Activated Sludge

The term activated sludge refers to suspended aerobic sludge consisting of flocs of active bacteria, which consume and remove aerobically biodegradable organic substances from screened or screened and pre-settled wastewater. Activated sludge systems can treat blackwater, brownwater, greywater, faecal sludge and industrial wastewater as long as the pollutants to be treated are biodegradable.

Activated sludge reactors are aerobic suspended-growth type processes (in opposition to fixed-film or attached-growth processes (e.g. RBCs). Different configurations of the activated sludge process can be employed to ensure that the wastewater is mixed and aerated in an aeration tank. Aeration and mixing can be provided by pumping air or oxygen into the tank or by using surface aerators. The microorganisms oxidize the organic carbon in the wastewater to produce new cells, carbon dioxide and water. Although aerobic bacteria are the most common organisms, facultative bacteria along with higher organisms can be present. The exact composition of bacteria depends on the reactor design, environment, and wastewater characteristics.

The flocs (agglomerations of sludge particles), which form in the aerated tank, can be removed in the secondary clarifier by gravity settling. Some of this sludge is recycled from the clarifier back to the reactor. The effluent can be discharged into a river or treated in a tertiary treatment facility if necessary for further use.

In the view of reuse of the effluent in agriculture, it is not beneficial to remove all nutrients while standards for pathogen removal are barely met.

As the system is also of high complexity and strongly mechanised, it is mainly adapted for centralised systems where energy, mechanical and technical spare equipment and skilled staff are available.

Activated sludge processes are one part of a complex wastewater treatment system (U.S. EPA 2002) . They are usually used following primary treatment ( including screening that removes settleable solids) , include one or more main aerated treatment chambers, aeration devices, a device for appropriate mixing to keep the sludge in suspension, a secondary clarifier to separate the biomass from the treated effluent and collect settled biomass, generally a non-linear, highly complex circulation regime (e.g. recirculation loops, by-passing etc.) and are sometimes followed by a final polishing step (see tertiary filtration and disinfection). The biological processes that occur are effective at removing soluble, colloidal and particulate materials. The reactor can be designed for biological nitrification and denitrification, as well as for biological phosphorus removal.

The design must be based on an accurate estimation of the wastewater composition and volume. Treatment efficiency can be severely compromised if the plant is under- or over-dimensioned. Depending on the temperature, the solids retention time (SRT) in the reactor ranges from 3 to 5 days for BOD removal, to 3 to 18 days for nitrification.

The excess sludge requires treatment to reduce its water and organic content and to obtain a stabilized product suitable for end-use or final disposal. It is important to consider this step in the planning phase of the treatment plant).

Large amounts of injected oxygen allow maintaining aerobic conditions and optimally mixing the active biomass with the wastewater to be treated. To maintain a relatively high amount of active microorganisms useful in removing organic substances from the wastewater, the sludge is separated from the effluent by settling in a secondary clarifier (UNEP 2004) or by membrane filtration and kept in the process by recirculation to the aeration tank. Several modifications of this basic process have been developed, including different aeration devices, different means of sludge collection and recycling to the aeration tank or primary clarifier, and process enhancement trough the addition of an inert media area on which biofilm can grow (combined fixed-film/suspended-growth process).

Although aerobic bacteria are the most dominant microorganisms in the process, other aerobic, anaerobic and/or nitrifying bacteria along with higher organisms can be present. Thus, besides the removal of organic matter, nutrients (organic ammonia, phosphorus) can also be removed biologically by nitrification/denitrification and biological uptake of phosphorus. The exact composition of microorganisms depends on the reactor design, the environment and the wastewater characteristics (TILLEY et al. 2008). To achieve optimal conditions for both, organic and nutrients removal, a sequences of changing aerobic and anaerobic chambers are used.

Detailed Treatment Process

After screening sand and similar heavy particles are removed next in a grit chamber where they settle to the ground. This chamber only wants to remove coarse grit and the wastewater spends only a relatively short period (some minutes) in it (UNEP & MURDOCH 2004). Smaller solids are removed in a settling or sedimentation tank. In this unit, the wastewater spends more time (about one hour) to allow for a good separation. The sludge from this mechanical primary treatment (including screening and settling in the grit chamber and the sedimentation tank) is called primary sludge and, as all excess sludge, requires an advanced further treatment chain.

