Phosphorus removal and recovery from waste water by
crystallisation
Dietfried Donnert1, Ute Berg1, Peter G. Weidler1, Rolf Nüesch1, Yonghui Song1, Manfred Salecker1, Iris Kusche2, Walter Bumiller2, Frank Friedrich1
1 Forschungszentrum Karlsruhe, ITC-WGT
Preprint from 2 Forschungszentrum Karlsruhe, BTI-V
Schriftenreihe "Geo- und Wassertechnologie" 03/02, Aedificatio Verlag, Freiburg i. Br., ISSN 1610-3645
1 Abstract
Batch and continuous running column experiments in the laboratory scale were car-ried out with the goal to develop an efficient process to remove phosphorus from waste waters enabling also simultaneously a recycling of the eliminated phosphorus. As an appropriate method the seed induced phosphorus removal, also referred as crystallisation, was selected, and calcite and tobermorite successfully tested as seed materials. The interaction with both materials and phosphorus leads to a fixation of the phosphorus onto the corresponding surface. The coating of the calcite surface with precipitated calcium phosphate compounds causes a further improvement of the fixation process and does not deteriorate the efficiency of the tobermorite. According to the results obtained, the method can be applied for secondary effluents in the range of 10 mg/L P as well as for waters from industrial processes with a phosphorus content of > 200 mg/L P.
2 Introduction
Phosphorus compounds are present in every domestic waste water, originating from detergents as well as from metabolism processes, diffuse runoff from agricultural land and inputs from the air. Already at very low concentrations of ≈ 10 µg/L P sec-ondary reactions, also known as eutrophication processes, may occur. Thus, many water bodies everywhere in the world are experiencing an increased number of algal blooms, which reduces the amenity value and ecological health of water bodies such as lakes, slow moving rivers and drinking water reservoirs [Bernhardt, 1978]. Apart from an internal release from the sediments [Hart et al., 2002] the main phosphorus source comes from the catchments inputs.
Therefore it is nowadays still a stringent task to provide an efficient phosphorus re-moval from waste waters like effluents from sewage treatment plants (STPs) and in-dustrial discharges. Hence most of the STPs in Germany have to meet an effluent standard of 1 - 2 mg/L P according to their size [Bundesanzeiger, 1996]. But, it is not only an important issue to remove the phosphorus from the waters, but also to pro-vide a recycling of the eliminated phosphorus as well. This is due to the following facts:
• Phosphorus is an essential nutrient for all forms of life and a key element for many physiological and biological processes. It cannot be replaced by another element [Steen, 1998].
• The world reserves of phosphorus will run out within the next 100 years. Be-sides, the reserves of so-called good ores, which can be used for the phos-phorus production without large pre-treatment measures e.g. for the removal of heavy metals, will already run out in about 30 years [Driver et al., 1999].
Therefore different methods enabling phosphorus recovery are currently developed and improved [Brett et al., 1997]. They are co-ordinated by a large industrial union (CEEP = Centre Européen d’Études des Polyphosphates e.V. in Brussels), which has been formed in the meantime with the goal to apply within the next decade 25 % of recycled phosphorus within the European industries [CEEP, 1998]. And, above that, the Dutch company Thermphos B.V. in Vlissingen (Netherlands), which is the only producer of elementary phosphorus in Western Europe, has the ambitious plan to use up to 40 % of recycled phosphorus as raw material in the near future. This amounts to about 17 kT/a from a total of 40 kT/a raw phosphorus produced. For that, about 25 % could be provided from STP effluents [Schipper, 2001]. The plans in Sweden are even more ambitious in recovering at least 75 % of the phosphorus in the waste waters until 2010 [Hultmann et al., 2001].
The German plans stand far behind such attempts. The simultaneous precipitation, which is mostly applied, does not offer good boundary conditions to recycle phospho-rus. Furthermore only first ideas are existing, which are mainly governed by eco-nomical considerations, disregarding the running out of the world resources men-tioned above. But, an approach for an efficient recycling seems important especially in Germany, because it may be considered as the country with the greatest phospho-rus recycling potential in the EU. Germany produces about one third of the total sludge within the EU and has high phosphorus elimination rates which other larger nations of the EU (F, I, E, GB) will not meet in the near future [Farmer et al., 1999]. In Germany, about 65.000 t P/y, i.e. 1.8 g P/cap•day, are discharged into the waste water systems [Jardin, 2002]. Generally two main sources for phosphorus recycling in the waste water treatment field are existing:
• from the sludge or from the ashes after sludge incineration, which has a great potential of about 89 % of the total phosphorus discharge into the waters in Germany, but needs expensive measures [Driver et al., 1999, Cornel, 2002]. Furthermore, the direct use of excess sludge in agriculture will be much more restricted in the near future in Germany and probably in the whole EU [Schnurer, 2002, Deutscher Bundesrat, 2002].
• from the water phase, which can be handled by cheaper and easier methods. The total potential can be estimated to about 39 % of the total discharge in Germany [Jardin, 2002] or 15 - 20 % of the raw phosphorus demand of the fertilizer production in Germany (0,9 - 1,2 Mio. t P2O5/a) [Cornel, 2002].
3
Selection of the crystallisation method for phosphorus
re-cycling
Currently the methods applied in Germany for phosphorus removal from waters, like sewage discharges, are mainly Bio-P and/or simultaneous or post precipitation [de-scribed e.g. in Helmer & Sekoulov, 1977]. But, these processes do not offer a good opportunity for recycling. Regarding phosphorus recycling, the international devel-opment has focused on two different principles:
• Struvite (MAP = Magnesium-Ammonium-Phosphate) precipitation mainly in Japan [Taruya et al., 2000] and
• Crystallisation of Ca-Phosphate compounds with different seeds [Brett et al.,1997].
