17 Activated sludge process modifications

Babita Khosla

Objectives:

Understand the conventional activate sludge process

Explain various types of modifications in the activated sludge process

Introduction:

Biological Waste Water Treatment: Wastewater treatment embraces numerous techniques to produce effluent suitable for the survival of flora and fauna, after having been polluted by chemicals, sanitary wastes, oils and the like. Various methods for treatment of wastewater have been described and invariably these methods are used in a sequence to suit the wastewater treatment requirement The treatment of water soluble compounds that cause depletion of oxygen in the wastewater as well as surrounding areas around it, have been the target of many a study and the most proven systems for treatment of such wastewaters has been by the “biological” route. Biological Wastewater treatment facilities are designed to emulate the purification process that occurs naturally in lakes, rivers and streams. The biological treatment process may be aerobic, anaerobic or anoxic. Suitability of the process is determined by a number of factors including:

• Composition of the wastewater
• Degree of stabilization required for environmental compliance
• Economic viability of the process

In aerobic treatment systems, microbes (bacteria, algae etc.) are used to condition themselves so as to utilize the soluble pollutants available in the wastewater as substrate. The microbes reproduce by cell division and metabolize to remove most of the organic matter present in the wastewater. Methods of aerobic biological wastewater treatment are divided into two classes:

• (a) Fixed film systems: In these systems, microbes are located on fixed locations and metabolism by bacteria takes place in the film covering the fixed biomass.
• (b) Dispersed systems: Here microbes usually bacteria are kept in motion and suspended biomass is used to treat the wastewater biologically. Activated Sludge Process is a typical dispersed wastewater treatment system.

18.1 Activated sludge process:

The basic principle of all the activated sludge processes is that microorganisms grow and they form particles that chunk together and make flocs. These flocs are allowed to settle down at the bottom of the tank, leaving a partially clear liquid which is free of organic material and suspended solids. Basically, 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 continuously stirred and fed with large quantities of air, to maintain oxygen and keep solids in suspension. After some time, mixed liquor flows to a clarifier where it is allowed to settle. A portion of the bacteria is removed as it settles which is called activated sludge, and the partially cleaned water is taken for further treatment. The resulting settled solids or the activated sludge, are returned to the first tank to begin the process again. This process was initially developed in England in the early 1900s but this process did not become widespread in the U.S. until the 1940s. But today a number of variations in the basic process have been developed. This issue includes descriptions of three of the most common variations: Extended aeration, sequencing batch reactors, and oxidation ditches. The activated sludge process is a popular biological treatment process for larger plants or small plants which are used today and this process make plants capable of producing a high quality effluent at lower price. Other advantages of the activated sludge process are the low construction cost and relatively small land requirement. This process is widely required by large cities and communities where large volumes of wastewater is required to be highly treated & economically too. Activated sludge process plants are also good choices too for isolated facilities, such as hospitals or hotels, cluster situations, subdivisions and small communities.

A basic activated sludge process consists of several interrelated components:

• An aeration tank where the biological reactions occur
• An aeration source that provides oxygen and mixing
• A tank, known as the clarifier, where the solids settle and are separated from treated wastewater
• A means of collecting the solids either to return them to the aeration tank, (return activated sludge) or to remove them from the process (waste activated sludge).

Aerobic bacteria flourish as they travel through the aeration tank. The bacteria gets multiply rapidly having sufficient food and oxygen. By the time the waste reaches at the end of the tank generally between four to eight hours, the bacteria produce new cells by consuming most of the organic matter. The micro-organisms gets settle down at the bottom of the clarifier tank, separating from the clearer water. This sludge is pumped back to the aeration tank where it gets mixed with the incoming wastewater or it can be removed from the system as excess and this process is called wasting. The relatively clear liquid above the sludge, the supernatant, is sent on for further treatment as required. The flow sheet of activated sludge process is shown in figure 18.1:

Fig. 18.1 Flow sheet of activated sludge process

Source: http://cdn.biologydiscussion.com/wp-content/uploads/2016/08/clip_image002_thumb4_thumb-2.jpg

Advantages:

Capable of removing 97% of suspended solids

• Biological nitrification without adding chemicals
• Oxidation and nitration achieved
• Biological phosphorous removal
• Solids and liquids separation
• Removes organics
• Cost effective
• Easily maintained mechanical work Self sustaining system

Disadvantages:

• Cleaning is a hassle
• Most plants need at least three tanks Temperature changes affect the tank greatly

