DK177729B1 - Method and plant for preparation of potable water - Google Patents

Abstract

The invention relates to a method for preparation of potable water, comprising the steps of thoroughly aerating crude water and arranging for drops of aerated water to drip directly onto an open sand filter. In a second aspect a plant is provided for preparation of potable water ac cording to said method.

Description

i DK 177729 B1

The present invention relates in a first aspect to a method for preparation of potable water. In a second aspect, the invention relates to a plant for preparation of potable water according to the method of the first aspect of the invention.

5 The provision of drinking water of a satisfactory quality has been of major concern to mankind throughout recorded history. In order to improve locally available water depreciated by the occurrence of unhealthy or unsavoury substances and sometimes even pathogenic agents, various attempts of purification have been made from an early 10 date. Thus, the Sanskrit text "Sushruta Samhita", dated to approximately 2000 B.C., mentions boiling and filtering through sand and coarse gravel as methods for purification of water, while the contemporaneous work "Ousruta Samhita" describes filtering through charcoal.

Sand filtration has been applied on a regular basis ever since 15 the first municipal water treatment plants were installed in the 19th century, but today most waterworks in industrialized countries also rely heavily on advanced automated and power-driven equipment, computerized control systems and the possible supply of a broad range of extraneous chemicals during the process of drinking water production.

20 The sand filters applied in water purification are classified into several types according to their mode of functioning. Slow sand filters use biological processes and depend on the formation of a gelatinous layer of diverse living organisms known as a "Schmutzdecke" in the uppermost few millimetres of the fine sand layer of the filter. To keep the 25 filter working, the Schmutzdecke must be rejuvenated by regular intervals. This is mostly done by scraping off the top layer of the filter to expose a new layer of fresh sand. As the name implies, the flow rate of slow filters are relatively low.

As opposed hereto, rapid sand filters use coarser sand and 30 show a higher flow rate, which means that a smaller expanse of area for sand filtration is occupied to yield a given level of production. Moreover, they are less sensitive to changes in raw water quality. Rapid sand filters are cleaned by frequent backwashing with little or no Schmutzdecke formed, so that only modest biodegradation takes place in this type of 2 DK 177729 B1 filter. To exert a proper filtering function, rapid sand filters require that the substances to be filtered are supplied with the water in the form of floe, i.e. flakes of precipitate that comes out of solution during the process of flocculation and which are able to trap particles and other 5 impurities. This is in contrast to sand filters that mainly rely on processes within the bulk of filter sand and seek to mimic the processes of a natural soil profile. An example of such a filter is given in the patent GB 191316050.

When using rapid sand filters for preparation of potable water, 10 various measures are therefore normally undertaken to induce flocculation. Typically, either flocculation chemicals such as salts of aluminium or iron are employed, or resort is made to precipitation of oxidized compounds in a settling container to spare the filter a substantial amount of non-flocculated compounds. The latter solution, 15 however, calls for an additional piping arrangement involving a pump or hydrophore. An example of the latter practice is given in the patent application US 2009/0020482, wherein an intermediate step of precipitation in a collection container is envisaged between the steps of aeration and sand filtration of the water to be treated.

20 In view of the above, the object of the present invention is to provide a method and a plant for preparation of potable water, wherein simple yet efficient sand filtration of water at a high flow rate is accomplished upon aeration without the obligation to rely on the supply of extraneous chemicals for flocculation or the intermediate step of precipi-25 tating oxidized compounds in a designated container.

To meet this object, according to the first aspect of the invention a method is provided for preparation of potable water, comprising the steps of aerating crude water to a degree of oxygen saturation exceeding 90%, preferably 95%, by leading said water to the top of an 30 apparatus for treatment of water, said apparatus comprising a plate with holes for forming drops by flow of the water through the holes at the initiation of the treatment process, and means arranged below said plate for causing division of the drops by contact therewith, wherein the means for causing division of the drops comprise a plurality of tubular 3 DK 177729 B1 elements in the form of pipes having pipe walls made of wire net, said tubular elements being placed in horizontal layers of several parallel tubular elements stacked in such a way that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the 5 longitudinal axes of the tubular elements in the one or more adjacent layers; and letting the water pass through said apparatus to the bottom thereof by the force of gravity; and letting divided drops of aerated water reaching the bottom of said apparatus leave the apparatus and drip directly onto an open sand filter, placed below said apparatus.

