Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
  • C02F1/5236—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using inorganic agents

Definitions

• This invention relates to processes for removal of phosphorous from aqueous streams, and in particular to a process whereby the phosphorous is present as orthophosphate and is removed by adsorption onto hydrated magnesium oxide in the form of calcium salts.
• One general approach to phosphorous removal has been to treat the phosphorous containing stream with metal salts, thus forming an insoluble metal phosphorous which is removed from the stream through sedimentation or other means.
• Some examples of this method include the use of aluminium or ferric salts, and also of lime. In this process insoluble phosphates of aluminium, iron or calcium are formed.
• Plant upgrades to utilise metal salts will entail modest capital costs as well as increased operating costs, but more importantly, the use of these chemicals for phosphorous precipitation produces very gelatinous sludge products which are difficult to handle and to_de-water.
• chemical precipitation to remove phosphorous from sewage streams increases sludge volume by 30-40%, and this entails increased transport and landfill disposal costs concomitant with the increased sludge volume.
• the sludge will also contain increased amounts of aluminium or iron which may detract from its application as a fertiliser supplement for agriculture. Again the use of these chemicals can, in some cases, compromise the performance of various biological treatments designed to remove carbon and nitrogen, since they may affect the operating pH of the bioreactors.
• a further consideration is that considerable tonnages of potentially valuable phosphorous are lost to the sludge. For example, it is estimated that a city with population 500,000 loses around 300 tonnes (of elemental phosphorous) per annum to its sewage sludge through the use of chemical precipitation.
• the other general approach to phosphorous removal constitutes the use of adsorbants. In this approach phosphorous is caused to precipitate in either an amorphous or crystalline form onto a suitable medium whereby it remains adsorbed thereon. Once the medium has become sufficiently loaded with phosphorous, it is removed and either employed further to recover the phosphorous and regenerate the medium, or may be employed directly as a source of phosphorous in, for example, certain fertiliser formulations.
• a phosphorous containing effluent stream is first decarbonated and then an appropriate amount of calcium hydroxide is added. Thereafter in one form the treated stream is passed through a sand filter and then through a fixed bed of media. Treated water flows out of the fixed bed. In another form, the treated stream is passed through a fluidised bed of media, then through a sand filter and finally through a fixed bed of media. Treated water flows out of the fixed bed.
• Kaneko et al describe a process for phosphorous removal using a granular activated magnesia clinker said to have a composition of 95% MgO, 3% CaO and 1% Si0 2 .
• the process comprises addition of sodium hydroxide and calcium sulphate to a secondary sewage effluent to adjust the pH to 8-9 and calcium concentration to 50-60mg/L.
• the treated solution is passed downwardly through a sand filter to removed suspended solids and then through a column containing the granular activated magnesia clinker to remove the phosphorous.
• phosphorous was removed (as phosphorous) from 2-5mg/L to levels less than
• the media consisted of a equimolar mixture of MgO and CaCO., and was 0.5-2.0mm in diameter with a mean diameter of 1.5mm.
• the 0.5mm grains had a phosphorous absorbing capacity of lOmg PO./gram of medium. Acid regeneration of the medium was unsuccessful due to excessive dissolution of the media itself. Since it was found that phosphorous fixation capacity was related to the specific surface area of the media, a pulverised material with specific surface area of
• the present inventions whilst recognising the utility of the prior art crystallisation/absorption processes for phosphorous removal, have surprisingly found that by using a magnesia of relatively fine particle size in a gently stirred bed, a highly efficient process for phosphorous removal from an aqueous stream is achieved.
• the present invention consists in a process for the removal of phosphorous, in the form of orthophosphate, from an aqueous stream, comprising introducing into the stream an effective amount of calcium ions, optionally decarbonating the stream, adjusting the pH to between about 5.0-8.0, introducing the stream into a reaction vessel, contacting the stream with an amount of magnesium oxide in the form of fine particles sufficient to fix the so-formed calcium phosphate, whilst agitating the magnesium oxide and removing a stream from the vessel which is depleted in phosphorous.
