Summary of 14 sludge digestion problems

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The most detailed summary of 14 sludge digestion problems in history, detailed below:

1. What is sludge digestion?

Sludge digestion is the use of microbial metabolism to stabilize the organic matter in the sludge. When the content of volatile solid MLVSS in the sludge drops below 40%, stabilization is considered to have been achieved. Sludge digestion can be treated with aerobic treatment or by anaerobic treatment.

2. What is the aerobic digestion of sludge?

Aerobic digestion of sludge is to aerate the sludge for a long time without adding organic matter, so that the microorganisms in the sludge are in the endogenous respiration stage for their own oxidation, aerobic digestion can make the biodegradable part of the sludge (about 80% of the total sludge) be oxidized and removed, the degree of digestion is high, the amount of residual sludge is small, and the treated sludge is easy to dehydrate.

Aerobic digestion takes much less time than anaerobic digestion, and the hydraulic residence time at room temperature is 10-12d, which is mainly used for sludge production occasions. The general blast volume is 4.2-16.8m3/(m3h▪), the sludge load is 0.04-0.05kg BOD5/(kgMLSSd▪), and the BOD5 removal rate is about 50%.

3. What are the characteristics and types of aerobic digestion of sludge?

1) Aerobic digestion supernatant BOD5, SS, CODcr and ammonia nitrogen concentration is low, digestion sludge is small, odorless, easy to dehydrate, convenient and simple disposal. The tank is simple, easy to manage, and has no risk of methane explosion.

2) Can not recycle biogas energy, high operating costs, large energy consumption, digested sludge gravity concentration, because aerobic digestion does not take heating measures, so the degree of decomposition of sludge organic matter with temperature fluctuations.

Aerobic digestion has two kinds: ordinary aerobic digestion and high temperature aerobic digestion. Ordinary aerobic digestion is similar to the activated sludge method, mainly relying on delayed aeration to reduce the amount of sludge, high-temperature aerobic digestion uses the heat released by microorganisms to oxidize organic matter to heat the sludge, and increase the sludge temperature to 40 ~ 70 ° C, so as to achieve the purpose of digesting the sludge under high temperature conditions. Compared with ordinary aerobic digestion, high-temperature aerobic digestion is faster, the residence time is shorter, and it can kill almost all pathogens without the need for further disinfection.

High-temperature aerobic digestion can use the heat generated by its own activities to reach high-temperature conditions under most natural climatic conditions, without the need for external heating sources, as long as the digester is covered with heat preservation.

4. What is anaerobic digestion? What is the difference between anaerobic treatment and high concentration wastewater treatment?

Anaerobic digestion of sludge is the process of using anaerobic microorganisms to hydrolyze, acidify, and produce methane, and most of the solid organic matter in the sludge is hydrolyzed, liquefed, and finally decomposed.

Methanogens eventually convert the carbon in the organic matter of the sludge into methane and release it from the sludge, stabilizing the sludge. The anaerobic digestion of sludge differs from the anaerobic treatment of high concentration wastewater. The organic matter in the wastewater is mainly present in the dissolved state, while the organic matter in the sludge is mainly present in the solid state. According to the different operating temperatures, sludge anaerobic digestion is divided into two types: medium temperature digestion (30-37 °C) and high temperature digestion (45 ~55 °C). Due to the high energy consumption of high-temperature digestion, large sewage treatment plants generally do not use it, so the common anaerobic digestion of sludge is actually medium-temperature digestion.

5. What are the basic requirements for the sludge anaerobic digester?

1) When using two-stage digestion, the ratio of the residence time of the first-level digester and the first-level digester can be 1:1, 2:1 or 3:2, of which the maximum is 2:1: the liquid level of the first-level digester must be able to meet the needs of the sludge self-flowing to the first-level digester, when the groundwater level is high, the anti-floating of the pool body must be considered, and it is best to choose when the groundwater level is low when cleaning the digester.