From the point of view on the product obtained it seems much more attractive to fo-cus on the production of Ca-Phosphate which offers better prospective for industrial applications. The method itself is already applied in full scale for side stream treat-ment in the Netherlands [Piekema & Gaastra, 1993] using sand as seed. But, prior to the crystallisation the carbonate in the water has to be removed (section 4.1).
Therefore the crystallisation method was chosen in our programme with the goal to avoid such pre-treatment steps. Two materials, calcite [Donnert & Salecker, 1999, Donnert, 2001] and tobermorite according to [Moriyama et al., 2001] were used as seed, in order to obtain Ca-Phosphate compounds deposited on the seed surface under the avoidance of expensive measures concerning pH-adjustment. The investi-gations were focused on both, secondary effluents and higher concentrated industrial waste waters.
4 Chemical
Fundamentals
4.1 Crystallisation
The most important mechanism of the ability of calcite and tobermorite to remove phosphorus from waters is crystallisation [e.g. Donnert & Salecker, 1999]: the domes-tic waste waters in most countries contain calcium and phosphate ions in higher con-centrations (e.g. in Germany: domestic waste waters about 80 mg/L Ca and 5 - 10 mg/L P). Thus, the waters are supersaturated with regard to calcium-phosphate compounds, i.e. the water is not in the equilibrium state. Theoretically, i.e. if the equi-librium state would have been attained according to the solubility product of HAP (hy-droxyl-apatite), the phosphorus concentration in the water should not exceed 6.0 x 10-8 moles/L P = 0.002 mg/L P. In Japan and other countries, the waters are softer, they contain much less calcium, but enough to enable a formation of Ca-P-compounds as well. The idea of crystallisation is therefore to initiate this “equilib-rium”, which means a removal of the phosphorus from the waste water by the addi-tion of seeding crystals.
A very interesting full scale development was performed in the Netherlands applying sand as seed [Piekema & Gaastra, 1993]. But, the process has the disadvantage that the dissolved carbon dioxide in the water has to be removed prior to the crystalli-sation step. This is effected by acidifying the water with sulphuric acid to pH ≈ 4.2 and stripping with air. The crystallisation is then carried out after another pH adjust-ment to pH ≈ 9.0 with NaOH. No statements for the market value of the recycled phosphorus are available up to now.
Calcite was also previously investigated and lead to applications in the waste water field [Donnert & Salecker, 1999]. But, in all cases a pH ≥ 9.0 had to be applied for a sufficient phosphorus retention which does not suit to ecosystems such as catch-ments and lakes. The aim should therefore be to maintain the pH of the water (pH < 9.0). This was achieved with calcite and to some extent also with tobermorite [Mori-yama et al., 2001].
4.2
Mechanisms of phosphorus bonding onto calcite
The application of calcite implies taking the carbonate balance into account. It is ex-pressed by the saturation index, a scale showing the relationship of calcium carbon-ate (or solid phases as gypsum, Ca-phosphcarbon-ate compounds) to the pH and hardness of the water:
( )
(
)
= − + L CO a Ca a SICaCO 2 3 2 . lg 3 a = activity, SICaCO3 = Calcite saturation Index with:
(
Ca)
Gl a(
CO)
Gl aL= + 32−
2
. L = solubility product, aGl = activity in the saturation state
Leading to ∆ SICaCO3 = 1/10 - ∆pH .
Hence, generally according to the water chemical fundamentals [Stumm & Morgan 1996] three different mechanisms have to be considered which are interacting as well as depending on the experimental conditions, especially on the saturation index of the water
- For all waters (for SICaCO3= 0 the only mechanism)
Sorption of phosphorus onto calcite (section 7.1.1.1), nearly no effect on the pH of the water
- For SICaCO3 > 0 (supersaturation of the water versus calcite)
Co-precipitation of Ca-phosphate compounds together with calcite fol-lowed by nucleation of Ca-phosphate onto calcite, slight decrease of the pH of the water
- For SICaCO3 < 0 (undersaturation of the water versus calcite)
Dissolution of calcite effecting an increase of the Ca concentration and the pH value followed by formation of Ca-phosphate
4.3
Mechanisms of phosphorus bonding onto tobermorite
The application of 11Å-tobermorite (Ca5Si6O16(OH)2 x 4H2O) as seed was investi-gated and further developed by [Moriyama et al., 2001]. A special pellet product con-sisting of tobermorite and calcareous material was developed and used especially for higher concentrated waste waters (about 50 mg/L P) from side streams of STP. The P-removal mechanism is as with calcite (section 4.2) mainly crystallisation, detected by deposition of Ca-P-phases on the tobermorite surface by XRD. But, additionally, a coagulation of Ca-phosphate powder crystals is assumed. But, due to dissolution of the seed an increase of the pH to about pH = 9 is occurring, and, the Ca-content had to be maintained by addition of Ca-ions.5
Materials used for the investigations
In the investigations two different types of seeds, calcite in various forms and tober-morite, were studied.
5.1 Calcites
applied
Some data regarding the BET area of different calcite materials used in the investiga-tions are given in Table 1 below. They show a broad variety concerning the SSA (specific surface area) of the calcites used between ≈ 1 m2/g up to about 70 m2/g. Also the prices vary to a large extent from € 25/t up to € 1200/t for the extremely fine grained calcite type SOCAL U3, which is manufactured by precipitation.
