PROCESS FOR TREATING POLLUTANT EFFLUENT, PARTICULARLY IN THE TANNING INDUSTRY, AND PLANT FOR IMPLEMENTING THE PROCESS

Abstract: A process for treating alkaline pollutant effluent containing sulphides and proteic substances, particularly in the tanning industry, characterised by comprising the following treatments in succession: a first sedimentation of the effluent to be purified, after subjecting it to flocculation, partial neutralisation of the free alkalinity of the supernatant from the preceding sedimentation, using sulphuric acid to adjust the pH to a value of 9,7-10, a first oxidation to convert the sulphates present in the effluent to be purified into intermediate oxidation products, coagulation and intermediate sedimentation of the proteic substances contained in the effluent which has undergone the previous first oxidation, by using sulphuric acid to further lower the pH of said effluent to the protein isoelectric point, separating the coagulated proteins from the liquid fraction, a second oxidation of said liquid fraction, neutralisation of the turbid liquid from the second oxidation treatment by calcium hydroxide until a pH value suitable for discharge is obtained, and final sedimentation.

Description: Process for treating pollutant effluent, particularly in the tanning industry, and plant for implementing the process This invention relates to a process for purifying pollutant alkaline effluent containing sulphides and proteic materials, and in particular the depilation effluent (liming effluent) and the effluent from the subsequent wash stages, in the tanning industry. The invention also relates to a plant for implementing the process. The liming effluent from the "lime work" is formed from the first liming bath (solution of lime, sodium sulphide, caustic soda and wetting and surface-active additives i water), the components of the depilated hide epidermis, th water added to form the second liming bath, and furthe components dissolved from the treated hides. The wash liquor, deriving from at least two washes, contains the organic an inorganic residues of the previous processing steps, including the sodium chloride used for initially treating th hides. Thus the processing effluent of the tanning industry, such as that described, represents a highly pollutant liquor, especially if the number of washes i„s small, and has a CO which, in relation to the number of washes, varies betwee about 30000 and 100000 mg 0 /l. The purification of such liquors is a complicate problem, which up to the present time has been solved b various methods. In one of these methods, said liquor is diluted by adding sewage from the town sewer. This however has a series of serious drawbacks, in that: - it does not solve the pollution problem unless successive active sludge purification treatment steps are applied, these being implemeήtable only with large costly plan, - it requires vast water quantities, which are available only in urban developments of adequate size. Another known method comprises auto-purification of the liquor in oxidation pools- or the disposal of sludge which has settled from the liquor in special dumps, together wit mechanical filtration of the liquid part and active-sludge treatment of the filtrate. However, this method can also no be considered general .in that it depends on particular territorial and regulatory situations, and often involves dispersion of substances having a high energy content whic could find wide use in other sectors, such as the livestoc or agricultural field. In order to recover these substances it has also bee proposed to treat tanning industry effluent with sodiu metabisulphite, to oxidise the sulphides present in it, an then to convert these into sulphates, to be able to the extract the proteins. However this method also has drawbacks , in that on the one hand it requires large quantities of sodiu ^ -6* metabisulphite, ie a substance which is of high cost and als results in an undesirable increase in the salinity of th 5 treated material, and on the other hand gives rise to th formation of colloidal sulphur, which alters the recoverabl proteins and reduces their commercial value. The object of the invention is to obviate all thes drawbacks by purifying'the pollutant effluent in particula 10 of the tanning industry by a process which: - involves low plant costs, - requires the use of low-cost reagents, - involves short oxidation times and can be implemented i small, compact plants of limited size, 15 _ does not use harmful substances, or give rise to th development of