A PROJECT REPORT ON
“WASTEWATER TREATMENT”
SUBMITTED TO
POLLUCON TECHNOLOGIES LIMITED, NOIDA
SUBMITTED BY ROSHAN MISHRA BIO CHEMICAL ENGINEERING HARCOURT BUITLER TECHNOLOGICAL INSTITUTE NAWABGANJ, KANPUR- 208002
TO WHOM IT MAY CONCERN
This is to certify that ROSHAN MISHRA, student of Harcourt Butler Technological Institute, Kanpur has completed his 6 weeks industrial training from Pollucon Technologies Limited, Noida under the guidance of Mr. S.K. Singh.
Mr. S.K. Singh Director Pollucon Technologies Limited D-39, Sector-7 Noida - 201301
ACKNOWLEDGEMENTS
I would like to thank Mr. S. K. Singh, Director, Pollucon Technologies Ltd., Noida, for his valuable time and guidance in completing my Industrial training and preparation of the Project Report.
ROSHAN MISHRA BIO CHEMICAL ENGINEERING HARCOURT BUTLER TECHNOLOGICAL INSTITUTE NAWABGANJ, KANPUR
COMPANY PROFILE The main objectives of the company are: • Sewage Treatment /Sewerage system / Sewage disposal • Water Treatment /Supply/distribution • Effluent Treatment • Process. Equipment • Air Pollution Control Devices. • Environmental Impact Assessment Studies • Environmental Management Plan. • Environmental Audits / Statements. • On Site Emergency Plan. • Packaged Effluent Treatment Plants.
Environmental Engineering The Rationale Mission The Spirit to contribute towards the welfare of human being by optimum use of available natural resources and optimum use and discharge of water to maintain equilibrium is realism at Pollucon Technologies Ltd. (PTL). PTL is committed to Design, Engineer, Deliver, review and continuously up grade innovative Environmental Engineering Solutions in the fields of Effluent / Water Treatment / Disposal System, Air Pollution Control Systems, Solid Waste Management by exercising quality and adding values for maintaining trustworthiness with all customers & clients. Being Honest, respectful & Co- Operative having the set of objectives as follows: To educate people and organizations about optimum water utilization and minimum possible disposal. To provide Technology for minimizing waste generation. To design and deliver the most efficient waste treatment systems. To provide diagnostic services to assess the present technology and identify improvement areas. To conduct application research for improvement of existing Technology and to create and develop better system.
THE CORPORATE M/s POLLUCON TECHNOLOGIES LIMITED (PTL) is an organization backed by the vision of experienced Engineers Scientists and Management Professionals having expertise in the fields of Survey & Study of Water supply, Water Treatment, Environmental and Chemicals Engineering. The specialty of the organization is turnkey projects in the field of Air, Water, Sewage and Solid Waste Management and support services being provided by a team of dedicated, efficient, highly qualified, experienced advisors with proven track records in their respective disciplines of competence. The company also has an added advantages of having requisite in house facilities such as Laboratory for Water, Wastewater testing, R&D, technical Library, drafting Printing, CAD & in House fabrication etc. The above competence supplemented by prompt response and innovative applications makes Pollucon Technologies Limited “a force to reckon with, when it comes to Water supply, Treatment & Environmental Engineering Solutions.
GENERAL OUTLINES OF SEWAGE TEATMENT Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater, both runoff (effluents) and domestic. It includes physical, chemical and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a waste stream (or treated effluent) and a solid waste or sludge suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with many toxic organic and inorganic compounds. Sewage is created by residences, institutions, hospitals and commercial and industrial establishments. It can be treated close to where it is created (in septic tanks, biofilters or aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant (see sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Industrial sources of wastewater often require specialized treatment processes (see Industrial wastewater treatment). The sewage treatment involves three stages, called primary, secondary and tertiary treatment. First, the solids are separated from the wastewater stream. Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne microorganisms. Finally, the biological solids are neutralized then disposed of or re-used, and the treated water may be disinfected chemically or physically (for example by lagoons and micro-filtration). The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge. Raw influent (sewage) includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The draining of household waste into greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. A lot of sewage also includes some surface water from roofs or hard-standing areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include stormwater runoff. Sewage systems capable of handling stormwater are known as combined systems or combined sewers. Such systems are usually avoided since they complicate and thereby reduce the efficiency of sewage treatment plants owing to their seasonality. The variability in flow also leads to often larger than necessary, and subsequently more expensive, treatment facilities. In addition, heavy storms that contribute more flows than the treatment plant can handle may overwhelm the sewage treatment system, causing a spill or overflow (called a combined sewer overflow, or CSO, in the United States). It is preferable to have a separate storm drain system for stormwater in areas that are developed with sewer systems. As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include sedimentation basins, wetlands, buried concrete vaults with various kinds of filters, and vortex separators (to remove coarse solids).
The site where the raw wastewater is processed before it is discharged back to the environment is called a wastewater treatment plant (WWTP). The order and types of mechanical, chemical and biological systems that comprise the wastewater treatment plant are typically the same for most developed countries: •
•
•
Mechanical treatment o Influx (Influent) o Removal of large objects o Removal of sand and grit o Pre-precipitation Biological treatment o Oxidation bed (oxidizing bed) or aeration system o Post precipitation Chemical treatment (this step is usually combined with settling and other processes to remove solids, such as filtration. The combination is referred to in the U.S. as physic
Primary treatment removes the materials that can be easily collected from the raw wastewater and disposed of. The typical materials that are removed during primary treatment include fats, oils, and greases (also referred to as FOG), sand, gravels and rocks (also referred to as grit), larger settleable solids and floating materials (such as rags and flushed feminine hygiene products). This step is done entirely with machinery. Many plants have a sedimentation stage where the sewage is allowed to pass slowly through large tanks, commonly called "primary clarifiers" or "primary sedimentation tanks". The tanks are large enough that sludge can settle and floating material such as grease and oils can rise to the surface and be skimmed off. The main purpose of the primary clarification stage is to produce both a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages.
