Screens: An Important First Step in a Wastewater Treatment Plant

By Phil Vella

No matter what size wastewater treatment plant you have, screening equipment at the headworks is a necessary requirement. Screens or pretreatment devices are designed to remove or reduce large solids like wood, cloth, paper and plastics from the waste stream. This not only allows downstream treatment process to be more efficient but also protects the equipment such as pumps.

Several different types of equipment can be used to meet these objectives and there is no one-size-fits-all solution that can be applied to every headworks situation. Some of the limitations of small systems are low flows, space and financial considerations. The following will focus on those options most likely to fit into a small wastewater treatment plant. 

As with most equipment at a plant, screens come in a variety of sizes, capacity, automation and cost. In general screens may be classified as coarse, fine and micro and are based on the size of the screening openings. The discussion here will focus on course screen technology with openings 6 to 36 mm (0.25 to 1.5 in.).
 

Manual Bar Screens

With the limitations of small systems, a manual bar screen may be a great option. These screens have vertical bars approximately 1 to 2 inches apart to catch the incoming debris. Although very basic, they do provide a good level of protection for the plant. An example is shown in Figure 1. However, as with most basic equipment, there are limitations.

Source: Islamic University of Gaza.

The most obvious limitation is that this is a manual operation and requires dedicated manpower and can be a burden to small systems. This is especially true during high flow events such as storms that may require more frequent raking and may also create more of a safety hazard for the operator.

Automatic Bar Screens

To limit the labor involved with manual bar screens, there are several automated options available. These can be classified into different group types. Chain Driven Screens, Catenary Screens, Reciprocating Rakes, Continuous Belt Screen and many variations of them. A summary of different types of screens with their advantages and disadvantages is given in Table 1. These options also commonly use vertical bars to capture the solids and remove them with an automated raking system. Since these are automated, the cost and other operational costs must be considered. In addition, these systems are larger than the manual screen so adding this to a facility may require civil engineering to modify the influent channel or the headworks building if placed there. You may have reduced the labor cost but have increased the capital and infrastructure expense.

Although course screens can remove large material at the head works, disposal of this material becomes and added cost and requires operation and maintenance.  In addition, the wet screenings collected are smelly that can attract vermin and result in odor complaints from the community. 

TYPE OF SCREEN	ADVANTAGES	DISADVANTAGES
CHAIN OR CABLE DRIVEN SCREENS	Design in the market for many years Simple channel construction High screenings loading rate Insensitive to Fat, Oil, and Grease (FOG) Low headroom required	Submerged components subject to wear and tear
RECIPROCATING RAKE SCREENS	No critical submerged components Widely used	Low screening loading rate High overhead clearance, particularly at deep channels
CONTINUOUS SELF-CLEANING SCREENS	Medium to low headroom required Allows a pivot design for servicing the unit above the channel	Several moving components Components subject to wear and tear
ARC SCREENS	Simple design Lower capital and operational cost No drive parts under water Utilizes 100% of channel width	Limited to small to medium flow plants Not suited for deep channels
CATENARY SCREENS	Simple to operate Easy to maintain	Chains are very heavy and difficult to handle Large footprint

Source: WEF, Manual of Practice 8, 2017

The following are examples of some of the screen options available to wastewater treatment plants.

Multi Rake Chain Driven Bar Screen (Automatic/Self Cleaning) | Source 

Multi Rake Automatic System

 

Reciprocating (Single/Basket) Rakes | Source

Catenary Bar Screen | Source

Arc Screen | Source

In summary, there is no right equipment choice for all headworks screening circumstances. Individual factors such as flow rate, solids loading, cost and infrastructure modifications must be considered. Choosing the correct option is important not only in protecting downstream equipment but also for efficient and effective solids removal resulting in proper wastewater treatment.