Phosphate removal in anaerobic liquors by struvite crystallization without addition of chemicals: Preliminary results

Pergamon P l h S0043-1354(97)00137-1

War Res. Vol. 31, No. 11, pp. 2925-2929, 1997 ~) 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/97 $17.00 + 0.00

RESEARCH NOTE PHOSPHATE REMOVAL IN ANAEROBIC LIQUORS BY STRUVITE CRYSTALLIZATION WITHOUT ADDITION OF CHEMICALS: PRELIMINARY RESULTS P. BATTISTONI '*@, G. FAVA 2@, P. PAVAN 3, A. MUSACCO 3 and F. CECCHP @ 'Institute of Hydraulics and 2Department of Earth and Material Science, University of Ancona, Via Brecce Bianche, 60131 Ancona, Italy, 3Department of Environmental Science, Calle Larga S. Marta 2137, 30123 Venice, Italy and 4Department of Chemistry, Chemical Engineering and Materials, 67100 Monteluco di Roio, L'Aquila, Italy (Received January 1995; accepted in revised form April 1997)

Abstract--The feasibility of phosphate removal from the supernatant of anaerobically digested sludge by struvite (MAP, MgNH4PO4) crystallization in a fluidized-bed reactor (FBR) was studied. Quartz sand was used as seed material. Three successive steps were investigated to highlight the process: 1, natural aging of the supernatant, with phosphate concentrations in the range 18-164mg/1, which gave good crystallization of struvite within 3 days; 2, the inhibiting effect of Mg and bicarbonate ions on hydroxyapatite (HAP, Ca~OH(PO4)J) formation, which leads to struvite formation under supersaturation conditions; and 3, phosphate removal via the process of complete crystallization on the seed material, and which is obtained by means of external continuous aeration. All runs were performed without the addition of chemicals. © 1997 Elsevier Science Ltd Key words--crystallization, fluidized-bed reactor, magnesium ammonia phosphate, phosphorus removal,

struvite

INTRODUCTION Today several biological nutrient removal (BNR) plants exist throughout the world. However, phosphorus (P) removal may prove more difficult because of its release during either sludge handling or anaerobic digestion (sludge liquors contain levels of P in solution up to 100mg P/I when anaerobic conditions and readily biodegradable COD are present). In this context two main problems arise: 1, MAP crystallization inside and outside the digester when there is a high Mg and NH4 content (Loewenthal et al., 1994; Maqueda et al., 1994; Pitman et al., 1991; Shao et al., 1992); and 2, BNR process performance decreases because of supernatant recirculation (Murakami et al., 1987). Solutions adopted include expensive processes of P reprecipitation by chemicals and air flotation of wasted sludge (Pitman et al., 1991). A way to overcome these problems is based on P recovery by crystallization. Van Dijk (1985) recovered HAP pellets from the effluent of wastewater treatment plants treated in a FBR, while Fujimoto et al. (1991) and Momberg et al. (1992) obtained *Author to whom all correspondence should be addressed.

MAP or HAP pellets from anaerobic liquors. The crystallization requires an appropriate pH (8.6-10.6) for operating at a metastable supersaturation zone near to the critical supersaturation curve (Joko, 1985). By crossing this curve, phosphate precipitation may be obtained. However, the buffering capacity of the anaerobic liquor requires large additions of alkali to reach the operative pH. Alternatively, the possibility of increasing the pH with aeration was demonstrated by Pitman et al. (1991). This phenomenon was attributed to CO2 stripping. Loewenthal et al. (1994) showed that the CO2 partial pressure controls struvite precipitation inside an anaerobic digester. Only Fujimoto et al. (1991), however, utilized aeration to reduce alkali addition in MAP crystallization (8 ml/l of 5% NaOH and 1,4 ml/l of 12.5% MgCI2 were still needed). In previous papers the authors (Cecchi et al., 1994; Pavan et al., 1994) proposed an integrated BNR system. Anaerobic fermentation of the organic fraction of solid waste was used to produce volatile fatty acids to improve the BNR process performance. In that context struvite crystallization in anaerobic liquors was preliminarily investigated using different size reactors and adding various chemical reagents. In this paper struvite crystallization on a real anaerobic

2925

Research Note

2926

pH 7.3

Table 1. Physico-chemicalcharacteristics of supernatant P04 Mg Ca NH4 COl HCO3 (mg/l) (mg/l) (mg/l) (mg/l) (mg CaCO3/t) (mg CaCO3/I) 18 53 184 220 0 1430

digester effluent with no chemical addition is reported. The best operative conditions to minimize the process retention time are also described.

