF is used for whitewater clarification and fibre recovery before the ETP, and again for ETP effluent polishing.
- Oil and gas industry: Produced water treatment and hydrocarbon removal from refinery oily water sumps. DAF is used alongside CPI (corrugated plate interceptor) separators for API separator effluent polishing.
- Textile and dyeing: DAF with chemical dosing for colour and surfactant removal. DAF is less commonly used than for food and oil sectors in textile ETPs but is employed where biological treatment alone cannot achieve CPCB colour removal requirements.
Need Help Designing a DAF System?
Spans Envirotech designs and commissions DAF units for dairy, food, oil and gas, and paper industry ETPs — including jar testing, chemical dosing optimisation, and CPCB compliance documentation.
Contact us: bd@spans.co.in · +91-98100 00233
Frequently Asked Questions
What is the typical hydraulic loading rate for a DAF unit?
Standard DAF units for industrial wastewater treatment are typically designed with a hydraulic surface loading rate of 3–6 m³/m²·hr (equivalent to 3–6 m/hr surface overflow rate). High-rate DAF systems using lamella plates or tube settlers can achieve surface loading rates up to 10–15 m³/m²·hr. The appropriate design rate depends on the nature of the suspended solids, the effectiveness of chemical coagulation, and the required effluent TSS quality. For dairy and food industry ETPs, 4 m³/m²·hr is a common conservative design point.
What is the air-to-solids (A:S) ratio in DAF design?
The air-to-solids (A:S) ratio is the mass ratio of dissolved air released in the flotation tank to the mass of suspended solids in the influent. For most industrial wastewater applications, the design A:S ratio is 0.005–0.060 mL air/mg solids (often expressed as 5–60 mL/g). For low-TSS influents or finely dispersed solids, a higher A:S ratio improves capture efficiency. The A:S ratio is controlled by adjusting the recycle ratio and the saturation pressure in the pressurisation system. Most industrial DAF systems operate at a saturation pressure of 4–6 bar (gauge).
What recycle ratio is used in industrial DAF systems?
The recycle ratio (ratio of pressurised recycle flow to total influent flow) for industrial DAF systems is typically 15–50%. A recycle ratio of 25–35% is most common for dairy and food industry applications. Higher recycle ratios improve bubble-to-particle contact and flotation efficiency but increase energy consumption for pressurisation and the size of the saturation tank. For high-TSS wastewater (TSS > 500 mg/L), a recycle ratio of 40–50% may be needed to achieve the target effluent quality within the CPCB TSS limit of 100 mg/L.
What chemicals are typically dosed before a DAF unit?
DAF performance is greatly enhanced by upstream coagulation and flocculation. Typical chemical dosing before a DAF includes: a coagulant (alum at 50–200 mg/L, ferric chloride at 30–100 mg/L, or a polyaluminium chloride (PAC) at 10–50 mg/L) to destabilise colloidal particles; followed by a cationic or anionic polyacrylamide flocculant (0.5–3 mg/L) to form larger, more buoyant flocs. For emulsified oil in food and dairy wastewater, pH adjustment (typically to pH 4.5–5.5) before coagulant addition improves oil separation efficiency by breaking oil-in-water emulsions.
How is DAF sludge managed in an industrial ETP?
DAF float (the sludge layer that accumulates at the water surface) has a solids content typically of 2–6% total solids (TS), which is higher than conventional primary clarifier sludge but lower than filter press cake. In dairy and food industry ETPs, DAF float can be digested in an anaerobic digester for biogas recovery, as it is rich in fats, proteins, and carbohydrates. If direct sludge dewatering is required, the DAF float can be blended with biological sludge and dewatered on a filter press or belt press. Disposal must comply with CPCB Hazardous Waste Management Rules if the sludge contains regulated heavy metals or chemicals.
This article summarises DAF design guidelines for industrial ETPs for informational purposes. Always verify current standards with your State Pollution Control Board and consult a qualified environmental engineer for site-specific design.