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Waste water disposal and treatment is a necessary utility for the civilised world. The earliest recorded evidences of waste water disposal systems are from the Bronze Age i.e. the Indus valley civilization. They had a fully functional running water and underground sewage system. The Romans had an open sewer system and the structures still exist as an evidence of human endeavours to supply safe water, remove wastes from water, and to protect public health.
Fig-1 Early Sewer systems. Courtesy- www.sewerhistory.org
It was only after Antony Von Leeuwenhoek and his rudimentary microscope, that the field of microbiology was born. Microorganisms and their roles were studied with great interest. Modern Biotechnology and its applications including sewage treatment, owe a lot to such humble beginnings in science. In 1850, the first comprehensive sewage system was laid out in the city of Chicago. Since then, the field of waste water management has come a long way with technological advancements and a relative comprehensive knowledge of the microbial life involved.
Modern definition of sewage treatment is that it's the process of removing contaminants from wastewater and household sewage, 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.
Usually domestic sewage refers to the effluent from households and it contains human excreta along with other waste water from day to day household activities. Whereas, Industrial sewage refers to the unwanted effluent from any industrial process. Most industries have an in-house water treatment facility that can effectively remove chemical and biological contaminants. This enables the industries to recycle the water back for their own needs.
IMPORTANCE OF SEWAGE TREATMENT
Waste water is rich in organic nutrients and when it accumulates, it can become a rich source of medium for microorganisms to grow on. This eventually leads to decomposition and foul gases being emitted. The most dangerous consequence of all is the accumulation and propagation of pathogens that can lead to dangerous epidemics.
Every year hundreds of billions of gallons of untreated sewage flow into our rivers, lakes, and coastal waters. Cases of sewage water contaminating fresh water resources are numerous and have resulted in serious health consequences in the local population. In the USA alone, Environmental Protection Agency, EPA estimates that over 7 million people suffer from mild to moderate illnesses caused by untreated sewage every year. Another ½ million get seriously ill. This is just a conservative estimate considering the fact that many cases go unreported. A recent study found that up to 1.5 million people get gastroenteritis at beaches in two counties in California each year. If this is the case, the number could be much higher. If this is the case in one of the most developed countries in the world, the plight of developing and underdeveloped countries can only be imagined. Epidemics like Cholera have wiped out thousands of lives over the years due to poor sanitation. These epidemics can be very sever in countries with already existing problems like malnourishment, high HIV incidence and so on.
Algal bloom is another tragic consequence of untreated water. The most common pathogens in sewage are bacteria, parasites, and viruses.
Chronic or Ultimate Effects
Ulcers and stomach cancer
Skin and Tissue infection
Death in those with liver problems
Death: Guillain-Barré syndrome
Fever, headache, chills, muscle aches, vomiting
Weil's Disease, kidney damage, liver failure, death
Death, Hemolytic Uremic syndrome
Fig-2 List of some of the pathogens commonly found in Sewage.
WASTE WATER TREATMENT METHODS
The usage of a septic tank is a tried and tested domestic sewage solution. The septic tank and soak away system is a time-honoured one that originated in France in the 18th century (the reason it's often referred to as French drain). It relies on natural bacterial processes for its successful operation. There is no external energy input and therefore no 'running cost'. It is widely in use in many parts of the world with varying differences in their functionalities. The problem with septic tanks is the increasing concentration of chemicals like caustic soda, sulphuric acid and other bleaching agents that are increasingly found in domestic effluents due to the use of household chemicals/cleaning agents. These chemicals are harsh on microorganisms that are supposed to naturally degrade the organic nutrients in the sewage, thereby effectively sabotaging the natural principle behind a septic tank. Nowadays, septic tanks are increasingly used as storage tanks in many developing nations, where there is no access to a centralised drainage system. From the storage tanks, trucks are used to transport the sewage to disposal sites.
Conventional large scale wastewater treatment processes typically follow the stage wise procedure outlined above. Waste sludge drawn from these operations is thickened and processed for ultimate disposal, usually either land application or landfilling. Chlorination of raw feed is sometimes done to combat odour and also to improve settling characteristics of suspended solids.
