Full Answer
What is activated sludge sewage treatment?
Activated sludge sewage treatment is ideally completed in a centralized treatment facility. The treatment of activated sludge can occur in most climates. However, colder environments reduce the treatment capacity. AOS professionals can discuss activated sludge treatment methods and other wastewater treatment services during a thorough consultation.
How much sludge is produced from activated sludge?
The activated sludge process produces about 70–100 kg/ML of waste activated sludge (that is kg of dry solids produced per ML of wastewater treated; one mega litre (ML) is 10 3 m 3 ). A value of 80 kg/ML is regarded as being typical.
How is primary effluent mixed with Return Activated Sludge?
Primary effluent (or plant influent) is mixed with return activated sludge to form mixed liquor. The mixed liquor is aerated for a specified length of time.
What is Step 2 of the wastewater treatment process?
Step 2: Grit Removal In this step, heavy but fine material such as sand and gravel is removed from the wastewater. This material is also disposed of in a landfill.
What stage is activated sludge used?
In an activated sewage system, the settled raw sewage and activated sludge are added to an aeration tank. After this stage the settled sludge can be removed from the tank, leaving behind the effluent. Following this process, the settled sludge is reactivated before reentering the tank to reuse it in the process.
What is activated sludge used for?
The activated sludge process is a means of treating both municipal and industrial wastewater. The activated sludge process is a multi-chamber reactor unit that uses highly concentrated microorganisms to degrade organics and remove nutrients from wastewater, producing quality effluent.
Is activated sludge secondary treatment?
Activated sludge is a common suspended-growth method of secondary treatment. Activated sludge plants encompass a variety of mechanisms and processes using dissolved oxygen to promote growth of biological floc that substantially removes organic material.
What is activated sewage sludge?
The activated sludge process is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa.
How is activated sludge used to clean sewage?
The activated sludge process in the treatment of wastewater involves blowing oxygen or air into raw, unsettled sewage. This process smashes the solids. The sewage is bubbled, and sewage liquor is discharged into an activated sludge chamber.
Which process is provided after activated sludge process?
Which is the next reactor after activated sludge reactor in the treatment process? Explanation: Clarifier (settling tank) is the next reactor where the solids (mostly cells, called sludge at this stage) are separated from the water.
What are the 3 stages of sewage treatment?
There are three main stages of the wastewater treatment process, aptly known as primary, secondary and tertiary water treatment.
What is secondary treatment of sewage?
Secondary treatment of sewage is called biological treatment because it involves living organisms such as aerobic or anaerobic microbes to digest the organic waste. [ 1] In this process, the primary effluent is first passed into an aeration tank where vigorous growth of aerobic microbes (flocs) takes place.
What is primary and secondary treatment for sewage?
Primary treatment works on sedimentation, where solids separate from the water through several different tanks. In contrast, secondary treatment uses aeration, biofiltration and the interaction of waste throughout its process.
What is activated sludge?
Activated sludge is also the name given to the active biological material produced by activated sludge plants. Excess sludge is called "surplus activated sludge" or "waste activated sludge" and is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance. This sewage sludge is usually mixed with primary sludge from the primary clarifiers and undergoes further sludge treatment for example by anaerobic digestion, followed by thickening, dewatering, composting and land application.
When was activated sludge discovered?
The activated sludge process was discovered in 1913 in the United Kingdom by two engineers, Edward Ardern and W.T. Lockett, who were conducting research for the Manchester Corporation Rivers Department at Davyhulme Sewage Works. This development led to arguably the single most significant improvement in public health and the environment during ...
What is a bioreactor and final clarifier?
The process involves air or oxygen being introduced into a mixture of screened, and primary treated sewage or industrial wastewater ( wastewater) combined with organisms to develop a biological floc which reduces the organic content of the sewage.
What is pure oxygen sludge aeration?
Pure oxygen activated sludge aeration systems are sealed-tank reactor vessels with surface aerator type impellers mounted within the tanks at the oxygen carbon liquor surface interface . The amount of oxygen entrainment, or DO (Dissolved Oxygen), can be controlled by a weir adjusted level control, and a vent gas oxygen controlled oxygen feed valve. Oxygen is generated on site by cryogenic distillation of air, pressure swing adsorption, or other methods. These systems are used where wastewater plant space is at a premium and high sewage throughput is required as high energy costs are involved in purifying oxygen.
