BIOLOGICAL AND CHEMICAL SINGLE STEP PURIFICATION OF DISCHARGED WATER

Abstract: A method for biological and chemical single-step purification of discharged water. A flocculant and dispersed water are used and biocatalyst particles with immobilized microorganisms or enzymes are added for the conversion of residual products in the discharged water.

Description: BIOLOGICAL AND CHEMICAL SINGLE STEP PURIFICATION OF DISCHARGED WATER The invention relates to a method for the purification of discharged water from industrial plants, and more specifically a combination of a specific biological and a general chemical purification process. The Authorities and the community are demanding increasingly stricter standards for the discharge of waste water into the environment. In the following the invention will be explained with reference to discharged water from the dairy industry, but it may be used with every relevant purification process, where substances are to be converted to environmently friendly compounds. There are today two main processes for the purification of discharged water based on biological and chemical techniques. The principle applied in biological purification is that living micro-organisms are used to metabolize pollutants into harmless compounds and gases. One of the principles with chemical purification of discharged water from the food industry is that by lowering the pH almost to the isoelectric point of proteins, these are precipitated. With the addition of a flocculant and dispersed air the precipitated proteins will float together with fat which due to the density will float to the surface in a water solution. Sugar and other soluble components are not cleansed through this chemical method. Chemical purification of discharged water from the food industry is in use or has been in use at several enterprises. Most plants use mineral acid together with a flocculant such as ferric chloride or CMC (carboxyl-methyl cellulose) to adjust pH and as a precipitant for protein which is removed together with fat through flotation or through the addition of dispersed air. If one should want to recover the protein in the chemical sludge for use in animal feed, other types of precipitants should preferably be used. There exists a batch-type plant where CMC is used as flocculant together with mineral acid, e.g. sulphuric acid and where the sludge is separated through flotation. Figure 1 is an example of such a plant. The discharged water contains proteins, sugar and fat. It is desirable to obtain a better purification result by also purifying the soluble sugar from the water phase. The procedure for purification of discharged water in process tanks where a flocculant and dispersed water are used is characterized in that biocatalyst particles with immobilizing micro-organisms or enzymes for conversion of waste products are added to the discharged water. The procedure is further characterized in that sugar, proteins or fat are converted to acid which precipitate the remaining proteins isoelectrically in a combined biological and chemical purification phase. The procedure is also characterized in that proteins and fat are recovered by means of flotation through the addition of a flocculant and dispersed water. In the process described in the invention this addition of mineral acid is avoided in that in its place bacteria are added, for instance lactic acid bacteria which are immobilized in approximately monodispersed and gasfree biocatalyst particles. In this context biocatalyst means a biological system which comprises micro-organisms or cells, as well as parts of these, for instance enzymes. With biocatalyst particles is meant biocatalysts which are fixed (immobilized) by means of a microscopic carrier, for instance a hydro-colloid cell so that the system may carry out metabolic processes. With the procedure according to the invention an automatic acid production takes place when these micro-organisms, for instance lactic acid bacteria, are converting sugar for instance into lactic acid in the discharged water. pH falls to the isoelectric point for proteins, and these precipitate. After an addition of a flocculant and dispersed air the desired purification of organic material is obtained through this combined use of biologic and chemical purification in one step. Thus proteins, fat and sugar are removed. A further advantage with this process may be that the flocculated mass may contain biocatalyst particles with for instance lactic acid bacteria which are a favourable component of animal feed. In the following the process is explained with reference to Figure 1 which shows a simplified diagram of a batchwise flotation plant for the purification of discharged water from the food industry. The flotation plant in Figure 1 is a downscaled industrial plant and is a batchwise plants with two process tanks (1) which are filled up alternately. In the experiments each process tank had a diameter of 1.4 m and they were able to treat up to 2.5 m 3 discharged water per batch which was supplied via a pipe (2). On average the consumption of tap water (9) was 10-15$ of the quantity of discharged water supplied (2). The flotation plant was PLC controlled (10) (Programmable Logic Control). It was operated in such a way that pH in the process tank was adjusted to 4.2 when acid was added. Alternatively, pH may be adjusted by converting lactic sugar to organic acid, for instance lactic acid by using biocatalyst particles. Flocculant (approx. 8 mg/1 CMC) was added and flotation began. Otherwise the plant was equipped with a battery of pump valves (3), an acid tank (4), a tank for the flocculant (5), compressor (6), dispersion water tank (7) as well as sampling points (8). The temperature and pH of the discharged water were continuously measured and recorded. During the production periods the temperature of the discharged water varied mostly between 10 and 50° C, with extreme values ranging from 5 to 55°. Normally the pΞ value in the discharged water was between pΞ 5 and pH 10. However, extreme values from pΞ 3.5 to Ph 12 were measured. Most of the time the pH value was on the alkaline side. A characterization of the crude water, based on water- chemical analyses of a mixture of weekly samples and a mixture of samples during a 24 hour period is shown in Table 1. 10 15 20 The mean value for total COD (Chemical Oxygen Demand) shows a concentration of 3200-3300 g/m . However, there were relatively large variations from week to week, and especially from one day to another. 24-hour emissions with concentrations up to 2.5 times higher than the mean values for organic compounds were registered. It is important to take these variations into account when dimensioning a purification plant. When testing various purification processes a batchwise flotation plant in laboratory scale was used. With the laboratory plant it is possible to test a larger number of precipitants in a relatively short time. In order to obtain optimal purification effect of the given flocculants, it is customary to adjust pH by adding a mineral acid, for instance sulphuric acid. There was a desire to use the laboratory plant to test the adjusting of pH with lactic acid as an alternative to sulphuric acid. A survey was performed in the plant of combinations of various precipitants and flocculants. Table 2 provides an overview of alternative combinations. Table 2 Alt, 2 3 4 5 Table 2. Alternative combinations in the laboratory flotation plant. The tests with the laboratory flotation plant showed that use of Ultrazine gave the poorest result. This process variant therefore was considered the least interesting alternative. The bar diagram in Figure 2 sums up which purifying effects may be achieved under optimal conditions with four other combinations. Ferric chloride (JCL) removed -3$ more of total COD than H 2 S0 4 +CMC, which in turn removed 1-2$ more of total COD than lactic acid + CMC. It is possible with biocatalyst particles to remove over 30$ more of total COD than with the other methods, since sugar is also removed through this method. Protein was removed somewhat more efficiently with ferric chloride than with the other combinations. However, this is not a very advantageous method when the process concentrate (the sludge) is to be used as animal feed. On the basis of the test results, table 3 gives a proposal for the precipitation, coagulation and flotation conditions for chemical purification of discharged water from dairies, and for combined biological and single-step chemical purification of discharged water from a dairy. Table 3 Flocculant Flocculant pH adjusting Desired Dispersion type dosage component pH water qty. The use of lactic acid direct as an alternative to sulphuric acid for pH adjustment is most interesting. Particularly because lactic acid is an advantage if the sludge is to be used for animal feed. The laboratory tests showed that the use of lactic acid resulted in a purification factor which was only insignificantly lower than with the use of sulphuric acid. By adding spherical biocatalyst particles together with e.g. immobilized anaerobic micro-organisms, e.g. lactic acid bacteria, the same precipitation of protein is achieved as by adding lactic acid when the bacteria converted the sugar in the discharged water to organic acids, for instance lactic acid. By adding bacteria to these particles it is thus possible to achieve good purification results since none of the other methods removes sugar. The addition of acid is avoided since the bacteria convert the sugar to lactic acid, and the sludge will after dehydration to process concentrate be well suited for use in animal food mixtures. By using gasifying micro-organisms, e.g. hetero- fer entable lactic acid bacteria, alternatively facultative anaerobic micro-organisms, the resulting gas production will help reduce the need for dispersed air used for the flotation of the protein material. The biocatalyst particles to be used may be produced by any appropriate method, and one method is e.g. described in a parallel patent application, cp. No. C29881. The invention is mainly described with reference to the use of lactic acid bacteria, but the process is not limited to using these bacteria. The type of bacteria may vary according to the substances and compounds one wishes to convert into harmless, eco-friendly products. In the place of bacteria, enzymes may also be used in the biocatalyst particles in order to metabolise special compounds.

