For the removal of residual suspended solids from the effluents from the conventional sewage treatment plants the following techniques are commonly used: 1. Granular-Medium Filtration 2. Microscreening.
Method # 1. Granular-Medium Filtration:
Although granular-medium filtration is one of the principal unit operations used in the treatment of potable water, the filtration of effluents from conventional sewage treatment plants is a relatively recent practice. However in spite of the short period of its use, filtration is already a well-established operation for achieving supplemental removals of suspended solids (including particulate BOD) from sewage effluents of biological and chemical treatment processes. Moreover, filtration has also been used to remove chemically precipitated phosphorus.
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Description of the Filtration Operation:
The complete filtration operation essentially involves two phases: filtration and backwashing. Both these phase are identified in the definition sketch shown in Fig. 18.1. Filtration is accomplished by passing the sewage to be filtered through a filter bed composed of granular material with or without the addition of chemicals.
Within the granular filter bed, the removal of the suspended solids contained in the sewage is accomplished by a complex process involving one or more removal mechanisms, such as straining, interception, impaction, sedimentation, and adsorption. The end of the filter run (filtration phase) is reached when the suspended solids in the effluent from the filter start to increase (break through) beyond an acceptable level, or when a limiting head loss occurs across the filter bed.
Once either of these conditions is reached, the filtration phase is terminated and the filter is backwashed to remove material (suspended solids) that has accumulated within the granular filter bed. This is usually done by reversing the flow through the filter (see Fig. 18.2). A sufficient flow of wash water is applied until the granular filtering medium is fluidized (expanded).
The material that has accumulated within the bed is then washed away. The wash water moving past the medium also shears away the material attached to the individual grains of the granular medium. In backwashing the filter, care should be taken not to expand the bed to such an extent that the effectiveness of the shearing action of the wash water is reduced. In most sewage treatment plants, the wash water containing the suspended solids that are removed from the filter is returned either to the primary settling facilities or to the biological treatment process.
Method # 2. Microscreening:
Microscreening involves the use of microstrainers or rotating-drum filters operating under gravity conditions (see Fig. 18.4). The filter consists of a special woven stainless-steel or polyester fabric fitted on the drum periphery. The drum rotates about a horizontal axis at a variable low-speed (upto 4 r.p.m) and is continuously backwashed.
The filter fabric has openings of size 20 to 35 μ. The drum operates submerged in the flowing sewage with submergence: 70-75% of height, or 60-70% of area. The sewage enters the open end of the drum and flows outward through the rotating screening fabric, leaving behind the suspended solids.
The collected solids are backwashed by high-pressure jets into a trough located within the drum at the highest point of the drum. Drum rotation and its back washing are continuous and adjustable. The pressure head develops due to intercepted solids which build up on the inside of the filter fabric and create a filtration mat capable or removing particles that are smaller than the size of the opening in the fabric.
The design parameters include screen size, hydraulic loading rate, head loss through screen, drum diameter, drum submergence, drum speed and backwash requirements.
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Typical values of these parameters are given below:
(a) Screen size – 20-35 μ
(b) Hydraulic loading rage – 3-6 x 10-3 m3/minute/m2 (Based on submerged surface area of drum)
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(c) Head loss through screen – 75-150 mm (Bypass should be provided when head exceeds 200 mm)
(d) Drum diameter – 2.5-5 m (3 m is used most commonly; smaller sizes increase backwash requirements)
(e) Drum submergence – 70-75% of height;60-70% of area
(f) Drum speed– 4.5 m/minute at 75 mm head loss; 35-45 m/minute at 150 mm head loss (Maximum rotational speed is limited to 45 m/minute)
(g) Backwash requirements – 2% of throughput at 350 kPa; 5% throughput at 100 kPa
The suspended-solids removal efficiency of microstrainers ranges from about 10 to 80 per cent, and a typical value is about 55 per cent. The problems encountered with microstrainers include incomplete solids removals and inability to handle solids fluctuations. It has, however, been found that by reducing the rotating speed of the drum and flushing the screen less frequently, the removal efficiency of the microstrainer is increased but its capacity is reduced.