New technology for biological denitrification of sewage-simultaneous nitrification and denitrification

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2 Simultaneous nitrification and denitrification

(1) Simultaneous Nitrification and Denitrifi-cation (Simultaneous Nitrification and Denitrifi-cation, SND) refers to the simultaneous presence of nitrification and denitrification in one reactor under low oxygen conditions, so that sewage denitrification can be achieved in one step The effect
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On the one hand, explaining SND's microenvironment theory from a physical point of view is currently a generally accepted view
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Theoretically, due to the limitation of oxygen diffusion, a dissolved oxygen (DO) gradient is generated in the microbial floc

On the other hand, from a biological point of view, microbiologists have reported the discovery of aerobic denitrifying bacteria and heterotrophic nitrifying bacteria
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Since most heterotrophic nitrifying bacteria are aerobic denitrifying bacteria at the same time, because of this, they can directly convert ammonia into the final gaseous product.
Therefore, from a biological point of view, aerobic simultaneous nitrification and denitrification is possible, and facts have also proved Denitrification under aerobic conditions does exist in different biological treatment systems, such as biological carousel, SBR, oxidation ditch, CAST, MBR, SMBR and other processes.
In these processes, the sludge not only has a long residence time and high concentration, but also An anaerobic or anoxic environment can also be formed in a local area of ​​the reactor, which is conducive to the progress of denitrification and saves reaction time and space
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These characteristics have improved the application value of simultaneous nitrification and denitrification in sewage treatment, which has attracted more and more attention

Compared with traditional biological theory, SND has great advantages.
It can simultaneously carry out nitrification and denitrification reactions in the same reactor.
It has the following advantages:

①The amount of aeration is reduced and energy consumption is reduced;

② OH produced by denitrification ^- can neutralize H ⁺ produced by nitrification in situ, and SND can effectively maintain the pH in the reactor;

③Because there is no need for anoxic reaction tank, capital construction costs can be saved, or at least the volume of the reactor can be reduced;

④It can shorten the reaction time and save the carbon source;

⑤Simplifies the design and operation of the system
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Therefore, the SND system provides the possibility of reducing investment and simplifying biological nitrogen removal technology in the future
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(2) Influencing factors

①The influence of dissolved oxygen (DO)
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It is very important for simultaneous nitrification and denitrification.

②Carbon source
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For the simultaneous nitrification and denitrification system, the integration of anoxic environment and aerobic environment and the simultaneous occurrence of nitrification and denitrification make the impact of organic carbon sources on the entire reaction particularly important.

Some scholars have also conducted research on the way of adding carbon sources
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For example, the traditional carbon source dosing method is often one-time dosing at the beginning of aeration, but with the consumption of COD, DO has a gradually rising trend, which is not conducive to the progress of denitrification

③Other factors
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There are many control factors that affect SND, such as ORP, temperature, pH, etc.

(3) Application status At present, there have been many research reports on SND
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For example, high-concentration ammonia-nitrogen wastewater is subjected to simultaneous nitrification and denitrification in a sequential batch reactor, to clarify the influence of different dissolved oxygen concentrations and influent carbon-nitrogen ratios on simultaneous nitrification and denitrification performance

In summary, many of the current simultaneous nitrification and denitrification (SND) studies, some are to ensure the presence of aerobic and anaerobic environments in the reactor by controlling the aeration time of the SBR reactor, and some are due to uneven oxygen supply in the reactor space The formation of aerobic and anaerobic areas, and the use of the oxygen concentration gradient generated on the thickness of the biofilm or the radius of the biofloc to form a microenvironment with aerobic surface and anaerobic inner layer
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These studies are based on the traditional denitrification principle and are divided into two stages: aerobic nitrification and anaerobic denitrification

Some other new denitrification processes include aerobic ammonia removal process, short-cut nitrosation denitrification process in the combined biofilm/activated sludge process, and high-salt short-cut nitrification-denitrification process
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