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The treatment of wastewater is mainly by the biological and physico- chemical methods, with the concentration in this essay been more on the chemical and biological treatment processes.
The chemical treatment for effluent (in some of its processes) is less energy efficient in comparison to the biological process of effluent treatment. The bioproduct for the biological effluent treatment process is biogas, and in the most commonly used form (anaerobic effluent treatment) it is very advantegous as there is reduced sludge production.
Wastewater can de defined as any water that has been unfavourably affected in quality as a result of the influence of human (Man Made) activities. This usually comprises of waste liquid that is discharged by domestic residences, industry, commercial properties, and/or agricultural activities. Wastewater/effluent in this essay will include Industrial and municipal/city wastewater/effluent (United Nations, 2003).
Industrial wastewater include any wastewater that has been discharged from any premises that is used in the carrying on of any trade or industrial activity (apart from run - off rain water and domestic water), while municipal wastewater is the water-carried wastes originating in the sanitary conveniences of domestic dwellings, industrial or commercial facilities and institutions, in addition to any groundwater, surface water and storm water that may be present (European Commission, 2009).
Waste water that has not undergone treatment generally contains numerous pathogenic microorganisms, high levels of organic material, toxic compounds and nutrients. Hence, it gives rise to environmental and health hazards. Thus, wastewater must immediately be carried away from sources of generation and treated aptly before final disposal. The crucial aim of waste-water treatment is to protect the environment in a way that is adequate with set public health and socio-economic standards (European Commission, 2009).
The quality of wastewater can be defined by its chemical, biological and physical characteristics (de la Sota, 2007). Content that is insoluble; such as oil, grease and solids, all make up physical properties of waste-water; solid contents are divided into inorganic and organic fractions. Colour, temperature, turbidity and odour also fall under the physical parameters (United Nations, 2003). The chemical properties that are characteristic of wastewater include the organic content; chemical oxygen demand (COD), biochemical oxygen demand (BOD), total oxygen demand (TOD), and total organic carbon (TOC). The inorganic chemical content include hardness, acidity, pH, alkalinity, acidity, concentrations of ionized metals such as iron and manganese, and anionic entities such as sulphates, nitrates, sulphides and phosphates. Biological parameters include fecal coliforms, coliforms, viruses and pathogens (United Nations, 2003).
Figure 1: Diagram showing the sources of wastewater and these include the domestic and industrial sources.
IMPORTANT CONTAMINANTS IN WASTE-WATER
Reason for importance
When untreated wastewater is discharged into aquatic bodies, it can result in the development of sludge deposits and anaerobic conditions.
They are principally made up of proteins, carbohydrates and fats. They are commonly measured in terms of BOD and COD. If discharged into inland rivers, streams or lakes, their biological stabilization can deplete natural oxygen resources and cause septic conditions that are detrimental to aquatic species.
These organisms can cause varying infections
These pollutants include organic and inorganic compounds, and they may be highly toxic, carcinogenic, and mutagenic.
These are usually added by commercial and industrial activities and must be removed for reuse of the waste-water.
These are constituents of effluents that tend to resist conventional waste-water treatment, and they include surfactants, phenols and agricultural pesticides.
Dissolved inorganic constituents
These include calcium, sulphate and sodium, and are often initially added to domestic water supplies, and may have to be removed for waste-water reuse.
Table 1: The above table shows the constituents of wastewater, and their varying function or uses in wastewater treatment.
There are different stages of wastewater treatment, and they include the preliminary, primary, secondary and tertiary treatment. There is also the type of treatment; physical, chemical and biological. I will focus more on the chemical and biological routes of effluent treatments, and also the combination of the above mentioned methods.
Figure 2: This shows the different treatments that have to be undergone by wastewater before it is suitable for reuse or discharge into aquatic bodies. There is the primary and secondary treatment in the above diagram. Source: (Earthpace, 2009).
Unit operations and processes of wastewater treatment
Figure 3: This figure show the different type of effluent treatment that are available, and the varying methods under each type of treatment. There is the physical, chemical and biological type of effluent treatment.
2.0 VARIATION IN EFFLUENT FLOW: WHY?
There are varying rates of flow for the different types of effluent (industrial and city), and this is due to a multitude of factors that include but are not limited to the following; climate, water conservation practices or requirement, size of community, type of industrial activity, living standards, degree of industrialization, water supply pressure and cost of water (Metcalf and Eddy, 2003). Thus there is expected to be a wide variation in the different effluent flow rate within a region/community.
