The two are different processes, although both are anaerobic environments, their main uses are different. Hydrolysis acidification is to break the chain, increase the BC ratio of the influent, and improve biodegradability; In AAO, although anaerobic tank A also undergoes some hydrolysis and acidification metabolism, it mainly provides an environment and place for anaerobic phosphorus release by polyphosphate accumulating bacteria! This article will specifically talk about the differences between the two types of pools!

1. Hydrolysis acidification tank

Hydrolysis acidification is a reaction process in anaerobic biological reactions that involves four stages: hydrolysis, acidification, acetic acid production, and methanogenesis. By controlling the reaction in two stages, hydrolysis and acidification, suspended organic matter and large molecular substances (carbohydrates, fats, and lipids, etc.) can be hydrolyzed into small molecules by microbial extracellular enzymes. The small molecular organic matter is then converted into volatile fatty acids by acidification bacteria. During this process, suspended solids can be hydrolyzed into soluble organic compounds, and difficult to biodegrade macromolecules can be converted into easily biodegradable small molecules.

Firstly, a large number of microorganisms in the hydrolysis reactor rapidly intercept and adsorb particulate and colloidal substances in the influent, which is a rapid physical process. Generally, it only takes a few seconds to tens of seconds to complete. Therefore, the response is rapid. The intercepted substances are adsorbed on the surface of hydrolyzed and acidified sludge, slowly decomposed and metabolized, and their residence time in the system is greater than the hydraulic retention time. Under the action of a large number of hydrolytic acidification bacteria, large molecules and difficult to biodegrade substances are converted into small molecules that are easy to biodegrade, and then released back into the liquid. Under high hydraulic loads, it flows out of the system with water. Due to the short generation period of hydrolysis and acid producing bacteria, which are often measured in minutes and hours, this degradation process is also rapid. Although the removal rate of dissolved BOD and COD in this process is only about 10% on the surface, the hydrolysis of particulate organic matter increases the concentration of dissolved organic matter in the system. Therefore, the removal rate of dissolved BOD and COD is much higher than 10%. However, due to the inability to strictly control the acidification process, there may still be a small amount of methane bacteria present in the sludge, which may produce a small amount of methane. However, the solubility of methane in water is also considerable, so the amount of methane released as gas formation is very small. It can be seen that the hydrolysis reactor integrates physical and chemical processes such as precipitation, adsorption, net capture, and biological flocculation with biodegradation functions such as hydrolysis, acidification, and methanogenesis.

The main factors affecting the hydrolysis (acidification) process

1. Types and forms of matrix

The type and morphology of the substrate have a significant impact on the rate of hydrolysis (acidification) process. For polysaccharides, proteins, and fats, under the same operating conditions, the hydrolysis rate decreases sequentially. Organic compounds of the same type, the larger the molecular weight, the more difficult it is to hydrolyze, and the corresponding hydrolysis rate in the tank is lower. For example, in terms of sugar substances, disaccharides are more easily hydrolyzed than trimeric sugars; Oligosaccharides are more easily hydrolyzed than polysaccharides. In terms of molecular structure, straight chains are easier to hydrolyze than branched chains; Branches are easier to hydrolyze than rings; Single ring compounds are more prone to hydrolysis than heterocyclic or multi ring compounds.

2. PH value of hydrolysis solution

The pH value of the hydrolysis solution mainly affects the rate of hydrolysis, the products of hydrolysis (acidification), and the morphology and structure of the sludge. A large number of research results have shown that hydrolysis (acidification) microorganisms have strong adaptability to pH changes, and the hydrolysis process can proceed smoothly within a pH range of 3.5-10.0, but the optimal pH value for Z is 5.5-6.5. When the pH shifts towards the acidic or alkaline direction, the hydrolysis rate will decrease. The pH value of the hydrolysis solution also affects the type and content of hydrolysis products.

3. Hydraulic retention time

Hydraulic retention time is one of the important parameters for controlling the operation of hydrolysis reactors. Its impact on the reactor varies depending on the function of the reactor. For reactors solely aimed at hydrolysis, the longer the hydraulic retention time, the longer the contact time between the hydrolyzed substance and the hydrolysis microorganisms, resulting in higher hydrolysis efficiency. Usually 3-4 hours.

4. Temperature

Hydrolysis reaction is a typical biological reverse, therefore, the effect of temperature changes on hydrolysis reaction conforms to the general biological reaction law, that is, within a certain range, the higher the temperature, the faster the hydrolysis reaction rate. However, research has shown that the hydrolysis reaction rate does not change significantly when the temperature varies between 10-20 oC, indicating that hydrolytic microorganisms have a strong adaptability to low temperature changes.

5. Particle size

Particle size is an important factor affecting the hydrolysis (acidification) rate of granular organic matter - the larger the particle size, the smaller the specific surface area per unit weight of organic matter, and the lower the hydrolysis rate. Due to the significant impact of the particle size of particulate organic matter on the hydrolysis rate and efficiency, some researchers suggest that wastewater or sludge with high concentrations of particulate organic matter can be crushed using pumps or grinders before entering the hydrolysis reactor to reduce the particle size of pollutants and accelerate the hydrolysis reaction.

