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This document overviews the characteristics of city and industrial effluents ,draws the reason for varying flow issues andÂ optimises the processes of wastewater treatment by evaluating the comparative merits of conventional and new developments in technology.A comparative approach on the relative merits of Chemical and Biological options to explore the possibilities of various microbial treatments in improving the wastewater quality with a view to understand the economic efficiencies of the treatment methods,by comparing new future emerging Membrane bioreactor technology.This document summarizes the comparison of the biological and chemical phosphorus removal depending on various factors of effluent quality,sludge production,needed plant volume,consumption of energy & chemicals and economic cost of treatment. new insights into the anaerobic degradation of very different categories of compounds, and into process and reactor technology will lead to very promising new generations of anaerobic treatment system such as Expanded Granular Sludge Bed (EGSB) and Staged Multi-phase Anaerobic Reactor systems (MARS). Extensive research activity in this field has led to significant improvement and diversification in the processes and methods used for wastewater treatment and sludge management. Public health hazards are often associated with wastewater reuse, and consequently it is essential to disseminate knowledge and information about the danger of raw wastewater reuse and issue safe reuse guidelines.
AIM:To compare and contrast the efficacy of various treatment methods to recycle wastewater.
1) To estimate the availability of potable water.
2) To understand the human fallacies in polluting water.
3) To understand the various biological processes generated while addressing the problems of waste water.
4) To understand the comparative merits of biological and chemical treatment methods.
Water constitutes over 70% of the Earth's surface and is a very important resource for all people and the environment. If water gets polluted it cant be the elixir of life anymore to aquatic and to the wild life that depend on it.Rivers and streams polluted with chemical contaminants account as one of the most crucial environmental problems. Water pollution is basically a human fallacy. This used water is called "wastewater". Growing population and rapid industrialisation has increased the volume of wastewater manifold eventually deteriorating the fresh water resources and surrounding environment due to inappropriate management.It is 99.94 percent water, with only 0.06 percent of the wastewater being dissolved and suspended solid material.
Wastewater may be classified into four categories:
* Domestic:wastewater discharged from residences and commercial institutions and similar facilities;
* Industrial:wastewater in which industrial waste predominates;
* Infiltration/inflow:extraneous water that enters the sewer system through indirect and direct means such as through leaking joints,cracks,or porous walls.Inflow is storm water that enters the sewer system from storm drain connections,roof headers,foundation and basement drains or through manhole covers;
* Storm water: runoff resulting from flooding due to rainfall.
The first two categories, domestic and industrial are not entirely separable.
Characteristics: Fresh, aerobic, domestic water has been said to have the odour of kerosene or freshly turned earth. Aged, septic sewage is considerably more offensive to the olfactory nerves.The characteristic rotten-egg odour of hydrogen sulfide and the mercaptans is indicative of septic sewage.Fresh sewage is typically grey in colour.septic sewage is black.The class of chemical compounds found in wastewater are limitless and so they are better known by the name of the test used to measure themÂ which are BOD5Â and COD test.Industrial processes generate a wide variety of wastewater pollutants.The characteristics and levels of pollutants vary significantly from industry to industry.
Waste-water quality is assessed based on physical, chemical, and biological characteristics. Physical parameters include colour, odour, temperature, and turbidity. Insoluble contents such as solids, oil and grease, are to be considered under this category.. Solids may be further subdivided into suspended and dissolved solids as well as organic (volatile) and inorganic (fixed) fractions. Chemical factors to be considered are: biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), and total oxygen demand (TOD). Inorganic chemical parameters include salinity, hardness, pH, acidity and alkalinity, as well as concentrations of ionized metals such as iron and manganese, and anionic entities such as chlorides, sulfates, sulfides, nitrates and phosphates. Bacteriological parameters include coliforms, faecal coliforms, specific pathogens, and viruses. Both constituents and concentrations vary with time and local conditions.
VARYING FLOW ISSUE:
Waste-water flow fluctuates with variations in water usage, which is affected by a multitude of factors including climate, community size, living standards, dependability and quality of water supply, water conservation requirements or practices, and the extent of meter services, in addition to the degree of industrialisation, cost of water and supply pressure. Wide variations in wastewater flow rates may thus be expected to occur within a community.
TABLE: VARIATIONS IN WASTE-WATER FLOW WITHIN A COMMUNITY
Source: Adapted from D.H.F. Liu and B.G. Lipták, Wastewater Treatment (Boca Raton: Lewis, 1999).
