EP3585735A1

EP3585735A1 - Method of performing chemical precipitation in water and waste water treatment plants

Info

Publication number: EP3585735A1
Application number: EP18757845.5A
Authority: EP
Inventors: Hans Ulmert
Original assignee: Ulmert Hans David Staffan
Current assignee: ULMERT, HANS
Priority date: 2017-02-21
Filing date: 2018-02-21
Publication date: 2020-01-01
Also published as: SE540532C2; WO2018156075A1; EP3585735A4; SE1700035A1

Abstract

A method of performing a precipitation process in a water or waste water treatment plant, characterized in that polymerized trivalent iron ions are used as coagulant and that the precipitation process comprises the polymerization of the trivalent iron ions to poly iron complexes in situ by the addition of OH-ions to trivalent iron ions, prior to the use of the poly iron complexes in the precipitation process,, whereby the polymerization is optimized by regulation of on one or some of the parameters reaction temperature, retention time and iron concentration so that for a given basicity the maximum charge of the complex is obtained while at the same time the flocculation ability of the complex is not inactivated, and whereby the degree of pollution in outgoing water from the precipitation step of the treatment plant is measured by on-line analysis of one or some of the parameters turbidity, colour, phosphorous, COD and TOC. A system for providing polymerized trivalent iron ions for use as coagulant in a water or waste water treatment plant is also provided.

Description

Method of performing chemical precipitation in water and waste water treatment plants

Field of the invention

The present invention relates to a method of performing chemical precipitation with iron salts in water and waste water treatment plants by regulating the basicity of the coagulant according to the preamble of the independent claim.

Background

Trivalent aluminum and iron salts (below referred to as ME-salts), such as Me-sulfate and Me- chloride, are used to purify raw water as well as waste water by means of chemical precipitation. The trivalent ion of the salts is hydrolyzed in the water and form a poorly soluble hydroxide precipitate according to:

Me ⁺ + H₂0→ Me(OH)²⁺ + H⁺

Me(OH)²⁺+ H₂0→ Me(OH)₂ ⁺+ H⁺

Me(OH)₂ ⁺+ H₂0→ Me(OH)_3(s) + H⁺

As the reaction requires that hydroxide ions are present in the water, it may, if appropriate, be necessary to add these by for example dosing sodium- or calcium hydroxide to obtain an optimal precipitation pH. During the hydroxide precipitation, flocks are formed in the water which adsorb and enclose dissolved and undissolved organic substances and particles in the water.

The formed precipitation (the sludge) can be separated from the purified water for example by filtration, sedimentation or flotation. For inter alia economic reasons, no more precipitant than needed for obtaining an acceptable quality of the purified water should be added.

A not insignificant factor is also that the amount of sludge increases with an increased addition of the coagulant and thereby also the handling and disposal costs. In some cases as much as 40 % of the formed amount of dry substance can consist of precipitated hydroxide. An optimization of the precipitation process is thus of the highest importance.

During the chemical precipitation the amount of dissolved organic substances in the water is reduced, which often affects the colour of the purified water. Thus for example the reduction of humus substances in raw water and lignin residue in waste water from pulp-mills entails a reduction of the colour in the water. The turbidity of the water is also reduced by chemical precipitation. The turbidity is caused by particles in the water. These may consist of inorganic substances, such as clays, but also organic compounds, for example remaining bacterial colonies after biological purification, emulsified oil or fibers. Together they constitute the amount of suspended solids (SS-Suspended Solids) at the same time as the organic part together with dissolved organic substance form the total amount of oxygen consuming substances in the water.

The total amount of organic carbon is analyzed as TOC (Total Organic Carbon) and the oxygen consuming substances as COD (Chemical Oxygen Demand). The analyses could be performed continuously and automatically.

In an aqueous solution of a monomeric Me(3) solution, complexes with hydroxide bridges can be created. With an increased basicity these complexes obtain increased charge according to the table below:

Aluminum sulfate and aluminum chloride are prepared by dissolving an aluminum containing material, for example aluminum hydroxide, in acid, i.e. H₂S0₄ respectively HC1 in a stoichiometric amount. An increased basicity can be obtained by using a deficit of acid in the dissolution, which then requires that the reaction occurs at increased pressure and temperature. An increased basicity can also be obtained by adding hydroxide ions to a monomelic aluminum sulfate or chloride solution.