Size of community (Population)
Variation of Waste-water flow (% of average daily flow rate)
20 - 400
1000 - 10 000
50 - 300
10 000 - 100 000
Up to 200
Table 2: This shows the varying flow rates of waste-water within a community. The rate of flow is affected by the activity and population size of the community Source: (United Nations, 2003).
3.0 EFFLUENT TREATMENT
Biological and physico-chemical characterisation is the first step that is needed to define the convenient treatment technology for effluents that have been generated industrially and domestically (de la Sota, 2007).
To attain varying degrees of contaminant removal, wastewater treatment procedures are combined into different phases, classified as primary, secondary, tertiary and sometimes preliminary and advanced treatment. They are discussed further below (United Nations, 2003):
Preliminary treatment: this step prepares the wastewater for further treatment by the reduction and elimination of non-favourable properties that may impede treatment operation, or greatly increase the maintenance of downstream processes and equipments. Some of the features of wastewater at this stage include presence of large solids, abrasive grit, peak hydraulic (usually at an unnecessarily high peak), and odours and in some instances organic loadings. At this phase, the processes consist of mostly physical units of operations; such as grit elimination that is necessary for the removal of coarse suspended matter, floatation for grease and oil removal, and comminution and screening for the elimination of rags and debris. Septage handling, odour control processes and flow equalization are also some operations carried out at this stage (United Nations, 2003).
Primary treatment: This stage comes before the secondary treatment, and like its precursor it also involves physical unit of operations, like sedimentation and screening (Earthpace, 2009). The treatment entails the removal of organic matter and suspended solids partially via the physical processes mentioned above. Primary treatment is intended for the production of effluent (liquid) that is appropriate for downstream biological treatment, and also the separation of solids as sludge that will be treated conveniently and economically before final disposal. Effluents at this stage have a high level of biological oxygen demand (BOD) (United Nations, 2003).
Secondary treatment: the main characteristic of this stage is the removal of suspended solids (that were not removed at the primary treatment stage), and soluble and colloidal organics (Earthpace, 2009). Biological processes are the main treatments that are carried out at this stage of effluent treatment, and they include; treatment by fixed - film reactors, lagoon systems, sedimentation and activated sludge (United Nations, 2003).
Advanced or Tertiary treatment: not the most common of the stages of wastewater treatment, as it usually goes beyond the phase of the conventional secondary treatment to get rid of considerable amounts of phosphorus, heavy metals, nitrogen, biodegradable organics, viruses and bacteria. The unit of operations in this instance are the biological and chemical processes. The chemical processes include flocculation, coagulation and sedimentation, activated carbon and filtration (United Nations, 2003).
3.1 BIOLOGICAL EFFLUENT TREATMENT
This treatment is used in the conversion of dissolved and finely divided organic matter in the wastewater into flocculent inorganic and organic solids. During this treatment, microorganisms, such as bacteria aid in the conversion of dissolved and colloidal organic matter into cell tissue and various gases - these are subsequently removed in sedimentation tanks.
There are different types of biological effluent treatments, and they are classified below. This classification is also further subdivided based on the place the treatment is taking place, either in an attached - growth system, a suspended growth system or the combination of the two.
This occurs in the presence of oxygen, and it sometimes follows some form of pre-treatment. It involves contacting wastewater with oxygen and microbes in a reactor, in order to optimize efficiency and growth of biomass.
Like the Aerobic process, but there is a deficiency of oxygen. Mostly used in organisms that does not require addition of oxygen.
Also like the aerobic process, but in the total absence of oxygen. Used in organisms that do not require oxygen.
This process is a combination of all the above mentioned processes (aerobic, anaerobic and anoxic),it could also involve the processes been run in a sequence
Table 3: This shows the varying classifications of processes that are utilized in the biological treatments of effluent.
In most cases biological treatment is used in combination with the physical and chemical effluent treatments.
BIOLOGICAL EFFLUENT TREATMENT PROCESSES
Activated sludge process
Oxygen is mechanically supplied to bacteria which feeds on organic material and provide treatment
Sophisticated process with mechanical and electrical parts, which also needs careful operator control. Produces large quantities of sludge for disposal, but provides high degree of treatment (when functioning properly)
Like WSPs, but with mechanical aeration
Not very common; oxygen requirement mainly from aeration and hence more complicated and higher O&M costs.