Secondary treatment Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal and industrial plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota requires both oxygen and a substrate on which to live. There are number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed film or suspended growth. Fixed-film treatment process including trickling filter and rotating biological contactors where the biomass grows on media and the sewage passes over its surface. In suspended growth systems—such as activated sludge—the biomass is well mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of
biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems. Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them to then be treated by conventional secondary treatment processes. Characteristics include typically tall, circular filters filled with open synthetic filter media to which wastewater is applied at a relatively high rate. They are designed to allow high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the normal range for conventional treatment processes. Tertiary treatment Tertiary treatment provides a final stage to raise the effluent quality before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called "effluent polishing".
ROLE OF BACTERIA IN WASTEWATER TREATMENT: Treatment plants should be designed to take advantage of the decomposition of organic materials by bacterial activity. This is something anyone can equate to lower costs, increased capacity, and an improved quality of effluent; even freedom from bad odours which may typically result when anaerobe bacteria become dominant and in their decomposition process, produce hydrogen sulphide gas and similar by-products. Considering the fact that the total organic load of wastewater or sewage is composed of constantly changing constituent, it would be quite difficult to degrade all of these organics by
the addition of one enzyme, or even several enzymes. Enzymes are specific catalysts and do not reproduce. What is needed is the addition of an enzyme manufacturing system right in the sewage that can be pre - determined as to its activity and performance and which has the initial or continuing capacity to reduce waste. At the present time, the addition of specifically cultured bacteria seems to be the least expensive and most generally reliable way to accomplish desirable results. When you add the right bacteria in proper proportions to the environment, you have established entirely new parameters of potential for the treatment situation.
WASTE WATER TREATMENT TECHNOLOGIES The principal biological processes used for wastewater treatment are divided into two main categories: 1) Suspended growth processes 2) Attached growth (or biofilm) processes. SUSPENDED GROWTH PROCESSES In suspended growth processes the microorganisms responsible for treatment are maintained in liquid suspension by appropriate mixing methods. Many suspended growth processes used in municipal and industrial wastewater treatment are operated with a positive dissolved oxygen concentration (aerobic), but applications exist where suspended growth anaerobic processes are used, such as for high organic concentration industrial wastewater and organic sludges. The most common suspended growth process used for municipal wastewater treatment is the Activated Sludge Process. ATTACHED GROWTH PROCESSES:In attached growth processes, the microorganisms responsible for the conversion of organic material or nutrients are attached on inert packing material. The organic material and nutrients are removed from the water flowing past the attached growth also known as the biofilm. Packing material in attached growth processes include rock, gravel, sand, slag, redwood and range of plastics and other synthetic materials. Attached growth processes are operated as aerobic or anaerobic processes. The packing can be submerged completely in liquid or not submerged, with air or gas space above the biofilm liquid layer. The most common aerobic attached growth process used is the Trickling filter. The major methods for wastewater treatment are listed below: • • • • • • • • •
Activated sludge systems Aerated lagoon Aerobic granular reactor Aerobic treatment system Anaerobic clarigester Anaerobic digestion API oil-water separator Anaerobic lagoon Bead Filter
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Belt press Bioconversion of biomass to mixed alcohol fuels Bioreactor Bioretention Biorotor Bioroll [2] Biolytix Carbon filtering Cesspit Chlorine disinfection Combined sewer Composting toilet Constructed wetland Dissolved air flotation Distillation Electrocoagulation Electrodeionization Electrolysis Expanded granular sludge bed digestion Facilitative lagoon Flocculation & sedimentation Fluidized Bed Biofilter Flotation process Froth flotation Fuzzy Filter Humanure (composting) Imhoff tank Iodine Ion exchange Living machines Membrane Bioreactor Nanotechnology N-Viro Ozone and Ultrasound Parallel plate oil-water separator Recirculating Sand Filter Reed bed Retention basin Reverse osmosis Rotating biological contactor Sand filter Septic tank Sequencing batch reactor Sewage treatment Submerged aerated filter Treatment pond Trickling filter Ultrafiltration (industrial) Ultraviolet disinfection
• •
Upflow anaerobic sludge blanket digestion Wet oxidation CLASSIFICATION OF WASTEWER TREATMENT TECHNOLOGIES
The wastewater treatment technologies can be broadly classified on the basis of mode of operation as: 1) Aerobic (in presence of oxygen) 2) Anaerobic (in absence of oxygen) THE RATIONALE FOR ANAEROBIC TREATMENT The rationale for and interest in the use of anaerobic treatment process can be explained by considering the advantages and disadvantages of this process. The principal advantages and disadvantages of anaerobic treatment are listed as follows: Advantages:
Less energy requirement since aeration is not required. Less biological sludge production Fewer nutrients required Methane production, a potential energy source Smaller reactor volume required Elimination of off-gas air pollution Rapid response to substrate addition after long periods without feeding.
Disadvantages: Longer start-up time to develop necessary biomass inventory May require alkalinity addition May require further treatment with an aerobic treatment process to meet discharge requirements Biological nitrogen and phosphorus removal is not possible Much more sensitive to the adverse effects of lower temperatures on reaction rates. May be more susceptible to upsets due to toxic substances Potential for production of odors and corrosive gases.
AEROBIC TREATMENT PROCESSES ACTIVATED SLUDGE PROCESS (ASP) The activated sludge process is a wastewater treatment method in which the carbonaceous organic matter of wastewater provides an energy source for the production of new cells for a mixed population of microorganisms in an aquatic aerobic environment. The microbes convert carbon into cell tissue and oxidized end products that include carbon dioxide and water. In addition, a limited number of microorganisms may exist in activated sludge that obtain energy by oxidizing ammonia nitrogen to nitrate nitrogen in the process known as nitrification.
