Anaerobic reactors sometimes produce a large amount of foam, which is semi-liquid and semi-solid. In severe cases, it can fill the gas phase space and enter the biogas pipeline, leading to difficulties in the operation of the biogas system.

 

The main reason for foam production is the unstable operation of the anaerobic system, because foam is mainly formed due to excessive CO2 production. When the reactor temperature fluctuates or the load changes suddenly, it can lead to unstable system operation and increased CO2 production, thus leading to foam production. If the factors causing instability are eliminated in time, the foam phenomenon will generally disappear. In the early stage of anaerobic sludge cultivation, due to high CO2 production and low methane production, foam may also appear. As the methane bacteria mature, CO2 production decreases, and foam generally gradually disappears. The presence of protein in the influent is one reason for foam production, while some intermediate products produced during the metabolism of microorganisms themselves can also reduce the surface tension of water and generate bubbles. A large amount of gas production during anaerobic biological treatment will produce a similar aeration effect to aerobic treatment, forming bubble problems. A sudden increase in gas production due to a sudden increase in load may also cause foam problems.

 

Calcium carbonate (CaCO3) precipitation: High calcium content in the wastewater being treated or the use of lime to supplement alkalinity will both increase the possibility of calcium carbonate precipitation. High concentrations of bicarbonate and phosphate are conducive to calcium precipitation.

 

Struvite (MgNH4PO4) precipitation: When the influent contains high concentrations of soluble orthophosphate, ammonia nitrogen, and magnesium ions, struvite precipitation will occur. Struvite precipitation in anaerobic treatment systems mainly occurs in pipe bends, pump inlets, and the inlet and outlet of secondary sedimentation tanks.

 

 

What are the three stages of anaerobic biological treatment?

 

Theoretical research suggests three stages: the anaerobic digestion process is divided into three parts: hydrolysis and fermentation stage, acetic acid and hydrogen production stage, and methane production stage.

 

The hydrolysis and fermentation stage and the acetic acid and hydrogen production stage can also be collectively referred to as the acid fermentation stage. In this stage, complex organic matter in wastewater is decomposed into simple organic matter, such as organic acids and alcohols, as well as inorganic matter such as CO2, NH3, and H2S, under the action of acid-producing bacteria. Due to the accumulation of organic acids, the pH of the wastewater drops to below 6. Afterwards, due to the decomposition of organic acids and nitrogen-containing compounds, carbonates and ammonia are produced, reducing acidity and raising the pH to around 6.6～6.8.

 

1. Hydrolysis and acidification stage. Complex, large-molecule, insoluble organic matter in wastewater is hydrolyzed into small-molecule, soluble organic matter under the action of extracellular enzymes, and then enters the cells. Hydrolysis produces volatile organic acids, alcohols, and aldehydes.

 

2. Hydrogen and acetic acid production stage. Under the action of hydrogen and acid-producing bacteria, various organic acids are decomposed and converted into acetic acid, hydrogen, and carbon dioxide.

 

3. Methane production stage. Methanogens convert acetic acid, hydrogen, and carbon dioxide into methane.

 

What are the advantages of hydrolysis and acidification?

 

1. The tank does not need to be sealed, nor does it need a three-phase separator, making operation and management convenient and simple.

 

2. After hydrolysis and acidification of macromolecular organic matter, small-molecule organic matter is produced, which has better biodegradability. That is, hydrolysis and acidification can change the biodegradability of the original wastewater, thereby reducing reaction time and energy consumption.

 

3. Hydrolysis and acidification are the early stages of anaerobic treatment and have not reached the final stage of anaerobic fermentation, so there is no unpleasant odor produced by anaerobic fermentation in the effluent, improving the environment of the wastewater treatment plant.

 

4. The time required for hydrolysis and acidification is short, so the required structure volume is small, generally equivalent to the sedimentation tank, saving infrastructure investment.

 

5. Acidification has a good effect on the degradation of solid organic matter, and produces very little excess sludge, realizing the one-time treatment of sludge and wastewater, and has some functions of a digester.

 

 

What are the main characteristics of anaerobic biological treatment?

 

1. Low energy consumption: Because anaerobic biological treatment does not require oxygen supply, energy consumption is about 1/10 of that of aerobic activated sludge processes, and it can also produce methane gas (CH4) with high calorific value. 0.35 standard liters of methane or 0.7 standard liters of biogas can be produced per gram of CODcr removed. The calorific value of biogas is 22.7 KJ/L, the calorific value of methane is 39300 KJ/m³, and the calorific value of general natural gas is 34300 KJ/m³.

