Colleagues in the water treatment field and newcomers to the industry, today we're going to chat about something that might seem insignificant in the wastewater treatment system but plays a crucial role—**inorganic carbon sources**.
When people hear the term "carbon source," their first thought is often organic carbon sources like glucose or sodium acetate, which they consider the mainstays of nitrogen and phosphorus removal. As a result, inorganic carbon sources are frequently overlooked. But honestly, without inorganic carbon sources, our biological treatment systems might "throw a tantrum," leading to a sharp decline in treatment efficiency.
Today, we'll break it down step by step to explain what inorganic carbon sources actually are and what role they play in wastewater treatment.
First, we need to clarify: what is an inorganic carbon source?
To put it bluntly, inorganic carbon sources are carbon compounds that lack carbon-hydrogen bonds—in other words, carbon sources without an "organic" flavor. In the field of wastewater treatment, the common inorganic carbon sources we encounter are actually just a few types, such as sodium bicarbonate (baking soda), sodium carbonate, calcium carbonate, and carbonic acid formed by dissolved carbon dioxide in water. These are all typical inorganic carbon sources. Unlike organic carbon sources, which require microbes to "struggle" to decompose before they can be utilized, inorganic carbon sources have simpler structures. Often, microbes can directly "absorb" them or process them slightly for use. This is a significant advantage in biochemical reactions within wastewater treatment.
Someone might ask, "Don't we already have organic carbon sources in our wastewater treatment system to provide energy for microorganisms? Why do we still need inorganic carbon sources?" You're hitting the nail on the head. The role of inorganic carbon sources isn't just that of a "backup player"—in many scenarios, it's actually the "main player.".
Let's start with the core process of wastewater treatment—the nitrogen removal process. As you all know, nitrogen removal primarily relies on two steps: nitrification and denitrification. During denitrification, microorganisms require organic carbon sources as electron donors, which is correct. However, the nitrification process cannot do without inorganic carbon sources! Nitrifying bacteria are autotrophic microorganisms. What does "autotrophic" mean? It means they don’t rely on consuming organic carbon for survival but instead absorb inorganic carbon sources and utilize the energy generated from oxidizing ammonia nitrogen to synthesize the cellular materials they need. Think about it—if the nitrification system lacks sufficient inorganic carbon sources, the nitrifying bacteria won’t get enough "food," and their growth and reproduction will be restricted. Consequently, the efficiency of converting ammonia nitrogen into nitrite and nitrate will be significantly reduced. The end result is超标出水氨氮超标，整个脱氮系统直接“趴窝”。.
Here’s a common practical example we encounter: many industrial wastewater streams, such as chemical wastewater and electroplating wastewater, have extremely low organic carbon content but high ammonia nitrogen levels. When these wastewaters enter the biochemical treatment system, the available organic carbon sources are insufficient for denitrification, let alone meeting the needs of nitrifying bacteria. If we only add organic carbon sources at this point, the nitrifying bacteria will still go "hungry." Therefore, we must supplement inorganic carbon sources like sodium bicarbonate to provide the nitrifying bacteria with "food." This ensures the smooth progress of the nitrification reaction and lays a solid foundation for subsequent denitrification and nitrogen removal.
In addition to supporting the nitrification reaction, inorganic carbon sources have another crucial function—maintaining the pH stability of the biochemical system. As we all know, microbial growth has very stringent pH requirements. The aerobic tank generally requires a pH between 6.5 and 8.5, while the anaerobic tank and anoxic tank each have their own suitable ranges. However, biochemical reactions in wastewater treatment, especially nitrification, produce a large amount of hydrogen ions, leading to a drop in system pH. When pH decreases, not only is the activity of nitrifying bacteria inhibited, but the functions of other microorganisms, such as heterotrophic bacteria in activated sludge, are also affected.