• orthophosphate levels may typically, and reliably be reduced from several hundred mg/L (as phosphorous) in the influent stream to 0.3mg/L (as phosphorous) or less in the effluent in a continuous manner.
• the final solid product from the reactor is significantly enriched in phosphorous and - depending on how the process is operated - may contain from between 5-18% or greater of elemental phosphorous. This represents a premium high grade source of this element suitable for recycling back into various industries.
• the aqueous phosphorous containing stream may originate from a number of sources including primary, secondary or tertiary treated sewage effluent and industrial waste or process streams. Such streams may also contain heavy metals which would be detrimental to the environment were they not removed prior to release of the final effluent. However, the basic nature of the magnesia will act to absorb these metals as hydroxide precipitates in addition to its ability in removing phosphorous. For those streams containing an excessive quantity of organic or insoluble particulate matter, it may be necessary to subject the stream to aerobic or anaerobic treatment, possibly followed by sand filtration to remove particulate matter.
• calcium salts may be used to supplement the stream with calcium
• two readily available sources are gypsum -CaS0 4 .2H 2 0 - and calcium chloride.
• a convenient manner in which to augment the influent stream with calcium to the required level is simply to divert a portion of the stream (typically 10-20% of the total flow) to a small stirred tank into which solid gypsum is added at an appropriate rate. This portion of the stream, now substantially saturated with calcium sulphate, is then recombined with the major portion of the influent. If a more soluble calcium salt such as CaCl 2 is to be used as the calcium source, then no partial diversion is necessary and a concentrated solution of this salt may simply be bled into the influent stream at an appropriate rate. In some cases it may be desirable to decarbonate the phosphorous containing stream prior to contacting it with the magnesia.
• Both bicarbonate and dissolved carbon dioxide may interfere with phosphorous deposition by competing for the available calcium leading to co-deposition of CaCO, - calcite.
• One means of decarbonating is by adjusting the pH of the stream to 4-4.5 using dilute mineral acid, followed by air stripping. Dilute sulphuric acid is excellent for this purpose. Following this process, it may be desirable to readjust the stream pH to higher values by judicious ⁇ addition of dilute sodium hydroxide, ammonia, lime or other suitable base.
• the decarbonation step may be performed before or after the calcium addition stage. However, it is preferred that any decarbonation is performed prior to supplementing the influent with calcium.
• reaction vessel preferably from a lower portion thereof.
• a reaction vessel that is cylindrical in shape with a conical bottom has been found to be suitable, as this configuration allows for uniform distribution of the effluent.
• any other reactor geometry and influent distribution system allowing for even distribution of the phosphorous containing influent could be employed.
• a quantity of fine magnesia previously introduced into the vessel is gently agitated by the upflowing stream, while some horizontal motion of the magnesia bed is produced by a slow speed stirrer. The purpose of this agitation is to prevent channel formation in the bed, which would lead to "short circuiting" of the system and consequently reduce the contact time.
• magnesia employed for the reactive bed should be in the form of fine particles so as to maximise the activity of this component.
• a preferred particle size is about 200 microns, preferably about 100 microns or less. If, however, the particles are too small, some of the bed is lost and carried out of the system with the effluent.
• Enviromag 75 - available from ICI - with mean particle diameter of around 10 micron and a particle size distribution such that 95% of the weight passes through a 75 micron screen is suitable. This material has a
• Magnesium oxides or materials containing a substantial amount of magnesium oxide from other sources, used either in their unhydrated or hydrated forms, and having different particle size distribution and chemical compositions could usefully be employed for the removal of phosphorous from waste or other process streams by the method embodied in this invention.
• the volume of the fluidised bed begins to increase continuously, and that the rate of increase of the bed volume varies approximately directly to the flow rate of liquid through the reactor, ie, to the "surface loading” and also directly as the concentration of the phosphorous in the influent stream.
• the bed volume stabilises and the settling properties of the bed improve dramatically.
