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Wastewater Treatment Plants | Sewage Treatment Plants

Treatment process of wastewater such as sewage, sullage and kitchen waste involves multiple stages for the removal of sediments, dissolved / suspended organic contaminants, oil, grease, reduction of biochemical oxygen demand (BOD), chemical oxygen demand (COD) and nitrogenous matter to less than the regulatory standards stipulated for discharge or reuse.

The primary objective of sewage treatment is to make sure that the treated effluent does not contaminate the soil and ground water when discharged back to the environment or is safe to recycle for the intended uses. Treated sewage can be used at homes, commercial places and factories for a multitude of purposes. It is estimated that fresh water consumption can be reduced up to 60% through recycling treated sewage.

The size of the sewage treatment plant and the technology used are primarily decided based on the sewage quantum and characteristics, output quality requirements, space availability, site conditions and applications of the treated effluent.

Types of Sewage Treatment Technologies and Processes

Sewage treatment plants (STP) can be designed in conventional, modular, packaged and containerized forms using a host of technologies referred here below.

Activated Sludge Process (ASP)
Fluidized Bio Bed Reactor (FBBR)
Submerged Aerobic Fixed Film Reactor (SAFF)
Sequential Batch Reactor (SBR)
Membrane Bio Reactor (MBR)

These plants can be structured on the ground, under the ground or on elevated platforms based on site conditions and user preference.

Process Automation and Remote Monitoring

While in terms of efficacy, a manual sewage treatment system is as good as an automated one if operated and maintained properly, critical process automation significantly reduces the operational burden and minimizes the possibility of human errors. At Korgen, we use modern electronics and IT/Cloud-based tools for partial or end-to-end automation and remote monitoring of sewage treatment plants.

How korgen fits in?

As one of the leading sewage treatment plant manufacturing companies with full-fledged operations in Chennai and key cities in Tamil Nadu, Korgen is a vertically integrated and professionally managed organization led by a dynamic team of qualified technocrats and environmental specialists.

With a combined experience of over five decades in designing, building, commissioning and servicing wastewater (sewage and sullage) treatment plants for homes, residential apartments, townships, educational institutions, malls, hotels, clubs, restaurants, resorts, parks, hospitals, commercial complexes, SEZs and industries, we are known for our formula-driven designs, advanced technological solutions, seamless project execution capabilities and efficient after sales service.

Our aerobic sewage treatment plants ranging from a modular 25 KLD plant up to 1000 KLD custom-built STPs are designed to be cost-effective and user-friendly. We use high quality material and branded equipment/components for durability, consistent performance, lower maintenance costs and maximized operational efficiency.

We help our customers choose the right sewage treatment technology and equipment by carrying out a thorough Need Analysis and Site Evaluation Study. This methodical approach and our deep domain expertise give our wastewater treatment systems a near-zero failure rate.

We also offer various types of after-sales service packages for sewage treatment plants such as plant operations, tailored O&M contracts and AMCs.

KORGEN is a completely system-driven organization governed by professional practices, stringent quality control processes and ethical standards. This helps us offer superior sewage treatment plants, robust preventive maintenance procedures, operation and maintenance (O&M) support and timely service to our customers.

Advantages of Korgen Sewage Treatment Plants

  • Built using cutting-edge technologies and formula-driven designs
  • Effectively reduces sediments, organic contaminants, oil, grease, BOD, COD and other nitrogenous contaminants
  • Customized plant layout based on site conditions
  • Reduced plant footprint and lifecycle costs
  • Delivers consistently efficient performance over the long term
  • High quality equipment and components ensure long plant life
  • Lesser power consumption, odourless environment, user-friendly operations and maintenance
  • Effective sludge processing and disposal methods
  • Quick capacity extensions to handle increased sewage volumes
  • Trained team of technicians and site operators
  • Modern IT-based and electronic solutions for end-to-end automation
  • Wide range of modular and custom-built treatment plants that are competitively priced
  • Affordable O&M Packages and efficient after-sales service network
Know more about Sewage and Waste Water Treatment

Why Sewage Treatment is necessary?

Why Sewage Treatment is necessary?

Burgeoning population, modern living standards and growing usage of water for agricultural and industrial purposes are among the major factors that cause a severe imbalance between the availability of fresh water and its consumption. Water usage has been constantly on the increase with no signs of abatement.

In the coming years, conservation of fresh water and recycling of wastewater will be one of the most debated environmental challenges. The World Economic Forum, in 2019, has listed water crisis as one of the largest global risks in terms of potential impact over the next decade.

