Dissolved-Air Flotation for Wastewater: Four Process Types, Pressurised DAF Design Parameters and Operational Control under HJ 2007-2010

1. Status and scope of the standard

HJ 2007-2010, Technical specifications for floatation process in wastewater treatment, was approved on 2010-12-17 and took effect on 2011-03-01. It is a first issuance and its Annex A on symbols is normative. The standard governs the design, construction, check and acceptance, and operation of flotation units in municipal or industrial wastewater treatment projects, and serves as a technical basis for feasibility studies, environmental impact assessment, process design, construction acceptance, operation and supervision. It was drafted by the Jiangsu Provincial Academy of Environmental Science, Southeast University, Jiangsu Pengyao Environmental Engineering Design Institute and Yangzhou Chenglu Environmental Engineering Co., Ltd. In practice it is applied together with HJ 2006-2010 on coagulation and flocculation and HJ 2008-2010 on filtration, issued in the same batch.

2. Deciding whether flotation is the right unit

2.1 Flow and quality thresholds

Clauses 4.1 and 4.2 set two hard boundaries. Flotation suits small to medium flows of industrial wastewater or combined municipal wastewater. Its target pollutants are hydrophobic suspended solids and destabilised colloidal particles; the raw water SS mass concentration may reach 5 000 to 10 000 mg/L, while the flotation tank effluent SS can generally be kept below 20 to 30 mg/L. Where the effluent is discharged directly it must satisfy national or local discharge standards; where it flows to a downstream treatment system it must satisfy that system's influent requirements. Equalisation is required where flow or quality fluctuates widely.

2.2 How the four process types divide the work

• Electrolytic flotation: provides oxidation-reduction and coagulation alongside flotation, adapts well and is easy to adjust; the plant is packaged, compact and quiet, but power consumption is high. Applied at Q below 10 to 15 m3/h, and particularly advantageous for high salinity, high conductivity and toxic or hazardous streams, including electroplating chromium(VI) and cyanide bearing wastewater.
• Impeller flotation: simple, fast separation, effective on high suspended solids, with easily adjusted air supply, but it demands much of the coagulation pretreatment. Typically applied at Q below 30 to 40 m3/h for industrial wastewater with high suspended solids and surfactants.
• Pressurised dissolved-air flotation: mature with extensive operating experience, high loading, good performance, large capacity, fully automatic, low sludge moisture and good effluent. Full-flow or partial-recycle saturation can be selected according to suspended solids concentration. Applicable across a wide range of flows and to high suspended solids, oil, micro-organisms, paper pulp and fibres.
• Shallow air flotation: high surface loading, fast separation, high efficiency, easy hydraulic layout, small footprint and shallow tank; steel units can be combined or elevated. Suited to suspended matter, fibres, activated sludge and oil separation at any scale, and to solids recovery from industrial wastewater.

3. Pressurised DAF: three flow sheets and one parameter set

3.1 Selecting a flow sheet

Clause 6.5.1 defines three basic process flow sheets: full-flow saturation, partial saturation and recycle pressurised saturation. The selection rule is explicit. Full-flow and partial saturation suit raw water with a relatively low concentration of separable suspended solids that contains no fibrous material. Recycle pressurised saturation suits raw water with high suspended pollutant concentrations, large flow, and coagulation or demulsification pretreatment. In addition, the contact chamber and separation chamber must respectively guarantee the air-water contact time and the sludge-water separation time; the level control chamber shall have a reliable, easily adjusted level regulator; the scum scraper shall provide adjustable stroke and speed with reciprocating operation; and each releaser shall have an independent quick-opening valve and a rapid-disconnect coupling.

3.2 Tank geometry and separation zone (clause 6.5.2)

• Effective water depth is generally 2.0 to 2.5 m;
• Length-to-width ratio is generally 2:1 to 3:1 for horizontal flow and shall be 1:1 for vertical flow;
• Single cell width should not exceed 6 m and length should not exceed 15 m;
• Upflow velocity in the contact zone: about 20 mm/s at the lower end and 5 to 10 mm/s at the upper end, with hydraulic retention time above 1 min. Clause 6.5.4.1 further requires the contact time tc to exceed 60 s, with the separation chamber depth H1 normally 1.8 to 2.2 m;
• The baffle is generally inclined at 70 degrees to the vertical;
• Surface loading of the separation zone, saturator flow included, should be 4 to 6 m3/(m2 h), with hydraulic retention time generally 10 to 20 min. Clause 6.5.4.2 gives an average downward velocity vs of normally 1 to 1.5 mm/s, and a length-to-width ratio of generally 2:1 to 3:1 for rectangular separation chambers.
• The recycle ratio, or saturator water ratio, shall be determined by calculation and is generally 15 to 30 percent.

