During the project development process, you will eventually narrow down a list of potential dissolved air flotation (DAF) system manufacturers. At this stage, the critical task is to determine which manufacturer and system best meet your specific needs.
How Should You Choose?
Whether you are an engineer specifying equipment for a client, a plant owner addressing wastewater treatment challenges, or someone seeking a deeper understanding of wastewater process equipment, this guide aims to provide a comprehensive understanding of key DAF system design considerations.
By exploring mechanical and process design elements in this series, you will gain valuable insights to evaluate DAF systems effectively. This knowledge will enable you to identify superior designs and, most importantly, make an informed purchasing decision. Given the significant investment, selecting the right DAF system is essential.
1. Aeration System: The heart of the DAF unit.
The aeration systems is the central component of a DAF, representing one of the largest capital and maintenance expenses. Therefore, understanding its design and functionality is crucial. Below is a breakdown of common pump types used in DAF aeration systems and their respective advantages and limitations.
Multistage Impeller Pumps
These pumps are often referred to as "whitewater pumps". They draw atmospheric air (or compressed air) into the pump, where impellers mix/shear the air with water to create micron-sized bubbles that dissolve into the solution. While effective in generating whitewater, multistage impeller pumps pose several challenges in wastewater environments:
Low Solids Tolerance: These pumps are prone to failures when handling oily, stringy, or gritty materials, which are common in wastewater applications.
Cavitation/Airlock: In general, these pumps are not designed to handle entrained air. Air can collect at the eye of the impellers which can cause loss of flow (airlock) and binding. Air being introduced to the pump for “white water” generation can cause cavitation leading to increased energy usage, increased noise/vibration, and in more extreme instances cause damage to seals, bearings and impellers.
Dependency on Manufacturer-Specific Components: These pumps tend to be “special order” or made in low quantities. Typically, these pumps must be sourced directly from the manufacturer which can lead to high replacement cost and extended downtime due to limited availability or long lead times.
Multistage impeller pumps can generate quality “white water”; however, they lack the robustness in a wastewater environment where solids, grit and oily materials are prevalent. With the high associated maintenance costs and lower reliability, most DAF manufacturers have moved away from the Multistage Impeller Pump.
Regenerative Turbine Pumps
Regenerative turbine pumps are another type of pump often marketed as "whitewater pumps". These pumps are often characterized as providing high discharge pressures associated with positive displacement pumps with the versatility of a centrifugal pump. They utilize a turbine-like impeller with radially oriented blades/vanes to draw in atmospheric or compressed air and mix with water to create microbubbles. When choosing a Regenerative Turbine pump for a DAF application it is important to consider:
High Pressure Capability: These pumps can generate high pressures and low flows in a compact design. They can operate at discharge pressures of 90-120 psi in DAF applications while being resistant to cavitation and other ill effects of air entrained fluids.
Clean Liquid Requirement: Due to tight internal clearances, these pumps typically require liquids with minimal abrasives or solid content, limiting their suitability for wastewater treatment.
Limited Parts Availability: Popular models, such as those manufactured by Nikuni, often have very small supplier networks, leading to limited options in accessing quick replacement parts.
Several manufactures utilize regenerative turbine pumps for “whitewater generation” on their DAF units, and their compact footprint make them a popular choice for upgrades of older Multistage technology. Tight tolerances and poor solids handling make them less suitable for certain wastewater environments and care should be taken to ensure proper application.
End-Suction Centrifugal Pumps
End-Suction Centrifugal pump are produced by many pump manufacturers and are available in a variety of materials and configurations that can be adapted to a wide variety of applications. Due to the large variety in available pumps, there is a high degree of versatility across various industries, including food processing, oil refining, and chemical manufacturing. As with any pump selection, application is important. Here are some important considerations when evaluating a system using an end suction centrifugal pump.
Simplified Whitewater Generation: In most DAFs that use an end-suction centrifugal pump for “whitewater” generation, the pump is utilized only for the pressurization of the recycle stream. The air and water are mixed in a separate vessel or saturation tube, meaning the pump is only doing what it was design for (pumping). This ensures that the pump can be selected for high efficiency and reliability. In some designs, manufacturers will use the pump in a similar manner to the Multistage pump where atmospheric air (or compressed air) is injected at the pump suction and the pump is used to dissolve the air. When the pump is utilized in this manner, it is prone to the same Cavitation/Airlock issues as the Multistage Pump.
Adaptability: These pumps can be selected based on the application to handle a wide variety of liquids (with or without solids) and can be fitted with various materials and alloys for corrosive environments.
Manufacturer Choice: Unlike specialized whitewater pumps, the end-suction centrifugal pump is typically not mixing the air in water. This allows more choice in manufacture and style of pump. Some DAF providers can work with the end user to select a pump that aligns with their preferred manufacturer/plant standard providing improved access to parts and service.
By assigning the task of whitewater generation to a static tube or vessel, end-suction centrifugal pumps focus solely on pressurization, enhancing reliability and efficiency.
When evaluating DAF systems, the aeration system reflects the manufacturer's design philosophy. Prospective buyers should inquire about the reasoning behind the selected pump type and its suitability for their specific wastewater treatment needs.
2. Controls and Automation: Enhancing Operational Simplicity
The operational efficiency of a DAF system often hinges on its control and automation features. A well-designed system should be intuitive and user-friendly, akin to the seamless functionality of modern smartphones.
Unfortunately, some DAF systems rely on overly complex operational procedures and have limited automation to reduce costs. This typically leads to increased labor, frustration, and hidden expenses. Often these systems require frequent operator intervention and manual procedures which take more time and require more knowledge from the operator:
Start/Stopping the system based on incoming flow.
Make regular adjustments to the DAF aeration system to achieve “whitewater” generation.
Manually adjust chemical dosing rates based on changing flows and/or pH.
Check the system for faults and evaluate performance based on visual inspection.
In contrast, a well-automated system streamlines operations significantly by:
Automatically starting and stopping the treatment system as determined by incoming flow and tank levels.
Automating the aeration features such as low-pressure detection, system warm-up and off-cycle air purging.
Automatically adjust chemical dosing rates based on flow rate to the system and pH.
Report and alarm system faults and provide warnings for reduced system performance.
Trend and store data for system flows, tank levels, pH turbidity, chemical consumption, and alarms.
When considering a DAF system, request details its operating procedure and automation. This will provide insights into its usability and the time/labor necessary to operate it. A system with intuitive controls not only saves time but also reduces operational errors and long-term costs.
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