Bacteria constitute the majority of microorganisms present in activated sludge. Bacteria that require organic compounds for their supply of carbon and energy (heterotrophic bacteria) predominate, whereas bacteria that use inorganic compounds for cell growth (autotrophic bacteria) occur in proportion to concentrations of carbon and nitrogen. Both aerobic and anaerobic bacteria may exist in the activated sludge, but the preponderance of species are facultative, able to live in either the presence of or lack of dissolved oxygen. Fungi, rotifers, and protozoan are also residents of activated sludge. The latter microorganisms are represented largely by ciliated species, but flagellated protozoan and amoebae may also be present. Protozoan serve as indicators of the activated sludge condition, and ciliated species are instrumental in removing Escherichia coli from sewage. Additionally, viruses of human origin may be found in raw sewage influent, but a large percentage appears to be removed by the activated-sludge process. The success of the activated-sludge process is dependent upon establishing a mixed community of microorganisms that will remove and consume organic waste material, that will aggregate and adhere in a process known as bio flocculation, and that will settle in such a manner as to produce a concentrated sludge (return activated sludge, or RAS) for recycling. Any of several types of activated sludge solids separations problems indicate an imbalance in the biological component of this process. In the ideal "healthy" system, filamentous organisms grow within a floc (a large aggregate of adherent, or floc-forming, microorganisms, such as bacteria) and give it strength, with few filaments protruding out into the surrounding bulk solution. In such a system, there is no interference with the compaction and settling rates of the activated sludge prior to its recycling. In the figure above, following are the terms: Q=
flow rate of influent
[m3/d]
QW = waste sludge flow rate
[m3/d]
Qr = Flow rate in return line from clarifier
[m3/d]
V=
volume of aeration tank
[m3]
S0 =
influent soluble substrate concentration (bsCOD)
[BOD g/m3] or [bsCOD g/m3]
S=
effluent soluble substrate concentration (bsCOD)
[BOD g/m3] or [bsCOD g/m3]
X0 = concentration of biomass in influent
[g VSS/m3]
XR = concentration of biomass in return line from clarifier
[g VSS/m3]
Xr = concentration of biomass in sludge drain
[g VSS/m3]
Xe = concentration of biomass in effluent
[g VSS/m3]
The activated-sludge process is a biological method of wastewater treatment that is performed by a variable and mixed community of microorganisms in an aerobic aquatic environment. These microorganisms derive energy from carbonaceous organic matter in aerated wastewater for the production of new cells in a process known as synthesis, while simultaneously releasing energy through the conversion of this organic matter into compounds that contain lower energy, such as carbon dioxide and water, in a process called respiration. As well, a variable number of microorganisms in the system obtain energy by converting ammonia nitrogen to nitrate nitrogen in a process termed nitrification. This consortium of microorganisms, the biological component of the process, is known collectively as activated sludge. The overall goal of the activated-sludge process is to remove substances that have a demand for oxygen from the system. This is accomplished by the metabolic reactions (synthesisrespiration and nitrification) of the microorganisms, the separation and settling of activatedsludge solids to create an acceptable quality of secondary wastewater effluent, and the collection and recycling of microorganisms back into the system or removal of excess microorganisms from the system.
THE PHYSICAL COMPONENTS OF THE ACTIVATED-SLUDGE PROCESS According to Activated Sludge, Manual of Practice #9 (Water Environment Association, 1987), the activated-sludge process contains five essential interrelated equipment components. The first is an aeration tank or tanks in which air or oxygen is introduced into the system to create an aerobic environment that meets the needs of the biological community and that keeps the activated sludge properly mixed. At least seven modifications in the shape and number of tanks exist to produce variations in the pattern of flow. Second, an aeration source is required to ensure that adequate oxygen is fed into the tank(s) and that the appropriate mixing takes place. This source may be provided by pure oxygen, compressed air or mechanical aeration. Just as there are modifications in the shape and number of aeration tanks that can be used in the activated-sludge process, different equipment systems exist to deliver air or oxygen into aeration tanks. Third, in the activated-sludge process, aeration tanks are followed by secondary clarifiers. In secondary clarifiers, activated-sludge solids separate from the surrounding waterwater by the process of flocculation (the formation of large particle aggregates, or flocs, by the adherence of floc-forming organisms to filamentous organisms) and gravity sedimentation, in which flocs settle toward the bottom of the clarifier in a quiescent environment. This separation leads ideally to the formation of a secondary effluent (wastewater having a low level of activated-sludge solids in suspension) in the upper portion of the clarifier and a thickened sludge comprised of flocs, termed return activated sludge, or RAS, in the bottom portion of the clarifier.
Next, return activated sludge must be collected from the secondary clarifiers and pumped back to the aeration tank(s) before dissolved oxygen is depleted. In this way, the biological community needed to metabolize influent organic or inorganic matter in the wastewater stream is replenished. Finally, activated sludge containing an overabundance of microorganisms must be removed, or wasted (waste activated sludge, or WAS), from the system. This is accomplished with the use of pumps and is done in part to control the food-to-microorganism ratio in the aeration tank(s).
THE BIOLOGICAL COMPONENT OF THE ACTIVATED-SLUDGE SYSTEM The biological component of the activated sludge system is comprised of microorganisms. The composition of these microorganisms is 70 to 90 percent organic matter and 10 to 30 percent organic matter. Cell makeup depends on both the chemical composition of the wastewater and the specific characteristics of the organisms in the biological community. Bacteria, fungi, protozoa, and rotifers constitute the biological component, or biological mass, of activated sludge. In addition, some metazoan, such as nematode worms, may be present. However, the constant agitation in the aeration tanks and sludge recirculation are deterrents to the growth of higher organisms. The species of microorganism that dominates a system depends on environmental conditions, process design, the mode of plant operation, and the characteristics of the secondary influent wastewater. The microorganisms that are of greatest numerical importance in activated sludge are bacteria, which occur as microscopic individuals from one micron in size to visible aggregations or colonies of individuals. Some bacteria are strict aerobes (they can only live in the presence of oxygen), whereas others are anaerobes (they are active only in the absence of oxygen). The preponderance of bacteria living in activated sludge are facultative—able to live in either the presence or absence of oxygen, an important factor in the survival of activated sludge when dissolved oxygen concentrations are low or perhaps approaching depletion. While both heterotrophic and autotrophic bacteria reside in activated sludge, the former predominate. Heterotrophic bacteria obtain energy from carbonaceous organic matter in influent wastewater for the synthesis of new cells. At the same time, they release energy via the conversion of organic matter into compounds such as carbon dioxide and water. Important genera of heterotrophic bacteria include Achromobacter, Alcaligenes, Arthrobacter, Citromonas, Flavobacterium, Pseudomonas, and Zoogloea.
