FIELD OF THE INVENTION

The invention relates to a method for decalcifying water, in particular effluent of a water purification process, namely, biologically purified wastewater for the purpose of replacing groundwater or surface water. Preferably, this method is used in the paper industry.

BACKGROUND OF THE INVENTION

In the industry, particularly the paper industry, reuse of process water is increasingly important. After internal reuse has been optimised and thereby the fresh water use (spring or surface water) has been reduced to a (too) far-reaching extent, the demand for biologically purified process water becomes large. As the process water system of a paper mill is closed further, the calcium concentration increases by acidification (rot by bacteria) therein. In the biological purification, only a part of the calcium

concentration is removed.

Reuse of partially purified water with or without partial softening is already being applied in various paper mills at less critical points. The quahty (temperature and residual pollution) of this water, however, does not make it possible to replace the employment of fresh water (ground and/or surface water) to a far-reaching extent. In this regard, primarily, the presence of calcium is the biggest problem because - at the current critical points of large-scale fresh water use— deposits of calcium (originating from calcium carbonate and calcium sulphate) cannot be accepted. In addition to replacing fresh water, reuse of biologically cleaned water can provide a possibility of lowering organic dirt load in the process water system by dilution. While this does not result in a lowering of the fresh water use, it does reduce the dissolution of CaCOe. Consequently, the concentration of calcium and the precipitation of calcium carbonate will present fewer problems both in the paper mill and in the purification process.

The large amount of low-grade heat contained in the water could be used for the heating of houses and offices. Application of heat exchangers is required then, but these calcify if the hardness of the water is too high.

Particular attention is to be paid to the salt load in the process water system of the paper mill. Along with the raw materials and

auxiliaries that are used in the paper industry, various salts enter the process water system. In that connection, typically, the concentration of chloride is limited to avoid corrosion of the (stainless steel) machinery.

Membrane removal of chloride is not possible if a high concentration of lime is present in the water.

There are various ways of softening water and so also effluent of a water purification. The trick, however, is to do this in a way proving to be so low in both investment and operational costs that reuse of water is not only integrally sustainable but also economically sound. Also, it is important that the softening be carried through so far that the softened water can serve as a replacement for ground and/or surface water and so meets high quality standards.

Methods for decalcifying water are known, e.g., from EP 1 120 380,

NL 7611903 and EP 0 335 043 Al. All these documents describe a method of decalcifying in which compressed air, via a diffuser or perforated plate / membrane, is contacted with water, whereby CO2 which is dissolved in the water is emitted to the compressed air, as a result of which CaC03 (and/or MgCOe and/or CaS04) precipitates from the water, and is removed. Here, recirculation of lime crystals may be applied, so that a crystallisation surface is presented and the crystallisation process proceeds more

effectively. The drawback of the methods described in these documents is that, firstly, the operational costs are high due to calcification of aeration elements, tanks, and the like, and, secondly, insufficient crystallisation surface is presented.

Also known is the Crystalactor®, an effectively working softening plant, i.e., a high degree of water quality can be achieved with it, which operates on the principle of crystallisation in a pellet reactor. But here too, there are various inherent disadvantages. Firstly, the investment is relatively high. Secondly, the crystallisation takes place on sand grains, so that the lime that is discharged is not pure and therefore has a lower (sustainability) value and sand needs to be supplemented continually.

A third disadvantage is the dosage of lye, being the driving force to raise the pH of the water and hence the crystallisation of the lime on the sand grains. The price of this lye makes for relatively high operational costs. If sodium hydroxide is used as lye, the softened water will eventually contain more sodium ions, which means one salt ion is being replaced with another.

Application of hme milk does not entail the drawback of introducing other ions into the water, but an expensive dosing installation is required and all calcium that is introduced with the hme milk also needs to be discharged again as calcium carbonate. For that matter, having regard to the

sustainability principle, application of chemicals is questionable anyhow.

Accordingly, there is still a need for an improved system and method for an effective water purification, more specifically, water decalcification.

SUMMARY

The present invention has for its object the provision of a method for removing lime from water that is low in investment and operational costs, yields a water quality that can replace fresh water and is integrally sustainable.

This object is achieved in the present invention by utilising a method whereby CO2 is driven from the water by stripping, whereby the stripping process is carried out in a series-arranged number of reactors having therein a high concentration of seed crystals without utilisation of compressed air or chemicals, has low maintenance and operational costs, and produces a pure lime.

The present invention now provides a method for removing lime and/or other pollutants from water, comprising the following steps:

a) treating water in a reactor vessel by mechanical surface CO2 stripping;

b) conducting the treated water to a next reactor vessel;

c) treating the treated water with mechanical surface CO2 stripping in the next reactor vessel; and

d) optionally repeating steps b) and c).

