Modelling of iron, sulfur and phosphorus interactions

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Modelling of iron, sulfur and phosphorus interactions in sewer-wastewater treatment plant integrated system Lisha Guo1, Keshab Raj Sharma1 and Zhiguo Yuan1 1

Advanced Water Management Centre, The University of Queensland, St-Lucia, QLD 4072, Australia (Email: [email protected]; [email protected]; [email protected])

Abstract A modelling study was done to investigate the interaction between iron (Fe), sulfur (S) and phosphorus (P), and the impact of sewer iron dosing on a downstream WWTP. In this purpose, an integrated model of sewer system and wastewater treatment plant (WWTP) was built. Simulation results of a real sewer-WWTP system were compared under three scenarios, respectively with: no iron dosing, iron dosing in WWTP only and iron dosing in sewer only. It was found that chemical iron dosing can improve both phosphate and sulfide removal. The Fe-P-S interaction shows that the iron precipitates formed in the upper stream facilities can be used in the downstream facilities, suggesting a better use of iron salt in the management of sewer-WWTP integrated system. Keywords Modelling; Iron dosing; Sewer-WWTP integrated system; Sulfide; Phosphate

INTRODUCTION Hydrogen sulfide (H2S) is an odorous gas that engineers aim to eliminate from sewer system, not only because of its unpleasant smell but also due to that fact that the oxidation of H2S to sulfate causes sewer pipe corrosion. Iron solutions, ferric (FeIII) and ferrous (FeII) salts, are commonly used to remove H2S from the bulk water of the sewer. Several studies have been carried out to investigate the precipitation efficiency of iron agents (Hvitved-jacobsen, et al., 2013; Padiva et al., 1995). However, it is also necessary to study the iron dosing effect from a large scale, i.e. the management and control of a full-scale sewer system, as well as its impact on end-of-pipe wastewater treatment plants (WWTPs). This paper presents a work of the modelling of iron (Fe) - sulfur (S) -phosphorus (P) interactions in the sewer-WWTP system. Different iron dosing strategies are tested in a real sewer system located in Elanora (QLD), Australia. Light is shed on how the sewer-WWTP integrated system, especially in terms of sulfide and phosphate removal, reacts under these strategies. This paper provides a framework for sewer-WWTP integrated modelling. Scenarios operating with agents other than iron or under different strategies will be feasible in the future. METHOD Elanora sewer network and WWTP The Elanora sewer system has three branches, namely B09, B47 and C27. B09 and B47 are composed of both gravity mains and rising mains, while C27 only has rising mains. The three branches are combined together and the sewage is delivered to the Elanora WWTP. The plant uses modified Ludzack-Ettinger (MLE) process, i.e. denitrification-nitrification treatment, with an average influent flowrate being around 23,000 m3/d. Ferric chloride (FeCl3) is added 243

Poster Session Adavanced physico-chemical WWTP modelling

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directly near the inlet of secondary settlers in order to remove phosphate (SPO4). Waste sludge from primary and secondary settlers is pumped to anaerobic digester. a. B09 (L: 9260m, D: 100-375mm ) B47 (L: 7723m, D: 100-1050mm)

WWTP

C27 (L: 22603m, D:100600mm) b. Internal recycle 1 Anoxic tank 1 (Denitrification)

Influent

Aerobic tank 1 (Nitrification)

Primary settler

Secondary settler 1

Sludge recycle 1 Anoxic tank 2 (Denitrification)

Aerobic tank 2 (Nitrification)

Underflow

Internal recycle 2 Sludge recycle 2

Thickner recycle Supernatant

Sludge thickner

Anaerobic digester

Effluent

Secondary settler 2

Underflow

Supernatant

Dewatering Waste sludge

Dewatering recycle

Figure 1. Map of the modelled system and the WWTP Model Sewer model The sewer system is modelled by SeweX, a modelling package focusing on sewer bioreactions. The SeweX model is a further development of the WATS model (Hvitvedjacobsen et al., 2013; Sharma et al., 2008), putting great improvement in the modelling of sulfur transformation under aerobic and anoxic conditions, through biological and chemical processes. In this paper, the modelling of iron reacting with sulfide and phosphate is improved. Especially, the sulfide precipitation with ferric salt is modelled as two steps, including ferric reduction with sulfide, Eq.1, and ferrous ion precipitated with sulfide ion, Eq.2. Fe3+ + S2Fe2+ + S2-

Fe2++ S0 + OHFeS (solid)

Eq.1 Eq.2

Ferric salt, like FeCl3, is acid solution because the ferric ion is easily hydrated. When FeCl3 is added into the water with SPO4 existing, two precipitates may form, i.e. ferric hydroxide and 244


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ferric phosphate. The pH-logFe profile is shown below. It can be seen that the formation of the two precipitates depends on pH.

