Screen Test: Size Matters
On-site influent sieve testing of screenings benefits preliminary wastewater treatment design.
BY JAY R. CONROY
The wastewater industry has responded to increasingly stringent plant effluent requirements with enhanced plant design and innovative technology. It is generally accepted that plant designs have defined performance requirements in order to meet effluent regulations but increasing emphasis on the analysis of a unique plant’s influent flow is needed to optimize treatment. In recent years, there has been focus on sampling to select and size appropriate grit removal equipment and now that focus is shifting to screenings testing.
Engineers, operators, and maintenance personnel alike know the benefits of removing inorganic and settleable solids as early in the treatment process as possible, but preliminary treatment equipment has conventionally been selected more on requirements of downstream processes than on influent flow characteristics. Specifically, as these processes increase in sophistication and sensitivity, plant design is driven toward finer upstream screening protection without further investigation into the types of solids entering the plant. Generally, this can result in higher capital outlays and larger headworks structures, and frequently increases disposal of the organic and fecal material the plant is designed to treat.
More detailed information about the contents of the waste flow is critical to properly determine the correct screen type, opening, and size for the application.
Many factors affect waste stream solids loading and particle size. The design of a collection system, constituents feeding the plant, stormwater infiltration, variations in flow, and headworks design all have a direct impact on the quantity, size, and consistency of screenings in the influent flow. Studies have shown that screening volume relative to flow can vary by a factor of as much as 70 times. This means even conservative sizing used by most screen manufacturers cannot properly account for fluctuations in screenings of this magnitude when calculations are based on peak flow and opening size alone. Having more detailed information about the contents of the waste flow is critical to properly determine the correct screen type, opening, and size for the application.
Through onsite testing with specialized equipment, the solids loading characteristics of an individual plant can be better defined. Previously, this was generalized as total suspended solids (TSS) or biological oxygen demand (BOD) but that does not identify solids at a given size. What is needed is stratification of solid sizes present in the waste stream relative to desired screen opening sizes. Analysis of this data helps identify proper screen openings and capture ratios required by downstream processes while determining the appropriate screen type, size, and operational sequence determined by the unique inputs to the individual plant. Small batch sieve testing is useful and can be advanced with specialized testing equipment to sample much larger volumes of water for more accurate data on solids size and amounts.
With the goal of selecting the proper screen opening, multiple sieves can be stacked and visual inspection can indicate which opening sizes remove the desired inorganic solids and at which point primarily organic material is captured. Valuable insight can also be gained regarding the need for more sophisticated screening handling equipment if a large amount of fecal material is identified on the desired opening sieve.
The volume of screenings collected on the sieves can give insight into the amount of screenings expected and specific opening sizes. This information can be used to properly size the screen and screening handling equipment. More sophisticated onsite testing equipment can sample larger volumes of influent, measure rate of blinding across multiple sieves, and sample from various points in the water column for advanced analysis but even basic sampling methods can provide valuable information to better design preliminary screening equipment in front of today’s advanced wastewater treatment plants. Before upgrading or designing a wastewater plant’s headworks, consider onsite testing of the plant’s unique flow to provide valuable information for a properly designed screening system.
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About the Author: Jay R. Conroy is president of Hydro-Dyne Engineering, a mechanical screen and grit removal equipment manufacturer located in Clearwater, Fla. He is a member of the Water and Wastewater Equipment Manufacturers Association (WWEMA) Board of Directors and Vice Chair of the WWEMA Marketing and Member Services Committee. WWEMA is a non-profit trade association founded in 1908 whose vision is to be the “voice of water and wastewater technology providers.” Its mission is to promote the advancement of technology solutions for clean water that ensure a future sustainable environment and to improve its members’ economic viability. More information about WWEMA can be found at www.wwema.org.