Wastewater Filtration Engineering: Eight Filter Types, Filtration Rates, Media Composition and Backwash Control under HJ 2008-2010

1. Where filtration sits in the train, and the influent thresholds

HJ 2008-2010, Technical specifications for filtration process in wastewater treatment, was approved on 2010-12-17 and took effect on 2011-03-01 as a first issuance. It applies to the design, construction acceptance and operation of filtration unit processes in municipal or industrial wastewater treatment projects. It was drafted by the Jiangsu Provincial Academy of Environmental Science, Southeast University, Yangzhou Chenglu Environmental Engineering Co., Ltd. and Jiangsu Pengyao Environmental Engineering Design Institute.

Chapter 4 sets three thresholds on influent suspended solids, and this is the first step in selection. Conventional filtration suits secondary biological effluent, chemically precipitated industrial effluent and flotation effluent, with influent SS below 20 mg/L. As pretreatment ahead of advanced treatment such as activated carbon adsorption, membrane processes or ion exchange, the filter influent SS must be below 10 mg/L. Direct filtration, also called micro-flocculation contact filtration, allows influent SS up to 60 mg/L, provided the media size is increased accordingly. Clause 4.1 also notes that filtration can be applied across a range of flows for the separation of relatively low concentrations of suspended solids, and clause 6.2.1 recommends its use in the advanced treatment unit of industrial and municipal works.

2. Choosing among eight filter types

Clause 6.1 lists the common types: rapid filters and their derivatives, that is double-valve filters, flap filters and dual-media filters, plus V filters, gravity valveless filters, pressure filters and rotary disc filters. Clause 6.2.2 gives a clear scale-based split. Rapid filters, including conventional rapid filters, double-valve filters, flap filters and V filters, suit large and medium plants; valveless and pressure filters suit small plants; rotary disc filters can be used at any scale for municipal or industrial wastewater. Clause 6.2.4 further requires that, except for valveless, pressure and rotary disc filters, the number of cells should in principle not be fewer than four. On media, clause 6.2.3 requires adequate mechanical strength and corrosion resistance, with quartz sand, anthracite, ceramsite and porcelain sand preferred.

Table 1 sets out the characteristics and applicability of each type, four items of which can be used directly to eliminate options. Conventional rapid filters should not exceed 50 m2 per cell, give relatively poor water washing, and should use surface washing or air-assisted washing where conditions allow. Flap filters need more equipment, carry higher first cost and higher operating power demand, and suit large and medium flows with high suspended solids. Dual-media filters impose demanding media selection, lose media easily and are hard to wash, tending to accumulate mud balls, so high-loss distribution and auxiliary washing are preferred. Pressure filters operate at variable head and constant rate, giving water quality inferior to declining-rate filtration, with cell area below 10 m2, and suit small flows without available head. Gravity valveless filters have small cells, poor washing and waste part of the water during backwash, and require available elevation head. V filters require influent SS below 15 mg/L and an automatic control system. Rotary disc filters tolerate shock loads and use cross-flow filtration with low head loss.

3. Filtration rate and media composition

Clause 6.3.1 requires that filters be designed at the normal filtration rate and checked at the compulsory filtration rate that applies during maintenance. Table 2 gives three sets of values:

• Single-layer coarse sand (quartz sand): size d10 = 0.8 mm, uniformity coefficient K80 below 2.0, depth 700 mm, normal rate 8 to 10 m/h, compulsory rate 10 to 12 m/h;
• Dual media: anthracite d10 = 1.0 mm, K80 below 2.0, depth 300 to 400 mm, plus quartz sand d10 = 0.8 mm, K80 below 2.0, depth 400 mm, normal rate 9 to 12 m/h, compulsory rate 12 to 16 m/h;
• Uniformly graded coarse sand (quartz sand): size d10 of 1.0 to 1.3 mm, K80 below 1.4, depth 1200 to 1500 mm, normal rate 8 to 10 m/h, compulsory rate 10 to 12 m/h.

The standard also reminds the reader that the filtration rate for wastewater should be higher than for water supply filtration, and the media size increased accordingly. In practice the values should be set from testing or from the operating experience of comparable filters, taking into account influent quality, required filtered water quality and filter configuration. Clause 6.2.6 gives a gradation check: the ratio of media depth L to effective size d10 shall be greater than 1 000 for fine sand and dual media, and greater than 1 250 for coarse sand and triple media.

4. Water and air distribution systems

4.1 Pipe and channel velocities (clause 6.4.2)

• Inlet pipe 0.8 to 1.2 m/s; outlet pipe 1.0 to 1.5 m/s;
• Wash water 2.0 to 2.5 m/s; drain 1.0 to 1.5 m/s;
• Initial filtrate discharge 3.0 to 4.5 m/s; air line 10 to 15 m/s.

