Everything you need to know about how to treat industrial sewage.
Contents:
- How to Treat Sewage from Distilleries?
- How to Treat Sewage from Cotton and Synthetic Textile Industry?
- How to Treat Sewage from Electroplating Industry?
- How to Treat Sewage from Tanning Industry?
- How to Treat Sewage from Viscose Rayon Industry?
- How to Treat Sewage from Petroleum Refining Industry?
1. How to Treat Sewage from Distilleries?
Sewage from Distilleries:
ADVERTISEMENTS:
Distilleries are primarily engaged in the production of alcoholic beverages from grains and industrial alcohol from molasses.
Processes Involved in Distilleries:
In making whisky and other alcoholic beverages from malt different kinds of raw materials such as barley, wheat, maize, mahua flowers, etc., are used from which wort is made. Wort is then distilled. The residues from the distillation process together with the wastes produced in preparing the wort constitute the sewage discharged from the distilleries.
The process of manufacture of alcohol from molasses is mainly used for the production of industrial alcohol. The molasses are diluted with water. The molasses are then allowed to ferment under controlled conditions of temperature and pH. Nutrient supplements in the form of ammonium sulphate or urea are also added.
Yeast sludge containing about 30 per cent solids settle down in the fermentation vats and constitutes a major source of waste. The liquid is then distilled for recovery of alcohol. The residues which are known as ‘spent wash’ which is hot when discharged, constitute the important source of sewage from the molasses distillery. Other sewage originates from the washing of the floors, presses, fermentation vats and other equipment.
Sources, Quantity and Characteristics of Sewage:
In the production of alcohol from molasses, 3 to 10 kg of molasses are used for producing 1 litre of alcohol; and for each litre of alcohol about 10 to 15 litres of spent wash is produced. The distillery spent wash is hot, highly coloured and acidic, apart from containing high percentage of dissolved inorganic and organic matter, the latter being particularly responsible for high biochemical oxygen demand (BOD) and polluting nature of the sewage. Spent wash also contains large amount of potash. Table 19.5 shows the average characteristics of the spent wash.
Note:
In general potash content ranges from 7 to 15 per cent of the total solids.
The yeast sludge in fermentation vats is another source of strong sewage in a distillery, but this sewage and other wash waters are discharged along with the dealcoholized spent wash. The cooling and condenser waters are generally six times the volume of the spent wash and contain little contamination since they are used in surface condensers.
Total sewage volume is 60 to 100 litres per litre of alcohol produced.
Pollutional Effects of Distillery Sewage:
ADVERTISEMENTS:
The pollutional effects of distillery sewage are due to high biochemical oxygen demand and colour. Stagnation of the sewage on land results in obnoxious conditions in the region. If the soil is porous the sewage may also affect ground water quality.
Discharge of untreated distillery sewage into water courses results in rapid depletion of oxygen content of water, making the environment unfit for fish life. The sewage may also impart colour and odour to the water and result in unsightly conditions in the water course.
Methods of Treatment and Disposal of Distillery Sewage:
The following methods may be adopted for the disposal of distillery sewage after necessary treatment:
1. Land Disposal:
ADVERTISEMENTS:
The spent wash may be treated by an anaerobic process such as anaerobic lagooning or anaerobic digestion followed by an aerobic process such as activated sludge process, trickling filtration or aerated lagooning. The treated effluent is diluted to have a biochemical oxygen demand (BOD) of less than 500 mg/l and used for irrigation or disposed of on land (see Fig. 19.5).
2. Disposal of Water Course:
The spent wash may be treated by anaerobic process followed by one of the aerobic processes such as extended aeration activated sludge process, trickling filtration or aerated lagooning and diluted with condenser water to keep the BOD of the final effluent below 100 mg/l before disposing of into a water course (see Fig. 19.6). It may be necessary to dilute the influent to the aerobic processes. This can be done by using part of the condenser water.
