Handbook of biological wastewater treatment 2nd ed a c van haandel, j g m van der lubbe (IWA publishing, 2012)

Handbook of Biological Wastewater Treatment Handbook of Biological Wastewater Treatment Design and Optimisation of Activated Sludge Systems Second Edition A.C van Haandel and J.G.M van der Lubbe www.wastewaterhandbook.com Published by IWA Publishing Alliance House 12 Caxton Street London SW1H 0QS, UK Telephone: +44 (0)20 7654 5500 Fax: +44 (0)20 7654 5555 Email: publications@iwap.co.uk Web: www.iwapublishing.com First published 2012 © 2012 IWA Publishing Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright, Designs and Patents Act (1998), no part of this publication may be reproduced, stored or transmitted in any form or by any means, without the prior permission in writing of the publisher, or, in the case of photographic reproduction, in accordance with the terms of licenses issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licenses issued by the appropriate reproduction rights organization outside the UK Enquiries concerning reproduction outside the terms stated here should be sent to IWA Publishing at the address printed above The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for errors or omissions that may be made Disclaimer The information provided and the opinions given in this publication are not necessarily those of IWA and should not be acted upon without independent consideration and professional advice IWA and the Author will not accept responsibility for any loss or damage suffered by any person acting or refraining from acting upon any material contained in this publication British Library Cataloguing in Publication Data A CIP catalogue record for this book is available from the British Library ISBN 9781780400006 (Hardback) ISBN 9781780400808 (eBook) Contents Preface xv Notes on the second edition xvii About the authors xxi Acknowledgements xxiii Symbols, parameters and abbreviations xxv Chapter Scope of text 1.0 1.1 1.2 1.3 1.4 Introduction Advances in secondary wastewater treatment Tertiary wastewater treatment Temperature influence on activated sludge design Objective of the text Chapter Organic material and bacterial metabolism 2.0 2.1 2.2 Introduction Measurement of organic material 2.1.1 The COD test 2.1.2 The BOD test 2.1.3 The TOC test Comparison of measurement parameters 9 10 12 15 16 vi Handbook of Biological Wastewater Treatment 2.3 Metabolism 2.3.1 Oxidative metabolism 2.3.2 Anoxic respiration 2.3.3 Anaerobic digestion 17 18 20 22 Chapter Organic material removal 25 3.0 3.1 3.2 3.3 3.4 3.5 Introduction Organic material and activated sludge composition 3.1.1 Organic material fractions in wastewater 3.1.2 Activated sludge composition 3.1.2.1 Active sludge 3.1.2.2 Inactive sludge 3.1.2.3 Inorganic sludge 3.1.2.4 Definition of sludge fractions 3.1.3 Mass balance of the organic material Model notation Steady-state model of the activated sludge system 3.3.1 Model development 3.3.1.1 Definition of sludge age 3.3.1.2 COD fraction discharged with the effluent 3.3.1.3 COD fraction in the excess sludge 3.3.1.4 COD fraction oxidised for respiration 3.3.1.5 Model summary and evaluation 3.3.2 Model calibration 3.3.3 Model applications 3.3.3.1 Sludge mass and composition 3.3.3.2 Biological reactor volume 3.3.3.3 Excess sludge production and nutrient demand 3.3.3.4 Temperature effect 3.3.3.5 True yield versus apparent yield 3.3.3.6 F/M ratio 3.3.4 Selection and control of the sludge age General model of the activated sludge system 3.4.1 Model development 3.4.2 Model calibration 3.4.3 Application of the general model Configurations of the activated sludge system 3.5.1 Conventional activated sludge systems 3.5.2 Sequential batch systems 3.5.3 Carrousels 3.5.4 Aerated lagoons 25 26 26 27 29 29 29 30 31 36 38 38 39 40 40 44 45 49 53 53 56 58 62 63 65 67 70 73 76 77 78 78 79 81 82 Contents Chapter Aeration 4.0 4.1 4.2 Introduction Aeration theory 4.1.1 Factors affecting kla and DOs 4.1.2 Effect of local pressure on DOs 4.1.3 Effect of temperature on kla and DOs 4.1.4 Oxygen transfer efficiency for surface aerators 4.1.5 Power requirement for diffused aeration Methods to determine the oxygen transfer efficiency 4.2.1 Determination of the standard oxygen transfer efficiency 4.2.2 Determination of the actual oxygen transfer efficiency Chapter Nitrogen removal 5.0 5.1 5.2 5.3 5.4 Introduction Fundamentals of nitrogen removal 5.1.1 Forms and reactions of nitrogenous matter 5.1.2 Mass balance of nitrogenous matter 5.1.3 Stoichiometrics of reactions with nitrogenous matter 5.1.3.1 Oxygen consumption 5.1.3.2 Effects on alkalinity 5.1.3.3 Effects on pH Nitrification 5.2.1 Nitrification kinetics 5.2.2 Nitrification in systems with non aerated zones 5.2.3 Nitrification potential and nitrification capacity 5.2.4 Design procedure for nitrification Denitrification 5.3.1 System configurations for denitrification 5.3.1.1 Denitrification with an external carbon source 5.3.1.2 Denitrification with an internal carbon source 5.3.2 Denitrification kinetics 5.3.2.1 Sludge production in anoxic/aerobic systems 5.3.2.2 Denitrification rates 5.3.2.3 Minimum anoxic mass fraction