2) The sludge anaerobic digester generally uses a reinforced concrete structure with good water tightness, air tightness and corrosion resistance, the diameter is usually 6 ~ 35m, the ratio of the total height to the diameter is 0.8 ~ 1.0, and the ratio of the inner diameter to the height of the cylinder is 2:1. The slope of the bottom of the pool is 8%, the height of the pool top from the mud surface is greater than 1.5m, the diameter of the top gas collection cover is generally 2m, the height is 1 to 2m, and the diameter and height of the large digester gas collection hood are preferably greater than 4m and 2m, respectively.

3) Sludge anaerobic digester is generally set up with sludge pipe, sludge pipe, supernatant drainage pipe, overflow pipe, circulating stirring pipe, biogas pipe, emptying pipe, sampling pipe, manhole, pressure measuring tube, temperature measurement tube, etc., the general sludge pipe is arranged above the mud level in the pool, its position, quantity and form should be conducive to mixing evenly, crushing scum, the minimum pipe diameter of the sludge pipe is 150mm, the pipe should be resistant to corrosion or anti-corrosion treatment, and the pipeline cleaning equipment is equipped.

4) The supernatant discharge pipe can be set at different heights 3 to 4, the minimum diameter is 75m, and there are measures to isolate from the atmosphere; the overflow pipe is one level larger than the sludge pipe, and the diameter is not less than 200m, and the overflow height should be able to ensure that the pool is in a positive pressure state; the empty pipe can share the same pipe with the sludge pipe; the minimum diameter of the sampling pipe is 100mm, at least one in the pool and the pool edge, and 0.5m below the mud level; the manhole should be set up twice, and the location is reasonable.

5) The walls and top cover around the pool must be insulated.

6. What are the influencing factors of the sludge anaerobic digester?

1) Temperature, pH, alkalinity and toxic substances are the main factors affecting digestion, and their influence mechanism is the same as that of anaerobic wastewater treatment.

2) Sludge age and dispensing rate. In order to obtain a stable treatment effect, a long mud age must be maintained. The degree of degradation of organic matter is a function of sludge age, not of organic matter entering the mud.

3) Sludge stirring. By stirring, the fresh sludge can be quickly and fully mixed with the original mature sludge in the pool, so as to achieve completely consistent distribution of temperature, substrate concentration and bacterial concentration, accelerate the digestion process, and improve gas production. At the same time, it prevents sludge delamination or sludge layers.

4) Carbon-nitrogen ratio C/N. The anaerobic digester requires the C/N of the substrate to reach (10 ~ 20): 1 is the best, the general C/N of the primary sedimentation pond sludge is about (9.4 ~ 10.4): 1, can be treated with anaerobic digestion alone, and the C/N of the remaining activated sludge discharged from the second sedimentation tank is about (4.6 ~ 5): 1, it should not be digested alone, and should be mixed with the primary sedimentation tank to improve the carbon-nitrogen ratio and then anaerobic digestion treatment together.

7. What is the dispensing rate of the sludge digester?

The dispensing rate is the percentage of the effective volume of the digester by adding fresh sludge to the digester every day, and the dispensing rate and the sludge age are reciprocal. The solid residence time of the digester is the same as the hydraulic residence time of the digestion tank without counting the discharge of the digestive juice, that is, the digestion time of the sludge, for example, when the sludge dispensing rate is 5%, the residence time of raw sludge in the digester is 20d, and the sludge volume dispensing rate is 0.05m3/(m3d▪).

The high dispensing rate and slow digestion rate may cause the accumulation of fatty acids in the digestion pool, so that the pH value decreases, the sludge digestion is incomplete, the gas production decreases, and the amount of sludge reduction is reduced.

The dispensing rate is low, the sludge digestion is relatively complete, the gas production rate is higher, but the digester volume is required to be large enough, which will reduce the capacity utilization rate of the digester tank, increase the cost of infrastructure In addition, in order to ensure that the ratio of microorganisms in the digester and the sludge organic matter is stable, the sludge handling rate and the sludge moisture content are also related, the sludge distribution rate with low water content should be appropriately reduced, and the sludge dispensing rate can be appropriately increased when the water content is low.

