Waste Water Treatment Engineering - Case Study Example

Introduction The Loganholme treatment plant deals with waste water treatment in Queensland. Waste water treatment involves the removal of contaminants from the waste water. Waste water is inclusive of domestic sewage and effluents. The process of waste water treatment involves the application of chemical, biological and physical contaminants. The aim of the process is always to produce a fluid waste stream that is environmentally safe and treated sludge that is safe when reused as fertilizer on the farm or disposed off. The focus of this report is to discuss the waste water treatment process and certain components in the system namely oxidation ditches chlorination, location of primary sedimentation tanks and the foaming and scum observed on the reactors during the site visit. The report is based on a visit of the Loganholme treatment plant in Queensland. A. Waste water treatment process The waste water is collected from homes and industrial establishments and directed into the Loganholme treatment plant. The treatment process of the waste water begins with the preliminary treatment stage. In this step any object with the potential to cause harm on mechanical equipment such as pieces of wood, rags and grit are removed. A communitor is used to grind the large objects. The waste water is then pumped across a bar screen to remove any remaining debris. Sand and grit are removed in an aerated grit tank in which they settle down. These materials are removed on daily basis so that they do not accumulate in the tanks (Henze, Harremoes, Jansen, 2001, 112). Primary treatment comes after the preliminary stage. In this stage the materials that settle at the tank bottom or float are removed. The sedimentation tanks used allow the objects to settle down by gravity. The water can stay in these tanks for 2 to 3 hours within which the sludge settles down. Pumps and mechanical scrappers are used to remove sludge from the primary sedimentation tank or primary clarifier. Surface skimming devices are used to remove the oil and grease that floats on top of the tank (Kolarik, Priestley, 1996, 71). After primary treatment, the water is taken through the secondary treatment stage. The secondary treatment stage is strategically designed to bring about the substantial degradation of the biological matter in the sewage that originates in human waste, detergents and soap and food wastes. The Loganholme treatment plant treats the liquor formed from the waste water by the use of biological aerobic processes. The bacteria in the swage water require oxygen and food in form of organic substances to survive. The protozoa and the bacteria in the waste water consume all the organic mater that is biodegradable such as organic short chain carbon molecules, fats and sugars among other substances. In the secondary treatment stage the microorganisms bind together the insoluble substances into big lumps called floc. The secondary systems of waste water treatment are categorized as suspended growth or fixed film systems. Here, activated sludge is used to clean the water. This happens in the oxidation ditches 1 to 4 in which the water flows from one ditch to the other until it ends up in the last ditch. In these ditches the biological sludge containing microorganisms is mixed with the waste water. The water moves form one oxidation ditch to the next until the microorganisms have eaten up all the organic particles. Biological solids settle out. Air containing oxygen is pumped into the waste water as microorganisms mix with organic particles. Growth of microorganisms results in their clumping together into activated sludge. Waste water and activated sludge form mixed liquor in which organic particles are eaten up by microorganisms (Petrovski, Dyson, Quill, McIlroy, Tillett, Seviour 2011, 98). Mixed liquor is then allowed to flow into other tanks called final clarifiers in which the activated sludge is allowed to settle out. The final clarifiers are basically settling tanks in which the waste water containing microorganisms is allowed to remain for a while. In the Loganholme treatment plant, these tanks are eight in number. The waste water moves from one tank to the next until all the activated sludge has completely settled out. Part of the settled sludge is returned to the aeration tank to increase the population of microorganisms that break down the organic substances. Excess sludge must be removed and disposed off after passing through the dewatering tank. In the dewatering tank all the water is removed from the sludge before it is pumped forward. The sludge is taken to a digester where anaerobic digestion takes place. When the water has been separated from the activated sludge, it is almost clean and the next step in line is to disinfect it. This process is accomplished with the use of chlorine in a chlorination process. The chlorine gas or salt is