in the pre-D reactor 5.3.3 Denitrification capacity 5.3.3.1 Denitrification capacity in a pre-D reactor 5.3.3.2 Denitrification capacity in a post-D reactor 5.3.4 Available nitrate Designing and optimising nitrogen removal 5.4.1 Calculation of nitrogen removal capacity 5.4.2 Optimised design of nitrogen removal 5.4.2.1 Complete nitrogen removal 5.4.2.2 Incomplete nitrogen removal vii 85 85 88 89 89 91 92 94 97 97 99 107 107 108 108 110 115 115 117 120 123 124 134 136 137 141 142 142 143 146 146 147 149 151 151 153 156 158 160 165 166 169 viii Handbook of Biological Wastewater Treatment 5.4.2.3 5.4.2.4 Effect of recirculation of oxygen on denitrification capacity Design procedure for optimized nitrogen removal Chapter Innovative systems for nitrogen removal 6.0 6.1 6.2 6.3 6.4 6.5 Introduction Nitrogen removal over nitrite 6.1.1 Basic principles of nitritation 6.1.2 Kinetics of high rate ammonium oxidation 6.1.3 Reactor configuration and operation 6.1.4 Required model enhancements Anaerobic ammonium oxidation 6.2.1 Anammox process characteristics 6.2.2 Reactor design and configuration Combination of nitritation with anammox 6.3.1 Two stage configuration (nitritation reactor–Anammox) 6.3.2 Case study: full scale SHARON - Anammox treatment 6.3.3 Single reactor configurations Bioaugmentation Side stream nitrogen removal: evaluation and potential Chapter Phosphorus removal 7.0 7.1 7.2 7.3 Introduction Biological Phosphorus Removal 7.1.1 Mechanisms involved in biological phosphorus removal 7.1.2 Bio-P removal system configurations 7.1.3 Model of biological phosphorus removal 7.1.3.1 Enhanced cultures 7.1.3.2 Mixed cultures 7.1.3.3 Denitrification of bio-P organisms 7.1.3.4 Discharge of organic phosphorus with the effluent Optimisation of biological nutrient removal 7.2.1 Influence of wastewater characteristics 7.2.2 Improving substrate availability for nutrient removal 7.2.3 Optimisation of operational conditions 7.2.4 Resolving operational problems Chemical phosphorus removal 7.3.1 Stoichiometrics of chemical phosphorus removal 7.3.1.1 Addition of metal salts 7.3.1.2 Addition of lime 7.3.1.3 Effects on pH 7.3.2 Chemical phosphorus removal configurations 7.3.2.1 Pre-precipitation 7.3.2.2 Simultaneous precipitation 172 177 181 181 183 184 187 188 189 190 191 193 195 195 198 199 203 204 207 207 208 208 212 214 214 220 225 228 229 229 231 233 238 239 239 239 241 242 243 245 247 Contents 7.3.3 7.3.2.3 Post-precipitation 7.3.2.4 Sidestream precipitation Design procedure for chemical phosphorus removal Chapter Sludge settling 8.0 8.1 8.2 8.3 8.4 8.5 Introduction Methods to determine sludge settleability 8.1.1 Zone settling rate test 8.1.2 Alternative parameters for sludge settleability 8.1.3 Relationships between different settleability parameters Model for settling in a continuous settler 8.2.1 Determination of the limiting concentration Xl 8.2.2 Determination of the critical concentration Xc 8.2.3 Determination of the minimum concentration Xm Design of final settlers 8.3.1 Optimised design procedure for final settlers 8.3.2 Determination of the critical recirculation rate 8.3.3 Graphical optimization of final settler operation 8.3.4 Optimisation of the system of biological reactor and final settler 8.3.5 Validation of the optimised settler design procedure 8.3.5.1 US EPA design guidelines 8.3.5.2 WRC and modified WRC design guidelines 8.3.5.3 STORA/STOWA design guidelines 8.3.5.4 ATV design guidelines 8.3.5.5 Solids flux compared with other design methods Physical design aspects for final settlers Final settlers under variable loading conditions Chapter Sludge bulking and scum formation 9.0 9.1 9.2 9.3 9.4 Introduction Microbial aspects of sludge bulking Causes and control of sludge bulking 9.2.1 Sludge bulking due to a low reactor substrate concentration 9.2.2 Guidelines for selector design 9.2.3 Control of bulking sludge in anoxic-aerobic systems 9.2.4 Other causes of sludge bulking Non-specific measures to control sludge bulking Causes and control of scum formation ix 252 253 255 259 259 260 260 263 264 266 270 270 271 274 274 278 281 283 286 286 286 287 287 288 291 293 297 297 297 301 301 303 305 309 310 315 Chapter 10 Membrane bioreactors 319 10.0 10.1 319 320 Introduction Membrane bioreactors (MBR) Appendix 9: Aerobic granulated sludge 757 effluent withdrawal period) Later research by de Kreuk et al (2004) and Liu et al (2004) showed that the main aspects in granule formation are: – Hydraulic selection pressure, resulting in process conditions which outcompete poor settling biomass in favour of biomass with excellent settling properties; – Initial high substrate concentrations in order to apply high gradients; – The conversion of easily biodegradable substrate into slowly biodegradable intermediate products, which stimulates the growth of slow growing organisms; – High shear forces, stimulating the growth of smooth and dense granules Figure A9.1 Difference in sludge settleability (i.e the sludge volume resulting after minutes at 4.0 g TSS · l−1) between aerobic granulated sludge and conventional activated sludge Courtesy of DHV BV The initial research and pilot studies focused on the