Table 1
Properties of quartz sand and different calcite qualities used for the experi-ments
(TC=Technical Calcite; MP=amount of micropores (pore width < 2nm) relative to SSA)
Product BET SSA
[m2/g]
MP [%] price
[€/t]
grain size [µm] Technical calcite (producer: Bassermann, Germany)
TC-B 1.0 17 45 20
Precipitated calcite (producer: Solvay, Austria)
Socal U3 70 9 1.250 0.02
Mined calcite (producer: Terrazzo, Germany) Juraperle 0.3 – 0.5 23 25 1200-1800
Biogenous marine calcite (producer: amb, France)
Coccolithe 4.5 7 200 1000-2000
Sand (divers types and producers)
Quartz Sand 0.1 n.d.* 8-35 600-1200 *n.d. not determined
5.2 Tobermorite
applied
The pellet material prepared by [Moriyama et al., 2001] was not available for the studies. On the other hand, a price of € 2000 per ton was given which seems unreal-istic for a technical application in Germany. Therefore in our experiments a waste product from the construction industry was applied. Mineralogy analysis revealed the material to be 11Å-tobermorite. It had a BET surface area of approx. 30 m2/g and a grain size of about 0.5 mm. The amount of micropores contributing to the SSA was around 22%. The price should be in the range of € 100 – 200 /t.
6 Experimental
part
The experiments carried out were started with agitation experiments with both, calcite and tobermorite, to get a comparative overview. Then continuous running column experiments were carried out in small scale. No correction of the pH-value being at pH ≈ 8.2 – 8.5 was applied. In contrast to this, all industrial applications mentioned earlier (section 4.1) were done at a pH ≥ 9. But this did not seem favourable for the boundary conditions given: treating e.g. agricultural runoffs should not require con-tinuous maintenance, additionally a pH ≥ 8.5 is not appropriate to guarantee the eco-logical aspect of the process, furthermore it is not allowed by law to discharge such
waters directly into a receiving water body. The same problem would occur concern-ing secondary effluents, i.e. a neutralisation would be necessary prior to discharge.
6.1
Agitation experiments
These experiments were carried out to get first information about the influence of the experimental conditions and the characteristics of the seed used on the phosphorus removal efficiency.
0.5 to 2.0 g calcite or tobermorite were shaken with 200 to 500 ml of water (see sec-tion 6.3). Aliquots were taken at different time steps to determine the time depend-ence of the reaction. For the equilibrium tests samples were shaken for about 48 hours on a horizontal shaking machine. In order to simulate a column experiment, calcite seeds as well as tobermorite seeds were also reused several times with new water. The P- and Ca-concentrations, the alkalinity and the pH-value were measured according to the German standards [DIN and DIN ENEN], details being given in sec-tion 6.3.4.
6.2 Column
experiments
After evaluating the results of the agitation experiments in the previous section 6.1 column experiments were carried out with the most promising materials. The idea of these set-ups was to investigate the coating effect described later in section 7.1.1.3 for different materials, i.e. to reproduce the batch experiments results.
The following experimental conditions were applied:
Columns: 50 cm2 x 10 cm, 2 bed volumes = 1 Litre
Water: see section 6.3 Residence time: one to eight hours.
6.3 Waters
investigated
The investigations were started with tap water and artificial lake waters, but later ex-tended to secondary effluent and to several other industrial waste waters.
6.3.1 Tap water, artificial lake waters
All the waters used, as the tap water of the Forschungszentrum, were spiked with NaH2PO4 to a content of ≈ 10 mg/L P, the order of magnitude of secondary effluents of STPs without P-precipitation or of agricultural runoffs. It was nearly in the equilib-rium versus CaCO3. Furthermore, lake waters were artificially composed in order to get waters with different saturation indices (Table 2). The exact composition of all these waters is given in the Annex, section 11.1.
Table 2
Saturation indices (SICaCO3), pH values, Ca and HCO3-concentrations
of tap water and artificial lake waters, all spiked to 10 mg/L P
pH c(Ca) c(HCO3) SICaCO3
[mmol/L]
Tap water (~ 10 mg/L P) 7.3 2.52 5.35 + 0.05 Lake Tegel (Berlin) 7.4 2.0 2.7 - 0.06 Mulde reservoir 7.2 5.4 2.7 - 0.42 Lake Müggel (Berlin) 7.5 2.0 1.31 - 0.40 Lake Müggel, no HCO3 7.5 1.8 0.02 - 2.20
6.3.2 Secondary effluent
In some experiments, the secondary effluent of the STP of the Forschungszentrum (Bio-P-treatment with simultaneous precipitation using FeCl3) was used. Again the water was spiked to a content of ≈ 10 mg/L P, the average phosphorus concentra-tion without simultaneous precipitaconcentra-tion, because the goal should be to replace the precipitation by the active filtration or crystallisation. The composition (mean values of 2001) is given in the Annex, section 11.2, the pH-value, the Ca- and the HCO3-concentrations and the SICaCO3 are listed in Table 3.
6.3.3 Supernatant from the Phostrip process
One goal for the application of the active filtration, especially in view of phosphorus recovery, seems the treatment of phosphorus enriched side streams from the sludge treatment processes in STPs. One application is this view was tested with the strip-per sustrip-pernatant from the Phostrip process [Kaschka & Weyrer, 2000]. Some data are summarised in Table 3, the exact composition is given in the Annex, section 11.2.
Table 3
Saturation indices, Ca- and HCO3-concentrations of Phostrip supernatant
(~ 50 mg/l P), tap water and secondary effluent FZK, both spiked to 10 mg/L P
pH c(Ca) c(HCO3) SICaCO3
[mmol/L]
Tap water (~ 10 mg/L P) 7.3 2.52 5.35 + 0.05 Phostrip water (~ 50 mg/L P) 7.4 1.45 6.42 - 0.25 Secondary effluent (~ 10 mg/L P) 7.5 2.30 5.45 - 0.18
6.3.4 Water analysis
All water samples were analysed after filtration (0.45 µm) on the Ks, the concentra-tions of Ca, Mg, Na, K as caconcentra-tions and the concentraconcentra-tions of Cl, SO4 and NO3 and the soluble reactive phosphorus (SRP) as anions. The methods applied were:
• Ks-titration with 0.1 molar HCl [DIN EN ISO 9963-1, (1995)].
