wastewater treatment

environmental science- aerobic and anaerobic wastewater treatment

1. Aerobic and anaerobic wastewater treatment Ezhilmathi . S 2015006015

2. Wastewater treatment It is a process used to convert wastewater into an effluent that can be returned to the water cycle with minimal environmental issues Instead of disposing of treated wastewater it is reused for various purposes, which is knows as water reclamation During the treatment process, pollutants are removed or broken down The infrastructure used for wastewater treatment is called a wastewater treatment plant or a sewage treatment plant in the case of municipal wastewater

3. Aerobic wastewater treatment Aerobic processes use bacteria that require oxygen, so air is circulated throughout the treatment tank These aerobic bacteria then break down the waste within the wastewater  Some systems utilize a pretreatment stage prior to the main treatment to reduce the chance of clogging the system Electricity is required for system operation

4. Advantages Minimum odor Large BOD removal providing a good quality effluent High rate treatment with less land requirement Final discharge may contain DO which reduces the immediate OD on receiving water

5. Disadvantages Energy cost of aeration at an adequate rate to maintain the DO levels needed Some organics cant be efficiently decomposed aerobically These biologically non-reactive components mainly composed of insoluble materials can account for up to 70% COD Reduction in storage capacity of lagoons and/ or ponds

6. Activated sludge process Process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa is a biological process that can be used for oxidizing carbonaceous biological matter, oxidizing nitrogenous matter (NH 3 and N 2 ), removing nutrients (N and P). Aeration methods - diffused aeration, surface aerators (cones) and pure oxygen aeration

7. Activated sludge process The sludge blanket is measured from the bottom of the clarifier The Sludge Volume Index is the volume of settled sludge in mm occupied by 1 gram of dry sludge solids after 30 mins of settling in a 1000 ml graduated cylinder. The Mean Cell Residence Time  is the total mass(kg) of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate (kg/day) of MLSS effluent The F/M is amount of BOD fed to the aerator (kg/day) divided by the amount of  MLVSS (kg) under aeration Some use Mixed Liquor Suspended Solids for expedience, but Mixed Liquor Volatile Suspended Solids is considered more accurate for the measure of microorganisms

8. Activated sludge process

9. Process Pre-treatment stage to remove large solids and other undesirable substances Aeration stage, where aerobic bacteria digest biological wastes Settling stage allows undigested solids to settle, forms a sludge that must be periodically removed from the system Disinfecting stage, where chlorine or similar disinfectant is mixed with water, to produce an antiseptic output

10. Trickling filter First used by Dibden and Clowes    It consists of rocks, lava, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic, or plastic media over which sewage flows downward and causes a layer of microbial slime (biofilm) to grow, covering the bed of media Aerobic conditions are maintained by splashing, diffusion, and either by forced-air flowing through the bed or natural convection of air if the filter medium is porous

11. Process Sewage flow enters at a high level and flows through the primary settlement tank The supernatant from the tank flows into a dosing device, often a tipping bucket which delivers flow to the arms of the filter The flush of water flows through the arms and exits through a series of holes pointing at an angle downwards This propels the arms around distributing the liquid evenly over the surface of the filter media Both absorption and adsorption of organic compounds and some inorganic species by the layer of microbial bio film

12. Process The filter media is typically chosen to provide a very high surface area to volume Passage of the waste water over the media provides DO which the bio-film layer requires for the biochemical oxidation of the organic compounds and releases CO 2  gas, water and other oxidized end products As the bio film layer thickens, it eventually sloughs off into the liquid flow and subsequently forms part of the secondary sludge Other filters utilizing higher-density media do not produce a sludge that must be removed, but require forced air blowers and backwashing

13. Rotating biological contactor RBC is a type of secondary treatment process   The primary treatment process means removal of grit and sand through a screening process, followed by settling This process involves allowing the wastewater to come in contact with a biological medium in order to remove pollutants It consists of a series of closely spaced, parallel discs mounted on a rotating shaft which is supported just above the surface of the waste water. Microorganisms grow on the surface of the discs where biological degradation of the wastewater pollutants takes place

14. Biofilter Biofiltration is a fixed–film process It is a bed of media on which microorganisms attach and grow to form a biological layer called biofilm Biofiltration is used to treat wastewater from a wide range of sources, with varying organic compositions and concentrations This process is versatile as it can be adapted to small flows (< 1 m3/d) as well as to flows generated by a municipality (> 240 000 m3/d)

15. Aerobic stabilization ponds It reduce the organic content (measured as BOD) and kill pathogens in the wastewater Ponds are depressions holding water confined by earthen structures After treatment, the effluent may be returned to surface water or reused as irrigation water if the effluent quality is high enough Waste stabilization ponds use no aerators High-performance lagoon technology with aerators has much more in common with that of activated sludge

16. Aerated lagoon This system consisting of a pond with artificial aeration to promote the biological oxidation of wastewaters