applied into the water and allowed to settle for approximately 15 minutes in which the chlorine reacts with all the pathogens present in the water (Jenkins, Richard, Daigger, 2004, 21). B1. Inlet Works  Waste water has many solids of various sizes either floating or suspended. Solids like rags and pieces of wood must be taken out because they interfere with mechanical equipment and treatment processes. Inlet works in the treatment plant help to remove these solids. They have screens in them that hold the solids back as the water is allowed to pass through. Inlet works represent a level in the pre-treatment stage where solid materials that are removed from the waste water easily are extracted to avoid damage in subsequent stages. Inlet works help to remove materials such as branches, tree limbs, trash and rags among others that could possibly clog and cause damage to pumps and sewage lines in primary treatment clarifiers. Inlet works include a bar screen through which the waste water passes in order to remove behind all the sticks, cans, rags, plastic packets among other solids brought through the stream of waste water. Removal of these materials through the inlet works is done with an automated mechanically raked bar screen. This commonly happens in modern plants that have a heavy load of waste water from highly populated areas. A screen that can be cleaned manually is used when the waste water treatment plant is less modern or smaller. The action of raking with a mechanical bar screen gets its pace from the rate of waste water flow or the accumulation of materials on the bar screen. From the inlet works the solids are taken away into a landfill or an incinerator (Jenkins, 2004, 30). To put the removal of solids at its optimum, it may be necessary to use mesh and bar screens with different sizes. Failure to remove the gross solids makes them to be trapped in moving parts and pipes in the treatment plant causing a lot of damage and inefficient performance of the plant. Inlet works may also include channels or chambers for grit and sand. Here, the velocity of waste water flowing in is adjusted to make the grit, broken glass, stones and sand to settle down. Grit removal is a must for larger plants as opposed to the smaller ones. B2. Oxidation ditches Oxidation ditches are circular basins that waste water flows through. The oxidation ditch is the place where activated sludge is added so that microorganisms in the sludge can start digesting B.O.D inside the waste water. An oxidation ditch is a wide tank that serves the purpose of aeration in the long term. It is made of a long circular or elliptical channel that has a rotor which serves as the aeration equipment. The rotor generates water flow and stirs the channel water to bring about oxygen supply. Through the process of stirring the water is disturbed and allowed to mix with oxygen in the air. When the water is well aerated the microorganisms can aerobically break down the organic particles within the water. The oxidation ditch occupies a wide area although its structure is simple and its operation is easy. The oxidation ditch holds mixed liquor which is mixed with oxygen by use of rotating biological contactors. B3. Chlorination The process of chlorinating wastewater involves the addition of chlorine to the water. Chlorination is done for purposes such as controlling odours and disinfection. Chlorine is applied either in gaseous or liquid form. The chemical form of chlorine that removes odour or destroys microbes is basically the same. If the chlorine is in gaseous form it must first be dissolved in water before it is added to the waste water. The liquid form exists as a salt that is soluble in water and so it is directly dissolved in waste water. Chlorine gas is cheaper and therefore it is the most commonly used for waste water treatment. However gaseous chlorine may not be used in treatment plants if it poses a health risk or is required in tiny amounts. Chlorinators or chlorinizers are special devices used in applying chlorine (Hug 2006, 55). Chlorination is a process that plays a very important role in the treatment of waste water through the removal of pathogens and other impurities of a chemical and physical nature. Chlorine brings benefit to the process of waste water treatment through disinfection, prevention of septicity and odor control, helping in the removal of grease and scum, controlling the bulking of activated sludge and dealing with foaming and filter flies. It also serves the purpose of stabilizing activated sludge before it is disposed of. The chlorination process also helps in foul air scrubbing, destruction of phenols, cyanides and removal of ammonia from the waste water. B4. Clarifiers Clarifiers are important in the waste water treatment system since they clarify the waste water. Clarification is accepted widely, and has been in use for many years in waste water treatment processes. Clarifiers