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United States Patent (19) 11

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United States Patent (19) 11 United States Patent (19) 11. Patent Number: 4,671,879 Solt et al. 45 Date of Patent: Jun. 9, 1987 54 PROCESS FOR REDUCING THE NITRATE 57 ABSTRACT CONTENT IN WATER The present invention provides a process for reducing 76 Inventors: George S. Solt, Hollington Wood, the nitrate content in water, wherein water containing Emberton, Olney, Buckinghamshire, nitrate is contacted with an anion exchanger containing MK46 5.JH, England; Abraham chloride and/or bicarbonate ions to remove the nitrate, Klapwijk, Langhoven 44, 6721 SK whereafter the exhausted anion exchanger is regener Bennekom, Netherlands ated, the spent eluate either (a) being divided into two or more portions at least one of which is rich in sulphate 21 Appl. No.: 753,031 and at least one of which is rich in nitrate, the sulphate 22 Filed: Jul. 8, 1985 rich portion being dumped to drain or being passed to a reaction vessel or vessels in which sulphate ions present 30 Foreign Application Priority Data therein are precipitated out as calcium salt by the addi Jul. 10, 1984 GB United Kingdom ................. 8417530 tion of a water-soluble calcium salt or by adding an 51) Int. Cl. ................................................ B01J 1/09 aqueous slurry of calcium hydroxide and in which deni 52) U.S. Cl. .................................... 210/610; 210/631; trification is carried out on the nitrate-rich portion by 210/677; 210/683; 210/903 means of appropriate anaerobic bacteria, a carbon 58) Field of Search ............... 210/610, 677, 683, 903, source for the bacteria also being introduced into the 210/611, 631, 656, 663, 668 reaction vessel and pH correction being carried out, if necessary, or (b) in which reductions of the sulphate 56) References Cited and nitrate ion concentrations are carried out on the U.S. PATENT DOCUMENTS recovered eluate without dilution into one or more 4,043,936 8/1977 Francis et al. .................. 20/60 X portions, the content of chloride and/or bicarbonate 4,098,690 7/1978 Semmens ... ... 210/903 X being supplemented by the addition of appropriate 4,370,234 1/1983 Marsland ... ... 210/903 X sources of chloride and/or bicarbonate, whereafter 4,479,877 10/1984 Guter ......... ... 210/903 X some or all the solution is recycled to the anion ex 4,522,727 6/1985 Weber ............................. 210/903 X changer for use as eluent in a subsequention exchange Primary Examiner-Tom Wyse cycle. Attorney, Agent, or Firm-Silverman, Cass, Singer & Winburn, Ltd. 19 Claims, 7 Drawing Figures Nutrient U.S. Patent Jun. 9, 1987 Sheet 1 of 4 4,671,879 U.S. Patent Jun. 9, 1987 Sheet 2 of 4 4,67 1879 U.S. Patent Jun. 9, 1987 Sheet 3 of 4 4,671,879 515 519 U.S. Patent Jun. 9, 1987 Sheet 4 of 4 4,671,879 610, 1611 6. 4,671,879 1. 2 stage may be carried out by two or more ion exchange PROCESS FOR REDUCING THE NITRATE columns in series, after which the first column will be CONTENT IN WATER found to contain primarily sulphate and may be regen erated separately to yield a sulphate-rich eluate. Alter DESCRIPTION natively, each ion exchange column may be equipped The present invention is concerned with an improved with an additional liquid distribution device present at process for reducing the nitrate content of water. an appropriate level within the column, which may be Two main methods of removing nitrates are well used to regenerate the exchangerlying above the device known. The first is by anion exchange, using an anion separately from that lying below it. exchange resin in a chloride or bicarbonate cycle. In the 10 The sulphate-rich portion of the eluate may be sent to chloride cycle, the resin used is regenerated with so drain in situations where it is easy to dispose of such a dium chloride and in the bicarbonate cycle with bicar liquor. bonate. The exchanger then removes all the sulphate Alternatively, the sulphate in such a portion of the together with the nitrate, replacing these ions with eluate may be precipitated by the addition of calcium in chloride or bicarbonate ions, respectively for the cycle 15 some appropriate form and the resulting slurry contain concerned. In the floride cycle, a part of the bicarbon ing calcium sulphate, together with surplus regenerant, ate content of the water is also replaced by chloride The can be allowed to settle and the supernatant liquor sent product water from the chloride cycle is, therefore, to drain. Alternatively, calcium sulphate is precipitated liable to be corrosive and of reduced palatability. Alter in the same way but the supernatant liquor is recycled to natively, the bicarbonate cycle avoids the removal of 20 the process. bicarbonate ions but involves a more expensive and In cases where the sulphate content of