Treatment of Wastewater
After every effort that can be made to reduce waste strength and volume, there still remains the problem of disposing the final remains of polluted waste into any water stream, thus the waste may be treated in various methods either singly or in combination and the best combination of methods differs from plant to plant.
Flow chart for ETP
Preliminary Treatment level
After the removal of gross solids, gritty materials, and excessive quantities of oil and grease, the next step is to remove the remaining suspended solids as much as possible. This step is aimed at reducing the strength of the wastewater and also facilitating secondary treatment.
Purpose: Physical separation of big sized impurities like cloth, plastics, wood logs, paper,
Common physical unit operations at the Preliminary level are:
Screening
- A screen with openings of uniform size is used to remove large solids such as plastics, cloth, etc. Generally, a maximum of 10mm is Coarse suspended matters such as rags, pieces of fabric, fibers, yarns, and lint are removed. Bar screens and mechanically cleaned fine screens remove most of the fibers. The suspended fibers have to be removed prior to secondary biological treatment; otherwise, they may affect the secondary treatment system. They are reported to clog trickling filters, seals, or carbon beads.
- Screening is the filtration process for the separation of coarse particles from influent
- Stainless steel nets with varying pore sizes can be utilized
- Screens are cleaned regularly to avoid clogging
Sedimentation
- Physical water treatment process using gravity to remove suspended solids from The suspended matter in textile effluent can be removed efficiently and economically by sedimentation. This process is particularly useful for the treatment of wastes containing a high percentage of settable solids or when the waste is subjected to combined treatment with sewage.
- The sedimentation tanks are designed to enable smaller and lighter particles to settle under gravity. The most common equipment used includes horizontal flow sedimentation tanks and center-feed circular clarifiers. The settled sludge is removed from the sedimentation tanks by mechanical scrapping into hoppers and pumping it out subsequently.
- In this tank, sludge is settled down
- Effluent is discharged from the plant through a fish pond
- Sludge is passed to the sludge thickening unit
- Clarification: Used for separation of solids from fluids
- Equalization: Effluent streams are collected into a sump pit. Sometimes mixed effluents are stirred by rotating agitators or by blowing compressed air from below. The pit has a conical bottom for enhancing the settling of solid particles
- Equalization makes the wastewater homogeneous
- Retention time depends upon the capacity of the treatment plant. (Generally, 8-16 hours)
Primary Treatment Level
Purpose: Removal of floating and settleable materials such as suspended solids and organic
Methods: Both physical and chemical methods are used in this treatment level.
Chemical unit processes
- Chemical unit processes are always used with physical operations and may also be used with biological treatment processes
- Chemical processes use the addition of chemicals to the wastewater to bring about changes in its quality.
Example: pH control, coagulation, chemical precipitation, and oxidation
Neutralization: Normally, pH values of cotton finishing effluents are on the alkaline side. Hence, the pH value of equalized effluent should be adjusted. The use of dilute sulphuric acid and boiler flue gas rich in carbon dioxide is not uncommon. Since most of the secondary biological treatments are effective in the pH 5 to 9, the neutralization step is an important process to facilitate.
pH Control:
- To adjust the pH in the treatment process to make wastewater pH neutral
- For acidic wastes (low pH): NaOH, Na2CO3, CaCO3or Ca(OH)2
- For alkali wastes (high pH): H2SO4, HCl
Chemical coagulation and Flocculation
- Coagulation refers to collecting the minute solid particles dispersed in a liquid into a larger mass.
- Chemical coagulants like Al2(SO4)3 {also called alum} or Fe2(SO4)3 are added to wastewater to improve the attraction among fine particles so that they come together and form larger particles called flocks
- A chemical flocculent (usually a polyelectrolyte) enhances the flocculation process by bringing together particles to form larger flocs, which settle out more quickly
- Flocculation is aided by gentle mixing which causes the particles to collide
Chemical coagulation and Mechanical flocculation: Finely divided suspended solids and colloidal particles cannot be efficiently removed by simple sedimentation by gravity. In such cases, mechanical flocculation or chemical coagulation is employed.
In mechanical flocculation, the textile wastewater is passed through a tank under gentle stirring; the finely divided suspended solids coalesce into larger particles and settle out. Specialized equipment such as clariflocculator is also available, wherein a flocculation chamber is a part of a sedimentation tank.
In order to alter the physical state of colloidal and suspended particles and to facilitate their removal by sedimentation, chemical coagulants are used. It is a controlled process, which forms a floc (flocculent precipitate) and results in obtaining a clear effluent free from matter in suspension or in the colloidal state. The degree of clarification obtained also depends on the number of chemicals used. In this method, 80-90% of the total suspended matter, 40-70% of BOD, 5days, 30-60% of the COD, and 80-90% of the bacteria can be removed. However, in plain sedimentation, only 50-70% of the total suspended matter and 30-40% of the organic matter settles out.
The most commonly used chemicals for chemical coagulation are alum, ferric chloride, ferric sulfate, ferrous sulfate, and lime.