How to reduce space in sewage treatment plant?
The space required for a sewage treatment plant can be reduced by using a membrane bioreactor to remove some wastewater from the mixed liquor prior to treatment. This results in a more concentrated waste product that can then be treated using the activated sludge process.
What is the mechanism of mixing sewage and sludge?
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 activated sludge biota. The rising oxygen and injected return sludge provide the physical mechanism for mixing of the sewage and sludge.
How is sewage treated in a short supply area?
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. Such shafts may be up to 100 metres deep and are filled with sewage liquor.
How does sewage sludge digest?
After amassing all the solids from the sewage sludge begins the sludge digestion process. This is a biological process in which the organic solids present in the sludge are decomposed into stable substances. This process also helps reduce the total mass of solids, while destroying any present pathogens to enable easy dewatering. The sludge digestion process is a two-phase process. In the first stage, the dry solid sludge is heated and mixed in a closed tank to enable anaerobic digestion by acid-forming bacteria. These bacteria hydrolyze the large molecules of proteins and lipids present in the sludge and break them down into smaller water-soluble molecules, which they then ferment into various fatty acids. The sludge then flows into the second tank where it is converted by other bacteria to produce a mixture of carbon dioxide (CO 2) and methane, after which the methane is collected and reused to power the digestion tank and generate power (depending on the quantity retrieved).
What is the first step in sewage treatment?
The first step in the sewage sludge treatment plan is called thickening. In this step, the sewage sludge is thickened in a gravity thickener to reduce its overall volume, thus enabling the easy handling of the sludge.
How much water is in dewatered sludge?
In most cases, dewatered sludge usually contains a significant amount of water, as much as 70 percent, in spite of its solidified state. Therefore, it is important to dry and dewater the sludge beforehand.
What is the purpose of wastewater treatment?
Mainly many sludges are treated using a variety of digestion techniques, the purpose of which is to reduce the bulk amount of organic matter and the number of disease-causing microorganisms present in the solids. Wastewater Treatment Plant like etp, zld stp plant etc are very good solutions for wastewater.
How is sewage treated?
While sewage sludge is usually treated using a standard plan of action , it is extremely important to factor in aspects like the origin of the sewage, the treatment process used to reduce the sewage to sludge, as well as the possible byproducts that can be retrieved from it for further use before choosing a sludge treatment plan.
What is the process of sludge digestion?
The sludge digestion process is a two-phase process. In the first stage, the dry solid sludge is heated and mixed in a closed tank to enable anaerobic digestion by acid-forming bacteria.
Why is it important to treat sludge?
Therefore, it is extremely important to properly treat such sludge in order to minimize its environmental repercussions.
How is complete mix activated sludge achieved?
In a rectangular tank, complete mixing is achieved by distributing the sewage and the return sludge uniformly along one side of the tank and withdrawing the aerated sewage, uniformly along the opposite side. In case of circular or square tank, complete mixing is achieved by mechanical aerator with adequate mixing capacity installed at the centre of the tank.
What happens when sewage is added to the returned sludge?
If the sewage is added to the returned sludge at more than two points along the aeration channel, the load coming on the returned sludge is changing. In the previous article, it has been stated that the B.O.D. of the mixed liquor is different at different points.
What is the disadvantage of a two stage aeration system?
The main disadvantage is that the sludge becomes septic soon and requires early removal and disposal from the settling tanks. Type # 4. Two-Stage Aeration: In two-stage aeration the sewage is allowed to flow in a pair of aeration and sedimentation units.
How long does it take for sewage to be aerated?
The sewage is aerated along with return sludge for a comparatively short period of 0.5 to 1.5 hrs, when the sludge absorbs the organic matter in the sewage.
What is secondary sludge?
Secondary sludge is either returned and wasted within each stage, or sometimes the excess sludge from the second stage is recycled to the first stage and from there, both the sludge are disposed off. This process is most suited for treating the strong industrial sewage which has high value of B.O.D. Type # 5.