If suspended solids in the spent wash are high then it is useful to separate them, dry and incinerate them or dispose of on land. The settled spent wash can then be taken for biological treatment.
2. How to Treat Sewage from Cotton and Synthetic Textile Industry?
Sewage from Cotton and Synthetic Textile Industry:
The textile industries require large volumes of water of high degree of purity and produce equally large volumes of liquid wastes or sewages which are complex and highly variable both in regard to quantity and characteristics. The sewage from a textile industry generally carries a significant pollution load and hence it should be disposed of only after treatment.
Sources, Quantity and Characteristics of Sewage:
Sources- The main sources of sewage in a textile industry are as follows:
(a) Preparation of yarn—slashing process waste;
(b) Textile Processing:
(i) Desizing,
(ii) Kier,
(iii) Bleaching,
(iv) Mercerization, and
(v) Dyeing and printing;
(c) Sewage from the washing operations in each step of textile process;
(d) Miscellaneous sewage streams, namely, cooling water and boiler blow down, housekeeping (floor washing and washing of equipment), and spills and leaks; and
(e) Sanitary sewage.
Quantity:
The quantity of sewage discharged from each unit process is very small compared to the quantity of sewage derived from the washings and rinsing of the cloth. The quantity of sewage varies considerably from one mill to another and depends on the factors such as source of water and its availability, the quantity and quality of fabric produced, and the types of processing and its sequence. The water usage of textile industries lies in the range of 57 to 800 m3 per 1000 kg of cloth produced and the sewage produced lies in the range of 49 to 430 m3 per 1000 kg of cloth produced.
Characteristics:
The characteristics of sewage produced from textile industries are shown in Table 19.7. The sewage produced from textile industries is subject to sharp variation in colour, pH value, alkalinity, total dissolved solids, BOD and COD. It contains various process chemicals used by the different sections of the industry. Further due to limited water usage in small textile industries the sewage produced from them are generally stronger than the sewage produced from the large industries as shown in Table 19.8.
Pollutional Effects of Sewage from Textile Industry:
Sewage from textile industry is of highly polluting nature and affect the quality of the receiving water in several ways as indicated below:
i. pH value:
The high alkalinity of the sewage causes an increase in pH value. Any increase in pH value of the receiving stream greater than 9.0 will have an adverse effect on aquatic life.
ii. Colour:
The soluble dyes and colours present in the sewage will persist in the stream and interfere with penetration of sunlight essential for photosynthesis.
iii. Turbidity:
The colloidal organic matter in the sewage will increase turbidity of the receiving water and along with the colours, dyes and oily scum will produce an unsightly appearance. The oily scum formed on the surface of water will interfere with the mechanism of oxygen transfer at the air/water interface.
iv. Oxygen Depletion:
The most serious effect of textile sewage on the receiving body of water will be depletion of dissolved oxygen. The organic matter in textile sewage such as starch, dextrin and inorganic chemicals such as sulphide and hydrosulphite and nitrite will exert an immediate oxygen demand, while dyes and colours will exert long term oxygen demand.
Such changes in the oxygen balance of receiving streams will be deleterious to fish life and will also interfere with self-purification. Toxic chemicals like sulphide, chlorine, chromium and aniline dyes will also affect the aquatic life.
Methods of Treatment and Disposal of Sewage from Textile Industry:
The treatment of sewage from textile industry is based on the ultimate disposal as follows:
(a) For Disposal into Municipal Sewers:
In this case only primary treatment including screening, grit removal, chemical coagulation, flocculation and sedimentation would be necessary.
(b) For Disposal into Streams:
In this case primary treatment as well as secondary treatment for BOD reduction would be necessary.
(c) For Land Disposal:
In this case primary treatment or secondary treatment or both followed by treatment with gypsum would be necessary.
The flow diagrams for the treatment processes indicated above are shown in Fig. 19.18.
Fig. 19.18 Flow Diagrams for Treatment and Disposal of Sewage from Textile Industry
How to Treat Sewage from Electroplating Industry?