• Cation concentrations: atomic absorption spectrometry [DIN EN ISO 7980, 2000, DIN 38406-13, 1992, DIN 38406-14, 1992] using an ICP-AES (Vista, Varian, Australia).
• Anion concentrations : ion-chromatography [DIN EN ISO 10304-1, 1995] (DIONEX DX 100).
• SRP concentration: Molybdenum blue reaction (DIN EN 1189, 1996) detected with a MPM 1500 spectrophotometer, WTW (1 cm cell, 690 nm).
The calculation of the saturation index SICaCO3 was done with the programme
BWASA [Eberle & Donnert, 1991]. 6.3.5 Analysis of the seeds
The mineralogical content of the used materials were checked by X-ray diffraction with a Siemens D5000 diffractometer in Bragg-Brentano geometry equipped with a Cu-anode and Graphit monochromator (Cu-Kα = 0.15406 nm). The ground samples
were gently pressed into the round (diameter 25 mm) intention of the sample holder. Specific surface area (SSA) and porosity were determined by measurement of the N2 isotherm at 77K with a Quantachrome Autosorb1-MP device.
The cross section of adsorbed N2 was assumed to 0.162 nm2. Prior to adsorption the samples were heated in vacuum at 90°C overnight. The SSA was calculated with the BET method [Brunauer et al., 1931], the microporosity was determined according to [de Boer et al., 1966].
The Infrared Spectroscopy (FTIR) measurements were carried out with a Bruker IFS 66 with a Globar source and a DTGS detector.
Surface specific analysis was carried out with a GoldenGate ATR (attenuated total reflection) diamond cell with single reflection. Grains of the different samples were pressed onto the diamond with the same pressure throughout the experiments, no further preparation was necessary.
7 Results
7.1
Results of experiments with calcite
General aspects of the possible mechanisms of phosphorus retention – adsorption and precipitation or co-precipitation with calcite - have already been described in sec-tion 4.2. For that, mainly the SICaCO3, which is described in this chapter, is the char-acteristic determining value, apart from the influences of the seed surface.
7.1.1 Results of agitation experiments
In order to evaluate the different mechanisms described in section 4.2, the experi-ments were carried out with waters of different saturation indices. Generally it has to be observed, that agitation experiments do not allow an absolute evaluation of the results kinetically, because they are very dependent on the shaking geometry and velocity, respectively. But, they can provide a useful information for comparison of different experimental conditions, if the experimental conditions are identical. For ki-netic measurements only column experiments can provide useful information.
7.1.1.1 Phosphorus adsorption/crystallisation as phosphorus removal mechanism
Adsorption is the dominating mechanism if SICaCO3 = 0, i.e. the water is in equilibrium with calcite. If the water is strongly supersaturated versus calcium phosphate com-pounds additional crystallisation effects may occur. The results of agitation experi-ments with tap water (c(P)o = 10 mg/L P , SICaCO3 ~ + 0.05), i.e. almost in equilibrium, in Table 4 show, that calcites with a higher SSA are required to effect an efficient phosphorus removal.
Table 4
Agitation experiments with two calcites and tap water
(time: 24 hours, calcite: 0.5 g/L, c(P)0: 10 mg/L, SICaCO3 ~ + 0.05)
Material grain size SSA P-load obtained in
equilibrium
[µm] [m2/g] [mg/g] [mg/m2]
Technical Calcite (TC-B) 2 – 63 1 0.04 0.04
Calcite Socal U3 0.015 – 0.025 70 6.75 0.10
The influence of the SSA, hence sole adsorption, is evident. Comparing the ratio of the P load per mass Socal U3 adsorbed approximately 170 times more P than TC. Considering this ratio per unit surface area, this ratio reduces to a factor of 2.5. Thus the Socal material is more finely dispersed in solution than the TC material, and therefore in a closer contact with phosphorus leading to the higher amount of ad-sorbed P per unit surface area. The equilibrium values are 1.3 and 3.2 µmol P/m2, respectively and are somewhat lower than the maximum of 7.9 µmol P/m2 for calcite reported by [Bertrand et al., 2001]. But, there also higher loads published e.g. for So-cal U1 with 20,0 µmol P/m2 [Berg, 2001].
7.1.1.2 Precipitation or dissolution of calcite as additional mechanism
The agitation experiments were carried out with several artificial waters (Table 2) with differing SICaCO
3, which were spiked with 10 mg/L ortho-P by addition of the
corre-sponding amount of NaH2PO4. Shaking was performed for about 24 hours with 0.5 g calcite and 1 L of the solution. The composition of the waters is described in the An-nex, section 11.1.
The results of these agitation experiments in Table 5 show the remarkable change of the phosphorus removal efficiency, especially regarding the calcite TC-B for waters
subsaturated or supersaturated versus calcite (see section 4.2). Especially the water Lake Tegel (Berlin), which is in a state near the calcite equilibrium, shows clearly the preference of the adsorption mechanism explained in section 4.2 in removing the phosphorus from the water. Regarding Socal U3 a considerable phosphorus load was obtained, which is attributed mostly to adsorption. The found maximum value of 0.23 mg P/m2 corresponds to 7.5 µmol P/m2 and is in very good agreement with the maximum loadings found for calcite by [Bertrand et al., 2001].
