What Is a Valveless Filter? Wastewater Treatment Terms Explained
A valveless filter is another type of rapid filter and one of the newer filter designs to come into general use in China over the past two decades. It carries no gate valves: inflow and backwashing are driven automatically by the hydraulics of a siphon, which is why it is called a valveless filter. The design suits small and medium-sized water supply schemes for industrial and mining enterprises, towns and rural areas, and it has been adopted by most township water supply projects in many provinces in recent years.
Whether the filter is of the gravity type or the pressure type, the operating principle is essentially the same. Its characteristics are:
(1) No valves are fitted for inflow, outflow, backwashing or drainage. Operation and backwashing are controlled entirely by hydraulics, and unlike siphon filters, single-valve and double-valve filters, no siphon pipe has to substitute for a valve and no vacuum-pumping equipment is required.
(2) Variable-head, constant-rate filtration. The change in head is reflected in the water level inside the siphon riser: as filtration proceeds the level rises steadily until the siphon forms and backwashing begins. Throughout the filter run the flow rate stays the same — the head varies, the quantity of water does not.
(3) The backwash water tank sits on top of the filter, which eliminates the need for a dedicated wash-water tank or wash-water pump. This saves floor space and power, and needs no operating attention.
(4) Low-head backwashing with a low-resistance underdrain system.
(5) Operation and washing are fully automatic, so no operating, management or repair staff are needed beyond routine maintenance.
Where a Valveless Filter Fits
(1) Because operation and backwashing are hydraulically automatic, the filter needs no operating or management staff and is very well suited to industrial and mining enterprises that have no water treatment technicians.
(2) The filter performs only iron and manganese removal by filtration, so it is ideal for treating groundwater. Where sedimentation or clarification is also required, a hydraulic circulating clarifier is the right partner, because the two match well in elevation and in treatment capacity.
(3) Iron and manganese removal in a valveless filter relies on contact oxidation, and jet-pump aeration can be used. There is also a free-fall aeration step between the distribution weir and the inlet channel.
Valveless filters are built in two forms: the gravity type and the pressure type.
How the Gravity Type Works
During filtration, raw water passes through the inlet distribution channel and the inlet pipe into the siphon riser, flows downwards, and is spread evenly over the filter media by the baffle beneath the cover. It then passes through the media and the support layer, enters the bottom space of the wash-water (clear-water) tank through the low-resistance underdrain, and rises through the passage into the clear-water tank. Once the level in that tank reaches the design height, the filtered water flows through the outlet pipe to the clear well.
As the run continues, the resistance of the media bed grows and the water level above the bed — that is, the level in the siphon riser — rises. When it reaches the mouth of the siphon auxiliary pipe, water starts to flow out through that pipe. 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 in the siphon increases until a siphon forms. The pressure above the media bed then falls and clear water from the clear-water tank flows in the reverse direction through the siphon, passing upwards through the bed and backwashing it. The wash water leaves through the water-seal well.
During washing the level in the clear-water tank falls. When it drops below the siphon-breaker bucket, the breaker pipe empties the bucket so that the pipe mouth is open to atmosphere, the siphon is broken, washing ends and filtration restarts.
Measuring the Filtration Rate
The rate is measured after washing, while the tank has not yet refilled. At that point the connecting pipe in the partition wall of the wash-water tank must be blanked off with a blind flange and a level scale placed in the tank. When filtration restarts the tank gradually fills, and the rise in level over a measured time gives the filtration rate. The rate multiplied by the filter area gives the filtered flow. Every cell should be measured in this way; where the flows differ significantly from cell to cell, the height of the distribution weirs in the inlet box can be adjusted so that the loading on each cell is similar.
Filter Media and Commissioning
The media are essential to filtrate quality and must be carefully selected; only material that meets the specified particle size and grading should go into the filter. Quartz sand should be used rather than building sand. Building sand has low strength, breaks down after repeated washing, loses particle size, shortens the filter run dramatically and may even be washed out during backwashing, thinning the bed and putting water quality at risk. The bed should be laid some 50 mm thicker than the design depth, because the material is loose when placed and compacts after use, and because some fine sand is carried away during washing.
Before a valveless filter is put into service, air must first be expelled from the media bed and only then should filtration begin. There are two ways to do this: limit the inflow so that trapped air has a chance to escape, or admit water from the connecting channel of the wash-water tank into the bottom of the filter and let it pass upwards through the bed, carrying 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 in succession until the filtrate is clear and transparent, after which the media are disinfected in the same way as for a rapid filter.
During the first run, the siphon formation time (from water spilling at the mouth of the auxiliary pipe to the start of backwashing), the washing duration, the filtered volume or rate and the filter run should all be measured and adjusted to the design values. This may involve changing the height of the breaker bucket, the opening of the wash-rate regulator, or the influent turbidity that determines the length of the run.
Construction and Welding Points
During civil works the elevations of several key points must be strictly controlled 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.
When pipework is welded, the mouth of the siphon auxiliary pipe should be flush with the inner wall of the siphon riser and must not protrude into the pipe, which would impair air extraction; the air-extraction pipe at the top of the siphon, however, should protrude about 20 mm into the pipe. Every weld on the siphon must be executed strictly so that no air leaks in.
The filtration rate is calculated from:
v = H / T
where v is the filtration rate in m/h, normally held at 8–10 m/h; H is the rise in the tank water level in m; and T is the time over which the level rises, in seconds.
Backwash Control
Before the first backwash, the wash-rate regulator installed below the siphon downpipe should be opened to about one quarter of the downpipe diameter, then opened progressively until the specified wash intensity is reached. The wash rate of a valveless filter varies — high at the start and lower later — so only an average intensity can be measured. When several cells share one wash-water tank, the intensity can be estimated from the fall in tank level between the start of washing and the moment the siphon breaks. The measurement carries some error: the other cells are still filtering and feeding the tank, and head losses in the connecting pipes mean the tank levels are not identical.
In operation, if the influent turbidity suddenly rises and the filtrate fails to meet requirements, the coagulant dose should be increased promptly to bring turbidity down; the filtration rate can also be reduced, or the filter force-washed several times, to protect filtrate quality.
After several years of service the sand has to be replaced or the filter overhauled. To avoid interrupting supply, cells are taken out of service one at a time, with the connecting pipe in the partition wall blanked off. Because the isolated tank then has less capacity, the wash water available may be insufficient to clean the bed thoroughly; the remedy is to refill the tank and force-wash several times.