Wastewater does not flow into a municipal wastewater treatment plant at a constant rate. The flow rate varies from hour to hour. In most cities, the pattern of daily activities sets the pattern of sewage flow and strength. Above-average sewage flows and strength occur in mid-morning.The constantly changing amount and strength of wastewater to be treated makes efficient process operation difficult.Also,many treatment units must be designed for the maximum flow conditions encountered which actually results in their being oversized for average conditions.Flow equalization is not a treatment process in itself,but a technique that can be used to improve the effectiveness of both secondary and advanced wastewater treatment processes.The purpose of flow equalization is to dampen the variations so that the wastewater can be treated at a nearly constant flow rate.Flow equalization can significantly improve the performance of an existing plant and increase its useful capacity.In new plants,flow equalization can reduce the size and cost of the treatment units.
WASTEWATER TREATMENT OPTIONS: may be classified into groups of processes according to the function they perform and their complexity:
CONVENTIONAL WASTEWATER TREATMENT PROCESSES:
General terms used to describe different degrees of treatment in order of increasing treatment level are preliminary, primary, secondary and tertiary and/or advanced wastewater treatment.
Preliminary treatment prepares wastewater influent for further treatment by reducing or eliminating non-favorable wastewater characteristics that might otherwise impede operation or excessively increase maintenance of downstream processes and equipment. These characteristics include large solids and rags, abrasive grit, odors, and, in certain cases, unacceptably high peak hydraulic or organic loadings. Preliminary treatment processes consist of physical unit operations, namely screening and comminution for the removal of debris and rags, grit removal for the elimination of coarse suspended matter, and flotation for the removal of oil and grease. Other preliminary treatment operations include flow equalization, septage handling, and odour control methods.
Primary treatment is designed to remove organic and inorganic solids by the physical processes of sedimentation and floatation. Nearly 30 - 40% of the pollutants are removed from the waste waters. Primary treatment acts as a precursor for secondary treatment.
The purpose of secondary treatment is the removal of soluble and colloidal organics and suspended solids that have escaped the primary treatment. This is typically done through biological processes, namely treatment by activated sludge, fixed-film reactors, or lagoon systems and sedimentation.
DEVELOPMENTS IN WASTEWATER TREATMENT METHODS:
Developments to the previous methods are made based on the necessity of treatment and novel processes are designed to produce an effluent of higher quality than normally achieved by secondary treatment processes or containing unit operations not normally found in secondary treatment.so the types of advanced wastewater treatment methods are divided into three major categories by the type of process flow scheme utilized:
* Tertiary treatment
* Physicochemical treatment
* Combined biological -physical treatment
Tertiary treatment may be defined as any treatment process in which unit operations are added to the flow scheme following conventional secondary treatment. Additions to conventional secondary treatment could be as simple as the addition of a filter for suspended solids removal or as complex as the addition of many unit processes for organic, suspended solids, nitrogen and phosphorus removal. Physicochemical treatment is defined as a treatment process in which biological and physical-chemical processes are intermixed to achieve the desired effluent. Combined biological-physical-chemical treatment is differentiated from tertiary treatment in that in tertiary treatment any unit processes are added after conventional biological treatment ,while in combined treatment,biological and physicochemical treatments are mixed.
Another way to classify advanced wastewater treatment is to differentiate on the basis of desired treatment goals. Advanced wastewater treatment is used for:
* Additional organic and suspended solids removal
* Removal of nitrogenous oxygen demand(NOD)
* Nutrient removal
* Removal of toxic materials.
In many, if not most instances today, conventional secondary treatment gives adequate BOD and suspended solids removals. But advance wastewater treatment is necessary because advanced wastewater treatment plant effluents may be recycled directly or indirectly to increase the available domestic water supply.
BIOLOGICAL TREATMENT OPTIONS: There are three basic categories of biological treatment: aerobic, anaerobic and anoxic. Aerobic biological treatment, which may follow some form of pretreatment such as oil removal, involves contacting wastewater with microbes and oxygen in a reactor to optimise the growth and efficiency of the biomass. Anaerobic (without oxygen) and anoxic (oxygen deficient) treatments are similar to aerobic treatment, but use microorganisms that do not require the addition of oxygen. Maintaining the required population of "workers" in a bioreactor is accomplished in one of two general ways:
* Fixed film processes: Microorganisms are held on a surface, the fixed film, which may be mobile or stationary with wastewater flowing past the surface/media. These processes are designed to actively contact the biofilm with the wastewater and with oxygen, when needed.
* Suspended growth processes: Biomass is freely suspended in the wastewater and is mixed and can be aerated by a variety of devices that transfer oxygen to the bioreactors contents .
It is also possible to combine both methods in a single reactor for more effective treatment.
FIXED FILM PROCESSES: The description of Trickling filters and Rotating Biological Filters are shown in the table below:
MEMBRANE BIOREACTOR: Membrane bioreactor (MBR) systems are unique processes, which combine anoxic- and aerobic-biological treatment with an integrated membrane ultrafiltration unit process that can be used with most suspended-growth, biological wastewater-treatment systems.
