How do you design a bar screen in wastewater?
A screen is a device with openings for removing larger suspended or floating matter in sewage that would otherwise damage equipment or prevent treatment units from functioning properly.
Factors to consider while designing screens:
1. Flow Velocity
The flow velocity ahead of and through a screen varies significantly, affecting its operation. The number of screens removed from sewage increases as the velocity of flow through the screen decreases. However, the lower the flow velocity, the more solids will be deposited in the channel where the screen is installed.As a result, the design velocity of the flow should be such that it allows for the complete removal of material of a given size without causing unnecessary depositions. The effective area of opening of the screen (which is the vertical projected area of the screen openings from the invert of the channel to the flow line) should be such that the velocity of flow produced through the screen opening does not exceed 0.9 m/s at maximum or peak flow, according to Indian Standard IS: 6280-1971.
For maximum or peak flows, however, velocities of 0.6 to 1.2 m/s through the screen apertures have been found to be satisfactory. When dealing with large amounts of storm water (or rain water), an approach velocity of roughly 0.8 m/s is recommended to avoid grit deposition at the bottom of the screen, while a lower value of 0.6 m/s is currently employed.Furthermore, to avoid solid deposition, the flow velocity in the approach channel should not be less than 0.3 m/s at low flows. A straight path in front of the screen promotes good velocity distribution and maximises the screen's efficiency.
2. Head Loss
As sewage passes through a screen, a certain quantity of head is lost.The loss of a head due to bar screens is a result of-
i) The shape of a bar, and
ii) The flow's velocity head between the bars.
The head loss through clean flat bar screens is calculated from the following formula:
h = 0.0729 (V2 - v2)
Where, v = velocity before the screen in mps, h = head loss in m, V = velocity through the screen in mps.
3. Materials
Steel bars, flats, or rods are fixed to an appropriate steel frame to create bar screens. The minimum cross-section of bars, flats, or rods utilised for bar screens is 10 mm x 50 mm, with the big dimension parallel to the flow direction. The spacing between the bars, flats, or rods should be consistent and preferably maintained by a sufficient number of spacers that are not in the way of the raking action.Fine screens are made of brass or bronze plates or wire mesh for rust and corrosion resistance. They have 1.5 to 3 mm perforations or apertures. For every 1000 m3 of average daily sewage flow from a separate system, fine screens typically have a net submerged open area of not less than 0.05 m2, with the comparable figure being 0.075 m2 for combined systems.
4. Additional Considerations
(i) The top of the screen must be at least 300 mm above the peak sewage flow level.
(ii) In the case of large sewage treatment plants, the screen channel can be divided to a maximum width of 1.5 m.
(iii) The submerged area of the screens, including bars and openings, should be roughly 25 to 35 percent larger than the approach channel's cross-sectional area. In the event of an inclined screen, the area should be calculated as the projected area normal to the flow direction. Manually cleaned screens have an inclination angle of 45° to 60° with the horizontal, while mechanically cleaned screens have an inclination angle of 60° to 90° with the horizontal.
(iii) The submerged area of the screens, including bars and openings, should be roughly 25 to 35 percent larger than the approach channel's cross-sectional area. In the event of an inclined screen, the area should be calculated as the projected area normal to the flow direction. Manually cleaned screens have an inclination angle of 45° to 60° with the horizontal, while mechanically cleaned screens have an inclination angle of 60° to 90° with the horizontal.
(iv) The screen should be implanted in such a way that any frame, if present, does not restrict sewage flow.
(v) A drop below the invert level of the screen channel upstream of the screen is supplied for the screen channel invert downstream of the screen. A minimum drop of 150 mm should be provided for hand-operated screens, while a minimum drop of 75 mm should be provided for mechanically driven screens.
(vi) The length of the screen channel should be sufficient to provide for good screen housing, adequate working space, flow stabilisation, and the avoidance of eddies.
For more information, contact Netsol Water.
Netsol Water is Greater Noida-based leading water & wastewater treatment plant manufacturer. We are industry's most demanding company based on client review and work quality. We are known as best commercial RO plant manufacturers, industrial RO plant manufacturer, sewage treatment plant manufacturer, Water Softener Plant Manufacturers and effluent treatment plant manufacturers. Apart from this 24x7 customer support is our USP. Call on +91-9650608473, or write us at enquiry@netsolwater.com for any support, inquiry or product-purchase related query.
