Upflow Anaerobic Sludge Blanket (UASB): Introduction to a Core Wastewater Treatment Process
When the reactor is in operation, the wastewater enters the bottom of the reactor through uniform water distribution and passes upward through the anaerobic sludge bed reactor. At the bottom of the reactor there is a high-concentration (up to 100-150 g/L), highly active sludge layer.
Above the sludge layer a suspended sludge zone is formed; the upper part of the reactor is equipped with a three-phase separator that completes the separation of gas, liquid, and solid phases; the separated biogas is led out from the top, the separated sludge automatically slides back down to the suspended sludge zone, and the effluent flows out from the clarification zone. Because a large amount of anaerobic granular sludge can be cultivated inside the reactor, it has a very high loading capacity; for general high-concentration organic wastewater, when the water temperature is around 30°C, the loading can reach 10-30 kg COD/(m3·d).
Successful UASB operation relies on three important prerequisites: ① well-settling granular or flocculent sludge is formed inside the reactor; ② the natural mixing effect produced by uniform gas production and water inflow; ③ a properly designed three-phase separator that retains well-settling sludge within the reactor. The formation of a good granular sludge bed yields high organic loading and removal rates, requires no mixing, and adapts to load shocks as well as variations in temperature and pH.
The UASB reactor has the following main characteristics:
① Granulation raises the average sludge concentration in the reactor to above 50 g VSS/L, and the sludge age is generally over 30 days;
② The hydraulic retention time of the reactor is correspondingly short;
③ The reactor has a very high volumetric loading;
④ It is suitable not only for high- and medium-concentration organic industrial wastewater but also for low-concentration municipal sewage;
⑤ The UASB reactor integrates biological reaction and settling separation in a compact structure;
⑥ No packing needs to be installed, saving cost and improving volumetric utilization;
⑦ Generally no mixing equipment is needed, as the upward water flow and the upward airflow from biogas provide the mixing action;
⑧ Its construction is simple and operation is convenient.
As shown in the figure, the UASB reactor integrates a bioreactor and a settling tank into one compact anaerobic reactor. Its main components include the influent distribution system, reaction zone, three-phase separator, effluent system, gas chamber, scum collection system, and sludge discharge system.
The influent distribution system is located at the bottom of the reactor and serves two main functions: ① to distribute the wastewater evenly across the entire bottom of the reactor; ② to provide a certain degree of hydraulic mixing. An effective influent distribution system is one of the keys to ensuring efficient UASB operation.
The reaction zone is the main site of biochemical reaction in the UASB reactor and is further divided into the sludge bed and the suspended sludge zone. The sludge bed zone concentrates most of the highly active granular sludge and is the main site of organic matter degradation; the suspended sludge zone is where flocculent sludge is concentrated.
The sludge bed is located at the bottom of the entire UASB reactor; it has a very high sludge biomass, with a sludge concentration (MLSS) generally of 40-80 g/L. The sludge in the sludge bed consists of granular sludge in which the active biomass (or bacteria) accounts for over 70%-80%. In normal operation, the granular sludge in the UASB reactor generally has a particle size of 0.5-5.0 mm, excellent settling performance, a typical settling velocity of 1.2-1.4 cm/s, and a typical sludge volume index (SVI) of 10-20 mL/g. The microbial composition of the granular sludge is relatively complex, mainly bacilli, cocci, and filamentous bacteria. The volume of the sludge bed generally accounts for about 30% of the total UASB reactor volume, yet it plays an extremely important role in the overall treatment efficiency of the reactor, degrading 70%-90% of all organic matter decomposed in the reactor.
Extracellular polymer is another important component; on the surface and inside of the granular sludge one can generally see transparent, shiny, mucilaginous substances, mainly polysaccharides, proteins, and glucuronic acid; their presence helps maintain the stability of the granular sludge.
The settling zone is located at the top of the UASB reactor and serves to let solid particles (mainly flocculent sludge from the suspended sludge layer) carried upward by the water flow settle down in the settling zone and slide down along the inclined wall at the bottom of the settling zone back into the reaction zone (including the sludge bed and suspended sludge layer), so as to prevent sludge loss while maintaining the sludge concentration in the bed. Another function of the settling zone is to ensure the effective height of the gas-collection chamber of the entire reactor by reasonably adjusting the water level height of the settling zone, preventing damage to the gas-collection space.