Claims: P a t e n t c l a i m s 1. A method for the purification of discharged water in process tanks where a flocculant and dispersed air in water are used, c h a r a c t e r i z e d i n t h a t biocatalyst particles with immobilized micro-organisms or enzymes are added, for the conversion of residual products in discharged water, so that sugar, proteins or fat is converted into acid which precipitates the remaining proteins isoelectrically in a combined biological and chemical purification phase. 2. A method according to claim 1, c h a r a c t e r i z e d i n t h a t proteins and fat are recovered by means of flotation through the addition of a flocculant and dispersed air in water. 3. A method according to claims 1-2, c h a r a c t e r i z e d i n t h a t the consumption of dispersed air in water may be reduced through the addition of gasifying micro-organisms immobilized in the particles. 4. A method according to claims 1-3, c h a r a c t e r i z e d i n t h a t flotation results in a process concentrate which e.g. may be used as animal feed. 5. A method according to claims 1-4, c h a r a c t e r ¬ i z e d i n t h a t biocatalyst particles become part of the process concentrate. 6. A method according to claims 1-5, c h a r a c t e r i z e d i n t h a t the temperature in the process water is kept between 10 and 50°C.

Inventors: EIDE OLA, OTERHOLM BJARNE, STORROE IVAR, EIDE, OLA, OTERHOLM, BJARNE, STORROE, IVAR
Applicants: NORSKE MEIERIER
Application year: 1993
Main classification: C02F3
IPC codes: A23K1, C02F1, C02F3