TECHNICAL FIELD

The present invention relates to a process foi"* treating settlings formed during the biological purification of municipal and industrial waste water and may foe used in various fields of industry, in particular in paper and pulp production.

BACKGROUND ART

There have been known processes for treating a surplus activated sludge with special reagents followed by its separation into water and thickened settlings by means of filtering or settling- In most cases- an aqueous solution of ferric chloride and lime milk /see O. I. Yakovleva eτ* al . , "Sewage Treatment", "Lesnaya Promyshlennost" , Moscow, 1975, pp. 40 - 43,-', as- well as various ammonium salts ,*see US Patent No. 4240904, Int. Ci . C02F 11/14, 1980/ and organic compounds /see USSR Inventor's Certificate No. 827424, Int. CI. C02F llj/1 , 1981/ are used for this purpose.

There have been also known processes foi ^-* treating a surplus activated sludge by separating into water and thickened settlings by means of filtering and using auxiliary filtering substances such as, for example inorganic compounds, for example chalk, gypsum /as in the above mentioned USSR Inventor"s Certificate No. 827424/, or high-molecular organic- compounds, for example copolymer of acr-ylamide with methacrylic acid derivatives /see GE Patent Specification No. 1466185, Int. 01. C02C 3/00, 1977/.

Of the major drawbacks of the aforementioned processes is the use of expensive chemical reagents, practically in the

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complete absence of their regeneration.

The closest prior art has been described in 0 process or treating a surplus activated sludge, comprising the steps of presettling followed by filtering, avoiding the step of treatment by reagents. Settlings formed a.s a result of dehydrating the surplus activated sludge are treated just on a filtering cloth with an aqueous alkaline solution ox* white sul te liquor. In this case, a highly concentrated alkaline solution of the surplus activated sludge is obtained folowed by its evaporation and sending to burn down see fJSSR Inventor's Certificate No- 1038237. Int. Cl. C02F 11/14, 1933/.

The main disadvantage of the known process consists an low effectiveness and high duration of a process for dehydrating the settlings by filtering, which is caused by an unfavorable structure of said settlings that firmly retain water and have a high specific filtration resist-ance. Moreover, the known process requires the use of expensive alkaline ι*eagents .-'see Yυ. N. Nelepin, "Sulfate Pulp Production", " esnaya Promyshlennost", Moscow, l£-80, p. 531/'.

SUMMARY OF THE PRESENT INVENTION

The present invention is made in order to solve the above problems included in the prior arts, and an object of this invention is t provide a new process for treating a surplus activated sludge enabling intensification of the process fo* dehydrating the settlings by filtering, avoiding the use of expensive chemical reagents, a complete utilisation of the process products so obtained t clarified alkaline hydrolysate and an ash residue) as well as utilisation of the sulfate-pulp production waste with minimal costs.

The above technical result is achieved in accordance with the present invention by treating a pre-tnickened surplus activated sludge with white slime obtained ar the stage of

causticising green liquor in the process of the sulfate-pulp production, with stirring and in a volume ratio of white slime : surplus activated sludge equal to 1 : 5 - 10 followed by dehydration by means of filtering, drying and burning down of the dehydrated settlings.

The present process differs from the known one in that it provides for the treatment of the pre-dehydrated surplus activated sludge with white slime obtained at the stage of cauεticising green liquor in the process of the sulfate-pulp production, with stirring and in a volume ratio of white slime : surplus activated sludge equal to 1 : 5 - 10.

White slime used in the present process for treating the surplus activated sludge is a sulfate-pulp production waste being a black suspension with a density of 1.25 - 1.35 Kgj/'cub. dm and having the following chemical composition, % wt. :

It is common knowledge that upon regeneration of a waste black liquor during the sulfate-pulp process, a fusion cake (a mineral part of the waste black liquor) is formed with sodium carbonate being its main component. Then, the fusion cake is dissolved and green liquor formed therewith is subjected to causticisation by adding calcium oxide to obtain a strong white liquor and a calcium carbonate precipitate, that is white slime. Usually, a subsequent utilisation of white slime requires its repeated washing with fresh water using special chemical reagents /see USSR Inventor's Certificate No. 827424, C 02 F 11/14, 1981/. The necessity of using fresh water and expensive organic compounds significantly complicates the

process and rises its cost-

Using white slime in accordance with the present process allows solution of two problems at once: on the one hand, to intensify and lower the price of the process for treating the surplus activated sludge and, on the other hand, to significantly simplify nd lower the price of the process for utilising white slime.

