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The following operations are generally adopted for treating the composite effluent of an average textile mill in the order given below:
(i) Equalisation.
(ii) Acid-dosing.
(iii) Coagulation.
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(iv) Sedimentation.
(v) Primary and secondary gypsum treatment.
(vi) Filtration.
(vii) Sedimentation.
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(viii) Sludge-drying.
(ix) Sometimes biological treatment
(i) Equalisation:
Effluent streams from various wet processing sections of the mill are collected into a tank generally made of cement concrete called “sump fit” via underground concrete or iron pipes.
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The sump pit is generally located outside the factory shed at a lower level and is mostly submerged in the ground. Sometimes gentle stirring is imparted to the mixed effluents in this pit either by means of rotating blades or by blowing compressed air from the bottom. Generally, the pit has a conical bottom for enhancing the settling of solid particles.
The sump pit serves following purposes:
(a) Mixing various effluent streams so that a composite stream emerges from it.
(b) Since the pit is open to atmosphere the hot effluents lose their heat from the open surface; thereby lowering their temperature.
(c) Because of mixing of various streams having varied properties, neutralisation of characteristics is brought about in this unit i.e., because of mixing of effluent from carbonisation section, which is acidic in nature with the effluent from kiers which is alkaline, neutralisation occurs so that pH of the composite stream is considerably reduced. Likewise some dilution also takes places when highly concentrated streams mix with the dilute streams.
(d) Sometimes this unit is provided with screens so that suspended solid particles can be separated from the outgoing composite stream.
(e) It reduces the BOD to some extent because of its surface being exposed to atmosphere and because of bubbling of compressed air from its bottom.
(f) Its conical bottom enhances the settling of solids so that sludge accumulates at the bottom. Since no coagulants are added in this unit, flocculation of solids is not considerable.
(g) Sometimes it is provided with skimmers so that oils and greases floating on the surface can be skimmed off.
(ii) Acid Dosing:
As can be seen from the preceding Table 22.13 that sometimes pH of the composite effluent exceeds the stipulated limit of 9 because of its high alkalinity. Highly alkaline nature of textile effluents can be mainly attributed to scouring and mercerisation section of wet processing of cloth.
In order to bring the pH down to the stipulated limit, dilute sulphuric acid (about 10%) dosage may be given to the composite effluent. Generally, the acid used for this purpose is a cheap waste product coming from acid slurry manufacturers.
pH of the effluent is checked before and after mixing of acid periodically so that its flow can be adjusted. This is achieved either by addition of acid from an overhead tank through regulating valve and distributors or by addition from a tank situated on the ground and pumping the acid through distributors. Sometimes, it is added manually in small batches also.
(iii) Addition of Coagulants:
Coagulants are added to the composite effluent for inducing precipitation and sedimentation of colloidal impurities, settleable solids and colour present in it by formation of flocs. Generally, mills are using sulphuric acid and alum or lime and copper as (FeSO4. 7H2O). However, use of ferric sulphate, ferric chloride and chlorinated coppers is not very uncommon.
It is reported that the alum treatment operates best when pH of the effluent is less than 8 and preferably at 7.5. Therefore, alum treatment should be preceded by treatment with sulphuric acid so that pH is brought within 8.
However, usually in the mills, pH is not brought to 8 because it becomes expensive due to large amount of acid required. Generally, the pH is brought to 9 and alum treatment is given thereafter.
A dose of 400-500 mg/litre of alum are required for the effluent. The alum dose is generally given by addition of alum solution coming from an overhead tank through distributors.
The overhead tank has two unequal sized compartments separated by a screen. In the bigger compartment a rotating agitator is provided so that lumps of alum added to water can be homogenously mixed and dissolved.
The alum solution so formed flows to the other smaller compartment through the screen which prevents the undissolved alum and other solid impurities from going with it. It is finally withdrawn from the second compartment and its flow is manually regulated by controlling a valve provided on the line.
In another method using copperas, the waste is treated with lime. If the waste contains required hydroxide alkalinity, addition of lime may not be followed. Otherwise lime is added to the effluents so that hydroxide alkalinity is raised.
