Let's first clarify, what is activated sludge for simultaneous nitrification denitrification (SND)? Simply put, microorganisms in ordinary activated sludge can perform both "nitrification" tasks (converting ammonia nitrogen into nitrate nitrogen) and "denitrification" tasks (converting nitrate nitrogen into nitrogen gas and escaping), without the need for separate tanks like traditional processes, saving water, electricity, and space. They are truly "efficient and versatile" in sewage treatment. But this' versatile 'is not something that falls from the sky. We need to gradually' nurture 'it step by step. Today, from preparation work to daily maintenance, we will explain it clearly to you. Even if you are a novice in the industry, following along can help you avoid detours.
1、 First, establish the basic conditions for "nurturing mud", so that microorganisms do not go hungry or live comfortably
To make the microorganisms in the sludge have SND activity, the first step is to create a comfortable "home" for them, as well as enough food to eat. This is similar to how we need to prepare flower pots and fertilizers before growing flowers. Let's talk about it from two aspects: "hardware" (devices) and "software" (water quality, nutrition):
1. Hardware equipment: There is no need to make it too complicated, but the key cannot be omitted
We don't need to start with a large-scale reactor that costs hundreds of thousands of yuan. Novices can start with a "mini version" of laboratory trials, such as a 10L or 20L cylindrical reactor made of glass or stainless steel. The key is to meet these requirements:
-Ability to control oxygen: The core of SND is "local hypoxia and overall aerobic", which means that the water cannot be completely devoid of oxygen, nor can there be too much oxygen. The water must have high and low oxygen zones so that microorganisms can nitrify and denitrify at the same time. So the reactor needs to be equipped with an aeration head (preferably using microporous aeration, with small bubbles and uniform oxygen distribution), and then equipped with a dissolved oxygen meter for real-time monitoring, making it convenient to adjust the aeration size at any time. Generally, the dissolved oxygen (DO) should be controlled between 0.5-2mg/L, neither too high nor too low.
-Must be able to stir: If the aeration is not enough, a mixer is also needed, otherwise the sludge will easily sink to the bottom, mix unevenly with water and nutrients, and microorganisms will not be able to eat. The stirring speed doesn't need to be too fast, as long as it allows the sludge to "float" in the water (i.e. maintain a suspended state). If it's too fast, it can actually "knock out" the microorganisms and affect their activity.
-Must be able to control temperature and pH: Microorganisms, like humans, are sensitive to cold, heat, acid, and alkali. The most suitable temperature for SND microorganisms to work is 25-30 ℃. If it is too low, they will become lazy and their activity will decrease; If it is too high, it may directly cause 'heat death'. So it's best to wrap the reactor with a constant temperature heating jacket or place it in a constant temperature water bath. The pH value also needs to be closely monitored. Nitrifying bacteria prefer a slightly alkaline environment (7.5-8.5), while denitrifying bacteria are slightly acid resistant, but overall it should be between 7-8. If the pH is low, add some sodium carbonate, and if it is high, add some dilute hydrochloric acid to prevent the value from "deviating".
2. Water quality and nutrition: Prepare food for microorganisms, don't feed blindly
The microorganisms in the sludge need to have "staple foods" (carbon and nitrogen sources) and "vitamins" (trace elements) to work, all of which come from the inflow. If the nutrients are not enough or the proportion is not correct, the microorganisms will either be "hungry and thin" or "malnourished", and cannot produce SND activity at all.
-Nitrogen source: Use the most common ammonia nitrogen: We raise SND sludge mainly to treat ammonia nitrogen, so the inflow must have sufficient ammonia nitrogen. Don't make it too complicated, just mix ammonium sulfate or ammonium chloride directly. The concentration should be controlled at 50-100mg/L, which is more suitable. If it is too low, microorganisms will not be able to survive, and if it is too high, it will "poison" them. Novices should start with this concentration range.