10 It has been found by the inventor that the above-mentioned apparatus for treatment of water works in conjunction with a sand filter in a most propitious way, offering remarkable advantages over traditional methods. By ensuring a thorough aeration in said apparatus, a fully satisfactory flocculation of undesired compounds is achieved, re-15 suiting in the formation of floe, which is deposited at the surface of the sand filter without substantially infiltrating this, so that it can be easily removed from the filter. An effective sand filtration is thus obtained making use of neither extraneous flocculation chemicals nor vessels for precipitation and their associated pumps.

20 Preferably, the sand filter is regularly cleaned by backwashing, allowing the use of a filter type showing a high flow rate.

In this way, a robust, inexpensive, simple and safe method is provided, which method requires only a small consumption of energy and materials, and wherein the capacity of the sand filter is fully turned 25 to account.

Preferably, when during the step of aeration, the water passes to the bottom of the apparatus for treatment of water, it is allowed to drip onto the sand filter without any intermediate collection or pumping whatsoever, but in cases where the crude water contains extraordinary 30 high levels of substances to be removed, it may be desirable to recycle it once or more to the top of said apparatus in order for appropriate oxidation to take place. The above apparatus is the subject of European patent EP 1 070 022, and the workings of some relevant embodiments thereof are disclosed in said document. Thus, a microorganism culture 4 DK 177729 B1 may be added to the water prior to initiating drop formation. Hereby, components of the crude water, which are not converted by the oxidation alone, may be removed or reduced by the metabolism of the microorganisms. In this connection, the concrete microorganism culture may 5 be selected depending on the content of components, whose amounts are to be reduced in the crude water.

According to a preferred embodiment of the invention, the apparatus for treatment of water is fit up so that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the 10 longitudinal axes of the tubular elements in the one or more adjacent layers by an angle of approximately 90°. In this way, good overall conditions for drop divisions to occur within the apparatus are generated.

In a specific embodiment of the invention, air, optionally enriched in oxygen, is led by passive or active flow in a vent pipe to the part of 15 said apparatus containing the tubular elements. In this manner, the degree of oxidation achieved in the apparatus for treatment of water may be further regulated. The active supply of oxygen to the apparatus could be used as an alternative to recirculation from the bottom to the top of said apparatus in instances of crude water heavily loaded with sub-20 stances to be removed.

Preferably, the sand filter has a filter medium comprising siliceous sand, optionally being mixed with or having superimposed on it a material of lower specific gravity and larger grain size. An upper layer of a material of the latter kind, an example of which could be hydroan-25 thrazite and/or activated carbon, may impart to the filter a greater capacity for adsorption of floe without caking of the filter, which may be relevant in case of extensive floe formation.

Depending on the desired capacity for water treatment per time unit and the content of substances to be removed from the water during 30 treatment, it may be advantageous according to one embodiment of the invention to arrange additional sand filters in a parallel and/or serial array relative to the apparatus for treatment of water.

According to one specific embodiment of the invention, the crude water is contacted prior to aeration with an iron-containing material in a 5 DK 177729 B1 closed system adapted for enriching the water with Fe(II) compounds, with a view to enhanced ensuing flocculation of pesticides and contaminant trace species such as arsenic with oxidized iron compounds. Said contacting is the subject of the patent application US 2009/0020482, 5 and preferred embodiments thereof are disclosed in said document.

Thus, the iron concentration in the water preferably is increased by contacting the water intimately with an iron material having a suitably large contact surface. An inexpensive material is iron swarfs, available as a wate product of machining, said swarfs having been calcinated prior to 10 their use to remove residual cutting oil. Another material is iron ore.

In practice, the contact between water and the iron material may be achieved by spraing a suitable bed of iron material, for example a layer of iron swarfs having a thickness of 10-30 cm and arranged in a perforated tray made of plastic or stainless steel. The thickness of the 15 bed of iron swarfs is adjusted so that the desired amount of iron to be added to the water is achieved. The tray is provided with a plurality of holes, e.g. having a diameter of 3-4 mm, at the bottom of the tray. The water is supplied to the bed of iron swarfs in a manner ensuring a good and uniform distribution. This may be accomplished in a manner well-20 known to a person skilled in the art, for example by means of a suitable number of nozzles or from an overhead distributor tray adapted as a drip tray with a plurality of holes, so that the bed of iron swarfs is constantly sprayed with water, and the content of iron in the water increases.