take solids out of the sewage water. The process of clarification results in the removal of turbidity, floating material and sediment from the water (Petrovski et al 2011, 56). The tank that is also called a clarifier has a scraper mechanism for collecting the settled solids to be taken out through pumping. In circular clarifiers there are floor scrapers linked to a rotating arm that scraps sludge in the direction of a trough above it on the central hopper. The scraper mechanisms have various forms depending on the treatment. In a circular clarifier the scraper may be a peripheral drive or cage drive (Petrovski et al 2011, 56). Rectangular clarifiers have a chain and flight mechanism or have equipment for removing scum and held by a moving bridge. Circular sedimentation tanks do better than rectangular ones because removal of sludge takes a short time and their maintenance is cheap. The clarifiers are not used in ne process but in a complex procedure with various combined processes. Gravity helps to settle the bigger suspended solids at the bottom of the clarifier. There are other materials that do not settle down in the tank. These are removed through coagulation, flocculation or sedimentation processes. Three processes namely rapid mix for coagulation, soli-liquid separation and moderate mixing for flocculation take place in clarifiers. These processes make up the water clarification or the conventional clarification process. Chemicals can also be applied in the clarifier to clarify the water as well as other methods including disinfection and filtration (Petrovski et al 2011, 56). Clarifiers exist in two types namely; circular center feed clarifier and circular peripheral flow clarifier. In the circular center feed clarifier, the waste water gets in through a center stilling well. This type of clarifier entry ensures that the water inside the clarifier has adequate resident time so that the solids can settle at the bottom. When the level of water rises it gets out of the tank through a weir mounted on the wall. In the circular peripheral flow clarifier the water flow enters the tank through the periphery. The water evenly moves around the tank and goes down the annulus in the tank in a spiral manner. This happens through the action of a baffle skirt designed specifically for that purpose. This allows the solids in the sewage to settle to the maximum towards the side where the sludge is collected from. B5. Sludge dewatering and disposal The production of liquid sludge only means that there is more treatment to be done before the sludge is finally disposed of. The water in sludge must be removed before disposal. Disposal of sludge is facilitated through transportation. Transportation is easily done when the sludge has the lowest content of water. Dewatering lowers liquid volume considerably. Digested sludge goes through big centrifuges spinning all through. The fast spinning of centrifuges removes a lot of the water from the sludge solids to form biosolids. Water obtained in this manner is taken to the plant to be reprocessed. Organic polymer is added to improve the consistency of sludge. The remaining product called “cake” is taken up by fertilizer making companies that make fertilizer pellets out of it. The sludge may also be taken to a digester where anaerobic digestion takes place. Sludge must be thickened or dewatered in order to lower its volume so that transportation from the site for disposal can be made easier. Currently, there is no designed process used to completely do away with the need for disposal of biosolids. However, in certain cities of the world, the sludge is superheated to make it into tiny pellets or granules with a high nitrogen content or organic material. The liquid removed from sludge is called centrate and it may be taken back to the process of treatment. Fertilizer pellets formed from the sludge are sold to turf farms and farmers as a fertilizer for use in farming. This process reduces the quantity of space needed for landfills used in disposal of sludge. A lot of the sludge obtained from commercial and industrial zones may have toxic substance contamination directed into sewers from industrial activities. Where these substances exist in high concentrations, the sludge produced may turn out to be unsuitable for use on farms. This means the sludge may need to be taken to a land fill or incinerator (Henze, Harremoes, Jansen, 2001). C. Offline position for Primary sedimentation tanks Primary sedimentation tanks are designed to make the waste water have the optimal speed for sedimentation. If the speed of water in the primary sedimentation tank is very high many of the particles in the waste water may not settle because they lack enough time for that and therefore they can flow away with treated water. The primary sedimentation tank at Loganholme WWTP’s is left offline so that the waste water can have a considerably low speed of flow. When