Detailed piping and instrumentation diagrams for dissolved air flotation systems: saturator pressure control, air dissolution loops, recycle hydraulics, chemical dosing integration, and sludge removal. Every valve, instrument, and safety interlock documented to ISA-5.1.
Free interactive DAF sizing calculator. Calculate dissolved air flotation surface area, recycle flow, saturator volume, and power.
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Design DAF systems for correct hydraulic loading. Surface area, rise rate and detention time calculations for your flow.
Dissolved air flotation is not a generic settling tank — it is a precision hydraulic process with pressurised subsystems
A DAF system P&ID must document far more than a simple tank with inlet and outlet. The process relies on a pressurised saturator vessel, controlled air dissolution, precisely engineered hydraulic loading rates, and chemical preconditioning — all interacting in real time. Omit any of these from the diagram and commissioning will fail.
The saturator operates at 4–6 bar, dissolving air into a recycle stream under Henry’s Law. The P&ID must show pressure control, level interlocks, air bleed, and safety relief — none of which exist in gravity settling systems.
Air-to-solids ratio (A/S) is the governing design parameter, typically 0.02–0.05. The P&ID documents compressor capacity, air flowmeter, and control valve to maintain this ratio across varying influent loads.
Surface loading rate, rise velocity, and contact-zone velocity gradient must all be within design envelopes. The P&ID defines nozzle geometry, effluent weir elevation, and bypass lines that protect these parameters.
Coagulant and flocculant dosing is not optional for most industrial DAF applications. The P&ID integrates rapid-mix and flocculation zones with precise injection points, dosing pumps, and online analysers.
The heart of every DAF: pressurised air dissolution engineered to ISA-5.1
The saturator vessel is where recycle water is pressurised and air is dissolved. A well-drawn P&ID captures every instrument loop, safety device, and manual isolation valve required for safe, efficient operation.
Recycle water is drawn from the clarified effluent zone by pump P-101, delivering 6–12% of total inlet flow at 4–6 bar. Discharge pressure is monitored by PT-101 and controlled via variable-speed drive or recycle control valve FCV-101.
An oil-free compressor delivers air to the saturator at a rate set by flow controller FIC-102. Air flow is measured by FT-102 and modulated via solenoid valve XV-102 to maintain target A/S ratio.
Level transmitter LIT-103 and level controller LIC-103 modulate inlet valve LCV-103. Pressure transmitter PT-103 interlocks with compressor start/stop. High-high pressure switch PSHH-103 trips the compressor and opens vent valve PSV-103.
Excess undissolved air is vented through automatic bleed valve XV-104 timed by XT-104. Pall-ring packing increases air–water contact time and dissolution efficiency to >90%.
| Parameter | Symbol / Tag | Typical Value / Range | Design Basis |
|---|---|---|---|
| Operating pressure | PT-103 | 4 – 6 bar(g) | Higher pressure = greater air dissolution per Henry’s Law |
| Retention time | — | 2 – 3 minutes | Ensures equilibrium air dissolution at design temperature |
| Air-to-solids ratio (A/S) | FIC-102 | 0.02 – 0.05 (wt/wt) | Governs bubble volume and solids flotation efficiency |
| Recycle ratio | FCV-101 | 6 – 12% of Qin | Balanced between air delivery and hydraulic loading |
| Dissolution efficiency | — | > 90% | Pall-ring packing and adequate retention |
| Design temperature | TE-103 | 10 – 25 °C | Lower temperature improves air solubility |
| Tag | Description | Type | Range / Setpoint | Interlock / Action |
|---|---|---|---|---|
| PT-101 | Recycle pump discharge pressure | Smart pressure transmitter | 0 – 10 bar | Low pressure alarm → pump fault |
| FT-101 | Recycle flow to saturator | Electromagnetic flowmeter | 0 – 150 m³/h | Flow control loop FIC-101 |
| FT-102 | Air flow to saturator | Thermal mass flowmeter | 0 – 50 Nm³/h | Ratio control with recycle flow |
| LIT-103 | Saturator level | Hydrostatic level transmitter | 0 – 2,500 mm | LIC-103 modulates LCV-103 |
| PT-103 | Saturator pressure | Smart pressure transmitter | 0 – 10 bar | Compressor start/stop interlock |
| PSHH-103 | Pressure switch high-high | SIL 2 pressure switch | Trip at 7.5 bar | Compressor trip + vent open |
| PSV-103 | Safety relief valve | Spring-loaded PSV | Set at 8.0 bar | Mechanical overpressure protection |
| TE-103 | Saturator temperature | RTD Pt100 | 0 – 50 °C | Temperature compensation for A/S calc |
Contact zone, separation zone, and effluent weir hydraulics documented on every P&ID
The flotation cell is where pressurised recycle water flashes to atmospheric pressure, releasing micro-bubbles that attach to flocculated solids. P&ID accuracy in this zone directly determines whether the DAF will meet its separation targets.