With decreasing SICaCO3 the amount of P removed by the technical calcite TC-B is in-creasing, which means, that the dissolution mechanism starts. Hence, more Ca is available in the TC-B system than in the Socal system and it can be assumed that the production of nuclei is higher for TC-B, thus leading to an enhanced crystallisa-tion of Ca-phosphates. This finding is supported by data of microporosity determined with N2 gas adsorption (extended BET treatment). The technical calcite contains twice as much micropores than Socal, and should show a higher rate of the release of calcium at the prevailing conditions of Lake Müggel.
Table 5
Results of shaking experiments with artificial waters (section 11.1)
with different calcite saturation indexes (SICaCO
3)
(time: 24 hours, calcite: 0.5 g/L, c(P)0: 10 mg/L) TC = Technical Calcite Water composition according to SICaCO3 P – Elimination [%] (first cycle) P adsorbed [mg / m2] SOCAL U3 TC-B SOCAL U3 TC-B
Lake Tegel (Berlin) -0.08 55 0.5 0.16 0.17 Lake Müggel (Berlin) - 0.40 33 2.5 0.09 0.83
Mulde reservoir - 0.42 44 3.6 0.13 1.00
Lake Müggel, no HCO3 - 2.20 80 50.0 0.23 16.67
7.1.1.3 Coating of calcite with Ca-Phosphate, application of used calcite
The experiments described in Table 5 were all carried out with unused calcite mate-rial. But, in reality, this material will be covered (coated) by precipitated calcium car-bonate and by calcium phosphates formed as described above according to the SI-CaCO3 of the water. This represents the actual state of throughput (e.g. column) ex-periments as well as the processes in barrier layers. Therefore, agitation exex-periments were performed with two types of calcite, Juraperle and Coccolithe (see Table 1), which were reused for three times and one time, respectively. Tap water spiked with phosphorus up to 10 mg/L P was applied.
Figure 1: Agitation experiments with 2 g/L Juraperle (1a) and 2 + 10 g/L Coccolithe
(1b), influence of one time (orig) and multiple use (recy); tap water, c(P)0 = 10 mg/L P The results are shown in Figure 1. It is evident that this coating had a decisive effect. The large differences to the unused materials vanished completely and the P elimina-tion rates improved considerably, due to the coating of the surface, which was al-ready reported by [Griffin & Jurinak, 1973; Freeman & Rowell, 1981], who found an increasing phosphorus removal with increasing experimental time. These findings should at least be valid in the concentration range investigated, because the transfer from adsorption to surface precipitation diminishes with lower phosphorus concentra-tions [Giannimaras & Koutsoukos, 1987]. The effects of this coating on the surface properties were investigated also by FTIR-ATR and ESEM investigations.
The FTIR-Diagram (Figure 2) shows clearly the formation of a hydroxyl-apatitic com-pound, which proves, that phosphate has been fixed at least on the surface of the seed.
Figure 2:Surface specific infrared spectroscopy (FTIR-ATR) on PO4 coated Calcite
seed crystals after reuse in c(P)0 = 10 mg/L P.
The results of ESEM investigations of one and two times used Juraperle are shown in Figure 3. The prismatic crystal in figure 3a corresponds most likely to apatite. And,
R e c y c lin g o f " J u ra p e rle " T im e [d a y s ] 0 2 4 6 8 1 0 1 2 1 4 1 6 0 2 4 6 8 1 0 1 2 o rig in a l 1 s t re u s e 2 n d re u s e 3 rd re u s e T a p w a te r, 1 0 m g /L P , 2 g /L J u ra p e rle , S I = + 0 ,5 [m g/ L P] R e c y c lin g o f " J u ra p e rle " T im e [d a y s ] 0 2 4 6 8 1 0 1 2 1 4 1 6 0 2 4 6 8 1 0 1 2 o rig in a l 1 s t re u s e 2 n d re u s e 3 rd re u s e T a p w a te r, 1 0 m g /L P , 2 g /L J u ra p e rle , S I = + 0 ,5 [m g/ L P] R ecycling of C ocolith c(P ) tim e [days] 0 2 4 6 8 10 12 14 16 mg/L P 0 2 4 6 8 10 12 2g/L orig 10g/L o rig 2g/L recy 10g /L recy Coccolith R ecycling of C ocolith c(P ) tim e [days] 0 2 4 6 8 10 12 14 16 mg/L P 0 2 4 6 8 10 12 2g/L orig 10g/L o rig 2g/L recy 10g /L recy Coccolith
after the second use (figure 3b) the whole calcite surface is covered by a layer of about 700 nm of a Ca-P-compound.
Figure 3a: ESEM image of Calcite
(Jura-perle) after first use. Figure 3b:
ESEM image of Calcite (Jura-perle) after second use.
Figure 3: ESEM images of Calcite Juraperle after the first (figure 3a) and the second
use (figure 3b); tap water, c(P)0 = 10 mg/L P
7.1.1.4 Influence of the DOC in the water
Several agitation experiments with secondary effluent and Juraperle water showed a remarkable retardation of the phosphorus removal efficiency in comparison to tap water (Table 6). This seems to be due to the DOC content of the water and its simul-taneous elimination, i.e. fixation onto the calcite surface. Coccolithe with a much higher BET surface (Table 1) showed a better phosphorus removal efficiency, but this could not be verified up to now by column experiments.