25 To meet the aforementioned object, according to the second as pect of the invention a plant for preparation of potable water is further provided, which plant comprises an aeration unit consisting of an apparatus for treatment of water, said apparatus comprising a plate with holes for forming drops by flow of the water through the holes at the 30 initiation of the treatment process, and means arranged below said plate for causing division of the drops by contact therewith, wherein the means for causing division of the drops comprise a plurality of tubular elements in the form of pipes having pipe walls made of wire net, said tubular elements being placed in horizontal layers of several parallel tu- 6 DK 177729 B1 bular elements stacked in such a way that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the longitudinal axes of the tubular elements in the one or more adjacent layers, said elements allowing divided drops to pass through the appa-5 ratus to the bottom thereof by the force of gravity; and a filter unit in the form of an open sand filter, said filter unit being disposed under the aeration unit, wherein the aeration unit and the filter unit are mutually arranged, so that water leaving the bottom of the aeration unit is allowed to drip onto the filter unit by the force of gravity. Said plant pro-10 vides the same or similar advantages as the first aspect of the invention and due to its modest requirements for energy and materials is suitable for use under harsh or primitive conditions.

Preferably, the aeration unit is mounted over the filter unit on a deck of aluminium rails allowing the water to flow freely from the former 15 to the latter.

In the operation of a municipal water treatment facility or the like, a system of multiple plants will be relevant. The crude water is led to a number of apparatuses for treatment of water, each being set up with one or more sand filters.

20 According to a specific embodiment, an assembly in the form of a closed system supplying Fe(II) compounds to the crude water by contacting this with an iron-containing material is incorporated before the aeration unit, preferably on top of the latter. This may be relevant, where enhanced flocculation of pesticides and contaminant trace species 25 such as arsenic with oxidized iron compounds is desired.

In the following, a preferred embodiment of the invention will be illustrated by reference to the non-limiting figure.

Figure 1 illustrates an embodiment of the plant according to the second aspect of the invention, said plant being for carrying out the 30 method according to the first aspect of the invention.

Referring now to the figure, the main features of the illustrated plant are referenced by numbers as follows: 1 is a deck of aluminium rails for carrying an aeration unit 4; 2 is a framework of a filter unit containing a sand filter 8; 3 is a pipeline for supply of crude water to the 7 DK 177729 B1 aeration unit 4 by the aid of a pump 3a; 5 represents angularly displaced layers of tubular elements for causing division of drops; 6 stands for water drops dripping from the aeration unit 4 to the sand filter 8; 7a shows a pump for leading away purified water from the framework of 5 the filter unit in a pipeline 7.

An overall description of a preferred embodiment of the method according to the method will now be given.

An amount of crude water is received in the top of the aeration unit 4, where drops are formed in a top plate provided with holes. By 10 the force of gravity said drops fall and impinge on a multitude of alternating layers 5 of tubular elements, mutually displaced by 90°, so that the drops are divided into droplets. The formation of droplets results in a substantially larger drop surface area relative to drop volume, so that enhanced enrichment with oxygen can take place. The height of the 15 stack of layers 5 is adjusted so that the initial drops are divided at least 50-60 times and preferably 60-80 times when falling through the aeration unit, in which case a satisfactory aeration is assured.

A consequence of the sweeping aeration performed in the aeration unit 4 is that methane, hydrogen sulphide, and corrosive carbon dioxide 20 is largely driven out of the water, while soluble Fe(II) and Mn(II) ions are oxidized to insoluble Fe(III) and Mn(IV) compounds, respectively.

Said insoluble compounds then form floe together with nickel, radon, various aromatic compounds, MTBE, and chlorinated solvents, the concentrations of which in the water are all strongly reduced during the 25 process of aeration.

From the bottom of the aeration unit 4 aerated droplets and floe entrained with these drip through the deck of aluminium rails 1 onto the surface of the sand filter 8. According to the present embodiment, a sand filter cleaned by backwashing is envisaged. Alternatively, use 30 might have been made of a filter of the slow sand filter type. In that case, a purifying Schmutzdecke consisting of bacteria, protozoa, fungi, rotifera, insect larvae and eventually algae and larger aquatic organisms such as bryozoa, snails and annelid worms would be cultured on a layer of fine sand, overlaid by a supernatant layer of unfiltered water.