While water scarcity has pushed countries into developing advanced technologies to desalinate sea water, it has also spurred them into finding ways to conserve water and also recycle and reuse waste water. The concept of sewage treatment and industrial effluent treatment was born out of this sheer necessity. It is estimated that fresh water consumption can be reduced up to 60% through recycling and reusing treated wastewater. This can also drastically reduce the rate of depletion of surface and ground water.

On the other hand, discharging huge volumes of untreated sewage containing chemical compounds, pathogens and other dangerous constituents into the environment (land, rivers, streams, lakes, ocean) can severely contaminate the ecosystem and harm the health of people, plants, animals, birds, fishes and other aquatic lives that live in or near the discharge location. Partially or under-treated sewage also negatively impacts the environment in the long term.

On account of these factors, wastewater treatment and recycling has today become a fundamental requirement and an ecological imperative. Fortunately, today’s technological developments have made treatment and recycling of wastewater (sewage, sullage, industrial effluent) easier, more economical and commercially viable.

Many of the developed nations have already taken a giant leap in wastewater treatment to the extent of even making it potable.

Design Basis for Sewage Treatment Plants

Design Basis for Sewage Treatment Plants

The basic design and dimensions of a Sewage Treatment Plant are highly formula-driven and should be primarily based on the following factors.

i. BOD, COD, Oil and Grease content in the Raw Sewage
ii. pH and DO level in the Water
iii. Other related influent characteristics
iv. Minimum, maximum and average daily flows,
v Minimum and peak hourly flows | Instantaneous peak flows
vi. Hydraulic Retention Time
vii. Site conditions and end-use of treated effluent
viii. Local weather conditions
ix. Mode of operation required by the client (Manual | Semi-Automatic | Fully Automatic)
x. Sludge Management Plans

Multi-stage Sewage Treatment Process

Multi-stage Sewage Treatment Process

Sewage treatment process essentially involves multiple stages to ensure effective removal / reduction of contaminants. The number and type of stages may vary based on the technology used for sewage treatment. However, the typical stages adopted in sewage treatment are classified as follows:

  • iPre-treatment
  • iiPrimary Treatment
  • iiiSecondary Treatment
  • ivTertiary Treatment
  • v Sludge Treatment
Pre-Treatment

At the pre-treatment stage, the effluent from the sewerage network is subjected to a physical treatment before it is allowed to flow into the collection-equalization tank. This includes processes like screening, comminution and sedimentation. In this process, mechanical devices such as bar screens, comminutors and grit channels are used to remove large floating objects and readily settleable coarse solids from the sewage stream such as trash, tree limbs, leaves, egg shells, bone chips, sand, mud, gravel, seeds, cans, rags, sticks, plastics, condoms, sanitary pads, bags, tampons, cloth, vegetable / fruit peels and other debris. The comminutor is used to grind and shred debris that passes through the screens.

If these large and heavy materials are not removed from the sewage stream, it can adversely impact the efficiency and performance of downstream treatment processes by forming heavy deposits in the aeration tank and clogging digesters, pipelines, conduits, pumps and damage other moving parts of the sewage treatment plant.

The material removed at this treatment stage is washed, pressed and disposed off in a landfill. This stage is carried out entirely using mechanical devices and hence is also known as Mechanical Treatment.

In larger plants, the raw sewage is also passed through a separate tank that deploys mechanized skimmers and other separator equipment to collect and remove fat, grease and oil from the sewage stream.

Primary Treatment

Once the pre-treatment of the sewage stream is completed, the sewage flows into the collection tank. This tank is also known as equalization tank since the raw sewage achieves a good degree of homogeneity inside this tank.

The key purpose of this primary treatment stage is to temporarily store and equalize the sewage flow and produce a generally homogeneous liquid capable of being treated biologically. Coarse Diffusers placed on the floors or on the side walls of the equalization tank that are connected to air blowers installed outside the tank pumps air into the tank in the form of coarse bubbles to homogenize the sewage. This equalized sewage (mixed liquor) is then pumped out for the next stage of treatment.

A liquid level controller can be, optionally, used to auto-transfer the equalized sewage to the next stage.

Secondary Treatment

Sewage from the primary treatment stage is pumped into the next tank where aeration is carried out. Secondary treatment is designed to substantially degrade and settle the biological (organic) content in the sewage that originate from human waste, food waste, soaps and detergents.

The purpose of the biological treatment is to reduce the BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) of the mixed liquor. Except for some specific reasons, the majority of domestic, municipal and industrial sewage treatment plants use aerobic biological processes to treat the mixed liquor.