A process for treating the surplus activated sludge thickened when p-resettling the precipitate, by means of white slime containing alkali residues is accompanied with practically a complete alkaline hydrolysis of the bioorganic part of the sludge (constituting no less than 83 % wt. 2^ dry residue) with the formation of salts of amino acids, amino alcohols and a number of protein derivatives. In turn, calcium carbonate constituting the major contents of white slime, is uniformly distributed, when stirred thoroughly, among undissolved large fibres and solid particles present in the surplus activated sludge in the amount up to 17 w . on dry basis. This results in the change of an unfavorable structure of the surplus activated sl ϊdge settlings which, under nor-mal conditions (that is, in the absence of treatment with special reagents), firmly retain water and have a high specific filtration resistance. Solid particles and large fibres of the surplus activated sludge .join with particles of calcium carbonate and lose water with the formation of a solid phase which is readily separated from an aqueous phase by filtration. Thxis, in accordance with the present process, white slime being the sulfate-pulp production waste, on the one hand, replaces an expensive alkaline agent and, on the other hand, performs functions of an auxiliary filtering substance.

The present process is carried out. m the following manne .

The surplus activated sludge is subjected to

pre- hickening in a radial or vertical pond. Most preferably, '' thickening is carried out up to the surplus activated sludge concentration of 18 - 40 g/1. Upon thickening, the surplus ' activated sludge is fed to a mixer where it is treated with white slime, while stirring. White slime is used in a volume ratio to the surplus activated slλjdge equal to 1 : 5 - 10 which allows achievement of a maximal efficiency of post-filtering. When treating the sludge with white slime taken in the amount less than that mentioned above, a filtering rate reduces significantly. It is not to the purpose of technology to use white slime in the amounts exceeding the claimed ratio. Most preferably, stirring is carried out at a temperature of between 75 and 104 C. It is also possible to use lower temperatures, up to room temperature, but in this case a process for treating becomes more prolonged. The treatment is carried out to a complete dissolution of a bioorganic part of the surplus activated sludge. Then, a mixture so obtained which contains an alkaline hydrolysate, undissolved fibres and solid particles present in the surplus activated sludge, ae well as calcium carbonate uniformly distributed among the particles of sludge settlings as a result of stirring, are fed to a vacuum filter or a pressure filter and subjected to dehydration. A clarified alkaline hydrolysate obtained upon filtration may be used, without a subsequent treatment, in the manufacturing cycle of the sulfate-pulp production at the stage of reagent regeneration. A dehydrated precipitate formed on a filter surface is readily removed from the filter surface, thereupon it is subjected to heat drying in drier drums and fed to burn down in an utilisation plant furnace. An ash residue comprising calcium oxide is used in the manufacturing cycle of the sulfate-pulp ( production at the stage of a green liquor causticising.

BEST MODS FOR CARRYING OUT THE INVENTION

To 5 I of an aqueous suspension of the surplus activated sludge having, after settling in a radial pond, a concentration of 80 g,-l, 0.75 1 of a white slime suspension is added and thoroughly stirred for 15 minutes at 95 C. A mixture so obtained is fed to a vacuum filter and subjected to dehydration of the settlings. In this case , a filtration rate is determined to be 40 I/sq. m/rnin. A clarified alkaline hydrolysate is fed to be used in the manufacturing cycle of the sulfate-pulp production. A dehydrated precipitate formed on a filter surface is removed and subjected to heat drying at 850 C, theι ^v eupon it is burned down. An ash residue obtained is fed to the stage of a green liquor causticising.

INDUSTRIAL APPLILIABILITY

The present process for treating the surplus activated sludge allows intensification of the filtration process, avoiding the use of expensive chemical reagents (at. the expense of using waste of the sulfate-pulp production), a complete utilisation of products obtained when treating the sludge (an ash residue and a clarified alkaline hydrolysate) as well as utilisation of the sulfate-pulp production waste, that is white slime, with simplifica ion of the production procedures and techniques and significant reduction of material costs. For the purpose of comparison, a filtration rate in accordance with the known process is 4 i/sq. m min. Besides, reduction in a dehydration process time according to the present process results in the decrease of energy consumed to realise thereof.