crucial role that phosphate accumulating bacteria played in the formation of aerobic granules (De Kreuk, 2006), but later research (and practical findings from full-scale installations) have shown that this is certainly not a perquisite A9.1 BENEFITS OF AEROBIC GRANULAR SLUDGE SYSTEMS The aerobic granulated sludge system has the following benefits compared to conventional aerobic treatment: • • • • • Easy and efficient nutrient removal; Lower energy requirements; Reduced footprint, due to the high settling velocity of the sludge granules; Reduction in investment and operational costs; Increased sustainability 758 Handbook of Biological Wastewater Treatment (a) Efficient nutrient removal An important feature of granular biomass is the enhanced capacity for biological nutrient removal Even when the AGS system is not explicitly design for nutrient removal, performance will be better than in a conventional activated sludge system The reason for this is that a dissolved oxygen (DO) gradient will develop during aeration, from the bulk liquid towards the centre of the sludge granule Oxygen will penetrate only partly into the granule, as it is consumed by autotrophic- and heterotrophic organisms in the outside layer The oxygen penetration depth depends on the DO concentration in the bulk liquid, the granule diameter and the oxygen uptake rate In the outer (aerobic) layers of the granule, nitrate and nitrite are produced, which diffuse towards the anoxic centre of the sludge particle where denitrification takes place This process is called simultaneous denitrification and can also be observed in conventional activated sludge systems containing zones with low DO concentrations (e.g in Carrousel® systems) In Figure A9.2 this process is schematically demonstrated Figure A9.2 Schematical representation of the different zones in an aerobic sludge granule and the processes that occur in each zone (left) and close-up of an aerobic granule (right) Courtesy of DHV BV As to aerobic granular sludge systems with biological phosphorus removal, during the anaerobic phase, typically pre-settled or raw influent is distributed over the bottom of the reactor, moving upward through the packed sludge bed in a plug-flow manner The easily (soluble) biodegradable substrate (Sbs) diffuses into the sludge granules and is fermented into VFA As described in Chapter 7, the bio-P organisms store the VFA as internal cell products, mainly PHB The reduction equivalents required for the conversion of VFA into PHB are supplied by the conversion of glycogen, while internally stored polyphosphate is first split into ortho-phosphate and then released to the liquid phase of the reactor In the anoxic- and aerobic phases, PHB will be used as a substrate for biomass growth and for the regeneration of glycogen and polyphosphate If not all of the VFA has been absorbed during the anaerobic phase, it will be used for direct growth of other heterotrophs, for growth of bio-P biomass, and partly for PHB production During the aerobic period, an oxygen gradient is formed from the bulk liquid towards the centre of the sludge granule and the processes of nitrification and simultaneous denitrification as discussed above take place Similar to the conventional bio-P system, during the anoxic- and aerobic phases “normal” heterotrophic biomass will grow on the slowly biodegradable organic substrate Sbp In Figure A9.3 typical concentration profiles in the sludge granules are shown that develop during the anaerobic feed period and the subsequent aerobic period The anaerobic sludge mass fraction fan in an AGS reactor can be defined as the length of the anaerobic feed time divided by the total cycle time and has a larger value (typically around 0.25) than the anaerobic sludge mass fraction of a conventional bio-P removal system (fan = 0.1−0.15) The hydrolysis under Appendix 9: Aerobic granulated sludge 759 anaerobic conditions of slowly biodegradable COD (Sbp) into easily biodegradable COD (Sbs) and VFA will be more complete and the fraction of influent COD available to bio-P bacteria in an AGS system will thus be higher than in a conventional bio-P system PHB + O2 → Biomass + CO2 Acetate + Poly-P PHB + PO43- PHB + NOx- → Biomass + CO2 + N2 NH4+ + O2 + CO2 + OH- → Concentration PO43- Concentration Biomass + NO3- PHB O2 PHB Acetate Granule NOx Granule Liquid Liquid Penetration depth Penetration depth Figure A9.3 Concentration profiles of selected key components during the anaerobic feed phase (left) and the aerobic phase (right) of an AGS process cycle, De Kreuk (2006) The high concentration of nitrifying, denitrifying and bio-P organisms in the aerobic granule result in an improved biological nutrient removal capacity compared to a conventional activated sludge system with a comparable sludge mass Table A9.2 shows the treatment performance of a Nereda® pilot plant treating a mixture of industrial (slaughterhouse) and municipal wastewater under typical Dutch climate conditions, where the sewage temperature varied between 7−20°C Table A9.2 Average treatment performance of the Nereda® pilot plant at STP Epe, the Netherlands - 2009 (based on data reported