the raw water difficult regeneration. is low, it may be possible to operate the cycle without An important objection to the ion exchange process precipitation of sulphate from the spent regenerant, is, however, the difficulty of disposing of the spent possibly with occasional purging of portions of recov regenerant, which is rich in excess regenerant and con 25 ered liquor to keep the recirculating sulphate within tains the nitrates removed from the water, without dam manageable limits. age to the environment. Calcium may be added in the form of dry or slurried The other well known nitrate removal process is the line or as calcium oxide or calcium chloride. The latter destruction of nitrate with denitrifying bacteria in an serves the dual purpose of removing surplus sulphate anoxic process. This process requires the addition of a 30 and, at the same time, of adding chloride ions to make nutrient carbon source and an electron donor, such as up for the loss of anions from the cycling regenerant, it methanol. Before denitrification can begin, oxygen dis solved in the water must first be consumed, with a cor is also possible to use calcium bicarbonate solutions to responding consuption of nutrient. Water produced by achieve this object. the anoxic process has to be reconditioned and the bio 35 The precipitation of calcium sulphate is known to be mass has to be removed from it. troublesome. In order to achieve a precipitate which However, this process is difficult to control and the settles readily, it may prove convenient to add a surface cost and toxicity of the methanol used as the nutrient agent to promote coagulation of the particles or to add carbon source and electron donor are also disadvanta seeding material. Another means of promoting a better geous factors. The dentrification process is liable to precipitate is to raise the pH of the solution, for example produce concentrations of nitrite, which is carcino by dosing with lime, and to cause calcium carbonate to genic, or other undesirable side products in the water. co-precipitate with calcium sulphate. The present invention provides a way of combining The recovered eluate or the nitrate-rich portion only elements from these two known method in order to is subjected to biological denitrification to remove ni avoid the main objections to both of them. 45 trate. In the course of this operation, bicarbonate may According to the invention there is provided a pro be created in the solution. cess for reducing the nitrate content in water wherein The solution may then be brought up to the appropri water containing nitrate is fed through anion exchang ate concentration by adding chloride and/or bicarbon ing means, including the steps of regenerating the anion ate and re-used in a subsequent ion exchange cycle. exchanging means with an eluent to produce a nitrate 50 Therefore, the present invention includes nitrate re containing eluate, biologically treating the eluate to moval by ion exchange, using either chloride or bicar reduce the nitrate content thereof, and utilising the bonate as the regenerating ion, or by regenerating with eluate of reduced nitrate content in a recycled eluent for a chloride/bicarbonate mix. In all these case, a modest regenerating the anion exchanging means. quantity of residual sulphate and/or nitrate may be For continued re-use of eluate, which is the basis of 55 permitted to accompany the recycled regenerant. the present invention, it is desirable to prevent accumu Alternatively, it is possible that anions such as sul lation of sulphate ions which are eluted from the ion phates can be used instead of chloride or bicarbonate as exchange means, though a certain level of sulphate in the regeneration ion. the recovered eluate can be tolerated. The biological denitrification reaction can be carried For example, the entire recovered eluate can be 60 out in a moving or fixed bed microbiological reactor, a bulked and dosed with calcium ions in order to precipi fixed bed reactor being operated in upflow or down tate sulphate, which is allowed to settle out before recy flow. This biological denitrification reaction is most cling the liquor. conveniently carried out in an upflow device containing In anion exchange, the affinity of the exchanger for a fluidised bed of biomass. This bed may or may not be sulphate is higher than that for nitrate so that chromato 65 seeded with sand, the object of such seeding being to graphic banding takes place. This banding may be uti provide a surface on which the biomass may settle and lised to obtain separate sulphate-rich and nitrate rich so create heavier particles which permit a higher up fractions of spent eluate. For example, the ion exchange flow velocity, while at the same time reducing the car 4,671,879 3 4. ry-over of suspended matter.
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