8. What are the cultivation methods for digesting sludge in sludge anaerobic digester?

The initiation of the sludge anaerobic digestion system is to complete the cultivation process of anaerobic digestion sludge, that is, anaerobic activated sludge or methane bacteria. There are two methods of culturing anaerobic digested sludge.

1) Step-by-step cultivation method: that is, the method of gradually putting raw sludge into the anaerobic digestion tank, so that the raw sludge can be gradually converted into anaerobic digestion sludge by itself. This method makes the activated sludge undergo a transformation process from aerobic to anaerobic, and the growth rate of anaerobic microorganisms is much lower than that of aerobic microorganisms, so the gradual cultivation process takes a long time, generally taking about 6 months to 10 months to complete.

2) Inoculation culture method: that is, the method of anaerobic inoculation of sludge into the sludge anaerobic digester with a total volume of 10%-30%. Inoculation sludge is generally taken from the sludge anaerobic digester of the municipal sewage treatment plant in operation, and when the transportation of liquid digestible sludge is inconvenient, dry sludge that has been mechanically dehydrated can be used. In places where anaerobic digestion sludge is lacking, it is possible to take the decomposed organic sediment from pits and ponds, or use human manure, pig manure, cow dung, distiller's lees or primary sedimentation pond sludge as a strain. The sludge is first dissolved with water, and then filtered with a 2mm × 2mm filter to remove large impurities, and then the static precipitation is carried out to remove part of the supernatant, and the solid concentration of 3% to 5% of the sludge is put into the digester as inoculation sludge.

9. Precautions for digesting sludge culture in anaerobic digestion tank?

1) The object of sludge anaerobic digester treatment is activated sludge, and there is generally no toxicity problem. However, in order to speed up the incubation start-up process, in addition to inoculating sludge, heating and heat preservation should also be done.

2) After fully stirring the inoculation sludge in the digester to heat it to the specified temperature, and then gradually adding the concentrated sludge, while continuing to do a good job of heating and stirring, so that the temperature in the digester is always in the best state.

3) When using the inoculation culture method, the amount of raw sludge in the early stage is related to the amount of inoculated digestion sludge and the culture time, and the early stage can be added according to 30% to 50% of the designed sludge intake, and then gradually increase the amount of sludge after generally cultivating to 60d.

4) Often measure the gas production and the concentration of VFA in the digestion solution in the pool and the pH straight, if the monitoring results find that the digestion is very abnormal, the amount of sludge should be reduced immediately, or other types of digestion sludge should be added as inoculation sludge for re-culture.

5) In order to prevent the occurrence of explosion accidents, nitrogen should be used to replace the air in the digester and gas pipeline system before inoculation, and after the biogas is generated, the nitrogen should be gradually replaced.

6) The object of sludge anaerobic digester treatment is activated sludge, in which nutrients such as wrestling, nitrogen, phosphorus and other nutrients are generally balanced and can adapt to the needs of the growth and reproduction of anaerobic microorganisms. Therefore, nutrients do not have to be added during the cultivation process of digesting sludge, such as the treatment of high concentrations of industrial wastewater.

10. The role of stirring in the sludge anaerobic digester is set

Mixing and stirring is one of the key conditions to improve the efficiency of anaerobic digestion of sludge, and there is no stirred anaerobic digestion tank, and there must be a layering phenomenon in the material liquid in the pool. By stirring, the delamination can be eliminated, the contact between the sludge and microorganisms can be increased, the sludge and the original liquid in the pool can be quickly mixed, and the separation of biogas and digestive juice can be promoted, while preventing the crust of the scum layer.

The volume utilization rate of the digester with good agitation can reach 70%, while the volume utilization rate of the digester with unreasonable agitation will drop to less than 50%. Agitation can be carried out continuously or intermittently, and most sewage plants use intermittent mixing. In general, stirring every 24h should not exceed lh. Agitation is usually carried out at the same time as the mud feed and steam are heated, and the stirring should be stopped when the digestive solution is discharged, so that the supernatant is separated by a static precipitation and discharged. When the bottom sludge discharge method is used, the stirring can be stopped during the discharge process, and when the upper sludge discharge method is used, the stirring must be carried out at the same time during the sludge discharge process.

