WO1992011206A1

WO1992011206A1 - Process and plant for purification of agricultural waste material

Info

Publication number: WO1992011206A1
Application number: PCT/NO1991/000156
Authority: WO
Inventors: Carl-Henrik Knudsen; Stein-Thore Larsen
Original assignee: Ticon Vvs A/S
Priority date: 1990-12-19
Filing date: 1991-12-19
Publication date: 1992-07-09
Also published as: NO912460D0

Abstract

A process and a plant for purifying agricultural waste liquid such as waste liquid from the processing of fruit, wherein the waste material initially is separated into a solid and water-soluble phase and where the aqueous phase further is provided with chitosan to flocculate the colloidally floating particles in the aqueous phase. The thus treated aqueous phase is passed through a pipe flocculator (2) and is subsequently treated by further adding of chitosan (4, 5, 12) and filtering through membrane filters (5) and coal filters (7) or vice versa. The process and plant is suited for purifying the waste water from olive oil pressing called alpechin.

Description

Process and Plant for Purification of A ricultural Waste Material

The present invention concerns a process for purifying waste material from agriculture, especially pressed fruit waste material and more especially waste from olives (alpechin) , and a plant being suited for performing the process according to the invention.

In countries with industry processing fruit such as grapes (wine and grape juice) , olives (olive oil) , plums (plum juice, wine) , apples (apple juice, cider) etc. it has been a problem to treat further the waste material from such a processes. The background for such problems have their basis in that the fruit meat has been unsuited for further treatment and that the water content in the fruit waste material, after the juice and the oils have been pressed away and separated, has contained very large quantities organic material, something which has made the waste liquid per se unsuited for further processing. This has been especially pronounced with waste material such as alpechin and silo liquid, where the alpechin contains large quan¬ tities organic components and metals, and where the silo liquid contains large quantities organic material which may overload surrounding environment and cause pollution problems.

Much of such waste has previously either not been purified or has been used for animal feed or fertilizer, but when such material contains e.g. metals it is unfit for sub- sequent use since the polluting materials will accumulate in the nutritional chain.

It has previously been found to be problematic to purify such material since the aqueous phase from the crushed fruit material which is not used for further processing to the end product, contains dissolved organic components which are difficult to separate. Flocculation with conventionally used flocculating agents have in this connection proved to be scarcely effective or ineffective.

A purpose with the present invention is thus to produce a process for purifying the aqueous phase from fruit waste material and to obtain a plant which may be used for such a purification.

To achieve such a purification it is necessary to pre¬ cipitate the organic substances from the aqueous phase from the fruit waste material. Such substances may comprise a number of inorganic and organic compounds such as colouring materials, proteins and sugars.

If the fruit waste material has a too solid consistency there is initially added water thereto. Fruit waste material and water is then separated by known methods such as centrifuga ion, filtering and/or decantation. The separating process may also optionally include heating of the aqueous phase, e.g. to 35-80°C, preferably to 35-50'C.

After such a separating step the aqueous phase will contain water-soluble organic compounds, proteins, sugars and organic and inorganic salts. The inorganic components comprise nutritional salts from the fruit and salts from the water with which the fruit was mixed before the milling in the first step.

To further precipitate the organic components being present in the aqueous phase, there is added chitosan.

Chitosan is a biopolymer which has in the above mentioned aqueous system surprisingly been shown to precipitate and flocculate the dissolved and hovering organic materials and salts in the aqueous solution from the initial purifying step mentioned above.

By sequentially precipitating the waste water from the purification steps with chitosan and then removing the precipitated material by using one or more of the above mentioned separating processes, the organic components will eventually be removed from the waste water, which then may be used as such for e.g. an addition to the dilution in the first purification step of the purification process or it may be led back to nature as purified water via irrigation pipes or directly to water courses etc. The number of purification steps being necessary to remove the organic components may be easily determined by using standard methods being known to the person skilled in the art and which may be performed on the waste water from each and any of the purification steps in the process.

I addition to the precipitation steps in the purification process there may be used different conventional separation- /purification steps such as membrane filtration and coal filtration. This also to optionally remove the components which have been precipitated with chitosan.

