-1Presented at the WISA 2000 Biennial Conference, Sun City, South Africa, 28 May to 1 June 2000
EXTERNAL NITRIFICATION IN BNR ACTIVATED SLUDGE SYSTEMS WITH VARYING AEROBIC MASS FRACTIONS R MOODLEY, MC WENTZEL and GA EKAMA Water Research Group, Department of Civil Engineering University of Cape Town, Rondebosch, 7700, Cape Town, South Africa; email:
[email protected] SUMMARY With external nitrification (EN), the nitrification process is removed from the suspended medium biomass of the biological nutrient removal (BNR) activated sludge (AS) system and transferred to an external fixed medium system. The EN system was successful in nitrifying 76% of the ammonia in the BNRAS system allowing a 50% reduction in the sludge age and a 100% increase in the treatment capacity of the system. Increasing the aerobic mass fraction by 50% stabilized and improved the anoxic P uptake resulting in a P removal of 0.015 mgP per mg influent total COD. The increased aerobic mass fraction increased the carbonaceous OUR from 35 to 38 mgO/l aerobic reactor/h, while causing the sludge settleability (DSVI) to deteriorate from 80 to 150 ml/g. Dosing nitrate to the main anoxic reactor reduced the carbonaceous OUR by 1.42 mgO/(l.h) per 1mgNO3-N/l influent dosed. ________________________________ 1.
INTRODUCTION
The biological nutrient removal activated sludge (BNRAS) system has become an established technology in wastewater treatment practice. However, implementation of BNR has brought with it a new set of difficulties (Ekama and Wentzel, 1999a), the main ones being (i) the long sludge age required for nitrification, (ii) filamentous organism bulking and (iii) the treatment /disposal of supernatants generated from the sludge and solids handling. This paper focuses on the long sludge age for nitrification and investigates means of reducing it by nitrifying externally to the BNR activated sludge system. In the BNR activated sludge (BNRAS) system, the requirement to nitrify governs selection of the two linked parameters, sludge age and the aerated mass fraction. The need for nitrogen (N) and phosphorus (P) removal sets a requirement for a part of the sludge to be unaerated, for anaerobic conditions to stimulate P removal and anoxic conditions for N removal. In N and P removal plants, the unaerated sludge mass fraction usually needs to be high, i.e. > 40%, causing the aerated mass fraction to be reduced , i.e. 80%), would lead to high nitrate recycle to the anaerobic reactor and consequently reduced BEPR. As a result of the poor nitrification efficiency in the external nitrification system, the nitrate recycled to the anaerobic reactor in Configuration 3 due to internal nitrification (in the main aerobic reactor) decreased the BEPR to 8.8 mgP/l. From sewage batch 26, the fixed media stone column was abandoned and replaced with a suspended medium external nitrification system for Configuration 4. This change to the external nitrification system showed an improvement in external nitrification and a decline in internal nitrification and hence the nitrate recycled to the anaerobic reactor decreased, improving the BEPR of Configuration 4 to 12.4 mg/l with 60% and 34% P uptake in the main anoxic and aerobic reactors respectively. 3.6
Filament Identification and Sludge Settleability
For the 13 monthly filament identifications on the system, the % frequency of dominance and occurrence were calculated as the number of times a particular filament was dominant (most abundant filament) and occurred (observed). The most frequently dominant and occurring filaments were M. parvicella, at 81% and 92% respectively, type 1851 at 44% and 30% and type 0092 at 13% and 15 % respectively. Type 021N also occurred occasionally but being a septic sewage filament (Jenkins et al., 1984) this was probably due to aging sewage during storage. Apart from type 021N, these filaments are almost always observed in full scale NDBEPR systems (Blackbeard et al., 1988) and are classified anoxic -aerobic (AA) filaments by Casey et al (1994). The average NOx concentrations leaving the anoxic reactor in the system was 2.8 mgNOx-N/l. The stone column external nitrification system (Configuration 3) was replaced with the suspended medium system (Configuration 4) which led to a significantly improved nitrification efficiency, the aerobic mass fraction was increased to 30% at the expense of the anoxic mass fraction (30%), the sludge age was increased from 8 to 10 days and a 1:1 mixed liquor a-recycle from the aerobic to the anoxic reactor was installed. All these changes led to increased nitrate load on the main anoxic reactor and a decreased denitrification potential, leading to high nitrate and nitrite concentration in the outflow of the anoxic to the aerobic reactor. The DSVI at the start of the investigation was around 125 ml/g and decreased to 80 ml/g by the end of Configuration 2. In the 2nd half of the investigation (Configurations 3 and 4) the DSVI increased to 150 ml/g as a result of changes in the system and the increase in the aerobic mass fraction and sludge age. This observation i.e. an increase in the NOx concentration leaving the anoxic reactor with the increase in aerobic mass fraction and with the simultaneous increase in the DSVI, supports the AA filament bulking hypothesis of Casey et al. (1994). 4.
CONCLUSIONS
Evaluation of the EN BNRAS system at laboratory-scale indicates that the scheme holds considerable potential for BNR system intensification through the reductions in sludge age and oxygen demand and significant improvement in sludge settleability. Because the suspended sludge is not required to nitrify, its anoxic fraction can be considerably enlarged at the expense of the aerobic mass fraction creating conditions that (i) allow it to achieve high N removal with domestic wastewaters with high TKN/COD ratios and (ii) promote anoxic P uptake polyphosphate accumulating organisms (PAOs) to develop. However, the size of the anoxic reactor must be limited such that the denitrification potential of this reactor closely matches the nitrate load. This investigation confirmed the observation of Hu et al., (2000) that the system can consistently produced a good settling sludge but the sludge age needs to be short and the aerobic mass fraction small. 5.