-Carbon source: Choose "easily digestible", don't choose too "hard": Denitrifying bacteria need carbon source as "energy" for their work. If the carbon source is not enough, nitrate nitrogen cannot be converted into nitrogen gas. It is best for beginners to choose easily degradable carbon sources, such as glucose and sodium acetate, as these microorganisms "eat quickly" and are easy to quickly cultivate activity. The ratio of carbon to nitrogen (i.e. C/N ratio) is crucial, generally controlled at 5-10:1. For example, if the ammonia nitrogen concentration is 50mg/L, the carbon source should be added in an amount of 250-500mg/L. If the ratio is not correct, either denitrification is not thorough, carbon sources are wasted, and water will become smelly.
-Trace elements: Don't ignore them, it's not enough. Many people easily forget to add trace elements, but in fact, microorganisms need "little things" such as iron, manganese, and zinc to activate enzyme activity. Without them, even if there is enough carbon and nitrogen, the activity cannot be increased. You don't need to buy expensive reagents. Just add a little tap water (which contains natural trace elements) or filter a small spoonful of garden soil soaked water when preparing the water. The amount doesn't need to be too much, as too much can lead to heavy metal toxicity.
-Don't bring "poison" when entering water: If there are heavy metals (such as chromium, lead), antibiotics, or high concentrations of salt in the water, microorganisms will immediately "stop cooking". Therefore, when preparing water, it is necessary to use distilled water or deionized water. Do not use industrial wastewater or polluted river water casually. Novices should first cultivate "clean" water and then slowly try to treat actual wastewater.
2、 Inoculating sludge is a shortcut, don't start from scratch
The mistake many beginners make is to start from an "empty pool" and wait for sludge to naturally grow, which takes several months and may not necessarily result in SND activity. The smart approach is to "inoculate", which means finding ready-made activated sludge as "seeds" and directly putting them into the reactor, equivalent to "standing on someone else's shoulder", which can save more than half of the time.
Where can I find 'good seeds'? Prioritize these two places
-Return sludge from urban sewage treatment plants: This is the easiest "seed" to obtain, especially for sewage treatment plants that treat domestic sewage. The sludge inside already contains nitrifying and denitrifying bacteria, but the activity may not be strong enough. Let's take it back and "domesticate" it a little. When you go, bring a clean bucket and fill it with 5-10L (depending on the size of the reactor, usually sludge accounts for 10% -20% of the reactor volume). Don't leave it for too long, take it back and use it on the same day. If left for too long, the sludge will "lack oxygen" and its activity will decrease.
-Ready made ordinary activated sludge in the laboratory: If you have friends who conduct sewage treatment experiments around you, you can borrow some of the ordinary activated sludge they have raised. Although it may not have SND activity, the basic microbial community is sound and can be domesticated quickly. Never use black or smelly sludge, as most of it contains dead bacteria and cannot be used as seeds.
2. Don't "collapse" during vaccination, let the sludge "adapt to the new home"
After receiving the inoculated sludge, do not pour it directly into the reactor. First, "wash the sludge" to remove impurities and residual nutrients from the sludge. The method is simple: pour the sludge into a centrifuge tube, add distilled water, centrifuge for 5 minutes (at a speed of 3000 rpm), discard the clear liquid on top, add some distilled water and stir well. Repeat 2-3 times until the clear liquid becomes transparent.
After cleaning, pour the sludge into the reactor and slowly add the prepared water (do not add too quickly, otherwise the sludge will "not adapt"). After adding, do not turn on the aeration first. Turn on the mixer and stir for 10 minutes to fully mix the sludge and water. Then turn on the aeration and adjust the dissolved oxygen to around 1mg/L. Let the sludge "breathe" first and adapt to the new environment. This step is called the "adaptation period before film hanging", do not skip it.
3、 Domestication is key: step by step "forcing" microorganisms to grow SND activity
After inoculating the sludge, the core next step is "domestication" - in simple terms, adjusting environmental conditions (such as DO, C/N ratio, hydraulic retention time) to allow nitrifying and denitrifying bacteria in the sludge to "work together" and gradually develop the habit of working synchronously. This process should not be rushed, usually taking 15-30 days. It requires patience, daily observation, recording, and gradual adjustment.