 

2. Low sludge production: Because the proliferation rate of anaerobic microorganisms is much lower than that of aerobic microorganisms, the amount of sludge produced by aerobic biological treatment systems per kilogram of CODcr treated is 0.25～0.6kg, while the amount of sludge produced by anaerobic biological treatment systems per kilogram of CODcr treated is only 0.02～0.18kg.

 

3. It can completely or partially degrade some macromolecular organic matter that cannot be degraded by aerobic biological treatment systems.

 

4. Anaerobic microorganisms are more sensitive to changes in environmental factors such as temperature and pH, and it is more difficult to manage and operate anaerobic biological treatment systems.

 

5. Wide water temperature adaptability: The water temperature for aerobic treatment is between 10～35℃, and cooling measures need to be taken at high temperatures; while the water temperature adaptability of anaerobic treatment is wide, divided into low-temperature anaerobic (10～30℃), mesophilic anaerobic (30～40℃), and high-temperature anaerobic (50～60℃).

 

 

What are the influencing factors of anaerobic biological treatment?

 

1. Temperature: There are two different optimal temperature ranges (around 55℃ and around 35℃). The so-called high-temperature anaerobic digestion and low-temperature anaerobic digestion correspond to these two optimal temperature ranges.

 

2. pH value: The optimal pH range for anaerobic digestion is 6.8～7.2.

 

3. Organic load: Because anaerobic biological treatment has a degradation effect on almost all organic matter in wastewater, when discussing anaerobic biological treatment, CODcr is generally used for analysis and research, unlike aerobic biological treatment, which must be based on BOD5. The organic load of anaerobic treatment is usually expressed by the volumetric load and a certain CODcr removal rate.

 

4. Nutrients: In anaerobic processes, the carbon-nitrogen-phosphorus ratio should be controlled at CODcr:N:P=(200～300):5:1. The optimal requirement of hydrogen sulfide for methanogens is 11.5mg/L. Sometimes, it is necessary to supplement some essential special nutrient elements. Methanogens have a specific need for sulfide and phosphorus, while iron, nickel, zinc, cobalt, and molybdenum have an activating effect on methanogens.

 

5. Oxidation-reduction potential: The oxidation-reduction potential can indicate the oxygen concentration in water. Non-methane anaerobic microorganisms can survive in an environment with an oxidation-reduction potential less than +100mV, while the oxidation-reduction potential suitable for methanogen activity is below -150mV. In the initial stage of cultivating methanogens, the oxidation-reduction potential should not be higher than -330mV.

 

6. Alkalinity: The alkalinity formed by the bicarbonate in wastewater has a buffering effect on pH changes. If the alkalinity is insufficient, sodium bicarbonate and lime should be added to ensure moderate alkalinity in the reactor.

 

7. Toxic substances.

 

8. Hydraulic retention time: The main impact of hydraulic retention time on anaerobic processes is reflected through the upward flow velocity. On the one hand, a higher flow velocity can increase the disturbance in the inflow area of the wastewater system, thereby increasing the contact between the biological sludge and the influent organic matter, and improving the removal rate of organic matter. On the other hand, in order to maintain a sufficient amount of sludge in the system, the upward flow velocity cannot exceed a certain limit.

 

 

In what aspects do nutrients affect anaerobic biological treatment?

 

The growth and reproduction of anaerobic microorganisms require the uptake of a certain proportion of CNP and other trace elements. However, because the utilization rate of carbon nutrients by anaerobic microorganisms is lower than that of aerobic microorganisms, it is generally believed that the carbon-nitrogen-phosphorus ratio in anaerobic processes can be controlled at CODcr:N:P=(200～300):5:1.

 

It is also necessary to supplement some essential special nutrient elements according to the specific situation, such as sulfide, iron, nickel, zinc, cobalt, and molybdenum.

 

Providing nitrogen sources during anaerobic treatment not only meets the needs of microbial synthesis but also helps improve the buffering capacity of the reactor. If the nitrogen source is insufficient, that is, the carbon-nitrogen ratio is too high, it will not only lead to slow anaerobic bacterial proliferation but also reduce the buffering capacity of the digestate, causing the pH to decrease. Conversely, if the nitrogen source is excessive, the carbon-nitrogen ratio is too low, and nitrogen cannot be fully utilized, it will lead to the accumulation of nitrogen in the system, causing the pH to rise; if the pH rises above 8, it will inhibit the growth and reproduction of methanogens, reducing digestion efficiency. Generally speaking, the nitrogen concentration must be maintained within the range of 40～70mg/L to maintain the activity of methanogens.