MATERIALS AND METHODS

Materials

The liquid from the dewatering section (belt press) of a 100 000 population equivalent civil wastewater treatment plant with nitrification, denitrification and anaerobic digestion of sludge was used. The physico-chemical parameters of the supernatant, as determined using Standard Methods (APHA, 1985), are reported in Table 1. Laboratory tests Natural aging. With this test the time behaviour of phosphate solutions was analysed. Anaerobic supernatants were phosphate enriched, using Na3PO4 from a 5000 mg/1 stock solution, and left in a thermostatically controlled room (25°C). Daily samples were taken and analysed. Supersaturation curves. To perform these tests synthetic and real liquors, with the appropriate amount of phosphate added, gradually had their pH increased by adding alkali and by aeration, respectively. Synthetic samples were prepared as distilled water solutions of CaC12, MgCh and NH4CI, and were organized in two classes. The first, termed HAP, is made up of 200, 100 and 50 mgCa/1; and the second, termed MAP, of 50 mg Mg/1 and 2130 mg N H J . The Joko (1985) methodology was modified, monitoring the solution transmittance at 400 nm and using the critical supersaturation point when the transmittance value reached the 80% (equivalent to a turbidity of 100-170 mg/1 of SiO2). Pilot test. A FBR, 58 mm i.d. and 0.42 m high, was used (Fig. 1). The column was flied with 610g quartz sand (0.21~).35 mm) to obtain an unexplained bed 0.15 m high. The bed was expanded to 0.3 m working at an upflow rate ranging from 1.8 to 5 l/min; the air flow rate adopted was 15 1/min. FBR tests were conducted in a discontinuous mode. The starting supernatant volume (storage tank plus void column; Fig. 1) was three times that of the expanded bed. Phosphates were analysed both on acidified raw effluent and on filtrate at 0.45/zm. NH4, Ca, Mg and alkalinity were analysed in each sample, and pH and air flow rate were continuously monitored.

RESULTS AND DISCUSSION The anaerobic supernatant has a Mg and Ca content (Table l) l0 times larger than the stoichiometric demand for M A P or H A P formation. The unusually low level of alkalinity is attributed to the large flow rate of final effluent, due to belt press washing, and its diluting effect. The initial approach of the study was to investigate the natural evolution of anaerobic liquor with phosphate contents in the range 18-164mg/1. The higher concentrations were used to simulate the anaerobic supernatants of a BNR process (Fig. 2). A decreasing phosphate concentration and an increase in pH were observed within 3 days. A loss of alkalinity from 50 to 35 mg CaCO3/1 was also evidenced (runs A - D , Table 2). This can be explained by considering the CO2 to be in equilibrium inside and outside the anaerobic digester, where the partial pressures were, respectively, related to 30-40% CO2 (biogas) and to 0.035% CO2 (air). P removal gradually decreases from 81 to 53% (Table 2) according to initial phosphate concentration. Prevalent M A P formation can be tested for with the ion specific molar loss (SML; initial minus final molar concentration with respect to phosphate molar loss). Similar events have been observed during the cooling of digested sludges in open lagoons with natural evolution of CO: (Salutsky et al., 1972). Along with these observations the suggestion was made that the process could be made faster by stripping CO2 with air. In order to establish the precipitation or crystalline nature of the solid obtained, the supersaturation curves of real supernatants and synthetic solutions were investigated (Fig. 3).

