The type and nature of any product depends upon medium or substrate used in production process. Based on type of the medium used, the range of waste materials are produced accordingly. Thus, treatment methods to be applied depends upon characteristics of waste and various factors like physical state of the effluent, degree of hazard of pollutant, chemical complexity of the effluent and so on.
The level of pollution is increasing day by day due to increased population, increased industrial development, and awareness on damages caused by pollution causing industrial units in order to treat them before disposal into environment.
Effluents from production units contain many hazardous toxicants that adversity affect the flora and fauna by interacting with the microbes and drastically decreasing the dissolved oxygen levels of that area. Effluent can be treated using biological, physical and chemical methods. Industrial effluent can be treated by oxidative ponds, spray irrigation, well disposal, and incineration, etc.
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Filtration and sedimentation are major processes in physical treatment of effluent. Chemical treatment process done by flocculation, coagulation, aggregation methods which are done by addition of various flocculants and coagulants like alum, ferric sulphide, calcium hydroxide, etc.
In biological treatment, trickling filters, biologically aerated filters, rotating biological contractors, activated sludge process come with aerobic way and anaerobic digesters, anaerobic filters, up-flow anaerobic sludge blankets come with anaerobic treatment processes.
Biological treatment consists of aerobic and anaerobic processes. It is an important part of any wastewater treatment plant. This method can be used to treat wastewater from either municipality or industrial waste containing soluble organic impurities or a mix of the two types of wastewater sources.
There is economic advantage of biological treatment over other treatment processes like chemical oxidation, thermal oxidation, etc. Biological treatment using aerobic activated sludge process has been in practice for well over a century.
Aerobic treatment process occurs in the presence of air and utilise aerobic micro-organisms which use molecular/free oxygen to assimilate organic impurities, i.e., convert them in to carbon dioxide, water and biomass. The anaerobic treatment process takes place in the absence of air by those microorganisms, called anaerobes which do not require air (molecular/free oxygen) to assimilate organic impurities. The end products of anaerobic treatment are methane and carbon dioxide gas and biomass.
Steps Involved in Aerobic Treatment Process:
Aerobic biological treatment is normally used for treatment of industrial effluent for their organic impurities. The major process used worldwide is activated sludge process. It is an aerobic biological process that utilises microorganisms such as bacteria, and protozoa for the decomposition of organic matter.
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This process involves the production of biomass of micro-organisms which effectively stabilise the organic content of wastes in presence of oxygen. This biomass is known as activated biomass which convert organic matter to carbon dioxide and water which are safe for disposal in the environment. This is effectively achieved by a series of metabolic reactions carried out by the micro-organisms.
Aerobic treatment can be used for waste treatment of various industries like sugar, soap, solvent and oil extraction industries, edible oil refineries, food, dairy, soft drink bottling, breweries, starch, pesticides and bulk drug pharmaceutical industries, chemical and mining industries, etc. This can also be used for sewage treatment and common industrial wastewater treatment.
Steps Involved in Anaerobic Treatment Process:
Anaerobic treatment is a biological wastewater treatment process which is used for treatment and reduction of organic wastes such as organic sludge or concentrated organic industrial waste. Organic solids are major components of industrial waste.
The quantity of organic solids are decreased in the sludge after anaerobic treatment process and this lead to easier disposal of waste due to less volume. During this treatment process, consortia of microbes convert organic matter in to biogas (methane) which can be used as a clean energy source.
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Anaerobic digestion process is a natural process that is effectively utilised to treat waste. This digestion occurs in anaerobic bioreactor or digesters, in which the anaerobic bacteria are maintained in high concentrations, thus increasing the effective sludge retention time. This process also results other by-products those are biogas (capable of providing heat) and compost for soil improvement.
Digests are commonly used for sewage, municipal wastewater or industrial wastewater treatment. They significantly help improve the quality of effluent being released into the environment and are a major component in water recycling from high-organic-load wastewater. In comparison to aerobic treatment, anaerobic biological treatment plants is used to treat high-strength wastewater to a level that will permit discharge to a municipal sewer system.
Anaerobic digestion process takes place in three steps or stages. The first stage involves liquification of solid material in the sludge. This process is called hydrolysis. The second stage is rapid and involves digestion of the soluble solids that resulted from the previous stage. This process is carried out by acids producing anaerobic bacteria.
The microbes involved in this stage are facultative aerobes, are heterotrophic and found in soil. They belong to diverse genera such as Escherichia, Flavobacterium, Alcaligenes, Aerobacter, Pseudomonas, etc., and can function in a large pH range. Final stage is called the gasification stage.