1. Phase 1: Allow nitrifying bacteria to "survive" and reduce ammonia nitrogen (first 7-10 days)
At the beginning of domestication, don't rush to let denitrifying bacteria work. First, "feed" the nitrifying bacteria and let them dominate in the sludge. At this point, we need to do this:
-Slightly increase the aeration: control the dissolved oxygen (DO) at 1.5-2mg/L to ensure sufficient oxygen in the water, which facilitates the conversion of ammonia nitrogen into nitrate nitrogen by nitrifying bacteria.
-Lower the C/N ratio a bit: There is no need to add too much carbon source, and the C/N ratio should be controlled at 3-5:1, mainly to allow microorganisms to "eat nitrogen", with a focus on observing the removal rate of ammonia nitrogen.
-The hydraulic retention time (HRT) should be longer: HRT is the time water stays in the reactor, initially set at 8-12 hours, to prevent the water from flowing too fast and give enough time for nitrifying bacteria to work.
Take samples every day to measure the ammonia nitrogen concentration in the inflow and outflow. If the ammonia nitrogen concentration in the outflow can be stably reduced to below 10mg/L, it indicates that the nitrifying bacteria have "come back to life" and the first stage meets the standard.
2. Phase 2: Slowly "reduce oxygen and increase carbon" to activate denitrifying bacteria (10-15 days in between)
After the nitrifying bacteria stabilize, it is time to "awaken" the denitrifying bacteria. The key is to create a "local hypoxia" environment and provide sufficient carbon sources for the denitrifying bacteria. At this point, it is necessary to adjust the conditions little by little and not change too much at once, otherwise the microorganisms will be "stressed":
-Gradually reduce DO: Every day, lower DO by 0.2-0.3mg/L, gradually reducing it from 2mg/L to 0.5-1mg/L, and stabilize it after reaching this range. When descending, it is important to observe the color of the sludge. If the sludge changes from yellow brown to black or the effluent becomes turbid, it indicates that the dissolved oxygen (DO) is too low. Quickly adjust it back a bit to prevent the sludge from dying from oxygen deficiency.
-Gradually increase the C/N ratio: At the same time, increase the amount of carbon source and gradually increase the C/N ratio from 3-5:1 to 5-8:1. Add a little bit every day, don't add too much at once, otherwise there will be an excess of carbon source in the water, which will breed miscellaneous bacteria and cause sludge to smell.
-Slightly shorten HRT: Slowly reduce HRT from 12 hours to 6-8 hours to allow microorganisms to work more compactly and increase their activity.
At this stage, two indicators need to be measured daily: ammonia nitrogen (to see if there are any problems with nitrification) and total nitrogen (to see if denitrification is effective). If the total nitrogen removal rate can remain stable at over 50%, it indicates that the denitrifying bacteria have started working and the SND activity has preliminarily emerged.
3. Phase 3: Stable conditions to "consolidate" SND activity (last 7 days or so)
When the total nitrogen removal rate stabilizes at over 50%, do not adjust the conditions significantly anymore. Maintain DO at 0.5-1mg/L, C/N ratio at 5-8:1, and HRT at 6-8 hours to allow microorganisms to "live and work" in this environment, gradually increasing SND activity.
At this point, it is necessary to observe the characteristics of the sludge: good SND sludge should be yellow brown in color, with relatively large flocs (visible in clusters under a microscope), good settling properties (after standing for 30 minutes, the sludge can settle down, and the clear liquid on top is very transparent), and no blackening or odor. Measure the total nitrogen removal rate once a day. If it can remain stable at 70% or even reach 80%, congratulations! The simultaneous nitrification denitrification activated sludge has been successfully cultivated!