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Fig. 2, Natural time evolution of phosphates (--) and pH (---) in anaerobic supernatants for different starting concentrations of phosphate (see Table 2).

Research Note

2927

Table 2. Evolution of anaerobic supernatant SML Removal (%) PO4 Mg Ca

Initial PO4 (mg/l)

Run A B C D

81 80 76 53

164 t12 49 18

1 I I 1

The supersaturation condition for each starting solution was obtained by addition of alkali to the synthetic system and air stripping the anaerobic supernatant. Results show that different HAP curves are related to Ca content (cf. HAP200, HAP100 and HAP50, Table 3). Comparing HAP200, MAP and supernatant curves (Fig. 3), a major indication is observable: the supernatant supersaturation curve is obtained at a pH lower than for MAP, but higher than for HAP200. The supersaturation curve clarifies the natural evolution of supernatant. Phosphate-pH plots of runs A - D (Fig. 4) show that the metastable zone, indicated as the area under the supersaturation curve, is involved. This allows only MAP crystallization. The ion SML values for the anaerobic supernatant solutions are always the same: PO4:Mg:Ca = 1:1:0.3, suggesting that HAP is not formed and a calcium salt constitutes the nucleation seed for MAP. This hypothesis can justify the relative positions of the supersaturation curves, while the inhibitory effect on HAP formation must be attributed to the presence of bicarbonate and magnesium. The role of these ions on calcium phosphate crystallization in effluent of civil wastewater treatment plants was shown in previous studies. A level of alkalinity of 550mg/l (Joko, 1985) weakly affects phosphate removal, while with levels from 500 to 1000 mg/l the effluent quality is seriously influenced (Kaneko et al., 1988). Furthermore, magnesium determines a crystallization lag-time which is proportional to the concentration in the range

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1-5 mg/l (Kaneko et al., 1988). The operative Mg:Ca ratio determined is 0.2, very close to the value of 0.3 of the anaerobic supernatant employed in this work (Table 1). Even points far from the supersaturation curve runs D and C show considerable phosphate removal (Table 2). The ion SML values in natural evolution (Table 2) and the supersaturation curve suggest that the same phenomenon, prevalent MAP formation, is involved either with a slow room air equilibrium or with the faster air stripping. O n the basis of these preliminary results the nucleation of struvite in a bench-scale FBR was investigated using supernatant with a low phosphate concentration ( < 50 mg/1). These are considered the most conservative conditions in order to verify the feasibility of MAP crystallization using air stripping instead of chemical addition. External gradual aeration (EGA) or external continuous aeration (ECA) operating modes were adopted with the aim of limiting phosphate precipitation. For evaluating the process performance the following approach was considered: the difference between the completely soluble initial concentration and the effluent total phosphate concentration (TPC), suspended plus soluble, defines the amount crystallized as that trapped on the seed. In addition, the difference between TPC and the effluent soluble phosphate concentration (SPC) defines the precipitated form. Thus, close TPC and SPC values are related to good performance, while the opposite indicates inadequate operative conditions. Figure 5 shows that EGA conditions (air stripping for 56% of the total time) are insufficient to obtain rapid phosphate removal. On the contrary, ECA conditions give faster removal (Fig. 6) and reveal that there is no risk of MAP precipitation. Air stripping increases the pH from 7.9 to 8.3-8.6, and both experiments were carried out on solutions stored for a maximum of 1 day. The two curves on Fig. 7 show percentage removal up to 80%, The best operative condition is identified as processing the supernatant with external continuous aeration, since this leads to Table 3. Chemical characteristics of systems

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2928

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Fig. 7. Removal efficiency with different methodology. recycling MAP crystals (Fujimoto et al., 1991) is substituted by the use of an external seed material. These results indicate the feasibility of the MAP crystallization process via air stripping.

