analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

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compartments (Section 6.1.1) pH values in the effluent were similar to values predicted for digestion of domestic wastewater by a stoichiometric pseudo-steady state model, indicating that low pH values in the outflow were not caused by inhibition of methanogenesis. • As there were more than 2 compartments, the acidogenic zone was able to increase to occupy more than just the first compartment as the total amount of sludge in the reactor built up, without compromising overall process stability (Section 6.1.1). • There were indications that the nature of flow in the reactor (pseudo-plug-flow with solids retention) allowed rapid removal of sour liquors or toxicants after going sour, and that this may have assisted in rapid recovery (Section 5.6.3). • The ABR is a sludge accumulating device, and therefore does not require continuous sludge removal. This has two advantages: firstly only infrequent solids handling is required; secondly, there is no requirement for continuous energy supply for sludge pumps as there would be in a non-accumulating UASB system. 6.8.2 Disadvantages The ABR design has many of the same disadvantages as other anaerobic systems treating dilute particulate wastewater: • low influent wastewater alkalinity concentration and low alkalinity generation potential of the wastewater result in poor pH buffering without alkalinity supplementation (Section 6.1.1.2) • Without alkalinity supplementation, anaerobic digestion proceeds at low metabolic rates resulting in low biomass generation rates and increased vulnerability to sludge washout. • The low organic load in dilute wastewater results in low biomass generation rates and therefore long start-up times. This also contributes to increased vulnerability to sludge washout. In addition, there are two specific disadvantages associated with the design of the ABR for this application: • There is inefficient use of reactor volume during early days of operation where most biological activity takes place in the first few compartments (Section 4.4.2). (However, this is compensated by the long desludging intervals that can be allowed as the remainder of the reactor space becomes active through sludge accumulation.) Thus the rate of treatment will be lower per unit volume than systems with continuous sludge removal. • There is a higher capital cost associated with the ABR than with a standard septic tank. The cost of a multi-compartment digester would be higher than in a single-stage digester. 164
7 CONCLUSIONS AND RECOMMENDATIONS "It is easier to get into something than to get out of it." – Donald Rumsfeld Given the heterogeneous nature of domestic wastewater, investigations on domestic wastewater treatment are best performed at pilot- or full- scale since laboratory scale experimentation is beset with difficulties related to supply and condition of the feed wastewater. Few studies have been published on the performance of ABR technology at pilot- or full- scale. Thus there was a gap in the understanding of how the technology would perform on a feed of real wastewater. An 8-compartment, 3 000 ℓ pilot-scale ABR was built and operated at two municipal wastewater treatment plants, Umbilo and Kingsburgh WWTP over a period of 5 years and chemical and microbiological data were collected on samples from inflow and outflow streams and from within compartments of the pilot-scale ABR. This thesis presents an analysis of these data. While pilot-scale research is less controlled and therefore more difficult to undertake and understand, these disadvantages were compensated for by obtaining information on the function of such a system on real domestic wastewater, including all those features that one would rather not have to contend with! Research into anaerobic baffled reactor (ABR) technology for treating domestic wastewater was undertaken to achieve the following objectives (Section 1.4): • To investigate the performance of a pilot-scale ABR in the treatment of wastewater of domestic origin and understand the mechanisms of treatment therein • To identify the critical parameters in the design of an ABR sanitation system • To determine whether the baffled design has any significant benefits over other anaerobic technologies in the treatment of domestic wastewater • To develop a dynamic mathematical model of the biochemical processes in an ABR treating domestic wastewater It was hypothesised that (i) phase separation in an ABR treating sewage is a benefit of the design over a single phase system, by allowing development of acidogenic and methanogenic zones in the ABR; and (ii) the critical parameter controlling effluent quality and sludge digestion rates in an ABR treating sewage was the applied hydraulic retention time (A-HRT) and that low effluent COD concentrations could be achieved at an A-HRT of 20 h. This chapter presents conclusions and recommendations that have arisen from this research. 7.1 CONCLUSIONS This section presents general conclusions made in the course of the research about mechanisms and observations related to the performance of the pilot-scale ABR treating domestic wastewater and more 165
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ANALYSIS OF A PILOT-SCALE ANAEROBIC
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i DECLARATIONS I, Katherine Maria F
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My deepest thanks must be extended
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vii TABLE OF CONTENTS ANALYSIS OF A
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5.3 Feed characteristics and loadin
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xi LIST OF TABLES Table 1.1: List o
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xiii LIST OF FIGURES Figure 1.1: Gr
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Figure 5.1 Installation of the ABR
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Figure 6.8: WEST® representation o
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xix LIST OF ABBREVIATIONS A-HRT App
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1 1 INTRODUCTION The work presented
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obtained from anaerobic systems rel
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• Lalbahadur (MTech, 2005) perfor
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1.7 ORGANISATION OF THE THESIS This
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9 2 LITERATURE REVIEW A detailed re
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• Disintegration of composite par
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Homoacetogens are one of the most v
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Most of the control in anaerobic di
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therefore the sensitivity of the ov
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∆Gº (W) [kcal] -10 -8 -6 -4 -2 L