What is activated aeration?
Activated Aeration: It is a combination of two tapered activated sludge plants in parallel using the same settled sewage as influent. The excessive activated sludge of one plant is used as seeding for another plant, which is called activated aeration.
What is activated sludge treatment?
Activated sludge treatment process is widely used in the treatment of municipal sewage and industrial waste waters due to the fact that it’s economically viable and reasonably safe to operate. Such a system can be used in large installations. However it’s very important to have compatible design parameters to be infused while designing Activated Treatment Plant. Moreover, air requirement, MLSS, MLVSS, etc. are very important parameters to be maintained in the system. The present paper deals with these aspects to impart an over view of the conceptualization along with system design.
What is activated sludge?
Activated sludge is a sludge particles reduced in wastewater by the growth of organisms in aeration tanks. The term ‘activated’ comes from the fact that the particles teem with bacteria, fungi, and protozoa and is different from primary sludge in the sense that the sludge particles contain many living organisms that can feed on the incoming wastewater. Described simply, screened wastewater is mixed with varying amounts of recycled liquid containing a high proportion of organisms taken from a secondary clarifying tank, and it becomes a product called mixed liquor. This mixture is stirred and injected with large quantities of air, to provide oxygen and keep solids in suspension. After a period of time, mixed liquor flows to a clarifier where it is allowed to settle. A portion of the bacteria is removed as it settles, and the partially cleaned water flows on for further treatment. The resulting settled solids, the activated sludge, are returned to the first tank to begin the process again. Today a number of variations of the basic process have been developed. This issue of Pipeline includes descriptions of three of the most common variations: Extended aeration, sequencing batch reactors, and oxidation ditches. The activated sludge plant is the most popular biological treatment process for larger installations. A glossary of terms that are used in activated sludge process is reflected as under: Aerobic – a condition where oxygen is present BOD – biological oxygen demand. Measure of oxygen organic material in the water requires.
What is the oxygen requirement for activated sludge?
The total oxygen requirement of the process may be formulated as follows: O2 required (g/d) = Q(SO - S) - 1.42 QwXr
What are the considerations for design of activated sludge plant?
The items for consideration in the design of activated sludge plant are aeration tank capacity and dimensions, aeration facilities, secondary sludge settling and recycle and excess sludge wasting
How does a secondary settling tank work?
Secondary settling tanks, which receive the biologically treated flow undergo zone or compression settling. Zone settling occurs beyond a certain concentration when the particles are close enough together that interparticulate forces may hold the particles fixed relative to one another so that the whole mass tends to settle as a single layer or "blanket" of sludge. The rate at which a sludge blanket settles can be determined by timing its position in a settling column test whose results can be plotted as shown in figure. Compression settling may occur at the bottom of a tank if particles are in such a concentration as to be in physical contact with one another. The weight of particles is partly supported by the lower layers of particles, leading to progressively greater compression with depth and thickening of sludge. From the settling column test, the limiting solids flux required to reach any desired underflow concentration can be estimated, from which the required tank area can be computed.
How much oxygen can an aerator give?
Assume oxygenation capacity of aerators at field conditions is only 70% of the capacity at standard conditions and mechanical aerators are capable of giving 2 kg oxygen per kWh at standard conditions .
What is activated sludge?
Activated Sludge Basics. The activated sludge process is a biological wastewater treatment process. This means that treatment occurs as pollutants are used as a food source by many different types of microorganisms.
How does activated sludge work?
Sludge Production - The activated sludge process produces excess sludge that must be removed or "wasted" from the system. The amount of sludge produced is a function of the SRT, or how long the sludge remains in the system. The longer the SRT, the less excess sludge is produced. Sludge production, also called "growth rate", can be expressed in terms of pounds of solids produced per pounds of BOD5 removed from the system. The equation for the growth rate factor is shown below. Depending on the type of activated sludge process, the growth rate factor can range from 0.1 to 1
What is RAS in wastewater?