4、 Daily maintenance: Don't let the "hard-earned mud" have problems
Raising SND sludge is not the end, but the beginning. If daily maintenance is not in place, the sludge is prone to "activity decline" or even "death", and the previous efforts will be in vain. These points must be closely monitored:
1. Must do daily: measure indicators, observe traits
-Measure three key indicators: dissolved oxygen (DO), pH value, and total nitrogen removal rate, at least twice a day (in the morning and afternoon). Once DO is found to be below 0.5mg/L, increase aeration, and above 2mg/L, decrease aeration; Add sodium carbonate if the pH is below 7, and dilute hydrochloric acid if it is above 8.5; If the total nitrogen removal rate decreases by more than 10%, check if the carbon source is added enough and if there are impurities in the influent.
-Observing the characteristics of sludge: Observe the color of the sludge daily (normally yellow brown), whether there is any odor (normally no odor, odor is due to lack of oxygen), and the sedimentation situation (the sedimentation ratio is best at 20% -30% within 30 minutes, if it is too high, it means there is too much sludge, and if it is too low, it means there is sludge loss).
2. Weekly mandatory tasks: sludge discharge and impurity removal
-Regular sludge discharge: Sludge will continue to multiply, and too much will settle at the bottom of the reactor, affecting mixing and aeration. How much excess sludge should be discharged once a week? Check the settling ratio. If the settling ratio exceeds 30%, discharge 5% -10% of the reactor volume of sludge and maintain the settling ratio between 20% -30%.
-Cleaning impurities: Even if the water is filtered thoroughly, a small amount of impurities will still settle at the bottom over time. Every week, during sludge discharge, clean the impurities at the bottom of the reactor to prevent them from affecting the mixing of sludge and water.
3. Don't panic when encountering problems: common problem-solving methods
-Black and odorous sludge: It is highly likely that the dissolved oxygen (DO) is too low and there is oxygen deficiency. Quickly increase the aeration and measure the pH. If the pH is also low, add some sodium carbonate, and it usually takes 1-2 days to recover.
-Sudden decrease in total nitrogen removal rate: First check the carbon source to see if it has been added too little. If the carbon source is sufficient, then check if the dissolved oxygen (DO) is too high (too high can inhibit denitrifying bacteria). Adjust the DO to 0.5-1mg/L and observe for 1-2 days. Generally, it can return.
-Serious sludge loss (sedimentation ratio below 10%): It may be due to mixing too quickly, which breaks the sludge, or the inflow water flow is too fast (HRT is too short), which flushes out the sludge. First, lower the mixing speed, then extend HRT for 1-2 hours, and at the same time, supplement with some inoculated sludge, and gradually recover.
5、 Common Misconception for Beginners: Don't Step on These 'Pits'
Finally, talk to beginners about common misconceptions and don't let them take a wrong path:
-Don't pursue "quick results": Many people hope to produce SND activity within 3 days after inoculating sludge, which is simply impossible. Microbial domestication takes time, and 15-30 days is a normal cycle. It's useless to rush, but taking it slow can actually be faster.
-Don't add reagents randomly: Some people think that "adding more nutrients is better". Adding too many carbon and nitrogen sources can result in excessive nutrients in the water, the growth of bacteria, and the odor of sludge; Some people think that "adding trace elements is good", adding too many heavy metal reagents at once can directly poison microorganisms, and adding them in proportion is enough.
-Don't stop aeration and mixing for a long time: Sometimes when you have to go out, you turn off aeration and mixing, and when you come back, you find that the sludge has completely sunk to the bottom and turned black. This is because of long-term lack of oxygen, and a large number of microorganisms have died. Therefore, even if you go out, you need to find someone to keep an eye on it, at least to ensure that aeration and mixing don't stop for too long (no more than 4 hours at most).
In short, the core of cultivating synchronous nitrification denitrification activated sludge is to "set up a good environment, select the right seeds, gradually domesticate, and carefully maintain". It is not so mysterious. Novices only need to follow the steps, observe and record more, and make fewer low-level mistakes, and they can definitely cultivate sludge with good activity. At the beginning, you may encounter some problems, but as long as you don't panic and adjust slowly, you can quickly get started. When you see the total nitrogen removal rate stabilize at over 70%, that sense of achievement is stronger than anything else!