by Berkhof et al., 2010) Parameter COD Total nitrogen TKN NH4-N NOx-N Total phosphorus PO4-P Suspended solids UoM −1 mg · l mg N · l−1 mg N · l−1 mg N · l−1 mg N · l−1 mg P · l−1 mg P · l−1 mg · l−1 Influent Effluent Limit Removal 585 75 75 52 10 193 55 0.5 ,1 0.4 ,15 125 5–8 – – – 0.5 – ,20 90% 90% 95% 99% – 90% 90% 90% 760 Handbook of Biological Wastewater Treatment (b) Lower energy requirements In comparison with the conventional activated sludge (CAS) process, the aerobic granular sludge process can have a better aeration efficiency, i.e when efficient fine bubble aeration is used, due to the possibility of operation at increased height Furthermore there are neither return sludge or nitrate recycle streams nor mixing and propulsion requirements Therefore significantly less energy is required for plant operation Depending on the site specific conditions, the energy requirements for AGS systems may be 20% to 50% lower compared to a CAS system In an internal evaluation performed by DHV the energy consumption of a Nereda® for biological nutrient removal (100,000 P.E at 136 g TOD per P.E.) was compared to that of a conventional Carrousel® for the following design values: – Daily flow rate = 110,000 m3 · d−1, peak flow rate = 11,500 m3 · h−1; – Influent composition: COD = 750 mg · l−1, TSS = 300 mg · l−1 TKN = 55 mg N · l−1 and total-P = 10 mg P · l−1; – Design temperature = 12 to 25°C The results are presented in Table A9.3 It can be observed that the predicted energy consumption of the Nereda® is significantly lower (2220 kWh · d−1 or 38%) than that of the Carrousel® configuration Table A9.3 Comparison of energy requirements of a conventional Carrousel® system and Nereda® in a nutrient removal configuration treating municipal sewage Parameter Influent lifting station Screen and sand/grit removal Biological reactor – mixers anaerobic zone – mixers pre-denitrification – nitrate recirculation – propulsors aerobic zone – aeration – final settlers – return sludge pumping station Sludge dewatering Miscellaneous small equipment Cable/frequency converter losses Total energy consumption Carrousel® Nereda® kWh · d-1 % kWh · d-1 % 150 73 4972 192 318 648 848 2534 60 372 93 228 291 5807 3% 1% 85% 3% 5% 11% 15% 44% 1% 6% 2% 4% 5% 100% 262 73 2397 – – – – 2397 – – 383 228 244 3587 7% 2% 67% – – – – 67% – – 11% 6% 7% 100% (c) Reduced footprint The increase in sludge concentration that is possible because of the high settling velocity of the aerobic sludge granules (as discussed earlier in this appendix) and the absence of a final settler result in a significant reduction in the required footprint of the treatment plant In Table A9.4 the difference in footprint between a Carrousel® and Nereda® for the case study discussed above are shown: the Nereda® Appendix 9: Aerobic granulated sludge 761 is respectively 100,000 m3 (54%) and 30,000 m2 (71%) smaller than the Carrousel® system Note that the total required sludge mass is slightly larger for the Nereda®, to compensate for the time required for settling and decanting Table A9.4 Comparison of footprint and system volume of a conventional Carrousel® system and Nereda® in a nutrient removal configuration treating municipal sewage Parameter Design reactor sludge concentration Aeration tank volume Additional volume of anoxic/anaerobic zones Settler area Total volume Total area UoM Carrousel® Nereda® g TSS · l−1 m3 m3 m2 m3 m2 138,000 13,000 11,000 184,000 42,000 84,000 – – 84,000 12,000 (d) Reduced investment and operational costs The concentrated biomass substantially reduces tank volume and easily reduces the plant footprint to 50%, which is further accentuated by the elimination of the final settler from the design Furthermore the number of mechanical equipment is reduced, as for example mixers, recirculation pumps and return sludge pumps are not required This lowers the direct investment costs for green field, retrofit or capacity extension applications and the existing treatment site might be utilized rather than having to purchase new land Operation and maintenance costs are lower as well, due to the reduction in mechanical equipment, reduction of chemical usage and the high energy efficiency of the process On the other hand, at present there are few suppliers of aerobic granular sludge systems, as the technology is still largely patent protected This implies that cost-competition between AGS vendors will not be very large, which partly negates the potential savings in investment costs (e) Sustainable The AGS technology may be considered as more sustainable than the conventional activated sludge system, mainly because energy demands are significantly lower and less equipment and construction material are required The Swedish Research Institute IVL executed an extensive lifecycle assessment (LCA) study in which the environmental impact of Nereda® was compared to that of a conventional activated sludge system for the aerobic polishing of anaerobic pre-treated wastewater from breweries (Giesen, 2010) It was concluded that the aerobic granular system was more sustainable for all investigated environmental parameters A9.2 SYSTEM DESIGN AND OPERATION A9.2.1 