GB2216114A - A continuous chemical precipitation process for water using lime - Google Patents

Abstract

A method for the continuous chemical precipitation of water, preferably waste water or water for consumption. In this process lime at least is added as a precipitation chemical. In the method precipitated sludge from a completed chemical precipitation process is recycled to the water prior to the precipitation process so that part of its calcium content is reactivated to thereby reduce the need for fresh lime. The result is that the cost of the chemical purification of the water can be reduced.

Description

METHOD IN A CONTINUOUS CHEMICAL PRECIPITATION PROCESS FOR WATER The present invention relates to a method in a continuous chemical precipitation process for water, preferably waste water or consumption water, in which process at least lime is added as a precipitation chemical. The invention is substantially characterised in that precipitated sludge from the completed chemical precipitation process is recycled to the water prior to the precipitation process such that part of its Ca content is reactivated, and thus the need for fresh lime is reduced. In this manner the cost of the chemical purification of the water can be kept low.

According to an essential characteristic feature, there is added, besides lime, magnesium as a precipitation chemical, part of the magnesium being reactivated when part of the Ca contentis reactivated during recycling of the precipitated sludge, such that the magnesium content of the recycled sludge increases.

The invention will now be described in detail below, reference being had to the accompanying drawing. Fig.

1 is a schematic side view of a plant section in which biologically purified waste water is treated. Fig. 2 is a schematic side view of a plant section in which simultaneous precipitation by means of lime and magnesium is carried out. Fig. 3 is a diagram illustrating the presence of bicarbonate, carbonate and hydroxide at different pH. Fig. 4 is a diagram illustrating the pH-increasing effect of the recycled sludge, provided by the reversible properties of the lime in the purification process. Fig. 5 illustrates the phosphorus content (Ptot) of Ca-Mg-precipitated waste water in a pH-Mg diagram.

Fig. 6 illustrates the oxygen-absorbing capacity of activated sludge at different pH.

The plant section shown in Fig. 1 is intended for precipitation of biologically purified waste water enter ing at arrow 1. The plant comprises a postprecipitation step in which lime and, preferably, also seawater are used as precipitating agents. The seawater is employed as a so-called coagulant aid in which the magnesium content is the effective component. On the Swedish west coast, the magnesium content of the seawater amounts to about 700 mg/liter, expressed as Mg.

A flocculation of the impurities in biologically treated waste water may be carried out by means of lime and seawater. A settleable floc is obtained at a pH of about 9.8 - 10.3. If large amounts of lime are added, a settleable floc can, of course, be obtained at a higher pH. In the pH range as stated, the lime in the water is present almost exclusively in carbonate form and is precipitated.

When the precipitated calcium carbonate is transferred to a lower pH, the amounts of sludge as formed are converted from insoluble calcium carbonate into soluble bicarbonate, and a large amount of the floc as formed is dissolved. The calcium-magnesium floc together with the phosphates etc. included therein constitute at least partially reversible flocculations when the waste water pH changes.

In order to maintain a magnesium content on the Swedish west coast of about 140 g of Mg/m3 of waste water in the waste water treatment plant, without sludge recycling according to the present invention, about 20% seawater must be admixed to the waste water. By recycling the chemical sludge according to the invention, it should be possible to reduce the seawater admixture to 5-10% of the amount of waste water, whereby the dimensions of the flocculation tanks 2 and the final settling tanks 3 need not be changed.

The admixture of return sludge 4 to the waste water from the biological purification step should be carried out as efficiently as possible and with a duration of the mixing process of 30-60 minutes. After such agitation, the sludge flocs are assumed to be largely dissolved, and the magnesium may again be assumed to be an efficient flocculant, together with freshly added seawater and lime. In a continuous process, the freshly added amount of seawater should be such that the amount of magnesium in the flocculation tanks will be about 70-140 g Mg/m3 of treated waste water. The salinity which is proportional to the Mg content, should be automatically controlled.

Seawater is pumped to the waste water treatment plant. The amount is assumed to be between 5 and 10% of the design amount of waste water. The seawater may be supplied at the inlet side of the waste water flow, in any case before the chemical postprecipitation step, alternatively before the chemical preprecipitation step.

The waste water from the biological purification step flows to a mixing tank 5 provided with an agitator.

The sojourn time of the waste water is here 30-60 minutes at qdim. In the tank 5, the waste water is thoroughly agitated.