harmful substances, - does not increase the salinity of the treated material, an in fact reduces it in the final product, and - leads to a drastic lowering in COD. 20 This aim is achieved according to the invention by process for treating alkaline pollutant effluent containin sulphides and proteic substances, particularly in the tannin industry, characterised by comprising the followin treatments in succession: - a first sedimentation of the effluent to be purified, after subjecting it to flocculation, - partial neutralisation of the free alkalinity of the supernatant from the preceding sedimentation, using sulphuric acid to adjust the pH to a value of 9,7-10, - a first oxidation to convert the sulphates present in the effluent to be purified into intermediate oxidation products, - coagualation and intermediate sedimentation of the proteic substances contained in the effluent which has undergone the previous first oxidation, by using sulphuric acid to further lower the pH of said effluent to the protein isoelectric point, - separating the coagulated proteins from the liquid fraction, - a second oxidation of said liquid fraction, - neutralisation of the turbid liquid from the second oxidation treatment by calcium hydroxide until a pH value suitable for discharge is obtained, and - final sedimentation. Advantageosuly the second oxidation can be conducted in the presence of a ferric salt functioning as a further oxidising and flocculating agent. The plant for implementing the method is characterised by comprising: - a first reactor in which the material suspended in the effluent to be treated is flocculated, - a first settling tank in which a first separation of the flocculated materials from the liquid phase takes place, - a second reactor in which the free alkalinity of hte supernatant from said first settling .tank is partly neutralised, - a first oxidation tank, - a third reactor in which coagulation of the proteic sustances contained in the oxidised liquor takes place, - a separator for separating the coagulated proteins from the liquid phase, - a second oxidation tank, - a fourth reactor in which the turbid liquid from the second oxidation tank is completely neutralised, - a second settling tank in which sedimentation of salts and hydroxides tades place. A preferred embodiment of the present invention is described in detail hereinafter with reference to the accompaying drawings, in which: Figure 1 is a schematic block diagram of the process according to the invention; and Figure 2 is a schematic isometric view of a plant for implementing the process according to the invention. As can be seen..from the figures, the process according to the invention is implemented with a plant comprising a reactor 2 to which the effluent to be treated is fed after being previously collected in conventional equalisation tands provided with air bubblers, not shown on the drawings for simplicity. The effluent to be treated is fed from the equalisation tanks to the reactor 2 by a centrifugal pump 1. A magnesium sulphate feed pipe is also connected to the reactor 2. The outlet of the reactor 2 is connected to a settling tank 3, which is itself connected to a further reactor 4 provided with an energetic mechanical stirrer and fitted with a gas offtake pipe. A sulphuric acid feed pipe is also connected to this reactor and is fitted with a valve 5 for controlling the pH in the reactor 4. The outlet of th reactor 4 is connected to an oxidation tank β comprising several compartments in series, to the first of which ther are connected an antifoaming agent feed pipe and a manganes sulphate feed pipe. All the comparments of the oxidation tan β are also connected to a pipe for feeding an air stream which, as will be apparent hereinafter, oxidises the sulphides present in the effluent to be treated. Within each compartment, the air feed pipe is configured as a porous ceramic or porous plastic aerator tube able to feed into the water in which it is immersed a stream of air in the form of very minute bubbles in a sufficient quantity to effect total sulphide oxidation during the oxidation treatment time. The outlet of the oxidation tank β is connected to a further reactor 7 provided with an energetic mechanical stirrer and fitted with a gas offtake pipe. A sulphuric acid feed pipe is also connected to the reactor 7, and is provided with a valve 8 for controlling the pH in the reactor 7. The outlet of the reactor 7 is connected to an apparatus for collecting the products which have coagulated in this reactor, the apparatus in the described