The invention will below be described more closely in connection with a purification plant for fruit waste material which has been shown in the attached figures wherein:

Fig. 1 shows a principle sketch of the purification plant according to the invention.

Fig. 2 shows an embodiment of the purification plant according to the invention where the location and sequence of each of the step components have been indicated.

Fig. 3 shows a plane view of a purification plant based on the principle sketch according to Fig. 1 and 2 as well as a sectional view of some parts of the plant.

Fig. 4 Shows the location of a purification plant according to fig. 3 in a purification station. Fig. 5 shows an embodiment of a pipe flocculator being suited for flocculation with chitosan in the plant according to the invention.

Fig. 6 shows a mixing device for the compounds with the liquid for treatment according to the invention.

Fig. 7 shows the purification effect of each step in the purification process and -plant according to the invention.

Fig. 8 shows schematically some fields of use for the end products subsequent to the purification process according to the invention.

After a pre-treatment of fruit in a conventional way for extraction of oils (olive oil) and/or juice (grape-, apple-, plum-juice) , the starting material for the processing according to the invention is present. To such a material there is added suitable quantities of water (for flushing/- dilution of the aqueous phase of the starting material and washing out of the solid material remaining after the pre- treatment) to a TOC (Total Organic Carbon)-value in the aqueous phase to below about 40.000 mg/1 and a BOC (Bio¬ logical Oxygen Consume)-value below about 45.000 mg/1. An alternate measurement for the water-soluble material in the aqueous phase is the COC (Chemical Oxygen Consume)-value which may lie below 90.000 mg/1, e.g. in the interval 90- 60.000 mg/1.

An alternative starting material for removing organic material according to the invention may also be silo liquid which may be diluted as mentioned above as well or be used directly in the purification process.

If the waste material contains solid particles it is subjected to centrifugation to remove the solid material (fruit meat, water-insoluble fibre material, fruit skins, cores, kernels etc.) and this solid material will be free from water-soluble components since these will have become flushed out in the initial washing step. If necessary there may be performed several water additions/sentrifugations to wash out the starting material.

Instead of sentrifugation other methods for removing particulate materials may be used such as e.g. filtering, precipitation and/or flotation, as mentioned above.

The aqueous phase from this initial separation step comprise the raw material for further processing by the purification method according to the invention. This liquid, below specified as the waste water, is treated in several consecutive purification steps, as explained below with reference to figs. 1-4 which show an example of a plant which may be used for performing the purification method according to the invention.

In this plant the waste water is pumped to an inlet tank 1 shown in fig. 1-4. In this inlet tank 1 the TOC-, BOC- or COC-values are subsequently adjusted to suitable levels as mentioned above and additionally the pH is optionally adjusted to a suitable value for precipitation with chitosan, e.g. within the pH-interval 5,5-7,0, preferably pH 6,5.

From the inlet tank(s) 1 the waste water is pumped via mixing devices 12 for mixing in chitosan, e.g. such mixing devices as shown in fig. 6, to a device for flocculation 2, e.g. a pipe flocculator as shown in fig. 5.

From these mixing devices 12 chitosan is admixed to the waste water in an amount of 100-200 g/m³ water and is then led into the flocculator 2.

The amount of waste water added to the plant is measured with a water-measuring device, e.g. of electromagnetic type, for not overloading the capacity of the plant and to maintain a continuous purification process.

From the flocculator 2 the water" is passed to a precipi¬ tation tank/flotation unit 3, and during this period there is added calcium in an mount of up to 200 g/m³ as well as optionally additional adjuvants for flocculation and to aggregate the flakes which have been produced with chitosan to larger and more solid flakes. Such adjuvants may e.g. be organic polymer materials such as "Prastol" and/or "Zetag" which may be added in an amount of 50-100 g/m³. Other adjuvants may also be added to the waste water together with the chitosan in the same amounts as for the pipe flocculator 2.

The additions of calcium in the flocculating steps may be performed with suitable water-soluble calcium salts such as calcium hypochlorite. Other calcium salts such as calcium nitrate may also be used and the choice thereof will lie within the competence of the person skilled in the art.