An aerial view depicts a typical wastewater treatment plant supporting primary and secondary treatment. Visible in this photograph are flocculation basins (rectangular), primary settling tanks (foreground), and secondary trickling filters (background).
Fig-3. An aerial view depicts a typical wastewater treatment plant supporting primary and secondary treatment. Visible in this photograph are flocculation basins (rectangular), primary settling tanks (foreground), and secondary trickling filters (background). Courtesy - www.wastewaterencyclopedia.com.
The main objective of a waste water treatment process is to safely dispose domestic and industrial effluents with no harm to public health and the environment. Reuse of the water purified from sewage is also an essential parameter in the context of sustainability. The most appropriate wastewater treatment to be applied before effluent use in agriculture is that which will produce an effluent meeting the recommended microbiological and chemical quality guidelines both at low cost and with minimal operational and maintenance requirements (Arar 1988). Pathogen removal has very rarely been considered an objective but, for reuse of effluents in agriculture, this must now be of primary concern and processes should be selected and designed accordingly (Hillman 1988).
Conventional waste water treatment processes involve a combination of physical, chemical and biological methods in varying frequencies. But all of them follow a stage wise approach to water treatment.
Preliminary treatment involves the removal of coarse solids and other large particles. The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Most of often physical equipment like a comminutor is used to pulverise the solids found in the feed. Grit chambers are used, through which the feed water is gushed with considerable velocity to remove grit. Grit removal is not done in smaller water treatment plants. Other types of physical equipments can also be used for such screening. Flow and other parameter measurements are taken at this juncture.
Primary treatment is used to remove settle able organic and inorganic solids using sedimentation and skimming. BOD (Biological oxygen demand) and COD (chemical oxygen demand) play a crucial role in determining the amount of organic/inorganic content of the feed. The BOD is reduced by 25% to 50% , while the TDS (total suspended solids) is reduced by 50% to 70 %. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation. The effluent from primary sedimentation units is referred to as primary effluent. The below table provides information on primary effluent from three sewage treatment plants in California along with data on the raw wastewaters.
Secondary treatment is used to remove the residual organic material and suspended solids. In most cases biodegradation is achieved by an aerobic biological treatment process. This effectively metabolises the organic matter in the waste water, resulting in the propagation of much needed degrading microorganisms and organic end products. Common processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contactors (RBC). A combination of two of these processes in series (e.g., biofilter followed by activated sludge) is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources.
i. Activated Sludge
ii. Trickling Filters
iii. Rotating Biological Contactors
QUALITY OF SECONDARY EFFLUENT AT SELECTED WASTEWATER TREATMENT PLANTS IN CALIFORNIA
Quality parameter (mg/I except as otherwise indicated)
Chino Basin MWD (No. 1)
Chino Basin MWD (No. 2)
Santa Rosa Laguna
Montecito Sanitary District
Biochemical oxygen demand, BOD5
Chemical oxygen demand
Electrical conductivity dS/m
Total dissolved solids
Sodium adsorption ratio
Total Hardness (CaCO3)
Fig-5 Source: Asano and Tchobanoglous (1987)
Tertiary and/or advanced treatment
Tertiary or advanced treatment is used to remove unwanted toxic constituents that are hitherto not removed during the conventional process. Individual These maybe nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. Because advanced treatment usually follows high-rate secondary treatment, it is sometimes referred to as tertiary treatment. However, advanced treatment processes are sometimes combined with primary or secondary treatment (e.g., chemical addition to primary clarifiers or aeration basins to remove phosphorus) or used in place of secondary treatment (e.g., overland flow treatment of primary effluent).
Disinfection is normally done using chlorination in a chlorine contact basin. It is cost-effective. Other methods like Ozone and UV irradiation can also be used.
