This article was first published on the official account on February 1, 2024: "Mr. Fat's Water Treatment Diary", if you want to learn more interesting and informative water treatment knowledge in advance, please search and pay attention (this number only drains and does not reply, if you want to communicate further, please move to the official account)!
Today, I will share a case of technical transformation of a sewage plant.
This case is based on a project that our company received some time ago, so far the project is basically finished, the effect is very good, not only to reduce the effluent index to meet the owner's requirements of the discharge standard, but also to save the owner a lot of operating costs, about 20%, the owner of the straight snot bubbles.
There is no way, now the landlord's house is almost out of surplus food.......
Okay, let's not bury the owner, let's officially talk about this technical transformation project.
The sewage plant is located in a coastal industrial park in a river basin in the north, mainly collecting and treating wastewater from enterprises in the industrial park, with a design scale of 50,000 m3/d, divided into three series, and the main pollutants in the water are COD, ammonia nitrogen, total nitrogen and total phosphorus.
Before the transformation, the sewage plant has been implementing the "urban sewage treatment plant pollutant discharge standard" in the first class B, the process used is APAN1AN2AN3OAN, that is, anaerobic tank + pre-anoxic tank 1 + pre-anoxic tank 2 + pre-anoxic tank 3 + aerobic tank + secondary sedimentation tank + rear deep bed denitrification filter, the specific process flow diagram is as follows.
▲Schematic diagram of the process flow of the sewage treatment plant in the industrial park
This process may be a bit complicated, so let's analyze the information of the process first.
First of all, most of the enterprises in the industrial park are biopharmaceutical, mechanical processing, auto parts and other enterprises, so the C/N in the inlet water is low, and the anaerobic unit and denitrification unit also need to add additional carbon sources for denitrification and phosphorus removal, so the original process set up carbon source addition at 1 of the anaerobic tank and anoxic tank.
Secondly, because the influent contains both ammonia nitrogen and nitrate nitrogen, and the nitrate nitrogen content is not low, directly entering the anaerobic pond will destroy the environment for anaerobic phosphorus release, so the influent water is divided into two parts, and a small part directly enters the anaerobic pond to supplement the carbon source.
Most of them directly enter the anoxic 1 pool, and after the denitrification and denitrification operation of the anoxic 1 and anoxic 2 pools, a part of the nitrate nitrogen is removed and then refluxed back to the anaerobic tank through the anoxic 2 pool, so as to reduce the influence of nitrate nitrogen in the raw water on the anaerobic phosphorus release of phosphate polyphosphate bacteria and the absorption of organic matter.
Again, because 60% of the total nitrogen composition of the raw water is nitrate nitrogen and 40% is ammonia nitrogen, a small part of the reflux nitrification solution after the aerobic tank treatment is refluxed to the anoxic 1 pool, and most of the reflux is returned to the anoxic 3 pool, and the reflux point is not set in the anoxic 2 pool, so as to prevent the nitrate nitrogen produced by nitrification from entering the anaerobic pool and inhibiting the function of phosphate bacteria.
Finally, the sludge reflux does not enter the anaerobic tank, but crosses it directly to the anoxic 1 pool, which is also based on the high nitrate nitrogen content in the effluent, which may damage the environment of the anaerobic tank.
After the basic questions are explained, do you think this process design is perfect?
Before the transformation, the discharge standard of the sewage plant was Class 1B, and this process could be operated improvised, but now the competent authorities require it to be upgraded to Class 1A, which is a bit difficult, even if the operating costs are increased, it has little effect.
What's more, the owner not only does not want to increase the operating costs, but also wants to take the opportunity of the renovation to cut down.
That's probably what happened when we took over.
Next, the fat brother went on to talk about the problem diagnosis and improvement measures of the existing process and facilities after on-site investigation.
There are many problems, one of the most common and serious of which is the high loss of carbon sources due to nitrification reflux.
For this point, in fact, I estimate that almost all sewage plants have more or less this problem, the root cause of which is that in the nitrification solution that flows back from the aerobic pond to the front anoxic pond, due to the high dissolved oxygen, it will form a de facto aerobic environment for a period of time after entering the anoxic pond.
The group of denitrifying bacteria working in the anoxic pond is a typical "slippery head", when there is aerobic, they hold steamed bread (organic matter) in their left hand, dip sugar (oxygen) in their right hand, and live a chic life.