Process configurations Different system configurations for aerobic granulated systems may be selected, depending on the situation at hand: (a) Greenfield applications (Figure A9.5); (b) Retrofit/upgrades of existing installations (Figure A9.6); (c) Hybrid capacity extension of existing installations (Figure A9.7) 762 Handbook of Biological Wastewater Treatment Figure A9.4 Examples of some full-scale Nereda® applications: edible oil industry (left) and Nereda® under construction at the STP Epe - The Netherlands (right) Courtesy of DHV BV (a) Greenfield applications Depending on the wastewater flow and characteristics, a typical greenfield AGS plant comprises multiple modular reactors (often three), operating out of sequence so that there is always one reactor available to receive the incoming wastewater Alternatively, if the quantity of wastewater to be treated is low, as will the case for many industrial applications, a single reactor preceded with a buffer tank can be selected as well Full-scale examples of Nereda® systems are shown in Figure A9.4 (a) Greenfield application Aerobic granulated sludge reactor Influent Aerobic granulated sludge reactor Effluent polishing (optional) Effluent Aerobic granulated sludge reactor Figure A9.5 Schematic representation of a greenfield AGS configuration (b) Retrofit/upgrades of existing installations As the physical design of an aerobic granular sludge reactor is quite flexible, it is often possible to convert the reactors of existing conventional continuous activated sludge systems or Sequencing Batch Reactors Appendix 9: Aerobic granulated sludge 763 (SBRs) into an AGS reactor As the application of aerobic granulated sludge allows an increase of the biomass concentration with (typically) a factor two, after the retrofit the treatment capacity of the original plants will be significantly increased and/or the effluent quality will be considerably improved (c) Hybrid capacity extension of existing installations In this application, the AGS reactor receives only part of the raw wastewater flow while the remaining part is treated by the existing conventional treatment system Depending on the specific requirements, several variants of this configuration can be distinguished: – If both the hydraulic- and biological capacity of the existing conventional treatment plant are to be increased, one or more AGS reactors can be operated in parallel with the existing biological reactors Depending on the local circumstances and effluent requirements, a post treatment step might be required for extensive phosphorus- and suspended solids removal; – If only the biological capacity needs to be increased, this can be easily implemented by constructing only one AGS reactor in parallel to the existing biological reactors If necessary, a storm water buffer for hydraulic optimisation can be considered; – Another variant of the hybrid capacity extension can be applied when a part of the organic load originates from a concentrated wastewater flow In this case it can be considered to treat the concentrated flow in a compact AGS system in parallel with the existing activated sludge reactors, although the application of AGS should be compared to other alternatives, such as anaerobic treatment Retrofit existing WWTP (b) Influent Existing activated sludge system converted into aerobic granulated sludge reactor Final settler (decommisioned) Effluent Figure A9.6 Schematic representation of retrofit/upgrade of existing activated sludge plants into an AGS configuration (c) Hybrid extension/upgrade Final settler Influent Existing activated sludge system Effluent (optional Sludge return Buffer tank (optional) Aerobic granulated sludge reactor Effluent polishing (optional) Figure A9.7 Schematic representation of an hybrid capacity extension of existing activated sludge plants using an AGS reactor 764 Handbook of Biological Wastewater Treatment An important additional advantage of a hybrid capacity extension with an aerobic granular sludge reactor is that the conventional activated sludge system can be seeded with granular surplus sludge, either with the AGS effluent or -waste sludge Because of this inoculation process, the sludge characteristics and settling performance of the existing treatment plant will gradually improve, resulting in increased capacity and improved treatment performance A9.2.2 Reactor configuration A schematic layout of a typical greenfield aerobic granulated sludge reactor is shown in Figure A9.8 When the fill- and draw stages are combined, then the critical engineering aspects are the proper design of the inlet distribution system and that of the effluent removal section In this design, the influent requires distribution over the entire reactor bottom to prevent short-circuiting between reactor inlet and -outlet The valve in the main effluent discharge line is only open during the aerobic fill phase, when effluent is simultaneously discharged The valve in the secondary line is opened directly after the end of the feed phase, in order to create some empty volume in the reactor to allow for expansion of the liquid column when aeration is resumed An alternative