Also, the invention comprises an apparatus in which the above- mentioned method can be carried out. This installation also comprises means for the discharge of the Hme removed from the water. In practice, the method can be expanded to include an acidification step, a filtration step and a disinfection step, and the installation to include means for these steps.

DETAILED DESCRIPTION

The invention comprises a method whereby CO2 is driven from the water by stripping, whereby the stripping process is carried out in a series- connected number of tanks. The CO2 stripping takes place in that the water is mechanically put into turbulent motion, as a result of which a large amount of air is mixed into the water. An additional benefit involved here is that this makes it possible for a high concentration of seed crystals, which is present in the water, to remain in suspension. By recirculation of the seed crystals, they grow, so that they are simple to separate from the water and the separated lime can be easily removed from the system in a condition of shovel consistency. Such a method and installation have the advantage of requiring little maintenance, little energy and no chemicals, and therefore being suitable for large volume flows.

The present invention accordingly provides a method for removing lime from water, comprising the following steps

a. treating water in a reactor vessel with a mechanical surface C0 ₂ stripper (MOS)

b. conducting the treated water to a next reactor vessel; c. treating the treated water with an MOS in the next reactor vessel; and

d. optionally repeating steps b) and c).

Additionally, the following process steps are carried out:

e. introducing and maintaining a high seed crystal concentration in the water to be treated

f. discharging lime from the installation

The step of treating the water with a mechanical surface CO2 stripper (MOS) involves putting the water into turbulent motion by causing a motion of the water just below the surface and simultaneously drawing in (headspace) air present above it. Putting the water into motion causes an intensive mixing of the (headspace) air and the water to a point deeply below the water surface level in the reactor tank. Putting the water into motion is preferably done by a rotor with blades, the shape of the blades being chosen such that a substantially rotating circumaxial circulation of the water is set going, so that the entire contents of the reactor uniformly come into contact with the air drawn into the liquid by the rotor.

The water to be treated preferably originates from an industrial process, for example paper making, and has passed a preferably biological water treatment plant before it can be conveyed to the softening plant of the present invention. This supplying pipe system and the pipe system that is part of the installation as described herein should preferably be as short as possible and preferably be made of HDPE (high density polyethylene) or one or more other lime sediment repellent materials to prevent the occurring deposition of lime (scahng) and, if occurring nonetheless, to be able to simply remove this lime scale or precipitate. If the water to be softened needs to be lifted to the reactor(s), preferably a pumping unit is used. With water to be softened having a calcium concentration of more than 200 mg/1, pumps of the centrifugal type will rapidly become less effective as a result of scaling. Regular but unwanted maintenance is necessary then. For the present application, however, a centrifugal pump can be used if seed crystals are added in the suction of the pump. These prevent scaling in the pump by their abrasive action and by presenting crystallisation surface. Pipes are preferably tightly dimensioned so that high flow velocities occur and settlement of lime in and/or scaling on inner walls of pipes are prevented. For the main pipe, velocities of 1-1.5 m/s are recommended, for the seed crystal recirculation pipe 1.5-2.5 m/s. The reactor must be of robust construction to be able to take up the water and turbulence forces of the MOS but does not otherwise call for application of special materials. The reactors are preferably arranged in the ground for the most part to avoid costly aboveground constructions.

The shape of the reactor may be chosen freely, while the size of the reactor, in correlation with the residence time of the water and other parameters, is determinative of the capacity for an efficient water softening. From a structural point of view, it is advantageous if the reactors are virtually identical but this is not requisite, so that use can be made of available tanks. The transition from one reactor to another proceeds with the help of pipes as also discussed above with respect to the supply. What is important is a high degree of plug flow through the reactor(s). Preferably but not necessarily, the installation proper consists of a series of two or more preferably virtually identical reactors, which are connected in series (i.e., the supply of a next reactor is coupled to the discharge of a preceding reactor). At the end of the series, the installation is completed with a lime/water separator (settling system). The reactors preferably have a content such that the water resides in the reactor for 30-60 minutes. The depth depends on the MOS used, which may lead to inefficiency if a basin is too shallow and to insufficient mixing action if a basin is too deep. Depending on the water to be softened, the number of series-arranged reactors and the size thereof should be so chosen as to bring about sufficient softening. Determinative of a sufficient degree of softening are the crystalhsation surface presented there and the stripping efficiency of the MOS. With an input calcium content of 300 mg/1, a set of three to five series-arranged reactors as further described herein has proven sufficient to decalcify the water to a far-reaching extent. In all reactors, precipitation of CaCOe will take place throughout the tank, because the MOS brings air deeply into the tank into intensive contact with the seed crystals suspended in the water. Everywhere in the tank, CO2 will strip to the air and thus form the driving force for the precipitation. The seed crystals prevent scaling both on the reactor tank walls and on the MOS and so these remain

maintenance-free.