As managers, we naturally don't want denitrifying bacteria to be too happy, we have to let them always maintain a hard and simple work style, and once the nitrification liquid entrainment of "sugar-coated shells" (oxygen) is too much, denitrifying bacteria will do everything possible to steal and play tricks, do not work well, the denitrification efficiency of the whole system will decline, not to mention, but also consume too much of our "steamed bread", all kinds of harm but no benefit.
According to the data obtained from the on-site investigation, the dissolved oxygen in the effluent of the aerobic pool 2 of the plant is as high as 5mg/L, and the internal reflux ratio is 200%, and the carbon source of this part of the additional oxygen consumption is about 10mg/L through theoretical calculation.
PS: The general chemical equation for aerobic oxidation of organic matter is as follows:
CxHyOz+（x+y/4-z/2）O2=xCO2+y/2H2O+△H
Don't underestimate the 10mg/L COD, if you can save this amount and multiply it by the daily processing capacity of 50,000m³, it is not a small amount of money.
So how to change it? My colleague's proposal is as follows:
(1) Change the anoxic 1 pool to a degassing tank, which is used to remove the excess oxygen entrained in the reflux of aerobic nitrified solution;
(2) Move the raw water inlet point of the original anoxic 1 pool to the anoxic 2 pool to ensure that the carbon source in the raw water is not consumed by the oxygen in the nitrifying solution;
(3) Cancel the reflux point of the anoxic 3 pool of aerobic nitrification reflux and change it to the degassing tank for excess oxygen removal;
(4) move the point of the anaerobic pool of anoxic pool 2 to the anoxic pool 3;
(5) Cancel the carbon source addition point of the anaerobic pool, and rely on the carbon source carried in the reflux fluid of the anoxic pool 3 to supplement the carbon source of the anaerobic pool.
The principle of the colleague's transformation plan is very simple, that is, the anoxic 1 pool in the original process is used as the degassing tank, so that the anoxic 2 pool can better play the denitrification and denitrification efficiency, and then the carbon source can be refluxed into the anaerobic tank, which can not only ensure the phosphorus release of the anaerobic tank, but also ensure that the carbon source in the anoxic pool is not wasted by oxygen.
▲Simplified flow diagram of the process transformation plan proposed by colleagues
There is no doubt that the retrofit plan of the colleague is very feasible, and if it is realized, it should not be a problem to achieve a grade 1A effluent, but it is not enough to save money.
How to save money? It's very simple, start from two aspects: one is the cost of electricity, and the other is the cost of medicine.
The original agent of this system only has a carbon source, and the carbon source is sodium acetate, because phosphorus removal and nitrogen removal have the minimum C/N and C/P requirements, which is not easy to reduce, so you can only start with the electricity fee.
If you want to reduce the electricity bill, you can naturally start from a variety of pumps and fans, and the fan will be changed from fixed frequency to frequency conversion, which can not only save electricity costs, but also control the DO value of the aerobic pond water output is not so high, but this does not reflect the characteristics of my transformation plan, so I will not discuss it too much.
In addition to the fan, it is the water pump, and the water pump is divided into two kinds, one is the raw water inlet pump, which must not be small, and the other is a variety of return water pumps, this has a lot of space, and the less the better.
According to this principle, the content of the proposed transformation is as follows:
(1) Cancel the anaerobic tank and change it to a degassing tank to remove excess oxygen in the reflux fluid;
(2) Cancel the reflux from the anoxic 2 pool to the anaerobic pool;
(3) Cancel the carbon source addition point of the anaerobic tank;
(4) Cancel the nitrification reflux from aerobic pond 2 to anoxic pool 1 and anoxic pool 3;
(5) Cancel the water inlet of the anaerobic pool, and divide it into 3 parts to enter the anoxic 1 pool, the anoxic 2 pool and the anoxic 3 pool.
My plan is mainly based on "cancellation", and the basic principle is to save everything that can be saved, including the nitrification liquid reflux system, the reflux system from the anoxic tank to the anaerobic tank. The carbon source dosing has also been changed from 2 points to 1, and the process flow chart is shown below.
▲ Schematic diagram of the process transformation plan I proposed
Of course, there are two questions that someone will ask after seeing my changes:
"First, if the anaerobic tank is canceled, how to remove phosphorus?"
"Second, the nitrification reflux system has been cancelled, how to ensure that the total nitrogen meets the standard?"
