Waste water treatment by Screening

The process of waste water treatment of physical water treatment is followed by step by step systematic process. First step is screening. Screening devices are followed by this screening process, steel bar screen process, coarse screen process and in-line grinders process, fine screen process, and micro screen process.

Screening by Bars

Bar screening process may be classified into (1) trash racks, (2) mechanically cleaned bar screens, and (3) manually cleaned bar screens.

Screening by Trash Racks

This Trash racks are apparently installed in surface water treatment plants intakes to protect or resist course screens from large debris or floating waste product and to prevent large debris from entering combined/storm water sewerage systems. The openings of racks are 1 to 4 inches. Generally, the bars are made of steel, and its shape and size depend on the expected structural loads, which are both static load (due to the head loss through the rack structure) and dynamic load (due to the impact effect of moving debris material). Then logged racks are cleaned after intermittently by mechanically driven rakes system process that are drawn across the outside of the steel bars. This raking mechanism should be sufficiently able to lift and separately move that largest expected object from the screening system.

Mechanically Cleaned Bar Screening process

Mechanically cleaned steel bar screens are generally installed in the head works points of sewage treatment plants to intercept large size debris. It may be followed by coarse screens  and in-line grinders system. The outer side clear openings between the bars are usually kept ½ to 1¾ inch. In sewage or water treatment plants, the front side approach channel should be perpendicular to the plane of the bar screen and straight direction of system of flow. Approach stream velocities should lie between 1.35 ft/sec (to avoid grit deposition) and 3.5 ft/sec (to avoid forcing material through the openings). Waste water velocities through the openings of bars should be limited to 3 to 5 ft/sec.


Inclined and Vertical Multirake Bar Screening process

Multirake steel bar screens are used when continuous and simultaneously heavy debris loads are expected in the screening system. The spaces between the continuous bars are kept clear by several rows of rakes mounted on continuous belts systematically. The rake speed/velocity and spacing is manually adjusted so that any particular place or position on the screen may be available to cleaned at intervals of less than 1 min.

The raking may be placed in front of the bars, behind them, or may loop around them. In the most common arrangement, the continuous belt and rakes are installed in front of the screen, and the ascending side of the belt is the cleaning side. At the top of the motion, a high-pressure water spray dislodges debris from the rake and deposits it into a collection device.

Manually Operated Cleaned Bar Screening system

Manually operated cleaned bar screens are sometimes randomly installed as temporary bypass channels for use purpose whenever the mechanically cleaned bar screen is break down for servicing. The reinforcement bars should be setup slope at 32o to 45° angle from the

Kirschmer’s Shape Factors for Bars shape and size

 Various size Bar Cross Section Shape Factor (SF)
Sharp-edged, rectangular bar     2.42
Rectangular with semi-circular upstream face bar     1.83
Circular bar     1.79
Rectangular with semi-circular faces upstream and downstream     1.67
Teardrop with wide face upstream     0.76

horizontal, and the total longitudinal length of screening bars from the top of the invert must be accessible from the rake. The clear to clear opening between the screening bars should not be less than 3 cm, and the flowing debris or waste water velocity/speed through it should be between 30 cm, and 60 cm/sec. The screenings will generally be dragged up over the top of the slopped bars and deposited into some sort of waste product container. The floor supporting to this container should be drained or grated for clear passing out of water.

Head Losses for Bar Screening Process

The maximum allowable headloss through screening process for dirty bar racks is normally kept about 45 cm. The minimum headloss for Bar racks can be calculated from Kirschmer’s formula .

The sloping angle is an important and considerable design factors, i.e. because sloping angle of bar generally directly increases the opening area/ surface contacted area exposed to the flow of waste water and helps to keep the velocity/speed through the opening gap of the bar to less than the desired maximum. Slopes or angles may be as flat as 30° from the horizontal plane with manual cleaning of the racks easier, although at present days, all type of racks is always mechanically cleaned under machine operated system.

Coarse Screening system

Coarse screens may be classified as traveling screens, rotating drum screens, rotating disc screens, or fish screens.

Traveling Screens

One of the most common type of screens is Traveling type of Coarse screen. They are physically used in water flowing intake points to protect treatment plant equipment from debris and at wastewater treatment plants to remove debris or floating material from the raw sewage. This type of screen bars is consisting of flat opened panels of wire mesh of specified size supported on steel structure frames. All panels are mechanically hinged connected with together to form a continuous chain loops that are vertically mounted on motor-driven shafts and sprockets. These types of screens are used to remove comparatively smaller size of debris material which is below 50 mm., and the mesh openings are generally kept about 3mm to 18mm. Traveling screens are regularly placed by bar racks to resist damage by large size of debris or waste product. In case of extreme weather condition that may be cold or different extreme climates, the approach velocity of traveling screens is generally kept downward value i.e 15 cm/sec in order to resist the formation of frazil frozen ice and to protect the resuspension of sedimentation of particles near the intake point, and to permit fish entry through the screen to swim away.

The dirty panels of traveling screens are required to be clean on regular basis after few days of interval by high-pressure water sprays or water jet, and the removed debris from the mesh is deposited into a drainage channel for removal from the traveling screen system.

 Rotary Drum Fine Screening system

In case of rotary drum fine screening system, steel made wedgewire is wrapped surrounding a horizontally placed cylindrical framework which is partially submerged in water. Basically, about 74% of the rotary drum diameter and 65% of its mesh surface contact area are submerged within the water. Whenever the drum rotates clock wise or anti clockwise direction, dirty wire is brought to the top portion of the drum, where it is cleaned by high pressure / high velocity jet water sprays and blades.

The direction of flow of water may be in along the drum rotation axis and out through the inner surface of the wedgewire or it may be through the outer surface of the wedgewire and out along the axis of the rotary drum system.

General common openings of mesh are kept 0.02 to 0.05 inch. (0.05 inch. is preferred for raw wastewater screening purpose), usually the wire diameter are kept 0.05 inch

Inclined attached, Self-Cleaning, Static Screening system

Inclined based, self-cleaning, static screening system consist of inclined planes of  steel wedgewire . This systematic wire runs horizontally through the attachment. The flow of waste water mixed with debris is introduced at the top of the screen, and the mixed water travels towards downwards direction along the screen surface portion. Now at this situation Solids particles are retained on the  inside surface of screen, and screened/cleaned water passes through it and is collected underneath  tank of the screen. As the  solid debris materials accumulate on the outer screen surface, it resist  the water flow, it may causes the water to move the solid debris downwards to the screen bottom.

General  opening size of screenings  are 0.02 to 0.08 inch. (determined by field tests), and the usual wire diameter is 0.07 inch.

Microscreening system

Microscreens screening system  are  generally  used as  secondary supporting based bottom suspended  solid particle removal devices , which is followed by  biological wastewater treatment process and secondary clarification process of waste water. The opening size of mesh screen are generally   kept  between 25 to 30 µm and range from about 12 to 65 µm. Mechanically hydraulic type of loading is applied  typically 6 to 12 gpm/ft2 of total  submerged area water flowing surface. The settled down suspended solids removal process from secondary clarifier effluents is about 45 to 65%. Effluent type of settled down bottom suspended solids concentrations are generally followed by  4 to 12 mg/L.