The pressurised recycle stream enters through a specially designed nozzle or distribution header at the tank inlet. Velocity must be controlled to <0.3 m/s to prevent floc shear. The P&ID shows nozzle arrangement, isolation valves, and pressure indicator PI-201 on the flash line.
Where bubble–floc agglomerates rise to form the sludge blanket. The P&ID documents surface area, effective depth, and effluent weir geometry. Lamella plates may be shown if a high-rate DAF-FB configuration is specified.
Ultrasonic sludge blanket level transmitter LIT-202 monitors blanket thickness and triggers scraper and sludge pump cycles. Setpoint typically 150–400 mm to balance capture efficiency and effluent clarity.
V-notch or rectangular weir plates ensure uniform horizontal velocity and prevent short-circuiting. The P&ID shows weir elevation, adjustable brackets, and effluent turbidity analyser AIT-203 downstream.
| Parameter | Conventional DAF | High-Rate DAF | DAF-FB (Lamella) | Notes |
|---|---|---|---|---|
| Hydraulic loading rate | 5 – 10 m/h | 12 – 20 m/h | 20 – 40 m/h | Based on total plan area |
| Surface loading rate | 2 – 5 m³/m²·h | 8 – 15 m³/m²·h | 15 – 30 m³/m²·h | Including lamella projected area |
| Bubble rise velocity | 3 – 8 m/h | 6 – 12 m/h | 10 – 18 m/h | Depends on bubble size (30–100 μm) |
| Contact zone residence | 60 – 120 s | 30 – 60 s | 30 – 60 s | Time for bubble–floc attachment |
| Separation zone depth | 2.0 – 3.0 m | 1.5 – 2.5 m | 1.2 – 2.0 m | Effective water depth |
| Scraper speed | 1 – 3 m/min | 2 – 5 m/min | 2 – 5 m/min | Chain-and-flight or bridge type |
Coagulant, flocculant, and pH correction loops tied into the DAF process
Raw wastewater seldom floats without chemical assistance. The P&ID must show every dosing line from storage tank to injection point, including backup pumps, calibration columns, isolation valves, and online analysers that close the control loop.
Ferric chloride, aluminium sulphate, or PAC is dosed into the rapid-mix chamber via metering pump P-301A/B (duty/standby). Dosing rate is controlled by flow-paced signal from inlet flowmeter FT-301 with trim from online turbidity AIT-301. Mixing intensity G-value 300–1,000 s-1 for 10–60 seconds.
Anionic or cationic polyelectrolyte is prepared in the polyelectrolyte station PPS-302 and dosed by progressive-cavity or diaphragm pump P-302A/B into the flocculation zone. G-value 20–75 s-1 for 5–20 minutes. Variable-speed mixer M-302 controlled by torque or amp feedback.
Acid (HCl or H2SO4) or caustic (NaOH) is dosed to maintain optimum coagulation pH, typically 6.5–7.5. pH transmitter AIT-303 and controller AIC-303 modulate dosing pump P-303. The P&ID shows day tank, overflow protection, and bunded containment.
Each dosing line includes non-return valve, pulsation dampener, isolation ball valves, calibration column, and pressure relief. The P&ID shows line flushing connections and drain to bund to prevent crystallisation and blockage.
