for Your Application
BY WENDI RICHARDS
Screening is a compo- nent of every wastewa- ter treatment plant and
water intake facility. The question is
whether it’s given the important status it deserves. Most plant operators
will agree that the more they remove
up front, the better off they are
with less maintenance downstream.
That’s a great goal but how do you
reach it with so many mechanically-cleaned screen choices available?
The best approach is to first
characterize what comes into your
screen; second, assess what’s downstream that needs protection; and
third, review the details of your plant
layout and hydraulic profile to see
CHARACTERIZE SCREENINGS AND FLOW
What is the variability in screened flow rates? Is a sewer
system combined, separated or both? Is flow by gravity or
pumped, or through a grinder? Are there fats, oil and grease
(FOG), certain sized solids, or fish in the flow? All these answers help determine the size, nature and rate of material
coming into the facility, which in turn helps select the appropriate equipment. Large material coming in needs heavy duty
equipment to take it out. On the flip side, smaller organic
material may need filtering and better washing before compacting. Finally, how clean and dry do the removed screenings
need to be for disposal?
SELECT A SCREEN TO DO THE JOB
There are all kinds of equipment at a facility that require
protection and the amount of protection determines the most
suitable type of screen as well as grid type and opening size.
For most wastewater plants, the trend is toward finer 1/4 inch
or 6 mm openings and larger. But integrated fixed-film acti-
vated sludge (IFAS), membrane bioreactors (MBRs), and oth-
er technologies may require finer openings. In general, most
pumps are protected by mechanical rake-type screens while
fine screens with higher screening removal rates are more
suitable for wastewater plant headworks and water intakes.
Coarse screens include chain- or cable-driven, reciprocating rake and catenary, and typically have openings larger
than 1/4 inch. These may be configured with single or multiple rakes. Medium to fine screens include continuous self-cleaning through-flow, rotary drum or inclined screw, and
band screens. Some installations require a coarse screen located ahead of a fine screen. Screen grids may consist of fixed
bars, wire wedge, slots, or perforations with a range of opening sizes and materials of construction.
Within a screen type, each manufacturer differs in materials of construction, frame design, protection of moving parts
from grit and debris, carrying capacity, and how the screen
is cleaned. The amount of screenings that are not removed
and carry over a screen — ending up downstream in the flow
— is partially related to the off-loading methods and screen
bypass. Rakes penetrate bars that return in front or behind
the screen, brushes also make contact with the grid, while
spray water makes no contact, thus reducing material forced
through screen openings.
PLANT LAYOUT AND SCREEN FIT
This is where the detailed engineering comes in. Once
The existing channel dimensions and hydraulic profile are extremely important when determining the
screening equipment that can fit.