this happens the level of turbulence and disturbance to the water goes down and the particles suspended within the water have time to settle at the bottom. The offline position ensures that water flows in the tank at a lower speed, is more still while in the tank and flows out slowly to increase the speed of sedimentation. The offline location of the tank there ensures that sedimentation of these particles happens at its maximum (Droste, 1997, 19). D. Foaming and scum on reactors The reactors at Loganholme waste water treatment plant have foam and scum. The foam is viscous and sticky with a brownish colour. It was seen floating on the tanks where it has accumulated over time and it takes a huge fraction of the volume of the reactors. The foam is formed from the existence of surfactants like household detergents that are biodegradable but at a very low rate. These surfactants originate from municipal and industrial waste water. Foam may also appear on the reactors because the extracellular polymeric substance produced by the microorganisms in the activated sludge is in excess. This happens when nutrients are limited. Filamentous organisms may proliferate and cause forming as well. It is also possible that the foam may result from the gas in aeration tanks, anaerobic digesters or the secondary clarifiers. Foaming in waste water treatment processes takes place when gas bubbles interact with hydrophobic and surfactant particles. These hydrophobic particles accumulate at the place where the water meets the air where they have the effect of strengthening the film of water existing between water bubbles. The same particles also collect surfactant that end up stabilizing the foam. The aeration process is responsible for generating the gas bubbles in the reactors. Mechanic mixing as well as biological processes such as anaerobic digestion and denitrification also form gas bubbles. In the process of waste water treatment, surfactants are known to originate in waste water streams that have surfactants that biodegrade at a very slow rate (Spellman, Drinan, 2003, 32). Foaming and scum formation in waste water treatment can be controlled in but with no universal effective methods. Particular measures must be taken based on the cause of the foam, the condition of operation and the microorganisms involved in the process. Foaming can be controlled through reduced sludge retention time or reduced time for mean cell retention. These can be applied in the operation of waste water treatment to clean out all filamentous bacteria, get rid of hydrophobic substances or certain substrates capable of causing foaming or creating a suitable environment for proliferation of filamentous bacteria. Chlorine which is an oxidizing agent could also be added in order to destroy filamentous bacteria as well as the other strains of bacteria. If the foam is not removed it may become a nuisance when it flows out of the tank onto the paths and other surrounding areas (Spellman, 2009, 66). Conclusion In conclusion, this report presents a discussion of the process followed in the Loganholme treatment plant to treat waste water. It also discusses oxidation ditches, chlorination, the offline location of the primary sedimentation tank and the formation of scum and foam on reactors. The process treats industrial and municipal water and removes all impurities. Waste water originates from households and industrial effluents. The waste water goes through primary treatment, secondary treatment then the process is completed with disinfection of the water and disposal of sludge. The waste water treatment plant must have all the processes discussed in this report for it to be complete and effective. Appendix An oxidation ditch (Hug 2006, 23) References Droste, R. (1997). Theory and Practice of Waste Water Treatment. J. Wiley. Henze, M. Harremoes, Jansen, J. (2001). Waste Water Treatment; Biological and Chemical Processes. Springer. Hug T. (2006). “ Characterization and controlling of foam and scum in activated sludge systems”. ETH Ph.D. Dissertation.  Jenkins, D, Richard, MG, Daigger, GT. (2004). Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems, 3rd edition. Lewis. Publishers. Kolarik, L. Priestley, A. (1996). Modern Techniques in Water and Waste Water Treatment. Csiro Publishing. Petrovski S, Dyson Z.A, Quill E.S, McIlroy S.J, Tillett D, Seviour R.J. (2011). An examination of the mechanisms for stable foam formation in activatedsludge systems. Water Research. 45 (5): 2146–2154.  Simon Judd – (2002) Process Science and Engineering for Water and Waste Water. IWA Publishing. Spellman, F. (2009). Handbook of Water and Waste Water Treatment Operations. Taylor & Francis. Spellman, F. Drinan, J. (2003). Waste Water Treatment Plant Operations Made Easy: Apractical Guide for Licensure. DEStech Publications Inc. Stephens, A., Fuller, M. (2009). Sewage Treatment: Uses, Processes and Impact. Nova Science Pub. Read More