| Chemical | Typical Dose | Injection Point | Control Strategy | Storage Requirement |
|---|---|---|---|---|
| Ferric chloride (FeCl3) | 50 – 300 mg/L | Rapid-mix inlet | Flow-paced + turbidity trim | PE tank, 7–14 days storage |
| Aluminium sulphate (Al2(SO4)3) | 80 – 400 mg/L | Rapid-mix inlet | Flow-paced + pH trim | GRP or PE tank, bunded |
| Polyelectrolyte (anionic) | 1 – 5 mg/L (make-up 0.1–0.5%) | Flocculation zone mid-point | Flow-paced or manual setpoint | Dry polymer station + day tank |
| Sodium hydroxide (NaOH) | 10 – 100 mg/L (as 30–50%) | Pre-coagulation or post | pH feedback control | Carbon steel or GRP, insulated |
| Sulphuric acid (H2SO4) | 5 – 50 mg/L (as 50–98%) | Pre-coagulation | pH feedback control | PE or GRP, bunded, vented |
Recycle ratio, pump sizing, and pressure boosting control narrative
The recycle stream is the energy input that drives the entire DAF process. Get the pump wrong — undersized, wrong curve, or poor NPSH — and the saturator will never reach design pressure. The P&ID documents every detail.
Typical recycle ratio is 6–12% of influent flow. Lower for high-TSS effluents where sludge blanket stability is critical; higher for low-TSS applications where bubble volume dominates separation. The P&ID shows the recycle draw-off point in the clarified effluent zone.
Recycle pump P-101 is typically a horizontal multistage centrifugal or vertical turbine pump selected for 4–6 bar discharge at design flow. NPSH available must exceed NPSH required by >1.5 m at maximum operating temperature. Duty/standby configuration is mandatory.
VFD on P-101 allows flow modulation to match influent variations. Flow controller FIC-101 receives setpoint from plant PLC based on inlet flow FT-001 multiplied by recycle ratio. Manual override is provided for startup and commissioning.
Low suction level switch LSLL-101 trips the pump to prevent cavitation. High discharge pressure PSH-101 alarms and closes discharge valve XV-101. Bearing temperature and vibration monitors feed the plant SCADA.
| Parameter | Specification | P&ID Reference |
|---|---|---|
| Pump type | Horizontal multistage centrifugal or vertical turbine | P-101A/B |
| Flow rate (each) | 6–12% of Qmax design | FT-101 |
| Discharge pressure | 4 – 6 bar(g) @ design flow | PT-101 |
| Speed control | VFD, 30–100% speed range | Drive interlock with FIC-101 |
| Material | SS316 or duplex SS for corrosive effluents | P&ID material notes |
| Mechanical seal | Cartridge double mechanical seal, Plan 53A | P&ID seal plan detail |
| Motor | IE3 efficiency, IP55, Class F insulation | P&ID motor datasheet xref |
Normal operation: Inlet flow transmitter FT-001 sends 4–20 mA signal to the PLC. The PLC calculates recycle setpoint = FT-001 × recycle_ratio_setpoint (default 8%). Flow controller FIC-101 compares setpoint to measured recycle flow FT-101 and modulates VFD speed on P-101A. If P-101A trips, automatic switchover to P-101B occurs with open XV-101A and close XV-101B sequence.
Startup sequence: Operator selects “DAF Start” on HMI. PLC checks LSLL-101 (not low), PSH-101 (not high), and saturator level LSHH-103 (not high). P-101A starts at minimum speed; speed ramps to achieve minimum flow of 3 m³/h. Compressor starts when PT-103 > 3.5 bar. Full automatic control engages after 5-minute stabilisation.
Shutdown sequence: Operator selects “DAF Stop&rdquo. Compressor stops. Recycle pump speed ramps down over 60 seconds to avoid water hammer. Discharge valve XV-101 closes after pump stops. Saturator drain valve XV-105 opens if maintenance mode selected.
Scraper mechanisms, sludge hoppers, and dewatering feed documented on the P&ID
Floated sludge is typically 2–5% dry solids — thicker than primary clarifier sludge but still a liquid. The P&ID must show how it is removed from the cell, transported to thickening or dewatering, and how level controls prevent both overflow and dry-running pumps.
Chain-and-flight or bridge scraper traverses the DAF surface, pushing floated sludge into the sludge hopper. Scraper motor M-401 has variable speed controlled by sludge blanket level LIT-202. The P&ID shows limit switches, overload protection, and emergency stop loop.
The sludge hopper is sized for 15–30 minutes residence at design sludge volume. Ultrasonic or radar level transmitter LIT-401 triggers sludge pump P-401A/B when high level is reached and stops at low level to prevent air ingress.
Progressive-cavity or self-priming centrifugal sludge pump transfers floated sludge to thickening or dewatering. The P&ID shows pump curves, NPSH verification, line flushing, and drain to sump. Flowmeter FT-401 logs mass balance data for regulatory reporting.
