Air scour

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. 

Backwashing can be by water alone or by an air scour and water. Usually where high volumes of water are encountered a horizontal type is used because the tank length can be increased proportional to the volume. However, horizontal tanks are restricted to lower levels of suspended solids because of limitations in media bed depth. They are also prone to the risk of the underbed shifting, with a consequential decrease in filtering efficiency. 

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. 

Granular solid filters (see Fig. 16-4) are cleaned by backwashing with water. The backwashing is usually preceded by an air scouring to assure better cleaning.30 The backflow water rate should be fast enough to fluidize the bed. 

Koch Hollow-fiber membrane of 0.1-pm pore size. They were arranged in a vertical position with intermittent air scouring and back-flushing. 

Limitations Solids collect around and at potted ends of fibers Requires air scour to clean Cannot perform true recycle High pressure drop limits fiber length Requires significant filtration for pretreatment... 

First, the treated wastewater is pumped through hollow fibre MF membranes (pore size 0.2 pm) in pressurised Memcor CMF modules. Every 25—30 min the microfilters are cleaned backwashed the residue or cake formed on the membrane surface is first loosened by air scouring and then flushed away during backwash. 

Membrane filters are Memcor CMF. Trans-membrane pressure (TMP) and flux are maintained by regular - every 30 min - air-scour backwash. Eventually, TMP increases over time (normally about 30-35 days) due to particulate and biological fouling. When the TMP reaches 1.5 bar, the membranes are cleaned with chemicals using solutions of sodium hypochlorite and/or citric acid sequentially. The membrane flux is 44 Imh and the MF filtrate has a SDI value of 3 to 4 and turbidity 0.1 to 0.2. 

Check that the AP switch is not out of calibration Initiate a two-step backwash sequence start at one-half the flow rate gradually increasing to the design flow rate Verify that the bed has not shrunk or compressed Make provisions for air scouring (media filters only) to break loose the particles trapped in the upper layers of the bed... 

Air scour blowers Positive displacement blowers supply low pressure air to the feed side to scour the membrane fibres during pre-backwash. 

During cleaning the flow is reversed to wa off the deposited solids, this is called back-flushing . This often fluidises the media, and frequently air scour is also used. The flow conditions are, therefore, veiy aggressive and turbulent, and particles attached to... 

Turbulence promoters, air scour and abrasives have also bem investigated as a means to reduce the boundary layer or deposit thickness during microfiltration [D mek, et al, 1974 Milisic and Bersillon, 1986]. In gareral, these are not so efifective during microfiltration as they can be during ultrafiltration. 

Air scouring is an intermittent operation with fairly high instantaneous demand. Superficial air flow rates (standard conditions) are between 1.0 and 1.5 m min The requited pressure is only 0.5 or 0.7 bar, while compressed plant air is usually available... 

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