Stripping arises as a result of a difference in CO2 concentration in the air with respect to that in the water, these concentrations being virtually equal under stable conditions. The driving force to remove CO2 is hence very small, and therefore has an effect only with a large air supply.

The concentration of seed crystals is kept at 5-100 g/1, preferably at 20 to 80 g/1 and most preferably at 30-50 g/1 in the reactors. Preferably the highest concentration that can still be kept in suspension by the MOS. This concentration can be simply determined experimentally by a person skilled in the art and depends on the geometry of the reactor, the MOS used, and the parameters with which this MOS operates (for example, speed). The MOS is in principle an electrically driven rotor, consisting of a drive shaft and rotor blades or vanes thereon, which is mounted just below the surface level of the water in the reactor, such that all parts of the rotor are below the surface and - at rest - are covered with a thin film of water. The rotor blades of the MOS, due to their shape and rotational speed, ensure an intensive mixing of air and water to a point deeply below the water level in the reactor tank. As a result, CO2 stripping occurs throughout the reactor. The shape of the rotor and the blades is chosen such that a liquid circulation rotating around the axis of the MOS occurs, so that the whole reactor contents uniformly come into contact with the air drawn into the liquid by the rotor. Openings in the rotor deck ensure that the reduced pressure arising behind a rotor blade is equalised, which lowers the driving energy. The rotor of the MOS is driven at a speed of 25-200 rpm, preferably 50-100 rpm, more preferably around 65-75 rpm. On the bottom of the reactor, a baffle or water deflector is mounted to avoid the water coming in a circular movement from the top to the bottom, as a result of which the effectiveness of the CO2 stripping could decrease. The shape of the reactor may, in principle, be chosen freely, with the proviso that a residence time per reactor of more than half an hour, but preferably from 0.5 to 1 hour, is desired and the surface of the reactor is sufficiently large to bring in sufficient air volume (at a flow rate of 200 m ³ /h, therefore, a total content of 100 times the number of reactors (for example, five), that is, around 500 m ³ is necessary then) and the shape of tank and MOS are such that the water in the tank can be stirred well and hence mixed well. The reactor surface must be large enough to allow the MOS to work efficiently. A smaller reactor calls for a smaller MOS. The depth of the reactor measured from the underside of the rotor should be 1 meter minimum and 4 meters maximum. A small depth leads to release of energy on the bottom of the reactor that is converted into an unwanted reaction force on the bearing construction of the MOS. Too great a depth can lead to insufficient turbulence in the whole of the reactor so that seed crystals are not, or insufficiently so, kept in suspension. An MOS has an air mix-in capacity of around 300-400 m ³ of air/kW of installed power. The MOS is preferably mounted on a bridge which is set up over the reactor. The distance from the bridge to the top of the rotor is chosen such that splashing water with suspended lime particles does not end up against the bridge. In practice, that distance will be about 1 meter. The MOS is so positioned relative to the water level in the reactor that the flat part of the rotor sits approximately at or up to 50 mm below the (stationary) liquid level in the reactor. The transition from one reactor to another proceeds with the help of pipes such as have also been discussed above with regard to the supply.

A passage from one tank to another, or from one compartment to another, should be large enough to allow the flow to pass, but not too large either, so as to prevent the compartmentation of the reactors being undone.

A lime/water separator is arranged after the last reactor.

This can be a normal settling tank with clearing installation or a parallel plate separator. If, rather than a radical softening, a partial softening is opted for, it is necessary, when choosing a lime/water separator, to take the residual scaling properties of the (partially) softened water into account. When softening has been radical, this is less critical. With a slight dosage of CO2 or other acid, scaling on the lime/water separator can be prevented without calcium redissolving.

The lime is discharged at the bottom of the (settling) tank by a continuously operating pump. The pump must operate continuously to prevent clogging on the suction side of the pump. The discharge is for the most part recycled to the first reactor as recirculation of seed crystals. The hourly produced lime particles are discharged as lime drain-off with an endless screw system, such that excess water can flow back to the settler and lime of shovel consistency is screw-conveyed to the lime depot. The floor of the lime depot is preferably constructed at an angle of about 3° and terminates in a gutter. Thus, after leak-out of water, the dry matter content of the lime can increase further. A rootlet above the lime depot prevents remoistening.