11. Stirring method of sludge anaerobic digester

1) Mechanical stirring in the pool: that is, there is a propeller in the pool, and the digestion mixture is stirred by the motor drive outside the pool, and the stirring strength is generally 10~20W/m3 the pool capacity, and the energy consumption required is about 0.0065KW/m3. The optimal stirring radius for each agitator is 3 to 6 m.

If the diameter of the digester is large, you can set up multiple agitators, in an equilateral triangle and other uniform arrangement, the advantage of mechanical stirring for large digestion tank is that the impact on the separation of sludge is small, the disadvantage is that the transmission part is easy to wear, and the airtightness problem of the bearing seal at the top of the digester is not easy to solve. The seal can be used to weld the water seal on the stirring shaft, and the water seal tank is provided on the top cover of the digester, and the water seal can play a sealing role by rotating in the water seal tank, and the water depth in the water seal tank can be determined according to the gas phase pressure in the digester.

2) Biogas stirring: that is, the compressor is used to extract the biogas from the top of the pool, and then flush in from the bottom of the pool, the circulating biogas is stirred, the biogas stirring is conducive to making the CO2 in the biogas as the substrate of methane production used by methanogenic bacteria, the stirring strength is generally 1 ~ 2m3 biogas / (m2h), the required energy consumption is 0.005 ~ 0.008KW / m3, the compressor used must ensure that it never leaks, so as not to suck in air or leak biogas caused by explosion.

3) Pump according to the bad digestion liquid stirring: usually in the pool set up a jet, by the pool outside the pump pressure of the circulating digestion liquid through the jet spray, from the throat vacuum into a part of the pool of digestive juice or cooked sludge, sludge and digestive juice together into the middle of the digestion pool to form a stronger stirring, the required energy consumption is about 0.005KW / m3, with the sludge pump to extract the digestive sludge for agitation can be combined with the heating of the sludge.

12. The characteristics and basic requirements of the pump of the anaerobic digester are stirred according to the bad

The method of using a water pump to pump part of the mixture in the digester and return it to the digester under pressure to achieve water inlet or sludge and fully mix with the original mixture is called pump circulation stirring. Pump circulation mixing equipment is simple, easy to maintain, in order to make the mixture mixed completely, the amount of circulation required is larger, 1m3 effective pool volume mixing required energy consumption is generally 0.005kW, in order to improve the mixing effect, usually in the digester set up a jet, by the water pump pressure of the mixed liquid through the jet jet spray, in the jet throat to form a vacuum, sucked into a part of the pool of digestive fluid or cooked sludge, the formation of a more intense stirring.

In order to prevent clogging, the minimum diameter of the circulating mixing pipe can not be less than 150m, the choice of jet must match the head of the pump, the head of the sewage pump used is generally 15 ~ 20m, and the ratio of the induced flow to the suction flow is generally 1: (3 ~ 5). The working radius of the jet is about 5m, when the diameter of the digester exceeds 10m, multiple jets can be set up to circulate stirring with water pumps, due to the violent agitation of the pump impeller and the high-speed jet of the water sealer nozzle, the sludge will be crushed, which is very unfavorable to the separation of the sludge water digestion sludge, and sometimes it will cause the supernatant SS to be too large. Therefore, this stirring method is more suitable for small digesters.

13. Biogas stirring method of sludge anaerobic digester

The agitation of the sludge anaerobic digester is to use a part of the biogas produced by the digester, which is pressurized by the compressor and then sent to the digester through the vertical pipe or the diffuser at the bottom of the pool, so as to achieve the purpose of mixing and mixing uniformly. There are three ways to mix biogas.

1) Gas lifting stirring: the biogas is pressed into the middle or bottom of the diversion pipe located in the digester, so that after the biogas and the digestion liquid are mixed, the sludge containing the biogas bubble is reduced and then rises along the diversion pipe, so that the digestive juice in the digester is continuously circulated and stirred to achieve the purpose of mixing.