Anaerobic tank in AAO

The figure shows the traditional A2/O process flow, with the first stage being an anaerobic tank. The main function of this tank is to release phosphorus from polyphosphate accumulating bacteria, and hydrolysis acidification reactions can also occur in this tank.

Phosphorus accumulating bacteria, also known as phosphorus absorbing bacteria or phosphorus removing bacteria, are a special type of bacteria in traditional activated sludge processes. Under aerobic conditions, they can absorb excess phosphorus from wastewater into their bodies, resulting in a phosphorus content several times higher than that of ordinary bacteria. These bacteria are widely used for biological phosphorus removal.

1) Phosphorus release under anaerobic conditions

In the absence of dissolved oxygen or nitrate nitrogen, facultative bacteria convert soluble BOD5 into low molecular weight volatile organic acid VFA through fermentation. Polyphosphate accumulating bacteria absorb these fermentation products or VFA from raw wastewater and transport them into cells, assimilating them into intracellular carbon energy storage substance PHB. The required ability comes from the hydrolysis of polyphosphate and the fermentation of intracellular sugars, leading to the release of phosphate.

2) Phosphorus uptake under aerobic conditions

Under aerobic conditions, the vitality of polyphosphate accumulating bacteria is restored and stored in the form of polyphosphate exceeding the required amount for growth. Energy is generated through the oxidation metabolism of PHB, which is used for phosphorus absorption and synthesis. The energy is captured and stored in the form of polyphosphate high-energy bonds, and phosphate is removed from water.

3) Discharge of phosphorus rich sludge

The generated phosphorus rich sludge is discharged in the form of residual sludge to remove phosphorus. From an energy perspective, polyphosphate accumulating bacteria release phosphorus under anaerobic conditions to obtain energy for absorbing dissolved organic matter in wastewater, and degrade and absorb dissolved organic matter under aerobic conditions to obtain energy for phosphorus absorption.

After mixing the raw water with the phosphorus containing sludge that enters the secondary sedimentation tank synchronously, some easily biodegradable large molecular organic matter is converted into small volatile fatty acids (VFA) under the action of facultative anaerobic fermentation bacteria. Polyphosphate accumulating bacteria hydrolyze the intracellular polyphosphate into orthophosphate and release it into water. The released energy can be used to sustain the survival of aerobic polyphosphate accumulating bacteria in the anaerobic environment, while absorbing the hydrolyzed small molecular organic matter to synthesize PHB and store it in the body.

The key to phosphorus removal is the establishment of anaerobic zones, where polyphosphate accumulating bacteria can quickly absorb low molecular weight substrates and assimilate and store these fermentation products under brief anaerobic conditions. Therefore, anaerobic zones provide a competitive advantage for polyphosphate accumulating bacteria.

In this way, polyphosphate accumulating bacteria that can absorb a large amount of phosphorus can selectively proliferate in the treatment system and achieve phosphorus removal by eliminating excess sludge with high phosphorus content. Another advantage of this selective proliferation is that it inhibits the proliferation of filamentous bacteria, avoiding the possibility of producing sludge with poor sedimentation performance. Therefore, anaerobic/aerobic biological phosphorus removal processes generally do not cause sludge swelling.

The main factors affecting anaerobic tanks are:

1. Temperature

The effect of temperature on phosphorus removal efficiency is not as significant as its effect on biological nitrogen removal process. Within a certain temperature range, biological phosphorus removal can operate successfully when the temperature change is not very large. Experiments have shown that the temperature for biological phosphorus removal should be greater than 10 ℃, as the growth rate of phosphorus accumulating bacteria slows down at low temperatures.

2. PH value

When the pH is between 6.5 and 8.0, the phosphorus content and phosphorus uptake rate of polyphosphate accumulating microorganisms remain stable. When the pH is below 6.5, the phosphorus uptake rate sharply decreases. When the pH value suddenly decreases, the concentration of phosphorus increases sharply in both aerobic and anaerobic zones. The larger the pH decrease, the greater the release amount. This indicates that the phosphorus release caused by pH decrease is not a physiological and biochemical reaction of polyphosphate accumulating bacteria themselves to pH changes, but a purely chemical "acid dissolution" effect. Moreover, the greater the anaerobic release caused by pH decrease, the lower the aerobic phosphorus absorption capacity, indicating that the release caused by pH decrease is destructive and ineffective. When the pH increases, there is a slight absorption of phosphorus.

3. Dissolved oxygen

Each milligram of molecular oxygen can consume 1.14 mg of easily biodegradable COD, which inhibits the growth of polyphosphate accumulating organisms and makes it difficult to achieve the expected phosphorus removal effect. The anaerobic zone should maintain a lower dissolved oxygen value to facilitate the fermentation and acid production of anaerobic bacteria, thereby enabling better phosphorus release by polyphosphate accumulating bacteria. In addition, less dissolved oxygen is more conducive to reducing the consumption of easily degradable organic matter, thereby enabling polyphosphate accumulating bacteria to synthesize more PHB.

