The conventional rapid filter, also called a four-valve filter, is a gravity rapid filter and the most classic and earliest-applied filtration structure in waterworks. Operating conditions are switched by valves, and it has its own backwash pump or wash-water tower rather than sharing wash water with other cells.

1. Main Construction

1. Filter structure: a rectangular single cell, with several cells arranged in parallel; wash-water drainage troughs are provided above

2. Media bed: single-layer quartz sand is most common, though anthracite-quartz sand dual media can also be used.

Single-layer sand: grain size 0.5-1.2 mm.

3. Support layer: gravel, laid above the distribution system to prevent media loss

4. Distribution system: a high-resistance distribution system (main pipe with perforated branches) giving good backwash distribution uniformity.

5. Pipe gallery and valves (four valves): each cell is fitted with an inlet valve, a clear water valve, a backwash inlet valve and a drain valve. Replacing the inlet and drain valves with siphons gives a two-valve filter, a derivative form of the conventional rapid filter.

2. Operating Cycle (filtration + backwash)

1. Filtration Phase (constant head, variable rate)

Settled water from the sedimentation tank passes downwards through the media bed, where suspended solids are retained. The clear water collects through the support layer and the high-resistance distribution system into the clear water main and is conveyed to the clear water tank. As impurities accumulate, head loss rises steadily; when it reaches the limit (2.0-2.5 m) or the effluent turbidity exceeds its limit, filtration is stopped and backwashing begins.

2. Backwash Phase

The inlet valve and clear water valve are closed and the drain valve and backwash water valve opened. High-pressure backwash water supplied by a dedicated backwash pump or wash-water tower flows upwards, expanding and fluidising the sand bed; the media grains rub against each other and the sludge is detached. The wash wastewater is discharged through the drainage troughs above.

Typical water-only backwash rate: 12-15 L/(s·m²); wash duration 5-7 minutes; bed expansion 45%-50%.

3. Key Design Parameters

• Single-layer sand filtration rate: 8-10 m/h; forced filtration rate 10-14 m/h (when some cells are being washed);

• filter run: 12-24 h;

• bed depth: 700-800 mm;

• cell area: preferably not more than 100 m², to prevent uneven backwash distribution.

4. Advantages and Disadvantages

Advantages

• Stable and reliable operation, mature technology and abundant operating experience;

• The high-resistance distribution system gives uniform backwash distribution unaffected by the operating condition of other cells;

• Flexible cell arrangement: as few as 2-3 cells can operate, unlike siphon filters which require at least 6;

• Cells are independent, so maintenance on one does not affect the others; single-layer sand or dual media can be selected as required;

• The tank is shallower than a siphon filter, keeping civil works costs manageable.

Disadvantages

• Each cell has many valves and the pipe gallery is complex, so valve maintenance is substantial;

• A backwash pump or wash-water tower is essential, increasing equipment investment and backwash power consumption;

• With conventional water-only washing the surface media tends to accumulate mud while the lower media is washed less effectively;

• When the degree of automation is low, manual operation is labour intensive.

5. Applications

Filtration for large, medium and small municipal waterworks and industrial water supply; can be combined with horizontal flow sedimentation tanks and inclined tube sedimentation tanks.

6. Key Comparison with Siphon Filters