4.2 Long-handle nozzle distribution (clause 6.4.1, item 3)

• Velocity at the inlet of the air distribution main: 10 to 15 m/s;
• Velocity at the outlet of the air ports in the distribution channel: about 10 m/s;
• Velocity at the inlet end of the water distribution main: 1.5 m/s;
• Velocity at the outlet of the water ports in the distribution channel: 1 to 1.5 m/s;
• A vent pipe should be provided on top of the main or channel, discharging above the filter water surface.

4.3 High-loss and low-loss systems (clause 6.4.3)

• High-loss perforated pipe system: the ratio of total orifice area to filter area should be 0.20 to 0.25 percent; velocity at the inlet of the distribution main or channel 1.0 to 1.5 m/s; at the inlet of the branch pipes 1.5 to 2.0 m/s; at the outlet of the branch orifices 5 to 6 m/s.
• Support gravel layer for high-loss systems (Table 3, top to bottom): gravel 2 to 4 mm at 100 mm, 4 to 8 mm at 100 mm, 8 to 16 mm at 100 mm, and a fourth layer of 16 to 32 mm gravel whose top surface shall stand 100 mm above the distribution system orifices.
• Low-loss nozzle system: the ratio of total slot area to filter area should be 1.0 to 1.5 percent, taking the lower bound where conditions allow; the opening ratio is generally kept at 1 to 1.5 percent.
• The discharge coefficient alpha should be determined by testing. Where no test data exist, Table 4 applies: nozzles 0.8, slotted circular grids 0.85, wooden grids 0.6, reinforced concrete grids 0.6, perforated plates 0.75 and filter spheres 0.78.

5. Backwashing: mode, intensity and duration

5.1 Selecting the washing mode (Table 5)

Clause 6.2.8 is explicit: combined air and water backwashing is the preferred washing mode. Table 5 assigns the mode and sequence by media composition. Single-layer coarse graded sand takes water washing or air washing followed by water washing. Single-layer coarse uniformly graded sand takes air washing, then simultaneous air and water washing, then water washing. Dual anthracite and sand graded media takes water washing or air washing followed by water washing. One relationship deserves attention: although combined air-water washing is stated as preferred, Table 5 still retains water-only washing for single-layer and dual-layer graded media. Which path applies depends on the media gradation, and the decision must be taken together with the intensity values in Tables 6 and 7.

5.2 Water-only backwash intensity (Table 6, water temperature 20 degrees C)

• Single-layer coarse graded sand: wash rate 12 to 15 L/(m2 s), bed expansion 45 percent, wash time 5 to 7 min;
• Dual anthracite and sand graded media: wash rate 13 to 16 L/(m2 s), expansion 50 percent, wash time 6 to 8 min;
• Note 1 to Table 6: where surface washing equipment is provided, the low end of the wash rate may be used. Note 2: allowance must be made for seasonal variation in water temperature and quality, and the wash rate adjusted accordingly.

5.3 Air-water backwash intensity (Table 7)

• Single-layer fine graded sand: air scour first at 15 to 20 L/(m2 s) for 2 to 3 min, then water wash at 8 to 10 L/(m2 s) for 4 to 5 min;
• Dual anthracite and sand graded media: air scour 15 to 20 L/(m2 s) for 2 to 3 min, then water wash at 6.5 to 10 L/(m2 s) for 4 to 5 min;
• Single-layer coarse uniformly graded sand, first set: air scour at 13 to 17 for 1 to 2 min, simultaneous air at 13 to 17 plus water at 3 to 4 for 4 to 3 min, then water wash at 4 to 8 for 2 to 3 min;
• Single-layer coarse uniformly graded sand, second set with surface sweep: air scour 13 to 17 for 1 to 2 min, simultaneous air 13 to 17 plus water at 2.5 to 3 for 5 to 4 min, then water wash at 4 to 6 for 2 to 3 min, with surface sweep washing at 1.4 to 2.3 L/(m2 s) throughout;
• Note to Table 7: where uniformly graded coarse sand is washed with air and water, the filter run should be 24 to 36 h.

Air for backwashing is preferably supplied directly by blowers; air compressor and receiver combinations may be used for small and medium filters, with compressor pressure generally at 0.6 MPa. Roots blower capacity is calculated from the air scour intensity in Table 7 and the filter area being washed. Washwater pump flow is calculated from Table 6 for water-only washing or Table 7 for air-water washing together with the filter area; where both modes are provided the pumps may be shared, with the flow taken as the larger of the two calculated values. Washwater tank volume should be 1.3 to 1.5 times the volume of one wash, with water depth not exceeding 3 m; the washwater storage tank may be sized at 1.5 times the largest single wash volume.