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Fig. 5. Performance of FBR operating under EGA conditions. the same performance in about half the time (150 min). The P removal evidenced is better than that previously reported by Fujimoto et al. (1991), who employed longer times to reach pH 8.3 and added magnesium. This result can be attributed to the difference between the operative conditions employed: the auto-nucleating environment created by

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The conclusions that can be drawn from these preliminary results are as follows: 1) it is confirmed that aging the anaerobic supernatant makes it possible to obtain crystallization of MAP starting from a typical CO2 partial pressure in the digester ( 3 5 ~ 0 % CO2) and reaching that of air (0.035% CO2). 2) The supersaturation curve shows a surprising MAP formation attributed to the inhibitory effect of magnesium and bicarbonate ions on HAP growth. 3) The combination of air stripping and quartz sand allows us to avoid the use of chemicals to reach appropriate pH values for struvite crystallization. 4) All the phosphate is crystallized as MAP and no precipitate is formed when external continuous aeration (removal of up to 80% over 120-150 min) is adopted. 5) Continuous external aeration represents the best operative condition for reducing the crystallization time. Acknowledgements--The authors would like to thank the

30

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Fig. 6. Performance of FBR operating under ECA conditions.

National Research Council (CNR) of Italy for financial support.

REFERENCES APHA, AWWA, and WPCF (1985) Standard Methods for the Examination of Water and Wastewater, 16th Edn. American Public Health Association, Washington, D.C. Cecchi F., Battistoni P., Pavan P., Fava G. and Mata-Halvarez J. (1994) Anaerobic digestion of OFMSW and BNR processes: a possible integration. Preliminary results. Wat. Sci. Technol. 30, 65-72. Fujimoto N., Mizouchi T. and Togami Y. (1991) Phosphorous fixation in the sludge treatment system of a biological phosphate removal--accomplishments and needs. Wat. Res. 25(12), 1471-1478.

Research Note Joko 1. (1984) Phosphorous removal from wastewater by the crystallization method. Wat. Sci. Technol. 17, 121-132. Kaneko S. and Nakajima K, (1988) Phosphorus removal by crystallization using a granular activated magnesia clinker. J. Wat. Pollut. Control Fed. 60(7), 1239-1244. Loewenthal R. E., Kornmuller, U. R. C. and Van Heerden E. P. (1994) Struvite precipitation in anaerobic treatment systems, pp. 498-507. In Seventh International Symposium on Anaerobic Digestion, Capetown, South Africa, 23-27 Jan. 1994. Maqueda C., Perez Rodriguez J. L. and Lebrato J. (1994) Study of struvite precipitation in anaerobic digesters. Wat. Res. 2g(2), 411-416. Momberg G. A. and Oellerman R. A. (1992) The removal of phosphate by hydroxyapatite and struvite crystallization in South Africa. Wat. Sci. Technol. 26(5/6), 967-976. Murakami T., Koike S., Taniguchi N. and Esumi H. (1987) Influence of return flow phosphorus load on performance of the biological phosphorus removal process, pp. 237248. In IA WPRC Specialized Conference on Biological

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Phosphate Removal From Wastewaters, Rome, Italy, 28-30 Sep. 1987. Pavan P., Battistoni P., Musacco A. and Cecchi F. (1994) Mesophilic anaerobic fermentation of SC-OFMSW: a feasible way to produce RBCOD for BNR processes. In Int. Syrup. Pollution of the Mediterranean Sea, Nicosia, Cyprus, 2-4 Nov. 1996. Pitman A. R., Deacon S. L. and Alexander W. V. (1991) The thickening and treatment of sewage sludges to minimize phosphorus release. Wat. Res. 25(12), 1285-1294. Salutsky M. L., Dunseth M. G., Ries K. M. and Shapiro J. J. (1972) Ultimate disposal of phosphate from wastewater by recovery as fertilizer. Effl. Wat. Treat. J~ October 509-519. Shao Y. J., Wada F., Abkian V., Crosse J., Horenstein B. and Jenkins D. (1992) Effect of MRCT on enhanced biological phosphorous removal. Wat. Sci. TechnoL 26, 967-976. Van Dijk J. C. and Braakensiek H. (1985) Phosphate removal by crystallization in a fluidized bed. Wat. Sci. Technol. 17, 133-142.