 

 

In what aspects does pH value affect anaerobic treatment?

 

Anaerobic microorganisms have certain requirements for the pH value within their activity range. Acid-producing bacteria have a wider range of pH adaptation, generally maintaining high activity between 4.5 and 8.0. Methanogens are more sensitive to pH and have a narrower adaptation range, being more suitable between 6.6 and 7.4, with the optimal pH being 7.0～7.2. Therefore, in the anaerobic treatment process, especially when acid production and methane production occur in one structure, the pH in the reactor is usually maintained between 6.5 and 7.2, preferably in the range of 6.8～7.2.

 

The optimal pH required for anaerobic treatment refers to the pH of the mixed liquor in the reactor, not the pH of the influent, because biochemical processes and dilution can quickly change the pH of the influent. The pH of the reactor effluent is generally equal to or close to the pH inside the reactor.

 

Wastewater containing a large amount of soluble carbohydrates, after entering the anaerobic reactor, will cause a rapid decrease in pH due to the production of acetic acid, while acidified wastewater entering the reactor will cause the pH to rise. Wastewater containing a large amount of protein or amino acids may have a slight increase in pH due to the formation of ammonia. Therefore, different pH values can be controlled for wastewater with different characteristics, which may be lower or higher than the pH required by the reactor.

 

 

What are the measures to maintain sufficient alkalinity in the anaerobic reactor?

 

1. Add alkaline sources: Alkaline sources that increase the system's buffering capacity can include sodium bicarbonate and lime.

 

2. Increase the reflux ratio: The effluent from normal anaerobic digestion facilities contains a certain amount of alkalinity, and returning the effluent can effectively supplement the alkalinity in the reactor.

 

 

What are VFA and ALK? What is the significance of the ratio of VFA to ALK?

 

VFA represents the content of volatile organic acids in the anaerobic treatment system, while ALK represents the alkalinity in the anaerobic treatment system.

 

During the normal operation of the anaerobic digestion system, ALK is generally between 1000～5000 mg/L (calculated as CaCO3), with a typical value between 2500～3500mg/L, and VFA is generally between 50～2500mg/L. It is necessary to maintain a balance between alkalinity and volatile acid concentration to keep the pH of the digestate within the range of 6.5～7.5. As long as the alkalinity and volatile acid concentration can be balanced, even if VFA exceeds 1200mg/L when the alkalinity exceeds 4000mg/L, the system can still operate normally. The main indicator of the balance between alkalinity and acidity is that the ratio of VFA to ALK remains within a certain range.

 

VFA/ALK reflects the accumulation level of intermediate metabolites in the anaerobic treatment system. The VFA/ALK of a normally operating anaerobic treatment device is generally below 0.3. If VFA/ALK suddenly increases, it often indicates that intermediate metabolites cannot be decomposed and utilized by methanogens in time, that is, the system has malfunctioned and measures need to be taken to solve the problem.

 

If VFA/ALK just exceeds 0.3, it won't cause a pH drop within a certain period, and there is still time to analyze the causes of the VFA/ALK increase and implement control measures. If VFA/ALK exceeds 0.5, the CO2 content in biogas begins to rise. If measures are not taken in time to control it, the pH will quickly drop, inhibiting the activity of methanogens. At this time, some alkaline sources should be added to increase the alkalinity in the reactor to raise the pH, providing time to find the exact cause and take control measures. If VFA/ALK exceeds 0.8, the pH in the anaerobic reactor begins to drop, and the methane content in the biogas is often only 42%~45%, and the biogas cannot be burned. At this time, a large amount of alkaline source must be added to the reactor to control and raise the pH. If the pH continues to drop below 5, all methanogens will lose their activity, and the anaerobic sludge needs to be recultured.

 

 

Why is VFA an important indicator reflecting the effectiveness of anaerobic bioreactors?

 

VFA represents the content of volatile fatty acids in the anaerobic treatment system, and volatile fatty acids are intermediate products of the anaerobic biological treatment system.

 

Anaerobic biological treatment systems achieve effective treatment of organic matter in wastewater or sludge, ultimately through the methanogenic process, and the organic matter that methanogens can utilize is volatile fatty acid VFA. If the anaerobic bioreactor operates normally, the VFA content will remain within a relatively stable range.