Aeration Tank

In this stage, the sewage in the aeration tank is sufficiently oxygenated by mixing atmospheric air to create a congenial environment for rapid multiplication of microorganisms. These microorganisms (Bacteria, Protozoahttps://en.wikipedia.org/wiki/Protozoa) then start digesting the biodegradable organic contaminants and convert them into carbon dioxide, water and a range of lower molecular weight organic compounds for their own growth and reproduction, a natural biological process. These microbes also bind much of the less soluble fractions into floc particles. This treatment is also known as Biological Treatment.

Apart from submerged diffusers, oxygenation can also be achieved using different types of aerators such as fixed aerators, floating surface aerators, spray aerators and air grids. Aerators help water absorb the desired quantity of oxygen and in maintaining the optimum level of MLSS (Mixed Liquor Suspended Solids) in the wastewater. All these aerators are available in different sizes and designs.

In case of insufficient organic matter, activated bacterial growth is achieved or accelerated by adding external organic matter into the aeration tank. Eg., Urea, Cow dung. This process has to be closely monitored and controlled so that the microbial volume is maintained at an optimum level.

Korgen uses a wide range of time-tested technologies for the growth of microorganisms in this stage based on site conditions, client preferences and other related factors.

Settling Tank

The aerated sewage now flows into the settling / clarifier tank where separation of biologically activated sludge takes place. This clarifier tank can be either circular or rectangular in shape with a conical bottom equipped with a feed well drum, sludge chamber, sludge scrapper and overflow weir. The settling tank is also known as sedimentation tank.

Settling-Clarifier tanks operate on the principle of appropriately slowing down the velocity of the influent flow so that the sludge and other suspended matter can sink to the bottom, collected and be removed. In addition, the scraper continually drives the collected sludge towards a hopper at the base of the tank. While the supernatant flows into the next treatment stage, the precipitated sludge is pumped out for disposal or further digestion.

This natural separation of sludge can be expedited and made more efficient by using flocculants, coagulants and advanced clarifier mechanisms such as lamella clarifiers, plate separators, tube settlers etc.

Based on specific process requirements, settling may also be done in two stages. viz. Primary Settlers and Secondary Settlers.

Recirculation of Activated Sludge (Biomass)

The aeration process creates activated sludge otherwise called as biomass. A portion of this activated sludge is recirculated back into the aeration tank to maintain adequate mixed level suspended solids (MLSS) levels in the aeration tank. This process is also known as reseeding the fresh sewage stream which helps to keep the ratio of biomass to food supplied in balance (F/M ratio).

The recirculation process also prevents sludge bulking and the remaining surplus sludge is removed from the process for further treatment using aerobic or anaerobic processes before disposal.

Tertiary Treatment

This is the final stage of sewage treatment that raises the effluent quality to the requisite standards before being discharged into the receiving environment or reused for the intended purposes. In this treatment process, the objective is to filter the residual particles (suspended solids, turbidity, residual organics, colour, odour, colloidal matter etc) and disinfect the treated sewage.

Tertiary treatment by itself is a multi-stage process wherein the number of stages is decided based on the application (end-use) of the effluent. This treatment is also known as Physical-Chemical Treatment.

The standard tertiary treatment module comprises the following plants. Here, the output water can be used for recharging ground water aquifers, toilet flushing, watering plants etc.

i. Dosing pump for chemical disinfection or UV Plant (elimination of pathogens)
ii. Pressure Sand Filter for filtration of suspended particles
iii. Activated Carbon Filter for removal of residual chlorine, colour, odour, organics

Advanced tertiary treatment module comprises the following units in addition to the above plants. If rightly designed and executed, the output water from the below treatment units should be pure enough to be used for washing, bathing and even for drinking purposes.

i. Micron Filters
ii. Ultrafiltration (UF) Units
iii. Reverse Osmosis (RO) Plants

Biological Nutrient removal (BNR)

Treated sewage sometimes may contain high levels of nutrients (nitrogen and phosphorous) that in certain forms may be toxic to aquatic lives even at low concentrations (e.g., ammonia) and excessive release of such nutrients into the receiving environment can also lead to a build-up called eutrophication which in turn results in rapid growth of weed and algae population.

The decomposition of this algae by microbes considerably depletes the level of dissolved oxygen in the water and most of the marine lives die due to this. In addition to causing deoxygenation, some algae species produce toxins that severely contaminate water sources. Nitrogen and phosphorous can be removed from sewage either biologically or using chemical processes.

In the biological process, nitrogen is reduced from ammonia to nitrate (nitrification process) and then from nitrate to nitrogen gas (denitrification process) which is then released to the atmosphere. These form conversions occur in carefully created and controlled oxygenated and anoxic environments facilitated by different types of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogenous matters.