Let's start with the first question, yes, if I change it this way, don't expect biological phosphorus removal, but there must be a reason for my change.
First of all, a considerable part of the total phosphorus in the influent water is insoluble granular phosphorus, which can be discharged out of the system with the remaining sludge, so in more than 90% of the time period, the total phosphorus value that actually needs to be removed by biochemical process or chemical precipitation process is ≤ 1mg/L, which is not too high, so no chemical phosphorus removal agent is added to the original process, and the biochemical system alone can barely reach the standard.
Then, compared with the energy consumption of simple chemical phosphorus removal and biochemical phosphorus removal, the biochemical phosphorus removal process is currently adopted, because the NOx-N concentration contained in the raw water is high, and a considerable part of the water needs to be refluxed from the anoxic pond to the anaerobic pond in order to effectively release phosphorus anaerobically.
After investigation, it was found that there were 3 pumps responsible for the backflow on site, each with a power of 15kw/h, and the electricity cost of this part of the water was 0.027 yuan/m³ according to the electricity cost of 0.75 yuan/kWh.
According to the metrological relationship of the chemical phosphorus removal reaction, 0.87mg/L Al is theoretically required to remove 1 mg/L of P, and the amount of PAC required to calculate the total phosphorus concentration required to remove the design total phosphorus concentration is about 20 mg/L.
According to the commercial PAC price of 1300 yuan/t, the process operation cost of direct PAC precipitation and phosphorus removal is 0.026 yuan/m³.
Obviously, the cost of direct use of PAC chemical precipitation for phosphorus removal is almost the same as the cost of biochemical phosphorus removal, but you should not forget that I have not calculated the cost of sodium acetate, the carbon source required for the biological phosphorus removal process, and it is obviously more convenient to manage chemical phosphorus removal, and the saved anaerobic tank space can also be used as a nitrification liquid reflux degassing tank.
Therefore, canceling the biochemical phosphorus removal process and changing it to a simple chemical phosphorus removal process not only saves money and convenience, but also strengthens the effect of system denitrification, which is simply a good thing to kill three birds with one stone, why not?
The follow-up company carried out the on-site pilot PAC phosphorus removal test according to my proposal, and the results proved that the total phosphorus concentration in the effluent could be ≤ 0.3mg/L with a PAC dosage of 17mg/L, and the operating cost was even lower than my original estimate.
Then let's talk about the second question, that is, after the nitrification liquid reflux system is canceled, will it affect the total nitrogen removal effect.
In fact, Fat Brother has said many times in the previous article about this problem, that is, the difference between the segmented multi-stage water inlet and the series single-stage water inlet process, through the system mass balance analysis, it can be known that the segmented multi-stage water inlet can indeed greatly reduce the reflux flow, and even cancel the nitrification liquid reflux system, and the sludge reflux system alone can achieve the same effect of removing total nitrogen.
So here you guys and let the fat brother be lazy, let's not expand too much, similar articles can refer to this one:
Turning decay into magic: this water treatment system reduces electricity consumption by 90% with just one change!
In addition to this problem, there are actually many other problems, such as poor denitrification efficiency of the rear deep bed denitrification filter.
However, the number of words in this article has exceeded 5000 words, and if you finish writing it all, you don't know how many words to write, if there are too many words, you are tired of reading it, and I am also tired of writing it.
So let's have a chance in the back of this question, and the fat brother will write it again.
Okay, that's all for today's diary.
Finally, thank you very much for your attention to Fat Brother's water treatment diary, if you have time, let's continue to talk about water treatment next time.
PS: Friends who haven't paid attention to the fat brother yet, move your little hands to pay attention~
--End--
I am Mr. Fat, focusing on the research and development of sewage treatment technology for more than 10 years, I will share my personal experience, experience and some interesting case stories about water treatment technology on this official account from time to time, welcome everyone to pay attention and share.
In addition, a single flower is not spring, a hundred flowers bloom in spring, Mr. Fat also wants us to learn more useful knowledge about water treatment, and contribute to the lucid waters and green mountains.
Therefore, I designed a systematic course for everyone to learn, and the course name is "Practical Training Course on the Principles and Design of Common Biochemical and Physical Technologies for Environmental Protection Wastewater Treatment". There are 62 lectures in total, and the table of contents is long, so I won't paste them separately.
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