Environment Technology

Anaerobic Biological Treatment Technology for Wastewater Treatment

2019-1-31

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Anaerobic biological treatment technology for wastewater treatment is a process in which facultative anaerobic and anaerobic microorganisms transform organic matter into methane and carbon dioxide under anaerobic conditions, also known as anaerobic digestion. Anaerobic biological treatment technology has always been favored by environmental protection workers in the water treatment industry. Because of its good removal effect, higher reaction rate and better adaptation to toxic substances, and more importantly, because it does not need to provide a large amount of energy consumption for oxygen transfer compared with aerobic biological treatment wastewater, anaerobic biological treatment is applied in water treatment industry and widely spread.

Generally speaking, there are many complex organic materials in wastewater, which are degraded by anaerobic decomposition in four stages:

(1) Hydrolysis stage: Because of its large molecular volume, macromolecule organic matter can not directly pass through the cell wall of anaerobic bacteria, and needs to be decomposed into small molecules by extracellular enzymes in vitro. Typical organic substances in wastewater such as cellulose are decomposed into cellobiose and glucose by cellulase, starch is decomposed into maltose and glucose, and protein is decomposed into short peptides and amino acids. After decomposition, these small molecules can enter the cell body through the cell wall for further decomposition.

(2) Acidification stage: The above-mentioned small molecular organic compounds enter the cell body and are transformed into simpler compounds and distributed outside the cell. The main products of this stage are volatile fatty acids (VFA), while some alcohols, lactic acid, carbon dioxide, hydrogen, ammonia, hydrogen sulfide and other products are produced.

(3) Acetic acid production stage: In this stage, the products of the previous step are further converted into acetic acid, carbonate, hydrogen and new cellular substances.

(4) methanogenesis stage: in this stage, acetic acid, hydrogen, carbonate, formic acid and methanol are converted into methane, carbon dioxide and new cellular substances. This stage is also the most important stage of the whole anaerobic process and the speed limit stage of the whole anaerobic reaction process.

The development of anaerobic technology has gone through three stages.

The first stage (1860-1899): the initial stage of development in which simple sedimentation and anaerobic fermentation are combined. In this stage of development, sewage sedimentation and sludge fermentation are concentrated in a decomposition tank (commonly known as septic tank), and sludge is not separated.

The second stage (1899-1906): the development stage of stratification of sewage sedimentation and anaerobic fermentation.

The third stage (1906-2001): the advanced development stage of independent construction. In this stage of development, the anaerobic fermentation chamber in the sedimentation tank was separated and an independent anaerobic digestion reactor was built.

Correspondingly, the reactor body of anaerobic biological treatment technology has gone through three times.

The first generation of anaerobic reactor is a low load system represented by common anaerobic digestion tank (CADT) and anaerobic contact process (ACP).

The second generation reactor was developed at the end of 1960s, aiming at maintaining a large amount of activated sludge and a long enough sludge age in the reactor, using the immobilization technology of biofilm and the cultivation of anaerobic sludge which is easy to precipitate. Such as anaerobic filter (AF), anaerobic fluidized bed (AFB), anaerobic biological turntable (ARBCP), up-flow anaerobic sludge bed (IAASB), anaerobic attached expanded bed (AAFEB), etc. UASB reactor is the most widely used reactor. Based on the research and application of the second generation reactor represented by UASB reactor, a new generation reactor has been developed.

The third generation of anaerobic reactor is based on the separation of solid residence time and hydraulic residence time, so that solid-liquid two-phase contacts can be fully maintained, so that a large number of sludge can be maintained and the wastewater and activated sludge can be fully mixed and contacted to achieve truly efficient purposes. At present, there are many studies: anaerobic granular sludge bulking bed (EGSB), anaerobic internal circulation (IC) and so on.

In this paper, several anaerobic technologies which are widely used are introduced.

1. Anaerobic Biofilter
The structure of anaerobic biofilter is similar to that of general biofilter, with fillers in the tank, but the top of the tank is sealed. Wastewater enters from the bottom of the pond and is discharged from the top of the pond. The filler is immersed in water, and microorganisms attach to the filler. The microbial biomass in the filter is high, and the average residence time can be as long as 150 days, so it can achieve a higher treatment effect. The filter filler can be gravel, pebble or plastic, with an average particle size of about 40 mm.