NITRIFICATION 2 NH4+ + 3O2 =2NO2- + 4H+ + 2H2O + energy Nitrosomonas 2NO2 + O2 =2NO3- + energy Nitrobacter
Nitrification generally occurs when the time that the sludge stays in the system (called the mean cell residence time, or MCRT) is increased. A longer mean cell residence time, therefore, allows an adequate population of nitrifying bacteria to be built up. However, because the oxygen demand for complete nitrification is high, both the necessary oxygen supply and power requirements for the system will be increased. Moreover, optimum pH for the growth of nitrifying bacteria is in the 8 to 9 range, with pH levels below 7 causing a substantial reduction in nitrification activity. In the process of converting ammonia to nitrate, mineral acidity is produced. In instances when insufficient alkalinity exists, the pH in the system will drop, potentially inhibiting nitrification. Finally, though nitrification occurs over a wide range of temperatures, a reduction in temperature produces a slower rate of reaction.
CALCULATION OF THE COMMON CHARACTERISTICS OF A COMPLETE-MIX ACTIVATED-SLUDGE SYSTEM WITH RECYCLE Characteristics of Primary Sedimentation Effluent wastewater flow rate Q=
1000
influent soluble substrate concentration (bsCOD) S0 =
192
nbVSS concentration in influent inert inorganic Total Suspended Solids (iTSS) total MLVSS concentration Sedimentation Retention Time (SRT)
X0,i = iTSS = XT = SRT =
30 10 2500 6
k= Y= kd = Ks = fd =
12.5 0.4 0.1 10 .15 0.85
m3/d BOD or bsCOD g/m3 g/m3 or mg/l g/m3 g/m3 or mg/l d
Kinetic Coefficients maximum rate of soluble substrate utilization biomass yield endogenous decay coefficient half-velocity constant fraction of biomass that remains as cell debris biomass fraction
Characteristics of Complete-Mix Suspended Growth Process effluent soluble substrate concentration (bsCOD) S = 0.56
g g g g g
COD/g∙d VSS/g COD used VSS/g VSS∙d COD/m3 VSS/g VSS
g bsCOD/m3
Hydraulic Retention Time (HRT)
τ=
0.197
d
daily sludge production
PX,T,VSS = PX,T,TSS = X/XT = Yobs,VSS = Yobs,TSS = R0 =
82.2 101.4 0.58 0.48 0.53 117.4
kg VSS/d kg TSS/d kg/d g VSS/g bsCOD g TSS/g bsCOD kg/d
fraction of biomass in the MLVSS observed solids yield removed oxygen requirement
TRICKLING FILTER
A schematic cross-section of the contact face of the bed media in a trickling filter Trickling filter consists of a fixed bed of rocks, gravel, slag, polyurethane foam, sphagnum peat moss, or plastic media over which sewage or other wastewater flows downward and is contacted with a layer or film of microbial slime covering the bed media. Aerobic conditions are maintained either by forced air flowing through the bed or natural convection of air if the filter medium is porous. The process mechanism involves adsorption of organic compounds in the sewage or other wastewater by the layer of microbial slime, diffusion of air into the slime layer to furnish the oxygen required for the biochemical oxidation of the organic compounds to release carbon dioxide gas, water and other oxidized end products. As the slime layer thickens, it becomes more difficult for air to penetrate the layer and an inner anaerobic layer is probably formed. For some plastic-mesh material filters this slime layer will build and eventually slough off the smooth plastic walls into the treated effluent as a sludge that requires subsequent removal and disposal. Other filters utilizing higher-density media such as sand, foam and peat moss do not produce a sludge that must be removed. The terms trickle filter, trickling biofilter, biofilter, biological filter and biological trickling filter are often used to refer to a trickling filter. These systems have also been described as intermittent filters, packed media bed filters, alternative septic systems, percolating filters, attached growth processes, and fixed film processes.
Types Two of the basic types of trickle filters are those applied to the treatment of sewage and those applied to the treatment of industrial wastewater. Sewage treatment trickle filters Onsite sewage facilities (OSSF) are recognized as viable, low-cost, long-term, decentralized approaches to sewage treatment if they are planned, designed, installed, operated and maintained properly (USEPA, 1997).
Sewage trickling filters are used in areas not serviced by municipal wastewater treatment plants (WWTP). They are typically installed in areas where the traditional septic tank system are failing, cannot be installed due to site limitations, or where improved levels of treatment are required for environmental benefits such as preventing contamination of ground water or surface water. Sites with a high water table, high bedrock, heavy clay, small land area, or which require minimal site destruction (for example, tree removal) are ideally suited for trickling filters. All varieties of sewage trickling filters have a low and sometimes intermittent power consumption. They can be somewhat more expensive than traditional septic tank-leach field systems, however their use allows for better treatment, a reduction in size of disposal area, less excavation, and higher density land development.
Configurations and components All sewage trickling filter systems share the same fundamental components: • • • • •
a septic tank for fermentation and primary settling of solids a filter medium upon which beneficial microbes (biomass, biofilm) are promoted and developed a container which houses the filter medium a distribution system for applying wastewater to be treated to the filter medium a distribution system for disposal of the treated effluent.