Iron(2)sulfate is found naturally in the mineral rosenite (plagionite) and melanterite, but also as a byproduct of iron in sulfuric acid and from preparation of titanium dioxide. Iron chloride can be prepared by dissolving iron ore (Fe₃0 ) in hydrochloric acid. During the reaction equal parts of iron(2) and iron(3) chloride are formed. Both iron(2)chloride and iron(2) sulfate can thereafter be oxidized so that the divalent iron transitions to trivalent. This can occur with the help of traditional oxidants such as peroxide and chlorine gas. Both iron(3) sulfate and iron(3)chloride are sold separately or in mixture for use as coagulant in chemical precipitation. Also mixtures of iron and aluminum salts are sold for the same purpose.

Even if aluminum salts, both polymeric, i.e. as complexes with hydrogen bridges, and monomeric, have a larger market share, some treatment plants choose to precipitate with iron(3)salts. The reason for this is mainly that they form iron(3)hydroxide with concurrent low residual content of free iron ions within a larger pH range than aluminum. It may be an advantage to precipitate with iron in those cases where it is found that an optimal purifying result is obtained with pH that is higher or lower than where aluminum has its optimum.

Polymeric iron chloride or polymeric iron sulfate is not commonly available for commercial use and is therefore not commonly used for chemical precipitation in water and waste water treatment plants. The reason for this is that polymerized iron chloride or iron sulfate after some time decomposes into insoluble, and for chemical precipitation inactive iron compounds such as FeO(OH). With polymerized iron coagulant is here understood that a complex with at least two iron ions Fe₂(OH)₂ ⁴⁺, corresponding to a basicity of at least 33% and/or a charge larger than 3+, has been formed. This means that these products are less suitable for being prepared in industrial scale to thereafter be transported to and stored at the consumer.

SE 95978T3, corresponding to European patent no 0095 978 proposes, according to a disclosed embodiment, that a polymeric iron chloride could be prepared in situ by diluting a ferric chloride solution 3-75 times in water at a temperature of at least 85°C while stirring. In another embodiment, of prepar- ing the solution, the commercially available aqueous solution of ferric chloride can be heated to at least 85°Cm while agitating it, for at most 10 minutes. The method that is proposed however gives no possibility to control the basicity, and the disclosure further does not specify which basicity is achieved by the method.

As regards the in situ polymerization of aluminum salts, Swedish patent SE 536998 (appl no

1300156-5) discloses preparation of poly aluminum chloride and poly aluminum sulfate by addition of OH ions to monomeric aluminium chloride or sulfate where after 5-45 minutes reaction time stable polymeric complexes are formed. The short reaction time (response time) and the stability of the complexes makes it possible to control the basicity and aluminum dosing with reference to on-line measuring of the degree of pollution. Thus the precipitation process may be optimized.

This Swedish patent however only concerns aluminum based coagulants. The preparation of poly iron is as previously stated more complex.

Accordingly, it is an object of the present invention to facilitate, enable, or provide a method for, the use of polymerized iron coagulants in chemical precipitation in water and waste water treatment plants.

It is a further object of the present invention to provide a method of optimizing the polymerization of trivalent iron ions in situ.

Summary of the invention

At least one of the above objects, or at least one of further objects which will be evident from the below description, are according to a first aspect of the present invention achieved by a method of performing a precipitation process in a water or waste water treatment plant, characterized in that polymerized trivalent iron ions are used as coagulant and that the precipitation process comprises the polymerization of the trivalent iron ions to poly iron complexes in situ by the addition of OH-ions to trivalent iron ions, prior to the use of the poly iron complexes in the precipitation process, whereby the polymerization is optimized by regulation of one or some of the parameters reaction temperature, retention time and iron concentration so that for a given basicity the maximum charge of the complex is obtained while at the same time the flocculation ability of the complex is not inactivated, and whereby the degree of pollution in outgoing water from the precipitation step of the treatment plant is measured by on-line analysis of one or some of the parameters turbidity, colour, phosphorous, COD and TOC.