Removal of phosphorous can also be achieved biologically. In this process, specific bacteria called polyphosphate-accumulating organisms are selectively enriched and they in turn accumulate large quantities of phosphorous within their cells. Separation of the resultant biomass from the treated water will result in formation of biosolids with high fertilizer value. Phosphorous removal can also be achieved by chemical precipitation using ferric chloride or aluminium sulphate. The resulting chemical sludge, however, is difficult to dispose off and the use of such chemicals in the treatment process is expensive in addition to making the operations relatively difficult.

Disinfection

The purpose of disinfection is to ensure that the treated effluent is free from all types of pathogenic microbes before it is safely discharged back into the environment or reused for other purposes.

The effectiveness of disinfection depends on the quality of the effluent being treated (e.g., cloudiness, pH etc), the type of disinfection process, disinfectant dosage (concentration and time) and other environmental variables. Generally, short contact times, low dosage and high-velocity flows militate against effective disinfection. Common methods of disinfection include Chlorination, UV Rays and ozonation.

Chlorination

Usage of liquid chlorine (Sodium Hypochlorite) is the most preferred method of disinfection on account of low cost and established effectiveness. In this method, chlorination of residual organic matter present in the treated effluent can generate carcinogens that are harmful to human health and to the environment as well. Besides, residual chlorine present in the treated effluent may also be capable of chlorinating organic matter in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be effectively dechlorinated.

Ultraviolet (UV) Rays

Ultraviolet (UV) rays can also be used for disinfection instead of chlorine or other chemicals. Since no chemicals are used in this process, the treated effluent will have no adverse effect on organisms that consume it. sum disadvantages of UV plants are frequent lamp maintenance/replacements and the need for a microfiltered effluent to ensure that the target microbes are not shielded from UV radiation by suspended solids present in the treated effluent.

Ozonation

Ozone, a very unstable and reactive element effectively oxidizes the organic matter it comes in contact with thereby destroying all the pathogenic microorganisms in the treated effluent. Ozone is considered safer than chlorine since it produces fewer by-products than chlorination. Unlike chlorine that needs to be stored on site, ozone is generated in-situ from the ambient air as and when required. One disadvantage of this method is the high cost of ozone generating equipment and the requirement of trained operators.

Oxygen Uptake

The treated effluent may be aerated, if required, to bring the level of dissolved oxygen up to the permissible limits before letting it out into the receiving environment. If re-use is intended, further polishing of this water may be required using appropriate treatment schemes. Eg. UF/RO Units.

Sludge Management

The processed sludge at the final stage containing coarse and fine biosolids generated as a by-product of the sewage treatment process must be further treated to reduce the volume and produce a usable end-product or dispose of in a safe and environment-friendly manner.

Reduction of sludge volume is achieved by removing the water content and increasing the sludge concentration. The water removed from the sludge will have high levels of ammonia and hence is returned to the aeration tank for retreatment.

The choice of dewatering method depends on the quantum of solids generated, client preference and other site-related conditions. While traditional ways such as sludge drying beds can be used, special equipment like gravity belts, filter presses and centrifuges can also be used to mechanically filter the sludge. In terms of final disposal, this de-moisturized / dry sludge can be used as natural fertilizers and for landfills.

Mechanical and Electrical Equipment used in STPs

Mechanical and Electrical Equipment used in Sewage Treatment Plants

Unless warranted under special conditions, Sewage Treatment Plants generally comprise all or select mechanical and electrical equipment from the following

Bar Screens Aerators Dosing Systems
Comminutor Diffusers UV Sterilizers
Grit Channels Transfer Pumps Ozonators
Detritors Clarifiers Sand Filters
Skimmers Tube Settlers Carbon Filters
Separators Decanters Micron Filters
Centrifuges Sludge Dewatering Machines UF Membrane Modules
Blowers Filter Presses Instrumentation Control
Mixers Sludge Dryers Automation Devices
Agitators Digesters Acoustic Enclosures

Technologies used for sewage treatment

Technologies used for Sewage Treatment

Fundamentally, there are only two types of biological processes to treat sewage. ie, Aerobic process and Anaerobic process. In both these processes, the objective is to ensure sufficient multiplication of microorganisms to consume and digest the organic matter in the sewage stream. While aerobic process achieves this by using oxygen, the anaerobic process uses anoxic environments (absence of oxygen) to achieve the same objective.

All sewage treatment technologies deploy either one of these biological processes in their treatment scheme, but in unique ways. The final choice of technology depends on various factors as explained in the design basis section and the preferences of the users. There are instances where both the biological processes are combinedly used in treatment schemes to address specific challenges.

At korgen, we use the following aerobic technologies for the treatment of wastewater / sewage.

