Filter Backwash in Water Treatment: Media Beds, Backwash Parameters and Bed Clogging
Granular media filters only work if the bed is cleaned properly. The media types, the backwash sequence and the bed-clogging mechanisms below determine whether a filter keeps delivering the same effluent quality month after month.
1. Multi-Media Filters
The multi-media filters used in water treatment are commonly anthracite-quartz sand-magnetite, activated carbon-quartz sand-magnetite, activated carbon-quartz sand, or quartz sand-ceramic. Three factors dominate the design of the media bed:
The different media must differ widely in density, so that backwashing does not mix the layers.
The media are chosen according to the intended use of the treated water.
Particle size must decrease from the upper layer to the lower layer, so that the lower layer stays effective and is fully used.
Taking a three-layer bed as an example, the top layer has the coarsest particles and is made of light, low-density media such as anthracite or activated carbon; the middle layer has intermediate particle size and density, usually quartz sand; the bottom layer has the finest, densest media, such as magnetite. Density differences largely fix the media selection for a three-layer filter. The top layer does the coarse filtration and the bottom layer the fine filtration, which is what makes a multi-media bed work: its effluent is clearly better than that of a single-layer bed. For drinking water, anthracite, resin and similar media are generally not permitted.
2. Quartz Sand Filters
A quartz sand filter uses quartz sand as its medium. It removes suspended solids effectively and also removes colloids, iron, organic matter, pesticides, manganese, bacteria and viruses. Its advantages are low filtration resistance, large specific surface area, good acid and alkali resistance, oxidation resistance, a working pH range of 2 to 13 and good fouling resistance.
The distinctive strength of a sand filter is that optimising the media and the filter design delivers self-adapting operation: the media adapt readily to raw-water concentration, operating conditions and pre-treatment process. During filtration the bed automatically forms a loose-on-top, dense-at-the-bottom structure that holds effluent quality under a wide range of conditions, and during backwash the media separate fully so that cleaning is effective. Sand filters offer high filtration velocity, fine filtration accuracy and large dirt-holding capacity, and are widely used for process water, domestic water, circulating water and wastewater pre-treatment in power generation, electronics, beverages, municipal water supply, petroleum, chemicals, metallurgy, textiles, paper, food, swimming pools and municipal engineering.
The equipment is simple, can run under automatic control, handles large flows, needs relatively few backwashes, filters efficiently, has low resistance and is easy to operate and maintain.
3. Activated Carbon Filters
An activated carbon filter uses carbon as its medium to remove colour, taste, residual chlorine and organic matter; its principal action is adsorption, and activated carbon is a manufactured adsorbent. Carbon filters are widely used to pre-treat water for domestic supply and for the food, chemical and power industries.
Because activated carbon has a highly developed pore structure and a huge specific surface area, it adsorbs dissolved organic matter such as benzene and phenol compounds strongly, and it also removes organic pollutants that biological and chemical methods handle poorly, including colour, off-odour, surfactants, synthetic detergents and dyes. Granular activated carbon removes more than 85 percent of ions such as silver, cadmium and chromate. After a carbon bed, suspended solids in the water are below 0.1 mg/L, COD removal is typically 40 to 50 percent and free chlorine is below 0.1 mg/L.
What Tells You a Filter Needs Backwashing
After a period in service, the media layer has intercepted and adsorbed a quantity of debris and dirt, and effluent quality starts to fall. The signs are a deterioration in filtered water quality, a rising pressure difference between inlet and outlet pipework, and a falling flow through the individual filter.
The principle of backwash is simple: water flows in reverse through the media bed so that the bed expands and becomes suspended, and the shear force of the water together with particle-to-particle collision and friction cleans the media, releasing the dirt trapped in the bed so that it leaves with the backwash water.
4. Why Backwashing Is Unavoidable
During filtration, suspended solids from the raw water are intercepted and adsorbed by the bed and accumulate there, so the pores gradually block and a filter cake forms on the surface; the head loss keeps rising. Beyond a certain limit the media must be cleaned to restore working performance.
As head loss rises, the shear force of the flow on the dirt attached to the media increases, and some particles are driven deeper into the bed. Suspended solids in the treated water then climb, water quality falls, and once impurities break through the bed the filter stops working. At that point the media must be cleaned to restore the bed's dirt-holding capacity.