In the aerobic zone, more dissolved oxygen is required to facilitate the decomposition of stored PHB substances by polyphosphate accumulating bacteria to obtain energy for absorbing soluble phosphates in wastewater and synthesizing cellular polyphosphate. The DO in the anaerobic zone should be controlled below 0.3mg/l, and the DO in the aerobic zone should be controlled above 2mg/l to ensure the smooth progress of anaerobic phosphorus release and aerobic phosphorus uptake.

4. Nitrite nitrogen in anaerobic tank

The presence of nitrate nitrogen in the anaerobic zone consumes organic matrix and inhibits the release of phosphorus by PAO, thereby affecting the uptake of phosphorus by polyphosphate accumulating bacteria under aerobic conditions. On the other hand, the presence of nitrate nitrogen can be utilized by Aeromonas as an electron acceptor for denitrification, thereby affecting its ability to produce acid through fermentation intermediates as electron acceptors, thereby inhibiting the phosphorus release and uptake capacity of PAO and the synthesis capacity of PHB. Each milligram of nitrate nitrogen can consume 2.86mg of easily biodegradable CO2, which inhibits anaerobic phosphorus release and is generally controlled below 1.5mg/l.

5. Mud Age

Due to the fact that biological phosphorus removal systems mainly achieve phosphorus removal by discharging excess sludge, the amount of excess sludge determines the phosphorus removal efficiency of the system, and the length of sludge age has a direct impact on the discharge of excess sludge and the uptake of phosphorus by sludge. The younger the sludge age, the better the phosphorus removal effect. This is because reducing the sludge age can increase the discharge of excess sludge and the phosphorus removal in the system, thereby reducing the phosphorus content in the effluent of the secondary sedimentation tank. However, for biological treatment processes that simultaneously remove phosphorus and nitrogen, in order to meet the growth requirements of nitrifying and denitrifying bacteria, the sludge age is often controlled to be relatively large, which is the reason why the phosphorus removal effect is not satisfactory. The sludge age of biological treatment systems aimed at phosphorus removal is generally controlled between 3.5 and 7 days.

6 COD/TP

In the biological phosphorus removal process of wastewater, the type and content of organic matrix in the anaerobic stage, as well as the ratio of nutrients required by microorganisms to phosphorus in the wastewater, are important factors affecting the phosphorus removal efficiency. The anaerobic release and aerobic uptake of phosphorus vary with different organic matter substrates. Organic compounds with smaller molecular weights that are easily degradable, such as volatile fatty acids, are easily utilized by polyphosphate accumulating bacteria. They decompose the polyphosphate stored in their bodies and release phosphorus, which has a stronger ability to induce phosphorus release. However, high molecular weight recalcitrant organic compounds have a weaker ability to induce polyphosphate release in polyphosphate accumulating bacteria. The more complete the release of phosphorus in the anaerobic stage, the greater the uptake of phosphorus in the aerobic stage. In addition, the energy generated by phosphorus releasing bacteria during the anaerobic stage is mainly used for their absorption of low molecular weight organic substrates as the basis for survival under anaerobic conditions. Therefore, whether the influent contains sufficient organic matter is an important factor related to whether polyphosphate accumulating bacteria can survive smoothly under anaerobic conditions. It is generally believed that the COD/TP ratio in the influent should be greater than 15 to ensure that polyphosphate accumulating bacteria have sufficient substrate and achieve ideal phosphorus removal efficiency.

7、 HRT

For a well functioning urban sewage biological nitrogen and phosphorus removal system, phosphorus release and absorption generally require 1.5-2.5 hours and 2.0-3.0 hours, respectively. Overall, it seems that the process of phosphorus release is more important. Therefore, we are more concerned about the residence time of sewage in the anaerobic stage. The HRT in the anaerobic stage is too short to ensure effective phosphorus release, and the facultative acidification bacteria in the sludge cannot fully decompose the large organic molecules in the sewage into lower fatty acids that can be taken up by polyphosphate accumulating bacteria, which can also affect phosphorus release; HRT is too long and unnecessary, as it increases infrastructure investment and operating costs, and may also have some side effects. In short, phosphorus release and uptake are two interrelated processes. Phosphorus accumulating bacteria can only absorb phosphorus better in the aerobic stage after sufficient anaerobic phosphorus release, and only phosphorus accumulating bacteria with good phosphorus uptake can release phosphorus excessively in the anaerobic stage. Proper regulation can form a virtuous cycle. The data obtained by our factory in actual operation is that the HRT for the anaerobic section is 1 hour 15 minutes to 1 hour 45 minutes, and the HRT for the aerobic section is 2 hours to 3 hours 10 minutes, which is more suitable.

Disclaimer: This article is translated from Environmental Protection Engineer. The copyright of this article belongs to the original author and does not represent the views of this website. It is only for learning and communication purposes and is not for commercial use. If there are third-party intellectual property rights in the content, images, audio, videos, etc. in the article, please contact us promptly to delete them.