• This "stabilisation" of the bed appears to be related to the degree to which the magnesia is hydrated and becomes converted to magnesium hydroxide.
• the rise in bed volume particularly when high phosphorous containing streams are pumped quickly through the reactor, necessitates some care in initial operation of the process until the bed stabilises.
• the process may be operated at surface loadings of 0.5-2.0metre/H with influent streams containing up to 90mg/L of phosphorous. Streams with higher phosphorous loadings may also be treated successfully, but may initially have to be pumped, depending on overall reactor volume, at slower rates.
• the phosphorous depleted effluent is drawn from the reactor, preferably from an upper portion thereof.
• overall retention times are from 0.5-2 hours.
• this retention time is governed by the settling properties of the reactant mass rather than by any inherent sluggishness of the chemical kinetics.
• Fig. 1 is a flow diagram showing one embodiment of the process of the invention
• Fig. 2 is a flow diagram showing a second embodiment of the process of the invention.
• Fig. 3 is a flow diagram showing a third embodiment of the process of the invention.
• Fig 4 is a flow diagram showing a fourth embodiment of the process of the invention.
• Fig. 5 depicts schematically an embodiment of the process as used for Examples 1 and 2;
• Fig. 6 depicts schematically a larger scale reactor and associated settling tank, used in Examples 6, 7 and 8;
• Fig. 7 is a plot of the performance of the process of Example 1.
• Fig. 8 is a plot of the performance of the process of Example 2
• Fig. 9 is a plot of the performance of the process of Example 4
• Fig. 10 is a plot of change in bed volume for Examples 6 and 7.
• Fig. 1 Although the reactor geometry of Fig. 1 was employed for the majority of the examples to be discussed below, other geometries and reactor configurations are suitable.
• One simple variation constitutes a vertical cylindrical reaction vessel - fitted with a suitable stirring impeller - again operated in upflow mode.
• the primary vessel is functioning both as a containment for the various important reactions, and as a settler, and consequently must be operated with the upflow velocity less than the settling velocity of at least the majority of the contained magnesia/calcium phosphate particles.
• a phosphorous containing stream 10 enters vessel 13.
• Sufficient acid 14 is added to the phosphorous containing liquid 12 so as to give a pH of about 4.5.
• Air 11 is then bubbled through the liquid 12 to complete decarbonation thereof.
• the liquid 12 is decarbonated, it is pumped into the lower conical end 23 of a reaction vessel 22 by pump 21. A portion of the decarbonated liquid 12 is diverted by pump 18 into another vessel 16. In this vessel, gypsum 15 is added with stirring so as to produce the requisite concentration of calcium in the liquid 17. This is then bled by line 20 back into line 19 so as to effectively provide a calcium loaded stream into the reaction vessel 22.
• Magnesia 24 is loaded into the reaction vessel 22 to form a bed to a height 26 which, with agitation by stirrer 25 in combination with the upward flow provided by pump 21, produces maximum contact between the magnesia bed and the phosphorous containing liquid stream. It must, however, be noted that the degree of overall agitation is controlled to a minimum consistent with the prevention of channelling.
• the liquid now at a depth 27 is drawn off by line 28 to stirrer vessel 32.
• anionic polymer 30 is added to flocculate solids in the liquid stream 31 contained therein whilst the liquid is agitated by stirrer 29.
• Flocculant 33 collects after a time and is removed 35 together with phosphorous loaded magnesia 36 from the reaction vessel 22 to be dewatered into a solid 34 and a liquid stream 37.
• the solid 34 is removed as a phosphorous enriched product 39 suitable as a fertiliser.
• the stream 37 is fed back into line 28.
• the phosphorous- depleted stream 40 passes into tank 41 where carbon dioxide 43 is introduced to give pH of about 7-8 for the liquid 42 contained therein. Once the pH is corrected, the phosphorous-depleted stream is discharged as effluent 44.
• a gypsum bed 45 is loaded into the reaction vessel 22 in a lower portion thereof and below the magnesia bed 24.