After flocculation and precipitation the contents of dissolved materials is significantly reduced as shown in fig. 7, but if required more steps with addition of chitosan, Ca and adjuvants may be performed.

From the precipitation step with removal of the precipitated material the waste water is conducted further to the contact filters 5 where there may be added additional calcium in a quantity of about 50-100 g/m³ and where residual flakes are filtered from the waste water. As shown in fig. 7 the TOC and BOC values after this step are significantly reduced.

For further purifying the waste water which now contains small amounts of the original waste material, there may be performed additionally one or more filtering steps 7,8 in a per se known manner with filtering processes such as membrane filtering 7 and filtering through active coal 8, or vice versa. An example of using the purification process for organic waste liquid according to the invention will below be given for the purification of alpechin (waste material from the pressing of olive oil) with reference to the enclosed figures for the purification plant.

For conventional production of olive oil the olive fruit is mixed with water in the weight ratio 1:1 - 1:2 and is ground to a pulp which is heated to 40-50 ° C . The solid material is separated from the mass either by pressing (old process) or by centrifuging (modern process) and is passed to the separator to remove the olive oil. The aqueous phase from this process is called "alpechin" in Spain, it has a strong brown colour and contains a number of organic and inorganic materials. Apart from the colouring components the organic part comprises mainly proteins and sugars. The inorganic components comprise nutritional salts from the fruit and salts from the water with which the fruit was mixed before the grinding.

Components in water which, in the instance of alpechin, it is possible to separate are present suspended in the aqueous phase in colloidal form. The suspension is stable based on the surface charge of the particles. The principle for the present purification method is that there to the aqueous phase is added a material (chitosan) which neutralizes the charge on the surface of the particles so that they coalesce (flocculate) into larger units (aggregates) and may thus be separated from the water.

It has previously been attempted to purify alpechin with conventional purification and flocculation compounds without any luck.

Tests with iron chloride:

When chlorides of trivalent iron are added to the water, there is usually produced flakes of iron hydroxide which make the impurities coagulate and simultaneously adsorb to the hydroxide. Tests with iron chloride for coagulating alpechin proved that the trivaleht iron immediately was reduced to divalent because of the reductive capacity of the alpechin. First at a large dilution with oxygen-containing water there were produced flakes of iron oxide and there was produced a clear liquid phase. This method requires so large quantities of water and energy for aeration that iron proved to be useless as a flocculation and coagulation compound.

Tests with aluminum sulphate:

Tests with aluminum sulphate proved that there neither here were produced flakes of aluminum hydroxide when the compound was added to undiluted alpechin. A prolonged aerating did not give any formation of flakes. Neither ozone treatment seemed to have any effect as an oxidizing compound for alpechin, and neither was there produced any improved precipitation by this treatment.

Aluminum sulphate did thus not prove to be any suitable flocculation and coagulation material for the same reasons as is indicated for iron chloride.

In addition neither iron chloride nor aluminum sulphate may be used for precipitation/coagulation if the precipitated material (sludge) is to be used as feed for animals.

Tests with sodium silicate: Sodium silicate has been tested as a coagulation material for alpechin. Added in larger amounts the whole solution coagulates to a gel which may be filtered first after stirring. At smaller additions the coagulation happens first after some time. The gel may only be sedimented or coagulated further first after thorough stirring. The silicates will make the sludge unsuited as feed for animals. Further, gel materials are difficult to process further among other things because such material consistencies will plug filters and membranes and prolonged and expensive purification processes are necessary to avoid this problem. The conclusion was that sodium silicate was useless as a precipitation agent for alpechin.

Tests with organic coagulation materials: Of organic coagulation materials there are many different types and depending on the properties which are necessary for such materials there is made a distinction between anionic, cationic and nonionic agents.

In addition to the fact that the coagulation agent for alpechin must have cationic properties on account of the charge on the surface of the colloid particles, it must also have non-toxic and non-polluting properties to be used later as inter alia feed for animals or fertilizer.