At this point, inorganic carbon sources like sodium bicarbonate and sodium carbonate come into play! As alkaline substances, they neutralize hydrogen ions produced during reactions when added to the system, thereby stabilizing the pH. For instance, sodium bicarbonate reacts with hydrogen ions to form carbon dioxide and water, effectively neutralizing acidity without generating harmful byproducts. It also supplements carbon sources for autotrophic bacteria—truly a win-win scenario. During on-site commissioning, we often encounter situations where the pH in the aerobic tank keeps dropping. Adding some baking soda quickly brings the pH back to normal, and microbial activity follows suit. This maneuver is practically a "standard procedure" for water treatment engineers.
Additionally, in the enhanced biological phosphorus removal (EBPR) process, inorganic carbon sources also play a role. Biological phosphorus removal primarily relies on polyphosphate-accumulating organisms (PAOs), which release phosphorus in the anaerobic phase while absorbing volatile fatty acids (VFAs) and other organic carbon sources for storage. In the aerobic phase, PAOs break down stored organic matter for energy and excessively absorb phosphorus, ultimately removing it from the system through sludge discharge. However, if the wastewater lacks sufficient organic carbon sources, particularly low VFA content, both the phosphorus release and uptake processes of PAOs can be affected. In such cases, appropriately adding inorganic carbon sources, such as bicarbonates, can assist in the metabolism of PAOs and improve phosphorus removal efficiency. It should be noted, however, that the primary carbon source for biological phosphorus removal remains organic carbon, with inorganic carbon sources primarily serving auxiliary and supplementary functions.
In addition, for some high salt and highly toxic industrial wastewater treatment systems, the advantages of inorganic carbon sources are even more apparent. The toxic substances in this type of wastewater can inhibit the activity of microorganisms, leading to a significant decrease in their utilization of organic carbon sources. The structure of inorganic carbon sources is stable and not easily destroyed by toxic substances. Moreover, autotrophic microorganisms have a relatively higher tolerance to toxicity. Therefore, adding inorganic carbon sources can help maintain the basic metabolism of microorganisms and ensure that the sewage treatment system will not collapse due to insufficient carbon sources.
At this point, some friends may ask, "How should inorganic carbon sources be added in practical applications? Are there any specific requirements
There must be some refinement to this! Firstly, it needs to be determined based on the water quality. For example, for wastewater with high ammonia nitrogen concentration and low organic carbon content, the demand for nitrifying bacteria should be given special consideration, and the dosage of inorganic carbon sources should be calculated. Generally speaking, in nitrification reactions, for every 1mg of ammonia nitrogen oxidized, approximately 7.14mg of bicarbonate ions (calculated as HCO ∝⁻) are required. This value is of great reference value in our actual debugging. Secondly, the method of addition is also important. It is best to add continuously rather than suddenly at once, which can avoid drastic fluctuations in system pH and carbon source concentration, and provide a stable growth environment for microorganisms. Also, it is important to pay attention to the combination with organic carbon sources. For example, denitrification requires organic carbon sources, while nitrification requires inorganic carbon sources. Only by combining the two can efficient denitrification be achieved.
Of course, inorganic carbon sources are not omnipotent, and they also have limitations. For example, it cannot replace organic carbon sources as the main electron donor for denitrification, as denitrifying bacteria are heterotrophic microorganisms that still rely on organic carbon sources for energy supply. Moreover, if the dosage is excessive, it can also bring some problems, such as high system pH or excessive total alkalinity of the effluent, which increases the burden of subsequent treatment. So in practical operation, we must adjust the dosage based on water quality monitoring data to achieve "precise feeding".
Having said so much, I believe everyone has a comprehensive understanding of the role of inorganic carbon sources in wastewater treatment. It is not a "substitute" for organic carbon sources, but the "best partner". Inorganic carbon sources play an important role in supporting nitrification reactions, stabilizing system pH, assisting biological phosphorus removal, and addressing difficult to degrade industrial wastewater.
In the years of struggling in the water treatment industry, we can always find that seemingly insignificant details are often the key to determining the treatment effect. Inorganic carbon sources are such a presence, and by valuing and utilizing them effectively, our sewage treatment system can operate more stably and efficiently. I hope today's sharing can be helpful to everyone, and I also welcome colleagues to chat in the comments section about the little tricks and experiences you have gained in using inorganic carbon sources in practical operations!
