by Marcia Sherony
For the past 15 years, the market trend in the wastewater industry has been to screen with finer and finer screen openings in the headworks of a municipal wastewater treatment plant. While this trend has continued down a relatively linear path, the drivers have varied and changed. The drivers are influenced by two main factors: new regulations and technologies.
With the advancement of screening designs, the physical separation of large solids in waste water is possible. But what are the consequences? Screening with very fine openings can complicate the simplest of steps in the waste treatment process. Screening is the first unit operation in a wastewater treatment facility. A screen performs a physical separation of liquid and solids.
Screening can remove rags, paper, plastics and metals to prevent damage and clogging of equipment, piping and appurtenances. There are basically three types of screen media: Bars, Perforated Plate and Woven Wire. One of these three types of screen media is used on virtually every screen. The screen media retains the solids larger than its opening, and allows smaller solids and liquid to pass through. Therefore, the smaller the opening, the greater the solids capture. Generally, the smaller the opening the larger the screen needed. Each type of screen media has unique design criteria.
Screen openings are available in most sizes. The real question is: what is really needed? A screen should be used to remove logs, timber, stumps, large debris, large solids, rags, small debris, floatables, paper, plastics and metals. A screen is the best technology for the coarser materials. Effective screening will improve downstream processes, improve flow conditions, prevent damage and clogging of equipment and piping, and reduce maintenance. Today, most large facilities use mechanically cleaned screens to remove larger material because these screens improve flow conditions, screenings capture, and reduce labor costs.
A screen’s effectiveness, however, is impacted by its overall design. Designs that allow for solids to pass through, over or around the screen, may allow for solids larger than the screen opening to by-pass the screen and foul downstream processes. Thus, increase maintenance costs and reduce effluent quality. A screen’s efficiency is measured by its reliability, maintenance and utility requirements. Maintenance requirements include both parts and labor for the screen plus maintenance on downstream equipment. For example, if a pump downstream of the screen clogs due to large solids, the labor to clean the pump is a cost that can be associated with ineffective screening. Utilities such as water and power requirements must also be considered.
Each plant must be evaluated for its needs. A plant should maximize the screen opening so organics are not taken from the biological process, yet it should maximize solids capture to keep undesirable solids from the biological process. Maximizing the solids capture will also help to protect downstream equipment and reduce maintenance. Maximizing the screen opening and maximizing solids capture is somewhat in conflict. The most acceptable balance of the two should be used.
The delineation between coarse vs. fine vs. very fine will continue to be a moving target. Physically, a fine or even a very fine screen can be used to separate solids from a wastewater flow; therefore, the separation can be accomplished. However, screening with very fine openings can complicate the simplest of steps in the waste treatment process. While fine screens can provide performance equivalent to a primary clarifier, the entire plant must be considered. Fine screens typically require upstream protection, therefore two screening steps are needed, coarse and fine. Fine screens often use water for cleaning, which increases utility costs and impacts the handling and disposal of residual solids. Cleaning of fine and very fine screens can result in increased maintenance requirements.
A variety of factors push the trend to screen finer. The main drivers are technological advances in biological processes, screening handling equipment and ever-tightening regulations. Recently, the movement toward finer screens has been accelerated with the advent of Membrane Bioreactor plants. MBRs offer the promise of a small footprint and excellent effluent quality, but membranes are intolerant of solids. Two existing technologies that are gaining popularity, as a result of their excellent solids removal abilities, are band screens and drum screens, both submerged and above ground.
Effective screening is the result of a number of factors. It is a balance of overall design, choosing the correct opening, acceptable reliability and maintenance costs and even plant effluent quality. Screens are inexpensive, and often determining the best screen for a plant is given much less scrutiny than other process steps. There is no one right answer as no two plants are identical. WW
About the Author:
Marcia Sherony is the Western & Industrial Regional Manager for Headworks Inc. and has been working with wastewater screens for over 20 years. Ms. Sherony has worked with market leaders in the screening business and has first hand knowledge of most technologies offered in the market. She has both industrial and municipal experience. Michael Schill, Headworks Inc., helped with editing of this article.