8 DK 177729 B1

In the present case, however, the sand filter is made up of relatively coarse sand, shows a high flow rate and is cleaned by frequent backwashing, so that an unbroken Schmutzdecke hardly ever develops.

The water flows through the filter medium under gravity, while most of 5 the floe, due to its substantial unit size effected by the thorough aeration, are deposited on the surface and in the uppermost part of the filter.

Despite the lack of a genuine Schmutzdecke, biological processes do occur in the sand filter, where oxidation of mainly iron, manganese 10 and ammonium takes place. The biological activity benefits from the fact that methane and hydrogen suphide, which would otherwise have severely restrained the diversity of species and the repertoire of biological processes, have been extensively removed already during the process of aeration.

15 After a period in use, the sand filter gets clogged with floe and must be backwashed. This involves reversing the direction of the water and possibly adding compressed air to loosen the floe. The backwash water is run into a settling tank through an opening (not shown) in the framework of the filter unit 2. The floe settles out and is disposed of as 20 waste material.

Example A plant for preparation of potable water according to the invention is installed and run for several months. The aeration unit is contained in 25 a cabinet of aluminum with a height of 250 cm and is supported by an aluminium deck approximately 50 cm over the upper surface of a filter medium bed contained in a filter unit. The medium is made up of coarse quartz sand. It is found that due to the superficial deposition of floe, the consumption of water for backwashing may be reduced by more than 30 60% relative to normal values within the art for operation of this type of sand filter. Thus, considerably less waste runoff is produced and the risk of washing away media, while it is fluidized during backwashing, is lessened.

Claims (8)

9 DK 177729 B1 A process for making potable water, comprising the steps of: a. Aerating raw water to an oxygen saturation rate greater than 90%, preferably 95%, by passing the water to the top of a water treatment apparatus, which apparatus comprises a plate with holes for forming droplets by flowing the water through the holes at the beginning of the treatment process, and 10 means arranged below the plate to cause droplet formation upon contact therewith, the means for causing droplet formation comprising several tubular elements in the form of tubes with wire mesh tubes, the tubular elements are arranged in horizontal layers of several parallel tubular elements stacked such that the longitudinal axes of the tubular elements in one layer are angularly displaced relative to the longitudinal axes of the tubular elements in the adjacent layer (s); and passing the water through the apparatus to the bottom thereof by the action of gravity; and 20 b. leaving split drops of aerated water reaching the bottom of the apparatus, leaving the apparatus and dripping directly onto an open sand filter located below the apparatus. A method according to claim 1, wherein the sand filter is regularly cleaned by return rinsing. A method according to any one of the preceding claims, wherein air, optionally oxygen-enriched, in step a.) Is conducted in a ventilation tube by passive or active flow to the portion of the apparatus containing the tubular elements. A method according to any one of the preceding claims, wherein the sand filter has a filter medium comprising silica sand, optionally mixed with or overlaid by a material of lower density and larger grain size. A method according to any one of the preceding claims, wherein additional sand filters are arranged in parallel and / or serially in relation to the water treatment apparatus. A method according to any one of the preceding claims, wherein the raw water prior to step a.) Is contacted with an ferrous material in a closed system enriched with water for Fe (II) - 5 compounds, for subsequent increase. flocculation of pesticides and pollutants such as arsenic with oxidized iron compounds. An apparatus for producing potable water, comprising: an aeration unit consisting of a water treatment apparatus, which apparatus comprises a plate with holes for forming droplets as the water flows through the holes at the beginning of the treatment process, and means arranged under the plate for causing droplet formation upon contact therewith, the means for causing droplet formation comprising a plurality of tubular elements in the form of tubes with tubular walls of wire mesh, the tubular elements being arranged in horizontal layers of several parallel tubular elements stacked so that the tubular elements are formed. the longitudinal axes of elements in a layer are angularly displaced relative to the longitudinal axes of the tubular elements in the adjacent layer (s), said elements allowing shared drops to pass through the apparatus to the bottom thereof by the action of gravity; and ii. a filter unit in the form of an open sand filter, which filter unit is arranged below the aeration unit, wherein the aeration unit and the filter unit are arranged so that water leaving the bottom of the aeration unit is allowed to drip down on the filter unit by the action of gravity. The plant of claim 7, wherein a closed system aggregate that delivers Fe (II) compounds to the raw water by contacting it with an ferrous material is inserted before the aeration unit, preferably on top of it.