example consisting of a rotary vacuum filter 9. This is connected to a second oxidation tank 10, of analogous type to the preceding oxidation tank 6, ie consisting of several compartments connected in series, into which aerator tubes open. A pipe for feeding an antifoaming agent, to be used only if necessary, is connected to the first compartment, and a pipe for feeding a solution of ferrous and ammonium polysulphates is connected to the last compartment. The outlet of the oxidation tank 10 is connected to a further reactor 11 provided with a mechanical stirrer. A lime feed pipe is connected to this reactor, and is provided with a valve 12 for controlling the pH in the reactor. The outlet of the reactor 11 is connected to a static settling tank 13. The process according to the invention, which uses the plant heretofore described, is as follows: the effluent to be treated (liquor), originating from the equalisation tanks and having for example a pH of about 13 13-5, is fed by the pump 1 to the reactor 2, into which magnesium sulphate is added until the material suspended i the liquor has flocculated. The liquor treated in this manner is then fed into th settling tank 3, in which initial separation of th flocculated materials from the liquid phase takes place, thi separation having the effect of limiting subsequent use o sulphuric acid, favouring sulphide oxidation and improvin the characteristics of the filtrate which separates in th rotary filter 9. The supernatant from the settling tank 3 is fed to th reactor 4, where sulphuric acid is added until the pH reaches a value of about 9,7-10. On reaching the correct pH value, the valve 5 interrupts the sulphuric acid flow. After a predetermined residence tipe in the reactor 4 under energetic stirring, the product under treatment is transferred into the oxidation tank β. A salt of a transition element (Fe, Co, Ni, Mn) is fed into the first compartment of the tank 6 to catalyse the oxidation of the sodium sulphide present in the treated effluent. Manganese sulphate is preferably used. The sodium sulphide is oxidised in the compartments of the tank 6 under the action of the oxygen contained in the air fed through the aerator tubes, and is converted into intermediate products such as sodium sulphites, sulphates and thiosulphates. The oxidised liquor from the tank 6 is fed into th reactor 7, into which sulphuric acid is also fed until th protein isoelectric point (pH = 5-5.5) is reached. Here th mechanical stirring of the liquor mixed with sulphuric aci causes their sufficient homogenisation with simultaneou protein coagulation. The entire contents then pass to th rotary filter 9 where the coagulated proteins are separate from a clear liquid, which is fed to the tank 10 in which th intermediate products (sulphites, sulphates an thiosulphates ) deriving from the sodium sulphide ar oxidised. A ferric salt (such as ferric sulphate) is fed into th last compartment of the oxidation tank 10 to act as a oxidising and flocculating agent, possibly in conjunctio with an aluminium salt ( such as aluminium sulphate ) o flocculation purposes. A solution of iron and aluminiu polysulphates ( 2% Fe 0 and 3% Al 0 ) is preferably used. The turbid liquid formed mainly of inorganic materia (acqueous sodium chloride and sulphate solution in whic partly precipitated calcium, iron and aluminium sulphate ar suspended) leaves the tank 10 and is fed into the reactor 11 where it is mixed under mechanical stirring with lim (Ca(0H) ) for its complete neutralisation. In the reactor 11 2 the metric pH probe regulates the lime quantiy so as to allo complete precipitation of the iron and aluminium hydroxide and to adjust the pH to a value suitable for discharge (pH 7-7.5). The product leaving " the reactor 11 is conveyed into th static settling tank 13, where sedimentation takes place o the salts except for the sodium chloride and sulphate, and o the hydroxides (CaSO +Fe(0H) +Fe(OH) +AK0H) ). Th 4 2 3 3 supernatant, which is perfectly clear, odourless and colourless, can be dispersed into suitable sewerage or can be fed to biological treatment for complete elimination of the residual organic load and conversion of the proteic and ammoniacal nitrogen into nitric nitrogen. The salt load of the final product leaving the static settling tank 13, which derives from sodium chloride and sulphate and remains unchanged even following the biological treatment, is in all cases considerably less than the salt load of the initial liquor. The process according to the invention is considerabl more advantageous