[Fe3+] from Ksp_FePO4

[Fe3+] from Ksp_Fe(OH)

3 Figure 2. pH-logC profile for ferric (FeIII) and SPO4 (P) system (Ksp is solubility product constant) The modelling of precipitation is still under study. Kazadi Mbamba et al. (2015) proposed a precipitation model based on solving of ordinary differential equations (ODE). The parameters of the precipitation rates were obtained based on experiments. However, in this study, which lacks of experimental data about the precipitation rate in full-scale sewer and WWTP, a simpler method, i.e. an algebraic equation (AE), is used. For example, in the solution contains both ferric salt and phosphate, i.e. a FeIII-P system, there are four possible assumptions as list in the following table. The general idea is to find out which is the correct assumption. A detail procedure is given in Appendix A. Table 1. Four possible situations under FeIII-P system No solids Only Fe(OH)3 Only FePO4 [Fe3+][OH-]^3 ≤KspFe(OH)3 >KspFe(OH)3 ≤KspFe(OH)3 3+ 3[Fe ][PO4 ] ≤KspFePO4 ≤KspFePO4 >KspFePO4

Two solids >KspFe(OH)3 >KspFePO4

WWTP model The activated sludge reactors of WWTP are modelled by a modified ASM2d that extends the original ASM2d with Fe and S transformations. The new components include element sulfur (SS0), sulfate (SSO4), dissolved sulfide (SH2S), soluble ferrous (SFeII), soluble ferric(SFeIII), ferrous hydroxide (XFeIIOH), ferric hydroxide (XFeIIIOH), ferrous sulfide (XFeIIS) and ferric phosphate (XFeIIIPO4). Importantly, this modified ASM2d model takes into account the oxidation of ferrous sulfide in the aerated tank discovered by Schippers and Jorgensen (2002). FeS + 2.25O2 + 2.5H2O Fe(OH)3 + SO42- + 2H+ Eq.3 Oxidation of element sulfur and dissolved sulfide is added following the work of Gutierrez et al. (2010). The precipitation uses the same way as sewer system. The anaerobic digester is modelled by a modified ADM1 that is extended with the hydrolysis of organic sulfur (Xsd), sulfide production by sulfate reduction bacteria (SRB), lysis of SRB, ferric reduction with sulfide and precipitation processes among iron, sulfide and phosphate. Again, the same method used in the sewer system is also applied for the precipitation modelling in digester. 245


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Interfaces An interface model is built to connect sewer effluent and WWTP influent based on mass balance of COD, N, P, S and Fe. The total cation (STcat) is solved based on pH and charge balance. The ASM-ADM interfaces following the same idea of Flores-Alsina et al. (2015). Scenarios Three scenarios are discussed in this paper, as shown in the table below. As said before, the WWTP currently doses FeCl3 at the inlet of secondary settlers (Case 2) to get the ferric concentration around 9.5 mg Fe/l. Case 3 is to study how the iron dosing in sewer system influences WWTP. Ferrous chloride (FeCl2) is added at the inlet of two branches, i.e. B09 and B47. It is supposed that the ferrous sulfide formed in sewer can be used to remove SPO4 in the aeration tank too as demonstrated in Eq. 3, indicating a multiple of iron in the sewer-WWTP system (Pikaar et al., 2014). To achieve this study purpose, the primary settler is removed in Case 3 in order to let ferrous sulfide go through and reach the bioreactor. Table 2. Iron dosing in three scenarios Case 1 Case 2 Sewer WWTP Note: “ ” means no iron dosing, “ ” means iron dosed.

Case 3

RESULTS Effect of iron dosing in WWTP Compared to Case 1, the effluent SPO4 is removed under Case 2, with sulfide gas produced in anaerobic digester also decreases. This is because within the digester, as pH decreases, XFeIIIPO4 is broken down, releasing ferric ion. The released ferric ion helps sulfide remove as Eq.1 and Eq.2. b. Digester H2S gas concentration

H2S gas (ppm)

SPO4 (mgP/l)

a. Effluent SPO4

Time (Day)

Time (Day)

Figure 3. Comparison of effluent SPO4 and digester H2S gas under Case 1 and Case 2

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Effect of iron dosing in sewer Figure 4 shows that SH2S within the two dosing sewer branches is well controlled, resulted in a large cut down on SH2S at the sewer outlet. Corresponding to the decrease of SH2S, XFeIIS is formed.

