Backwash/cleaning cycle

In the petroleum industry, mixed-media filters are usually of the pressure type. Either vertical or horizontal pressure vessels are used. Fig. 5 provides a cross-section of a typical horizontal filter system using mixed media. Note that a pipe lateral-underdrain system is used with silica gravel, providing support for the filter bed. An all-water backwash is used typically the backwash flow rate b 15 to 20 gal/min/sq ft, depending upon the water temperature. A surface wash device b used to aid in the cleaning cycle. It provides increased agitation of the upper portion of the filter bed, improving backwash efficiency. 

Fixed bed batch size on cycle time load, backwash/clean, regenerate, swing on-stream. Load typical flow rate 2 to 3.5 BV/h with the load time based on the ratio of the adsorption isotherm to the feed concentration of the target species (usual range varies with the application 18 to 100 min, while for water treatment 70 to 1(X) days). Backwash with velocity to fluidize the bed (see liquid fluidization, Section 16.11.7.5) velocity 0.8 BV/h. Time such that <5% feedrate used in backwash. Usually, carbon is removed and regenerated about four times per annum. Try to match the loading cycle to the regeneration cycle. 

The service, backwash and cleaning cycles described above are general in nature and depend on each membrane manufacturer s system design. 

Air scouring is often used with larger filters to improve the cleaning efficiency. The wash cycle is normally a reduced flow backwash in conjunction with air scouring, at the rate of 2 cu ft air/sq ft bed surface area/min, followed by a full-flow rinse with filtered water to resettle the bed. 

As noted in Section 6.1.2, in most applications the control of CP, and fouling, dictates the use of crossflow. However, for dilute feeds and low-pressure membranes it has been accepted that batch cycles of deadend operation with solids accumulation removed by periodic backwash requires potentially lower energy. Usually, deadend is at FF and the TM P cycles from a minimum to maximum or over a specified cycle time during the batch. If fouling occurs it is evident through a steady rise in TM Pmin or Rm. Occasional chemical cleaning may restore Rm. 

Cycle time clean by backwashing with permeate every 6, 24,170, 360, 1000 h (depending on the criteria, the membrane, operating conditions, and the amount of pretreatment) example, 8 h on and 1 to 2 h off to clean or short pulses every 150 to 300 s so that steady-state flux operation is never achieved. 

Rotating microscreen batch, fluid loading 3 to 6 L/s m of submerged area usually 66% area submerged solids loading 0.05 to 0.1 g/s m headless 7 to 14 cm to max. of 45 cm. Clean by backwash at 2 to 5 % volumetric throughput capacity. Screen pack downstream of extruders screen pack area directly proportional to rate of extrusion. Cycle time for the screen to blind > time between shutdowns. For example, screen pack area 0.17 m s/kg and screen pack diameter 20 cm s/kg but must be custom selected for the polymer. 

The PEC (programmable logic control) controlled membrane unit is cleaned periodically by permeate backwashing based on TMP to maximise the interval of chemical cleanings. At the end of the Hquid backwash cycle, air is introduced at the base of the modules to scour the membranes to loosen the dirt and debris from the surface of the membrane fibres. To facflitate scouring and debris removal of the fibres inside the modules, air is introduced within the base of the modules through a series of air connections in the bottom manifold. As the air continues to loosen the dirt on the surface of the... 

Over long periods of operation, membrane fouling is generally not totally reversible by the hydraulic backwash procedure. As the number of filtration cycles increases, the irreversible fraction of membrane fouling also increases (see Fig. 6.13). In order to obtain the desired production flow rates (or flux), an increase in TMP is required. When this pressure reaches a maximum allowable mechanical resistance of the membrane, chemical cleaning is required for the membrane to regain most of its permeability. Regardless of the membrane system... 

For relatively low solids content feeds, it is feasible to operate without continuous cross flow or surface shear, and this can reduce energy costs. Such applications include water treatment and pretreatment for RO desalination and reclamation. This mode of operation is called dead-end filtration or frontal filtration, and the key feature is that the deposition of retained species is allowed to grow. A t5q)ical cycle commences with a clean membrane (after backwash), and at constant flux the TMP rises according to Eq. (10.4). After a specified period (Q, or at a predetermined AP iax. the flux is stopped and the deposit is removed by backwashing and (usually) vigorous aeration. The contents of the membrane... 

The idealized TMP cycle is depicted in Figure 10.2b for a cycle that is limited by AP ax rather than a specified cycle time. In reality the backwash efficiency will be < 100% so that APnjin (the cleaned membrane TMP) may slowly rise over time, eventually requiring chemical cleaning (see Section 10.5.2). During a filtration cycle the deposit resistance grows so that at time t it is given by... 

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