UASB Reactor

In countries with a warm climate throughout the whole year, high wastewater allow and favour an anaerobic treatment of the entire sewage flow, not only the sludge portion (TBW 2001b). Anaerobic treatment systems such as UASB (see also anaerobic digestion general) do not require an energy consuming aeration system and can be constructed much simpler than aerobic treatments. They convert the organic matter into biogas, which can be recovered. The nutrient-rich effluent can be used for agricultural irrigation (ROSE 1997). Sludge, even is partily stabilised (mineralised) and can be used as an organic soil fertiliser after composting or drying (see also planted and unplanted drying bed).

UASB reactors can treat all type of hyigh-strenght wastewater. It can be used at large-scale (e.g. agro-industrail wastes) or as decentralised treatment systems for domestic wastewaters; yet domestic treatment is still relatively new and not always successful as domestic wastewater generally lower strength.

Treatment Process

UASB Reactors are constructed out of concrete or another watertight material and can be designed in a circular or rectangular way. Wastewater is pumped from the bottom into the reactor where influent suspended solids and bacterial activity and growth lead to the formation of sludge. The sludge blanket is comprised of microbial granules (1 to 3 mm in diameter), i.e., small agglomerations of microorganisms that, because of their weight, resist being washed out in the upflow. The microorganisms in the sludge layer degrade organic compounds. As a result, gases (methane and carbon dioxide i.e. biogas ) are released. The rising bubbles mix the sludge without the assistance of any mechanical parts. Upstream velocity and settling speed of the sludge is in equilibrium and forms a locally rather stable, but suspended sludge blanket (SASSE 1998). Sloped walls deflect material that reaches the top of the tank downwards. The clarified effluent is extracted from the top of the tank in an area above the sloped walls. A gas-liquid-solids separator (GLSS) separates the gas from the treated wastewater and the sludge (ROSE 1997, SANIMAS 2005).

After several weeks of use, larger granules of sludge form which, in turn, act as filters for smaller particles as the effluent rises through the cushion of sludge. Because of the upflow regime, granule-forming organisms are preferentially accumulated as the others are washed out. Fortunately, these bacteria are also more efficient for biogas production than flocculated biomass (WENDLAND 2008).

UASB Reactors are constructed out of concrete or another watertight material and can be designed in a circular or rectangular way. Critical elements for the design of UASB reactors are the influent distribution system, the gas-solids separator, and the effluent withdrawal design. The gas that rises to the top is collected in a gas collection dome and can be used as energy (see also use of biogas) for cooking, heating or other, but scrubbing before use is required (UNEP 2004). If the biogas is converted to electricity, the heat produced as a by-product can be reused to heat the reactor, favouring anaerobic digestion .

To maintain the reactor well-mixed and allowing the formation of granules and a good contact of the active sludge blanket and the influent sewage, it is critical that the influent is equally distributed in the bottom before moving upwards (see Figure 1). Besides these design requirements, the main influencing parameters are pH, temperature, chemical oxygen demand (COD), volumetric COD loads, HRT and flow, upflow velocity, concentration of ammonia and start-up phase (TBW 2001b). 

pH Value

The pH-value needs to be between 6.3 and 7.85 (TBW 2001b) to allow bacteria responsible for anaerobic digestion to grow. The pH-value is also important because at high pH-values, ammoniac (NH₄+) dissociates to NH₃ which inhibits the growth of the methane producing bacteria. 

Temperature

For an optimal growth of these bacteria and thus a optimal anaerobic digestion, the temperature should lie between 35 to 38°C. Below this range, the digestion rate decreases by about 11% for each 1°C temperature decrease and below 15°C the process is no longer efficient (ALAERTS et al. 1990 in TBW 2001b), although bacterial activity can still be noticed at temperatures less than 10°C (TBW 2001b). 

COD Loads

Influents should have concentrations of above 250 mg COD/Lm, as for lower rates, anaerobic digestion is not beneficial. Optimum influent concentrations are above 400 mg COD/L and an upper limit is not known (TBW 2001b). 

Hydraulic Retention Time (HRT)

The hydraulic retention time (HRT) should not be less than 2 hours. Anaerobic microorganisms, especially methane producing bacteria, have a slow growth rate. At lower HRTs, the possibility of washout of biomass is more prominent (BAL & DHAGAT 2001). The optimal HRT generally lies within 2 to 20 hours (TBW 2001b).

Upflow Velocity

The upflow velocity in UASB is an important design parameter as the process plays with the balance of sedimentation and upflow (SASSE 1998). On one hand, sludge should not be washed out the reactor, and on the other hand, a minimum speed needs to be maintained to keep the blanket in suspension, and also for mixing (TBW 2001b). An upflow velocity of 0.7 to 1 m/h must be maintained to keep the sludge blanket in suspension. Primary settling is usually not required before the UASB.

The operators should take proper health and safety measures while working in the plant, such as adequate protective clothing. Effluent and sludge still pose a health risk and should not be directly handled.  

However as nitrogen and phosphorous are not effectively reduced in anaerobic technologies, this primary treatment approach is particularly apt when used in parallel with agriculture or aquaculture (ROSE 1997). As in all anaerobic treatment processes, sludge is stabilised and if not used in agriculture, has good dewatering characteristics and can be treated in thickening ponds and drying beds or by composting before safe reuse or disposal (ROSE 1997; WSP 2008). The effluent can be used in fertigation but as it pathogen removal is only low the WHO 2006 guidelines must be considered for agricultural application.

To meet higher effluent standards, the effluent may be post-treated in pond systems (e.g. wastewater stabilisation ponds), constructed wetlands or anaerobic treatment units (e.g. trickling filter). The most common post-treatment alternatives for effluents are maturation ponds (see also waste stabilisation ponds where nutrients are further reduced, their primary function however being pathogen removal (TBW 2001b).

The significantly lower level of technology required by the UASB process in comparison with conventional advanced aerobic processes means that they are also cheaper in construction and maintenance. Capital costs for construction can be estimated as low to medium and comparable to baffled reactors (SANIMAS 2005). Operation costs are low, as usually no costs arise other than desludging costs and the operation of feeding pump (SANIMAS 2005).

The UASB is a Centralized Treatment technology that must be constructed, operated and maintained by professionals. A skilled operator is required to monitor the reactor and repair parts, e.g., pumps, in case of problems. 

UASB reactors require several months to start up. The sludge not only needs to form but also needs to adapt to the characteristics of the specific wastewater (TBW 2001b). As domestic or municipal wastewater already contains the composition of nutrients and micronutrients required for bacterial activity and growth, they are generally less problematic than industrial wastewaters. High organic loading in connection with lower hydraulic loading rates quicken the granulation process in the starting phase (SASSE 1998).

To keep the blanket in proper position, the hydraulic load must correspond to the upstream velocity and must correspond to the organic load. The latter is responsible for development of new sludge (SASSE 1998). This means that the flow rate must be controlled and properly geared in accordance with fluctuation of the organic load.

Sludge production is relatively low (WSP 2008). Desludging is infrequent and only excess sludge is removed every 2 to 3 years (TILLEY et al. 2014) .