After this primary treatment, the main unit containing the activated sludge follows. The pre-treated wastewater is mixed with the concentrated underflow activated sludge from the secondary clarifier in an aerated tank. Aeration is provided either by mechanical surface agitators or by submerged diffusers of compressed air (WSP 2008). Aeration provides oxygen to the activated sludge and at the same time thoroughly mixes the sludge and the wastewater (UNEP & MURDOCH 2004). During aeration and mixing, the bacteria form small clusters or flocs (TILLEY et al. 2008). Under these conditions, the bacteria in the activated sludge degrade the organic substances in the wastewater. They use the organic substance for energy, growth and reproduction. The end products are carbon dioxide (CO2), water (H2O) and new cells.

After a few hours in the aeration chamber, the mixture then enters the secondary settling tank (clarifier), where the flocculated microorganisms settle and are removed from the effluent stream. The settled microorganisms (the activated sludge) are then recycled to the head end of the aeration tank to be mixed again with wastewater and continue to grow and form new sludge and to degrade organics. To maintain an optimal amount of sludge in the system, the rate of recirculation of settled sludge varies from 20 to 100%. Excess sludge produced each day (waste activated sludge) must be processed in a further treatment chain together with the sludge from the primary treatment facilities. A conventional excess sludge treatment chain consists in anaerobic digestion, thickening, incineration and the safe disposal, e.g. in a landfill. A more sustainable way would be to compost the sludge (either before or instead of digestion) in order to reuse the nutrients in agriculture.

Hydraulic retention times in the whole systems range from some hours up to several days for the liquid phase. Proceeding of excess sludge can take somehow longer depending on the type of thickening and anaerobic digestion applied. The effluent from a properly designed and operated activated-sludge plant is of high quality, usually having BOD and TSS concentrations equal to or less than 10 mg/L (CRITES & TCHOBANOGLOUS 1998). The removal of both, biological oxygen demand (BOD) and suspended solids (TSS) generally lies within 80 to 100% depending on the influent concentrations, the system set-up and temperature (UNEP 2004; SANIMAS 2005; WSP 2008).

Nutrients such as nitrogen and phosphorus are also removed in activated sludge process but require a set-up of different aerated and non-aerated chambers in hybrid activated sludge systems. Biological removal of nitrogen is first achieved by the transformation of organic nitrogen into ammonia, followed by the aerobic conversion of ammonia (NH4+) to nitrite (NO2-) and then nitrate (NO3-) and the anaerobic transformation of nitrate to gaseous nitrogen (N2), which is then released to the atmosphere. The transformation of ammonia to nitrate via an intermediate step of nitrite is called nitrification. The transformation of nitrate to gaseous nitrogen is referred as denitrification. Thus, a combination of both, aerobic and anaerobic (anoxic) processes are required to achieve complete elimination of nitrogen from the wastewater. In many activated sludge treatment systems, An anaerobic tank is either integrated after the aerated basin and before clarification (post-denitrification); or just before the aeration tank (pre-denitrification). In the case of pre-denitrification, nitrification takes place in the aerated tank after the aerobic pre-tank. Denitrification only occurs when the effluent from the aerated tank, containing nitrite, is re-circulated like the sludge.

The removal of phosphorus in activated sludge systems can be done chemically or biologically. Biological elimination of phosphorus in conventional wastewater treatment system occurs through the uptake of phosphorus by some bacterial cells. However, only little phosphorus can be removed this way, as the phosphorus mass fraction in volatile sludge is only about 2.5% (HAANDEL&LUBBE 2007). This results in an effluent concentration of about 2 to 7 mg P/L for municipal sewage with a COD concentration of 500 mg/L (HAANDEL&LUBBE 2007). However, it will in general be required to lower the effluent phosphorus concentration to a value ≤ 1 mg P/L.

Another biological process is the enhanced biological phosphorus removal. Enhanced biological phosphorus removal is based on the cultivation of some special phosphorus accumulating bacteria, which, compared to 2.5% P in conventional activated sludge, can lead to up to 38% of P accumulation in the sludge (HAANDEL&LUBBE 2007). 

When both nitrogen and phosphorus are to be removed, the combination becomes even more complex. Enhanced biological phosphorus removal requires generally an anaerobic stage (for PAO cultivation), an anoxic stage (for denitrification) and an aerobic stage (for nitrification and phosphorus accumulation) in series.

Nowadays, activated sludge systems, where valuable nutrients (phosphorus and nitrogen) and organic matter are incinerated instead of re-circulated to the food production in agriculture are not perceived as sustainable any more. The introduction of nitrogen removal into an activated sludge plant increases the reactor volume significantly and leads to higher energy consumption of approximately 60 to 80% for aeration (MAURER 2003). The elimination of phosphorus requires either the addition of chemicals and subsequent disposal of inorganic sludge or an increase of complexity and reactor volume for enhanced biological phosphorus removal. 