Thus the present invention is at least in part based on the realization by the present inventor that the polymerization rate, i.e. the time it takes for a maximum degree of polymerization to be achieved, is dependent on the iron concentration, temperature and basicity. Thus, in contrast to the polymerization of aluminum ions, which as discussed above provide stable complexes in a short reaction time, i.e. 5- 45 minutes, for polymerization of iron ions one or some of the parameters reaction temperature, retention time and iron concentration needs to be regulated if the desired polymerization and maximum charge is to be achieved while at the same time a deactivation of the flocculation ability should not start. The reaction time, i.e. retention time, should be long enough that the desired complexes, with their basicity and charge, are fully formed. Thus, for obtaining complexes with a desired basicity and charge a certain amount of reactants in the form of trivalent iron ions and OH-ions are reacted. The reaction time, i.e. retention time, must then be long enough that the reaction proceeds to its conclusion, i.e. until the complexes with the desired basicity and charge are formed. If the reaction time is too short there is a risk that only part of the reactants will have reacted and that any formed complexes have a basicity and charge that is lower than the desired basicity and desired charge. If, on the other hand the reaction, i.e. retention time, is too long the complexes with the desired basicity and charge will start to decompose so that the flocculation ability of the complexes is inactivated or lost.

An increase of any of the parameters entails a decreased polymerization time. After the degree of polymerization which corresponds to the desired ratio OH/Fe has been achieved a deactivation of the flocculation ability of the iron begins. In order to hold the reaction time constant at varying basicity and dosed iron amounts either the reaction temperature or the iron concentration, or both, can be varied. It is also possible to vary the retention time so that it becomes shorter at high basicity and vice versa. The key is to ensure that desired polymerization is achieved while at the same time a deactivation of the flocculation ability should not start.

In one embodiment the retention time is varied, for example by varying the reactor volume, however, then one must take into account that the reaction time, i.e. retention time, may amount to several hours at low basicity. If a changed degree of pollution requires, for an optimal purifying result to be achieved, that the basicity is low, the response time of the change will be long. The latter means that optimum basicity cannot be achieved fast enough, i.e. it may take several hours to reach the optimum (low) basicity for the water to be purified, and thereby the time to reach an optimal purification effect is delayed.

According to another embodiment this problem can be solved by solely or additionally regulating, in this case increasing the reaction temperature whereby the reaction rate increases and the response time decreases.

In embodiments of the method the polymerization of trivalent iron ions results in a poly iron complex, i.e. a polymerized trivalent iron ion, with higher charge than 3+. The basicity of the poly iron complex may thus be at least 33 %, corresponding to the complex Fe₂(OH)₂ ⁴⁺ which has the charge 4+, or higher complexes. The formed complex is used to optimize the precipitation process in water and waste water treatment plants with respect to the degree of pollution in the clarified water phase after precipitation, operational cost and sludge production. This is obtained by a regulation of the parameters addition of iron and basicity, and where applicable also pH.

The polymerization being optimized by regulation of one or some of the parameters reaction temperature, retention time and iron concentration so that for a given basicity the maximum charge of the complex is obtained while at the same time the flocculation ability of the complex is not inactivated.

In other words the polymerization is performed so that for a given basicity, i.e. when the precipitation process in the water or waste water treatment plant requires a certain given basicity of the coagulant based on the properties of the water to be precipitated in the precipitation process at that time, the polymerization process is allowed to proceed so that the maximum charge of the complex, and thus also the basicity, is obtained while at the same time the flocculation ability of the complex is not inactivated. This requires regulating one or some of the parameters reaction temperature, e.g. the temperature in the reaction vessel or solution where trivalent iron ions are polymerized to poly iron complexes, retention time, e.g. the time available for the trivalent iron ions to be polymerized before they are used, and the iron concentration, e.g. the concentration of the trivalent iron ions, so that the reaction can proceed until poly iron complexes of the desired basicity and hence the desired charge, are ob- tained.

Thus, for a given desired basicity, if a given amount of OH ions, that is added to a given amount of (monomeric, basicity = 0) trivalent iron ions, is to result in the polymerization and formation of poly iron complexes with the maximum charge possible for that amount of OH ions and trivalent iron ions, i.e. for the given desired basicity, then one or some of the parameters reaction temperature, retention time and iron concentration are according to the method regulated so that complexes with the maximum charge, i.e. the highest possible basicity, for these amounts of OH ion and iron ions, are formed, while at the same time the flocculation ability of the complexes is not inactivated.