would be the use of moving or floating decanters as commonly used in SBR systems In general, the physical design of an AGS reactor is quite flexible, which allows the reuse of existing reactors in plant retrofits In Table A9.5 typical values of several AGS design and performance characteristics are summarised Secondary discharge line Aeration Blower Effluent Excess Sludge Wastewater Feed Pump Figure A9.8 Schematic layout of a typical (greenfield) aerobic granulated sludge reactor The specific organic loading rates (in kg COD · kg−1 TSS · d−1) used for the design of aerobic granular sludge systems and conventional activated sludge systems are comparable However, the high biomass settling rate and the increased biomass concentration will result in an increase in volumetric loading rate (kg COD · m−3 · d−1) compared to conventional treatment systems A9.2.3 Operation of AGS systems The aerobic granular sludge process is operated in a sequencing batch cycle mode comprising the following process stages: Appendix 9: Aerobic granulated sludge (1) (2) (3) (4) (5) (6) (7) 765 Pulse feed or anaerobic fill, optionally combined with effluent discharge; Draining (optional); Aeration; Anoxic (optional); Settling; Effluent discharge (optional); Sludge discharge An example of a typical process cycle for nutrient removal is shown in Table A9.6 Table A9.5 Typical value ranges for several design and process parameters in aerobic granulated sludge systems Parameter Value Avg upflow velocity(1) Max upflow velocity(1) Settling velocity Reactor sludge concentration Batch size Organic loading rate Parameter Value 2−3 m · h−1 m · h−1 10−50 m · h−1 8−12 g TSS · l−1 DSVI5 DSVI30 DO setpoint (bulk) Reactor height 40−80 ml · g−1 30−60 ml · g−1 2−2.5 mg O2 · l−1 4−12 m 5−70% of reactor volume 0.1−0.3 g COD g−1 TSS · d−1 Type of aeration Effluent TSS concentration Fine bubble 10−20 mg TSS · l−1 Note: (1) only relevant when operated under simultaneous filling and effluent withdrawal Table A9.6 Example of the process cycle of an aerobic granulated sludge system designed for nutrient removal (4-hour process cycle: in practice the length of the process cycle may range from 2−9 hrs) No Total Description Anaerobic fill Drain Aerobic process Anoxic process Settling Effluent discharge(1) Sludge discharge(1) Cycle time Duration Minutes Fraction of cycle time 60 165 10 0 240 25% 2% 69% 0% 4% 0% 0% 100% Note: (1) in this example fill and draw takes place simultaneously while excess sludge is withdrawn during the aerobic process stage 766 Handbook of Biological Wastewater Treatment (1) Pulse feed or anaerobic fill In this phase the wastewater is pumped into the reactor The high organic loading rate applied in this phase favours the slow-growing organisms, which are often capable of storing easily biodegradable substrate as cell-internal polymer, whereas the fast growing organisms are not When the AGS system is operated according to an optimized SBR cycle, which is typically recommended for greenfield applications, the effluent will be withdrawn simultaneously, while in retrofit situations this may not be the case The influent needs to be carefully and evenly distributed over the reactor bottom to prevent short-circuiting between reactor inlet and -outlet In practice the filling rate will be limited to a value of 2−3 m · h−1 Effluent discharge take places using floating decanters or fixed effluent collection channels or pipes, as shown in Figure A9.8 (2) Draining (optional - depending on reactor configuration) When the reactor is not equipped with a floating decanter but with a fixed effluent pipe and simultaneous fill and draw is applied, then an additional drain period is required Aeration without a prior drain discharge period would result in the discharge of mixed liquor due to the expansion (‘gas hold-up’) of the liquid volume Therefore prior to resuming aeration, a small volume of additional effluent is withdrawn from a secondary discharge pipe located beneath the effluent weir in order to create volume for liquid expansion during aeration without spills (3) Aeration During the aeration phase the biological conversion processes take place The outer layer of the granules will be aerobic and here nitrification will occur The produced nitrate is denitrified in the anoxic core of the granules, where substrate is available as a result of the feed/fill phase Biological phosphorus uptake takes place in large part of the granule, both under aerobic and anoxic conditions Depending on the influent characteristics and applied process conditions, part of the biomass may be present in traditional flocculent form, i.e outside the granules In the flocculent sludge, the same processes occur that take place in the aerobic zone of a conventional activated sludge system (4) Anoxic reaction phase (optional) Depending on the TKN/COD ratio in the influent, the concentration of easily biodegradable COD in the influent (Sbsi) and the applied bulk oxygen concentration, it might be necessary to include an anoxic stage to meet the the nitrate or total nitrogen effluent limits In this (optional) anoxic phase, all readily biodegradable material present