Activated Sludge Process (ASP)

Fluidized Bio Bed Reactor (FBBR)

Submerged Aerobic Fixed Film Reactor (SAFF)

Sequential Batch Reactor (SBR)

Membrane Bio Reactor (MBR)

Activated Sludge Process (ASP)

Activated sludge process is a time-tested conventional technology that works well in the treatment of wastewater with high volume flow rates. This process uses many variants of aeration methods namely tapered aeration, step aeration, extended aeration, dual-stage aeration, re-aeration, biosorption, complete-mix etc.

In this process, different types of aerators such as floating aerators, fixed aerators and fine diffusers are used to mix air (oxygen) in the mixed liquor and expedite the suspended growth of bacteria. The MLSS concentration in the aeration tank is maintained between 3,000 to 4000 mg/l at any point of time.

The major variables of the activated sludge process are the loading rates, mixing regime and the flow scheme. Activated Sludge Process requires larger plant footprints, extensive civil works and relatively longer HRTs (Hydraulic Retention Time).

Fluidized Bed Bio Reactor (FBBR)

Fluidized Bed Bio Reactor, a hybrid aerobic process, is also known by other names such as Fluidized Aerobic Bioreactor (FAB), Moving Bed Bio Reactor (MBBR) and Dynamic Bio Bed Reactor (DBBR).

In the FBBR technology, specially designed floating media made of UV-stabilized inert polypropylene vastly augments the surface area for high-rate attached growth of microorganisms in addition to their suspended growth.

The density of the media is optimized in a manner where it remains floating and moves around within the aeration tank all the time due to the turbulence created by the air injected into the tank through uniquely designed air grids.

Injection and uniform distribution of air and the random movement of FBBR media inside the aeration tank lead to quicker sludge dispersion and higher MLSS concentration that enhances the biodegradation rate of the organics present in the sewage.

Rapid and higher volume of bacterial growth makes the FBBR process more resistant to shock loads and sludge bulking that commonly plague suspended growth systems. Depending on influent characteristics and volume, multiple reactor tanks may also be used in the treatment scheme.

A properly designed FBBR-based sewage treatment plant can handle higher BOD loads, tolerates fluctuations in the BOD-COD content without impairing the plant performance and also significantly reduces the hydraulic retention time that results in smaller plant footprints, modular designs, lesser sludge generation, reduced odour and minimal civil works. FBBR delivers a far better quality of treated effluent than the conventional ASP.

In terms of capital and operational costs, FBBR is economical when compared with ASP.

Submerged Aerobic Fixed Film Reactors (SAFF)

The SAFF (Submerged Aerobic Fixed Film Reactor) process involves the usage of a specially developed fixed film media made of UV-stabilized polypropylene. This chemically compatible stationary media is submerged and fixed to the bottom of the aeration tank (reactor) and is designed to expedite the biological degradation process.

Cross-corrugated surface of the media provides an ideal condition and augmented space for the attached growth of microorganisms which in turn consumes and rapidly digests the incoming organic constituents. SAFF is again a hybrid aerobic process where attached and suspended microbial growth occur simultaneously.

Injection of air is carried out through tubular diffusers or air grids and the sludge built on the media surface is detached by the rising mass of fine air bubbles through the media. Use of this stationary media helps in maintaining relatively higher MLSS concentration and substantially reduces the size of the aeration tank.

A properly designed SAFF-based sewage treatment plant can handle higher BOD loads, tolerates fluctuations in the BOD-COD content without impairing the plant performance and also significantly reduces the hydraulic retention time that results in smaller plant footprints, modular designs, lesser sludge generation, reduced odour and minimal civil works. SAFF delivers a far better quality of treated effluent than the conventional ASP.

In terms of capital and operational costs, FBBR is economical when compared with ASP.

Sequential Batch Reactors (SBR)

Sequential Batch Reactors, also known as Sequencing Batch Reactors, is basically an activated sludge process that uses a set of tanks that operates on a fill-and-draw basis. The SBR process carries out three key processes of sewage treatment in a single reactor tank, namely, equalization, aeration and clarification thereby significantly reducing the space requirements. Multiple reactor tanks can be used to handle continuous flow of sewage and the number of tanks depend on the expected volume of sewage and the detention time allowed for each batch.

In SBR, the screened sewage stream let into the reactor tank is treated in batches. The treatment cycle consists of five distinct stages. ie., Fill, Aerate, Settle, Draw and Idle. A specific volume of sewage is filled into the reactor tank, aerated for a discrete period of time and is allowed to settle thereafter. Once the settling process is complete, the supernatant is drawn out from the tank and all but a small portion of the sludge is removed from the reactor tank.