In order to detect whether or not the flocculation ability of the complex is inactivated or not, the degree of pollution in outgoing water from the precipitation step of the treatment plant is measured by on-line analysis of one or some of the parameters turbidity, colour, phosphorous, COD and TOC. If the flocculation ability of the complex has been inactivated, this will result in an increased degree of pollution in the outgoing water.

It is also contemplated that other methods can be used to determine if the flocculation ability of the complex has been inactivated. One contemplated example is to analyse the conductivity of the coagulant after it has been formed from the polymerization of the trivalent iron ion and before it is used in the precipitation process.

The polymerization of the trivalent iron ions to poly iron complexes is performed at a temperature, i.e. reaction temperature, of 0 to 80°C, such as 10 to 70°C, 10 to 60°C, 20 to 70°C or 20 to 60°C. Further, where an increased reaction rate is needed the reaction temperature may be at least 20°C and at the most 80°C.

In an especially advantageous embodiment the degree of pollution is measured by on-line analysis of one or some of the parameters turbidity, colour, phosphorous, COD and TOC in outgoing and/or incoming water from the precipitation step of the treatment plant.

In some embodiments of the method the optimization of the polymerization takes into account operation cost including sludge production. Thus, for a given degree of pollution in the outgoing water from the precipitation step of the treatment plant the polymerization should be performed so, by regulating one or some of the parameter reaction temperature, retention time and iron concentration, the operation cost, including the cost for disposal of sludge, becomes low. If for example the retention time is not high enough the polymerization of the trivalent iron ions will not proceed full to form the poly iron complexes, in this case the coagulant that is used in the precipitation may contain unreacted trivalent iron ions. To reach the goal set for the degree of pollution in the outgoing water from the precipitation step the amount of coagulant may then need to be increased which increases the consumption and cost of the trivalent iron ions and also the sludge production and sludge disposal costs.

In some embodiments of the method the precipitation process and/or optimization of the polymerization comprises that the basicity (degree of polymerization) and the dosing of the coagulant is regulated based on flow and based on on-line measurements of the degree of pollution in incoming untreated water and/or in the clear water phase and where the regulation also takes into account stored regulation data and water temperature. The taking into account of stored regulation data and water temperature data may comprise looking up, in a table associating regulation data for the basicity and dosing with water temperature data, a water temperature measured online in the incoming untreated water and/or in the clear water phase and obtaining the regulation data associated with the water temperature data.

Here the dosing of the coagulant refers to the dosing of the polymerized trivalent iron ions, i.e. the poly iron complexes.

In some embodiments of the method measurements of the degree of pollution are made by measuring one or some of the parameter turbidity, colour COD, TOC and phosphorous.

In embodiments of the method the basicity is regulated through the addition of OH-ions. The OH-ions may be provided by sodium hydroxide, calcium hydroxide or magnesium hydroxide.

In some embodiments of the method the regulation of the basicity by OH-ions is made in a pressurized system. This is advantageous because by pressurizing the system the formed polymerized trivalent iron ions, i.e. the coagulant, which has a high viscosity and therefore may be difficult to pump, can be transported to the precipitation process where the coagulant is used, by the pressure. The pressure may thus be used to force the coagulant from inter alia one or more of a mixing tank, on or more reaction tanks, and the accompanying pipes connecting the tanks and or leading to the point where the coagulant is added to the incoming untreated water for the precipitation process.

At least one of the abovementioned objects is further obtained by a second aspect of the present invention pertaining to a system for providing polymerized trivalent iron ions for use as coagulant in a water or waste water treatment plant, the system being installed in situ in the water or waste water treatment plant, the system comprising:

- a mixing tank provided with a stirrer for mixing trivalent iron ions with OH-ions to produce a mixture in which the trivalent iron ions are polymerized to poly iron complexes, the mixing tank being provided with means, such as pumps, for controlling the amount of trivalent iron ions and OH ions that enter the mixing tank,

- a plurality of reaction tanks connected to the mixing tank for receiving the mixture from the mixing tank and for holding the mixture while the trivalent iron ions are polymerized to poly iron complexes, and

- an outlet connected to at least one of the reaction tanks for discharging the polymerized trivalent iron ions for use as coagulant in the water or waste water treatment plant,

wherein the mixing tank and/or the reaction tanks are provided with means, such as a combined heating/cooling jacket, for regulating the reaction temperature, and/or

wherein the reaction tanks are connected to the mixing tank such that the mixture can bypass one or more of the reaction tanks as it flows from the mixing tank to the outlet for regulating the retention time of the mixture in the reaction tanks.

