Aeration Fundamentals and Where BAF Fits: Oxygen Transfer, Pump Sizing and Media Choices
Aeration is the process of contacting water with air to dissolve oxygen or to strip dissolved gases and volatile substances from water. In the activated-sludge process for wastewater, the dissolved oxygen of the mixed liquor must be replenished by aeration. The aeration methods used in activated-sludge aeration tanks are divided into two categories: bubble aeration and surface aeration.
Three ways to make the interface
Ways to realize aeration include: spraying liquid into the air, as in biological filters; diffusing air bubbles through the liquid, as in blown aeration; and continuously renewing the liquid surface to promote air transfer at the interface to the liquid phase, as in mechanical aeration. The purpose is always to obtain sufficient dissolved oxygen in the water body. Mechanically, bubble aeration is compressed air passing through pipes and diffusers to form fine bubbles that rise through the water layer, while surface aeration uses impellers or brushes to violently agitate the water surface, continuously presenting a new interface to the atmosphere. Both provide mixing in addition to oxygen supply, keeping activated sludge suspended.
Application splits into two fields: aquaculture aeration, and wastewater-treatment aeration. The same physics serves both, but the second one also buys you grit management: bubble aeration is used in grit chambers to wash grit and prevent sewage putrefaction.
What drives the transfer rate
Factors affecting the gas-transfer rate between gas and liquid phases include the gas partial pressure in the gas phase and its concentration in the liquid phase, temperature, interfacial area, and water composition. At the same temperature and partial pressure, a gas's solubility in water, the equilibrium concentration, is constant; when the gas concentration is higher than equilibrium it escapes from water, and when lower it dissolves into water.
Stripping is the mirror image of dissolution, and it solves taste and odor problems. Reservoirs or lakes often develop odor due to the reproduction of algae, protozoa, and plankton or the decay of plant and animal residues. The odor-causing substances are usually volatile organics, removable with aeration devices such as fountains, multi-stage waterfalls, and perforated-plate or coke-tray drops that disperse water into films or droplets.
Groundwater treatment is the other classic duty. Groundwater sometimes contains hydrogen sulfide, carbon dioxide, or excess iron and manganese, and can also be treated by aeration. Free carbon dioxide is corrosive; when water alkalinity is low, after aeration the water can be filtered through a limestone bed to convert carbon dioxide into carbonate, requiring a longer contact time in the bed. Dissolved iron and manganese in groundwater are generally low-valent bicarbonates; upon contact with air, as free carbon dioxide decreases and dissolved oxygen increases, they convert to precipitates. Iron and manganese content is generally not high and requires little oxygen, so aeration requirements are low while contact-time requirements are high. Get that backwards and you will size a blower for a duty that actually needed detention.
Pump selection: the GLM(B) approach
The core component of an aeration system, the aeration pump, uses negative pressure at its suction inlet to draw in gas, so no air compressor or ejector is needed. The high-speed rotating impeller mixes liquid and gas, eliminating the need for mixers. Due to in-pump pressurized mixing, gas and liquid dissolve fully, with dissolution efficiency of 80-100%; thus no large pressurized dissolved-air tank or expensive reaction tower is needed. The gas-liquid ratio is about 1:9, with air intake of 8-10%, and series use can increase intake. A single GLM(B)-series aeration pump can aspirate, mix, dissolve and deliver highly dissolved liquid directly to the point of use, with pump flow of 1-50 m3/h and treatment capacity of 1-150 m3/h. Using it improves dissolved-air liquid production efficiency, simplifies the device, saves space, and greatly reduces initial investment and operating and maintenance costs.
Where BAF takes it next
The biological aerated filter, also called BAF, is a biofilm wastewater-treatment technology invented in the late 1980s and early 1990s. It can effectively remove SS, COD, BOD, and other harmful substances, with a maximum treatment capacity of hundreds of thousands of tons per day, and has gradually developed to enable nitrogen and phosphorus removal. It is widely used in European and American countries.
A certain amount of small-particle granular filter media is placed in a filter tank, and because a highly active biofilm grows on the media surface, aeration can occur in the tank. Three things happen in sequence. The biological oxidative degradation process: as sewage passes through, the oxidative degradation by the media rapidly purifies the sewage. The interception function: as the sewage flows, the compacted media together with the biofilm's flocculation intercept suspended solids, ensuring detached biofilm does not float out with the water. The backwash process: after running for a period, as head loss increases, the filter is backwashed to release intercepted suspended solids and renew the biofilm.
In operation, sewage flows top-down or bottom-up; as it passes the filter layer, the blower below begins aeration so air and sewage contact counter-currently or co-currently and the biofilm reacts with organic pollutants, which can also effectively perform nitrification and denitrification, omitting the secondary sedimentation process.
The stated advantages stack up as follows: a porous granular ceramsite carrier with large surface area, easily attached by microorganisms; bottom-up air feeding giving good air distribution and high oxygen-transfer efficiency; large biofilm-water contact area improving treatment efficiency; long sludge age and lower sludge yield with good settleability; filtration combined with biological flocculation and degradation, reducing oxidation steps; high biological activity with layered media giving stability and resistance to low temperature and shock loads; low cost, and by omitting secondary sedimentation, saved construction cost; and higher microorganism concentration because the granular media provides a good environment for survival and growth.