Land treatment (Soil Aquifer Treatment - SAT)
Sewage is supplied in controlled conditions to the soil
Soil matrix has quite a high capacity for treatment of normal domestic sewage, as long as capacity is not exceeded. Some pollutants, such as phosphorus are not easily removed
Oval - shaped channel with aeration provided
Requires more power than WSPs, but less land, and it is easier to control than processes such as ASP
Waste stabilization ponds (WSPs) ('oxidation ponds or lagoons')
Large surface-area ponds
Treatment is essentially by action of sunlight, encouraging algal
Ponds growth which provides the oxygen requirement for bacteria to oxidize the organic waste. Requires significant land area, but one of the few processes which are effective at treating pathogenic material. Natural process with no power/oxygen requirement. Often used to provide water of sufficient quality for irrigation, and very suited to hot, sunny climates.
Reed (or constructed wet lands) beds
Sewage flow through an area of reeds
Treatment is by action of soil matrix and particularly, the soil/root interface of the plants. Requires significant land area, but no oxygenation requirements.
Rotating biological contractor (biodisk)
Series of thin vertical plates which provide surface area for bacteria to grow
Plates are exposed to air and then sewage by rotating with about 30% immersion in sewage. Treatment is by conventional aerobic processes. Used on a small scale in Europe
Trickling ('or percolating') filters
Sewage passes through a loose bed of stones and the bacteria on the surface of the stone treats sewage.
An aerobic process in which bacteria take up air from the atmosphere (no external mechanical aeration)
Upflow anaerobic sludge blanket (UASB)
Anaerobic process using blanket of bacteria to absorb polluting load
Suited to hot climates. Produces little sludge, no oxygen or power requirement, but produces a poorer quality effluent than other processes such as ASP.
Table 4: Biological effluent treatment methods that are most commonly used are briefly explained in the table shown above.
Figure 4: Rotating Bed contractor (RBC), used in the conventional aerobic biological treatment of effluents
Figure 5: View of section of a trickling filter, used in the biological treatments of effluents. The treatment process is aerobic, with mechanical agitation absent.
During this treatment process, under the anaerobic condition there is the production of a relatively small amount of excess biological sludge (very advantageous), production of energy rich biogas. There is also the development of different reactors that are compact in nature, thus making this method very successful in its application in industrial and domestic effluent treatments, but mainly used in the industrial wastewater treatment (Driessen, Wouters, Habets, & Buisman, 2001). This treatment method in some processes enables the re-use of resources that are valuable e.g. the process water; it also minimizes the quantity of waste product.
This method has also been effectively utilized in the successful elimination of inorganic compounds (metals and sulphates, nitrogen: through denitrification), as well as organic compounds (COD), it also aids the reduction of the toxicity of aquatic bodies (Schultz, 2005) .
The cost for this process is relatively low in operating costs and capital, in comparison to the chemical effluent removal process (Driessen, Wouters, Habets, & Buisman, 2001).
In the anaerobic effluent treatment process, some of the bioproducts that are produced include biogas, and some other valuable resources that depend on the origin of the wastewater (Driessen, Wouters, Habets, & Buisman, 2001).
3.1.4 ENERGY USE
In some processes using this type of effluent treatment, operations can be carried out under thermophilic temperatures (45 - 55oC), thus there is an added conservation of energy as the effluents do not have to be heated (i.e. heating treated effluent becomes unnecessary) (Driessen, Wouters, Habets, & Buisman, 2001).
In some other processes like the aeration in biological treatments, large amount of energy is usually consumed. Thus purchasing new equipments that are energy efficient will increase the advantages of this process.
3.2 CHEMICAL EFFLUENT TREATMENT
This treatment involves the use of chemical reactions in wastewater treatment. Some changes take place during this process, and like the biological treatment, this process can be used with both the physical and biological effluent treatment.
There are different chemical processes, and they are outlined in the table below:
Chemical precipitation: This entails the coagulation chemically of raw wastewater before the sedimentation stage. This process promotes the flocculation of solids (finely divided) into settle-able flocs, thus enhancing the efficiency of the removal of suspended solids, phosphorus and BOD5, in comparison to normal (plain) sedimentation in the absence of coagulation.
Adsorption with activated carbon: The adsorption process involves the collection of substances that are soluble, within a solution on a surface that is suitable.
Adsorption using carbon that has been activated (char is heated to a high temperature and then activated by exposure to an oxidizing gas), usually follows the normal biological effluent treatment, and is normally aimed at eliminating a large portion of DOM (dissolved organic matter) that is remaining.