Seeding in Activated Sludge Plants: The effluent from the aeration tank containing the flocculent microbial material known as the sludge, is separated in a secondary settler or clarifier. The separated sludge exists without contact with the organic matter and becomes “activated”. A portion of the activated sludge is recycled back to the aeration tank as ‘”seed”, and the rest is wasted. Majority of activated sludge plants are started by either seeding microorganisms obtained from nearby operating plants or using specific microbial populations. In activated sludge systems, there have been commercially-oriented reports of bacterial cultures or mixtures of cultures which could be useful in improving biodegradation of specific target effluents. Addition of specialized strains to activated sludge to enhance removal of pollutants present in the influent is not yet widely applied, because bio-augmentation is less predictable and controllable. The source of “seeding” has a significant impact on the time required for a start-up. The most preferable source of seed includes:

Sludge underflow from the liquor Aeration basin mixed liquor Aerobic digester mixed liquor

A good quality seed material contains high to moderate number of high microscopic life which forms an active biological population and exhibits good flocculation. Seeding is the major step in starting the digestion process in an Activated Sludge process. The bacteria which appear in the seed are the major contributors to the direction, rate of process and the fate of the pollutants. The “Seed” is the starting point for the bacterial digestion and growth process. Without seeding, the process cannot start up. Other advantages of seeding are- reduction of process volume through decrease in critical sludge age, prevention of sludge bulking and removal of specific pollutants by specifically adapted bacteria. Seeding is usually performed at start-up of a plant and recycling of activated sludge, after acclimatization, so adjusted to maintain level of bacterial culture required for requisite MLSS (Mixed Liquor Suspended Solids). However, when changes in influent load are experienced, MLSS drops steeply and re-seeding of bacteria is required, as per process requirement.

Problems encountered in Activated Sludge systems:

Variation in the influent loading affect the performance of the activated sludge process quite adroitly and is the major cause of process upset and requirement for re-seeding of bacteria to maintain the influent treatment process requirements. Some of the typical problems encountered in activated sludge systems relate to:

1. Foaming of sludge: Two types of foaming are evident:

Chemical foam: This is caused due to either the presence of detergent/ foaming chemicals in wastewater, or a combination of high pH and Oil and grease – high pH promotes the formation of “soap”.

Biological foam: Quite a common phenomenon occurring due to filamentous foam generating bacteria that also affect sludge settling and scum formation. This is known as ‘”bulking of sludge”

2. Sludge settling problems: This is on account of formation of dispersed organisms due to two reasons:

• High organic loading rates
• Development of filamentous type organisms

3. Low BOD removal: BOD removal efficiency depends upon a number of factors – type of microbes, temperature, pH etc.

4. Extreme variation in loading: One major problem encountered by designers and operators of many wastewater treatment plants is extreme variation in loading, constituent concentrations and flow rates of wastewaters entering the treatment plant. The principal technique used to smoothen out this variation is “Equalization”.Equalization involves retaining of wastes for sufficient length of time to obtain an effluent which is more uniform. Equalization is of two types – Flow equalization and Waste strength equalization. Although both types are required simultaneously, one technique of equalization can be done at a time.

5. Occurrence of xenobiotic compounds: Xenobiotic compounds form a significant percentage of recalcitrant compounds not easily amenable to biological degradation – usually referred to as “non biodegradable” compounds. Such compounds also can be biologically treated using proper microbes and proper conditions.

18.2 Modifications in Activated Sludge Process:

The need to handle an explicit type of wastewaters entering the activated sludge plants has called for certain modifications in the systems resulting in the following activated sludge process variations:

Early Process Modifications:

1. STEP AERATION: In this process, the influent wastewater is introduced at various points along the length of the aeration tank. Sludge return varies between 25 and 50 percent. Aeration or the oxygen requirement during step aeration (3 to 7 hours) is about half that required for the conventional process. This result from more effective biomass utilization in the aeration basin, allowing organic loadings of 30 to 50 pounds biochemical oxygen demand per 1,000 cubic feet per day as compared to loadings of 30 to 40 pounds biochemical oxygen demand per 1,000 cubic feet per day permitted for conventional systems.

Fig. 18.2 Step Aeration Plant Source:  https://www.intechopen.com/source/html/49024/media/image32.png

2. CONTACT STABILIZATION: The contact stabilization activated sludge process is characterized by a two-step aeration system. Aeration of short duration (½ to 2 hours) is provided in the contact tank where raw or primary-settled wastewater is mixed with the activated sludge in the contact tank. The effluent from the contact tank is then settled in a final settling tank. The settled activated sludge to be recycled from the final clarifier is drawn to a separate re-aeration in a stabilization basin for 3 to 8 hours of aeration time. It is then returned to the contact aeration basin for mixing with the incoming raw wastewater or primary settled effluent. In addition to a shorter wastewater aeration time, the contact stabilization process has the advantage of being able to handle greater shock and toxic loadings than conventional systems because of the buffering capacity of the biomass in the stabilization tank. During these times of abnormal loadings, most of the activated sludge is isolated from the main stream of the plant flow. Contact stabilization plants will not be used where daily variations in hydraulic or organic loadings routinely exceed a ratio of 3:1 on consecutive days or for plants with average flows less than 0.1 million gallons per day without prior approval of the owner.