The feed volume and length of time allowed for each stage are computed based on various parameters including desired loading-detention time, settling characteristics of the microorganisms, volume of the reactor tank, number of tanks in operation and the diurnal variations in the influent flow rate.

Aeration is carried out to agitate the influent and ensure effective transfer of oxygen. Diffusers, surface aerators and air grids are used to achieve this. Measurement of dissolved oxygen is done using level sensing devices and timers to help in timing the process stages as required. Sludge wasting is periodically undertaken to maintain the optimum volume of sludge in the reactor tank and also to control the sludge age.

Settling process in the SBR occurs under quiescent conditions in the reactor tank. i.e., without inflow or outflow. The decanting function is facilitated using various mechanisms, including a pipe fixed at a prefixed level and the supernatant flow regulated by an automatic valve or pump or an adjustable floating weir at or just beneath the liquid surface. This drawing mechanism is designed and operated in a manner that prevents floating matter from being discharged.

The time period between ‘Draw’ and ‘Fill’ is termed idle. This idle time is used for sludge management functions.

The major advantages of using SBR-based sewage treatment plant are i. the flexibility it offers in terms of matching reaction times to the concentration and degree of treatment required for the wastewater stream, ii. the option to facilitate growth and digestion of desirable microorganisms under varying biological environments. ie., aerobic, anaerobic and anoxic conditions and finally, iii. the ease with which the plant capacity can be augmented by just adding more reactor tanks.

Membrane Bio Reactors (MBR)

Membrane Bio-Reactor (MBR) technology essentially combines two basic sewage treatment processes into a single one. ie., biological degradation of organic matter and separation. MBR is considered as the most advanced and globally acclaimed wastewater treatment technology,

Membrane Bio Reactors significantly reduce the plant layout by combining many conventional stages such as aeration, clarification and tertiary filtration into a single unified process that greatly reduces the overall plant footprint, operational difficulties and costs. The low-pressure UF (Ultrafiltration) membranes used in an MBR are either submerged in the aeration tank or mounted outside / adjacent to the aeration tank. With membrane pore sizes between 0.04 and 0.1 microns, MBRs deliver a distinctly superior quality of effluent that is virtually particulate-free.

By eliminating the need for sludge settling, MBRs operate at very high MLSS concentration (10,000 to 14,000 mg/litre) which is three to five times greater than MLSS levels maintained in other technologies. The higher biomass concentration in the MBR process allows for effective removal of both soluble and particulate biodegradable materials at higher loading rates.

MBR requires very short hydraulic retention time (HRT) and considerably less civil works. The increased sludge retention time (SRT) ensures complete nitrification even under extremely cold weather conditions. The quantity of sludge generated in MBR Is only 1/4th of the other technologies on account of accelerated bacterial digestion.

While advanced automations make the plant operations less labour-intensive and more consistent, the overall cost of building and operating a Membrane Bio Reactor is higher than the other wastewater treatment technologies. However, this technology is becoming increasingly popular primarily due to its superior treatment efficiency and MBR is considered the most appropriate choice in places where space is a limiting factor.

General applications of treated sewage

General Applications of Treated Sewage

  • Used for gardening, fountains, toilet flushing and vehicle washing
  • For construction activities
  • Airconditioning and cooling tower makeup
  • Agriculture and landscape irrigation purposes
  • Recharging ground water aquifers | Harvesting
  • Discharge into stream, river, bay, ponds, lakes and wetlands
  • Other industrial processes
Note:

Treated sewage water coming from STP with membrane-based tertiary treatment modules and MBRs can be used for higher purposes too.

Pre-Treatment

At the pre-treatment stage, the effluent from the sewerage network is subjected to a physical treatment before it is allowed to flow into the collection-equalization tank. This includes processes like screening, comminution and sedimentation. In this process, mechanical devices such as bar screens, comminutors and grit channels are used to remove large floating objects and readily settleable coarse solids from the sewage stream such as trash, tree limbs, leaves, egg shells, bone chips, sand, mud, gravel, seeds, cans, rags, sticks, plastics, condoms, sanitary pads, bags, tampons, cloth, vegetable / fruit peels and other debris. The comminutor is used to grind and shred debris that passes through the screens.

If these large and heavy materials are not removed from the sewage stream, it can adversely impact the efficiency and performance of downstream treatment processes by forming heavy deposits in the aeration tank and clogging digesters, pipelines, conduits, pumps and damage other moving parts of the sewage treatment plant.

The material removed at this treatment stage is washed, pressed and disposed off in a landfill. This stage is carried out entirely using mechanical devices and hence is also known as Mechanical Treatment.

