Wastewater Treatment Basics: What Is Aeration?
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.
Ways to realize aeration include: (1) spraying liquid into the air, e.g., biological filters; (2) diffusing air bubbles through the liquid, e.g., blown aeration; (3) continuously renewing the liquid surface to promote air transfer at the interface to the liquid phase, e.g., mechanical aeration. The purpose is always to obtain sufficient dissolved oxygen in the water body.
Aeration is mainly applied in two fields: (1) aquaculture aeration; (2) wastewater-treatment aeration.
Water contacts gas to dissolve oxygen or release dissolved gases and volatile substances. In wastewater activated-sludge processes, the mixed liquor's dissolved oxygen must be supplied by aeration. A water-treatment method where water and air fully contact to exchange gaseous substances and remove volatile substances from water, or to release gases from water (such as removing odor, carbon dioxide, or hydrogen sulfide); or to dissolve oxygen into water to raise dissolved-oxygen concentration, achieving iron and manganese removal or promoting aerobic microbial degradation of organics.
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. 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 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.
Bubble aeration is compressed air passing through pipes and diffusers to form fine bubbles that rise through the water layer. 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. Bubble aeration is also used in wastewater-treatment-plant grit chambers to wash grit and prevent sewage putrefaction.
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 (air intake 8–10%); 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. Pump flow 1–50 m³/h; treatment capacity 1–150 m³/h. Therefore, using the GLM(B)-series aeration pump improves dissolved-air liquid production efficiency, simplifies the device, saves space, and greatly reduces initial investment and operating and maintenance costs.
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 and has strong practicality.
A certain amount of small-particle granular filter media is placed in a filter tank. Because a highly active biofilm grows on the media surface, aeration can occur in the tank; when sewage passes through, the oxidative degradation by the media rapidly purifies the sewage — this is the biological oxidative degradation process. At the same time, 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 — this is the interception function. After running for a period, as head loss increases, the filter is backwashed to release intercepted suspended solids and renew the biofilm — this is the backwash process.
Specifically, the BAF first adds high-specific-surface-area granular media to favor microbial survival and growth. When the BAF starts operating, 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 on the media surface reacts with organic pollutants in the water (a biological oxidative degradation process), which can also effectively perform nitrification and denitrification, omitting the secondary sedimentation process. The granular media and biofilm flocculation intercept suspended pollutants, which are immediately treated and do not re-enter the flow — achieved by interception technology. The BAF also requires regular maintenance.
Compared with ordinary wastewater-treatment technology, the BAF has clear advantages: ① porous granular ceramsite carrier with large surface area, easily attached by microorganisms, increasing degrading-microorganism quantity; ② bottom-up air feeding gives good air distribution and high oxygen-transfer efficiency; ③ large biofilm-water contact area improves treatment efficiency; ④ long sludge age and lower sludge yield with good settleability; ⑤ combines filtration in biological flocculation and degradation, reducing oxidation steps; ⑥ high biological activity, layered media by flow direction gives stability and resistance to low temperature and shock loads; ⑦ low cost and, by omitting secondary sedimentation, saves construction cost; ⑧ higher microorganism concentration as the granular media provides a good environment for survival and growth.
