Activated Sludge

An activated sludge system is a biological wastewater treatment process in which a mixed culture of microorganisms is maintained in suspension in an aerated reactor and used to biodegrade organic matter and oxidise ammonia from the wastewater. The process relies on continuous aeration to supply oxygen to the microbial community, a secondary clarifier to separate treated effluent from the biological sludge, and a sludge recycle system to maintain the required biomass concentration in the reactor. Activated sludge is the most widely deployed secondary wastewater treatment technology globally, used in treatment plants ranging from small community systems to large municipal and industrial facilities.

IWTE designs conventional plug flow and completely mixed activated sludge systems with separate aeration tanks and secondary clarifiers, oxidation ditch systems using a continuous loop reactor for extended aeration with inherent simplicity and good sludge stabilisation, sequencing batch reactors that perform biological treatment and settling in a single vessel using timed operational cycles, and modified activated sludge configurations incorporating anoxic and anaerobic zones for biological nutrient removal. The configuration is selected based on the treatment objectives, available footprint, operational complexity preferences and the variability of the influent flow and load.

Nitrogen removal in activated sludge systems requires two biological steps. Nitrification converts ammonia to nitrate in the aerobic aeration zone, carried out by slow-growing autotrophic bacteria that require adequate dissolved oxygen and a sufficiently long sludge retention time. Denitrification then converts nitrate to nitrogen gas in an anoxic zone where dissolved oxygen is absent and heterotrophic bacteria use nitrate as an electron acceptor in the presence of organic carbon. Pre-anoxic configurations recycle mixed liquor from the aerobic zone back to an anoxic zone ahead of aeration, achieving total nitrogen removals of up to 85 percent.

An oxidation ditch is an extended aeration activated sludge system in which the mixed liquor circulates continuously around an oval or racetrack-shaped reactor channel, aerated by surface aerators or submerged diffusers. The extended aeration configuration operates with a long sludge retention time of 15 to 25 days, achieving complete nitrification, partial denitrification and a well-stabilised waste sludge with low volatile solids content. Oxidation ditches are recommended for applications where operational simplicity, robust nitrification and reduced sludge processing requirements are priorities, and are a well-proven technology for small to medium municipal treatment works.

A sequencing batch reactor (SBR) is an activated sludge system that performs all treatment steps including mixing, aeration, settling and decanting in a single tank operating in timed cycles. A typical SBR cycle comprises a fill phase, react phase with aeration and mixing, settle phase, decant phase and idle phase. SBRs eliminate the need for a separate secondary clarifier, offer operational flexibility to adjust cycle times in response to flow variation and are well suited to sites with limited footprint or variable influent flow patterns. IWTE designs SBR systems for municipal and industrial applications with fully automated PLC cycle control.

Activated sludge systems generate a continuous stream of waste activated sludge that must be processed and disposed of or reused. Waste sludge is typically thickened to reduce volume, then stabilised through aerobic or anaerobic digestion to reduce volatile solids and pathogen content, and dewatered by centrifuge, belt press or filter press to produce a cake for disposal or agricultural land application as biosolids. IWTE designs sludge handling systems as an integral element of the overall activated sludge plant, sizing thickening, digestion and dewatering equipment to match the waste sludge production rate at design loading.

Conventional secondary activated sludge treatment achieves BOD below 20 mg/L and TSS below 30 mg/L, meeting standard secondary treatment discharge standards. With biological nutrient removal configured into the process, total nitrogen can be reduced to below 10 mg/L and total phosphorus to below 1 mg/L. Further effluent quality improvement for stringent discharge or reuse standards can be achieved through downstream tertiary filtration, membrane polishing and disinfection, which IWTE designs and supplies as part of the complete treatment plant scope.

Yes. IWTE provides complete activated sludge system design and supply encompassing process design, aeration tank sizing, aeration system design and supply, secondary clarifier design, sludge recycle and wasting system, sludge handling equipment, PLC and SCADA control, effluent quality monitoring and commissioning. The system is engineered as an integrated scope with a single point of responsibility for performance. IWTE also provides operator training, process optimisation and ongoing service support for all activated sludge systems we supply.