Backwashing pressure

Backwash pressure can be increased to compensate for heavy solids loadings, which require higher pressure for thorough cleaning. The superiority of the higher-pressure system is manifested by the following (12) ... 

Nakatsuka and Ase (1995) found backwash most effective if the backwash pressure is more than double the operating pressure. An increase in crossflow velocity also lead to higher flux, but at the cost of higher energy consumption. Hagmeyer et al. (1996) optimised the backwash interval to 30 sec every 30 minutes. Efficiency could be further increased with the duration and frequency, but at the cost of recovery. 

Pressure, Flux, Frequency, and Duration of Backwash Backwashing conditions such as pressure, flux, frequency, and duration, in practice, are usually obtained by trial and error. Kennedy et al. (1998) studied the backwash conditions in order to maximize the net flux per filtration cycle. They found that increasing the backwash to filtration pressure ratio (Ph/Pf) above 2.5 did not result in any significant increase in flux restoration within the range of backwash pressures tested (0.2—1.6 bars) (Fig. 6.15). Their conclusion that applying high backwash pressures (8 limes the filtration pressure) cannot restore irreversible flux decline was in agreement with membrane suppliers recommendations. 

Increasing the backwash duration from 0.5 to 1 min had almost negligible effect on flux restoration. However, when the backwash duration was increased to 2 min, a significant effect on flux restoration was observed for aU backwash pressures tested. Increasing the backwash pressure increases the water consumption, which reduces the system recovery. It is, therefore, important to optimize the backwash pressure and duration to achieve as high a degree of flux restoration and system recovery as possible. 

Net flux serves as a tool to determine the optimal backwashing conditions since it reflects the operation efficiency by taking into account water production and consumption, including the time spent for backwashing. Kennedy et al. (1998) observed that, for all backwash pressures and durations tested, the net flux increased up to a pressure ratio of 2.5, then decreased, even though flux restoration increased (Fig. 6.15, right). The reason is that above a pressure ratio of 2.5, the flux decline that was recovered by backwashing was small compared to the quantity of water consumed to realize the flux restoration. 

Figure 6.15 (a) Effect of backwash pressure on flux restoration and water consumption, and (b)... 

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

The pressure filter with tubular elements has also been used as a thickener, when the cake, backwashed by intermittent reverse flow, is redispersed by an agitator at the bottom of the vessel and discharged continuously as a slurry. In some cases the filter cake builds up to a critical thickness and then falls away without bio whack. 

In diatomaceous-earth filtration, the powdered filter aid is built upon a relatively loose septum to screen out suspended soHds. The filter becomes clogged, and pressure losses become excessive backwashing is then necessary. The smallest removable particle is 0.5—1 p.m (see Diatomite). 

Dyna Sand Filter. A filter that avoids batch backwashing for cleaning, the Dyna Sand Filter is available from Parkson Corporation. The bed is continuously cleaned and regenerated by recycling solids internally through an air-lift pipe and a sand washer. Thus a constant pressure drop is maintained across the bed, and the need for parallel fdters to low continued on-stream operation, as with conventional designs, is avoided. 

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds. 

Step 5. Cake Discharge - At this point the air pressure is released, the cake outlet is opened and the leaf stack is vibrated to discharge the cake. The cake outlet opening must be interlocked with a pressure sensor to avoid opening under pressure. On some filters the cloth or mesh screen may be backwashed with water after cake discharge to dislodge and remove any cake residue that adhered to the medium. 

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

Backwashing requires locating a source that will supply the necessary flow and pressure of wash water. This water can be provided either by a reservoir at a higher location or by a pumping station that pumps treated water. Sometimes an automated... 

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed. 

Mechanical Force. Liquid can be readily expelled from a spongelike particulate mass of solid by using various pressing techniques. With this method, mechanical energy is used to force the liquid containing the particulate matter through a porous bed. The particulate matter is held in the pores in the bed. When the pressure drop reaches a certain level, replacement or backwashing takes place. This process may be either intermittent or continuous. 

Improved results may be obtained by substituting the volcanic lava for BIRM at a bed depth of 30 to 36 inches (76-91 cm) and a flow rate of 4 to 5 gpm/sq ft of media bed surface area. BIRM acts as a catalyst and normally requires only a periodic backwash to remove surface debris (backwash rate in pressure filter tank is 10 gpm/sq ft). 

Pressure filters are available in either horizontal or vertical tank (vessel) arrangements, with manual or automatic backwashing facilities and with tanks constructed of steel or noncorrosive construction materials. Flow through the filters is normally from the top and then down through the bed. 

Vertical pressure filters using only water for backwash are generally loaded with a single layer of sand or anthracite, but sometimes multiple layers of filter media are used. 

Air Bacfiflush A configuration unique to microfiltration feeds the process stream on the shell side of a capillary module with the permeate exiting the tube side. The device is run as an intermittent deadend filter. Every few minutes, the permeate side is pressurized with air. First displacing the liquid permeate, a blast of air pushed backward through the membrane pushes off the layer of accumulated solids. The membrane skin contacts the process stream, and while being backwashed, the air simultaneously expands the capillary and membrane pores slightly. This momentary expansion facilitates the removal of imbedded particles. 

Particle size, shape, and density to give low pressure drops when operated as a filter bed as well as good backwash characteristics. 

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