• the influent flows firstly through the layer of gypsum where it becomes enriched in calcium ions prior to it being introduced to the upper bed region where the high pH and surface area provided by the magnesium oxide particles allow deposition of amorphous calcium phosphate.
• This mixed bed configuration obviates the need to supplement the stream with calcium prior to introduction to, the reactor.
• it has been found that the more dense calcium sulphate layer remains substantially separate from the less dense upper strata containing the magnesium oxide and deposited phosphorous compounds.
• gypsum Since the gypsum is appreciably soluble in water, it may be necessary from time to time to replenish the lower layer with fresh gypsum.
• Another viable operating configuration is where the influent (supplemented in Ca) is pumped into a continuously stirred reactor, which may be of relatively small dimensions, containing a slurry of magnesia.
• the total reaction mixture is then allowed to overflow into a conventional settling or thickening apparatus (eg, a conical tank or an inclined plate settler) where, after the solids have settled the clear phosphorous depleted liquor is drawn out of the system.
• the thickened phosphorous laden slurry is then pumped, either wholly or in part, back to the reaction vessel.
• the magnesium oxide reactant is consumed by the process, it is necessary to add small quantities of unused MgO to the reaction mixture.
• the process consists of separate reaction and settling stages, and in some instances may offer advantages over the "single stage" mode of operation described previously.
• the process may, in some cases, be appropriate to employ an in line mixer in place of the reaction vessel.
• Fig. 3 this embodiment is shown schematically, with features like those of the embodiments of Figs. 1 and 2 like-numbered.
• the calcium dosed decarbonated phosphorous containing stream is pumped by pump 21 to reaction vessel 100, which contains a magnesia bed 101. This is continuously agitated by stirrer 102. After a time the loaded magnesia/reaction mixture is allowed to overflow 103 into a conventional settling or thickening apparatus, which in this case is a conical tank 104. After the solid 106 settles out, the clear phosphorous depleted liquid is drawn 107 into a tank 32 for deflocculation as previously described. The solid 106 is drawn out as a thickened slurry to be returned to tank 100 or, if the magnesia is fully loaded, to dewatering as previously described.
• advantage is taken of the propensity of the bed volume to increase as it becomes laden with phosphorous.
• the orthophosphate containing influent - previously supplemented with calcium and/or decarbonated if required - is introduced into the bottom end, which is preferably cone shaped to facilitate uniform liquid distribution, of a cylindrical reactor previously charged with magnesium oxide.
• the reaction mixture becomes increasingly enriched in deposited phosphorous, and as a consequence of this and the upflow velocity within the reactor, the overall bed volume increases.
• the bed volume increasingly occupies more of the total reactor volume and the "line of demarcation" between the reactive mass and the clear supernatant, now substantially depleted in orthophosphate, steadily rises up the cylinder.
• the bed is allowed to rise to a particular height in the reactor, and the phosphorous enriched slurry withdrawn through an appropriately sized port either continuously or in aliquots drawn off at set time periods. Since the bed volume rises at a much slower rate than the rate of influent flow through the reactor, the volume of the withdrawn slurry typically amounts to only 3-5% of the overall flow and the major portion of the effluent leaves the reactor via a port situated higher up the reaction vessel. Of course, it is necessary to replace the "lost" bed volume with fresh magnesium oxide, and this operation can be performed ion either a continuous or semi-continuous manner.
• the process in the reactor operated at a high pH whereby magnesium oxide is contacted with the phosphorous containing stream should be of from about 8 to 12.
• the pH will be from about 9 to about 11.
• the selection of pH is dictated by the solubility of magnesium hydroxide.
• the pH of the effluent may be reduced by dosing dilute mineral acid - eg, sulphuric acid - or by final treatment with carbon dioxide - ie, recarbonation.
• the present process represents a fundamentally different approach to those described in the prior art, and specifically, the present invention relies on the use of fine grained magnesia and on a reaction vessel in which the magnesia is present as an agitated rather than a fixed bed.