Fig. 18.3 Contact stabilization activated sludge process Source: http://img.bhs4.com/ab/f/abfa1adbfcc8a4cde49bd136bfd2bf815c847012_large.jpg

3. Tapered Aeration: When the oxygen supply and demand in the aeration tank is almost on proportionate terms – high at the influent end and low at the effluent end, the system has a tapered aeration. The conventional process is modified such that the oxygen demand decreases along the length of the aeration tank. The best results have been known by supplying 55-75% of the total air supply in the first half of the tank.

5. High rate Activated Sludge Process: In the high rate process, the aeration tank contains smaller concentrations of biological floe and the aeration period is short. The short aeration period does notallow the microorganisms to shift to the endogenous (death & decay) phase but keeps them between the lag phase and declining phase.

Complete mix process: The completely mixed system rapidly distributes influent flow and return sludge to throughout the reaction zone. Contents are uniformly mixed and aerated to maintain a consistent concentration. The aeration tank becomes an equalization tank. By this technique, the microorganisms in the activated sludge are utilized to the maximum and therefore, aeration costs are usually higher

Fig. 18.5 Complete mix process  Source: https://www.intechopen.com/source/html/49024/media/image30.png

7. Extended aeration: When sufficient aeration time is provided to metabolize the organics through cell growth and oxidize cell mass, the system is called extended aeration system The system consists of an aeration tank with a longer detention time than the conventional process, and is followed by a settling tank of several hours detention period, so as to achieve complete stabilization of the organic matter under aerobic conditions.

8. Kraus Process: Kraus improved the performance of the conventional sludge process by aerating a mixture of anaerobic digester supernatant and a part of the return sludge in a separate tank as shown in Fig. 18.6. This mixed liquor became highly nitrified and had good settling properties. The nitrified mixture was brought into contact with the raw waste water in the aeration tank and about 90% BOD removal was obtained. Kraus used a “dual system” of aeration in which fine bubble aeration at the bottom and coarse aeration at the top was employed. There are other several modifications in conventional systems which are discussed below:

• 1) Trickling Filters: Trickling filter is an attached growth process, i.e., process in which microorganisms responsible for treatment is attached to an inert packing material. Packing material used in attached growth processes include rock, gravel, slag, sand, redwood, and a wide range of plastic and other synthetic materials. The wastewater in trickling filter is distributed over the top area of a vessel containing non-submerged packing material. Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attached biofilm. During operation, the organic material present in the wastewater is metabolized by the biomass attached to the medium. The biological slime grows in thickness as the organic matter abstracted from the flowing wastewater is synthesized into new cellular material. The thickness of the aerobic layer is limited by the depth of penetration of oxygen into the microbial layer.

The micro-organisms near the medium face enter the endogenous phase as the substrate is metabolized before it can reach the micro-organisms near the medium face as a result of increased thickness of the slime layer and loose their ability to cling to the media surface. The liquid then washes the slime off the medium and a new slime layer starts to grow. This phenomenon of losing the slime layer is called sloughing. The sloughed off film and treated wastewater are collected by an under drainage which also allows circulation of air through filter. The collected liquid is passed to a settling tank used for solid-liquid separation.

• 2) Stabilization ponds: The stabilization ponds are open flow through basins specifically designed and designed to treat municipal sewage and biodegradable waste present. They have a long detention period of few to several days. Wastewater Stabilization Ponds (WSPs) are large, man-made water bodies in which fecal sludge, black water or greywater are treated by natural processes because of solar light, wind, microorganisms and algae. The ponds used can be individual or linked in a series for efficient treatment. There are three types of ponds i.e. anaerobic, facultative and aerobic, each of having different treatment and design characteristics. The process is low-cost, BOD and pathogen removal is high. But large surface areas and expert design are very important. The effluent still consists of nutrients like N and P and is therefore appropriate for the reuse in agriculture, but not for direct recharge in surface waters.

Aerated lagoons: Aerated lagoons in which oxygen is added by mechanical aeration as compared than algal photosynthesis are called aerated lagoons. Aerated lagoon is large, mixed aerobic reactor and is very similar to facultative ponds in waste stabilization pond systems; with only the difference that natural oxygenation is increased. Oxygen is provided mechanically which keep the microorganisms suspended and mixed with water to achieve a high rate of organic degradation

As natural oxygenation is enhanced, ponds can be deeper (thus smaller in surface) and are suited also for colder climates compared. There are two types of aerated ponds: common aerated lagoons (enhanced facultative ponds) and completely mixed aerated ponds are in essence activated sludge systems without sludge. The effluent of aerated ponds may be reused or used for recharge, but settled sludge requires a further treatment or correct disposal.