Disinfection: This process involves the selective destruction of microorganisms that are disease causing in nature. Has a great significance in wastewater/effluent treatment as a lot of disease causing microorganisms (harmful to humans) is usually found in wastewater.
Dechlorination: This process entails the elimination of combined and free chlorine that are residue in chlorinated effluent before it is discharged into aquatic bodies or reused.
This is very significant in wastewater that is chlorinated as the chlorine (when it reacts with organic compounds) produce undesired toxic compounds that have adverse effects that could be long-term and potentially toxic to aquatic animals and microorganisms.
Other chemical applications: There are many other chemical processes used in effluent treatment, and they include grease removal, Ph Control e.t.c
Figure 6: A once-through chemical treatment system
Figure 7: A typical granular activated carbon contactor
The disinfection process under this treatment method is very important, as most disease causing microorganisms are usually eliminated before the final discharge of the water into aquatic bodies/ before its reuse.
In comparison to the biological and physical effluent treatment processes, chemical effluent treatment processes have more disadvantages because it is an additive process, in that it causes a net increase in the dissolved constituents of wastewater.
In some processes under the chemical treatment methods, there is usually a high operational costs and it also demands great attention to detail on the part of the operator, this in turn increases manpower cost thus increases cost of total operation.
The bioproduct here in some instance
3.2.4 ENERGY USE
The most commonly used process for treatment of effluents in most industrial wastewater treatment plants is usually the biological treatment and this is sometimes mixed with the chemical and physical methods. As highlighted in the advantages, the biological method in most instances is more cost effective and saves energy in comparison to the chemical treatment methods. That is not to say the chemical treatment has no advantages; it does have advantages, but most of its advantages have not been highlighted in this essay, as not many information can be found on the advantages and bioproduct from the process (chemical treatment of effluents).
In recent times, computational methods that are advanced in fluid dynamics are being utilized for the optimization of existing treatment systems and the design of novel ones; this is done to remove all sources of inefficiencies. For instance, improved sedimentation tanks, chlorine contact tanks and UV disinfection systems are designed currently using the above mentioned computational fluid dynamics.
Energy management is also an important part of the wastewater treatment facilities design and operation. In some processes such as the aeration in biological effluent treatment, utilizes a large amount of energy, therefore the design of energy recovery schemes and the selection of equipments that are energy efficient are increasingly assuming a great importance in this field.
Changes in technology in industries, has seen the gradual change in industrial effluents. In most instances, industrial effluent contains larger quantities of novel synthetic organic compounds and significant proportion of heavy metals. Thus new effluent treatment technology could be required for the compounds to be removed. Some new treatment methods include the use of high rate clarification, vortex separators, membrane filtration and membrane bioreactors.
The movement towards the advance of proprietary treatment methods in light of the privatization of many treatment facilities is also observable.
NEW DEVELOPMENTS: There have been a number of new treatment methods that have only been developed recently, and they include the following:
Oxidative co- metabolic transformation by methanotrophs
Oxidative co - metabolic transformation by nitrifiers
Biological Activated Carbon Oxidative Filters (usually characterized by biofilms)
The above mentioned and many others, are new technologies that are being proposed or are currently showing great promise in their usage in effluent treatment. In the biological treatment, the use of organisms that have been genetically modified (that have a greatly improved catabolic potential) in advanced wastewater treatment is needed.
Wastewater treatment is a very important field, as it contributes greatly to the maintenance of the environment, and also when done properly, the health and safety of the general public can be assured. In countries where there is shortage of water, most of these treatments methods are been used, and in most developing countries that cannot afford the more expensive technology, the lower cost treatments methods are usually utilized.
In wastewater treatment, the use of the combined methods (physical, chemical and biological), in my opinion will be the best. The combination of the most energy efficient processes, with minimum amount of toxic waste could be used in the treatment of both industrial and domestic effluents. Although this might seem to cost more, in most instances these type of combination methods are already been utilized.
The processes with the least amount of sludge production, which produces bioproduct that can be utilized, that are energy efficient and cost effective, should be used in the treatment of wastewater in developing countries.
In conclusion, there is still a lot to be done in the improvement of the wastewater treatment processes, and with improvement in technology we can only hope for processes that are energy efficient, cost less and not harmful to the environment. It should also be noted that the different effluent treatment methods are used in different industries, mostly based on the type of industrial activity taking place.