than conventional processes in that: - it results in low plant costs, - it requires the use of low-cost reagents, - it involves short air-oxidation times and can b implemented with small, compact plants of limited size, - it does not use harmful products, or give rise to th development of harmful substances, - it results in a considerable lowering of the COD of th treated effluent and reduces its salinity. The following example, conducted with an experimenta plant on an e flulent sample from a tanning works, wi further clarify the invention. The treated effluent, having an original COD of 66.000 mg 0 /1, was fed into the reactor 2, in which it received about 500 p.p.m. of Mg( SO ) in a 25% solution. After about 10 minutes of residence in the reactor 2, in which complete flocculation of the suspended material took place, the treated effluent was fed into the settling tank 3 where, over a period of about 1 hour, magnesium hydroxide, calcium sulphate and animal tissue particles of mucillaginous 3 • consistency sedimented in a quantity of 7-2 kg/m of effluent, and had the following composition: moisture 50 % ash 33 % proteic nitrogen 3,9% The supernatant from the settling tank 3 was fed into the reactor 4 and H SO was added under energetic stirring for 5 minutes, until the pH had reached 9,8. The product was then transferred into the oxidation tank 6, into which Mn( SO) in a 15% aqueous solution was also fed to the extent of 20 mg of salt per litre of product to be treated. After a residence time of about 2 hours, sufficient to oxidise the sulphides, the oxidised liquor was transferred into the reactor, 7, into which sulphuric acid was also fed in such a quantity as to reduce the pH to 5 and thus cause protein coagulation. After a residence time of about 10 minutes under continuous stirring, the coagulated proteic material was in abundant quantity and had a caseous appearance. Its sedimentation would have required 8-10 hours, and because of this it was preferred to use more rapid separation by means of the rotary filter 9 using kieselguhr, which enabled about 25 kg of an easily agglomerable light- coloured paste to be extracted per cubic metre of liquor, and which separated very easily from the kieselguhr support for subsequent drying. Its composition in a sample dried under vacuum at a temperature of 60 C was as follows: moisture 8,5% ash 21 % proteic nitrogen 61 % The contained proteins were absorbable to the extent of 51% and represented an excellent byproduct for livestock use, in contrast to the byproduct obtained after the firs sedimentation, the low protein content of which made i however suitable for spraying over agricultural ground. The liquor separated by the rotary filter 9 was fed t the oxiation tank 10, into the last compartment of which iron was fed to the extent of 200 p.p.m. in the form of a iron and aluminium polysulphate ( 2% Fe 0 and 3%A1 0„ ). Afte 2 3 2 3 about 2 hours of treatment in this tank, the turbid liqui was fed into the reactor 11 together with lime, and subjecte to continuous mechanical stirring until complet neutralisation had -taken place (pH = 7-7,5) with complet precipitation of the iron and aluminium hydroxides. After remaining for about 2 hours in the static settlin tank, during which sedimentation of the iron and aluminiu salts and hydroxides occurred, the supernatant, which wa clear, colourless and odourless, had the followwin composition: chlorides (Cl~) 12 g/1 sulphates (SO ) 6,2 g/1 . 4 proteic nitrogen 1 ,3 g/1 ammoniacal nitrogen 0,08 g/1 calcium (Ca ++ ) 1,2 g/1 sodium (Na + ) 8,75 g/1 and the following characteristics: COD 3900 mg 0 It 2 BOD 800 mg 0 /1 5 2 " The overall salinity derived mainly from sodiu chloride, which was already present in the original liquor and from sodium sulphate, resulting from oxidation of th sodium sulphide. These two salts were present in th following quantities: Na SO < 7,1 g/i 2 4 NaCl 16,1 g/1 The residual liquor was subjected to further biologica tratment to completely eliminate the residual organic loa and convert the protein and ammoniacal nitrogen into nitri nitrogen. In this manner, a liquid was obtained having th following characteristics: COD ' 180 mg 0 /l BOD 30 mg 0 /I 5 2