The system may further comprise means for analyzing whether the flocculation ability of the polymerized trivalent iron ions, i.e. the poly iron complexes, has been inactivated, for example sensors for online measurements of the degree of pollution of the outgoing treated water as described above, and/or sensors for analyzing the polymerized trivalent iron ions in the reaction tanks and/or the outlet.

For both the method and the system a computer program, where the user ^'s current costs for hydroxide, coagulant and disposal of sludge has been input together with data from pollution measurements and water temperature, can be used to regulate basicity and coagulant dosing, optionally also regulate the reaction temperature, retention time and iron concentration, so that an optimal purification result is obtained and provide the answer where the breaking point is between increased and decreased basicity, respectively, versus an increased and decreased coagulant dosing, respectively, lies. Reaching the set purification requirements of course overrides the operational cost. As each separate water often has specific flocculation properties and the requirements for treated water is different, experience values regarding these properties and set purification requirements must be included in the program.

The method according to the first aspect of the present invention may in an alternative aspect be formulated as a method of performing a precipitation process in a water or waste water treatment plant, comprising the steps of:

i) providing a solution of trivalent iron-ions and a solution of hydroxide-ions to a mixing tank;

ii) adjusting the flow rates of the solutions, whereby the ratio of hydroxide-ions to iron-ions is substantially constant and according to a predetermined basicity to be obtained, the basicity corresponding to the degree of polymerization of polymerized iron complexes formed by the reaction of the trivalent iron ions with the hydroxide ions ;

iii) adjusting one or more of the temperature of the mixing tank, the retention time in the mixing tank, and the iron concentration of the solution of trivalent iron-ions so that the polymerized iron complexes in the mixing tank obtain the predetermined basicity while at the same time the flocculation ability of the complexes is not inactivated,

iv) using the polymerized iron complexes as coagulant in the precipitation process in the water or waste water treatment plant,

v) obtaining a measurement of at least one of the parameters turbidity, colour, phosphorus, COD and TOC of water treated by the precipitation process, and using the measurement to obtain a first indication or first determination whether the flocculation ability of the polymerized iron complexes has been inactivated, and adjusting the one or more of the temperature of the mixing tank, the retention time in the mixing tank, and the iron concentration of the solution of trivalent iron-ions in step (iii) based on the first indication or first determination.

The measurement may also be used to obtain a second indication or second determination whether the polymerized iron complexes in the mixing tank have obtained the predetermined basicity, and wherein the adjusting of the one or more of the temperature of the mixing tank, the retention time in the mixing tank, and the iron concentration of the solution of trivalent iron-ions in step (iii) is further made based on the second indication or second determination.

The polymerized iron complexes may be diluted with water before being used as coagulant.

The retention time is adjusted by adjusting the flow rate of the solutions, or by addition or removal of one or several further tanks to the mixing tank.

Brief description of the drawing

A more complete understanding of the abovementioned and other features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawing, wherein Fig. 1 describes embodiments of the method according to the first aspect of the invention as well as embodiments of a system according to the second aspect of the invention. Detailed description

Below, embodiments of the method and system according to the first and second aspects of the invention will be described. These embodiments are described in illustrating purpose in order to enable a skilled person to carry out the invention.

The description in the text refers to Fig. 1. Existing equipment for dosing of coagulant solution, i.e. the dosing that has previously taken place directly to the water of waste water that is to be precipitated chemically, may be used. However, a system according to the second aspect of the present invention is shown in Fig. 1 for performing the method.

A coagulant solution 1 is provided containing an iron(3) salt, for example monomeric iron sulfate or iron chloride. The solution 1 is pumped by a pump 2 to a mixing tank 3. The added amount of flow and iron content should be known or predetermined.