2) Vertical pipe stirring: According to the diameter of the digester, several vertical pipes are evenly arranged in the pool, and the pressurized biogas is distributed to each vertical pipe through the biogas distribution main pipe, and then sprayed out from the lower end of the vertical pipe, which plays the role of stirring and mixing.

3) Diffusion stirring: The compressed biogas produces a rotating flow of digestive juice in the digester through the gas diffuser installed at the bottom of the digester, which plays a role in stirring and mixing.

14. Heating method of sludge anaerobic digester

Sludge anaerobic digestion is generally digested at a medium temperature of 35 ° C, in order to maintain a moderate temperature in the digester, it is necessary to heat up the sludge. The common heating methods of anaerobic digesters are preheating in the heat exchanger outside the digester, heating directly in the digester with steam, and installing hot water heating coils inside the digester. There is also a way to build a preheating feeding pool outside the digester to heat the raw sludge and then add it to the digester.

Methane bacteria are very sensitive to temperature fluctuations, and the temperature fluctuations of digesting sludge should generally be controlled within 1 °C of soil. Temperature fluctuations, that is, the effect of heating is related to the number of mud feeds, the duration of mud feeding and the amount of mud each time, and the small number of mud feeds will inevitably lead to a large amount of mud each time to overload the heating system, and insufficient heating will cause a decrease in temperature. Therefore, the sludge should be as uniform and close to continuous as possible.

15. What are the main reasons for the decline in gas production of sludge anaerobic digester?

Taking the warm anaerobic digestion of sludge in urban sewage treatment plants as an example, when the moisture content of raw sludge is about 96% and the distribution rate is 6% to 8%, the gas production per cubic meter of raw sludge is 10 to 12m3. If high temperature digestion is adopted, under the same conditions, the gas production per cubic meter of raw sludge can reach 22~23m3; the gas production per cubic meter of raw sludge is 13~15m3 when the dispensing rate is 13%~15%.

1) The organic matter dispensing load is too low: under normal other conditions, the biogas production is directly proportional to the organic matter input, and the more organic matter is put in, the more biogas production. Conversely, the less organic matter is put in, the less biogas is produced. The reason for the decline in gas production is often due to the poor operation of the concentration tank, the poor concentration effect, and the loss of a large number of organic solids with the supernatant of the concentration tank, resulting in a decrease in the concentration of sludge entering the digester, that is, the amount of organic matter in the case of the same volume of sludge is reduced. At this time, the control of the sludge thickening process can be strengthened to ensure that the qualified concentration effect is achieved.

2) Reduced methane bacteria activity: due to some reason, the activity of methane bacteria is reduced, the rate of decomposition of VFA is reduced, and the biogas production is also reduced. Excessive hydraulic load, excessive loading of organic matter, excessive temperature fluctuations, uneven mixing effect, poisons in the inlet water and other factors can reduce the activity of methane bacteria, and the specific causes should be analyzed and corresponding countermeasures should be taken.

3) Excessive amount of mud discharge: the number of anaerobic microorganisms in the digester is reduced, which destroys the balance between microbial volume and nutrient content, and reduces the gas production, and the countermeasure is naturally to reduce the amount of mud discharge.

4) The effective volume of the digester is reduced: due to the accumulation of liquid surface scum in the pool and the accumulation of sediment at the bottom of the pool, the effective volume of the digester is reduced, the overall digestion effect is reduced, and the gas production is also reduced. At this time, the digester should be emptied for cleaning, and the operation of the slag elimination facility and the sand removal efficiency of the sedimentation tank of the pretreatment facility should be checked at the same time, and the existing faults should be eliminated in time.

5) Biogas leakage: The phenomenon of gas leakage in the pipelines or facilities of the digester and gas transmission system makes the measured gas production smaller than the actual gas production, at this time, the leakage point should be immediately found and repaired to prevent larger accidents such as biogas explosions.

6) The temperature in the digester decreases: the amount of mud entering is too large or the heating facility fails to reduce the temperature in the digester, and the gas production is also reduced. At this time, the countermeasure is to heat the sludge in the digester to the specified temperature, while reducing the amount of sludge in and sludge discharge.

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