This is required because copperas (FeSO4.7H2O) forms ‘Flocs’ of Fe(OH)2 with the hydroxide alkalinity and this ‘Flocs’ react with dyes and other solid impurities so that the mixed solid particles can settle easily in the subsequent sedimentation unit.
The above two treatments prove much more effective if only dye-house liquor is treated. The extra advantage of this treatment lies in the fact that it reduces BOD of the waste by about 30-35 per cent in case of alum and about 85 per cent in case of copperas by removal of organic solids. Whereas increase in TDS of the effluent because of the chemical treatment is varying from 2 to 18 per cent.
Both these treatments will produce ‘Flocs’ of solids which can be easily settled so that the effluent, after treatment, is sent to sedimentation tank. Sometimes, as mixing tank precedes the sedimentation tank where the effluent is given intense agitation by means of rotating agitators so that the coagulants can very well mix with it.
Table 22.14 gives the approximate economics and dosages to be given for alum, copperas and gypsum treatments.
(iv) Sedimentation:
This tank is generally a cement concrete tank having a conical bottom. Its function is to allow the flocculated solids to settle at the bottom to form a sludge. Mostly it is submerged in the ground and effluent is fed into it in its conical bottom at a height which clears the sludge level. This type of feeding enhances the solid separation and prevents them from being carried away with the clear supernatant liquid.
In some cases, this tank also functions as a coagulant-mixing tank in which periodically coagulants are added directly. To enhance dissolution of coagulants in water, sometimes air is blown from the bottom to impart mild turbulence to the content. It has a peripheral launder or weir so that clear supernatant liquid can be removed from the top.
In few cases, the tank is at an elevated position and is provided with gently rotating raker arms on its conical bottom to enhance sludge removal from bottom and radial top arms to facilitate removal of clear liquid from the top into the surrounding peripheral launder. Such unit is many times called ‘clariflocculator’; for it serves dual purpose of flocculation of solids and subsequent separation of liquid from the sludge.
Sedimented solids which accumulate on the conical bottom as sludge are removed from the bottom under the static head of liquid via underground masonry channels or pipes; and are also collected either in the sludge-well or directly in the sludge-bed.
Generally, two sedimentation tanks are provided for about 1 million litres of effluent flow per day and for higher flows, four or more tanks are provided which are operated cyclically. A common sludge well is usually provided from which sludge is pumped to the sludge bed.
These sludge-beds are rectangular cement-concrete structures offering large surface areas so that the sludge can be easily sun-dried. There are usually three sludge beds and they are operated in such a way that while one will be receiving the fresh sludge, the other two will be drying it.
The clear supernatant liquid remaining on the surface of uniformly spread sludge in the sludge-bed is again taken back to the sump pit. After the sludge is completely dried, it is manually scrapped off from the bed and disposed off.
(v) Primary and Secondary Gypsum Treatment:
The clear supernatant liquid from the sedimentation tank is now treated with gypsum lumps. As can be seen from the preceding Table 22.13 that the composite effluent of a textile mill has sodium ion concentration of about 90% of total cations present and this exceeds the stipulated limit of 60%.
Treatment of the effluent with gypsum serves following dual purposes:
(1) It reduces the sodium ion concentration to the desired level by increasing the calcium ion concentration in the effluent. The gypsum lumps dissolve in the effluent as it traverses a rectangular channel which is filled with gypsum lumps.
In order to increase the contact time of effluent with the gypsum lumps for enhancing the dissolution and to bring about proper mixing, the channel is provided with several rectangular concrete baffles. They reduce the effective cross-section for flow and thereby increase the velocity of the effluent. This in turn increases the turbulence.
(2) Gypsum treatment reduces the pH of the effluent upto the extent that it reacts with the sodium carbonate present in the effluents and precipitates out as calcium carbonate.
(vi) Filtration:
This treatment follows the primary gypsum treatment and it precedes the secondary gypsum treatment. The effluent from the primary gypsum treatment is fed into a cement concrete rectangular structure through perforated pipes.
The bed of this structure is generally covered with rubble, gravels and sand for separating the suspended and precipitated particles from the effluent. The effluent percolates through the various layers of sand, gravel etc., so that suspended solid particles are filtered out. Since the effective cross-section for flow of the effluents is very large, the velocity of the effluents is proportionately reduced.