• Roques et al teach their half burnt dolomite be added as a finishing step to biological treatment of sewage. At that stage there will be a substantial amount of organic material present in addition to the dissolved phosphorous. This organic material will be adsorbed onto their medium thereby occluding the active surface area and consequently impairing its ability to fix large quantities of phosphorous. This fact is recognised by the authors when they refer to the half burnt dolomite as an aid to settling of the biological floe.
• An influent 320 previously decarbonated and containing phosphorous and supplemented with calcium ions, was pumped into the bottom 323 of a 2.5L conical reactor using a peristalsic pump (not shown) .
• the reactor had been previously charged with magnesium oxide to form a bed 324 to a height 326.
• a glass wool plug 321 in the bottom 323 of the reactor 322 was used to retain the bed 324 in the reactor and to prevent it flowing back into the feedline.
• a stirrer 325 was used to agitate the contents of the reactor 322. After a time, the level of the contents of the reactor rose to a height 327. At this point, effluent was allowed to drain by gravity 328 through a perspex tube 329 containing a wad of glass wool 339.
• the phosphorous depleted stream 331 flowed into a 3L receiving vessel 332. Effluent filtered and depleted in phosphorous was then removed 340 from the vessel 332 .
• Tertiary effluent from Quakers Hill sewage treatment plant was enriched in phosphorous from the ambient l-2mg/L using disodium hydrogen phosphorous, so as to give the reactor an influent phosphorous concentration of between 10 and 40 mg/L.
• the calcium level was also supplemented using CaCl 2 with typically the calcium ion concentration between 70 and
• Bicarbonate and dissolved carbon dioxide which could interfere with phosphorous deposition by competing for available calcium and depositing calcite (CaCO ) in the reactor was present in the sewage at around 120mg/L HCO, and was removed by air stripping.
• the pH was lowered from around 7 to 4-4.5 using dilute sulphuric acid and then air bubbles - from a fish tank aerator - were blown through the liquid for around one half hour.
• the pH was then adjusted back to 6.5-7.5 using dilute sodium hydroxide solution.
• alkaline materials such as lime or ammonia solutions could have been employed. Note that the subsequent exposure to the MgO bed would have neutralised the residual acid. However, this may also have led to some redissolution of previously deposited phosphorous, and possibly lead to difficulties in interpreting the reactor and process performance.
• the liquid 320 was then pumped to the bottom 323 of the conical reactor 322 using a peristaltic pump at a flow rate of between 500 and lOOOmL/H.
• the reactor employed had a conical bottom section of volume 1.5 litres and a cylindrical top section of volume 1 litre giving total reactor volume of 2.5 litres.
• the reactor was charged with lOOg of fine particulate magnesium oxide 324 - commercially available as Enviromag-75 (ICI) and a small plug of fine glass wool 321 was fitted into the bottom of the conical section to prevent the solid reactor contents flowing back into the feed line.
• the reactor was also fitted with a stirrer 325 designed so as to gently agitate the reactor contents. This was found necessary to prevent channel formation in the bed which "short circuited" the system and significantly reduced the performance of the process.
• the stirring speed was about 40rpm.
• the experiment was run for 1500 hours with influent P levels between 2mg/L (unsupplemented sewage from the sewage treatment plant) and 45mg/litre.
• the system was operated at ambient temperature, ie 20-25 degrees C.
• Effluent samples were regularly taken from the receiving vessel and analysed for orthophosphate using the molybdenum blue method described in standard 4500-P of the 1992 edition of the American Water Works Association publication "Standard Methods for Examination of Water and Wastewater".
• the level of total effluent orthophosphate that contained within fine particles which were carried over from the reactor plus the truly soluble "ionic" phosphorous
• Examples 1 and 2 demonstrated the applicability of the process for the removal of orthophosphate from tertiary treated sewage.
• fluoride was reduced from 1.Omg/L in the influent to 0.7mg/L in the effluent, while silica was reduced from 3.5 to 2.5mg/L. It is also important to note that no biological growth was noted in the reaction bed despite the fact that the influent originated from sewage.
