UASB ANAEROBIC REACTOR

I. Introduction

Anaerobic biological treatment utilizes the metabolic characteristics of anaerobic microorganisms and uses reduced organic matter as hydrogen acceptor without the need for external energy. Meanwhile, it produces methane gas with energy value. Anaerobic biological treatment is not only suitable for high concentration organic wastewater, but also for low concentration organic wastewater, such as municipal wastewater.

Anaerobic biological treatment process has low energy consumption; high organic volume load, generally 5-10 kg COD/m3.d, the highest up to 30-50 kg COD/m3.d; little excess sludge; low nutritional demand of anaerobic bacteria, strong toxicity resistance, high molecular weight of degradable organic matter; strong shock load resistance; biogas produced is a clean energy source.

Nowadays, the whole society advocates circular economy and pays attention to the recycling and reuse of industrial wastes. It is obvious that anaerobic biological treatment is the best technology to recycle wastewater. In recent years, sewage anaerobic treatment technology has developed rapidly. Various new processes and methods, including anaerobic contact method, up-flow anaerobic sludge bed, baffle anaerobic method, anaerobic biological filter, anaerobic expanded bed and fluidized bed, as well as the third generation of anaerobic process EGSB and IC anaerobic reactor, have been developed rapidly.

Up-flow Anaerobic Sludge Bed (UASB) process has the dual characteristics of anaerobic filtration and anaerobic activated sludge process, as a technology that can convert pollutants in sewage into renewable clean energy, biogas. The sewage with different solid content has strong adaptability, and its structure, operation, maintenance and management are relatively simple, the cost is relatively low. The technology has matured, and is increasingly valued by the sewage treatment industry, and is widely welcomed and applied.

In this paper, the operation mechanism and process characteristics of UASB as well as the design and start-up of UASB are briefly described.

II. Origin of UASB

In 1971, Professor Lettinga of Wageningen Agricultural University in the Netherlands invented a three-phase separator by means of the physical structure design and the difference of the action of gravity field on different density substances. By separating the residence time of activated sludge from that of wastewater, a prototype of upstream anaerobic sludge bed (UASB) reactor was formed. In 1974, CSM Corporation of the Netherlands discovered the biopolymer structure formed by the self-immobilization mechanism of activated sludge, i.e. granular sludge, when treating sugar beet wastewater in its 6m3 reactor. The emergence of granular sludge not only promoted the application and development of the second generation anaerobic reactor represented by UASB, but also laid the foundation for the birth of the third generation anaerobic reactor.

III. UASB Working Principle

UASB consists of sludge reaction zone, gas-liquid-solid three-phase separator (including sedimentation zone) and gas chamber. A large amount of anaerobic sludge is retained in the bottom reaction zone, and sludge with good sedimentation and coagulation properties forms a sludge layer in the bottom. The sewage to be treated flows from the bottom of the anaerobic sludge bed to mix with the sludge in the sludge layer, and the microorganisms in the sludge decompose the organic matter in the sewage and convert it into biogas. Biogas emits continuously in the form of micro-bubbles. During the rising process, micro-bubbles merge and gradually form larger bubbles. Due to the stirring of biogas, a sludge with thinner sludge concentration is formed on the upper part of the sludge bed and water rises together into the three-phase separator. When biogas meets the reflecting plate at the lower part of the separator, it reverses. Surrounding the jet plate, it passes through the water layer and enters the gas chamber, concentrating on the methane gas in the gas chamber. It is exported by the conduit. The solid-liquid mixture is reflected into the sedimentation zone of the three-phase separator. The sludge in the sewage flocculates, the particles gradually increase and settle under the action of gravity. The sludge sludge sludge sludge deposited on the inclined wall slides back to the anaerobic reaction zone, which accumulates a large amount of sludge in the reaction zone. After separation with sludge, the treated effluent overflows from the upper part of the overflow weir in the sedimentation zone, and then discharges the sludge bed.

The basic requirements are as follows:

(1) Provide favorable physical, chemical and mechanical conditions for sludge flocculation, so that anaerobic sludge can obtain and maintain good sedimentation performance;

(2) A good sludge bed can often form a relatively stable biological phase, maintain a specific micro-ecological environment, resist strong disturbance, and the larger flocs have good sedimentation performance, thus improving the sludge concentration in the equipment;

(3) By setting up a sedimentation zone in the sludge bed equipment, the fine particles of sludge can be further flocculated and precipitated in the sludge layer of the sedimentation zone, and then flowed back into the sludge bed.  

IV. Flow pattern and sludge distribution in UASB

The flow pattern in UASB is quite complex. The flow pattern in the reaction zone is highly correlated with the gas production and the reaction zone. Generally speaking, in the lower sludge layer of the reaction zone, due to the result of gas production, more gas is passed through some sections, forming an upward flow, which drives some mixed liquids (sludge and water) to move upward. At the same time, the medium around the gas and water moves downward, causing reverse mixing, which results in short flow of water. It is easy to form dead corners away from the rising air and current. In these dead corners, there is also a certain amount of gas production, which forms a slow and weak mixing of sludge and water. Therefore, different degrees of mixing zones are formed in the sludge layer, and the size of these mixing zones is related to the degree of short flow. Mixed liquids in suspension layer form strong mixing because the movement of gas currency drives the liquid to rise and fall at a higher speed. In the case of less gas production, sometimes there is a clear boundary between sludge layer and suspension layer, but in the case of more gas production, this interface is not obvious. Relevant experiments show that the flow in the sediment area is plug flow, but there are still dead zone and mixed zone in the sediment area.  

The sludge concentration in UASB is related to the organic loading rate of the equipment. It is the relationship between sludge distribution and load in UASB when treating sugar-making wastewater. It can be seen from the figure that the sludge concentration in the sludge layer is higher than that in the suspension layer, and the difference between the sludge concentration in the upper and lower parts of the suspension layer is smaller. This indicates that the sludge concentration in the reaction zone is close to the fully mixed flow pattern. When the organic load is high, the boundary between the sludge layer and the suspension layer is not obvious. Experiments show that 90% of organic matter has been converted through the height of 0.4-0.6m at the bottom. Therefore, anaerobic sludge has very high activity, which changes the concept that anaerobic treatment process is slow for a long time. In the anaerobic sludge, the accumulation of a large number of highly active anaerobic sludge is the main reason for the huge treatment capacity of this equipment, which is attributed to the good sedimentation performance of sludge.  

UASB has a high volumetric organic loading rate, which is mainly due to the large amount of anaerobic sludge in the equipment, especially in the sludge layer. The stability and efficiency of the process depend largely on the formation of sludge with excellent settling performance and high methane activity, especially granular sludge. On the contrary, if the sludge in the reaction zone exists as loose flocculent, the sludge often floats and loses, which makes UASB unable to operate steadily under higher load.