• 4) Oxidation ditch: The oxidation ditch is a modified form of “extended aeration” of activated sludge The ditch consists of a long continuous channel oval in shape with two surface rotors placed across the channel. In some areas, where more land is available, sewage is treated in large round or oval ditches with one or more horizontal aerators typically called brush or disc aerators which drive the mixed liquor around the ditch and provide aeration. These are oxidation ditches, often referred to by manufacturer’s trade names such as Pasveer, Orbal, or Carrousel. They have the advantage that they are relatively easy to maintain and are resilient to shock loads that often occur in smaller communities (i.e. at breakfast time and in the evening). Oxidation ditches are installed commonly as ‘fit & forget’ technology, with typical design parameters of a hydraulic retention time of 24 – 48 hours, and a sludge age of 12 – 20 days. This compares with nitrifying activated sludge plants having a retention time of 8 hours, and a sludge age of 8 – 12 days.

• 5) Sequencing batch reactor (SBR): The Sequencing Batch Reactor (SBR) is a different configuration of the conventional activated sludge systems, in which the process can be operated in batches, where the different conditions are all achieved in the same reactor but at different times. 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 oxidise 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. 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. At least two tanks are needed for the batch mode of operation as continuous influent needs to be stored during the operation phase. Small systems 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.

Fig. 18.11 Sequencing batch reactorSource: https://kyocp.files.wordpress.com/2016/04/sbr-process-cycle-pic.jpg

Advantages

• Little land required.
• High effluent quality.
• Fully automatised.
• Resistant against shock-loads and applicable for a large range of organic and hydraulic loading rates.

Disadvantages

• Requires continuous supply of energy.
• Highly mechanised equipment (control panel).
• Effluent and sludge might require further treatment.

18.3Anaerobic Treatment:

The anaerobic waste treatment process is an effective method for the treatment of many organic wastes. The treatment has a number of advantages over aerobic treatment process, namely, the energy input of the system is low as no energy is required for oxygenation, lower production of excess sludge (biological synthesis) per unit mass of substrate utilized, lower nutrient requirement due to lower biological synthesis, and degradation leads to production of biogas which is a valuable source of energy.

Anaerobic Reactor

Various types of anaerobic units that have been developed are as follows:

Upflow anaerobic filters packed with pebbles, stones, PVC sheets, etc. as media to support submerged biological growths (fixed film). The units are reported to work well but a likely problem is accumulation of solids in the interstices.

Downflow anaerobic filters packed with similar media as above but not to be confused with usual trickling filters which are aerobic. In the anaerobic units, the inlet and outlet are so placed that the media and fixed film stay submerged.

UASB type units in which no special media have to be used since the sludge granules themselves act as the ‘media’ and stay in suspension. These are commonly preferred.

Fluidized bed units filled with sand or plastic granules are used with recirculation under required pressure to keep the entire mass fluidized and the sludge distributed over the entire reactor volume. Their power consumption is higher.

So, in the end we can say that activated sludge process is the conventional method for the treatment of wastewater and there are several modifications in this process both aerobic and anaerobic treatments.

Summary:

In this lecture we learnt about: Conventional activated methods for the treatment of wastewater Modifications in the activated sludge system

References:

• Mahvi AH, Mesdaghinia A, Karakani F (2004). Feasibility of continuous flow sequencing batch reactor in domestic wastewater treatment. American J App Sci, 1(4): 348-53.
• Mahvi AH (2008). Sequencing batch reactor: a promising technology in wastewater treatment. Iran J Environ Health Sci Eng, 5(2):79-90.
• Metcalf & Eddy Inc (2003). Wastewater Engineering: Treatment and Reuse. 4th ed. McGraw-Hill Inc, New York, pp.: 6-17.

Web links:

1. http://nptel.ac.in/courses/105104102/Lecture%2024.htm
2. http://wss.hry.nic.in/WriteReadData/Notice/Sewage%20Treatment%20Processes.pdf
3. https://www.vssut.ac.in/lecture_notes/lecture1424353637.pdf
4. https://i0.wp.com/www.thewatertreatments.com/wp-content/uploads/2009/11/oxidation-ditch1.jpg
5. http://www.thewatertreatments.com/page/6/
6. https://www.sswm.info/category/implementation-tools/wastewater- treatment/hardware/semi-centralised-wastewater-treatments/