Screening and Intake

The book Reynolds only reviews a short description of screen, specifically bar screen. Generally, if the intake is located to suck reservoir or river a two layer bar screen may be employed. First layer of bar screen is coarse bar screen. This screen has clear spaces between screen up to 75 mm (3″). As the clear space is quite big, it may be used to remove large debris (woods, rocks), or plastics as shown below.

screen outside the pump pipe

bar screen

Second layer of the screen is fine traveling screen, which is located behind the coarse bar racks. This screen has up to 13 mm clear spaces. Since the space is smaller than coarse screen, its function is to remove fine contaminants, debris, plastics that are not hang on coarse rack screen.

Of course after a long time of operation, sooner or later, the spaces between racks will be full or clogged by the debris hang on them. This of course will increase the headloss and thus reduce the volume of water may pass the screen while on the other hand will cause more debris stuck on between debris already hang on the racks. The example is displayed below.

clogged screen

Intake is simply a system either can be canal (open channel) or pipe (close channnel) to pass on water by deploying gravitation or the water is sucked by pump. Intake system along with the screen is crucial organ of water and wastewater treatment plant. It determines the necessary unit that need to be employed to treat the particular water source.

Reynolds stated that the package comprising of screening, grit removal, flow equalization and neutralization may be the most important thing of wastewater treatment. It explains that without grit chamber or flow equalization, experiences say that digestion or aeration may not takes place very well. Of course, the unexpected result will likely happen since wastewater (depends on what type of wastewater: residencial, industry) contains bacteria or biomass that fragile to the change of the loadings. The presence of grit or shock loadings may prevent the biological process. So I may say that the keypoint of the package is actually locating on the grit chamber and flow equalization.

Type of Screens

Above, it has been mentioned two layers of screens. In the front is coarse screen to remove large debris, wood or plastics, and then followed by fine traveling screen to remove smaller debris or plastics. So we have Coarse Screens (up to 75 mm openings) and Fine Screens (up to 13 or 15 mm).

There are two major types of screens that operated in around the world: mechanically-cleaned screens and hand-cleaned screens. The usage of these types of screens of course will be adjusted according on the case of water we are dealing with as it reflects on the operation and maintenance cost. It is common that in industry, mechanically-cleaned screes are used for the sake of simplicity. However, some industries equip themself of hand-cleaned screens to take place the mechanical if it experiences damage. Often, more than one screens are stored, with one is in operation while the other is operated when the primary is under maintainance or cleaning.

Usually, manually-cleaned bar screens, the bar spacing is ranging from 25-50 mm, and the bars are mounted at a 30 to 75o angle to the horizontal with 30-45o being typical. The bar spacing for mechanically-cleaned bar screens are ranging from 12-38 mm with the angle to the horizontal is 45-90o with 60o being typical.

Rule of Thumbs (Design Criteria)

The approach channel should be designed to form uniformity of water flow at least 0.6 m ahead the screen, there may be other options by other books and authors. The velocity of water before colliding bar screens at least 0.5 m/s to prevent grit settling. So, it is assumed that 0.5 m/s of velocity will prevent the grit to settle, the hydrodynamic plays the role here, this mechanism will be covered later. The velocity of water accross the bar screens is expected to range between 0.6 m/s to 0.9 m/s.

Calculating Screen

There are several stages to calculate screens or bar screens. First is the bar screen itself, and second is the headloss (energy loss of water, reflected by the difference of water level before and after the screens).

We can compute headloss with the following formula:

Headloss formula for bar screens

Va is approaching velocity towards bar screens
Vb, the velocity accross the bar screens
g, gravity

What you have to know in advance are the intake dimension, the bar openings and bar thickness. See the example below.

Suppose we want to design a mechanical screen, and we are supplied with area for intake 10m3. The boss gives 10 meter in length for intake, that would make 1 m2 for the cross section or Width (W) and height (H). After we search throughout catalogs, local company and internet, we are interested to purchase a bar screen with 10 mm thick and the openings are 30 mm wide. If we want the approaching velocity is 0.6 m/s, then compute the velocity of water accross the bar screens and calculate the headloss. Suppose the flow is 0.5 m3/sec, design the dimension of intake and determine the amount of bar screens.

Oke, so we have Q (flow), 0.5 m3/sec and we have 10 m in length (L). We want the approaching velocity is 0.6 m/s, which means :

Q = A x V ; Q is flow (m3/sec), A is cross section (m2 and V is velocity (m2/sec)

In this case, V is Va, that makes Q = A x Va

0.5 m3/sec = A x 0.6 m/sec; A = 0.83 m2

We have found the cross section area, now let’s assume the width and height. Suppose we want the ratio of W and H is 2:1, thus

W = 2H

W x H = 2H x H = 0.83 m2; H = 0.64 ~ 0.65 m, and we finally obtain W = 1.3 m

With bar thick 0.1 cm (0.001 m)and openings between bars are 0.3 cm (0.003 m), find out how many bars needed to fill the canal.

The formula is {(0.003+0.001) x t} + 0.003 = 1.3 m (W), t = 324.25 ~ 325 bar screens.

Okay, now its time to check the velocity accross the bar screens.

AaVa = AbVb

Aa = cross section of canal = 0.83 m2
Va = approaching velocity = 0.6 m/sec
Ab = openings between bar racks = {(0.003 m x 325 bar screens) + 0.003 m} x 0.65 m = 0.635 m2
Vb = Velocity of water accross in between bar screens, m/sec

0.83 m2 x 0.6 m/sec = 0.635 m2 x Vb

Vb = 0.78 ~ 0.8 m/sec, which is in the range of 0.6-0.9 m/sec.

It’s time to calculate the headloss based on the formula above.

HL = (0.82 - 0.62)/(2 x 9.81 x 0.7) = 0.02 m = 2 cm

Calculation Summary

So we have obtained several points from calculation above,
1. designing and determining width and height based on known flow (Q)
2. calculating the amount of bar screens needed,
3. calculating the crossing velocity (Vb) and check it with rules of thumbs
4. computing the headloss

These points are pictured by images below:

intake and screens

sideview intake and bar screens

Assignment

1. Mention and describe types of screens, compare among them (if any) according to your own opinion based on other handbooks, journals or from internet
2. Find vendor and their website (seller or vendor of bar screens)

Technical guidance
1. Maximum point is 10
2. Discussion in the class with student and teacher will be held at Monday, 3 September 2007

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