2. Anaerobic Contact Process
Anaerobic contact process, also known as anaerobic activated sludge process, is to install a settling and separating device behind the digestion tank. The mixed liquid after anaerobic digestion of the digestion tank is discharged to the settling tank separating device for sludge water separation. The clarified water is discharged from the upper part, and the sludge is refluxed to the anaerobic digestion tank. This not only avoids sludge loss but also increases the volume load of digestion tank, thus greatly shortening the hydraulic retention time. The general load of anaerobic contact process is 2-10 kg COD/(m3˙d) at medium temperature, 0.25 kg COD/(kgVSS˙d) at sludge load, and 10-15 g/L at MLVSS in tank.

3, UASB
The sludge bed of ASB reactor mainly consists of anaerobic sludge with good settling performance, and the concentration can reach 50-100g/L or higher. The sediment suspension zone is mainly formed by the rising and stirring action of the gas generated in the reaction process. The sludge concentration is low. Generally in the range of 5-40 g/L, there are three-phase separators on the upper part of the reactor: gas (methane), solid (sludge) and liquid (waste water). The separator first deflects the rising process of the generated methane bubbles and enters the gas chamber through the water layer, which is discharged by the conduit. After degassing, the mixed liquid is further separated from the solid and liquid in the settling area, and the sludge settled returns to the reaction area, which accumulates a large number of microorganisms in the reaction area. The wastewater to be treated enters from the bottom water distribution system, and the treated water after clarification is discharged from the sedimentation zone. A kind of granular anaerobic sludge with good settling power and high specific methanogenic activity can be obtained in UASB reactor, so it has certain advantages over other reactors: the relative density of granular sludge is smaller than that of artificial carrier, the sludge can be fully contacted with the substrate by the generated gas, and the equipment and energy consumption of stirring and reflux sludge can be saved; the application of three-phase separator can save auxiliary removal. Gas device; granular sludge sedimentation performance is good, avoid attaching precipitation separation device and reflux sludge equipment: reactor does not need to add fillers and carriers, improve the volume utilization rate.

4. EGSB
In the early 1990s, Wageningen Agricultural University of the Netherlands began to study the anaerobic expanded granular sludge bed (EGSB) reactor. When using UASB reactor to treat domestic sewage, Professor Lettinga et al. changed the structure design and operation parameters of UASB reactor in order to increase the contact of sewage sludge and utilize the volume of reactor more effectively, so that the granular sludge bed in the reactor expanded sufficiently at the rising velocity of high liquid surface, resulting in the early EGSB reactor. In fact, the EGSB reactor is an improved UASB reactor. The difference is that the former has a higher liquid upwelling velocity, which makes the whole granular sludge bed in an expanding state. This unique feature enables it to have a larger ratio of height to diameter. The EGSB reactor is mainly composed of the main part, the inlet water distribution system, the gas-liquid-solid three-phase separator and the outlet water circulation. Among them, the influent distribution system distributes water evenly to the bottom of the whole reactor and produces a uniform upward flow rate: the three-phase separator is the most critical structure of the EGSB reactor, which can effectively separate water, methane and sludge, so that sludge can be effectively retained in the reactor; the effluent circulation part is to improve the upward flow rate on the liquid surface of the reactor, so as to make granular sludge. Full contact with sewage to avoid dead angle and short flow in the reactor.

5. IC
IC internal circulation anaerobic reactor is a patent product of Parker Company in the Netherlands. At present, Parker Company has more than 300 IC reactors in the world. Compared with UASB, which has only one stage three-phase separator at the top, IC internal circulation reactor has two stages three-phase separator. IC reactor is actually composed of two UASB stages, with high UASB load at the bottom and low load at the top. Because a large amount of biogas is collected in the first stage separation, the disturbance to the wastewater is reduced, which makes the separation of gas, water and mud better in the second stage and three phase separation. The two-stage separation lC reactor ensures the optimal sludge retention time, which is beneficial for the treatment of some chemical wastewater, because the anaerobic sludge production of these wastewater is very small. IC reactor has a self-adjusting stripping internal circulation structure. The circulating wastewater is mixed with raw water to dilute the influent concentration. The energy generated by internal circulation makes the sludge mix more fully at the bottom, thus increasing the sludge activity. The internal dilution of wastewater in IC inner cycle can reduce the fluctuation of production load. The volume load of IC reactor (15-30 kg COD/m3) is twice that of UASB (7-15 kg COD/m3).

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