At the inlet side of the mixing tank 5, return sludge 4 is supplied from the sludge cones of the final settling tanks 3. The amount of return sludge 4 should be controlled by e.g. variable-speed pumps or mammoth pumps with variable air supply. The amount of return sludge should be 10-25% of the design flow of waste water.

Immediately downstream of the waste water outflow from the mixing tank 5, the lime sludge 6 from the bottom cone of a lime water producer 7 is supplied. The lime water producer is preferably of the type as disclosed in GB patent 1,541,531. This is the lime sludge 6, with partially insoluble particles therein, which settles in the sludge cones of the lime water producer 7, the particles constituting the nuclei or "seeds" on which floc is built up during the subsequent lime precipitation.

The waste water from the mixing tank 5, which now has a pH of from 8.5 to 9.5, is supplied to the first flocculation tank 2 of the three or four chambers which are normally included in a postprecipitation step.

At the inlet opening of the second flocculation tank 2, lime water 8 from the upper outlet portion of the lime water producer 7 is supplied, from which outlet portion completely dissolved Ca(OH)2 is discharged.

The lime water is supplied automatically, the supply being controlled by a pH electrode disposed in the last flocculation tank 2. At 9, limestone powder, CaO or Ca(OH)2, is added, and at 10, dilution water is supplied to dissolve the lime.

The final settling tanks 3 operate in conventional manner, and the excess sludge in the trimmed and continuously functioning plant is transferred to the sludge treatment plant.

As mentioned above, the method according to the invention may also be used for pre-precipitation. In this case, however, pH is not allowed to exceed about 9.5.

The chemical postprecipitation is suitably carried out with lime in the form of hydrated lime, Ca(OH)2, or burnt dolomite. The flocculation of impurities, mainly phosphorus impurities, is carried out in the flocculation tanks 2 provided with agitators, and the flocs are then normally precipitated in settling tanks 3 whose bottom is provided with sludge cones.

The return sludge 4 which in the sludge pockets has a pH of from 10 to 11.5, will, at the downstream end of the mixing tank 5, have a pH of from 8.5 to 10 for the mixed return sludge 4 together with the main flow of waste water.

When pHis reduced from 10 - 11.5 to 8.5 - 10, there occurs, as shown in the diagram of Fig. 3, a conversion in the mixing tank water of the return sludge and the main flow of waste water from hydroxides (down to about pH 10.7) to a mixture of hydroxides and carbonates (down to about pH 9.7) and, finally, to a mixture of carbonates and bicarbonates (down to about pH 8.5).

The hydroxides and carbonates precipitated in the return sludge are now partly dissolved. The calcium content is again activated and contributes to the pH increase during flocculation and settling in the chemical precipitation step, together with freshly added lime.

Thus, the process is reversible as to parts of the Ca content of the return sludge.

The return sludge discharge from the sludge pockets of the final settling tanks 3 is continuously supplied to the inlet of the mixing tank 5. Now, there is an increase in the amount of sludge in the flocculation and final settling tanks 2 and 3 and in the sludge pockets of the latter. The sludge concentration in the flocculation and final settling tanks increases, which promotes the precipitation process and provides for larger contact surfaces for efficient flocculation and precipitation of impurities, and which causes improved utilisation of freshly added lime, a reduction of the fresh lime in the flocculation tanks 2 and an increase of the phosphorus separation. The higher the sludge concentration, the quicker the establishment of an equilibrium pH and the larger the surfaces for precipitation of fresh lime, resulting in an improved utilisation of volume.

As illustrated by the graphs in Fig. 4, the sludge recycling implies that the pH of the waste water supplied to the flocculation tanks is increased to a balance level requiring a minimum amount of fresh lime. The amount of calcium required for maintaining the outlet pH accompanies the purified outlet waste water from the chemical precipitation step. This amount of calcium will, of course, be lost in the process.

The graphs in the diagram shown in Fig. 4 are drawn on the basis of the tests accounted for in the Table below. The X-axis refers to ppm Ca(OH)2 or burnt dolomite, while the Y-axis refers to the pH level. Graph A refers to return sludge, with no addition of lime, according to tests 4 - 11, mixed with biologically treated inlet waste water. Graph B refers to a fresh addition of 100 ppm Ca(OH)2 according to tests 4 - 11. Graph C refers to biologically treated waste water mixed with return sludge, with no addition of lime, according to tests 20 - 25. Graph D refers to a fresh addition of 150 ppm burnt dolomite according to tests 20 - 25.