Sewage from Electroplating Industry:
The electroplating industry uses small quantities of water but discharges highly toxic sewage and hence it needs to be treated properly before its final disposal.
Sources, Characteristics and Quantity of Sewage:
Sources:
In plating operations generally there are two sources of sewage namely, batch solutions and rinse waters which are distinctly different in volume and chemical quality. Batch solutions from vats (large tanks used for plating) are highly concentrated and are seldom discharged. Rinse waters are comparatively much more dilute but form the bulk of the sewage.
Quantity and Characteristics:
The quantity and characteristics of the sewage vary considerably from one plating shop to the other and within the same shop from day to day. This is primarily because of varying efficiencies in handling operations. Typical characteristics of sewage resulting in different plating operations are shown in Table 19.13.
Methods of Treatment of Sewage from Electroplating Industry:
In view of the various chemicals being present in the sewage from electroplating industry the treatment of the sewage consists of the following:
(a) Segregation of the cyanide bearing effluents and treatment by alkaline chlorination,
(b) Treatment of the hexavalent chromium bearing effluents with ferrous sulphate at low pH to reduce the chromium to trivalent state,
(c) Raising the pH to 8.5 and above of all the acidic metallic effluents including that of the trivalent chromium for precipitating the toxic metals,
(d) Separation and drying of sludge solids by sand beds and disposal, and
(e) Recovery of metals wherever possible.
(a) Segregation of Cyanide Bearing Effluent:
Cyanide bearing effluents are segregated from acid bearing effluents to avoid the evolution of the deadly hydrocyanic acid gas. Cyanide bearing effluents are treated to remove the cyanides completely or to a concentration below 0.2 mg/l. For the removal of cyanide alkaline chlorination process is most widely used.
(b) Reduction of Hexavalent Chromium to Trivalent State:
As hexavalent chromium is very toxic and cannot be easily separated from solution, chromium (VI) bearing effluents are first reduced to chromium (III) and treated further. Under Indian conditions chromium (VI) salts are reduced to chromium (III) salts best by reduction with ferrous sulphate in the pH range of 2.0 to 2.5. Chromium (III) salts may be precipitated along with other toxic metals in the effluents by raising the pH to around 8.5.
(c) Separation and Drying of Solids:
The effluents from the electroplating industry are divided into three streams, namely, alkaline effluents bearing cyanide, acid effluents bearing hexavalent chromium and acid effluents bearing all heavy metals except hexavalent chromium.
The cyanide effluents can be treated by alkaline chlorination in one reactor, the chromium effluents by ferrous sulphate reduction in another reactor and the two treated effluents, mixed together along with acid effluents containing other toxic metals in a third reactor to precipitate the heavy metals at a pH of 8.5 and above.
To simplify the process chromium (VI) effluents can be carried along with the other metal effluents, treated with ferrous sulphate to reduce the chromium (VI) ions and the two streams mixed in a third reactor for adjusting the pH.
How to Treat Sewage from Tanning Industry?
Sewage from Tanning Industry or Tannery:
The sewage from a tanning industry or tannery has high pollution load and hence if it is disposed of without proper treatment serious water pollution and acute insanitary conditions may take place.
Sources, Characteristics and Quantity of Sewage:
The sewage from a tanning industry comprises soaking effluent, liming effluent, unhairing and fleshing effluent, deliming effluent, spent bate liquor, spent pickle liquor, vegetable tanning effluent, chrome tanning effluent and dyeing and fat-liquoring effluent. The composite effluent from a tannery is highly coloured and foul smelling.
It is highly alkaline with large amount of suspended and dissolved impurities. BOD of the effluent varies from 2000 to 3000 mg/l. The quantity and typical characteristics of the individual and composite effluent from a tannery are shown in Table 19.14.