A solution 5 or suspension containing OH ions is also continuously pumped by a pump 4 to the mixing tank 3. The solution/suspension for example consists of or comprises sodium hydroxide, calcium hydroxide or magnesium hydroxide. The flow of the suspension to the reaction tank is regulated in relation to the pumped flow of iron salt solution 1 so that the amount of added OH ions corresponds to the desired basicity of the iron hydroxide complexes that are formed when the coagulant solution 1 and the solution/suspension 5 is mixed and allowed to react.

The mixing of solution and suspension takes place by an intensive stirrer 6 placed or arranged in the mixing tank 3. The mixture is thereafter led to one or more reaction tanks 7a-7d where the final formation of polymeric, i.e. polymerized, iron complex takes place. The required reaction time is dependent on inter alia temperature, iron content and basicity. The higher the basicity, i.e. the more OH- ions that are added, and temperature, the faster the reaction, i.e. the polymerization takes place.

The polymerization/reaction rate can therefore be affected or adjusted by cooling or heating the mixture. In order to provide control of the temperature (and thus the reaction time), at least one of the reaction vessel, i.e. reaction tanks 7a-7d, and the mixing vessel i.e. the mixing tank 3, may be provided with a combined heating/cooling jacket 8a-8e as shown in Fig. 1 for controlling the temperature thereof.

Although Fig. 1 shows the use of four reaction tanks 7a-7d, embodiments of the method according to the first aspect of the invention may use fewer, such as one, two or three, or more, such as five or six, reaction tanks. Furthermore, the reaction tanks may have different sizes, and depending on the desired reaction time, i.e. retention time needed for the polymerization at different basicity the mixture from the mixing tank 3 may, when longer retention times are needed, be led through all or a plurality of reaction tanks 7a-7d, while, for shorter retention times, be led through only two reaction tanks 7a and 7d bypassing the others, as shown by the optional routing 7e from the outlet of the first reaction tank 7a to the inlet of the last reaction tank 7d.

Both mixing 3 and reaction tank(s) 7a-d may advantageously be closed and pressurized to overcome the problem that the product, i.e. the polymerized iron complex, may be gel-like and thereby difficult to dose and pump. The pressure, i.e. overpressure, is advantageously created by the pumps that control the dosing of the iron and the OH solutions. The gel, i.e. the polymerized iron complex, 9a can thereby be pressed out of the reaction step, i.e. from the reaction tank 7d, by the overpressure and be fed to the inmixing tank (13) of the treatment plant to there, by the help of an intensive stirrer (14) be mixed with the water (15) that is to be purified by chemical precipitation.

Alternatively, or additionally as is shown in Fig. 1, there is also a possibility to dilute the iron coagulant 9b, i.e. the polymerized iron complex, with pure water 10 in a separate tank 11 provided with a stirrer 12 before the iron coagulant is introduced into the inmixing tank 13 of the treatment plant.

In the inmixing tank 13, micro flocks are formed which in the following flocculation tank(s) 16a, 16b are developed to larger flocks. The flocculation tank(s) 16a, 16b, are usually provided with a slow- moving flocculation stirrer 17a, 17b. The flocculated water is thereafter led into a sedimentation tank 18 where a separation of the flocks occurs, whereby a sludge 19 and a clear water phase 20 are formed. The clear water phase is obtained at the surface 21 of the water.

With the prevailing techniques the one who has chosen to precipitate with an iron based coagulant can often only adjust the coagulant dose (g Fe/m3) to optimize the purification result and operational cost. The coagulant dosing is adjusted, either manually or automatically (on-line), dependent on the purification result. In order to achieve a fast and simple indication of the purification result, the colour and/or turbidity in the water after the flock separation can be continuously measured. The purification result can also be measured by analyzing P, COD or TOC. These measurement results can also be supplemented with measurements on incoming water, which can give an early indication that the coagulant dosing and/or basicity may need to be adjusted.

The method according to the first aspect of the invention however entails that a second parameter is introduced, the basicity of the coagulant, with the possibility of regulating the basicity in situ and at need. The invention now enables plants to test and use a polymer iron coagulant in large scale.