in the bulk liquid (Sbs) and a large part of the internal cell polymers will have been depleted Denitrification will thus proceed at much lower rate This phase may be considered as equivalent to a post-D zone in a Bardenpho or UCT configuration (5) Settling phase The settling phase in an AGS is relatively short and is applied mainly to allow for liquid/solid separation in advance of effluent discharge, i.e in order to ensure that the liquid phase in the upper part of the reactor is free of suspended solids before the fill and discharge phase starts (6) Effluent discharge (optional) Instead of simultaneous fill and draw, effluent discharge can also be applied as a separate phase after the settling period In SBR systems, the use of floating decanters is effective in reducing the effluent discharge period compared to fixed decanters, as effluent discharge may already start during the sludge settling phase, as soon as the distance between the sludge bed and the decanter device has become large enough to prevent sludge entrainment with the effluent Due to the high settling velocity of the Appendix 9: Aerobic granulated sludge 767 granulated sludge, it will often be possible to reduce the duration of the dedicated effluent discharge period (i.e after the settling phase) to a very low value (7) Sludge discharge Sludge will generally be discharged at the end of the settling phase or during the aeration phase The advantage of the first option is the selective sludge discharge, which enables the retention of the aerobic granules with the highest settling velocities However, an advantage of the latter option is the more constant concentration of the discharged sludge, which makes it much easier to determine the actual quantity of solids discharged and to control the sludge age: i.e by means of hydraulic sludge control as explained in Section 3.3.3.6 A general disadvantage of batch operated systems is the difficulty in handling of peak flows However, due to the high settling velocity of the aerobic granular sludge it is possible to increase the hydraulic loading rate during peak flow without compromising effluent quality, also because, contrary to conventional activated sludge systems, all biomass will remain in the reactor (no sludge transfer to the final settler) However, in some cases it may be beneficial to reduce the length of the process cycle during peak flow periods A9.2.4 Start-up of aerobic granular sludge reactors Depending on the biomass growth rate and the size of the AGS system, the start up period might take several months, required to transform the original suspended activated biomass into granules This is a general disadvantage of all granular biomass systems However, due to the significantly higher sludge growth rate of aerobic biomass, the start up period of aerobic granular sludge is much lower compared to that of anaerobic granular sludge (i.e when no or insufficient anaerobic granular biomass is available to seed the reactor) The transformation from activated sludge flocs into aerobic sludge granules requires an initial high sludge selection pressure, e.g through the application of high upflow velocities during the filling phase, in order to selectively retain those organisms in the system that settle well As illustrated in Figure A9.9, for the Nereda® pilot in Ede it took only a few months to increase the aerobic sludge fraction to more than 70% using only a small quantity of granular seed biomass from lab reactors Generally, the biomass in a fully adapted AGS system will contain between 70−95% of granules An alternative and preferred option to accelerate startup is to supply seed sludge, as is customary with the slow growing anaerobic granular sludge systems Granular biomass is stable and can be stored easily for long periods While the granular structure will survive for years, the effect of storage on the biological activity is dependent on the method and duration of storage When aerobic granular biomass is kept under cooled conditions (about 4°C), it can be stored for several weeks without losing its biological activity When sufficient high quality aerobic granular seed sludge is available, then the start-up period can be reduced to two weeks A9.3 GRANULAR BIOMASS: EVALUATION AND POTENTIAL Since the first release of this book in 2007, the development of the aerobic granular biomass technology has advanced considerably Was the technology at that stage a promising emerging technology, nowadays it can be considered as “proven”, although the number of installed systems is at present still limited compared to other systems However, since 2005 the AGS technology has been implemented successfully more than ten times for both industrial and municipal wastewater Furthermore, at the time of publication (2011) more 768 Handbook of Biological Wastewater Treatment systems were in the design or construction phase An important finding was that the AGS system has been proven under harsh African conditions in which it has been demonstrated that due to extensive degree of automation that can be applied (i.e fully automated process cycles), also relatively low-skilled staff can operate