Wastewater suspended solids contain a great deal of organic matter. If it stays in the bed for a long time, bacteria and micro-organisms multiply there and anaerobic putrefaction sets in, so the media must be cleaned periodically.
5. Setting the Backwash Parameters
Bed expansion. During backwash the bed must expand enough to open sufficient voids for the dirt to leave quickly with the water, so a higher expansion rate is preferable. If expansion is excessive, however, the number of particles per unit volume falls and collisions become less frequent, which hurts cleaning. For a dual-media bed the expansion rate is 40 to 50 percent.
In service, the media depth and expansion height should be checked from time to time, because normal backwashing inevitably loses or wears some media, which then has to be topped up. A stable bed keeps effluent quality steady and preserves backwash effectiveness.
Backwash water rate and pressure. Design practice normally specifies a backwash water intensity of 40 m³/(m²·h) at a pressure not exceeding 0.15 MPa.
Backwash air rate and pressure. Design practice normally specifies a backwash air intensity of 15 m³/(m²·h) at a pressure not exceeding 0.15 MPa.
During backwash, the air introduced collects at the top of the filter and most of it should leave through the twin-orifice air release valve. Check the valve for free passage regularly in service; the practical test is how freely the float rises and falls.
6. Combined Air and Water Backwash
Air first, then water. Lower the water level in the filter to 100 mm above the media surface, admit air for several minutes, then backwash with water. This suits beds that are heavily fouled at the surface but lightly fouled inside. The isolating valves must close fully; if they do not, once the water level drops below the media surface the upper bed is no longer wetted, and the dirt is not expelled effectively during the up-and-down agitation — it migrates deeper into the bed instead.
Air and water together. Air and backwash water are admitted simultaneously from below a static bed. As the air rises it coalesces into large bubbles inside the sand layer and breaks up again into small bubbles on meeting the media, scouring the media surface as it does so. The backwash water loosens the bed and suspends the media, which helps the air scour it. The expansion produced by water and by air reinforce each other, so the combined effect is stronger than either alone. Because the water and air pressures and intensities differ, the sequence must be respected, and backwash water must be kept out of the air pipework.
Final rinse. When the combined backwash ends, stop the air but keep the backwash water at the same flow for a further 3 to 5 minutes, which drives out bubbles left in the bed. Watch the twin-orifice air release valve at the top throughout.
7. Why Media Beds Clog and Ball Up
Dirt held on the surface of the bed is not always removed effectively within the cycle. If the backwash air is unevenly distributed, the expansion height becomes uneven, and where the air scour is weak the oil and grease on the media surface are not removed. In the next filtration cycle the local load rises, impurities sink from the surface into the bed, the agglomerates grow, and they spread deeper into the bed until the whole filter fails. Uneven backwash air is common in practice, usually caused by perforation of the underdrain air piping, blockage or damage of individual nozzles, or deformation of the nozzle spacing.
Fine particles at the bed surface collide less often and carry less momentum during backwash, so they clean poorly. Attached sand grains readily form small mud balls which, when the bed re-stratifies at the end of backwash, sink into the lower media and migrate deeper as they grow.
Oil carried in the raw water is intercepted by the filter, partly survives backwash and accumulates, and is the main cause of bed clogging. The decision on when to backwash can be based on a limiting head loss, on effluent quality or on filtration time, according to the raw-water characteristics and the required effluent quality.
8. Fabrication and Acceptance Points
The parallelism between the effluent trough and the media support plate must be within 2 mm.
Levelness and flatness of the support plate must be within plus or minus 1.5 mm. A one-piece machined plate is best; for large-diameter shells, or where raw material or transport limits intervene, a two-piece plate is acceptable.
The joints between support plate and shell matter especially for air backwash. To eliminate the radial gap left by plate machining and shell rolling errors, a curved ring plate is welded segment by segment, with full penetration welds at all contact areas. The same treatment applies to the radial gap between the central pipe and the support plate. These measures ensure that filtration and backwash communicate only through the nozzles or the distributor gaps, which in turn keeps the flow distribution even in both directions.
Holes machined in the support plate must be within plus or minus 1.5 mm radially, and must be machined, not hand-cut: an oversize fit between nozzle stem and hole makes the nozzle hard to install or secure.
Nozzle material is best in nylon, with ABS second. The media above impose a heavy compressive load, so the nozzle must be strong enough not to deform. An elastic rubber gasket is required on both the upper and lower contact faces between nozzle and support plate.