The two main UF system categories are:
Submerged systems use membranes that are a hollow fibre or flat sheet construction and these are located within the bioreactor, or separate tank. The main feature is that the flow of effluent is from the 'outside to inside' of the membrane.
Cross-flow systems which use tubular or plate & frame membrane systems, are usedÂ where the biological sludge flows along the surface of the membrane.
Most industrial wastewater falls into the second category. For the treatment of this wastewater, cross-flow MBR has been found to have many advantages
One of the discussion points within the MBR industry is the on going argument concerning energy costs. There is no doubt that submerged MBR systems absorb less energy, but this only applies when membrane fouling is not a factor. However, considerable progress is being made in the development of 'low energy' cross-flow MBR systems.Â One approach is the, so-called,Â "airlift" technique.
Advantages of Cross-Flow MBR:
1. Virtual zero solids discharge in the final effluent.
2. The application of a very high MLSS (mixed liquor suspended solids) within the bioreactor which equates to smaller tanks and lower investment cost.
3. The ability to handle wastewater in which salts will tend to precipitate. The membranes can be easily cleaned in situ (CIP).
4. Flux rates ranging up to 170 l/h.m2Â are achievable. Less membrane area means a lower investment cost.
5. The use of low sludge loading (F/M ratio) results in extended sludge age and, by definition, less generation of excess biological matter.
6. Very small foot print required when compared to conventional biological treatment plants.
7. Cross-flow membranes can be retrofitted onto existing activated sludge plants.
8. The availability of portable containerized systems.
9. The membranes are separate from the biological aerated tank (bioreactor) and will not affect the aeration efficiency. They can be easily removed for inspection.
10.The development of the semi-cross flow MBR will result in even lower
Â Â Â Operating costs.
With this outlook, MBR technology will be the key technology for future wastewater treatment.
CHEMICAL VS BIOLOGICAL TREATMENT:
Biological process removes solid organic matter and dissolved organic matter. The chemical process removes solid organic matter and phosphorus. Chemical precipitation cleans the water very rapidly, say in less than 15mins after the initiation of the process, we have clean water, whereas with biological treatment it will take 3 hrs to achieve the same. Biological process lasts comparatively long time and is dependent on the effectiveness of the microorganisms. Chemical and Biological treatments work in different ways and achieve different results. We have to determine the major causes of oxygen deficiency in waters and select the treatment in accordance with local environmental requirements. Purification in itself demands resources. Biological treatment requires a long residence time and energy is consumed when air is blown into the sewage water. Consequently the Biological plant is big and more complex. The microorganisms have to be adapted to the pollution which sensitises the whole system. Chemical purification requires addition of coagulants, which is done in a small pool and at lower energy consumption. The total energy used for chemical treatment is only 15% of that required for biological treatment, even if the energy used for production and distribution of coagulants is included. In terms of the total ecological stress, the chemical process is favoured. Life cycle assessments show the biological process to be a larger consumer of resources and therefore it is more negative interms of its entire environmental impact. If there is no need to remove dissolved organic matter a biological process could do greater ecological damage than chemical process due to the total energy consumption and the raw materials used when constructing the plant. The environment makes demands on the purification process and if advanced purification is necessary both biological and chemical methods must be used. In Norway, the existing chemical plant built entirely within bedrock was extended with the nitrogen removal system due to the increased nutrient load. With the unique combination of chemical and biological processes this plant occupies less than half the volume of the conventional biological process.
CONCLUSION AND RECOMMENDATIONS:
Effective wastewater collection and treatment are of great importance from the standpoint of both environmental and public health. Any wastewater treatment plant needs significant investment and Operation Maintenance and control, and therefore any decision to implement such a facility should be carefully considered. It is not a good idea to conclude that any treatment method is better than the other.Each one has its own advantages and disadvantages.The choice of which method is to be used will depend on the society,the discharge requirements and the costs they are ready to incur.As a final conclusion,the winning concept these days should not be based on which process should be used in isolation to the other,but rather using the advantages of both processes together to obtain best results,while at the same time minimizing their disadvantages. However, new insights into the anaerobic degradation of very different categories of compounds, and into process and reactor technology will lead to very promising new generations of anaerobic treatment system such as Expanded Granular Sludge Bed (EGSB) and Staged Multi-phase Anaerobic Reactor systems (MARS). Extensive research activity in this field has led to significant improvement and diversification in the processes and methods used for wastewater treatment and sludge management. Public health hazards are often associated with wastewater reuse, and consequently it is essential to disseminate knowledge and information about the danger of raw wastewater reuse and issue safe reuse guidelines.
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