17. Maturation pond A self-purifying reservoir used to stabilize effluent from mechanized secondary treatment Maturation ponds receiving wastes with low BOD concentrations may be clear enough to accomplish pathogen reduction through exposure to UV solar radiation  Maturation ponds may be used in combination with a rainwater reservoir to form an ecological, swimming pond It should be saturated with DO throughout the entire pond and shallow enough for light to penetrate the entire depth of the pond It must be shallow with a great surface area so that more oxygen can dissolve into the water giving the bacteria enough oxygen to properly function By adding the fish the ponds bug issues are controlled in a natural and eco- friendly

18. Facultative lagoons Organic waste from food or fiber processing may be catabolized in a system of constructed ponds where adequate space is available to provide an average waste retention time exceeding a month A series of ponds prevents mixing of untreated waste with treated wastewater and allows better control of waste residence time for uniform treatment efficiency

19. Anaerobic lagoon An anaerobic lagoon filled with animal waste that undergoes anaerobic respiration The manure settles into two layers: solid, or sludge layer and the liquid layer The volatile organic compounds are converted into CO 2  and CH 4 This allows for preliminary sedimentation of suspended solids as a pretreatment process These substances are emitted through two main pathways: gas emissions and lagoon overflow Gas emissions are continuous The most prevalent gasses emitted by the lagoon are: NH 3 , H 2 S, CO 2  and CH 4

20. Anaerobic wastewater treatment Anaerobic bacteria transform organic matter in the wastewater into biogas that contains large amounts of methane gas and carbon dioxide Energy-efficient process  Often used to treat industrial wastewater that contains high levels of organic matter in warm temperatures  It can be used as a pretreatment prior to aerobic municipal wastewater treatment

21. Types of anaerobic digester The following is a partial list of digesters can be categorized according to two main criteria: by whether the biomass is fixed to a surface or can mix freely with the reactor liquid and by the organic loading

22. Upflow anaerobic sludge blanket It is a suspended-growth high-rate digester, with its biomass clumped into granules that will settle relatively easily and with typical loading rates in the range 5-10 kg COD/m 3 /d It is a methanogenic (methane-producing) digester that evolved from the anaerobic clarigester

23. Process Forming a blanket of granular sludge which suspends in the tank Wastewater flows upwards through the blanket and is processed (degraded) by the anaerobic microorganisms The upward flow combined with the settling action of gravity suspends the blanket with the aid of  flocculants The blanket begins to reach maturity at around three months Eventually the aggregates form into dense compact biofilms referred to as "granules " Biogas with a high concentration of methane is produced as a by-product, and this may be captured and used as an energy source, to generate electricity  The heat produced as a by-product of electricity generation can be reused to heat the digestion tanks UASB reactors are typically suited to dilute waste water streams (3% TSS with particle size >0.75mm )

24. Expanded granular sludge bed It is a variant of the  UASB concept Faster rate of upward-flow velocity is ei ther accomplished by utilizing tall reactors, or by incorporating an effluent recycle for the wastewater passing through the sludge bed The increased flux permits partial expansion ( fluidisation ) of the granular sludge bed, improving wastewater-sludge contact as well as enhancing segregation of small inactive suspended particle from the sludge bed It is appropriate for low strength soluble wastewaters (< 1 - 2 g soluble COD/l) or for wastewaters that contain inert or poorly biodegradable suspended particles

25. Anaerobic filter The digestion tank contains a filter medium where anaerobic microbial populations can establish themselves They produce a less solid residue   than other types of filter

26. Up-flow Anaerobic Hybrid Reactors It i nvolves the introduction of a new phase of attached-biomass into a regular suspended-growth system It contains both suspended and attached-growth biomass which is developed by introducing porous materials into a regular activated sludge unit It combines a UASB reactor with an anaerobic filter This combination is an advanced form enabling improved solid retention time in the treatment of waste water This waste water can be built up in the secondary chamber and must be removed daily

27. Fluidized bed reactor A type of reactor device that can be used to carry out a variety of multiphase chemical reactions   Fluid  (gas or liquid) is passed through a solid granular material  at high enough velocities to suspend the solid This process, known as fluidization, imparts many important advantages

28. Merits High degree of waste stabilization Low production of excess biological sludge that can be directly dried on sludge drying bed without further treatment Low nutrition requirements No oxygen requirement Production of valuable by product, methane gas Organic loading is not limited Less land requirement Non feed conditions for few month do not affect adversely to the system (can work seasonally)

29. Disadvantages Incomplete break-down of organic compounds No thorough nutrient removal Again later aerobic purification with nutrient removal is often needed Most efficient purification in the mesophilic range (30-37°C) whereby the influent must be heated in most cases Less robust system with regards to toxicity and inhibition Risk of odor problems

30. References en.wikipedia.org www.sswm.info https://greentumble.com http://nptel.ac.in www.extension.umn.edu emis.vito.be