Table 6
Results of agitation experiments with the calcite qualities Juraperle and Coccolithe 25 g/L Calcite, secondary effluent and tap water, both spiked to 10 mg/L P
P-concentrations [mg/L] andP-removal [%]
Seed
Material Water used in 0 minutes in 10 minutes in 24 hours Juraperle tap water 10.0 - 8.7 13.0 4.2 58.0
Juraperle secondary effluent 10.5 - 9.9 5.7 6.6 37.1
Coccolithe secondary effluent 11.1 - 9.7 11.7 4.5 59.5
7.1.2 Results of column experiments with calcite and sand in tap water
Quartz sand (Table 1) was also included into the experiments, because it could serve as an uphold if very fine calcite particles should be used either in filtration or in cap-ping (see [Ute Berg et al., 2002], sections 1, 2.1 and 2.3). Figure 4 a shows, that even this sand removed phosphorus to a certain extent. This is not an entirely new effect (Brett et al., 1997), but was up to now only observed for pH-values ≥ 9 of the water. And, a small addition of calcite Socal U3 (SSA 70 m2/g) caused a considerable improvement of the phosphorus removal rate in Figure 4 b over a very long period. This improvement could possibly be favoured by increasing the amount of Socal in the column.
a) Quartz Sand : 10 hours detention time
b) Sand K14 + 2 % Socal U3, 2.5 hours detention time
Figure 4: Column experiments with Quartz Sand K12 and Quartz Sand/2% Socal U3
Tap water (SICaCO
3 = +0.05, c(P)
0 = 10 mg/L; columns: 50 cm2 x 10 cm, 2 bed volumes = 1 Litre)
Figure 5 shows results of experiments with calcite Juraperle with a low SSA of 0.2 m2/g, which exhibits for two different residence times a considerable high phosphorus binding capacity over a long period up to 4000 bed volumes. This seems to be due to the coating effects described in section 7.1.1.3. This explains also the slow beginning before a considerable coating is formed after several bed volumes of water have passed through the column, even with a longer detention time. The shortening of the detention time from 4 hours to 1.5 hours had therefore no remarkable effect on the phosphorus removal rate of the material.
a) 4 h detention time b) 1.5 h detention time
Figure 5: Column experiments with calcite Juraperle
Tap water (SICaCO
3 = +0.05, c(P)
o = 10 mg/L P; Columns: 50 cm2 x 10 cm) 7.1.3 Results of column experiments with calcite Juraperle and secondary
efflu-ent
According to the results given in Table 6 there was not a great chance for a success-ful performance of a column experiment with Juraperle and secondary effluent. The only chance would have been that a coating effect (section 7.1.1.3) would enhance the phosphorus removal efficiency. But, there was no success, only an average
P-0 20 40 60 80 100 0 500 1000 1500 2000 bed volumes P -E lim in . % 0 20 40 60 80 100 0 800 1600 2400 3200 4000 bed volumes P -E lim ina tion % 0 20 40 60 80 100 0 1000 2000 3000 4000 5000 6000 bed volumes P-El im in a ti o n % 0 20 40 60 80 100 0 100 200 300 400 500 600 700 800 bed volumes P-El im in a ti o n %
removal efficiency of about 5 - 8 %. Therefore, the experiment was stopped after a throughput of 800 bed volumes.
7.2
Results of experiments with tobermorite
The experiments were carried out the same way than with calcite (section 6.1). 0.5 to 2.0 g tobermorite were shaken with 200 to 500 ml tap water spiked with NaH2PO4 to ≈ 10 mg/L P. Aliquots were taken at different time steps to determine the time de-pendence of the reaction. For the equilibrium tests samples were shaken for about 48 hours on a machine. P-and Ca concentrations, alkalinity, and pH were measured according to the German standards (DIN EN).
7.2.1 Hydrochemistry
Generally an increase of the pH-value was observed at the beginning of the dissolu-tion of CaO and Silicate, resulting in a considerable increase of the electrical conduc-tivity of the water. According to the column experiments described in section 7.2.3, this effect was reduced after some time. Both effluents from the tap water and the secondary effluent columns had then only a pH ~ 8.0.
7.2.2 Results of the agitation experiments
The results are listed in Table 7. Besides tap water and secondary effluent addition-ally a laundry effluent with a quite high phosphorus and TOC concentration was in-vestigated as an example of an industrial waste water.
Table 7
Results of agitation experiments
25 g/L tobermorite, secondary effluent and tap water, both spiked to 10 mg/L P, laundry effluent
P-concentrations [mg/L] and P-removal [%]
Water used
in 0 minutes in 10 minutes in 24 hours tap water 10.2 - 0.5 95.1 0.1 99.0
secondary effluent 10.5 - 1.8 82.9 0.1 99.0
laundry effluent 48.9 - 16.0 67.3 1.3 97.3
TOC-concentrations [mg/L] and TOC-removal [%]
laundry effluent 150 - 90 40.0 50 66.7
7.2.2.1 Phosphorus removal results
Compared to calcite (Table 6), the phosphorus removal efficiency of the tobermorite is much higher. In comparison, the reaction is also much faster. And, even in case of the highly polluted laundry effluent, a considerable efficiency of the P-removal was found, which should make it worth to follow up with column experiments.
7.2.2.2 Coating of the tobermorite
There was no significant change of the phosphorus removal efficiency after several reuses in agitation experiments as described for calcite in section 7.1.1.3. The
re-moval rates were quite constant in all cycles. But, infrared spectroscopy (FTIR-ATR) revealed the coating of the tobermorite with Ca-Phosphates (Figure 6).
Figure 6: Surface specific infrared spectroscopy (FTIR-ATR) on PO4 coated
tober-morite seed crystals
7.2.2.3 Influence of the DOC
The retarding influence of the DOC onto tobermorite was much less than regarding calcite (section 7.1.1.4), which can be seen in the results summarised in Table 7. The difference between tap water and secondary effluent after 10 minutes reaction time is very small, the final value is identical. And, even in case of the high polluted laundry effluent, a considerable efficiency of the P-removal was found, together with a re-moval of the TOC.
