UASB Reactors: Granular Sludge, the Three-Phase Separator, and Where Designs Actually Fail

How the reactor is supposed to behave

Wastewater enters at the bottom of a UASB reactor through a distribution system designed to spread it evenly, then travels upward through an anaerobic sludge bed. At the base sits a highly active sludge layer at high concentration, up to 100-150 g/L. Above it forms a suspended sludge zone, and at the top a three-phase separator splits gas, liquid and solid. Biogas leaves from the roof, separated sludge slides back into the suspended zone, and clarified effluent exits from the clarification zone at the top.

Because the reactor grows a large inventory of anaerobic granular sludge, it carries a very high loading. For typical high-strength organic wastewater at around 30°C water temperature, loading reaches 10-30 kg COD/(m3·d). None of that happens by accident. Three conditions have to hold together: well-settling granular or flocculent sludge forms inside the vessel; uniform gas production and influent flow generate the mixing; and a properly designed three-phase separator keeps the well-settling sludge where it belongs. When all three hold, the reactor needs no mechanical mixing and rides out shocks in load, temperature and pH.

Why granulation changes the economics

• Biomass: granulation lifts average sludge concentration above 50 g VSS/L, with sludge age generally beyond 30 days.
• Retention: hydraulic retention time is correspondingly short.
• Loading: volumetric loading is very high.
• Range: suitable for high and medium strength industrial wastewater, and for low-strength municipal sewage as well.
• Layout: biological reaction and settling separation share one compact structure, with no packing to buy or install.
• Mixing: upward water flow and rising biogas do the mixing, so no mixer is needed.
• Build: simple construction, convenient operation, high volumetric utilisation.

The reactor integrates a bioreactor and a settling tank in one shell. Its parts are the influent distribution system, the reaction zone, the three-phase separator, the effluent system, the gas chamber, the scum collection system and the sludge discharge system. The influent distribution system does two jobs, spreading flow across the whole floor and providing hydraulic mixing, and getting it right is one of the keys to efficient operation. Everything downstream depends on it.

Inside the reaction zone

The reaction zone divides into a sludge bed and a suspended sludge zone. The bed concentrates most of the active granular sludge and does most of the degradation; the suspended zone holds flocculent sludge. In the bed, sludge concentration (MLSS) generally runs 40-80 g/L, and 70%-80% of that biomass is active bacteria rather than inert material. Granules in normal operation measure 0.5-5.0 mm, settle well, show a typical settling velocity of 1.2-1.4 cm/s and a sludge volume index (SVI) of 10-20 mL/g. Bacilli, cocci and filamentous bacteria make up most of the population. The bed occupies only about 30% of the reactor volume yet degrades 70%-90% of all organic matter removed in the vessel, which is the single most striking number in the whole design.

Extracellular polymer is the glue. On the surface and inside the granules you can see transparent, shiny, mucilaginous material, mainly polysaccharides, proteins and glucuronic acid, and its presence is what keeps the granule stable under shear.

The suspended sludge layer sits above, roughly 70% of total volume, at a lower concentration of 15-30 g/L and made of highly flocculent, generally non-granular sludge. Its settling velocity is markedly lower than granular sludge and its SVI generally sits between 30-40 mL/g. Bubbles rising from the bed keep it mixed without any mechanical help. Flocculent concentration decreases from bottom to top, and the layer handles 10%-30% of the reactor’s organic degradation.

The separator decides the outcome

The three-phase separator usually sits at the lower part of the settling zone, sometimes at the top, and is built from a settling zone, a reflux gap and a gas chamber. It routes biogas to the gas chamber, treated effluent to the outlet, and solid particles back to the reaction zone. Anaerobic organisms grow slowly and the reaction makes a great deal of gas, so a poorly performing separator washes sludge out with the effluent, drops the reactor’s sludge concentration, and eventually fails the reactor completely. In function the separator is the secondary clarifier of conventional treatment and the sludge return line rolled into one, which is why its design deserves as much attention as the biology.

Around it sit the supporting parts. The effluent system collects flow uniformly after the settling zone and discharges it. The gas-collection chamber, also called the gas hood, captures biogas from the anaerobic reaction. The scum collection system clears the liquid surface of the settling zone and gas chamber. The sludge discharge system removes surplus sludge evenly. The settling zone itself lets solids carried upward settle back down the inclined wall into the reaction zone, and by setting its water level it also protects the effective height of the gas-collection space.

Structure and granule types

Open UASB reactors leave the top unsealed, or cover it loosely to contain odour, and suit medium and low strength wastewaters; they are simple to build and easy to install and maintain. Closed reactors seal the top, forming a gas chamber between liquid level and tank roof, and are used for high-strength wastewater where gas capture matters.

Granules appear oval, spherical or filamentous. Average diameter is 1 mm, generally 0.1-2 mm and at most 3-5 mm. Bottom granules usually grow on an inorganic core wrapped in biofilm, black core with a greyish-white, pale yellow or dark green surface, while granules higher in the reaction zone carry relatively higher volatility. The granules are soft, with a certain toughness and viscosity.

Three types are commonly distinguished. Type A is dominated by Methanosarcina barkeri, often with filamentous Methanobacterium wrapped around the outside; it is dense and small, about 0.1 mm, and forms readily when acetic acid concentration in the reactor is high, gradually converting to type B once that concentration drops. Type B is dominated by filamentous Methanobacterium, also called bacillary granules, with a regular surface wrapped by filamentous methanogens of various morphologies; it turns up with very high frequency in all sorts of UASB reactors and measures about 1-3 mm at a density of 1.033-1.054 g/cm3. Type C consists of spherical agglomerates formed by loose filamentous bacteria entangled and adhering to inert particles, also called filamentous granules; these are large and heavy, generally 1-5 mm, with a specific gravity of 1.01-1.05 and a settling velocity generally of 5-10 mm/s, and they form when an appropriate amount of suspended solids is present.

Bacteria inside a granule are layered, hydrolytic fermentative bacteria outside and methanogens inside, which is what makes the granule an ecosystem rather than a lump of sludge. That arrangement supports interspecies hydrogen and interspecies acetate transfer and is the reason granular sludge is so active. Building the inventory takes time: cultivating high-concentration, highly active granular sludge in a UASB reactor generally takes 1-3 months, across start-up, granular-sludge formation and granular-sludge maturation.