Test No. Seawater Return sludge pH Addition Chem. pur. outlet waste water % from sample after ad- of lime No. mixing of CA(OH)2 pH Ptot Mg biol. pur. (K) (ppm) (ppm) waste water or burnt (mixing dolomite time about (BD) 30 min.) 4 20 5 20 4 8.7 200 K 9.8 0.41 150 6 20 5 8.5 200 K 10.0 0.40 155 7 20 6 8.5 150 K 9.7 0.52 140 8 20 7 8.7 100 K 9.5 0.54 160 9 20 8 9.1 100 K 9.9 0.56 140 10 20 9 9.2 100 K 9.7 0.54 155 11 20 10 9.3 100 K 9.7 0.44 165 20 0 0 7.6 300 BD 11.4 0.32 8 21 0 20 8.9 200 BD 11.0 0.34 14 22 0 21 8.8 150 BD 9.9 0.68 16 23 0 22 9.3 150 BD 10.4 0.55 16 24 0 23 9.3 150 BD 10.4 0.26 22 25 0 24 9.7 150 BD 10.3 0.62 34 Since the handling of lime according to the present invention is reversible, although with some minor losses, a large portion of the fresh lime is required for replacing the calcium lost in the chemically purified outlet waste water.

To further reduce the addition of fresh lime seawater (because of its magnesium content) was added to the biologically purified waste water, as will appear from the first test series according to the Table. If the waste water contains magnesium, the phosphorus impurities can be precipitated at a lower pH than without magnesium, generally at pH 9.5 - 10.5.

To obtain the same effect in the second series of the tests according to the Table, burnt dolomite was used as a precipitating agent, with no addition of seawater. The burnt dolomite used for the tests contained about 22 ppm Mg. About 1/3 of the magnesium, i.e. about 7 ppm Mg, was released. In this test series the magnesium content of the return sludge in the precipitation process proved to be reversible, and the magnesium content increased by the number of return sludge discharges in the process. Consequently, when using burnt dolomite as the precipitating agent, the pH level in the precipitation process can, by means of the waste water magnesium content which increases during the sludge discharge, be brought down to an equilibrium level which implies a minimum of fresh lime in the form of dolomite so as to obtain the desired purification level for phosphorus and other impurities.

With reference to Fig. 2, a simultaneous precipitation process will now be described in detail, wherein the waste water from the pretreatment is not biologically purified. The waste water is, at arrow 11, supplied to the activated sludge tank 12 which, more precisely, is an aeration tank.

Upon precipitation of nutrient salts, in this case phosphates, from a waste water where lime and magnesium are used as flocculants, the flocculation can be carried out at a lower pH than if lime only is used. Magnesium can be added by means of seawater or burnt dolomite containing Mg in soluble form.

When using burnt dolimite, the required magnesium contents of the waste water are provided by sludge recycling according to the invention.

The diagram in Fig. 5 illustrates the flocculation in collodial form of phosphorus impurities at varying pH and the magnesium content of the waste water at phosphate residual contents which are constant in the graphs. The percentages along the X-axis refer to the seawater percentage in the waste water, while the figures beneath the percentages refer to ppm Mg. The Y-axis refers to the pH level. The graphs define Ptot equaling to 0.3, 0.6 and 0.8 ppm.

At high magnesium contents of the waste water, the flocculation pH is 9.6 or slightly lower. In the aeration tanks 12 in an activated sludge process during degradation of biological material and also in the return sludge during activation in the activated sludge process, pH is normally held at about 7. To provide a combination of biological degradation and chemical precipitation of phosphates in the same purification step, the so-called simultaneous precipitation process is used, which normally implies that iron salts or aluminium sulphate are added to the aeration tanks in an activated sludge process.

In the method according to the present invention, lime and magnesium are used as flocculants in an activated sludge process where pH in the aeration tanks is held at 9.6 or slightly lower. Although such a pH level is relatively high, the microorganisms are still active, at same time as microorganisms other than those predominant at pH 7 are active.

Fig. 6 shows a diagram of the absorption of oxygen in activated sludge at different pH, where the X-axis is a time axis and the Y-axis defines the oxygen content in magnesium per liter. As will appear from the diagram, the oxygen absorbing capacity is excellent up to pH 9.6. The Table below shows the oxygen absorbing rate of activated sludge at different pH. The Table accounts for practically performed tests.