 

Too low VFA will reduce the materials that methane can utilize, and the degree of organic matter decomposition in the anaerobic reactor will decrease; while too high VFA exceeding the amount that methanogens can utilize will cause excessive accumulation of VFA, which will in turn cause the pH in the reactor to decrease, affecting the normal function of methanogens. At the same time, after methanogens are damaged for various reasons, their utilization rate of VFA will also decrease, which will in turn cause the accumulation of VFA, forming a vicious cycle.

 

Therefore, all anaerobic reactors should use VFA as a control indicator for analysis and timely monitoring.

 

 

What is an upflow anaerobic sludge blanket reactor (UASB)?

 

The full name of the upflow anaerobic sludge blanket reactor is Upflow Anaerobic Sludge Blanket, abbreviated as UASB. Its basic characteristics are that a gas-solid-liquid three-phase separator is set in the upper part of the reactor, and the lower part is the sludge suspension zone and sludge bed zone.

 

 

What is an Expanded Granular Sludge Bed (EGSB)?

 

Expanded Granular Sludge Bed, abbreviated as EGSB, is developed based on the UASB reactor. The structure of the EGSB reactor is very similar to that of the UASB reactor. The difference is that the EGSB reactor uses a hydraulic load as high as 2.5～6m³/(㎡·h), which is much larger than the commonly used hydraulic load of about 0.5～2.5m³/(㎡·h) in UASB. Therefore, in the EGSB reactor, the granular sludge bed is in a partially or fully "expanded" state, that is, the volume of the sludge bed increases due to the increase in the average distance between the particles. In order to improve the hydraulic load (i.e., the upflow velocity), the EGSB reactor adopts a larger height-to-diameter ratio and a larger recirculation ratio.

 

 

What is granular sludge?

 

The formation of granular sludge is actually a form of microbial immobilization. Its appearance is relatively regular spherical or oval black granules. The particle size of granular sludge is generally 0.1～3mm, with individual large ones being 5mm, the density is 1.04～1.08g/cm³, slightly heavier than water, and has good sedimentation performance and methanogenic activity for degrading organic matter in water.

 

Observed under an optical microscope, granular sludge has a porous structure with a layer of transparent gel on the surface, on which methanogens are attached. The cell density is higher in the part of the granular sludge near the outer surface, while the internal structure is loose and the cell density is lower. Granular sludge with a larger particle size often has a cavity, which is caused by cell autolysis due to insufficient nutrition inside the granular sludge. Large and empty granular sludge is easily broken, and its broken fragments become the core of new granular sludge. Some large granular sludge is also prone to float because the gas generated inside is not easily released.

 

 

What are the methods to produce granular sludge in upflow anaerobic reactors?

 

The key to the successful operation of the UASB reactor is the presence of granular sludge. There are three methods to produce granular sludge in the UASB reactor:

 

1. Direct inoculation method: A certain amount of granular sludge is taken from other operating UASB reactors and directly put into the new UASB reactor. Then, the amount of wastewater treated is gradually increased from small to large until the design water volume is reached. This method has the fastest reactor start-up time, but it is generally only used to start small UASB reactors.

 

2. Indirect inoculation method: Anaerobic activated sludge taken from an operating anaerobic treatment device, such as digested sludge from a municipal wastewater treatment plant, is put into the UASB reactor. Then, the optimal growth conditions for anaerobic microorganisms are created, and nutrient water with appropriate nutrient components is used for cultivation. After granular sludge is formed, the amount of wastewater to be treated is gradually increased from small to large until the design water volume is reached.

 

3. Direct cultivation method: Anaerobic activated sludge taken from an operating anaerobic treatment device, such as digested sludge from a municipal wastewater treatment plant, is put into the UASB reactor. Then, it is directly cultivated with the wastewater to be treated. After granular sludge is formed, the amount of wastewater to be treated is gradually increased until the design water volume is reached. This method takes a longer time for reactor start-up, which can be as long as 3～4 months. This method is often used for large UASB reactors.

 

 

What are the characteristics of mature anaerobic sludge after cultivation?

 

After cultivation, the mature sludge is dark gray to black, has a tar smell but no hydrogen sulfide smell, the pH is between 7.0～7.5, and the sludge is easy to dewater and dry. The treatment effect on the influent is high, the gas production is large, and the methane content in the biogas is high. The basic indicators and parameters of the cultivated mature anaerobic digested sludge are shown in the table below.