Tests have now surprisingly shown that the compound chitosan, which is a form of deacylated chitin and which formerly has been used in photographic emulsions, is very well suited for precipitation, coagulation and flocculation of alpechin. Chitosan is further a product of chitin which is a natural product, is suited for feed or fertilizer on account of its nutritional value and has additionally adsorbing properties for e ulgated oil.

In tests with chitosan it was shown that concentrations of 100-200 g/m³ fluid gave flocculation and precipitation of the colloids and materials of the alpechin. The chitosan was added in the form of an acetic acid solution for adjustment of the pH to a value in the interval 5,5-7,0, preferably pH 6,5. As a measurement for the purifying effect during the coagulation tests there has been used the total contents of organic material (TOC-values) , a value which is commensurable with both the chemical (COC-values) and the biochemical (BOC-values) oxygen consume (see fig. 7). As an example of a material which may be purified with the process according to the present invention there is given alpechin. This example has been' chosen because purification of alpechin previously has been problematic (see above) and because the alpechin has previously not been suited as a feed or fertilizer material, inter alia because of its cuprite-content and content of poly phenols which makes it toxic. This has previously represented large problems because the waste from the olive oil industry has had a tendency to accumulate without any possibility for further processing or further use. Based on this there has been performed different analyses of alpechin to determine the contents of the different components in this material.

Chemical analysis of alpechin:

There has been done chemical analysis of alpechin from three mills, two lagoons and of fresh alpechin. The result has been summarized in table 1.

Table 1: Chemical anal sis of al echin

Of the inorganic materials in alpechin potassium dominates and this is a very important nutritional compound. To determine the level more closely, the potassium content from several mills was analyzed. The results are given in table 2.

Table 2: The potassium content in alpechin.

Classic at Sevilla 4,97 g/1

Modern at Rute 3,92 g/1

Modern north of Cordoba 3,35 g/1

Modern i Puerto Gentil 4,04 g/1

The analysis shows a relatively constant level for the potassium content of about 4 g/1.

The oil content in alpechin.

The oil content in alpechin was determined by extraction with tetrachloromethane (CC1₄) . After evaporation of the tetrachloromethane the oil residues were weighed. In a sample of alpechine from a lagoon to the mills of Alcala la Real the oil content was determined to 0,018%. The result shows that the oil content was very low. The sample must accordingly have been taken just below the surface of the lagoon.

When alpechin is stored for some time (a couple of days) there is formed a crust on the surface. If samples of this crust is studied under a microscope, small drops of oil are observed. From 1 litre of alpechin having been stored in a basin, there was taken samples for oil analysis from the surface with a crust, from the bottom with sludge and additionally of a filtrated sample after coagulation. The results were: Surface with crust: 0,2%, bottom with sludge: 0,03%, after coagulation: 0,004%. The results show that to obtain the greatest quantity of oil from alpechin before membrane filtration, floatation is preferred over sedi- mentation.

The copper content in alpechin: ^•

The copper content in alpechin from a mill in Alcala la Real was determined to be 0,36 mg Cu/1. In the reductive environment in alpechin copper is present in monovalent form as crystals of copper oxide (cuprite CU2O) . Cuprite is insoluble in water and has a red colour. The red colour which may be observed on the surface of alpechinlagoons may be caused by cuprite.

In addition to the fact that copper is used against fungus attacks, copper is also toxic towards algae and other lower vegetation, but in concentrations of on tenth of what has been found in alpechin. Cuprite is present as indicated in the form of crystals and may be enriched in the sludge in the bottom of the lagoon. If this sludge is used as fertilizer it may poison the soil.

During a conference in Cordoba on May 31 - June 2, 1990 it was apparent that by using alpechin as fertilizer there was found a poisoning of the soil. The cause was claimed to be the contents of polyphenols in alpechin. Polyphenols are produced in nature and is a natural conserving agent with a temporary toxicity, whereas the toxic action of copper is permanent.

Whether fertilizer being used on trees is the real cause of the copper content of alpechin and what role the copper may have on nature will be discussed below.