ACKNOWLEDGMENTS
This research was financially supported by the Water Research Commission, Water Sanitation Services SA (Pty) Ltd , the National Research Foundation and University of Cape Town and is published with their permission. The writers wish to express theirs appreciation to Mr Peter Tapscott for filament identification during the investigation. R MOODLEY et al.
-8Acknowledgment is also due to the Water Research Commission Steering Committee under the chairmanship of Mr Greg Steenveld, for guidance of this research. 6.
REFERENCES
Blackbeard J.R., Gabb D.M.D., Ekama G.A and Marais G.v.R. (1988). Identification of filamentous organisms in nutrient removal activated sludge plants in South Africa. Water SA, 14(1), 29-34. Bortone G., Saltarelli R., Alonso V., Sorm R., Wanner J. and Tilche A.(1996) Biological anoxic phosphorus removal -The DEPHANOX process. Water Sci. Technol., 34(1-2), 119-128. Bortone G., Sorm R., Wanner J. and Tilche A.(1997) Improvement of anoxic phosphate uptake with the DEPHANOX process. Procs. 2nd ACE CR International Conference, Jilhlava, Czech Rep., 1-18 Casey T.G., Wentzel M.C., Ekama G.A., Loewenthal R.E. and Marais G.v.R (1994). An hypothesis for the cause and control of anoxicaerobic (AA) filament bulking in nutrient removal activated sludge systems. Water Sci. Technol., 29(7), 203-212. Clayton J.A., Ekama G.A., Wentzel M.C. and Marais G.v.R. (1991). Denitrification kinetics in biological nitrogen and phosphorus removal systems treating municipal wastewaters. Water Sci. Technol., 23(4/6-2), 1025-1035. Ekama G.A., Wentzel M.C. (1999a). Difficulties and developments in biological nutrient removal technology and modelling. Water Sci. Technol., 39(6), 1-11. Ekama G.A. and Wentzel M.C.(1999b). Denitrification kinetics in biological N&P removal activated sludge systems treating municipal wastewaters. Water Sci. Technol., 39(6), 69-77. Hu Z., Wentzel MC., Ekama., (1999). External Nitrification in biological nutrient removal activated sludge systems. (Submitted to Water SA). Jenkins D., Richard M.G. and Daigger G.T. (1984). Manual on the cases and control of activated sludge bulking and foaming. Published by Water Research Commission, P O Box 824, Pretoria, 0001, RSA Kerrn-Jespersen J.P. and Henze M. (1993). Biological phosphorus uptake under anoxic and oxic conditions. Water Research, 27(4), 617-624. Kuba T., van Loosdrecht M.C.M., Brandse F.A. and Heinen J.J. (1997). Occurrence of denitrifying phosphorous removing bacteria in modified UCT-type wastewater treatment plants. Water Research, 31(4), 777-786. Lee S-E., Koopman B.L., Bode H and Jenkins D (1983). Evaluation of alternative sludge settleability indices. Water Research, 17(10), 1421-1426.
Mellin H.K.O., Lakay M.T Wentzel M.C.and Ekama G.A (1997). The effect of high temperatures (30oC) on biological nutrient removal performance, Research Report W91, Dept. of Civil Eng., Univ.of Cape Town, Rondebosch, 7700, W.Cape, South Africa Moodley R., Wentzel M.C and Ekama G.A (1999). External nitrification in biological nutrient removal systems. Research Report W100, Dept. of Civil Eng., Univ.of Cape Town, Rondebosch, 7700, W.Cape, South Africa Musvoto E.V., Casey T., Ekama G.A., Wentzel M.C. and Marais G.v.R. (1992). The effect of a large anoxic mass fraction and concentrations of nitrate and nitrite in the primary anoxic zones on low F/M filament bulking in nutrient removal activated sludge systems. Research Report W77, Dept. Of Civil Eng., Univ. of Cape Town, Rondebosch, 7700, W. Cape, South Africa. Pilson R.A., Ekama., Wentzel M.C. and Casey T.G. (1995). The effect of temperature on denitrification kinetics and biological excess phosphorous removal in biological nutrient removal systems in temperature climates (12 oC - 20 oC). Research Report W86, Dept. of Civil Eng., Univ.of Cape Town, Rondebosch, 7700, W.Cape, South Africa. Randall E.W., Wilkinson A and Ekama G.A (1991). An instrument for the direct determination of oxygen utilization rate. Water SA., 17(1), 11-18. Sneyders M.J., Wentzel M.C. and Ekama G.A. (1998). The effect of dosing unstabilized landfill leachate to a nutrient removal activated sludge system. Research Report W95, Dept. of Civil Eng., Univ.of Cape Town, Rondebosch, 7700, W.Cape, South Africa. Sorm R., Bortone G., Saltarelli R., Jenicek P., Wanner J and Tilche A (1996). Phosphate uptake under anoxic conditions and fixed media nitrification in nutrient removal activated sludge system. Water Research, 30(10), 1573-1584. Standard Methods (1985) Standard methods for the examination of water and wastewater (16th Edn.), APHA, AWWA, WEF, Alexandria, VA, USA. Wentzel M.C., Ekama G.A., Dold P.L. and Marais G.v.R. (1990). Biological excess phosphorus removal – Steady state process design. Water SA, 16(1), 29-48. Wentzel M.C., Mbewe A., Lakay M.T and Ekama G.A (1999). Batch test for characterization of the carbonaceous material in municipal wastewaters . Water SA., 25(3), 327-335.
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