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For systems with low potential for
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design of the digester in which the
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2.2.2 Expanded granular sludge bed
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fact that for most non-ideal flow s
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hydrolysis steps to convert them to
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from the clarified zone between the
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Table 2.4: Typical pathogen surviva
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Table 2.5: Studies using anaerobic
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• There is no requirement for bio
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Kennedy and Barriault (2005) perfor
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methanogenic (Akunna and Clark, 200
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and 6 h with no significant differe
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2.5.1.12 Granule formation in ABRs
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• A baffled reactor is described
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2.5.3.4 Scanning electron microscop
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spacing on flow patterns in a singl
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Figure 3.5: Orthographic projection
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PLC calculated the fraction of that
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epresent buulk conditionns. Compart
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The rreactor was initially commmiss
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Phase I Phase II Phase III Phase IV
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Table 4.1: Characteristics of degri
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Table 4.2: Pilot-scale ABR approxim
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COD [mgCOD/ℓ] 2000 1800 1600 1400
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4.3.5 Phosphorus Figure 4.8 present
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Observation of sludge in compartmen
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Another interesting observation is
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4.4.3 pH pH measurements were perfo
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esult in low pH values. The reasons
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4.5 SUMMARY: PHASE I - OPERATION AT
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than the optimal range for methanog
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5 EXPERIMENTAL PHASES II-IV: KINGSB
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sludge on a number of occasions. Wh
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On days 99 and 100 of Phase III, a
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5.3.2 Analysis of variations in fee
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Table 5.2: Pilot-scale ABR average
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5.4.1.2 Phase III In Phase III, (Fi
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Phase II: Alkalinity [mgCaCO3/ℓ]
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Eq. 5-1 However, at a COD/SO4 2- ra
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wastewater was probably higher than
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5.5.2.2 Damping Figure 5.12 and Fig
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• The outflow COD values were not
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• Information about the volume of
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As with the total volume of sludge
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It is not clear why the rate of acc
Page 137 and 138: most compartments was exactly 6.5,
Page 139 and 140: Soluble COD [mg/ℓ] 600 500 400 30
Page 141 and 142: pathogens and parasites, which may
Page 143 and 144: % Probe of DAPI % Probe of DAPI % P
Page 145 and 146: later compartments at all. Furtherm
Page 147 and 148: and 4, with decreasing observations
Page 149 and 150: • The mechanism of sludge build-u
Page 151: Table 5.6: Inflow and outflow chara
Page 154 and 155: Behling et al. (1997) studied the p
Page 156 and 157: organisms in previous compartments.
Page 158 and 159: velocities. Unfortunately, these tw
Page 160 and 161: Thus for a fixed sludge load, at hi
Page 162 and 163: sampling was not possible since in
Page 164 and 165: Wentzel (2006) recommends a COD to
Page 166 and 167: 6.2.1.4 Calculation of CH4 producti
Page 168 and 169: organically bound N) or that N accu
Page 170 and 171: All VFA is consumed in the process,
Page 172 and 173: However, these assumptions are clea
Page 174 and 175: The calculated pH value is compared
Page 176 and 177: influent alkalinity will increase d
Page 178 and 179: The original objective to develop a
Page 180 and 181: • It was not possible to simulate
Page 182 and 183: employed for a medium strength wast
Page 184 and 185: 6.6.4 Alternative baffle design Thi
Page 186 and 187: 6.7.3 Monitoring requirements The m
Page 190 and 191: generally about anaerobic digestion
Page 192 and 193: 7.1.1.6 Effect of sludge age on bio
Page 194 and 195: 7.1.2.4 Critical design parameters
Page 196 and 197: 172
Page 198 and 199: Batstone D. J., Keller J., Angelida
Page 200 and 201: Gopala K. G. V. T. (2007). Treatmen
Page 202 and 203: MacLeod F. A., Guiot S. R. and Cost
Page 204 and 205: Sasse L. (1998). Decentralised wast
Page 206 and 207: Wanasen (2003). Upgrading conventio
Page 209 and 210: METTHODS OFF SAMPLINNG AND ANNALYSI
Page 211 and 212: 3.6 Enumeration of total coliforms
Page 213 and 214: that the means are the same, or con
Page 215 and 216: And the significance of the regress
Page 217 and 218: ADDITIONAL RESULTS 193 APPENDIX A3
Page 219 and 220: Table A3. 1 cont. Phase I Phase II
Page 221 and 222: each of the eight compartments were
Page 223 and 224: 3.2 Pathogen indicator organisms A
Page 225 and 226: APPLIED VS. APPARENT HYDRAULIC RETE
Page 227 and 228: 19 h apparent HRT 14 h apparent HRT
Page 229 and 230: 205 APPENDIX A5 CALCULATION OF SOLI
Page 231 and 232: i.e. the average SRT of an accumula
Page 233: X X = P Inert ⋅ where Xinert is t
Page 236 and 237: iodegradable organic material may b
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Substituting Eq. A6- 1 in Eq. A6- 7
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1.5 Implementation of steady-state
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X = 7⋅ Y = 7 Z = 7⋅ A = 7⋅ (
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Dama, P., Bell J., Naidoo, V., Foxo
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Lalbahadur T. (2004) Characterisati
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