Methods of Treatment of Sewage from Tannery:
The treatment of sewage from tannery consists of the following:
(a) Segregation of salt bearing sewage and chrome tanning sewage,
(b) Primary treatment of sewage, and
(c) Secondary treatment of sewage
(a) Segregation:
Segregation of various effluents according to their characteristics simplifies the treatment and wherever possible such segregation should be effected. Effluents from soaking and pickling may preferably be segregated and allowed to evaporate in a shallow impervious pond. Chrome tanning effluent may be segregated and mixed with lime liquor.
(b) Primary Treatment:
This involves removal of suspended solids and partial reduction of BOD and other harmful constituents. Screening for removal of coarser impurities and hair and settling for at least 4 hours in continuous tank form the essential primary treatment of tannery sewage. The sludge from settled vegetable tannery effluents can be dried on sand beds and used as fertilizer for growing forage crops or a soil conditioner.
The sludge from settlement of chrome tannery effluents, after drying, may be used as land fill or disposed of by dumping on fallow land. Hydrogen sulphide from lime liquor may be removed by aeration using manganese salts as catalyst.
(c) Secondary Treatment:
Secondary treatment methods that can be used for treating settled tannery sewage include aerated lagoon, or anaerobic lagoon followed by aerated lagoon. Under Indian conditions anaerobic lagoon followed by aerated lagoon with minimum detention times of 10 days and 6 days respectively would give good results and the treated effluent could be disposed of.
In urban areas where the facility of sewerage system is available the above indicated primary treatment of tannery sewage would be adequate for final disposal along with city sewage. However, in rural areas due to non-availability of sewerage system both primary and secondary treatments as indicated above would be necessary depending upon the mode of ultimate disposal.
How to Treat Sewage from Viscose Rayon Industry?
Sewage from Viscose Rayon Industry:
The rayon industry falls under the group of chemical industries concerned with the manufacture of artificial and synthetic textile fibres. Rayon is manufactured by three different chemical processes, namely the viscose process, the cellulose acetate process and the cuprammonium process. In India most of the units produce viscose rayon. The sewage from a rayon industry carries a significant pollution load and hence it should be disposed of only after treatment.
Sources, Quantity and Characteristics of Sewage:
Sources:
The main sources of sewage in a viscose rayon industry are as follows:
(a) Carbon Disulphide Plant—Mostly in the form of cooling and condensate waters.
(b) Acid Plant—cooling and condensate waters.
(c) Alkaline Waste Streams—from dialysers, ripening and deaerating vessels, filter presses from desulphurizing process and washing operations.
(d) Acidic Wastes—derived mainly from the revolving buckets of spinning machines, spin bath make-up tanks, evaporators and washing processes.
(e) Bleaching Processes Wastes—combined waste stream from bleaching and antichlor wastes.
(f) Miscellaneous Wastes—from spills and leaks, floor washings, water treatment plant and sanitary wastes.
The acidic and alkaline waste streams constitute the bulk of the sewage and its pollution load from a viscose rayon industry.
Quantity:
The quantity of sewage produced depends on the size of the unit, its production, availability of water and its usage, and it varies from about 350 m3 to 1800 m3 per tonne of rayon produced.
Characteristics:
The sewage consists mostly of various chemicals used such as caustic alkali, sulphuric acid, sulphides, traces of sulphur and zinc. The organic constituents of the sewage are mostly in the form of hemicelluloses. The characteristics of the acidic and alkaline sewages produced in the process are shown in Tables 19.9 and 19.10 respectively and those of the combined sewage are shown in Table 19.11
Pollution Load:
The pollution load contributed by the sewage from a viscose rayon industry is mostly in the form of inorganic chemicals like sulphuric acid, sodium sulphate, zinc sulphate and sodium sulphide. The organic constituent of the pollution load is, however, quite low as may be seen from the low biochemical and chemical oxygen demand of the sewage.