Oxygen absorbing rate pH 3 g O2/m xh g O2/kg SSxh Activated sludge 6.8 31 17.22 -"- 8.5 24 13.33 -"- 9.3 29.25 16.25 -"- S.6 26.53 14.74 The activated return sludge 14 from the sludge cones of the secondary settling tanks 13 is mixed efficiently with the water supplied to the aeration tanks 12, and both lime and magnesium are dissolved to a certain extent and can be used again for precipitation or flocculation.The lime water 15 from the upper outlet portion of the lime water producer 16, and magnesium are therefore added further downstream in the aeration tanks 12 so as to successively, towards the downstream ends of the tanks, increase pH and the magnesium content such that the desired pH, viz. 9.6 or slightly lower, is obtained, which yields a maximum separation of biological material and phosphates from the waste water. The recycling of activated sludge should be carried out so many times that the magnesium content in the aeration tanks 12 amounts to 170 mg Mg/l. In the simultaneous precipitation process where lime and magnesium are used as flocculants, the volumes of the aeration tanks 12 at the waste water inlet ends correspond to the volumes of the separate mixing tanks 5 with a waste water sojourn time of 30 - 60 minutes, as shown in Fig. 1.The dash-dotted line 17 largely separates the inlet end from the downstream end in the aeration tank 12.

As shown in Fig. 2, all the waste water is efficiently aereated in the entire aeration tanks 12, and at the same time a satisfactory agitation is obtained.

Air is supplied at 18.

The chemical floc formed by means of lime and magnesium is more voluminous than the biological floc formed simultaneously in the same waste water, the two flocs being formed jointly in the aeration and final settling tanks in the simultaneous precipitation process and being settled in the settling tanks and recycled (returned) to the aeration tanks, for which reason a higher concentration of sludge and thus greatly increased contact surfaces between the biological floc and the chemical calcium-magnesium floc with accompanying phosphorus impurities etc. are obtained.

The invention is not limited to the embodiment described above and shown in the drawing, but may be modified within the scope of the appended claims.

Claims (10)

1. A method in a continuous chemical precipitation process for water, preferably waste water or consumption water, in which process at least lime is added as a precipitation chemical, c h a r a c t e r i s e d in that precipitated sludge from the completed chemical precipitation process is recycled to the water prior to the precipitation process, such that part of its Ca content is reactivated, and thus the need for fresh lime is reduced.

2. A method as claimed in claim 1, c h a r a c t e r i s e d in that, besides lime, magnesium is added as a precipitation chemical, part of the magnesium being reactivated in connection with the reactivation of the Ca content, when the precipitated sludge is being recycled, such that the magnesium content of the recycled sludge increases.

3. A method as claimed in claim 2, c h a r a c t e r i s e d in that magnesium is added as a precipitation chemical in the form of magnesiumcontaining seawater.

4. A method as claimed in claims 1 and 2, c h a r a c t e r i s e d in that lime and magnesium are added as precipitation chemicals in the form of dolomite containing both Ca and Mg in releasable form.

5. A method as claimed in any one of claims 1 - 4, c h a r a c t e r i s e d in that said recycled sludge is thoroughly mixed with water for a relatively extensive period of time, before the water-sludge mixture is subjected to the chemical precipitation process.

6. A method as claimed in claim 5, c h a r a c t e r i s e d in that said thorough mixing is carried out in a separate tank (5) ahead of the tanks (2, 3) in which the precipitation process occurs in case postprecipitation of pre-precipitation is involved.

7. A method as claimed in claim 5 or 6, c h a r a c t e r i s e d in that before the chemical precipitation process, the water-sludge mixture is given a pH of 8.5 or higher.

8. A method as claimed in any one of claims 2 c h a r a c t e r i s e d in that the recycled sludge is thoroughly mixed with the water preferably in the front portion of an activated sludge tank (12) in whose rear portion the precipitation process occurs in the form of simultaneous precipitation.

9. A method as claimed in claim 8, c h a r a c t e r i s c d in that the pH of the activated sludge tank is maintained at 9.6 or lower.

10. A method as claimed in claim 8, c h a r a c t e r i s e d in that the entire activated sludge tank (12) is subjected to efficient aeration.