Dissolved Oxygen (DO)

Overview



Dissolved Oxygen (DO): Amount of free oxygen present in the mixed liquor that is accessible for use by the microbial populations present within the sludge.



Dissolved oxygen, is among the most important physico-chemical parameters for the efficient functioning of the biological treatment process. Oxygen is a primary requirement for the growth of all aerobic populations within the activated sludge system, and thus a key player in the metabolic processes that govern nutrient removal by constituent populations.



When aerobic organisms utilise organic nutrients, they simultaneously consume dissolved oxygen. Each molecule of glucose (the basic building block of carbohydrates) requires 6 molecules of oxygen for complete conversion to carbon dioxide and water by aerobic bacteria.



If the dissolved oxygen is not replaced, aerobic growth will eventually stop allowing only the slow anaerobic processes to continue. Microbial activity is not only oxygen-limited in the case of aerobic microorganisms, it is also restricted by the availability of adequate supplies of carbon, nutrients such as nitrogen and phosphorus, trace elements and growth factors.



DO levels fluctuate seasonally and over a 24-hour period, and vary with water temperature and altitude. Cold water holds more oxygen than warm water & water holds less oxygen at higher altitudes. Thermal discharges, such as water used to cool machinery in a manufacturing plant or a power plant, raise the temperature of water and lower its oxygen content. Aquatic animals are most vulnerable to lowered DO levels in the early morning on hot summer days when stream flows are low, water temperatures are high, and aquatic plants have not been producing oxygen since sunset.



DO is measured either in milligrams per liter (mg/L) or "percent saturation." Milligrams per liter is the amount of oxygen in a liter of water. Percent saturation is the amount of oxygen in a liter of water relative to the total amount of oxygen that the water can hold at that temperature.



The cost of aerating wastewater is extremely expensive and it is important to maintain the wastewater in the activated sludge tank as near as possible to the critical dissolved oxygen concentration in order that maximum micro-organism efficiency can be achieved at minimum cost. This can be determined through calculation of the Aeration efficiency of the plant.



METHODS OF AERATION



    1.DIFFUSED AERATION



All have an air compressor and tube that connects to a diffuser located at the bottom of the pond or lake. As air is forced down the tubing, the diffuser breaks it up into tiny bubbles that are released into the pond. The bubbling action of the air rising to the surface causing the water to de-stratify, meaning the poor, oxygen deprived water at the bottom of the lake is mixed with the oxygen rich water above, causing noxious and harmful gases to be released into the atmosphere. At the surface, where the bubbles break, additional oxygen transfer is made adding to the overall oxygen content of the water.



Diffused aerators are too costly and inefficient in shallow ponds, and are best for ponds deeper than 8 feet, since efficiency is derived from the micro bubbles that are being forced from the diffuser and floating to the surface. The deeper the water, the more destratification/ mixing the bubbles can do. They also offer the benefit of keeping electricity out of the water, since the air compressor is usually located on shore and only the air tubing enters the water.



     2. SURFACE AERATION



Surface aerators or floating aerators are units that utilize a pump mounted beneath a float that pumps water from the pond into the air or right at the surface. Surface aerators are best for use in shallow ponds since the water pulled through the pump is taken from right below the surface. Oxygenation using floating aerators occurs when the water that is splashed into the air makes contact with the ponds surface when it comes back down. This interaction allows for the venting of gasses and the transfer of oxygen.



Diffused aeration systems include:



•           a low pressure, high volume air compressor (blower),



•           air piping system, and



•           diffusers that break the air into bubbles as they are dispersed through the aeration tank.



The most commonly used blowers are positive displacement type blowers, and centrifugal blowers (single and multi-stage).



Some aeration equipment combines diffusers with mechanical aerators. Submerged turbine aerators use sparger rings to deliver diffused air below mechanical mixers, as bubbles rise, the mixers shear the course bubbles and provide mixing as well.



CONSIDERATIONS AND EFFECTS OF AERATION



The  2 methods of aerating wastewater are through mechanical surface aeration to entrain air into the wastewater by agitation, or by introducing air or pure oxygen with submerged diffusers. A variety of factors need to be considered when designing an aeration system, but even though the Ideal aeration requirements are clearly outlined, selecting the best aerators for the plant design is still challenging.



Mechanical aerators are rated based on the amount of oxygen (kg) they can transfer into liquid per kWh consumed, which is called “aeration efficiency”.Mechanical aerators are rated based on the amount of oxygen (kg) they can transfer into liquid per kWh consumed, which is called  “Aeration efficiency".



Aeration raises the dissolved oxygen content of the water. If too much oxygen is injected into the water, the water becomes supersaturated, which may cause:



•           Corrosion (Corrosion can occur whenever water and oxygen come into contact with metallic surfaces)



•           air binding in filters (spaces between the filter media particles begin to fill with bubbles effectively clogging filters)



•           slow removal of the hydrogen sulfide (H2S removed by the physical scrubbing action of aeration. If the water has a high pH, the H2S will ionize, precluding removal by aeration)



•           algae production,



•           overuse of energy.



Calibrating the DO Meter



Calibration of a DO Meter is extremely important for gaining accurate results. This process involves immersing the meter in an oxygen saturated solution and in an oxygen deficit solution. The zero DO solution consists of a saturated solution of sodium sulfite with a trace of cobalt chloride. This solution should be a null reading with the DO being less than 0.5 mg/L or to the accuracy of the meter.



Aeration can also help remove certain dissolved gases and minerals through oxidation, the chemical combination of oxygen from the air with certain undesirable metals in the water. Once oxidized, these chemicals fall out of solution and become suspended material in the water. The suspended material can then be removed by filtration.



The constituents that are commonly affected by aeration are:



•           Volatile organic chemicals, such as benzene, found in gasoline, or trichloroethylene, dichloroethylene, and perchloroethylene (solvents are used in dry-cleaning or industrial processes)



•           Carbon dioxide



•           Hydrogen sulfide (rotten-egg odor)



•           Methane (flammable)



•           Iron (will stain clothes and fixtures)



•           Manganese (black stains)



•           Various Chemicals involved in odour causation