For those in the wastewater treatment field, there's a common question many have pondered: Should a sedimentation tank be installed after the hydrolysis and acidification tank? Some argue it's redundant and simply a waste of money, while others insist it's necessary—without it, the subsequent processes could be compromised. Today, we'll break this down in plain language, ensuring clarity for both newcomers and seasoned experts alike. No matter your experience level, this discussion will leave you well-informed.
First, it's essential to understand the purpose of the hydrolysis and acidification tank. Simply put, it serves as a "pre-treatment specialist" in wastewater treatment, specifically targeting refractory organic matter. For instance, large-molecule substances in industrial wastewater or fats and fibers in domestic sewage are broken down by microorganisms in the tank into smaller molecules—akin to mincing a large piece of braised pork belly—making them easier for the subsequent aerobic tank to "digest and absorb." This step also enhances the biodegradability of the wastewater, effectively doubling the efficiency of subsequent treatment processes while simultaneously generating biogas, making it a "waste-to-resource" stage.
But here's the problem: the hydrolysis and acidification tank doesn't operate quietly. The sludge inside must remain suspended to ensure full contact with the wastewater, allowing microorganisms to function effectively. However, this agitation results in the water carrying numerous fine sludge flocs, partially degraded suspended solids, and even some aged, detached biofilms. If these substances directly flow into the next treatment unit—such as an UASB, contact oxidation tank, or MBR membrane tank—the consequences could be severe.
Let’s first discuss the consequences of not installing a sedimentation tank. If a UASB reactor is connected downstream, excessive suspended solids in the water will deposit around the UASB distributor. Over time, this can clog the distribution holes, leading to uneven water flow distribution. Additionally, the granular sludge inside may be washed away, disrupting the originally efficient anaerobic reaction process. As a result, treatment effectiveness plummets, necessitating frequent shutdowns for dredging, which is both time-consuming and costly.
If the system is a contact oxidation tank, the situation won't be much better. The biofilm in the contact oxidation tank grows on the packing material. When there is an excess of suspended solids in the water, they will adhere to the surface of the biofilm, blocking the pathways for oxygen and nutrients. As a result, the biofilm cannot breathe or "eat," gradually aging and detaching. The microbial population in the tank decreases over time, leading to incomplete wastewater treatment.
There is also the commonly used MBR membrane tank, which has higher requirements for suspended solids in the influent. The sludge-water mixture from the hydrolysis acidification tank is directly fed into the membrane tank. Those tiny sludge flocs quickly clog the membrane pores, leading to a decline in membrane flux and a sharp rise in transmembrane pressure (TMP). A membrane that could last three to five years might need replacement within a year, significantly increasing costs. Additionally, the frequency of backwashing must be increased, doubling the operational workload.
Some might argue, "The sludge concentration in my hydrolysis and acidification tank isn't high, and having a bit of residue in the water shouldn't be a big issue, right?" But that line of thinking is flawed. Even if nothing seems amiss in the short term, these suspended solids will gradually accumulate in subsequent treatment processes. It's like a household drain—occasional hair loss might not cause a blockage, but daily buildup will eventually lead to a clog. Moreover, excessive suspended matter in the water can compromise the accuracy of downstream water quality tests. For instance, COD and BOD measurements may show higher results, leading you to mistakenly believe the treatment is ineffective and potentially steering you down the wrong path.
What are the specific benefits of adding a sedimentation tank? First and foremost, it directly enables "sludge-water separation." The sedimentation tank acts like a "filter," allowing the sludge-water mixture from the hydrolysis and acidification tank to "settle" slowly within the tank. Heavier sludge flocs sink to the bottom, while clear water flows out from above, directly reducing the burden on subsequent processes. The settled sludge can also be recycled back into the hydrolysis and acidification tank. On one hand, this helps maintain the sludge concentration in the tank, strengthening the microbial population and improving degradation efficiency. On the other hand, it reduces sludge discharge volume and lowers sludge treatment costs, achieving two goals at once.
Secondly, adding a sedimentation tank can enhance the stability of the entire treatment system. In the wastewater treatment industry, the biggest concern is "fluctuation"—even minor changes in influent quality or flow rate can disrupt downstream processes. With the buffering effect of the sedimentation tank, even if the suspended solids in the hydrolysis and acidification tank occasionally exceed standards, the sedimentation tank can intercept most impurities, preventing fluctuations from propagating to the next stage. This is equivalent to adding an "safety lock" to the entire system.
Of course, it is not necessary to install a sedimentation tank in all cases; it depends on the specific working conditions. For example, in some small-scale domestic sewage treatment plants, the hydrolysis and acidification tank is directly connected to a compact integrated unit, which inherently includes a simple sedimentation function. In such cases, a separate sedimentation tank may not be required. Similarly, for certain industrial wastewater, the suspended solids content is extremely low, and the operational load of the hydrolysis and acidification tank is also minimal, resulting in negligible sludge discharge. If calculations confirm that this has no impact on subsequent processes, it may also be considered acceptable to omit the sedimentation tank. However, such scenarios are relatively rare. In most cases, especially for projects with high water treatment volumes, high suspended solids content, and stringent requirements on influent quality for subsequent processes, sedimentation tanks are indispensable.
In addition, the design of the sedimentation tank also needs to be carefully considered. You cannot simply dig a pit and use it as a sedimentation tank. The area, depth, and hydraulic retention time of the sedimentation tank should be determined based on the effluent volume and suspended solids concentration of the hydrolysis acidification tank. For example, if the residence time is too short, the sludge may not sink completely; If it is too long, it will occupy too much land and increase investment. And the sludge discharge pipe at the bottom of the pool should be designed reasonably, with regular sludge discharge to prevent sludge from solidifying at the bottom of the pool and affecting the sedimentation effect.
To sum up, whether or not to add a sedimentation tank to the hydrolysis acidification tank depends on the "requirements of the subsequent process" and the "impact of suspended solids in the effluent". If the subsequent process is sensitive to incoming suspended solids, or if the outgoing suspended solids pose a risk to the system, the sedimentation tank must be added. This is not a waste of money, but a necessary investment to ensure the long-term stable operation of the entire sewage treatment system. On the contrary, if the working conditions are special and after professional calculation, it is indeed not necessary, then it can be saved, but caution must be exercised not to sacrifice the big for the small, and in the end, spend more maintenance and rectification costs.
Wastewater treatment is a systematic project, and each link is interrelated. The combination of hydrolysis acidification tank and sedimentation tank may seem like a small issue, but it actually affects the treatment effect and operation and maintenance costs of the entire project. So when designing, don't make hasty decisions. Instead, consider the actual situation and consult with professionals to avoid detours and truly make the sewage treatment system work.