Methods of Treatment and Disposal of Sewage from Viscose Rayon Industry:
The treatment of the sewage from viscose rayon industry consists of the following:
(1) Segregation of the acidic and alkaline sewage streams and reduction in sewage strength by recovery of process chemicals. From the alkaline sewage caustic soda is recovered by dialysis. From the acidic sewage sodium sulphate and zinc are recovered. Sodium sulphate is obtained by concentration, evaporation and crystallization. Recovery of zinc is made by either chemical precipitation with caustic soda at a critical pH of 9.0, as zinc hydroxide, followed by sedimentation and filtration or by ion exchange process.
(2) Equalization of the acidic and alkaline sewage streams for self-neutralization with provision for further treatment with lime if pH correction to the requisite degree is found necessary.
(3) Primary treatment for removal of floating matter, oil and grease and suspended and settleable solids.
(4) Secondary aerobic treatment for biochemical oxygen demand (BOD) reduction either by conventional trickling filter and activated sludge process or low cost systems like an aerated logoon or oxidation pond.
The degree of treatment to be provided will be determined by the ultimate disposal of the sewage which may be into municipal sewers or inland surface waters or directly into the sea.
For discharge into municipal sewers the treatment may be limited to the primary stage only; and for discharge into inland surface waters secondary treatment for biochemical oxygen demand (BOD) reduction will be necessary. The degree of dilution available in the stream will be yet another factor to be considered.
For disposal into the sea, the degree of treatment will be determined by the prescribed standards of effluent quality. In-view of the considerable degree of dilution available, the sewage from viscose rayon industry may be discharged into the sea after primary treatment. This would require a well-designed submarine outfall system with suitable arrangements for efficient diffusion and dispersion of the sewage from the outfall system into the sea.
The secondary treatment consists of the following:
(a) Lagooning;
(b) Conventional aerobic treatment including trickling filter, activated sludge process; and/or
(c) Low cost sewage treatment such as aerated lagoon and oxidation pond.
The flow diagram for the treatment of sewage from viscose rayon industry is shown in Fig. 19.19.
How to Treat Sewage from Petroleum Refining Industry?
Sewage from Petroleum Refining Industry:
Large quantities of water are used in refining processes in the petroleum refining industry; these may be for cooling purpose, power plant or refining processes. The sewage from the petroleum refineries pose pollution problems if discharged untreated, and hence a sewage treatment system is an essential part of every petroleum refinery installation.
Sources and Characteristics of Sewage:
Sources:
The major water pollutant in refinery operations is oil, besides chemicals that may be formed during treatment of petroleum products by chemical processes for removal of undesirable constituents.
The effluent of petroleum refineries may be classified under the following five categories:
(a) Effluent free from oil and dissolved organic material;
(b) Effluent accidentally contaminated with oil;
(c) Effluents continuously contaminated with oil but not contaminated with other soluble organic material;
(d) Process effluent; and
(e) Sanitary and domestic effluent.
(a) Effluent Free from Oil and Dissolved Organic Material— These originate from:
(i) Boiler blow-down,
(ii) Water treatment plant effluent, and
(iii) Storm water from oil-free areas.
(b) Effluent Accidently Contaminated with Oil — These originate from:
(i) Storm water from tank farms, and
(ii) Blow-down from circulating cooling water system.
(c) Effluents Continuously Contaminated with Oil but Free from Soluble Organic Material—These originate from:
(i) Storm water from oil processing areas, and
(ii) Ballast water.
(d) Process Effluents — These originate from:
(i) Desalter,
(ii) Condensate from steam stripping operations,
(iii) Pump gland cooling water,
(iv) Barometric condenser water containing emulsions, and
(v) Wash water from treating plants.
(e) Sanitary and domestic effluents—These originate from:
(i) Toilets, and
(ii) Canteen.
Characteristics:
General characteristics of the composite oil refinery sewage are likely to be as shown in Table 19.12.
