Treating Wastewater With Diffusers
Fine bubble diffusers are used to aerate wastewater for sewage treatment and reduce pollution. The goal is to reduce the amount of remaining sludge, which depends on the amount of solids generated and other conditions. Fine bubble diffusers are used in aerobic digestion, primarily in larger scale wastewater treatment.
The aerobic digestion employs living organisms to naturally reduce the amount of organic waste and the amount of disease-causing microorganisms. Aerobic digestion can also be achieved by using diffuser systems or jet aerators to oxidize the sludge. Aerobic digestion is a bacterial process occurring in the presence of oxygen.
A fine bubble wastewater diffuser is typically manufactured in various forms such as a disc or tube. They are made from porous ceramic plastic, or increasingly from perforated membranes made from EPDM (ethylene propylene diene Monomer) rubber. Average bubble diameters of 0.9 mm or less are possible nowadays, using special polyurethane (PUR) or special recently developed EPDM membranes.
With fine bubble, each unit has thousands of aeration holes. Each unit has thousands of very small air bubbles which rise slowly from the bottom of a wastewater treatment facility. With EPDM units, there can be 8,000 holes per disc through which air is diffused. The EPA classifies a fine bubble as anything smaller than 2mm in diameter.
The biological process uses air to speed up bacterial growth that consumes waste materials, such as phosphorus and nitrogen in the wastewater. Bacteria rapidly consume organic matter and convert it into harmless carbon dioxide. The oxygen, combined with the sewage, allows the bacteria to create enzymes which break down the waste so that it can settle in clarifiers or be filtered by membranes.
Pumps move air into the diffuser grid, which typically includes hundreds of units. Air pumped through the wastewater aerators membranes is released into the water at about 30,000 to 50,000 bubbles per second.
An important consideration for selecting diffuser type is to make sure it will produce the required oxygen transfer rate. Fine bubble diffusers have mostly replaced coarse bubble diffusers in most industrial nations and in much of the developing world.
Fine bubble aeration is a highly efficient way to transfer oxygen. This type of aeration has a very high oxygen transfer efficiency (OTE). They’re typically the more cost-efficient diffusion method, increasing the oxygen transfer efficiency, which reduces the amount of power the plant needs to pump air.
In general, wastewater consultants recommend finer bubbles and a deeper release in the tank will generate a greater oxygen transfer rate. Further, smaller bubbles take longer to reach the surface. Thus not only is the surface area maximized, but so is the amount of time each bubble spends in the water, allowing more time to transfer oxygen to the water. By releasing millions of fine bubbles instead of fewer coarse bubbles, fine bubble aeration maximizes the surface area of the bubbles and transfers more oxygen per bubble.
In addition, fine bubble diffusers can be spread out evenly in a 'grid arrangement' on the floor of the treatment tank, allowing the operator greater flexibility This can be used to create zones with high oxygen concentrations (oxic or aerobic), zones with minimal oxygen concentration (anaerobic) and zones with no oxygen (anoxic). This allows for more precise removal of specific contaminants.
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