Secondary Treatment Level
Methods: Biological and chemical processes are involved in this level.
Biological unit process
- To remove, or reduce the concentration of organic and inorganic compounds.
- The biological treatment process can take many forms but all are based around microorganisms, mainly bacteria.
The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by simple sedimentation. It also removes appreciable amounts of oil and phenol. In secondary treatment, the dissolved and colloidal organic compounds and colors present in wastewater are removed or reduced to stabilize the organic matter. This is achieved biologically using bacteria and other microorganisms. Textile processing effluents are amenable for biological treatments [3]. These processes may be aerobic or anaerobic. In aerobic processes, bacteria and other microorganisms consume organic matter as food. They bring about the following sequential changes:
- Coagulation and flocculation of colloidal matter
- Oxidation of dissolved organic matter to carbon dioxide
- Degradation of nitrogenous organic matter to ammonia, which is then converted into nitrite and eventually to
Anaerobic Processes
- The anaerobic treatment processes take place in the absence of air (oxygen).
- Utilizes microorganisms (anaerobes) that do not require air (molecular/free oxygen) to assimilate organic impurities.
- The final products are methane and biomass
Anaerobic treatment is mainly employed for the digestion of sludge. The efficiency of this process depends upon pH, temperature, waste loading, absence of oxygen, and toxic materials. Some of the commonly used biological treatment processes are described below:
Aerated lagoons: These are large holding tanks or ponds having a depth of 3-5 m and are lined with cement, polythene, or rubber. The effluents from primary treatment processes are collected in these tanks and are aerated with mechanical devices, such as floating aerators, for about 2 to 6 days. During this time, a healthy flocculent sludge is formed which brings about oxidation of the dissolved organic matter. BOD removal to the extent of 99% could be achieved with efficient operation. The major disadvantages are the large space requirements and the bacterial contamination of the lagoon effluent, which necessitates further biological purification.
- The function of aeration is oxidation by blowing air
- Aerobic bacteria are used to stabilize and remove organic material present in the waste.
SCHEMATIC DIAGRAM OF AERATION
Trickling filters
The trickling filters usually consist of circular or rectangular beds, 1 m to 3 m deep, made of well-graded media (such as broken stone, PVC, Coal, Synthetic resins, Gravel, or Clinkers) of size 40 mm to 150 mm, over which wastewater is sprinkled uniformly on the entire bed with the help of a slowly rotating distributor (such as rotary sprinkler) equipped with orifices or nozzles. Thus, the wastewater trickles through the media. The filter is arranged in such a fashion that air can enter at the bottom; counter-current to the effluent flow and a natural draft is produced. A gelatinous film, comprising of bacteria and aerobic micro-organisms known as Zooglea, is formed on the surface of the filter medium, which thrives on the nutrients supplied by the wastewater. The organic impurities in the wastewater are adsorbed on the gelatinous film during its passage and then are oxidized by the bacteria and the other micro-organisms present therein.
Activated sludge process
This is the most versatile biological oxidation method employed for the treatment of wastewater containing dissolved solids, colloids, and coarse solid organic matter. In this process, the wastewater is aerated in a reaction
tank in which some microbial floc is suspended. The aerobic bacterial flora brings about biological degradation of the waste into carbon dioxide and water molecules, while consuming some organic matter for synthesizing bacteria. The bacteria flora grows and remains suspended in the form of a floc, which is called Activated Sludge. The effluent from the reaction tank is separated from the sludge by settling and discharge.
A part of the sludge is recycled to the same tank to provide an effective microbial population for a fresh treatment cycle. The surplus sludge is digested in a sludge digester, along with the primary sludge obtained from primary sedimentation. Efficient aeration for 5 to 24 hours is required for industrial wastes. BOD removal to the extent of 90-95% can be achieved in this process.
Sludge thickening unit
- Here sludge is dried and
- A partial amount of sludge is returned back to the aeration tank from thickening unit through recycle tank called return sludge tank and disperse
SCHEMATIC DIAGRAM OF SLUDGE THICKENING UNIT
Oxidation ditch
This can be considered as a modification of the Conventional Activated Sludge process. Wastewater, after screening in allowed into the oxidation ditch. The mixed liquor containing the sludge solids is aerated in the channel with the help of a mechanical rotor. The usual hydraulic retention time is 12 to 24 hrs and for solids, it is 20-30 days. Most of the sludge formed is recycled for the subsequent treatment cycle. The surplus sludge can be dried without odor on sand drying beds.
Oxidation pond
An oxidation pond is a large shallow pond wherein stabilization of organic matter in the waste is brought about mostly by bacteria and to some extent by protozoa. The oxygen requirement for their metabolism is provided by algae present in the pond. The algae, in turn, utilize the CO2 released by the bacteria for their photosynthesis. Oxidation ponds are also called waste stabilization ponds.