A further aspect of the present invention concerns a method of performing a precipitation process in a water or waste water treatment plant, wherein polymerized trivalent iron ions are used as coagulant and that the precipitation process comprises the polymerization of the trivalent iron ions in situ, whereby the polymerization is optimized based on one or some of the parameters reaction temperature, reaction time and iron concentration.

The optimization of the polymerization may take into account operation cost including sludge production.

The precipitation process and/or optimization of the polymerization may comprise that the basicity (degree of polymerization) and the dosing of the coagulant is regulated by flow and by on-line measurements of the degree of pollution in incoming untreated water and/or in the clear water phase and that the regulation also takes into account stored regulation data and water temperature.

Measurements of the degree of pollution are made by measuring one or some of the parameter turbidity, colour, COD, TOC and phosphorous.

The basicity is regulated through the addition of OH-ions.

The regulation of the basicity (i.e. addition of OH-ions) may be made in a pressurized system. Feasible modifications of the Invention

The invention is not limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent application is intended to encompass all adjustments and variants of the preferred embodiments described herein, thus the present invention is defined and limited only by the wording of the appended claims and the equivalents thereof. Thus, the equipment may be modified in all kinds of ways within the scope of the appended claims. It shall also be pointed out that all information about/concerning terms such as above, under, upper, lower, etc., shall be interpreted/read having the equipment oriented according to the figures, having the drawings oriented such that the references can be properly read. Thus, such terms only indicates mutual relations in the shown embodiments, which relations may be changed if the inventive equipment is provided with another structure/design. Reference signs in claims, where provided, are provided for pointing out examples and are not limiting the scope of the claims. It shall also be pointed out that even thus it is not explicitly stated that features from a specific embodiment may be combined with features from another embodiment, the combination shall be considered obvious, if the combination is possible. Throughout this specification and the claims which follows, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer, feature or steps or group of integers, features or steps but not the exclusion of any other integer, feature or step or group of integers, features or steps. Likewise terms such as "a", "an", "first", "second" etc. do not exclude a plurality.

Claims

1. A method of performing a precipitation process in a water or waste water treatment plant, characterized in that polymerized trivalent iron ions are used as coagulant and that the precipitation process comprises the polymerization of the trivalent iron ions to poly iron complexes in situ by the addition of OH-ions to trivalent iron ions, prior to the use of the poly iron complexes in the precipitation process, whereby the polymerization is optimized by regulation of on one or some of the parameters reaction temperature, retention time and iron concentration so that for a given basicity the maximum charge of the complex is obtained while at the same time the flocculation ability of the complex is not inactivated,

and whereby the degree of pollution in outgoing water from the precipitation step of the treatment plant is measured by on-line analysis of one or some of the parameters turbidity, colour, phosphorous, COD and TOC.

2. Method according to claim 1, characterized in that the optimization of the polymerization takes into account operation cost including sludge production.

3. Method according to any preceding claims, characterized in that the precipitation process and/or optimization of the polymerization comprises that the basicity (degree of polymerization) and the dosing of the coagulant is regulated based on flow and based on on-line measurements of the degree of pollution in incoming untreated water and/or in the clear water phase and where the regulation also takes into account stored regulation data and water temperature.

4. Method according to claim 3, characterized in that measurements of the degree of pollution are made by measuring one or some of the parameter turbidity, colour COD, TOC and phosphorous.

5. Method according to any preceding claim, characterized in that the regulation of the basicity by OH-ions is made in a pressurized system.

6. Method according to any preceding claim, characterized in that the polymerized trivalent iron ions are diluted with water before being used in the precipitation process.

7. A system for providing polymerized trivalent iron ions for use as coagulant in a water or waste water treatment plant, the system being installed in situ in the water or waste water treatment plant, the system comprising:

- a mixing tank (3) provided with a stirrer (6) for mixing trivalent iron ions (1) with OH-ions (5) to produce a mixture in which the trivalent iron ions are polymerized to poly iron complexes, the mixing tank being provided with means, such as pumps (2, 4), for controlling the amount of trivalent iron ions and OH ions that enter the mixing tank,

- a plurality of reaction tanks (7a-7d) connected to the mixing tank for receiving the mixture from the mixing tank and for holding the mixture while the trivalent iron ions are polymerized to poly iron complexes, and