By treating septic tank effluent before it is distributed into the ground, higher treatment levels are obtained and smaller disposal means such as leach field, shallow pressure trench or area beds are required. Systems can be configured for single-pass use where the treated water is applied to the trickling filter once before being disposed of, or for multi-pass use where a portion of the treated water is cycled back to the septic tank and re-treated via a closed-loop. Multi-pass systems result in higher treatment quality and assist in removing Total Nitrogen (TN) levels by promoting nitrification in the aerobic media bed and denitrification in the anaerobic septic tank. Trickling filters differ primarily in the type of filter media used to house the microbial colonies. Types of media most commonly used include plastic matrix material, open-cell polyurethane foam, sphagnum peat moss, recycled tires, clinker, gravel,sand and geotextiles. Ideal filter medium optimizes surface area for microbial attachment, wastewater retention time, allows air flow, resists plugging and does not degrade. Some residential systems require forced aeration units which will increase maintenance and operational costs.
A typical complete trickling filter system
Industrial wastewater treatment trickle filters Wastewaters from a variety of industrial processes have been treated in trickling filters. Such industrial wastewater trickling filters consist of two types: •
Large tanks or concrete enclosures filled with plastic packing or other media.
•
Vertical towers filled with plastic packing or other media.
The availability of inexpensive plastic tower packings has led to their use as trickling filter beds in tall towers, some as high as 20 meters. The treated water effluent from industrial wastewater trickling filters is very often subsequently processed in a clarifier-settler to remove the sludge that sloughs off the microbial slime layer attached to the trickling filter media (see Image 1 above). Currently, some of the latest trickle filter technology involves aerated biofilters which are essentially trickle filters consisting of plastic media in vessels using blowers to inject air at the bottom of the vessels, with either downflow or upflow of the wastewater.
AERATED LAGOONS An aerated lagoon or aerated basin is a holding and/or treatment pond provided with artificial aeration to promote the biological oxidation of wastewaters. There are many other biological processes for treatment of wastewaters, for example activated sludge, trickling filters, rotating biological contactors and bio filters. They all have in common the use of oxygen (or air) and microbial action to bio treats the pollutants in wastewaters. Types of aerated lagoons or basins There are many methods for aerating a lagoon or basin: • • • •
Motor-driven floating surface aerators Motor-driven submerged aerators Motor-driven fixed-in-place surface aerators Injection of compressed air through submerged diffusers
Floating surface aerators
A Typical Surface-Aerated Basing (using motor-driven floating aerators) Ponds or basins using floating surface aerators achieve 80 to 90% removal of BOD with retention times of 1 to 10 days. The ponds or basins may range in depth from 1.5 to 5.0 metres. In a surface-aerated system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kWh. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units. Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C
ANAEROBIC METHODS UPFLOW ANAEROBIC SLUDGE TREATMENT PROCESS ( UASB) Upflow Anaerobic Sludge Blanket (UASB) technology, normally referred to as UASB reactor, is a form of anaerobic digester that is used in the treatment of wastewater. The UASB reactor is a methanogenic (methane-producing) digester that evolved from the anaerobic clarigester. A similar but variant technology to UASB is the expanded granular sludge bed (EGSB) digester. UASB uses an anaerobic process whilst 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 3 months. Small sludge granules begin to form whose surface area is covered in aggregations of bacteria. In the absence of any support matrix, the flow conditions create a selective environment in which only those microorganisms, capable of attaching to each other, survive and proliferate. Eventually the aggregates form into dense compact biofilms referred to as "granules". Biogas with a high concentration of methane is produced as a byproduct, and this may be captured and used as an energy source, to generate electricity for export and to cover its own running power. The technology needs constant monitoring when put into use to ensure that the sludge blanket is maintained, and not washed out (thereby losing the effect). The heat produced as a by-product of electricity generation can be reused to heat the digestion tanks. The blanketing of the sludge enables a dual solid and hydraulic (liquid) retention time in the digesters. Solids requiring a high degree of digestion can remain in the reactors for periods up to 90 days. Sugars dissolved in the liquid waste stream can be converted into gas quickly in the liquid phase which can exit the system in less than a day. UASB reactors are typically suited to dilute waste water streams (3% TSS with particle size >0.75mm).
ANAEROBIC DIGESTION Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is widely used to treat wastewater sludges and organic wastes because it provides volume and mass reduction of the input material. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertiliser. The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide,
hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Methanogenic bacteria finally are able to convert these products to methane and carbon dioxide. The technical expertise required to maintain anaerobic digesters coupled with high capital costs and lower process efficiencies have so far limited the level of its industrial application as a waste treatment technology. Careful control of the digestion temperature, pH, and loading rates is crucial to obtaining efficient breakdown of the material, and disturbances to a digest can lead to process failure. Ensuring that the quality of input materials to the digesters is maintained and that the process effectively monitored is essential for ensuring that a digester's performance is reliable.