7.2.3 Results of column experiments
According to the promising results found in the agitation experiments, column ex-periments were started. Figure 7 shows a long time experiment with secondary efflu-ent of the FZK STP, spiked with NaH2PO4 to ≈10 mg/L P. The residence time was one bed volume per hour, the other experimental conditions are given in section 6.2. The results show that a good phosphorus removal efficiency up to more than 3000 bed volumes was attained. Starting the experiment, the pH of the effluent changed from pH = 7.6 to about pH = 9.0, but gradually decreased to pH = 8.3. Hence, a to-bermorite filtration seems to be able to replace a simultaneous precipitation in a treatment plant. But, for its verification, more experiments on larger scale have to be carried out to enable technical and cost evaluations.
0 2 4 6 8 10 12 0 500 1000 1500 2000 2500 3000 3500 bed volumes mg/l P outflow inflow
Figure 7: Column experiment with tobermorite and secondary effluent of FZK, spiked
to ≈ 10 mg/L P. Column: 50 cm2 x 10 cm, residence time 1 bed volume/h
Furthermore, experiments with a side stream water from a Phostrip plant were car-ried out. The results in Figure 8 show a quite good phosphorus removal efficiency and confirmed the general ability of the tobermorite for a successful treatment of this water. 0 20 40 60 80 100 0 500 1000 1500 2000 2500 bed volumes
P-Elimination % 2nd column attached
Figure 8: column experiment with tobermorite for the treatment of a Phostrip water
from a STP in Austria, c(P)o = 50 mg/L
8 Discussion
8.1
Summary on the experiments with calcite
The corresponding experiments, which comprise the fundamental data in all parts of section 4.2 and additionally the results given in section 7.1, can be summarised as follows:
• The efficiency of the calcite in the first cycle depends mainly on the SSA - BET of the calcite and the SICaCO3 of the water, but this effect is surpassed and equalised by “coating” with Ca-Phosphate mineral phases. This causes an im-provement of the phosphorus removal rate.
• Thus it seems possible to apply a cheap coarse material possibly upon a layer with sand for the water treatment, which should enhance the feasibility of a full scale application
• Organic carbon in the water even in lower concentrations (10 mg/L TOC in secondary effluent) caused a deterioration of the phosphorus removal. This seems due to a simultaneous TOC removal and the partial covering of the sur-face by the organic compounds.
8.2
Summary on the experiments with tobermorite
The experiments, which comprise the fundamental data in all parts of section 7.2 and additionally the column experiments in section 7.2.3, can be summarised like the following:
• The efficiency of the tobermorite depends mainly on the SSA - BET and the pH increase due to a slight dissolution in the water, together with crystallisa-tion effects according to the SICaCO3 of the water. Contrary to calcite (section 7.1.1.3) no improvement by the “coating effect” of the surface was observed. • There was only a slight retarding influence of the organic carbon in the water
at lower concentrations of about 10 mg/L TOC, as experiments with secondary effluent showed. But, there was a retarding effect with a higher polluted laun-dry effluent. This seems due to a simultaneous TOC removal. But, the effects are much smaller than with calcite Juraperle.
• Therefore, it seems suitable to apply a tobermorite filtration instead of a pre-cipitation to get coarse particles of Ca-Phosphate on tobermorite, which can be used for recycling.
9
Outlook and further planning
Regarding calcite, especially the influence of the TOC in the waters, has to be further pursued, also regarding the nature of the TOC. Since the calcite is much cheaper than the tobermorite, it should be the goal of further investigations to attain a solution. Regarding tobermorite, it is planned to extend the investigations on other phospho-rus rich industrial effluents.
Regarding secondary effluent, a tobermorite filtration seems to be able to replace a simultaneous precipitation in a treatment plant. But, for its verification, more experi-ments on larger scale have to be carried out to enable exact technical and cost evaluations.
10 REFERENCES
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Berg, U., Donnert, D., Weidler, P.G., Neumann, T., Ehbrecht, A., Reinhardt, M., Schweike, U. (2002): Calcium carbonate as active barrier component for the reten-tion of phosphorus in sediments. Geo- und Wassertechnologie 3/02, Adificatio-Verlag, Freiburg i.Br., ISSN 1610-3645
Bertrand I., Grigon N., Hinsinger P., Souche G. and B. Jaillard (2001): The Use of Secondary Ion Mass Spectrometry Coupled with Image Analysis to Identify and Lo-cate Chemical Elements in Soil Minerals: the Example of Phosphorus. Scanning, 23, 279-291.
Bernhardt, H. (1978): Phosphor – Wege und Verbleib in der Bundesrepublik Deutschland. Verlag Chemie Weinheim -. New York
Besch, W.-K., Hamm, A., Lehnhardt, B. et al. (1992): Limnologie für die Praxis – Grundlagen des Gewässerschutzes, 3. Edition, ecomed-Verlag, Landsberg am Lech. Brett, S., Guy, J., Morse, G.K., Lester, J.N. (1997): Phosphorus removal and recov-ery technologies. ISBN 0 948411 10 0, Selper Publications, London 1997.
