Sewage treatment includes a variety of treatment techniques, each of which has its own protocols, benefits, tools, and effects. It's crucial to keep track of the different strategies and apply them to the right situations. It enables plants to meet regulatory requirements, maximise productivity from a treatment system, and control operating costs and workload.
An explanation of the anoxic conditions in sewage treatment is provided below, along with information on the role of anoxic tanks in STPs, and the usage of anoxic tanks in the removal of nitrogen.
What does anoxic means?
Environments under anoxic circumstances lack molecular or free oxygen (O2), yet bound oxygen may be present.
What is the role of anoxic treatment tanks in the sewage treatment?
Based on the presence or lack of nitrogen, anoxic conditions differ from anaerobic conditions in sewage treatment. Despite the absence of molecular oxygen, anoxic environments may nevertheless include nitrates or nitrites. Due to the fact that the structures of nitrates (NO3) and nitrites (NO2) include oxygen molecules, they both include bound oxygen. As long as there is no free oxygen present, these substances may be present in an anoxic environment.
What part does the anoxic condition play in the treatment of sewage? How are anoxic tanks useful for the removal of nitrogen?
Nitrogen removal from sewage is helped by anoxic zones. To prevent nutrient pollution when the facility releases its effluent back into the environment, some wastewater has a high nitrate and nitrite concentration, which the treatment process must break down.
When bacteria break down the nitrogen products in anoxic zones, the molecules are separated, releasing oxygen, which the bacteria need to survive. Diffusers or surface aerators are not necessary for the bacteria in anoxic tanks like denitrification basins, since the biodegradation of nitrogen products naturally releases oxygen.
Dissimilation (biological denitrification) by a combination of facultative heterotrophic bacteria is the main mechanism in anoxic tanks.
NH3 → NO3- → NO2- → NO → N2O → N2 (gas)
Assimilation process
Biomass growth is a secondary mechanism and absorption is constrained. It requires an aerobic zone to give nitrification and an anoxic (low or zero DO) zone to offer denitrification, 12–14% by biomass weight.
Methanol, ethanol, glycerine, acetic acid, and untreated sewage can all be used as an organic carbon food source. The rate at which mixed liquor circulates through the anoxic zone regulates the amount of denitrification.
Recycling in anoxic zones
The methods include SBR, on/off aeration systems, alternating aerobic and anoxic zones, oxidation ditches with alternating zones, step feed, etc.
Anammox
NH4+ + NO2- → N2 (gas) + 2H2O
Cost
1: 3.6 mg recovered per mg NO3-N removed and returns half of alkalinity utilized in nitrification;
2: Equal oxidising power of 2.86 mg recovered per mg NO3-N removed;
3: Production of sludge will rise, depending on the type of food source used.
Operational difficulties
Low recycle rate, oxygen carryover from the aeration zone to the anoxic zone, a lack of readily available (soluble) food, a high concentration of MLVSS, and temperature should be considered.
1: Carried over oxygen
Nitrate (NO3-), an electron acceptor, must be removed by denitrification bacteria using an electron donor.
3N2 + 5HCO3 + 7H2O + OH- = 6NO3- + 5CH3OH
Enzymes utilized by denitrifying bacteria are suppressed in the presence of oxygen, which affects the assimilation process. Oxygen competes with nitrate as an electron acceptor.
2: Content of MLVSS and food
• The anoxic tank volume, SDNR (NO3-N/biomass), and F/M ratio (BOD/biomass; influent flow rate variables) all affect nitrate removal.
• Higher F/M values and greater BOD in the anoxic zone result from reduced hydraulic retention times, but to improve SDNR, the food must be easily biodegradable.
Conclusion
Keep in mind that anaerobic methods are beneficial for sewage that contains a high concentration of organic matter, when deciding which is ideal for your sewage treatment system. Because of this, treatment facilities frequently use anaerobic treatment first to get rid of a substantial portion of the organic waste in the sewage, before continuing with aerobic treatment.
Anoxic, aerobic, and anaerobic techniques can all be used together to treat sewage in specific situations, because they help to minimise sludge. According to recent research, alternating conditions may result in a 50% reduction in the sludge production of commonly activated sludge (CAS).
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