In larger plants, the raw sewage is also passed through a separate tank that deploys mechanized skimmers and other separator equipment to collect and remove fat, grease and oil from the sewage stream.

Primary Treatment

Once the pre-treatment of the sewage stream is completed, the sewage flows into the collection tank. This tank is also known as equalization tank since the raw sewage achieves a good degree of homogeneity inside this tank.

The key purpose of this primary treatment stage is to temporarily store and equalize the sewage flow and produce a generally homogeneous liquid capable of being treated biologically. Coarse Diffusers placed on the floors or on the side walls of the equalization tank that are connected to air blowers installed outside the tank pumps air into the tank in the form of coarse bubbles to homogenize the sewage. This equalized sewage (mixed liquor) is then pumped out for the next stage of treatment.

A liquid level controller can be, optionally, used to auto-transfer the equalized sewage to the next stage.

Secondary Treatment

Sewage from the primary treatment stage is pumped into the next tank where aeration is carried out. Secondary treatment is designed to substantially degrade and settle the biological (organic) content in the sewage that originate from human waste, food waste, soaps and detergents.

The purpose of the biological treatment is to reduce the BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) of the mixed liquor. Except for some specific reasons, the majority of domestic, municipal and industrial sewage treatment plants use aerobic biological processes to treat the mixed liquor.

Aeration Tank

In this stage, the sewage in the aeration tank is sufficiently oxygenated by mixing atmospheric air to create a congenial environment for rapid multiplication of microorganisms. These microorganisms (Bacteria, Protozoa) then start digesting the biodegradable organic contaminants and convert them into carbon dioxide, water and a range of lower molecular weight organic compounds for their own growth and reproduction, a natural biological process. These microbes also bind much of the less soluble fractions into floc particles. This treatment is also known as Biological Treatment.

Apart from submerged diffusers, oxygenation can also be achieved using different types of aerators such as fixed aerators, floating surface aerators, spray aerators and air grids. Aerators help water absorb the desired quantity of oxygen and in maintaining the optimum level of MLSS (Mixed Liquor Suspended Solids) in the wastewater. All these aerators are available in different sizes and designs.

In case of insufficient organic matter, activated bacterial growth is achieved or accelerated by adding external organic matter into the aeration tank. Eg., Urea, Cow dung. This process has to be closely monitored and controlled so that the microbial volume is maintained at an optimum level.

Korgen uses a wide range of time-tested technologies for the growth of microorganisms in this stage based on site conditions, client preferences and other related factors.

Settling Tank

The aerated sewage now flows into the settling / clarifier tank where separation of biologically activated sludge takes place. This clarifier tank can be either circular or rectangular in shape with a conical bottom equipped with a feed well drum, sludge chamber, sludge scrapper and overflow weir. The settling tank is also known as sedimentation tank.

Settling-Clarifier tanks operate on the principle of appropriately slowing down the velocity of the influent flow so that the sludge and other suspended matter can sink to the bottom, collected and be removed. In addition, the scraper continually drives the collected sludge towards a hopper at the base of the tank. While the supernatant flows into the next treatment stage, the precipitated sludge is pumped out for disposal or further digestion.

This natural separation of sludge can be expedited and made more efficient by using flocculants, coagulants and advanced clarifier mechanisms such as lamella clarifiers, plate separators, tube settlers etc.

Based on specific process requirements, settling may also be done in two stages. viz. Primary Settlers and Secondary Settlers.

Recirculation of Activated Sludge (Biomass)

The aeration process creates activated sludge otherwise called as biomass. A portion of this activated sludge is recirculated back into the aeration tank to maintain adequate mixed level suspended solids (MLSS) levels in the aeration tank. This process is also known as reseeding the fresh sewage stream which helps to keep the ratio of biomass to food supplied in balance (F/M ratio).

The recirculation process also prevents sludge bulking and the remaining surplus sludge is removed from the process for further treatment using aerobic or anaerobic processes before disposal.

Tertiary Treatment

This is the final stage of sewage treatment that raises the effluent quality to the requisite standards before being discharged into the receiving environment or reused for the intended purposes. In this treatment process, the objective is to filter the residual particles (suspended solids, turbidity, residual organics, colour, odour, colloidal matter etc) and disinfect the treated sewage.

Tertiary treatment by itself is a multi-stage process wherein the number of stages is decided based on the application (end-use) of the effluent. This treatment is also known as Physical-Chemical Treatment.