Claims: - 1 6 - CLAIMS 1. A process for treating alkaline pollutant effluen containing sulphides and proteic substances, particularly i the tanning industry, characterised by comprising th following treatments .in succession: - a first sedimentation of the effluent to be purified, afte subjecting it to flocculation, - partial neutralisation of the free alkalinity of th supernatant from the preceding sedimentation, usin sulphuric acid to adjust the pH to a value of 9,7-10, - a first oxidation to convert the sulphates present in th effluent to be purified into intermediate oxidatio products, - coagualation and intermediate sedimentation of the protei substances contained in the effluent which has undergon the previous first oxidation, by using sulphuric acid t further lower the pH of said effluent to the protei isoelectric point, - separating the coagulated proteins from the liqui fraction, - a second oxidation of said liquid fraction, - neutralisation of the turbid liquid from the secon oxidation treatment by calcium hydroxide until a pH valu suitable for discharge is obtained, and - final sedimentation. 2. A process as claimed in claim 1 , characterised in that the material suspended in the liquor is flocculated by feeding magnesium sulphate into a reactor (2) containing said liquor. 3. A process as claimed in claim 1, characterised by adding a quantity of sulphuric acid to the supernatant from the preceding first sedimentation treatment sufficient to adjust its pH to 9,8. 4. A process as claimed in claim 1 , characterised in that the first oxidation is conducted in the presence of a catalyst consisting of a salt of a transition element. 5. A process as claimed in claim 4, characterised in that the first oxidation is conducted in the presence of manganese sulphate. 6. A process as claimed in claim 1, characterised in that the first and/or second oxidation is conducted with air. 7. A process as claimed in claim 1, characterised in that the coagulated proteins are separated from the liquid fraction by filtration. 8. A process as claimed in claim 1, characterised in that the second oxidation is conducted in the presence of a ferric salt functioning as a further oxidising and flocculating agent. 9. A process as claimed in claim 8, characterised in that the second oxidation is conducted in the presence of ferric sulphate. 10. A process as claimed in claim 8, characterised in that the second " oxidation is conducted in the presence of an aluminium salt functioning as a flocculating agent. 11. A process as claimed in claim 10, characterised in that the second oxidation is conducted in the presence of aluminium sulphate. 12. A process as claimed in claim 8, characterised in that the second oxidation is conducted in the presence of a solution of iron and aluminium polysulphates. 13. A process as claimed in claim 12, characterised in that the second oxidation is conducted in the presence of a solution of iron and aluminium polysulphates containing 2% Fe 0 and 3 A1 2 0 3 - 14. A plant for implementing the process claimed in one or more of claims 1 to 13, characterised by comprising: - a first reactor ( 2) in which the material suspended in the effluent to be treated is flocculated, - a first settling tank ( 3 ) in which a first separation of the flocculated materials from the liquid phase takes place, - a second reactor (4) in which the free alkalinity of th supernatant from said first settling tank ( 3 ) is partl neutralised, - a first oxidation tank (6 ), - a third reactor ( 7 ) in which coagulation of the protei sustances contained in the oxidised liquor takes place, - a separator (9) for separating the coagulated proteins fro the liquid phase, - a second oxidation tank (10), - a fourth reactor (11 ) in which the turbid liquid from th second oxidation tank (10) is completely neutralised, - a second settling tank (13) in which sedimentation of salt and hydroxides tades place. 15. A plant as claimed in claim 14, characterised in tha the first and second oxidation tanks (6,10) compris compartments in series. 16. A plant as claimed in claim 14, characterised in tha the reactors (4,7 and 11 ) are provided with a metric pH prob which controls the valves (5,8 and 12) for feeding th sulphuric acid and lime into the correspondig reacto respectively. 17. A plant as claimed in claim 15, characterised in tha the separator (9) for separating the coagulated proteins fro the liquid phase consists of a rotary vacuum filter. 18. A plant as claimed in claim 1 , characterised in tha an air feed pipe is. disposed within each compartment of th tanks (6,10). 19. A plant as claimed in claim 18, characterised in that bank of aerator tubes of porous material is connected to eac air feed pipe.

Inventors: PEZZI ALFREDO, PEZZI, ALFREDO
Applicants: MARCELLO GRIMANI GIUSTINIAN BE, MARCELLO GRIMANI GIUSTINIAN, BENEDETTO
Application year: 1987
Main classification: C02F9
IPC codes: C02F1