- 7 November 2007 -

Wastewater treatment: Advanced suspended growth technology

Activated sludge processing has been around for some time. Andrew Whittaker, Process Engineer for Severn Trent Services, explains how a modification of this procedure - advanced suspended growth technology for wastewater treatment - can meet total nitrogen standards.

The activated sludge process has been applied to sewage treatment works for a number of years. The first plants appeared in the early 1960s and they were mainly extended aeration plants or oxidation ditches. The most important advantages claimed were freedom from the fly and odour problems associated with bacteria beds and a reduced requirement for land area. However, the process is relatively sensitive and can require skilled control to maintain optimal operation and freedom from bulking. In recent years improvements in control systems have reduced concerns over operation.

Sequencing Batch Reactor (SBR) technology is a modification of the standard activated sludge process, where aeration, settlement and decanting functions are carried out within a single SBR basin. This removes the requirement for a separate clarifier. Instead, the batch system facilitates a quiescent settle period within the SBR basin and therefore settlement is not impeded by hydraulic limitations under peak flow conditions. Developments have recently taken place and in the USA in particular this process has been developed into plants that have stable operating characteristics, produce high quality effluent and low sludge yield.

The Surge Anoxic Mix (SAMT) process is a modification of a staged activated sludge treatment process, which was developed in the USA . The system is based on a sequencing batch reactor (SBR) with fixed decanter and jet aeration. Key problems associated with traditional activated sludge processes are variations in flow, reliability, sludge production, and manpower requirements. The advances offered by the SAMT technology are shown to address these issues. Using the jet aerator and fixed decanter negates the need for routine maintenance within the SBR basin, which is a concern normally expressed with the SBR process.

The development of the SAMT variation on SBR technology provides an anoxic zone upstream of the aeration tank (SBR basin) and incorporates a high recycle ratio. The SAMT tank functions as a selector, providing organism selection and floc control to ensure a well settling sludge. The anoxic zone offers good denitrification, therefore providing nitrate control and preventing a rising sludge blanket. The nitrification / denitrification processes are intensified and this helps to reduce the overall footprint, as well as considerably reducing the energy requirements of the system. Furthermore the SAMT tank acts as a balancing tank for organic equilisation, which helps to buffer incidents of shock load, therefore offering a more robust and stable process.

Severn Trent Services has been working to provide treatment solutions at a number of Waste Water Treatment Works in the UK .

Description of the SAMT process

The basis of the SAMT process is a sequencing batch reactor (SBR). The sewage is treated batch-wise, with aeration being followed by a period of quiescent settlement within the same SBR basin. Key components of the SBR are shown in Figure 2.

The key components are:

. The decanter;

. The jet aerator;

. The SAMT tank;

. The surge chamber and surge chamber weir.

Decanter

The treated effluent is removed by a unique and proven fixed decanter, which has no moving parts in the tank, therefore making maintenance easier than for other designs. When the tank is being mixed, the decanter is sealed by a positive air seal. This is very effective in preventing mixed liquor entering the decanter. After the settlement period is complete the air seal valve is opened to allow the decanter to draw from the clarified liquor above the settled sludge. The long decant weir length facilitates a low draw velocity, which does not disturb the settled sludge layer and prevents contamination of the decanted liquors. The decant outlet valve also opens and the treated effluent is discharged. The decant is terminated when the top water level is just above the lip of the decanter to prevent any scum being drawn into the effluent. At the end of the decant, the outlet valve and air seal valve are closed. This seals the air in the decanter to prevent ingress of mixed liquor.

Aeration

Aeration and mixing are provided by a jet aerator. This is an efficient aeration and mixing device. During operation mixed liquor is pumped through a series of venturi jets located along the length of the jet aerator header. Process air is entrained in the mixed liquor within the header and is sheared into small bubbles in the jets. The stream of mixed liquor and bubbles is forced into the aeration tank where mixing and aeration take place. This system of aeration is very efficient and the efficiency is not significantly reduced in dirty water conditions (Alpha factor effects are minimal). This compares favourably with fine bubble diffusers, where oxygen transfer efficiency in sewage may be less than half of that in clean water. The use of a jet aerator facilitates both aerobic and anoxic mixing, which allows simultaneous nitrification / denitrification within the SBR basin.

SAMT tank

The SAMT tank functions as both a balancing tank and an anoxic zone. It is located before the SBR tank and receives the incoming flow. When the level in the SAM tank reaches a preset level the jet aerator motive pump starts and transfers the accumulated feed to the SBR tank via the jet aerator.

Surge chamber and surge chamber weir

This is located at top water level in the SBR tank, and is connected to the SAMT tank via the surge jet. When the SBR tank is full the contents overflow into the SAMT tank via the surge chamber and surge jet. This causes very intense mixing in the SAMT tank.

Operation of the SAMT system

Settled sewage enters the SAMT reactor. This is an unaerated chamber, which will contain some activated sludge. The level in the chamber rises and at a pre-set level the combined jet motive / SBR fill pump starts and transfers the mixture from the SAMT tank to the aeration tank via the jet aerator. When the aeration tank (SBR basin) is filled, the mixed liquor overflows the surge weir and returns to the SAMT tank via the surge jet. This causes intense mixing in the SAMT tank, which then functions as an anoxic zone. The mixed liquor in the SBR basin is then subjected to alternating periods of aerobic and anoxic conditions. After a pre-set period, the pump is stopped and the mixed liquor in the aeration tank is allowed to settle. Settlement takes place in totally quiescent conditions. At the end of the settlement period the supernatant effluent is removed by the decanter. During the settlement and decanting periods the SAMT tank acts as a balancing tank for the incoming crude sewage. The SAMT tank contains activated sludge from the previous cycle and its contents are subjected to denitrification and high floc loading.

The net result of the above is as follows:

. The activated sludge is subjected to periods of high floc loading, inhibiting the growth of filamentous organisms and giving a readily settled sludge.

. The SAMT tank provides an ideal environment for denitrification, which prevents rising sludge and reduces energy costs by recovery of oxygen.

. Any scum which forms in the SBR basin (aeration tank) overflows the surge weir and is mixed in the SAMT tank. It is then broken down aerobically in the react phases.

. Settlement takes place in totally quiescent conditions. There is no danger of solids being scoured from the plant during high flow events.

In its simplest form the SAMT process consists of a single self contained package plant. For larger flows concrete walled tanks are utilised. The only moving part is the jet motive / fill pump, which is a conventional submersible pump mounted on rails. Typically two pumps are provided for each SBR stream, on a duty/standby basis.

Conclusion

In conclusion, the SAMT process is a natural development of the SBR system, providing optimised operating conditions for the activated sludge process. In particular this process is capable of producing an effluent of consistently high quality. Effluent quality is unaffected by wide variations in influent flow and loads.

There are over 200 SBR plants throughout the world which employ the fixed decanter and jet aerators. Approximately 70 of these plants have benefited by employing SAMT technology.

Contact
Andrew Whittaker
Process Engineer
www.severntrentservices.com