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In this stage, the organic acid produced in the previous stage is used by certain microbes as substrate and methane and carbon dioxide gases are produced. This stage is slower and is also called methanogenesis as it leads to production of methane. Microbes involved in this process are also anaerobic and belong to the Methanococcus, Methanobacterium, and Methanoscarcinagenera.
Steps Involved in Sewage Treatment Process:
Sewage is wastewater carrying domestic wastes, agricultural wastes or overland flow. This is carried in sewers or drains. Sewage contains both organic and inorganic wastes; it also contains grit, solids, oil and grease. Sewage is treated in sewage treatment plants to make it suitable for further disposal. Sewage treatment process involves three sequential treatment steps these include primary, secondary and tertiary treatment.
Wastewater treated in sewage treatment plants can be used to recharge ground water and for agricultural purposes. Treatment of sewage can be done by following steps – pre-treatment, primary treatment, secondary treatment and tertiary treatment procedures involved in treating sewage. This treatment is done by analysing the physical, chemical and biological contaminants present in sewage coming from a particular source.
Wastewater coming from industry as effluent is passing through screening unit to separate out larger components under primary treatment, then after grit chamber to sediment settalable grits. Wastewater coming from grit chamber is pass through primary settling tank or clarifier, in this step sludge settle down at bottom to reduce the volume.
In secondary treatment, wastewater is aerated to increase dissolve oxygen and then treated in biological reactors. After treatment in reactor, water goes to secondary clarifier to settle sludge. Some part of sludge (activated sludge) is recycled to reactor to maintain microbial count. Water from secondary clarifier/tank is chlorinated to polish the treated water. Sludge is thicked/concentrated and digested in anaerobic digester and then disposal of sludge takes place.
Steps Involved in Treatment Process of Sugar Industry Effluent:
The sugarcane received from the field contains about 70% moisture on an average. Majority of this water has to be discharged as factory wastewater. The effluent characteristics from a typical sugar plant are presented in Table 9.1.
The pollution load of sugar mills can also be reduced if we could use advance technological practices with in the plant. Judicious use of water in various plant practices, and its recycle, wherever practicable, will reduce the volume of waste to a great extent. Volume of mill house waste can be reduced by recycling the water used for splashing. Figure 9.4 shows treatment steps for sugar mill effluent waste.
Dry cleaning of floors or floor washings using controlled quantity of water will also reduce the volume of waste to a certain extent. The organic load of the waste can only be reduced by a proper control of the operations. Overloading of the evaporators and the vacuum pans and the extensive boiling of the syrup lead to a loss of sugar through condenser water, this in turn increases both volume and strength of the waste effluent.
Wastewater as influent is passing through filtration unit to separate out separable components as physical treatment. In next step, this wastewater is flocculated with treatment of polyelectrolyte. Floes of waste are treated with lime and settle down in sedimentation step. For adsorption step, activated charcoal can be used as adsorbent, followed by air stripping to increase dissolved oxygen and remove fouling smell from treated water.
Steps Involved in Treatment Process of Vegetable Oil Industry Effluent:
Vegetable oil or refined oil processing industry is a major source of environmental pollution in developing countries. The waste coming out from oil refinery creates serious environmental problem to aquatic life due to its high organic content.
In vegetable oil industry, wastewater mainly comes from the degumming, deacidification, deodourisation and neutralisation steps. In the neutralisation step sodium salts of free fatty acids and sulphuric acid generates highly acidic and oily wastewaters.
Effluent characteristics generated by oil processing industry, depends largely on the type of oil processed. Effluent is high in COD, oil and grease, sulphate and phosphate content, resulting in both high inorganic as well as organic loading of the relevant wastewater treatment.
Vegetable oil industry effluent contains oil and fat coming from different parts of processing unit. It is important to remove fat for easy digestion by microbes. For this purpose, a flotation system is used to separate fat present in effluent followed by recirculation of water.
After separation of fat, pH of wastewater is maintained using equalisation tank. In this step BOD reduction is happening. This treated water is pumped in to aerobic digester for further BOD reduction and treatment to get treated water, which can be used for different purposes.
Steps Involved in Treatment Process of Potato Processing Industry Effluent:
The potato processing industry generates large quantities of organic wastes. Treatment of this industrial effluent is targeted to remove organic materials from discharge. Proper treatment of potato processing wastewaters is necessary to minimise their undesirable impact on the environment.
The type of processing unit depends upon the product selection, for example, potato chips, frozen French fries and other frozen food, dehydrated mashed potatoes, dehydrated diced potatoes, potato flake, potato starch, potato flour, canned white potatoes, prepeeled potatoes, and so on. The major processes in all products are storage, washing, peeling, trimming, slicing, blanching, cooking, drying, etc.
