Valveless Filters: Siphon Hydraulics, Bed Depth and Commissioning Checks

A filter with no valves at all

The valveless filter is a rapid gravity or pressure filter in which every function, including backwash, is driven by the hydraulics of a siphon. There is no inlet valve, no outlet valve, no backwash valve and no drain valve. That is not a simplification for its own sake: it removes the actuators, the instrument air and the operator attention that valve sequencing requires, and it is why the design has been adopted by most township water supply projects in several provinces.

It suits small and medium schemes for industrial and mining enterprises, towns and rural areas, particularly where nobody on site has a water treatment background. The gravity and pressure versions share the same operating principle, and the differences are in the vessel rather than the logic.

How the siphon takes over

During filtration, raw water enters the distribution channel and passes into the siphon riser, flows down and is spread evenly over the medium by the baffle under the cover. It works through the media and the support layer, enters the bottom space of the wash-water tank through a low-resistance underdrain, and rises into the clear-water tank. When the level there reaches the design height, filtered water leaves through the outlet pipe.

As the run continues, bed resistance grows and the water level above the bed rises with it, because the level in the riser is the head gauge. When that level reaches the mouth of the siphon auxiliary pipe, water spills into it. The descending flow creates a vacuum and entrains air, and air is continuously drawn out of the siphon through the air-extraction pipe, so the vacuum deepens until a siphon forms. Pressure above the bed falls, clear water from the clear-water tank flows back through the siphon, passes upward through the bed and washes it. Wash water leaves through the water-seal well.

Termination is equally mechanical. During washing the clear-water tank level falls; when it drops below the siphon-breaker bucket, the breaker pipe empties the bucket, its mouth opens to atmosphere, the siphon breaks and filtration restarts. One consequence is worth noting at the design stage: because the head varies while the flow does not, the filter runs at variable head and constant rate, so the run length is set by the available head rather than by a flow controller.

Where it fits, and where it does not

The filter performs iron and manganese removal by filtration, and it does so by contact oxidation, so it is at its best on groundwater. Jet-pump aeration can be used, and there is a free-fall aeration step between the distribution weir and the inlet channel. Where sedimentation or clarification is also needed, a hydraulic circulating clarifier is the right partner, because the two match in elevation and in capacity without an intermediate pumping stage.

What it cannot do is respond to a variable load. There is no way to change the filtration rate without changing the media or the head, and there is no automatic coagulant control. If the influent turbidity swings, the run length swings with it, and the answer is a dose change made by a person rather than a signal from a controller.

Media selection and bed depth

The media decide filtrate quality and deserve more attention than they usually get. Only material meeting the specified particle size and grading should go into the filter, and quartz sand should be used rather than building sand. Building sand has low strength, breaks down under repeated washing, loses its grading, shortens the run dramatically, and can be washed out altogether during backwash, which thins the bed and puts water quality at risk.

Lay the bed about 50 mm deeper than the design depth. The material is loose when placed and compacts in service, and some fine sand is carried away during washing, so the extra depth is what keeps the design depth after a year of operation. Before the filter goes into service, air must be expelled from the bed: either limit the inflow so trapped air can escape, or admit water from the connecting channel of the wash-water tank into the bottom of the filter and let it rise through the bed and carry the air out. On first commissioning the media are still dirty, so once the wash-water tank is full the filter should be force-washed several times until the filtrate runs clear, after which the media are disinfected as for any rapid filter.

Measuring the filtration rate honestly

The rate is measured after washing, while the tank has not yet refilled. Blank off the connecting pipe in the partition wall with a blind flange, place a level scale in the tank, restart filtration and time the rise. The rate multiplied by the filter area gives the filtered flow. Every cell should be measured this way, and where flows differ significantly between cells, the height of the distribution weirs in the inlet box can be adjusted so the loading is similar.

The arithmetic is simple: v = H / T, with v in m/h, H the rise in tank level in metres and T the time over which it rises in seconds. Normal operation sits at 8-10 m/h. During the first run, record the siphon formation time from spillage at the auxiliary pipe mouth to the start of backwash, the washing duration, the filtered volume and the run length, then adjust to design values. That may mean changing the height of the breaker bucket, the opening of the wash-rate regulator, or accepting a different influent turbidity.

Construction and welding points that decide reliability

Several elevations must be controlled during civil works and re-checked on completion: the mouth of the siphon auxiliary pipe, the overflow of the wash-water tank, the bottom of the inlet distribution box, the distribution weir and the bottom of the U-shaped inlet pipe. A 20 mm error at the auxiliary pipe mouth changes the point at which the siphon trips, and therefore the run length.

When the pipework is welded, the mouth of the auxiliary pipe should be flush with the inner wall of the riser and must not protrude, because a protruding lip impairs air extraction. The air-extraction pipe at the top of the siphon, by contrast, should protrude about 20 mm into the pipe. Every weld on the siphon must be airtight; a pinhole leak will prevent the vacuum from ever building, and the filter will simply sit there at maximum head without washing.

Backwash control in practice

Before the first backwash, open the wash-rate regulator below the siphon downpipe to about a quarter of the downpipe diameter, then open it progressively until the specified wash intensity is reached. The wash rate of a valveless filter is not constant — it is high at the start and lower later — so only an average intensity can be measured. Where several cells share one wash-water tank, estimate the intensity from the fall in tank level between the start of washing and the moment the siphon breaks, accepting that the figure carries error: the other cells are still filtering into the tank, and head losses in the connecting pipes mean the levels are not identical.

In service, if the influent turbidity rises suddenly and the filtrate fails, increase the coagulant dose promptly, or reduce the filtration rate, or force-wash several times to protect quality. After several years the sand has to be replaced or the filter overhauled, and cells are taken out of service one at a time with the partition-wall connecting pipe blanked. The isolated tank then has less capacity, so the wash water available may not clean the bed thoroughly; refilling and force-washing several times is the remedy.