Natural biological treatment systems
Natural biological systems are a low cost and innovative way to treat wastewater. These processes are land intensive, but more then make up for this inadequacy in terms of being cost effective and non reliance of any high tech equipment. They are most effective at removing pathogens. The most commonly used systems are,
1. Wastewater stabilization ponds
Wastewater stabilization pond systems are essentially ponds with three different stages for different forms of treatment. A primary facultative pond is used to treat weaker waste water. Effluent from the primary pond is sent into a secondary treatment pond for further treatment. If the water contains pathogens, a tertiary maturation pond is used for the purpose of decontamination.
2. Overland treatment of wastewater
Suspended and colloidal organic materials in the wastewater are removed by sedimentation and filtration through surface grass and organic layers. Removal of total nitrogen and ammonia is inversely related to application rate, slope length and soil temperature. Phosphorus and trace elements removal is by sorption on soil clay colloids and precipitation as insoluble complexes of calcium, iron and aluminium. Overland flow systems also remove pathogens from sewage effluent at levels comparable with conventional secondary treatment systems, without chlorination. A monitoring programme should always be incorporated into the design of overland flow projects both for wastewater and effluent quality and for application rates.
3. Macrophyte treatment
These essentially use maturation ponds that use emergent aquatic plants for treating effluents. Macrophytes take up large amounts of inorganic nutrients (especially N and P) and heavy metals (such as Cd, Cu, Hg and Zn) as a consequence of the growth requirements and decrease the concentration of algal cells through light shading by the leaf canopy and, possibly, adherence to gelatinous biomass which grows on the roots.
4. Nutrient film technique
The nutrient film technique (NFT) uses the hydroponic plant growth system in which plants are grown directly on an impermeable surface to which a thin film of wastewater is continuously applied. Root production on the impermeable surface is high and the large surface area traps and accumulates matter. Plant top-growth provides nutrient uptake, shade for protection against algal growth and water removal in the form of transpiration, while the large mass of self-generating root systems and accumulated material serve as living filters.
Figure 10: Nutrient film technique variation of hydroponic plant production systems (Jewell et al. 1983)
VARIATION IN FLOW RATES
Flow rates are highly variable in a waste water treatment plant. These variations are primarily observed in municipal sewage treatment plants where the bulk of the feed comes from domestic sources. These short term variations in flow rate follow a diurnal flow pattern. Logically flow rate is at its highest during the late morning peak usage in domestic households and gradually decreases as the day progresses. It peaks again in the evening followed by another decreasing pattern. The magnitude of peaks depends on the local population, demographics and other social factors but it does follow a diurnal pattern. Municipal sewage plants are designed to meet these kinds of requirements. Small communities with small sewer systems have a much higher ratio of peak flow to average flow than do large communities. These variations, especially in smaller communities can make it impossible for the recycled water to be used for irrigation, as this involves varying levels of water purifications, which makes its use for agriculture impractical.
The science of wastewater treatment has come a long way from its humble beginnings of open sewer systems. Conventional methods, especially those on a large scale use a combination of physical, chemical and biological methods which is the most logical way to go about. But, in this entire quest for advanced technologies, have we lost our focus on simple naturally available purification mechanisms? The humble septic tank was a simple and effective way for treating domestic sewage. Due to rapid urbanisation and limited land availability it has become impractical in the larger cities. But, smaller municipalities and rural organisations are increasingly adopting urban ways of waste management which is unsustainable. Biogas method is an environmental friendly and sustainable approach to domestic waste management. It doesn't require much expertise and a valuable by-product is obtained, which can be used for household cooking needs. Industrial waste water treatment is regulated and monitored strictly in the developed countries, but the situation is abysmal in developing and under developed countries. Heavy metal pollution of ground water resources is the biggest hazard facing many developing countries. In Bangladesh, high levels of toxic arsenic can be found in underground drinking water resources. There is light at the end of the tunnel with newer biotechnological advancements in the field of bioremediation. Developments like nanostructure silica that can effectively remove any kind of toxic metals from water are being developed. But the problem is with the access to these kinds of cutting edge technologies for countries in need of it. This is where I feel the UN can play a major role in technology transfer to countries in need with considerable subsidies.