There is increasing demand for quality improvement of water resources in parallel with the demand for better finished products. These requirements have obliged the potato industry to develop methods for providing effective removal of settleable and dissolved solids from potato processing wastewater, in order to meet national water quality limits. In addition, improvement and research have been devoted to the reduction of wastes and utilisation of recovered wastes as by-products.
Because potato processing wastewater contains high concentrations of biodegradable components such as starch and proteins, in addition to high concentrations of Chemical Oxygen Demand (COD), Total Suspended Solids (TSS) and Total Kjeldahl Nitrogen (TKN), the potato processing industry presents potentially serious water pollution problems.
An average-sized potato processing plant producing French fries and dehydrated potatoes can create a waste load equivalent to that of a city of 200,000 people. About 230 million litres of water are required to process 13,600 T of potatoes.
This equals about 17 L of waste for every kilogram of potatoes produced. Raw potato processing wastewaters can contain up to 10,000 mg/l COD. Total suspended solids and volatile suspended solids can also reach 9700 and 9500 mg/l, respectively. Wastewater composition from potato processing plant depends on the processing method, to a large extent.
In general, the following steps are applied in potato processing- washing the raw potatoes; peeling, which includes washing to remove softened tissue; trimming to remove defective portions; shaping, washing, and separation; final processing or preservation; and packaging. Amount of different components of potato waste is given in Table 9.2.
Following are the steps involved in the treatment of waste generated during above mentioned potato processing steps in potato processing industry are shown in Fig. 9.6.
First step of treatment process is physical treatment in which solid components settled down in grit chamber settling tank. After this organic solids are recovered using separator. Oil and fat content present in effluent is separated by flotation and fat separation system as presence of oil and fat will reduce the growth of micro-organisms. This waste is now pumped in to activated sludge reactor for further treatment and BOD reduction.
Steps Involved in Treatment Process of Dairy Industry Effluent:
The dairy industry effluent is the largest source of food processing wastewater. Various steps in the milk processing plant generate vast amount of waste, including production, processing, packaging, transportation, storage, distribution, and marketing, impact the environment.
This is highly diversified nature of industry, in which various product processing, handling, and packaging operations create wastes of different quality and quantity. If produced waste is not treated properly, it could lead to increased disposal and severe pollution problems. Steps involved in dairy waste treatment process are shown in Fig. 9.7.
Wastes generated from the dairy industry contain high concentrations of organic material such as proteins, carbohydrates, and lipids, high concentrations of suspended solids, high Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD), high nitrogen concentrations, high suspended oil and/or grease contents. BOD and COD values of different type of milk processing waste is given in Table 9.3.
Effluent coming from dairy industry may be divided into three major categories:
1. Processing Wastewater:
This type of effluent include water used in the cooling and heating processes.
2. Cleaning Wastewater:
This effluent is mainly coming from the cleaning of equipment that has been in contact with milk or milk products, spillage of milk and milk products, whey, pressings and brines.
3. Sanitary Wastewater:
This is normally piped directly to a sewage works.
Following are the steps involved in the treatment of waste generated during milk processing in milk processing unit.
Wastewater coming from different part of processing units of plant, contains fat. This fat containing effluent is treated in fat separator to separate it out followed by mixing and pH stabilisation to maintain pH required for micro organisms in next step. In further step wastewater coming from equalisation tank is treated in activated sludge process, in which complex organic matter is digested by micro-organism to reduce BOD.
Steps Involved in Distillery Waste Treatment:
In India bulk of the alcohol is being produced from sugar cane molasses. Molasses is a thick viscous by-product of the sugar industry which is acidic in nature, rich in salts, dark brown in colour and it also contains sugar which could not be crystallised. Distillery waste treatment steps are shown in Fig. 9.8.
The liquid wastes from distillery can mainly be divided into three category:
1. Spent wash- It is generated from the separation (centrifuge) and distillation process.
2. Cleaning water- It is generated from the different plant components, i.e., fermenters, distillation columns, floors.
3. Refrigeration water- It is generated from cooling after distillation.
Characteristics of distillery wastewater is given in Table 9.4. wastewater from distillery unit is dark brown and containing high BOD and COD values. This wastewater is known as slop. Thickening of wastewater is required for anaerobic digestion. Influent is pass through slop thickening system and slop produced can be used as fodder biomass to animals. In next step water is treated in activated sludge reactor in which micro-organisms will digest organic components of waste and thus BOD is reduced.
After activated sludge process, water transfer to sedimentation tank where floccules is settled and BOD is reduced further and we get treated water.