Proper Flow Meter Selection Will Enhance DAF Process Performance

Dissolved Air Flotation (DAF) is a wastewater treatment process for removing suspended solids, oils, fats and other contaminants using air bubble flotation. This method dissolves air under pressure into water, mixes the dissolved air and water with the waste stream and releases the air from solution while in contact with the contaminants. Air bubbles form, attach to the solids, increase their buoyancy and float the solids to the water’s surface. Air is injected under pressure into a recycle stream of clarified DAF effluent. This recycle stream is then combined and mixed with incoming wastewater in an internal contact chamber where dissolved air comes out of solution in the form of very fine bubbles that attach to the contaminants. The bubbles and the contaminants rise to the surface and form a floating bed of material that is removed by a surface skimmer into an internal hopper for further handling.

Smart Measurements

Making smart measurements of the compressed air system will reduce maintenance and energy costs as well as ensure proper operation for all equipment requiring consistent air delivery. The measurement equipment is relatively inexpensive. Monitoring power use, air flow and air pressure ensures a properly working system, but other measurements like dew point and temperature can also help a system operate with better results. By measuring the air flow, pressure and energy use, users can generate a baseline of the system operation. Typical flow data in a compressed air system is measured in cubic feet per minute (CFM). Volumetric flow meters are typically used to measure compressed air since they do not require any straight runs of pipe before or after the meter installation (see Image 1). They also are unaffected by turbulent flow conditions, which can create inaccurate or no flow readings in a velocity type of flow meter. Some volumetric flow meters will read accurate flow readings even with differing system pressure by using pressure compensation into a flow monitor. One of the greatest benefits for using a volumetric flow meter, also called a variable area (VA) meter, in a compressed air system is that the meter is totally unaffected by water or moisture in the compressed air. Unlike velocity meters or thermal mass flow meters, the volumetric technology is only able to read flow ranges within a 10:1 flow ratio, also known as the effective flow range of the flow meter. Thermal mass flow meters are also an excellent choice for measuring flow in the compressed air line as long as the meter is installed after the air dryer and the air dryer is functioning properly. Thermal mass meters typically measure a flow range of 100:1 independent of temperature and pressure changes so they can read very low flow rates, which enable detection of air leaks in the system. All thermal mass flow meters are not the same. There are a few flow meter manufactures that use a small capillary tube to measure the air mass flow. When using this capillary thermal mass flow technology, measurements must be taken to make sure the air system is filtered to less than 1,000 microns and supply is very dry. Knowledge of the pressure drop these flow system will create is also something that will have to be considered in the design of the DAF air system before use. When users implement smart metering, they can optimize velocity and operating pressure to reduce pressure drop and increase performance of the DAF.

Raw Water Intake

Raw water intake into the DAF is another critical measurement that requires consideration before implementing a flow strategy. Since most treatment systems start with flash-mixing sulfuric acid and ferric chloride into the raw water to adjust pH, and the water will be disinfected for bacterial using some form of oxidation like sodium hypochlorite and then sodium hydroxide, care should be taken to ensure that the flow meter selection takes material corrosion protection in mind. Depending on the plant process, some raw water intakes may include a percentage of solid wastes, such as raw water from poultry processing, making the selection of flow meters with no moving parts or even non-invasive flow meters like clamp-on ultrasonic meters more attractive. Similar to the air flow rate flow meter, the influent raw water flow meter will require an electronic output for communication to the DAF programmable logic controller (PLC) or control interface unit to set the ideal rate for the optimum air to solids ratio. Adjusting the raw water intake to the DAF with the correct amount of air flow to the air diffuser will require accurate, reliable measurements for optimum control. The most important operational parameters for contaminant removal by dissolved air flotation are air pressure, total raw water/recycle or slip stream flow rate, influent total suspended solids (TSS) including oil and grease, bubble size and dispersion. Air pressure, recycle and influent TSS are normally related in an air-to-solids (A/S) ratio expressed as Equation 1. The A/S ratio is most important in determining effluent TSS. Recycle flow and pressure can be varied to maintain an optimal A/S ratio. Typical values are 0.02-0.06. Many factors impact the design and implementation for the DAF system, and several process equipment designers and fabricators offer technologies for different system designs. A few of the most important design criteria are TSS and fats, oils and grease (FOG). Knowing the combined TSS and FOG content in parts per million (ppm) of the wastewater and the process flow rate are critical in design. By using proper flow meter selection to accurately measure the process variables involved will assure clean water discharge and long process life as well as reduced maintenance.