Applications Anaerobic digestion is particularly suited to wet organic material and is commonly used for effluent and sewage treatment. Anaerobic digestion is a simple process that can greatly reduce the amount of organic matter which might otherwise be destined to be landfilled or burnt in an incinerator. Almost any organic material can be processed with anaerobic digestion. This includes biodegradable waste materials such as waste paper, grass clippings, leftover food, sewage and animal waste. The exception to this is woody wastes that are largely unaffected by digestion as anaerobes are unable to degrade lignin. Anaerobic digesters can also be fed with specially grown energy crops such as silage for dedicated biogas production. In Germany and continental Europe these facilities are referred to as biogas plants. A co-digestion or cofermentation plant is typically an agricultural anaerobic digester that accepts two or more input materials for simultaneous digestion. In developing countries simple home and farm-based anaerobic digestion systems offer the potential for cheap, low-cost energy for cooking and lighting. Anaerobic digestion facilities have been recognised by the United Nations Development Programme as one of the most useful decentralised sources of energy supply. Pressure from environmentally-related legislation on solid waste disposal methods in developed countries has increased the application of anaerobic digestion as a process for reducing waste volumes and generating useful by-products. Anaerobic digestion may either be used to process the source separated fraction of municipal waste, or alternatively combined with mechanical sorting systems, to process residual mixed municipal waste. These facilities are called mechanical biological treatment plants Utilising anaerobic digestion technologies can help to reduce the emission of greenhouse gases in a number of key ways: • • • • •
Replacement of fossil fuels Reducing methane emission from landfills Displacing industrially-produced chemical fertilisers Reducing vehicle movements Reducing electrical grid transportation losses
Methane and power produced in anaerobic digestion facilities can be utilised to replace energy derived from fossil fuels, and hence reduce emissions of greenhouse gases .This is due to the fact that the carbon in biodegradable material is part of a carbon cycle. The carbon released into the atmosphere from the combustion of biogas has been removed by plants in order for them to grow in the recent past. This can have occurred within the last decade, but more typically within the last growing season. If the plants are re-grown, taking the carbon out of the atmosphere once more, the system will be carbon neutral.This contrasts to carbon in fossil fuels that has been sequestered in the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in the atmosphere. Digestate liquor can be used as a fertiliser supplying vital nutrients to soils. The solid, fibrous component of digestate can be used as a soil conditioner. The liquor can be used as a substitute for chemical fertilisers which require large amounts of energy to produce. The use of manufactured fertilisers is therefore more carbon intensive than the use of anaerobic digestate fertiliser. This solid digestate can be used to boost the organic content of soils. The process There are a number of bacteria that are involved in the process of anaerobic digestion including acetic acid-forming bacteria (acetogens) and methane-forming bacteria (methanogens). These bacteria feed upon the initial feedstock, which undergoes a number of different processes converting it to intermediate molecules including sugars, hydrogen & acetic acid before finally being converted to biogas. Different species of bacteria are able to survive at different temperature ranges. Ones living optimally at temperatures between 35-40°C are called mesophiles or mesophilic bacteria. Some of the bacteria can survive at the hotter and more hostile conditions of 55-60°C, these are called thermophiles or thermophilic bacteria. Methanogens come from the primitive group of archaea. This family includes species that can grow in the hostile conditions of hydrothermal vents. These species are more resistant to heat and can therefore operate at thermophilic temperatures, a property that is unique to bacterial families. As with aerobic systems the bacteria in anaerobic systems the growing and reproducing microorganisms within them require a source of elemental oxygen to survive. In an anaerobic system there is an absence of gaseous oxygen. In an anaerobic digester, gaseous oxygen is prevented from entering the system through physical containment in sealed tanks. Anaerobes access oxygen from sources other than the surrounding air. The oxygen source for these microorganisms can be the organic material itself or alternatively may be supplied by inorganic oxides from within the input material. When the oxygen source in an anaerobic system is derived from the organic material itself, then the 'intermediate' end products are primarily alcohols, aldehydes, and organic acids plus carbon dioxide. In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of methane, carbon dioxide with trace levels of hydrogen sulfide. In an anaerobic system the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane. Populations of anaerobic bacteria typically take a significant period of time to establish themselves to be fully effective. It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations. This process is called 'seeding' the digesters and typically takes place with the addition of sewage sludge or cattle slurry.
Stages The key process stages of anaerobic digestion There are four key biological and chemical stages of anaerobic digestion: 1. 2. 3. 4.
Hydrolysis Acidogenesis Acetogenesis Methanogenesis
In most cases biomass is made up of large organic polymers. In order for the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts or monomers such as sugars are readily available by other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore hydrolysis of these high molecular weight polymeric components is the necessary first step in anaerobic digestion. Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids. Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such as volatile fatty acids (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilized by methanogens. The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products. The process of acidogenesis is similar to the way that milk sours. The third stage anaerobic digestion is
acetogenesis. Here simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen. The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here methanogens utilise the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water. It is these components that makes up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pH and occurs between pH 6.5 and pH 8. The remaining, non-digestable material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate. Anaerobic digesters can be designed and engineered to operate using a number of different process configurations: 1. Batch or continuous
2. Temperature: Mesophilic or thermophilic 3. Solids content: High solids or low solids 4. Complexity: Single stage or multistage
Batch or continuous A batch system is the simplest form of digestion. Biomass is added to the reactor at the start of the process in a batch and is sealed for the duration of the process. Batch reactors suffer from odour issues that can be a severe problem when they are emptied. Typically biogas production will be formed with a normal distribution pattern over time. The operator can use this fact to determine when they believe the process of digestion of the organic matter has completed. As the batch digestion is simple and requires less equipment and lower levels of design work it is typically a cheaper form of digestion. In continuous digestion processes organic matter is constantly or added in stages to the reactor. Here the end products are constantly or periodically removed, resulting in constant production of biogas. Examples of this form of anaerobic digestion include, continuous stirred-tank reactors (CSTRs), Up flow anaerobic sludge blanket (UASB), Expanded granular sludge bed (EGSB) and Internal circulation reactors (ICR). Temperature There are two conventional operational temperature levels for anaerobic digesters, which are determined by the species of methanogens in the digesters: •
Mesophilic which takes place optimally around 37°-41°C or at ambient temperatures between 20°-45°C where mesophiles are the primary microorganism present
•
Thermophilic which takes place optimally around 50°-52° at elevated temperatures up to 70°C where thermophiles are the primary microorganisms present
There are a greater number of species of mesophiles than thermophiles. These bacteria are also more tolerant to changes environmental conditions than thermophiles. Mesophilic systems are therefore considered to be more stable than thermophilic digestion systems. A drawback of operating at thermophilic temperatures is that more heat energy input is required to achieve the correct operational temperatures. This increase in energy is not be outweighed by the increase in the outputs of biogas from the systems. It is therefore important to consider an energy balance for these systems.
