By: Jay R. Conroy, President, Hydro-Dyne Engineering
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 on preliminary treatment is shifting to screenings testing.
Engineers, operators, and maintenance personnel alike know the benefits in 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 influent flow characteristics. Specifically, as these processes increase in sophistication and sensitivity, plant design is driven towards finer upstream screening protection without further investigation into the type of solids presented to the plant. Generally, this can result in higher capital outlays, larger headworks structures, and frequently increases disposal of the organic and fecal material the plant is designed to treat.
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 on-site testing with specialized equipment, the solids loading characteristics of an individual plant can be better defined. Previously this was generalized as TSS or 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 what 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. Volume of screenings collected on the sieves can give insight into amount of screenings expected and specific opening sizes and this information can be used to properly size the screen and screening handling equipment. More sophisticated on-site 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 on-site testing of the plant’s unique flow to provide valuable information for a properly designed screening system.
Jay R. Conroy is an environmental engineer and the President of Hydro-Dyne Engineering. With more than 20 years of experience in the water and wastewater industry, Mr. Conroy has helped design coarse screens, fine screens, screenings handling and grit removal equipment for applications around the world.