SH2S (mgS/l)

a. SH2S concentration in the sewer without iron dosing (Case 1 and Case 2)

Time (Day)

SH2S (mgS/l) S-FeS (mgS/l)

b. SH2S and S-FeS concentration in the sewer with iron dosing (Case 3)

Time (Day)

Figure 4. SH2S and FeS concentration in the sewer under iron dosing

Figure 5 shows that the XFeIIS is oxidized into ferric ion which precipitates SPO4 in the aerated tanks. Similar as the iron dosing in WWTP (Case 3), sulfide gas is also decreased in the digester. b. Digester H2S gas concentration

H2S gas (ppm)

SPO4 (mgP/l)

a. Effluent SPO4

Time (Day)

Time (Day)

Figure 5. Comparison of effluent SPO4 and digester H2S gas under Case 1 and Case 3

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DISCUSSION Primary settler It must be noted that KLa of Case 3 is increased by 10% compared to Case 2 in order to achieve the similar DO level and ammonia removal. This is because without primary settler, more organics flow into the bioreactor, consuming more oxygen. Therefore in order to meet this increased DO demand in the aeration tank, an increased KLa value was used in Case 3. Figure 6 shows that the three scenarios get similar results in terms of effluent ammonia and COD.

Figure 6. Effluent COD and ammonia under three scenarios Sulfide production in digester In the modified ADM1 model, the sulfide production by SRB only includes the use of hydrogen as electron donor, a process that is well proved (Batstone et al., 2006). However, it must be noted that SRB can also utilize organics (Fedorovich et al., 2003), which deserves to be considered in the future study. Actually, one major source of sulfide in digester comes from the hydrolysis of organics, especially protein. Therefore, one extra component, organic sulfur (Xsd), was added. The authors use this simple method, instead of adding sulfide release with organic (protein) degradation in order to avoid the complex of mass balancing of sulfur in the model. Sulfur is a tracer element in many organics and little information was found about its content. Also, organics is one product of bacteria decay and the sulfur contained in these products may not necessarily the same as the organics fed to the digester. CONCLUSION A sewer-WWTP integrated model was built and a scenario analysis was carried out focusing on the Fe-S-P interactions and the impact from upstream sewer iron dosing on the integrated system. Iron dosing in the sewer system, which is aimed to remove H2S, also benefits phosphate removal in both the sewer system and the WWTP.

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REFERENCES

Batstone D. (2006) Mathematical modelling of anaerobic reactors treating domestic wastewater: Rational criteria for model use. Reviews in Environmental Science and Bio/Technology, 5, 57-71. Fedorvich V., Lens P. and Kalyuzhnyi S. (2003) Extension of Anaerobic Digestion Model No. 1 with Processes of Sulfate Reduction. Applied Biochemistry and Biotechnology, 109, 33-45. Flores Alsina X., Solon K., Kazadi-Mbamba C., Tait S., Gernaey K., Jeppsson U. and Batstone D. (2015) Modelling phosphorus (P), sulphur (S) and iron (Fe) interactions during the simulation of anaerobic digestion processes. In Proceedings of the 14th World Congress on Anaerobic Digestion. Hvitved-jacobsen, T., Vollertsen, J. and Nielsen, A. H. (2013) Sewer processes: microbial and chemical process engineering of sewer networks, CRC press. Kazadi Mbamba, C., Flores-Alsina, X., Batstone, D. and Tait, S. (2015a). A generalized chemical precipitation modelling approach in wastewater treatment applied to calcite. Water Research, 68, 342-353. Padiva N.V., Kimbell W.A. and Redner J.A. (1995) Use of iron salts to control dissolved sulfide in trunk sewers. Journal of Environmental Engineering, 121, 824-829. Pikaar I., Sharma K. R., Hu S., Gernjak W., Keller J. and Yuan Z. (2014). Reducing sewer corrosion through integrated urban water management. Science, 345, 812-814. Schippers, A. and Jørgensen, B.B. (2002) Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments. Geochimica et Cosmochimica Acta, 66 (1), 85–92. Sharma, K. R., Yuan, Z., De haas, D., Hamilton, G., Corrie, S. and Keller, J. (2008) Dynamics and dynamic modelling of H2S production in sewer systems. Water Research, 42, 2527-2538.

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Appendix A Total_FeIII Total_P from ODE

[Total_FeIII]*[Total_P]0

No

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