Solids Typically there are two different operational parameters associated with the solids content of the feedstock to the digesters: •
High-solids
•
Low-solids
Digesters can either be designed to operate in high solid content, with a total suspended solids TSS) concentration greater than ~20%, or a low solids concentration less than ~15%.[55] High-solids digesters process thick slurry that requires more energy input to move and process the feedstock. The thickness of the material may also lead to associated problems with abrasion. High-solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture. Low-solids digesters can transport material through the system using standard pumps that require significantly lower energy input. Low-solids digesters require a larger amount of land than high-solids due to the increase volumes associated with the increased liquid: feedstock ratio of the digesters. There are benefits associated with operation in a liquid environment as it enables more thorough circulation of materials and contact between the bacteria and their food. This enables the bacteria to more readily access the substances they are feeding off and increases the speed of gas yields. Digestion systems can be configured with different levels of complexity: •
One-stage or single-stage
•
Two-stage or multistage
A single-stage digestion system is one in which all of the biological reactions occur within a single sealed reactor or holding tank. Utilising a single stage reduces construction costs, however facilitates less control of the reactions occurring within the system. Acidogenic bacteria, through the production of acids, reduce the pH of the tank. Methanogenic bacteria, as outlined earlier, operate in a strictly defined pH range. Therefore the biological reactions of the different species in a single stage reactor can be in direct competition with each other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are pond-like earthen basins used for the treatment and long-term storage of manures. Here the anaerobic reactions are contained within the natural anaerobic sludge contained in the pool. In a two-stage or multi-stage digestion system different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and more quickly grow and reproduce than methanogenic bacteria. Methanogenic bacteria require stable pH and temperature in order to optimise their performance. Typically hydrolysis, acetogenesis and acidogenesis occur within the first reaction vessel. The organic material is then heated to the required operational temperature (either mesophilic or thermophilic) prior to being pumped into a methanogenic reactor. The initial hydrolysis or acidogenesis tanks prior to the methanogenic reactor can provide a buffer to the rate at which feedstock is added. Some European countries require a degree of elevated heat treatment in order to kill harmful bacteria in the input waste. In this instance their may be a pasteurisation or sterilisation stage prior to digestion or between the two digestion tanks. It should be noted that it is not possible to completely isolate the different reaction phases and often there is some biogas that is produced in the hydrolysis or acidogenesis tanks.
Residence The residence time in a digester varies with the amount and type of feed material, the configuration of the digestion system and whether it be one-stage or two-stage.
In the case of single-stage thermophilic digestion residence times may be in the region of 14 days, which comparatively to mesophilic digestion is relatively fast. The plug-flow nature of some of these systems will mean that the full degradation of the material may not have been realised in this timescale. In this event digestive exiting the system will be darker in colour and will typically have more odour. In two-stage mesophilic digestion, residence time may vary between 15 and 40 days. In the case of mesophilic UASB digestion hydraulic residence times can be (1hour-1day) and solid retention times can be up to 90 days. In this manner the UASB system is able to separate solid in hydraulic retention times with the utilisation of a sludge blanket. Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents enabling the bacteria and the food to be in contact. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks.
Products There are three principal products of anaerobic digestion: biogas, digestate and water. Biogas is the ultimate waste product of the bacteria feeding off the input biodegradable feedstock, and is mostly methane and carbon dioxide, with a small amount hydrogen and trace hydrogen sulphide. Most of the biogas is produced during the middle of the digestion, after the bacterial population has grown, and tapers off as the putrescible material is exhausted. The gas is normally stored on top of the digester in an inflatable gas bubble or extracted and stored next to the facility in a gas holder. Biogas may require treatment or 'scrubbing' to refine it for use as a fuel. Hydrogen sulphide is a toxic product formed from sulphates in the feedstock and is released as a trace component of the biogas. Volatile siloxanes can also contaminate the biogas; such compounds are frequently found in household waste and wastewater. In digestion facilities accepting these materials as a component of the feedstock, low molecular weight siloxanes volatilise into biogas.
Digestive Digestive is the solid remnants of the original input material to the digesters that the microbes cannot use. It also consists of the mineralised remains of the dead bacteria from within the digesters. Digestive can come in three forms; fibrous, liquor or a sludge-based combination of the two fractions. In two-stage systems the different forms of digestive come from different digestion tanks. In single stage digestion systems the two fractions will be combined and if desired separated by further processing. Wastewater The final output from anaerobic digestion systems is water. This water originates both from the moisture content of the original waste that was treated but also includes water produced during the microbial reactions in the digestion systems. This water may be released from the dewatering of the digestate or may be implicitly separate from the digestate.
The wastewater exiting the anaerobic digestion facility will typically have elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), these are measures of the reactivity of the effluent and show an ability to pollute. Some of this material is termed 'hard COD' meaning it cannot be accessed by the anaerobic bacteria for conversion into biogas. If this effluent was put directly into watercourses it would negatively affect them by causing eutrophication. As such further treatment of the wastewater is often required. This treatment will typically be an oxidation stage where air is passed through the water in a sequencing batch reactors or reverse osmosis unit.
ANAEROBIC LAGOONS Anaerobic lagoons are used to dispose of animal waste, particularly that of cows and pigs. The waste is washed into the lagoon by flushing the animal pens with water. Solid waste, particularly the fibrous type of cows, is sometimes separated before the wastewater enters the lagoon to prevent the build up of solid material. Anaerobic organisms naturally present in the manure and the environment decompose the waste in the anaerobic conditions of the lagoon. Areas with cold winters are inappropriate for anaerobic lagoons because the activity of the microorganisms is highly dependent on temperature. It is critical to have the proper size for the lagoon, with volume being more important than surface area. A minimum of two meters is necessary for anaerobic conditions, but the depth should not exceed 6 meters. Sometimes a secondary lagoon is used to accept wastes while the primary lagoon is undergoing maintenance or for other purposes. If the anaerobic lagoon system is being used for energy production, the primary lagoon has a cover floating on the surface of the water. The cover captures the biogas produced by anaerobic bacteria. The biogas produced by anaerobic lagoons is 50 to 75% methane, with carbon dioxide making up most of the rest. The gas is usually used to produce electricity using a microturbine or reciprocating engine, but it can also be used for water or space heating. The gas usually undergoes pretreatment, particularly dehydration, prior to combustion. Sometimes the carbon dioxide, which is incombustible, is also removed.
