Sequencing Batch Reactor (SBR) Upgrading for domestic wastewater treatment

The Sequencing Batch Reactor (SBR) is a different configuration of the conventional activated sludge systems, in which the process can be operated in batches, where the different conditions are all achieved in the same reactor but at different times. 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 oxidise 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 (ASANO et al. 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 and MURDOCH UNIVERSITY 2004). At least two tanks are needed for the batch mode of operation as continuous influent needs to be stored during the operation phase. Small systems 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 not to 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 and MURDOCH UNIVERSITY 2004). Pollutants removal efficiency: BOD5: 95%, COD: 90%, TSS: 95%, Pathogen: N/A.

Figure 1: Sequencing Batch Reactor. Source: Own illustration (2015) 

Mechanical equipment, such as pumps, aerates and mixers, require continuous maintenance and control, and supply of oxygen and sludge is essential (WSP 2008). Control of concentrations of sludge and oxygen levels in the aeration tanks is required and technical appliances (e.g. pH-meter, temperature, oxygen content, etc.) need to be maintained carefully. To make sure that optimal living conditions for the required bacteria are guaranteed and a satisfying effluent quality is met, the influent as well as the effluent should be supervised and controlled constantly (e.g. by a centralised computerised monitoring system).

Construction and maintenance costs are high as activated sludge treatment units are highly mechanised. Operation costs have been usually expensive due to the requirement of permanent professional operation, high electricity consumption (pumping and aeration) and costly mechanical parts (SANIMAS 2005), but in the last years the development of cheaper and more energy efficient equipment has reduced significantly the operational cost.

The following are some of the known experiences in India with regard to the use of Sequencing Batch Reactors (SBR):

• Mundhwa Sewage Treatment Plant, Pune, India. The results are yet to be available as a published source, but it is stated that the raw BOD, SS and TKN of 205, 262 and 45 mg/l are reduced to less than 10 mg/l with phosphorous being 2.3 in the inlet and 0.7 in the outlet. The raw sewage MPN Faecal coliform of 230,000/100 ml was reduced to 7,500/100 ml but was still much higher than the NRCD limitations of 1000/100 ml.
• Sewage Treatment Plant (SBR) Kalpataru Construction Overseas, Mumbai. Capacity 65 m³/day. For the Commercial and residential building at the Camlin compound in Andheri, Mumbai. Sequential Batch Reactor (SBR) was chosen because of its compact footprint and ability to achieve enhanced nutrient removal. Also the output water needed to be used for a multiple of uses – right from toilet flushing, landscaping and cooling towers.
• Magarpatta City: SBR treating 3,000 m³/d of wastewater produced by the city. The treated water is used for lake recharge and secondary uses.
• Noida City: SBR built in Noida City to treat 35,000 m³/d of wastewater corresponding to the current and the expected wastewater to be produced by some areas of the city in the next years.

Fill-and-draw batch processes similar to the SBR are not a recent development as commonly thought. Between 1914 and 1920, several full-scale fill-and draw systems were in operation. Interest in SBRs was revived in the late 1950s and early 1960s, with the development of new equipment and technology. Improvements in aeration devices and controls have allowed SBRs to successfully compete with conventional activated sludge systems.

The Sequencing Batch Reactor (SBR) process has been successfully applied to more than 1,300 plants in the U.S., Canada, and Europe within the last 25 years. In particular, the number of SBR plants in North America is growing rapidly. Many of these facilities have been constructed for small communities, producing less than 4,500 m³/d of wastewater, although larger plants (up to 870,000 in Dublin, Ireland) have used SBR technology with similar effluent quality results (TOPRAK 2005).

Further examples of the compact design of the SBR process can be found in Bangkok, Thailand (average daily flow of 200,000 m³/d and peak flow of 500,000 m³/d) utilise tanks stacked on 4 levels to achieve a treatment plant footprint of 6,000 m².