the treatment works The benefits of using aerobic granular biomass are evident: lower costs, excellent treatment performance, reduced energy consumption and easy operation Granule fraction in biomass 100% 80% 60% 40% 20% 0% 50 100 150 200 250 300 350 Elapsed time after startup (days) Figure A9.9 Observed degree of granulation in the biomass after start-up of the first Nereda® pilot installation located at the Ede STP, The Netherlands (De Bruin et al., 2005) The research field is still relatively new and not all interactions between wastewater composition, operational conditions and system performance are yet fully understood However, the available research clearly shows that the aerobic granular sludge technology is capable of handling both dissolved and particulate organic pollutants Furthermore, it seems the granules are less vulnerable to toxic compounds than the suspended flocs in conventional activated sludge systems This is due (I) to the relative small penetration depth of toxic compounds into the granule, so that the bacteria in inner parts are partly protected and (II) to the relatively large and heterogeneous micro-organism population, in which latent available capacity inside the granule can (partly) replace the affected micro-organism population at the outside of the granule In South Africa, a Nereda® installation has been in operation from 2009 on wastewater containing very high levels of particulate matter, due to the contribution of septic tank waste The treatment plant is designed for a capacity of 4,000 m3 · d−1 and easily meets local discharge limits with an average effluent quality of NH4-N , 1.5 mg N · l−1, NO3-N between 5−10 mg N · l−1, PO4-P , mg P · l−1 and SS , 10 mg · l−1, which allows it to be directly reused for irrigation purposes Detailed performance data of this plant can be found in Table A9.7 The wastewater plant itself is shown in Figure A9.10 Appendix 9: Aerobic granulated sludge 769 Figure A9.10 Full-scale municipal sewage Nereda® application (4000 m3 · d−1) at the Gansbaai STP in South Africa Courtesy of DHV BV Table A9.7 Average performance of the 4000 m3 · d-1 municipal sewage Nereda® located at Gansbaai, South Africa (based on data provided by DHV BV) Parameter COD NH4-N(1) NOx-N Total phosphorus PO4-P Suspended solids UoM −1 mg · l mg N · l−1 mg N · l−1 mg P · l−1 mg P · l−1 mg TSS · l−1 Influent Effluent Limit Removal % 1240 81 – 19.2 – 690 42 1.4 8.7 – 4.7 5.5 75 15 – 10 25 96.6% 98.3% – – – 99.2% Note (1): The NH4-N/TKN ratio was 0.75 When comparing the aerobic granular sludge technology to another (more or less) recent development, the membrane bioreactor (MBR), the following remarks can be made: – Though the footprint of both systems is comparable, the aerobic granular sludge system is technologically much simpler than the MBR, requiring less instrumentation and process control Furthermore the skill level required for operation is lower, better meeting the capacity in the developing countries in for example Asia, Africa and Eastern Europe; 770 Handbook of Biological Wastewater Treatment – The investment- and operating costs, energy consumption and environmental profile for the AGS system are all much more favourable Furthermore the AGS system will often be able to achieve the required effluent quality at much lower investments and operational costs than the MBR; – Mainly because of the retention of suspended solids, including bacteria and viruses, the MBR yields a better effluent quality, whereas AGS requires additional effluent disinfection or polishing However, especially when the AGS is equipped with relatively simple mechanical polishing filters or a reed bed, the removal of suspended solids and the associated COD, nitrogen and phosphorus content for both technologies is quite similar and in any case effluent quality is better than limits currently applied in many countries (COD , 50 mg · l−1, total nitrogen , 10 mg N · l−1, total phosphorus , to mg P · l−1); – For difficult wastewaters (e.g of a predominantly industrial origin: high strength, saline or with toxic contributions) the MBR might be more stable as the membrane will prevent biomass loss even in the effect of toxic shocks, whereas for the AGS under certain conditions there might be a risk of degranulation On the other hand, as mentioned earlier, the bacteria inside the granule are partially protected against toxicity; – Both technologies can be considered important instruments in upgrading or extending existing wastewater treatment facilities without extending the actual plant footprint; ... mg COD · mg−1 COD kg COD · d−1 mg COD · mg−1 COD xxxvi MSxvu mwmeoh mwmp mwms mXa MXa MXah MXan MXap mXau MXau mXbpu MXbpu MXchem mXe MXe MXen MXep mXeu MXeu mXi MXi mXiu MXiu mXmu MXmu MXn mXt... TSS mg ISS · mg−1 COD kg TSS · kg−1 TSS mg N · mg−1 VSS mg COD · mg−1 COD mg COD · mg−1 COD mg COD · mg−1 COD mg COD · mg−1 COD mg COD · mg−1 COD mg COD · mg−1 COD mg P · mg−1 VSS (–) mg P · mg−1.. .Handbook of Biological Wastewater Treatment Handbook of Biological Wastewater Treatment Design and Optimisation of Activated Sludge Systems Second Edition A. C van Haandel and J. G. M van der Lubbe