PLANTS VISITED 0.35 MLD MBBR ( MOVING BED BIO REACTOR) , JANAKPURI WEST The purpose of MBBR system is to increase the amount of biomass in a biological treatment reactor by providing a media upon which it can grow. Thus the media of an MBBR system is central to its operation. It must perform the required task of acting as a carrier or residence for the biomass while also giving a long service life. The MBBR process employs a submerged ring media onto which micro organisms attach. The biomass retained on the ring media provides effective treatment for the effluent. The ring media are kept in motion by coarse bubble aeration. The air introduced into the tank is sufficient to ensure thorough mixing and turnover of the media within the reactor. The media can be used in aerobic, anoxic, and anaerobic zones. The MBBR does not incorporate return sludge. ADVANTAGES OF MBBR PROCESS: • High effluent quality. • Small footprint. • Simplicity of design, installation and operation. • Site specific designs for small to large populations. • Retrofits activate sludge plants to improve capacity and effluent quality. • Easily retained media. • Low capital and operating costs. • Robust package treatment plant for small communities.
40 MGD SEWAGE TREATMENT PLANT, DELHI JAL BOARD, NILOTHI SL. NO.
UNIT
QUANTITY
FUNCTION
1
Bar screen unit
4
It consists of conveyor belt. It is used for the removal of large floating solids.
2
Grit chamber unit.
4
There are four grit chambers in the plant which consists of a bridge. The bridge is divided into three parts: Gear Box, Shaft, and rubber equipment. Grit chamber mashes thick particles. Rake classifier is also present here which takes up the particles which are not mashed up by grit chamber (like pieces of stones).
3
Manual bar screen unit
4
It also removes floating solids. It is operated manually.
4
Primary sedimentation chamber unit
1
It removes primary settable solids. From here waste water goes to the aeration tank. In this step settable are removed by gravitational settling under quiescent conditions. The sludge formed at the bottom of the tank is removed as underflow and the cleared liquid produced is known as overflow. There are generally three types of settling: Settling of dilute suspensions of discrete particles, settling of dilute suspensions of flocculent particles and zone settling which includes hindered settling and compressive settling. Raw pump house is present where raw
sludge comes by gravity and water moves in the upward direction. There is also a pump which is called as return sludge pump for return sludge.
5
Aeration chamber
14 aerators
14 aerators are installed for providing adequate amount of oxygen. Aeration depends upon the amount of upcoming water. All the aerators are not operated at the same time. It depends on the amount of water supply. The main function of this unit is to control MLSS (Mixed Liquor Suspended Solids) by mixing oxygen through aeration. The rate at which dissolved oxygen is used , depends on: a. Quantity of organics. b. The ease with which they are biodegraded. c. Dilution capacity of the stream.
6
Secondary Sedmentation Tank
1
From the aeration tank the sewage flows to the final sedimentation tank since there are no floating solids, provisions for the removal of the scum or floatage are not needed. The suspended particles in the aeration tank are light in weight and are thus markedly influenced by currents. Therefore in these secondary settling tanks a considerable length of overflow weir is desirable to reduce the velocity of approach.
7
Sludge digestion tank
6
It consists of circular tank with
hoppered water and having a fixed or floating type of roof over its top. The raw sludge is pumped from raw sludge pump house into the tank and when the tank is put into the operation it is seeded with digested sludge from another tank. A screw pump with an arrangement for circulating sludge from bottom to top of the tank or viceversa is commonly used, for stirring the sludge.
The gases of decomposition (mainly CH4 and CO2) are collected in Gas dome or in gas holders for subsequent use. The digested sludge which settles down to bottom of the Tank is removed under hydrostatic pressure periodically once or twice a week. The supernatant liquor being higher in BOD and suspended solids content is sent back for the treatment along with raw sewage in the treatment plant.
The digestion tanks are cylindrical shaped tanks with dia ranging between 3 to 12 m. the bottom hoppered floor of the tank is given a slope of about 1:1 or 1:3 (1H:3V) the depth of the digestion tank is usually kept at about 6m. the capacity of the digestion tank is a function of sludge production, digestion period, degree of digestion required, loss of moisture and conversion of organic matter. If the progress of the sludge digestion is assumed to be linear then the capacity of digestion tank is given as: V=[(V1+V2)/2]*T
Where V= volume of digestion in m3 V1= raw sludge added per day(m3 per day) V2=equivalent digested sludge produced per day on completion of digestion(m3 per day)=V1/3 T= digestion period (days).
From the digester the sludge is taken as fertilizer and manure. Blowers are used for mixing and not for aeration in digester.
BULKING AND FOAMING SLUDGE IN AN ASP PLANT Foam formation and poorly settling sludge are two most common problems of ASP process. A sludge that exhibits poor settling characteristics is called as bulking sludge. Filamentous micro-organisms (fungi) are found to be responsible for bulked sludge. To control these organisms’ chlorination and reduction of sludge age to less than 6 days is done. POWER SUPPLY IN NILOTHI PLANT Dual fuel engine is used for power supply in which diesel and gas are used in the ratio 20:80. There are three generators of 600 KVA which generates about 1200KVa electricity. There are also many step-down and step-up transformers for the electricity supply and control. To cool the air water coolers are present.
BIBLIOGRAPHY:
Rose George, The Big Necessity: The Unmentionable World of Human Waste and Why it Matters
Khopkar, S. M. (2004). Environmental Pollution Monitoring And Control. New Age International.
Beychok, M.R. (1971). "Performance of surface-aerated basins". Chemical Engineering Progress Symposium Series 67 (107): 322–339.
Appropriate Technology for Sewage Pollution Control in the Wider Region, Caribbean Environment Programme Technical Report #40 1998
Massoud Tajrishy and Ahmad Abrishamchi, Integrated Approach to Water and Wastewater Management for Tehran, Iran, Water Conservation, Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press (2005)