A method according to the invention further has the advantage over the known prior art that due to the reactors being arranged in series, a quasi plug flow arises whereby the incoming water with a relatively high bicarbonate content has only limited influence on the situation in the next reactor. Consequently, the pH can rise per reactor and come closer to the ultimate pH value of 8.4 at which in the Ca, CaC03, CO2, HCO3 equilibrium a complete CaCOe does not take part anymore because it has precipitated.

In practice, the process will proceed as follows. The supply pump can be directly switched on to full capacity, at simultaneous maximum speed of all MOS units. Directly, lime will precipitate, which is preferably completely recirculated as seed crystals until the desired lime concentration in the reactors has been achieved. During the increase of the seed crystal concentration, the lime concentration in the effluent will decrease steadily. In-line pH measurement, measurement of the conductivity, and measuring the calcium concentrations after the last reactor or after the settler can provide for process monitoring, whereby the speed of the MOS can be regulated based on the desired final calcium concentration or on the amount of influent.

The high seed crystal concentration in the reactors also provides the advantage that from the residual concentration of phosphate in the purified wastewater (the water to be softened) calcium phosphate is formed, which adheres to the crystals and is therewith removed from the water. Phosphate as a nutrient for bacteria is not wanted in the process water of a paper mill. The large extent of air induction by the MOS provides that the residual concentration of nitrite compounds in the purified wastewater is converted into nitrate. Nitrate as an oxygen-donating substance can have a positive effect on the anoxic or anaerobic process water system of a paper mill.

The invention further relates to an apparatus for decalcifying water.

This object is achieved in the present invention by utilising an apparatus comprising two or more series-arranged reactors (3), with a baffle

(5) arranged on the bottom, which are suitable for holding water, especially biologically purified wastewater, the reactors being each provided with a water supply system (1) and a water discharge system (6), the water discharge system (6) of a first reactor forming the water supply system (1) of the next reactor, an MOS (4), and, connected to the water discharge system

(6) of the last reactor, a settling tank (7) having connected thereto a hme discharge (8) and a discharge for purified water (9). The distance between the underside of MOS (4) and the reactor bottom is preferably 1-4 meters. The flat rotor top is located 0-50 mm under the water surface given quiescent water.

Such an apparatus has the advantage of being eminently suitable for carrying out the method of the invention.

An apparatus according to the invention further has the advantage over the known prior art that it facihtates a decalcification process that is highly sustainable and that leads to a high degree of water softening. The apparatus according to the present invention may be described in a preferred embodiment as follows (see Figure 1). Central is a reactor (3) which is depicted in Figure 2 as a single reactor, but which is series- connected with one or more preferably similar reactors (see Fig. 1), whereby the outflow of a first reactor constitutes the inflow of a next successive reactor. As set out hereinabove, this series-connected array of reactors may be realised in one and the same housing, or the series consists of effectively discrete components connected by pipes. It will be clear that Fig. IB is the top plan view of Fig. 1 A.

The first reactor is fed with biologically purified wastewater via an influent pipe (1) which is passed into the reactor (3) via an endless screw or pump (2). The reactor (3) is provided with an MOS (4) whose capacity must suffice (> 10 m ³ of air per m ³ of water to be softened per reactor) to be able to realize the required stripping and mixing (as discussed in detail hereinabove). In a specific embodiment for the softening of about 200 m ³ h of water having a calcium concentration of about 300 mg/1 to a concentration of about 50 mg/1, an MOS is used having the following properties: motor power: 15 kW, air induction capacity: 5000 m ³ /h; speed rotor: 67.7 rev/min; diameter rotor: 1400 mm; number of blades rotor: 6. More specifically, this is a rotor of the type Landy (Landustrie, Sneek, Netherlands). The rotational speed of the MOS is controlled via a frequency converter which in turn is connected to and controlled by a quality analyser (e.g., calcium meter or conductivity meter). In the reactor (3), also a baffle (5) is present, whose shape is such as to substantially counteract circulation of the water from the top to the bottom in cross section of the reactor. In a preferred embodiment, the baffle comprises three or four baffle plates which are at right angles to the bottom surface of the reactor and are attached to each other in the centre of the reactor.

At the end of the last series-connected reactor, the water that is full of lime particles is discharged via a pipe (6) to the settling tank (7). At the top of this settling tank is an overflow (9) where the purified water can be discharged. At the lower end of the settling tank, the amount of lime to be drained is discharged via a Hme discharge screw (8) to a hme depot (12) with a drainage gutter (13). The most part of the settled lime is recycled via a pump (10) and a pipe (11) to the reactor (3) to serve as seed crystal.