3.3 Saturator and auxiliary equipment

• Design working pressure is generally 0.4 to 0.5 MPa, with hydraulic conditions that guarantee air-water contact; the automatic control must be coordinated with the air compressor and the saturator pump.
• Step-ring packing is normally used, with a packing height of one half of the tank height and not less than 0.8 m.
• The controlled liquid level is one quarter to one half of the tank height, measured from the bottom.
• Hydraulic retention time should exceed 2 to 3 min, taking the lower value where packing is installed, and shall be confirmed by calculation.
• Saturators are normally vertical with a height-to-diameter ratio above 2.5 to 4, taking the higher value where conditions allow; horizontal vessels are permitted where the hydraulic conditions are satisfied.
• The diameter follows equation (34) using the hydraulic loading per unit tank cross-sectional area, generally 80 to 150 m3/(m2 h), or 100 to 200 m3/(m2 h) for packed vessels. The height follows equation (35) as the sum of head height, distribution zone of 0.2 to 0.3 m, storage zone of normally 1.0 m and packing layer of 1.0 to 1.3 m for step rings.
• Pressure gauge, level gauge and safety valve shall be provided with automatic level and pressure control; a rapid drain valve at the bottom and a vent valve at the highest point; a strainer on the inlet line of the saturator pump; and a pressure vessel test certificate is required before the vessel may be placed in service (clause 7.2).
• The saturator pump should be a multi-stage pump with a relatively high head, working at 0.4 to 0.6 MPa; the air compressor works at 0.6 to 0.7 MPa and must be sized for the maximum air demand of the saturator (clauses 7.1 and 7.3).

4. Two routes for the air demand calculation

Clause 6.5.3 sets the principal process indices. With test data, equation (19) applies: Qg = Q R ae psi / (1000 gamma), where Q is the treated flow, R the recycle ratio under the test conditions, ae the released air volume under the test conditions, gamma the air unit weight and psi the water temperature correction factor of 1.1 to 1.3. Without test data, equation (20) applies: Qg = Q Cs (f P - 1) R / (1000 gamma), where Cs is the air solubility at one atmosphere at the given temperature, f the saturation efficiency of 0.8 to 0.9 and P the absolute saturation pressure in atmospheres.

Table 3 gives the air unit weight and solubility in water: at 0 degrees C, 1.252 g/L and 29.2 mL/(L atm); at 10 degrees C, 1.206 and 22.8; at 20 degrees C, 1.164 and 18.7; at 30 degrees C, 1.127 and 15.7; at 40 degrees C, 1.092 and 14.2. Solubility therefore falls by more than half as the water warms from 0 to 40 degrees C. That is the quantitative basis for clause 6.2.6, which requires the influence of water temperature to be taken into account in design, and for clause 12.5, which permits the recycle flow or saturation pressure to be raised during cold periods.

The air-to-solid ratio alpha depends on the hydrophobicity of the suspended particles. The standard gives a typical range of 0.005 to 0.006 and states that it is normally determined by testing. The recycle ratio is back-calculated with equation (22); the rated air compressor capacity follows equation (23) with a safety factor psi prime of 1.2 to 1.5; the saturator flow follows equation (24) using the solubility coefficient KT from Table 4: 0.038 at 0 degrees C, 0.029 at 10 degrees C, 0.024 at 20 degrees C, 0.021 at 30 degrees C and 0.018 at 40 degrees C. A saturation efficiency of 0.9 may be taken for saturators filled with step rings.