The standard tertiary treatment module comprises the following plants. Here, the output water can be used for recharging ground water aquifers, toilet flushing, watering plants etc.

i. Dosing pump for chemical disinfection or UV Plant (elimination of pathogens)
ii. Pressure Sand Filter for filtration of suspended particles
iii. Activated Carbon Filter for removal of residual chlorine, colour, odour, organics

Advanced tertiary treatment module comprises the following units in addition to the above plants. If rightly designed and executed, the output water from the below treatment units should be pure enough to be used for washing, bathing and even for drinking purposes.

i. Micron Filters
ii. Ultrafiltration (UF) Units
iii. Reverse Osmosis (RO) Plants

Biological Nutrient removal (BNR)

Treated sewage sometimes may contain high levels of nutrients (nitrogen and phosphorous) that in certain forms may be toxic to aquatic lives even at low concentrations (e.g., ammonia) and excessive release of such nutrients into the receiving environment can also lead to a build-up called eutrophication which in turn results in rapid growth of weed and algae population.

The decomposition of this algae by microbes considerably depletes the level of dissolved oxygen in the water and most of the marine lives die due to this. In addition to causing deoxygenation, some algae species produce toxins that severely contaminate water sources. Nitrogen and phosphorous can be removed from sewage either biologically or using chemical processes.

In the biological process, nitrogen is reduced from ammonia to nitrate (nitrification process) and then from nitrate to nitrogen gas (denitrification process) which is then released to the atmosphere. These form conversions occur in carefully created and controlled oxygenated and anoxic environments facilitated by different types of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogenous matters.

Removal of phosphorous can also be achieved biologically. In this process, specific bacteria called polyphosphate-accumulating organisms are selectively enriched and they in turn accumulate large quantities of phosphorous within their cells. Separation of the resultant biomass from the treated water will result in formation of biosolids with high fertilizer value. Phosphorous removal can also be achieved by chemical precipitation using ferric chloride or aluminium sulphate. The resulting chemical sludge, however, is difficult to dispose off and the use of such chemicals in the treatment process is expensive in addition to making the operations relatively difficult.

Disinfection

The purpose of disinfection is to ensure that the treated effluent is free from all types of pathogenic microbes before it is safely discharged back into the environment or reused for other purposes.

The effectiveness of disinfection depends on the quality of the effluent being treated (e.g., cloudiness, pH etc), the type of disinfection process, disinfectant dosage (concentration and time) and other environmental variables. Generally, short contact times, low dosage and high-velocity flows militate against effective disinfection. Common methods of disinfection include Chlorination, UV Rays and ozonation.

Chlorination

Usage of liquid chlorine (Sodium Hypochlorite) is the most preferred method of disinfection on account of low cost and established effectiveness. In this method, chlorination of residual organic matter present in the treated effluent can generate carcinogens that are harmful to human health and to the environment as well. Besides, residual chlorine present in the treated effluent may also be capable of chlorinating organic matter in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be effectively dechlorinated.

Ultraviolet (UV) Rays

Ultraviolet (UV) rays can also be used for disinfection instead of chlorine or other chemicals. Since no chemicals are used in this process, the treated effluent will have no adverse effect on organisms that consume it. sum disadvantages of UV plants are frequent lamp maintenance/replacements and the need for a microfiltered effluent to ensure that the target microbes are not shielded from UV radiation by suspended solids present in the treated effluent.

Ozonation

Ozone, a very unstable and reactive element effectively oxidizes the organic matter it comes in contact with thereby destroying all the pathogenic microorganisms in the treated effluent. Ozone is considered safer than chlorine since it produces fewer by-products than chlorination. Unlike chlorine that needs to be stored on site, ozone is generated in-situ from the ambient air as and when required. One disadvantage of this method is the high cost of ozone generating equipment and the requirement of trained operators.

Oxygen Uptake

The treated effluent may be aerated, if required, to bring the level of dissolved oxygen up to the permissible limits before letting it out into the receiving environment. If re-use is intended, further polishing of this water may be required using appropriate treatment schemes. Eg. UF/RO Units.

Sludge Management

The processed sludge at the final stage containing coarse and fine biosolids generated as a by-product of the sewage treatment process must be further treated to reduce the volume and produce a usable end-product or dispose of in a safe and environment-friendly manner.

Reduction of sludge volume is achieved by removing the water content and increasing the sludge concentration. The water removed from the sludge will have high levels of ammonia and hence is returned to the aeration tank for retreatment.

The choice of dewatering method depends on the quantum of solids generated, client preference and other site-related conditions. While traditional ways such as sludge drying beds can be used, special equipment like gravity belts, filter presses and centrifuges can also be used to mechanically filter the sludge. In terms of final disposal, this de-moisturized / dry sludge can be used as natural fertilizers and for landfills.

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