• Example 3
• This experiment was conducted using the same equipment as in the previous tests, but employed secondary treated sewage from Round Corner sewage treatment plant near Sydney, Australia. This is a small plant employing only secondary biological activated sludge treatment and consequently the influent to the MgO reactor contained a higher loading of organic matter and suspended solids than in the previous tests. This influent also contained ambient phosphorous levels of around 8mg/L total phosphorous, including 7mg/L of phosphorous as orthophosphate since no specific phosphorous removal is undertaken at this plant.
• the residual reactor solids from this experiment weighed 67gm and contained 2.2% P (6.6% P0 4 ). As with the two previous examples, despite the sewage origin of the influent, no biological growth was noticed on the bed.
• the influent contained no calcium apart from that present in the make up water which was around 5-10mg/L.
• the experiment was run for 514 hours over which time 17.1gm of elemental P was deposited on the bed.
• the final dry weight of the reactor solids was 260gm, which assayed at 19.5% P0 4 (6.5% P), 20% Ca, 47% Mg(0H) 2 and an estimated 21% of undissolved calcium sulphate.
• This experiment was conducted with a high orthophosphate influent, ie, 20-60mg/L, supplemented appropriately with calcium, ie, 120-230mg/L, in order to ascertain the level to which the final reactor solids could be enriched in elemental phosphorous, consistent with a high percentage of phosphorous removal from the influent stream.
• the reactor was the same as that used in the previously described Examples, and again the influent flow rate was 500-1000mL/hour. The trial was conducted over 1218 hours during which time 39.4gm of elemental P (present in the solution as orthophosphate) was passed through the reactor, and 37.7gm of elemental P was deposited on the bed. This represented 95.6% recovery of phosphorous.
• Final solids recovered from the reactor at the conclusion of this experiment weighed 205gm dry, and contained 45% P0 4 (18% P) , 29.7% Ca and only around 16% magnesium hydroxide. This product represents a high grade phosphorous raw material.
• the reactor 422 employed has a cone shaped bottom 423 as before, but had a significantly larger volume at 30 litres.
• the reactor dimensions are illustrated in Fig. 6.
• Influent 420 was pumped into the cone portion at a flow rate of 25-28 L/H, and this corresponded to a reactor surface loading of lm/H.
• Magnesium oxide had been previously charged to the reactor to give a bed 424 to a depth 426. This was retained in the reactor by a non-return leaf spring 429.
• Influent 420 was controlled by manifold 421.
• the reactor effluent was gravity fed 428 to a 5 litre settler 434 into which anionic polymer 431 (Alfloc 627) was fed at a rate giving 2-3mg/L polymer in the effluent.
• This treatment successfully flocculated the small amount of particulate matter 433 carried over in the reactor effluent.
• a stirrer 430 was used to agitate this tank. Once deflocculated, the phosphorous depleted contents 432 were removed via line 436.
• Example 6 This example followed the flowsheet of Fig. 1, using an artificial influent containing 15mg/L phosphorous and supplemented in Ca to llOmg/L.
• the reactor was charged with 50 grams of magnesium oxide and influent pumped into the system at 25L/H over a period of 30 hours.
• the soluble phosphorous content of the 0.45 micron filtered effluent never exceeded 0.2mg/L, while the phosphorous content from the unfiltered flocculated effluent from the settler was typically 0.2-0.5mg/L.
• the retention time in the settler was very short at about 12 min and longer settling times would probably have improved the quality of this effluent.
• Example 2 it was found that the presence of carbonate in the influent had no deleterious effect on the phosphorous removal, with soluble phosphorous in the reactor effluent never exceeding 0.26mg/L. The presence of carbonate did, however, have a dramatic impact on the volume of the reaction bed, leading to significantly smaller bed volumes than was the case when carbonate was absent from the " influent. This effect is illustrated in Fig. 10 in which the bed volumes for both Examples 6 and 7 are plotted against the progress of the two experiments.

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