5. Shared structural details

• Perforated collection pipe (clause 6.2.3): normally arranged in the separation chamber 20 to 40 cm above the floor, with an in-pipe velocity of 0.5 to 0.7 m/s; holes point downwards at 45 degrees to the vertical in a staggered arrangement, at 20 to 30 cm spacing and 10 to 20 mm diameter.
• Scum withdrawal (clause 6.2.4): the interval depends on the scum quantity and should not be too short, generally 0.5 to 2 h; scum moisture content 95 to 97 percent, with the blanket thickness controlled at about 10 cm.
• Scum scraper (clauses 6.2.5 and 6.5.8): mechanical removal is preferred, the travelling speed should be kept within 5 m/min, and the scraping direction should oppose the flow so that any falling scum lands in the contact chamber. Bridge scrapers are used for rectangular tanks and planetary scrapers for circular tanks.
• Scum trough (clause 6.5.6): the cross-section is selected from the sludge discharge per unit time, including the water carried out when the level is raised, and is generally not less than 200 mm; a sufficient slope towards the discharge port is required, generally greater than 0.03 to 0.05; where the trough exceeds 5 m in length, discharge shall run from both ends towards the centre, assisted by flushing pipes where necessary.
• Dissolved-air releasers (clause 6.5.7): type TJ or TV may be selected, the number following equation (39), n = Qr / q, where q is the output of a single releaser at the selected saturation pressure; a quick-opening valve shall be installed on the line from the saturator to the releasers, and provision made for rapid dismantling.
• Collector orifice hydraulics (clause 5.6): the pipe diameter is set from the distributed flow at 0.4 to 0.5 m/s, and the orifice velocity is back-calculated taking the orifice head loss h as 0.3 m.

6. Parameter boundaries for electrolytic and impeller flotation

Clause 6.3.2 gives the electrolytic flotation design parameters: plate thickness 6 to 10 mm, thicker where required for soluble anodes; net plate spacing 15 to 20 mm; current density generally below 150 to 200 A/m2; clarification zone height 1 to 1.2 m; separation zone retention 20 to 30 min; scum layer thickness 10 to 20 cm; and single cell width not greater than 3 m. The effective electrode area follows equation (1), S = E Q / i, where the specific current E and current density i should normally be determined by testing and may otherwise be taken from Table 2.

The sector values in Table 2 differ widely: leather and fur wastewater E of 300 to 600 A h/m3 with i of 50 to 100 A/m2; chemical wastewater 100 to 400 and 150 to 200; meat processing wastewater 100 to 270 and 100 to 200; artificial leather wastewater 15 to 20 and 40 to 80; dyeing and printing wastewater 15 to 20 and 100 to 150; chromium(VI) wastewater 200 to 250 and 50 to 100; phenolic wastewater 300 to 500 and 150 to 300. Dyeing wastewater and artificial leather wastewater share the same low specific current band of 15 to 20 A h/m3, yet their current densities differ by a factor of nearly two. Electrode area must therefore be sized project by project with equation (1) rather than copied from another sector.

Two hard operational constraints also apply. Clause 6.3.1 states that where iron-anode electrocoagulation flotation treats chromium(VI) wastewater, the influent chromium(VI) mass concentration should not exceed 100 mg/L; clause 12.2 further gives the pH for that duty as 4 to 6.5 and allows dosing of sodium sulphate or sodium chloride where the raw water conductivity is low, so as to raise conductivity and reduce the electrolysis voltage. Clause 6.4.2 sets the impeller flotation parameters: impeller diameter D of 200 to 400 mm and not more than 600 mm, rotational speed 900 to 1 500 r/min, peripheral speed 10 to 15 m/s, clearance between impeller and guide vanes adjusted to less than 7 to 8 mm, tank water depth generally 2 to 2.5 m and not more than 3 m, and a square tank with a side not exceeding 6 times the impeller diameter.

7. Two items that must be settled separately

First, clause 6.5.2 gives a separation zone surface loading, saturator flow included, of 4 to 6 m3/(m2 h), whereas clause 6.5.4.2 gives an average downward velocity vs of normally 1 to 1.5 mm/s. The units differ. Converted, 4 m3/(m2 h) corresponds to about 1.11 mm/s and 6 m3/(m2 h) to about 1.67 mm/s, so the upper bound slightly exceeds the vs limit of 1.5 mm/s. Both constraints act on the same cross-section, so the stricter value should govern in design, or the value should be confirmed by testing.

Second, chapter 10 lists three compliance red lines. Clause 10.6 states that sludge and residues from the treatment of chromium(VI) and cyanide bearing wastewater are hazardous solid wastes and shall be handed to qualified organisations for dedicated treatment and disposal. Clause 10.4 requires safety rails and non-slip ladders at flotation tanks together with life jackets and life rings. Clause 10.5 requires ventilation for electrolytic flotation, and clause 10.7 requires periodic pressure testing of pressurised saturators by the national pressure vessel authority. These are mandatory safety and solid waste management requirements and must be given a dedicated section in design documents and operating procedures rather than being mentioned in passing among process calculations. This article is based on the issued draft; the formal text published by the China Environmental Science Press prevails for formal use.