Anaerobic digestion
Sludge is the watery residue from the primary sedimentation tank and humus tank (from secondary treatment). The constituents of the sludge undergo slow fermentation or digestion by anaerobic bacteria in a sludge digester, wherein the sludge is maintained at a temperature of 35oC at pH 7-8 for about 30 days. CH4, CO2, and some NH3 are liberated as the end products.
Tertiary / Advanced Treatment
Purpose: Final cleaning process that improves wastewater quality before it is reused, recycled, or discharged to the environment.
Mechanism: Removes remaining inorganic compounds, and substances, such as nitrogen and phosphorus. Bacteria, viruses, and parasites, which are harmful to public health, are also removed at this stage.
Methods:
- Alum: Used to help remove additional phosphorus particles and group the remaining solids together for easy removal in the
- Chlorine contact tank disinfects the tertiary treated wastewater by removing microorganisms in treated wastewater including bacteria, viruses and
- The remaining chlorine is removed by adding sodium bisulfate just before it’s
It is worthwhile to mention that textile waste contains significant quantities of non-biodegradable chemical polymers. Since the conventional treatment methods are inadequate, there is the need for an efficient tertiary treatment process.
Oxidation techniques
A variety of oxidizing agents can be used to decolorize wastes. Sodium hypochlorite decolourizes dye baths efficiently. Though it is a low-cost technique, it forms absorbable toxic organic halides (AOX) [4].
Ozone on decomposition generates oxygen and free radicals and the latter combines with coloring agents of effluent resulting in the destruction of colors [5]. Arslan et al. investigated the treatment of synthetic dyehouse effluent by ozonization, and hydrogen peroxide in combination with Ultraviolet light [6]. The main disadvantage of these techniques is it requires an effective sludge-producing pre-treatment.
Electrolytic precipitation & Foam fractionation
Electrolytic precipitation of concentrated dye wastes by a reduction in the cathode space of an electrolytic bath has been reported although extremely long contact times were required.
Foam fractionation is an experimental method based on the phenomena that surface-active solutes collect at gas-liquid interfaces. However, the chemical costs make this treatment method too expensive.
Membrane technologies
Reverse osmosis and electrodialysis are important examples of the membrane process.
The TDS from wastewater can be removed by reverse osmosis. Reverse osmosis is suitable for removing ions and larger species from dye bath effluents with high efficiency (up to > 90%), clogging of the membrane by dyes after long usage and high capital cost are the main drawbacks of this process.
The dyeing process requires the use of electrolytes along with the dyes. Neutral electrolyte like NaCl is required to have high exhaustion of the dye. For instance, in cotton dyeing, NaCl concentration in the dyeing bath is in the range of 25-30 g/l for deep tone and about 15 g/l for a light tone but can be as high as 50 g/l in exceptional cases. The exhaustion stage in reactive dyeing on cotton also requires a sufficient quantity of salt.
The reverse osmosis membrane process is suitable for removing high salt concentrations so that the treated effluent can be re-used again in the processing. The presence of electrolytes in the washing water causes an increase in the hydrolyzed dye affinity (for reactive dyeing on cotton) making it difficult to extract.
In electrodialysis, the dissolved salts (ionic in nature) can also be removed by impressing an electrical potential across the water, resulting in the migration of cations and anions to respective electrodes via anionic and cationic permeable membranes. To avoid membrane fouling it is essential that turbidity, suspended solids, colloids, and trace organics be removed prior to electrodialysis.
Electrochemical processes
They have lower temperature requirements than those of other equivalents non-electrochemical treatment and there is no need for additional chemicals. It also can prevent the production of unwanted side products. But, if suspended or colloidal solids were high concentration in the wastewater, they impede the electrochemical reaction. Therefore, those materials need to be sufficiently removed before electrochemical oxidation [9].
Ion exchange method
This is used for the removal of undesirable anions and cations from wastewater. It involves the passage of wastewater through the beds of ion exchange resins where some undesirable cations or anions of wastewater get exchanged for sodium or hydrogen ions of the resin. Most ion exchange resins now in use are synthetic polymeric materials containing ion groups such as sulphonyl, quaternary ammonium group, etc.
Photocatalytic degradation
An advanced method to decolorize a wide range of dyes depending upon their molecular structure. In this process, photoactive catalyst illuminates with UV light, generates highly reactive radical, which can decompose organic compounds.
Adsorption
It is the exchange of material at the interface between two immiscible phases in contact with one another. Adsorption appears to have considerable potential for the removal of color from industrial effluents.
Owen (1978) after surveying 13 textile industries has reported that adsorption using granular activated carbon has emerged as a practical and economical process for the removal of color from textile effluents.
Thermal evaporation
The use of sodium persulfate has a better oxidizing potential than NaOCl in the thermal evaporator. The process is eco-friendly since there is no sludge formation and no emission of toxic chlorine fumes during evaporation. The oxidative decolorization of reactive dye by persulphate due to the formation of free radicals has been reported in the literature.
Effluent Treatment Practices: Textile industry encompasses a range of unit operations, which use a wide variety of natural and synthetic fibers to produce fabrics.
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