Methods of Treatment and Disposal of Sewage from Petroleum Refining Industry:
The general treatment methods for the sewage from various operations in a petroleum refinery are as follows:
(1) Primary Treatment:
It consists of free oil removal, stripping and extraction:
The removal of oil from the sewage is carried out principally in two stages-the first stage featuring gravity separation and the second stage floatation with or without addition of chemicals/coagulants.
Stripping is a physical method in which with the help of steam, gases such as hydrogen sulphide, mercaptans and ammonia and to some extent phenol and free cyanides are removed.
Extraction is another physical method which is used to remove phenolic compounds in refinery operations. In this process solvents such as tricresyl phosphates and mixed organic esters are used for extraction of phenol from the sewage. In some of the refineries use of petroleum fractions or crude oil is also made for extraction.
(2) Secondary Treatment Methods:
The secondary treatment methods may be classified as follows:
(a) Chemical method, and
(b) Biological method
(a) Chemical Methods:
The main purpose of a chemical methods is to remove emulsified oil with addition of flocculating agents and also to remove suspended solids and toxic substances, thereby conditioning the effluents for further treatment by biological method. Sedimentation is normally employed to remove suspended solids after chemical treatment.
Various chemical methods available may be further classified under the following four categories:
(1) Neutralization method,
(2) Precipitation and clarification method,
(3) Chemical oxidation method, and
(4) Regeneration method
(1) Neutralization:
Neutralization methods are applied to many types of refinery wastes as indicated below:
(i) Dilute acid or alkaline wastes from various refining processes and from water treatment plant are neutralized to produce a neutral effluent.
(ii) Spent caustic solution is neutralized with either spent acid or acidic stack gases.
(iii) Sulphuric acid sludges are neutralized with alkaline wastes.
(iv) Spent catalyst is neutralized with spent caustic wastes.
(v) Hydrofluoric acid wastes are neutralized with caustic soda solution.
(2) Precipitation and Clarification Method:
Precipitation method is used in the treatment of several types of refinery wastes such as:
(i) Sulphide water is precipitated with iron salts.
(ii) Solutions of sulphonates are precipitated by the addition of lime.
(iii) Acidic oils in spent caustic solutions are precipitated by neutralization.
(iv) Chromates from cooling water are reduced to trivalent with ferrous sulphate, sulphur dioxide or hydrogen sulphide and precipitated with lime as chromium hydroxide.
(3) Chemical Oxidation Method:
This method may be used for:
(i) Treatment of phenols and cyanides in wastes; and
(ii) Combustion of certain wastes.
(4) Regeneration Method:
Production of spent caustic solutions is reduced by the regeneration of a chemical like monoethanolamine which is used to absorb hydrogen sulphide and mercaptans. It is regenerated by steam stripping for further use.
(b) Biological Method:
This is intended to remove the biodegradable organic substances and toxic substances like phenol. Biological treatment also reduces the residual oil after primary treatment (but the oil is not recoverable). The biological treatment of the sewage is carried out by using the conventional methods like trickling filter, activated sludge process and low cost treatment methods like aerated lagoon and oxidation pond.
Trickling filters can tolerate oil upto 100 ppm, whereas activated sludge process can tolerate oil upto 25 ppm. Presence of oil in oxidation ponds retards reaeration. Besides oil, biological treatment methods are influenced by BOD, toxic substances like chromium, lead, nickel, hydrogen sulphide, etc., temperature, pH and nutrients.
(5) Tertiary Treatment Methods:
Tertiary treatment methods used includes activated carbon filtration process and ozonation which are effective in removal of the taste and odour and organic substances from biologically treated sewage. However, these methods are extremely costly. As such in most cases it is preferred to use oxidation ponds after biological treatment to serve as tertiary treatment. This is one of the cheapest methods of treatment where enough land is available.
The treated sewage which satisfy the relevant tolerance limits are finally disposed of by controlled dilution into the neighbouring stream river or sea.
Fig. 19.21 shows general plan for the treatment and disposal of effluents from petroleum refining industry.