To achieve specific effluent goals for BOD, nitrogen and phosphorus, different adaptations and modifications have been made to the basic activated sludge design. Well known modifications include sequencing batch reactors (SBR), oxidation ditches, deep shafts, extended aeration, moving beds and membrane bioreactors.  

Sequential Batch Reactors (SBRs)

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 (e.g. 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 U.S. EPA (1999).

Oxidation Ditches

Oxidation ditches are large round or oval ditches (channel reactors) with one or more horizontal aerators to guarantee oxygen supply, and to mix and move the content around the ditch. Screened influent enters the oxidation ditch, is aerated and circulates at about 0.25 to 0.35 m/s (SANIMAS 2005). Operation can be continuous or intermittent. Primary sedimentation is usually not required, but secondary sedimentation tanks are generally used. The required treatment volume per capita is about 1 m³ (SANIMAS 2005). Oxidation ditches are suitable for areas where land availability is high. They have the advantage that they are relatively easy to maintain and are resilient to shock loads that often occur in smaller communities (e.g. at breakfast time and in the evening). Typical hydraulic retention time is between 24 to 48 hours with a sludge age of 12 to 20 days (Wikipedia 2010). For more information on oxidation ditches, please refer to U.S. EPA (2000), WSP (2007) or WSP (2008).

Deep Shafts

Where land is in short supply, sewage may be treated by injection of oxygen into a pressured return sludge stream, which is injected into the base of a deep columnar tank buried in the ground. This type of activated sludge reactor is called deep shaft. Such shafts may be up to 100 m deep. As the sewage rises the oxygen forced into solution by the pressure at the base of the shaft breaks out as molecular oxygen. This provides a highly efficient source of oxygen for the microorganisms contained in the activated sludge. The rising oxygen and injected return sludge provide the physical mechanism for mixing. Mixed sludge and wastewater influent is decanted at the surface and separated into supernatant and sludge components. The efficiency of deep shaft treatment can be high but they require skilled professionals for construction, operation and maintenance; and additionally a large amount of energy (adapted from Wikipedia (2012)).

Operation and maintenance of activated sludge system is generally carried out by skilled labourers, which should be sufficiently well trained regarding any health risks. Because of space requirements and odours, Centralized Treatment facilities are generally located in the periphery of densely populated areas. Although the effluent produced is of high quality, it still poses a health risk and should not be directly handled but undergo an appropriate disinfection treatment before discharge (e.g. UV-light, chlorination) . In the excess sludge pathogens are substantially reduced, but not eliminated.

Excess sludge contains even higher amounts of microorganisms, as well as phosphorus and heavy metals if they are present in the influent wastewater. This can be the case because the wastewater treated in such reactors generally comes from an array of different sources (domestic, industrial and stormwater), which makes it a hard-to-treat mixture and therefore also a difficult resource to recycle. Hence, the sludge generally needs to be thickened and incinerated with the ashes being stored in a controlled landfill.

Highly trained staff is required for maintenance and trouble-shooting. The mechanical equipment (mixers, aerators and pumps) must be constantly maintained. A continuous 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. As well, the influent and effluent must be constantly monitored and the control parameters adjusted, if necessary, to avoid abnormalities that could kill the active biomass and the development of detrimental organisms which could impair the process (e.g., filamentous bacteria).

Two of the most serious problems with the activated-sludge process are (1) a phenomenon known as bulking, in which the sludge from the aeration tank will not settle, and (2) the development of biological surface foam (CRITES & TCHOBANOGLOUS 1998). Bulking can be caused either by organisms that grow in filamentous form instead of flocs and will not settle, or the growth of microorganisms that incorporate large volumes of water into their cell structure, making their density near that of water. Foaming is caused most often by the excessive growth of an organism called Nocardia (CRITES & TCHOBANOGLOUS 1998). Filamentous organisms can be controlled by the addition of chemicals (e.g. chlorine or hydrogen peroxide) to the recycled activated sludge; the alteration of the dissolved-oxygen concentration in the aeration tank; the addition of nutrients and growth factors to favour other microorganisms etc. Nocardia can be controlled by avoiding the recycling of the skimmed foam or the addition of a chemical agent (e.g. polymers or chlorine) on the surface (CRITES & TCHOBANOGLOUS 1998).