The dry matter content of alpechin is determined to be about 50-60 g/1. When the copper content of alpechin was determined to about 0,36 mg/1, this corresponds to a copper content in the dry matter of about 6 mg/kg. In a sample of dry matter which was precipitated by using a coagulation agent and filtered from the aqueous phase, the copper content was determined to be 67 mg/kg, i.e. about 10 times higher than in alpechin from a lagoon where most of the sludge had sedimented. This confirms that the copper is present in crystalline form and is enriched in the bottom sludge.

Copper is an important trace metal, but is toxic in high concentrations. Microorganisms and lower vegetation has a lower tolerance for copper, whereas animals and higher vegetation have higher tolerance limits. Since alpechin may be contemplated as an animal feed material, there will below be given some tolerance limits and necessary intake of copper.

One single dose of one gram is deadly for poultry. For bovines and sheep the required daily dose of copper is considered to be 0,5 g CuS0₄ per 100 kg body weight, while a 10 times larger quantity is toxic. For horses the copper content should not exceed 8 mg/kg in the feed daily. For pigs the required copper content in the feed is 6 mg/kg.

Feed material has a content of solids of about 10%. As a ready to use feed from alpechin the copper contents should accordingly be 6,7 mg/1.

In the dry matter after the coagulation according to the invention there was found 67 mg Cu/kg. As feed the same material will contain 6,7 mg Cu/kg, and will accordingly not contain toxic amounts of copper if this material is used as a feed for animals.

The aqueous phase that is exited from the last step in the purification process according to the invention will not contain any copper.

Results from coagulation tests with alpechin:

To examine the effect of chitosan as a coagulation agent there was performed a comparison between this and the material which exhibited the second best results in the coagulation tests, i.e. silicate. The results are given in table 3.

Table 3. Results from coagulation tests:

Coagulation agent Organic carbon Purifying effect

Silicate 27,3 mg/1 15,5 %

Chitosan 20,5 mg/1 36,5 %

The results from table 3 show that chitosan gave the best coagulating effect. The precipitation was measured to 17 g dry material per litre alpechin. Another test with the above mentioned 25 1 sample, gave a precipitated amount of dry matter of 23 g/1.

These coagulation tests must be regarded to be of an orienting nature. It has been shown that when fresh alpechin is stored there happens a coagulation/denaturi-. zation of the proteins. A coagulation happens also when heating. It is thus expected that there will be found other results with fresh water/alpechin mixture directly fro the mills than with cold and old alpechin. Tests which may be important for the purification process ought accordingly to be performed on the site where the purification plant is constructed. Such tests will, however, be of a routine nature and several thereof have previously been known and used.

As mentioned above silicate as a precipitation agent will result in the forming of a gel representing a significant problem at a subsequent filtering of the aqueous phase after the flocculation step. Conversely, chitosan gives no such gel formation and it has accordingly been performed tests with a subsequent treatment in the tube flocculator 2.

Membrane filtration. There has been performed extensive laboratory tests with membrane filtering and alpechin with different membrane types and varying operating pressures (15-60 bars) .

The ideal membrane for purification/concenration of waste water from the olive oil production should be able to concentrate organic compounds while the mineral materials are passed through and remain in the clear permeate. Is preferred to remove a significant part of the organic material without simultaneously removing minerals.

It is also in this case a question of pressure-resistant membranes in the transition area between reverse osmosis and ultrafiltration, i.e. membranes being suited for pressures above 40 bars and with a molecular "cut-off" of about 600.

Such components and operating conditions will exist during the operation of the purification plant which may be used for performing the purification process according to the present invention. The above indicated data are only given as an example, and other operating conditions may be determined by the person skilled in the art depending on the properties of the material which is to be purified.

One example of the pipe dimensions in the shown purifi¬ cation plant according to the invention may be 20,3 cm (8") in diameter, and by combining couplings in parallel and series of the pressurized pipes, the plant may be operated flexibly and may have an operating capacity for a supplied amount of liquid in the area of 1-9 m³/hour. A part of the concentrate material may be recirculated in the plant.

In the plant for purification of fruit waste shown in figs. 1-4 there will below as an example be specified dimensions for the purification of alpechin with continuous operation of the plant.