Brunauer, S., Emmett P.H., and Teller E. (1931): Adsorption of Gases in Multi-molecular Layers. J. Am. Chem. Soc. 60, 309
Bundesanzeiger (1996): Rahmen-AbwassereVwV vom 31. Juli 1996, 48, 164a CEEP (Centre Européen d’Ètudes des Polyphosphates) (1998): Phosphates, a sus-tainable future in recycling. Dépôt légal D/1998/3158/15
Cornel, P. ( 2002): Rückgewinnung von Phosphor aus Klärschlamm und Klärschlammaschen in
Wiemer, K.; Kern, M. (Hrsg.), "Bio- und Restabfallbehandlung VI", Eigenverlag Witzenhausen-Institut, 2002, ISBN 3-928673-38-6
de Boer J.H., Lippens B.G., Linsen B.G., Broekhiff J.C.P., van den Heuvel A., Os-inga, Th. J. (1966): The t-Curve of Multimolecular N2-Adsorption.Journal of Colloid and Interface Science, 21, 405
Deutscher Bundesrat (2002): Über die Zukunft der landwirtschaftlichen Verwertung von Klärschlamm. Beschluss 313/02
DIN 38406-13, -14 (1992): German standards for analysis of Sodium and Potassium. DIN EN 1189 (1996): German standards for analysis of phosphorus.
DIN EN ISO 9963-1 (1995): German standards for analysis of alkalinity. DIN EN ISO 10304-1 (1995): German standards for analysis of anions.
Donnert, D., Salecker, M. (1999): Elimination of Phosphorus from Waste Water by Crystallisation.Env. Techn. 20, 735-742.
Donnert, D. (2001): Phosphat – Elimination und Rückgewinnung aus Abwasser und Rückhaltung in Sedimenten. FZK - Nachrichten 33 (2001), Nr. 1, 40-50
Eberle, S.H., Donnert, D. (1991): Berechnung des pH-Werts der Calcitsättigung eines Trinkwassers unter Berücksichtigung der Komplexbildung. Z. Wasser- Abwasserforsch. 24, (6), 1991, 258-268
Farmer A.M. (Editor). (1999): Implementation of the 1991 EU Urban Waste Water Treatment Directive and its Role in Reducing Phosphate Discharges (summary of Report). Scope Newsletter, 34, 34pp. http://www.ceep-phosphates.org
Freeman, J.S., Rowell, D.L. (1981). The adsorption and precipitation of phosphate onto calcite. J. Soil Sci. 32, 75-84.
German Standards DIN EN 1189 (1996) for phosphorus. DIN EN ISO 7980 (2000) for Calcium.
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Helmer, R., Sekoulov , I. (1977): Weitergehende Abwasserreinigung. ISBN 3-8078-8055-0. Deutscher Fachschriftenverlag Mainz-Wiesbaden
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11
Annex: Tables of the water compositions
11.1
Composition of artificial lake waters (section
6.3.1)
Lake Tegel Mulderes ervoir Lake Müggel Lake Müggel without carbonate
Addition of mmol/L mmol/L mmol/L mmol/L
NaHCO3 2.7 2.7 1.31 1.31 CaCl2•2H2O 1.07 0.6 0.71 0.71 CaSO4•2H2O 0.7 1.07 1.11 1.11 MgSO4•7H2O 0.4 0.7 0.45 0.45 CaCO3 Merck 0.23 3,72 0.18 --- NaH2PO4•H2O 0.33 0.33 0.33 0.33 pH ~ 7.4 ~ 7.2 ~ 7.5 ~ 7.5 SICaCO 3 - 0.06 - 0.42 - 0.40 -2.20
11.2
Composition of the tap water and waste waters (sections
6.3.2 and
6.3.3)
KS4.3 , KB8.2: titration equivalents to pH = 4.3 or to pH = 8.2 in mmol/L Tap Water FZK Phostrip water Secondary effluent FZK pH 7.1 7.04 7.10 SICaCO3 + 0.05 - 0.25 - 0.18 Concentrations [mg/L] Anions Cl 28,2 36.2 142.9 P 0.02 30.7 1.7 SO4 74.6 23.7 112.0 Cations Ca 101.0 57.8 92.1 Mg 13.1 20.7 14.4 K 3.3 28 22.0 Na 14.5 36.4 106.0 Si 5.6 - - DOC 1.0 65.0 8.8 sensor BSB - 6.0 3.1 COD - 32 73 DIC 64.2 77.1 65.4 Equivalents in [mmol/L] KS 4.3 4.6 - - KB 8.2 0.56 - -
Contents
1 ABSTRACT... 1
2 INTRODUCTION ... 1
3 SELECTION OF THE CRYSTALLISATION METHOD FOR PHOSPHORUS RECYCLING ... 2
4 CHEMICAL FUNDAMENTALS... 3
4.1 Crystallisation... 3
4.2 Mechanisms of phosphorus bonding onto calcite... 4
4.3 Mechanisms of phosphorus bonding onto tobermorite ... 4
5 MATERIALS USED FOR THE INVESTIGATIONS... 4
5.1 Calcites applied... 5 5.2 tobermorite applied... 5 6 EXPERIMENTAL PART... 5 6.1 Agitation experiments ... 6 6.2 Column experiments ... 6 6.3 Waters investigated ... 6
6.3.1 Tap water, artificial lake waters ... 6
6.3.2 Secondary effluent... 7
6.3.3 Supernatant from the Phostrip process... 7
6.3.4 Water analysis ... 8
6.3.5 Analysis of the seeds ... 8
7 RESULTS... 8
7.1 Results of experiments with calcite... 8
7.1.1 Results of agitation experiments ... 9
7.1.2 Results of column experiments with calcite and sand in tap water... 12
7.1.3 Results of column experiments with calcite Juraperle and secondary effluent.... 13
7.2 Results of experiments with tobermorite ... 14
7.2.1 Hydrochemistry... 14
7.2.2 Results of the agitation experiments ... 14
8 DISCUSSION ... 16
8.1 Summary on the experiments with calcite ... 16
8.2 Summary on the experiments with tobermorite... 17
9 OUTLOOK AND FURTHER PLANNING ... 17
10 REFERENCES ... 18
11 ANNEX: TABLES OF THE WATER COMPOSITIONS... 21
