IC and UASB are the two most common structural forms in anaerobic reactors. Today we will talk about the structure and principles of UASB.

1. Principle of UASB anaerobic reactor

In the UASB reactor, wastewater is introduced as evenly as possible into the bottom of the reactor, and the wastewater flows upward through a sludge bed containing granular sludge or flocculent sludge. Anaerobic reactions occur during the contact between wastewater and sludge particles. The biogas produced under anaerobic conditions (mainly methane and carbon dioxide) causes internal circulation, which is beneficial for the formation and maintenance of granular sludge.

Some gases formed in the sludge layer adhere to the sludge particles and rise towards the top of the reactor. The sludge that rises to the surface collides with the bottom of the gas emission plate of the three-phase separator, causing the sludge flocs with attached bubbles to degas. After the release of bubbles, the sludge particles will settle on the surface of the sludge bed, while the gas will be collected in the collection chamber of the three-phase separator.

A baffle (gas reflector) is installed under the gap of the gas collection chamber unit to prevent biogas bubbles from entering the sedimentation zone, otherwise it will cause turbulence in the sedimentation zone and hinder particle sedimentation. The liquid containing some remaining solids and sludge particles enters the sedimentation zone through the separator gap.

Due to the increase in flow area near the water surface in the inclined wall sedimentation zone of the three-phase separator, the upward flow velocity decreases near the discharge point. At the same time, as the flow rate decreases, the sludge flocs can coagulate and settle in the sedimentation zone. The accumulated sludge flocs on the three-phase separator will exceed their frictional force on the inclined wall to a certain extent, and slide back to the reaction zone, where this part of the sludge will react with the incoming organic matter.

2. Structure of UASB reactor

The USAB reactor includes an inlet and distribution system, a reactor tank, and a three-phase separator. If considering the entire anaerobic system, it should also include biogas collection and utilization systems. However, due to the uncertainty of the pathways and goals of biogas utilization, there are also significant differences in its utilization systems.

The most important equipment in the USAB reactor is the three-phase separator, which is installed at the top of the reactor and divides it into a lower reaction zone and an upper precipitation zone. The main purpose of a three-phase separator is to effectively separate the biogas generated from the sludge bed as much as possible in order to achieve sedimentation effect on the sludge floc particles in the upflow in the settler.

Especially under high load conditions, installing a reflector plate under the gas collection chamber is to prevent biogas from escaping into the sedimentation chamber through the gaps between the gas collection chambers. In addition, the baffle is also beneficial for reducing liquid turbulence caused by high gas production in the reaction chamber.

The design of a three-phase separator should be such that as long as the sludge layer does not expand to the settler, sludge particles or flocculent sludge can slide back into the reaction chamber. It should be recognized that sometimes the expansion of sludge into the sedimentation tank is not a bad thing. On the contrary, the expanded sludge layer existing in the settler captures and disperses the sludge particles/flocs, while also having a certain removal effect on biodegradable dissolved COD.

On the other hand, it is important to have a certain amount of free space for the expansion of the sludge layer to prevent the loss of heavier sludge under temporary organic or hydraulic load shocks. Both hydraulic and organic (gas production rate) load rates can affect the expansion of sludge layers and sludge beds.

The principle of the USAB system is based on the formation of sludge flocs with good settling performance, combined with the installation of a sludge settling system in the reactor to separate gases, liquids, and solids, forming and maintaining sludge with good settling performance (granular or flocculent sludge), which is the fundamental point for the good operation of the USAB system.

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