The pump chamber 1 for supplied alpechin may have a surface area of 1,3 m³ with a centrifugal pump a with a capacity of 6,0 m³/hour. After the pump chamber 1 there is placed a tube flocculator 2, as shown in fig. 5, where the structure is spiral-formed with both an inlet and an outlet at the bottom of the spiral body. The flocculator 2 may of course also have another configuration and ought to be able to have an operating pressure in the pipes. After the flocculation unit 2, there is placed a flotaion unit 3 in the case of purification of alpechin, where the said flotation unit may be constructed with an area of 3,7 m². From the flotation chamber 3 the alpechin is led to a contact filter-pumping chamber 4 and 5 with a centrifugal filter-pump c and a flushing pump d with a capacity of 22 m³/hour and 190 m³/hour, respectively. From this chamber the liquid is pumped to a contactfilter 5 comprising a continuous upstream filter with an area of 2,25 m² and a surface load at 22 m³/hour of 10 m/hour. From this contact filter 5 the filtered liquid is passed back to the re- circulation chamber 4 and sludge from the filtering is passed to a sludge tank 6. From the chamber 4 the liquid is then optionally passed to a further membrane filter unit 7 with a filtering pressure of 40-60 bars and comprising as an example 4 units ø200 pressure pipes with 2 spiral elements in each pipe. The pipes may be combined via a connection in parallel or series. The total filtration surface may in this instance be 260 m². The pressure inside this membrane filtration unit 7 may be produced by using a pump f. From the filtration unit 7 liquid may be exited via an exit chamber 8, optionally containing a filter with active coal e.g. with a surface area of 1,3 m², where the surface load nay be adjusted to less than 3,2 m/hour. From each purification step excess liquid may be passed to a sludge tank 9 with an area of 4,6 m². As mentioned in the introduction a part of the purified water from this process may be returned to each step which requires supply of liquid. The pressure in each of the purification devices and steps is produced at several points by using suitable pumps (b,c,d,e,f,g,h) , said operating pressure preferably being within the interval 1-60 bars.

Addition of chemicals in each step where there is required addition of either chitosan, calcium, flocculation adjuvants etc. may be performed via reservoirs for chemicals 11 being guided by separate pumps and which also may be guided automatically via an electronic guiding unit for the process. A structure for such mixing units is shown in fig. 6.

The plant shown in figs. 1-4 may preferably be made fully automatican be guided by using a control panel with PLS. Important operating and measurement data are transferred to a control panel, and the signals are transferred to datalogs for storage and processing. The amount, pH, pressure amd temperature for the waste water is measured and registered continuously.

The above indicated example is given for treatment of alpechin, but nothing prevents that the same purification process and the same considerations are taken as a basis for purification of any type of waste material from agriculture and growth/treatment of fruit. The process according to the invention is in this case excellently suited for reducing the amount of proteins and the amount of organic material in the aqueous phase from such flushed waste material which has been mentioned initially.

A suitable form of the tube flocculator in the plant according to the invention, shown in fig. 5, may work under pressure and is constructed from a spiral-shaped tube with internal baffle plates for increasing the internal surface area and to create a longer flow distance in the floccu¬ lator. The baffle plates may be provided with penetrating apertures to increase the mixing effect between the flocculation agent and the waste water to create flakes. Inlet and outlet in the flocculator is preferably arranged in the same part of the flocculator, in which case the spiral tube forming the flocculator is wound back in a double helix so that inlet and outlet exist at the same end of the flocculator. This is, however, not a necessary feature of the flocculator.