Return Activated Sludge (RAS) - The activated sludge returned to the aeration tank from the final clarifier, expressed in terms of flow (typically gallons per minute). RAS provides the mass of microorganisms necessary to consume the organic material in the influent wastewater and maintain the desired level of MLSS in the aeration tank. RAS flow rate is expressed as a percentage of the influent flow and can vary from 50% to 200% of the influent flow. The RAS flow rate is typically optimized based on operational experience. The RAS flow rate is typically set at a constant rate or at a rate proportional to the influent flow.
How thick is a sludge blanket?
The optimum sludge blanket level must be determined by experience and must provide adequate settling depth and sludge storage. Typically, secondary clarifiers allow for 2-3 feet of depth for thickening, 3 feet for a buffer zone between the thickened sludge and the clarification zone, and 8 feet for clarification. The operator shoudl keep good records of the sludge blanket levels in order to avoid the loss of biomass from the clarifier. The sludge blanket level can be monitored manually or automatically.
What is flocculation in wastewater treatment?
In the third step of treatment, flocculation and separation, the microorganisms stick together to form large particles that will settle out of the purified wastewater in the secondary clarifier. Flocculation occurs when mixing allows the organisms to contact one another, but does not cause conditions so turbulent that the flocculated material is torn apart. Settleability and compaction of the floc particles depends on the density, size and shape of the particles as well as the efficiency of the clarifier. Settleability is affected by the abundance of filamentous bacteria, thos that form strings as they grow rather than forming floc. An excessive growth rate of these bacteria may cause a bulking condition in which the mixed liquor does not compact well, taking up much more volume in the clarifier. This condition may be caused by many factors, among which are improper Dissolved Oxygen environment and nutrient imbalance, and may result in solids loss in the clarifier effluent.
How to determine detention time in clarifier?
Final Clarifier Detention Time - The detention time in the final clarifier (in units of hours) is determined by dividing the volume of the final clarifier by the flow to the clarifier. (Return sludge flow is generally not included in the calculation.)
Why is my activated sludge bulking?
If your activated sludge process is experiencing sludge bulking problems, it may be due to filamentous microorganisms. If you suspect this is the problem, you can look at a sample of your sludge under a microscope. Healthy sludge has filaments in it. Just because your sludge contains filaments does not mean that your bulking problem is being caused by them. Bulking sludge is identified as a sludge that does settle but compacts very slowly. This may be caused by an excessive number of filamentous microbes. The key word is "excessive". In healthy sludge, the filaments act as a framework for the flocs. Filaments can serve to catch and hold small particles, acting somewhat like a net. This will in turn reduce effluent turbidity. In fact, if there are not enough filaments, the flocs may lack the necessary framework to form properly. This may result in pin floc. However, if too many filaments are in the sludge, the framework serves to prevent the sludge from compacting. Sludge bulking has been reported to be the most common operational problem affecting over half of the activated sludge facilities in the U.S.
What is activated sludge?
The activated sludge is a process with high concentration of microorganisms, basically bacteria, protozoa and fungi, which are present as loose clumped mass of fine particles that are kept in suspension by stirring, with the aim of removing organic matter from wastewater. From: Resource-Efficient Technologies, 2016.
How does activated sludge work?
The activated sludge process (Fig 12.4) uses microorganisms to feed organic contaminants that are in wastewater to produce a high-quality effluent. The basic principle behind all activated sludge processes is that as microorganisms grow, they form particles that clump together. These particles, which are referred to as floc, are allowed to settle to the bottom of the tank, which results in a relatively clear liquid free organic material and suspended solids. The screened wastewater is mixed with varying amounts of recycled liquid that contains a high proportion of organisms that are taken from a secondary tank, and it becomes a product that is called mixed liquor. The next step for the mixture is to stir and inject it with large quantities of air to provide oxygen and keep the solids in suspension. After a period of time, the mixed liquor flows to a clarifier where it is allowed to settle. During this settling a portion of the bacteria is removed and the partially cleaned water flows on for additional treatment. The settled solids that resulted, the activated sludge, are then returned to the first tank to begin the process again. The basic activated sludge process consists of several interrelated components:
What is the major phenomenon responsible for PPCPs elimination in activated sludge process?