The three-phase separator is important equipment in the UASB reactor, generally installed at the lower part of the settling zone but sometimes placed at the top of the reactor. It consists of a settling zone, a reflux gap, and a gas chamber. Its main function is to separate the three phases: gas (biogas produced during the reaction), solids (sludge in the reactor), and liquid (wastewater being treated). To ensure normal UASB operation, besides cultivating highly settling, highly active granular sludge, the separation performance of the three-phase separator is also decisive to UASB success. Its function is to direct the biogas into the gas chamber, the treated effluent into the effluent zone, and the solid particles into the reaction zone. Because anaerobic organisms proliferate slowly and the anaerobic reaction produces large amounts of gas, poor three-phase separator performance will cause large amounts of sludge to be carried away with the effluent, reducing the sludge concentration in the reactor and eventually leading to complete UASB failure. Having a three-phase separator is also one of the main features of the UASB anaerobic wastewater treatment process; it is equivalent to the secondary settling tank in conventional wastewater treatment and simultaneously provides sludge reflux. Therefore, the rational design of the three-phase separator is an important element in ensuring normal operation.
The main function of the effluent system is to uniformly collect the effluent after it has passed through the settling zone and discharge it from the reactor.
The gas-collection chamber, also called the gas hood, is a component of the three-phase separator whose main function is to collect the biogas produced by the anaerobic reaction.
The main function of the scum collection system is to remove scum from the liquid surface of the settling zone and the gas chamber.
The main function of the sludge discharge system is to uniformly remove the excess sludge from the reactor.
(1) Structural forms of the UASB
Depending on the wastewater quality, the structure of the UASB reactor differs; the main forms are the open UASB reactor and the closed UASB reactor.
① Open UASB reactor
Its top is left unsealed, or only covered with a loosely sealing plate (mainly to prevent odor release); it is mostly used for treating medium- and low-concentration organic wastewater, as shown in Figure 9-2(a). Its construction is relatively simple and easy to install and maintain.
② Closed UASB reactor
Whenever the top is sealed with a cover, a gas chamber is formed between the liquid level and the tank roof inside the UASB reactor; it is mainly suitable for the treatment of high-concentration organic wastewater.
1) Properties and formation of granular sludge
The ability to form well-settling, highly active granular sludge inside the reactor is an important characteristic of the UASB reactor; the formation and maturation of granular sludge is also a prerequisite for ensuring efficient and stable UASB operation.
① Appearance of granular sludge:
It may be oval, spherical, filamentous, etc.;
its average diameter is 1 mm, generally 0.1-2 mm, with a maximum of 3-5 mm;
the granular sludge at the bottom of the reaction zone mostly uses inorganic particles as a core wrapped by a biofilm; the core of the granules is mostly black, while the surface of the biofilm appears grayish-white, pale yellow, or dark green, etc.; the granular sludge in the upper part of the reaction zone has a relatively higher volatility; the granular sludge is soft, with a certain toughness and viscosity.
Various microorganisms, inorganic minerals, and organic extracellular polymers
Microorganisms: hydrolytic fermentative bacteria, hydrogen-producing acetogenic bacteria, and methanogens
The suspended sludge layer is located above the sludge bed and accounts for about 70% of the total UASB reactor volume; its sludge concentration is lower than that of the sludge bed, typically 15-30 g/L, and consists of highly flocculent sludge, generally non-granular. Its settling velocity is significantly lower than that of granular sludge, with a sludge volume index generally between 30-40 mL/g; the bubbles rising from the sludge bed provide good mixing of this layer. The flocculent sludge concentration in the suspended sludge layer decreases gradually from bottom to top, and this layer undertakes 10%-30% of the organic matter degradation of the entire UASB reactor.
2) Types of granular sludge
In this granular sludge the methanogenic bacteria are dominated by Methanosarcina barkeri, often with filamentous Methanobacterium wrapped around the outside; it is relatively dense, with a very small particle size of about 0.1 mm.
Type-B granular sludge is dominated by filamentous Methanobacterium, also called bacillary granules; its surface is regular, wrapped around by filamentous methanogens of various morphologies; it appears with very high frequency in various UASB reactors; its density is 1.033-1.054 g/cm3 and particle size is about 1-3 mm.
Type-C granular sludge is spherical agglomerates formed by loose filamentous bacteria entangled and adhered to inert particles, also called filamentous granules; Type-C granules are large and heavy, generally 1-5 mm in size, with a specific gravity of 1.01-1.05 and a settling velocity generally of 5-10 mm/s.
When the acetic acid concentration in the reactor is high, Type-A granular sludge is easily formed; after the acetic acid concentration drops, Type-A gradually transforms into Type-B; when an appropriate amount of suspended solids is present, Type-C is easily formed.
3) Bioactivity of granular sludge
The bacteria in granular sludge are distributed in layers: hydrolytic fermentative bacteria dominate the outer layer, while methanogens are in the inner layer; granular sludge is essentially an ecosystem of mutual dependence and optimization between organisms and environmental conditions. Various bacteria form a very complete food chain, facilitating interspecies hydrogen and interspecies acetate transfer, hence its high activity.
4) Cultivation conditions for granular sludge
Cultivating high-concentration, highly active granular sludge in a UASB reactor generally takes 1-3 months; it can be divided into three stages: start-up, granular-sludge formation, and granular-sludge maturation.