Where gravity thickening precedes dewatering, the P&ID shows thickened sludge transfer to screw press SP-501, centrifuge, or belt press. Polymer dosing P-501 may be added pre-dewatering. Cake discharge and filtrate return paths are clearly shown.
| Control Mode | Trigger | Typical Settings | Advantages | Disadvantages |
|---|---|---|---|---|
| Timer-based | Fixed interval | Scraper: 15–30 min cycle; Pump: 2–5 min every 30–60 min | Simple, no level instrumentation | Poor response to load variation |
| Level-based | Sludge blanket level | Start at 250 mm; stop at 100 mm | Responds to actual sludge production | Requires reliable level sensor |
| Continuous | Always running | Scraper at fixed speed; pump speed modulated by level | Stable blanket, best effluent quality | Higher wear, energy consumption |
| Hybrid (recommended) | Level primary, timer backup | Level triggers; timer ensures minimum removal every 2 h | Robust, failsafe, optimal effluent | More complex control logic |
Comprehensive instrument list with tags, ranges, setpoints, and interlocks
A DAF P&ID typically carries 30–50 instruments depending on complexity. Below is the master instrument list for a standard industrial DAF system, compiled to ISA-5.1 tagging convention with SIL-rated safety devices identified.
| Tag | Description | Type | Range | Setpoint / Action | SIL |
|---|---|---|---|---|---|
| FT-001 | Inlet wastewater flow | Electromagnetic flowmeter | 0 – 500 m³/h | Totaliser + flow-paced dosing | — |
| AIT-001 | Inlet pH | Glass electrode pH analyser | 2 – 12 pH | Alarm < 6.0 or > 9.0 | — |
| AIT-002 | Inlet turbidity | Optical turbidity (NTU) | 0 – 1,000 NTU | Trim signal for coagulant dosing | — |
| PT-101 | Recycle pump discharge pressure | Smart pressure transmitter | 0 – 10 bar | Low alarm → pump fault | — |
| FT-101 | Recycle flow | Electromagnetic flowmeter | 0 – 150 m³/h | FIC-101 setpoint = 8% × FT-001 | — |
| PSHH-101 | Recycle pump discharge high-high | Pressure switch | Trip at 7.5 bar | Trip pump + close XV-101 | SIL 2 |
| LSLL-101 | Recycle pump suction low-low | Capacitance level switch | Trip at 200 mm | Trip pump to prevent cavitation | SIL 2 |
| FT-102 | Air flow to saturator | Thermal mass flowmeter | 0 – 50 Nm³/h | FIC-102 maintains A/S ratio | — |
| LIT-103 | Saturator level | Hydrostatic level transmitter | 0 – 2,500 mm | LIC-103 modulates LCV-103 | — |
| PSHH-103 | Saturator pressure high-high | SIL 2 pressure switch | Trip at 7.5 bar | Compressor trip + vent open | SIL 2 |
| PSV-103 | Saturator safety relief valve | Spring-loaded PSV | Set at 8.0 bar | Mechanical overpressure protection | SIL 3 |
| TE-103 | Saturator temperature | RTD Pt100 | 0 – 50 °C | A/S calculation compensation | — |
| PI-201 | Flash line pressure (atmospheric) | Local pressure gauge | -0.5 – 1.0 bar | Visual check only | — |
| LIT-202 | Sludge blanket level | Ultrasonic sludge blanket metre | 0 – 1,000 mm | Scraper speed & pump control | — |
| AIT-203 | Effluent turbidity | Optical turbidity (NTU) | 0 – 100 NTU | Alarm > 10 NTU, > 20 NTU high | — |
| FIT-301 | Coagulant dosing flow | Calibrated sight glass / mag metre | 0 – 500 L/h | FIC-301 flow-paced control | — |
| AIT-303 | Rapid-mix pH | Glass electrode pH analyser | 2 – 12 pH | AIC-303 modulates acid/caustic pump | — |
| LIT-401 | Sludge hopper level | Ultrasonic level transmitter | 0 – 1,500 mm | Start/stop sludge pump P-401 | — |
| FT-401 | Sludge pump discharge flow | Electromagnetic flowmeter | 0 – 50 m³/h | Totaliser for mass balance | — |
| PSL-401 | Sludge pump suction pressure low | Pressure switch | Alarm at -0.2 bar | Alarm → check hopper level | — |
instruments per DAF P&ID
SIL-rated safety devices
tagging convention
valve & instrument documented
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