Biodegradation is suggested as the major phenomenon responsible for PPCPs elimination in activated sludge process. Therefore, to improve the degradation of PPCPs, an effective strategy must be adapted to increase the decomposition.
Why do activated sludge plants foam?
Foaming in activated sludge plants is caused by high SRTs, warm temperatures, low F:M ratios and high MLSS levels, as well as oil and grease and/or surfactants in the influent. Abundance of actinomycetes such as Nocardia or Microthrix are commonly related to foaming in activated sludge plants, and have been identified in a full-scale MBR plant subject to variable OLRs ( Smith, 2006 ). However, foam in MBR plants has been observed in the absence of actinomycetes. The degree of foaming is reported as being related to the protein EPS concentrations (Nakajima and Mishima, 2005). Foaming sludges also appear to yield lower membrane permeabilities ( Chang and Lee, 1998 ), attributed to the higher hydrophobicity of foaming activated sludge ( Section 2.3.6.4 ). Foaming thus provides an indication of sludge fouling propensity.
How many Ascaris eggs are in activated sludge?
The concentration of parasites in activated sludge treated effluents is usually low at less than 5 eggs/l, though higher numbers of around 60 Ascaris eggs/l have been reported ( Bhaskaran et al., 1956 ). Numbers of protozoan (oo)cysts in activated sludge effluents can be high despite relatively high removal rates.
Overview
The activated sludge process is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa. It uses air (or oxygen) and microorganisms to biologically oxidize organic pollutants, producing a waste sludge (or floc) containing the oxidized material. The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items…
Purpose
In a sewage (or industrial wastewater) treatment plant, the activated sludge process is a biological process that can be used for one or several of the following purposes: oxidizing carbonaceous biological matter, oxidizing nitrogenous matter: mainly ammonium and nitrogen in biological matter, removing nutrients (nitrogen and phosphorus).
Process description
The process takes advantage of aerobic micro-organisms that can digest organic matter in sewage, and clump together (by flocculation) as they do so. It thereby produces a liquid that is relatively free from suspended solids and organic material, and flocculated particles that will readily settle out and can be removed.
The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:
Plant types
There are a variety of types of activated sludge plants. These include:
There are a wide range of types of package plants, often serving small communities or industrial plants that may use hybrid treatment processes often involving the use of aerobic sludge to treat the incoming sewage. In such plants the primary settlement stage of treatment may be omitted. In these plants, a biotic floc is created which provides the required substrate. Package plants are designed and fabricated by specialty engineering firms in di…
Aeration methods
Sewage liquor is run into deep tanks with diffuser grid aeration systems that are attached to the floor. These are like the diffused airstone used in tropical fish tanks but on a much larger scale. Air is pumped through the blocks and the curtain of bubbles formed both oxygenates the liquor and also provides the necessary mixing action. Where capacity is limited or the sewage is unusually strong or difficult to treat, oxygen may be used instead of air. Typically, the air is generated by …
Recent developments
A new development of the activated sludge process is the Nereda process which produces a granular sludge that settles very well (the sludge volume index is reduced from 200–300 to 40 millilitres per gram (192–288 to 38 US fl oz/oz)). A new process reactor system is created to take advantage of this quick settling sludge and is integrated into the aeration tank instead of having a separate unit outside. About 30 Nereda wastewater treatment plants worldwide are operational, under construction or under design, varying in size from 5,000 up to 858,000 person e…
Issues
Sludge bulking can occur which makes activated sludge difficult to settle and frequently has an adverse impact on final effluent quality. Treating sludge bulking and managing the plant to avoid a recurrence requires skilled management and may require full-time staffing of a works to allow immediate intervention.
The discharge of toxic industrial pollution to treatment plants designed primarily to treat domestic sewage can create process upsets.
History
The activated sludge process was discovered in 1913 in the United Kingdom by two engineers, Edward Ardern and W.T. Lockett, who were conducting research for the Manchester Corporation Rivers Department at Davyhulme Sewage Works. In 1912, Dr. Gilbert Fowler, a scientist at the University of Manchester, observed experiments being conducted at the Lawrence Experiment Station at Massachusetts involving the aeration of sewage in a bottle that had been coated wit…