The process can be operated in batches, where the different conditions are all achieved in the same reactor but at different times (UNEP & MURDOCH 2004). The treatment consists of a cycle of five stages: fill, react, settle, draw and idle. During the reaction type, oxygen is added by an aeration system. During this phase, bacteria oxidize the organic matter just as in activated sludge systems. Thereafter, aeration is stopped to allow the sludge to settle. In the next step , the water and the sludge are separated by decantation and the clear layer ( supernatant ) is discharged from the reaction chamber (METCALF & EDDY 2007). Depending on the rate of sludge production, some sludge may also be purged. After a phase of idle the tank is filled with a new batch of wastewater (UNEP & MURDOCH 2004). At least two tanks are needed for the batch mode of operation as continuous influent needs to be stored during the operation phase. (Very) small systems (. serving small settlements) may apply only one tank. In this case, the influent must either be retained in a pond or continuously discharged to the bottom of the tank in order to not disturb the settling , draw and idle phases. SBRs are suited to lower flows because the size of each tank is determined by the volume of wastewater produced during the treatment period in the other tank (UNEP & MURDOCH 2004). For more information on SBR activated sludge systems, please consult WSP (2007) or . EPA (1999).
A UASB is not appropriate for small or rural communities without a constant water supply or electricity and skilled labour. It is particularly adapted for densely populated urban areas as it has low land requirements. The technology is relatively simple to design and build, but developing the granulated sludge may take several months. The UASB reactor has the potential to produce higher quality effluent than Septic Tanks , and can do so in a smaller reactor volume. Although it is a well-established process for large-scale industrial wastewater treatment and high organic loading rates up to 10 kg BOD /m3/d, its application to domestic sewage is still relatively new. It is often used for brewery, distillery, food processing and pulp and paper waste (TARE & NEMA n .y.) since the process typically removes 80 to 90% of COD . Where the influent is low-strength or where it contains too many solids, proteins or fats, the reactor may not work properly. Temperature is also a key factor affecting the performance. UASB reach high treatment levels regarding organics and the produced biogas can be used for energy conversion. Pathogens , however, as well as nutrients are not removed. Due to the low nutrient removal, the effluent is adapted for reuse in agriculture after further treatment or considering some special health protection measures.
UASB are not adapted for colder climates.