A suitable form for the static mixing units 12 which may be used in the plant according to the invention is shown in fig. 6 and comprises a mixing chamber I containing a dual inner chamber with a funnel-shaped configuration and where the narrow part II of the funnel has a circumferentially spaced number of nozzles III, preferably 8 nozzles. Adjacent to the mixing chamber's funnel-shaped penetrating aperture there is situated at least one side-pipe IV in the outer chamber V of the mixing chamber I, through which side pipe IV there may be added the required chemicals. The fluid to which the chemicals are to be added is passed through the funnel-shaped aperture and will achieve an advantageous mixing of added chemicals when passing the funnel-shaped aperture of the funnel-shaped section and the nozzles III. On account of the narrowing form of the penetrating pipe in the mixing chamber I there is produced a kind of ejector-effect which will enhance the mixing capacity between addition/chemical fluid and main fluid downstream from the exit of the narrowing funnel-shaped section II in the mixing chamber. The mixing device has itself a pipe-formed cross-section and may optionally also be equipped with internal spiral-formed grooves or ridges which will produce a vortex in the main fluid before it flows out of the narrowing part II of the funnel-section for further to enhance the mixing effect downstream from the mixing device. The flow direction through the mixing unit is indicated with an arrow.

Claims

C l a i m s

1. Process for purifying agricultural waste liquid, preferably waste material and waste liquid from the processing of fruit, where the fruit initially has been subjected to a processing such as grinding, pressing etc., c h a r a c t e r i z e d i n that the waste material optionally is subjected to an initial separation of solid, water-insoluble material and water-soluble material by a flushing method, whereupon the aqueous fraction containing the water-soluble organic and inorganic parts from the fruit waste material is added an organic flocculating agent to precipitate the organic materials from the aqueous phase, whereupon the precipitated flakes are removed and the remaining aqueous phase is optionally subjected to filter¬ ing, preferably membrane filtering and/or filtering through active coal.

2. Process according to claim l, c h a r a c t e r i z e d i n that the flocculation agent is chitosan.

3. Process according to claim 2, c h a r a c t e r i z e d i n that chitosan is added in an amount of 100-200 g/m³, optionally together with a water- soluble calcium-compound such as calcium hypochlorite in an amount of preferably 200 g/m³, and optionally together with other flocculation-enhancing agents such as "Prastol" and/or "Zetaq", preferably in an amount between 50 and 100 g/m³.

4. Process according to any of the claims 1 - 3, c h a r a c t e r i z e d i n that the initial separation step of solid particles is performed by using mechanical de- watering such as filterpressing or centrif gation, sedi- mentation, flotation, filtering and membrane filtering.

5. Plant for performing the process according to claims 1 -

4, c h a r a c t e r i z e d i n that it comprises in a downstream sequence an inlet for fluid which is to be treated, at least one inlet tank^* (1) for collecting said fluid, a main pipe for passing the liquid flowing through a flocculator (2) with upstream devices (12) for addition of flocculating agents, preferably chitosan, calcium and/or flocculation adjuvants, at least one flotation device and/or sedimentation device (3) for removing flocculated material, and optionally further devices for filtering the remaining fluid, e.g. contact filters (4,5), membrane filters (7) and/or filters of active coal (8) .

6. Plant according to claim 5, c h a r a c t e r i z e d i n that it further comprises pump units (b,c,d,e,f,g,h) for the provision of pressure, e.g. 1-60 bars, in parts of the plant.

7. Plant according to claims 5 - 6, c h a r a c t e r i z e d i n that it comprises static mixing units (12) situated at the addition points for chemicals in the pipes, said mixing units comprising a mixing chamber (I) comprising a funnel-shaped inner liquid- providing pipe with its narrow end (II) in the flow direction for providing an ejector-effect, and an adjacent addition side pipe (IV) for addition materials such as chemicals and salts, said side pipe (IV) opening into a separate chamber (V) in the main pipe, but outside of the narrowing part (II) of the mixing chamber (I) , said separate chamber (V) running into a number of nozzles (III) located about the exit of the narrowing part (II) of the pipe for addition of chemicals to the main stream of fluid.

8. Plant according to claim 7, c h a r a c t e r i z e d i n that the narrowing part of the main pipe (II) comprises grooves and/or extending ridges being located in a spiral-formed configuration to provide a vortex in the pipe.

9. Plant according to any of the claims 5 - 8, c h a r a c t e r i z e d i n that the pipe flocculator is shaped as a spiral pipe with an- inlet and an outlet on the same side and with an internal configuration which option- ally may comprise aperture-bearing internal baffle plates.