Consequently, the drag that the liquid exerts on the solid particles is also reduced. This helps in removal of the solid particles from the effluent. When the filtering bed is exhausted because of clogging of its pores by solid particles, it has to be regenerated by back-flushing with water or replaced by a new bed of solids.
Because of the large size of the filter and relatively smaller amount of suspended solids in the effluent, the bed lasts for a considerable period before getting exhausted. Sometimes the bed is covered with gypsum lumps in which case it acts as a gypsum-treatment unit or it is completely empty in which case it acts as a settling unit.
Secondary Gypsum Treatment:
Filtration treatment is generally followed by secondary gypsum treatment. The effluent passes through a rectangular channel having rectangular baffles as in a primary treatment past the gypsum lumps.
Sometimes this channel is not filled with gypsum lumps in which case flow of the effluent through this channel will ensure better mixing of gypsum from primary treatment and a greater contact time for enhancing the precipitation of calcium carbonate.
(vii) Sedimentation:
The effluent from the secondary gypsum treatment section flows to the circular cement-concrete reservoir having a conical bottom. The entry of the effluent is in the conical bottom surpassing the maximum sludge level through a distributor.
Effluent is allowed to stand still for a long time in this tank so that carbonate precipitates and other solid impurities settle down forming sludge in the conical bottom.
The sludge is eventually taken out from the bottom under the static head of liquid content and is transferred to the sludge-well. The clean supernatant liquid is sent either to the public sewers or for the biological treatment to reduce BOD in case it is to be discharged into the inland surface waters.
(viii) Biological Treatment:
The main aim of the biological treatment is to bring the BOD of the effluent down to the permissible limit. BOD of the composite waste of a textile mill varies from 300 to 500 ppm. Since the stipulated limit for BOD is 500, hardly any treatment is required when the effluent is to be discharged into the public sewers or on open land used for irrigation purpose.
However, when the effluent is to be discharged into the inland surface water, BOD has to be reduced to 30 ppm in which case effluents have to be treated by biological treatments. Two biological treatments most commonly reported are the activated sludge process and biological filtration.
(ix) Activated Sludge Process:
In this process, the effluent is mixed and aerated with a flocculent suspension of micro-organisms termed activated sludge. This consists predominantly of a variety of bacteria in different stages of development, aggregated together with organic debris, protozoan and occasionally fungi. The bacteria feed on the impurities in the wastewaters. The process is carried out in one or more tanks or channels to provide the necessary contact time.
The mixed liquor then flows to a settling tank where the activated sludge is separated before being recycled to mix again with the incoming waste. In a properly working plant, the overflow from the settling tank is clear and is suitable for being discharged.
There is a continual growth of activated sludge during the treatment process and when the required concentration in the mixed liquor has been reached, the surplus is removed continuously for disposal.
There are mainly two types of activated sludge plants:
(a) Those with a single aeration compartment in which the incoming waste rapidly and completely mixes with full volume of mixed recycled liquor.
(b) Those with a number of aeration compartments in series or with a long channel through which the incoming waste flows with only a small part of the total volume mixed, undergoing aeration.
Biological Filtration or Trickling Filter:
In the process of biological filtration, the wastewaters are evenly distributed over the surface of a bed of suitably graded inert medium such as coke, hard clinker, broken rock or gravel etc.
As the liquid percolates through the bed, bacteria and other micro-organisms develop as a slimy coating on the surface of the medium, feeding on the organic impurity of the waste, oxidising some to carbon dioxide and other products and some to new bacterial cells.
In a mature filter, there is a balanced community of bacteria, worms, fungi, protozoan, etc. The amount of slimy coating in a filter depends, among other things, on the rate of application of oxidisable organic matter.
If the load on filter is too great or if too concentrated liquid is applied, the coating grows to such an extent that it seals the spaces between the inert medium and leads to “flooding”. This prevents access of air and leads to deterioration of the performance of a filter.
To minimise such difficulties a good filter medium should be uniform in size and should have similar dimensions in three directions at right angles. Recirculation of filter effluent is sometimes employed. It is then often possible to treat a larger volume of waste without any ‘flooding’.