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. 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.

Activated sludge systems are highly efficient for organic matter and nutrient removal, though pathogen removal is low. 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 treatment occurs by biological processes, activated sludge could be considered as a naturally based technology. Yet, it does not fit the definition entirely because of the need for high and ongoing energy inputs that make the technology expensive to operate and maintain (ROSE 1997). 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.

Treatment Process and Basic Design Principles

Activated sludge reactors are one part of a complex wastewater treatment system (U.S. EPA 2002), including a primary treatment (including screening or screening and pre-settling), 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; excess sludge treatment; and generally a non-linear, highly complex circulation regime (e.g. recirculation loops, by-passing etc.).

Screening removes materials such as napkins, rags and other coarse particles that may damage mechanical equipment further down the treatment plant. 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: One portion of the waste organic matter gives energy to the bacteria as it is oxidized to CO₂. Another portion is transformed into new cells using the energy obtained from the oxidation.

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. More sustainable way would be to compost the sludge (either before or instead of digestion) in order to reuse the nutrients in agriculture.

Activated sludge processes can be operated either in high-rate or extended-aeration mode. In the high-rate mode (high nutrient input per unit of microbial biomass), organic waste consumed by the activated sludge produces a high amount of excess sludge. In extended-aeration systems (low-rate: low nutrient input per unit of microbial biomass), biological oxygen demand (BOD) removal is higher and little excess sludge is produced. Yet, extended-aeration processes are slower and can thus treat less wastewater at a time (UNEP & MURDOCH 2004). Depending on the flow rate of wastewater, several parallel trains of primary (screening and settling) and secondary (aeration tanks and secondary clarifier) stages can be employed (UNEP & MURDOCH 2004). 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 therefore 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. Generally speaking, all bacteria contain a fraction of phosphorus in their biomass due to its presence in cellular components, such as membrane phospholipids and Deoxyribonucleic acid (DNA). Therefore, as bacteria in a wastewater treatment plant consume nutrients in the wastewater, they grow and phosphorus is incorporated into the bacterial biomass. 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.

Initially, the advanced techniques used for phosphorus removal were all based on physical-chemical treatment methods. Chemical-physical removal is achieved when iron salts (in the form of the soluble FeCl₃ or FeSO₄) are added before the aerated main tank. The iron salts dissociate rapidly in the water, as the iron Fe³⁺ preferably associates with the phosphorus, forming non-soluble Fe₃PO₄, which than precipitates and accumulates in the sludge. This has the advantage that also some other nutrients (i.e. nitrogen) are co-precipitated, thus reducing the required volume of the treatment unit for nitrification/denitrification. However, as a certain overdosing of metal salts is necessary to obtain the required low effluent phosphorus values, this procedure can result in high costs of chemicals and a significant increase of excess sludge production. Furthermore, the accumulation of ions (increased salt content) may seriously restrict the reuse possibilities of the effluent. A more modern 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). These special polyphosphate-accumulating organisms (PAO) can develop in the sludge under strictly anaerobic conditions, growing on some fermentation products (volatile fatty acids, VFA). When they are transferred with the sludge to an aerobic environment, they start to use the energy they stored with the VFA in their cells and store phosphorus instead. This additionally stored phosphorus is removed from the effluent with the settled sludge. In practice, a biological phosphorus removal is accomplished by the sequencing of anaerobic, anoxic and aerobic chamber and a complex recirculation system (CRITES & TCHOBANOGLOUS 1998).

When both nitrogen and phosphorus are to be removed, the combination becomes even more complex. As it is vital for the growth of PAO that volatile fatty acids are present in the anaerobic reactor, it is of crucial importance that the return of nitrate to the anaerobic reactor is avoided: In that case, the volatile fatty acids would be used for denitrification by non-PAO organisms (HAANDEL&LUBBE 2007). The process can be further improved by combining it with a fermenter to assure VFA supply. In summary, 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. At the same time, N and P are essential for agriculture and have to be produced technically from natural resources. Phosphorus is gained from rock phosphates, which deplete in quantity and quality and nitrogen fixation is determined by the cost and availability of fossil fuels (MAURER 2003). Therefore, it becomes more and more interesting to recover these valuable nutrients from wastewater streams and reusing it for agricultural purposes, avoiding the costs of extensive wastewater treatment and saving resources.

Types of Suspended-growth Activated Sludge Systems

Different versions of the original reactor set-ups have been tested. The most common are plug-flow reactors (PFR) and continuous stirred-tank reactors (CSTR) processes (CRITES & TCHOBANOGLOUS 1998). In plug-flow systems, the different aerobic and anaerobic stages correspond only to zones, while completely stirred processes require a single tank for each step.

Oxidation Ditches

Typical plug-flow activated sludge systems are 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 of is between 24 to 48 hours with a sludge age of 12 to 20 days (http://en.wikipedia.org/wiki/Activated_sludge). 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 providing 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 http://en.wikipedia.org/wiki/Activated_sludge [Accessed: 23.03.2010]).

Sequential Batch Reactors (SBRs)

The process can also be operated in batches, where the different conditions are all achieved in the same reactor but at different times (UNEP & MURDOCH 2004). This type of reactor is called sequential batch reactor (SBR). 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 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 & MURDOCH 2004). For more information on SBR activated sludge systems, please consult WSP (2007) or U.S. EPA (1999).

Costs Considerations

Construction and maintenance costs are very high as activated sludge treatment units are highly mechanised. Also operation is expensive due to the requirement of permanent professional operation, high electricity consumption (pumping and aeration) and costly mechanical parts (SANIMAS 2005).

Operation and Maintenance

Mechanical equipments (pumps, aerates, 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 centralized computerized monitoring system). 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).

Health Aspects

Operation and maintenance of activated sludge system is generally carried out by skilled labourers, which should be sufficiently well trained regarding any health risks. The effluent water from the system is of high quality regarding chemical pollutants; helminth eggs, bacteria or viruses are not removed in suspended-growth treatment processes (BAHRI 2009). Even though activated sludge treatment plants should be constructed far away from housings, the effluent should undergo an appropriate disinfection treatment before discharge (e.g. UV-light, chlorination). 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.

At a Glance