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7.1.1.6 Effect of sludge age on biomass retention It was shown that sludge age or solids retention time (SRT) is not a fixed value for an accumulating system. It is possible to have poor biomass retention, but high sludge accumulation rates if the net rate of accumulation (in – out + generation – degradation) of biodegradable solids is larger than the rate of accumulation of the micro-organisms that should degrade them (Section 6.3). 7.1.1.7 Effluent stream composition A steady-state model with a sludge retention factor was used to predict the effluent composition of an anaerobic system treating domestic sewage. It was calculated that outflow pH values of between 5.9 and 6.1 could be expected for a system with no alkalinity supplementation. Increases in free and saline ammonia and alkalinity were predicted, and predicted values were found to be similar to those obtained experimentally (Section 6.4.1). 7.1.1.8 Diurnal variation in feed concentration A sampling campaign was undertaken to determine the extent of diurnal variations in feed condition on the pilot-scale ABR. The pilot-scale ABR was monitored hourly for 24 hours. Significant diurnal variation of inflow COD and alkalinity concentrations was observed. The regular sampling time for normal operation (i.e. not during this sampling campaign) coincided with higher inflow COD concentrations than the daily average. Thus inflow sample measurements for regular operation may have overestimated the average inflow COD concentration to the pilot-scale ABR by up to 100 mgCOD/ℓ (Section 5.5.3). Additional data are required to support this observation. 7.1.1.9 Attenuation of diurnal concentration variations in outflow stream The outflow COD and alkalinity concentration profiles showed less variation around the mean value than the inflow profile. Thus the pilot-scale ABR played some role in damping oscillations in determinand concentrations due to diurnal oscillations of feed strength. (Section 5.5.3) 7.1.1.10 Alkalinity supplementation It was calculated that dosing digester feed with a source of alkalinity such as lime or Na2CO3 such that the feed alkalinity concentration increased to around 1 000 mgCaCO3/ℓ is required to maintain the digester pH above a value of 6.5 (Section 6.4.5). 7.1.2 Conclusions relating to objectives and hypotheses of this study This section specifically addresses the project objectives and hypotheses. 7.1.2.1 Phase separation It was hypothesised that phase separation in an ABR treating sewage is a benefit of the design over a single phase system, since it allows development of acidogenic and methanogenic zones in the ABR. Examination of pH value and soluble COD concentration profiles across the compartments of the ABR showed that compartment 1, and at times compartment 2 experienced lower pH values than subsequent compartments, while significantly higher soluble COD concentrations were experienced in compartment 1 only. In addition, no observations of acetoclastic methanogens Methanosaeta were observed in compartment 1, but during Phase IV operation, these Archaea were observed in later compartments. These results suggest that a degree of phase separation did occur in the ABR, but that it was not complete phase separation since hydrolytic and acidogenic processes occurred and indeed were rate limiting in later compartments (Section 6.1.1.3). 168
Nevertheless, it is concluded that the compartmentalised structure of the ABR had advantages over UASB reactors and septic tanks in the treatment of domestic wastewater since: • Solids retention in compartment 1 reduces the requirement for pre-settling and separate digestion of wastewater solids • Sludge beds in later compartments were protected from transient high organic loads through solids retention in the first compartment • The initial acidogenic zone was not contained exclusively to the first compartment when after a long period of continuous operation, undigested particulate organics overflowed to the second compartment, without any detrimental effects on the outflow stream (Section 6.8.1). 7.1.2.2 A-HRT as the critical design parameter It was hypothesised that 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. It was found that the ability of the system to retain active biomass depended on the upflow velocity in the reactor and that this controlled the quality of the effluent and the rate of sludge accumulation. Although this was dependent on the feed flow rate, and therefore the A-HRT, the relationship between A-HRT and upflow velocity is dependent on reactor geometry and therefore would be different in another system. Therefore it was concluded that both A-HRT and upflow velocity should be regarded as critical design parameters for design of an ABR treating domestic wastewater. (Sections 6.1.2, 6.1.4.1 and 6.6.2.1). It was observed that at a 22 h A-HRT, a well-balanced and stable anaerobic biomass was not able to establish in the pilot-scale ABR. However, it was concluded that this was due to the upflow velocity being too high, and was not specifically due to the low A-HRT value. It was proposed that stable digestion and acceptable effluent quality at A-HRT values of 20 h and lower could be achieved, but only with reactor geometries that resulted in lower upflow velocities. 7.1.2.3 Performance of ABR treating domestic wastewater and mechanisms of treatment The first objective of this study was to investigate the performance of a pilot-scale ABR in the treatment of wastewater of domestic origin and understand the mechanisms of treatment therein. Principle mechanisms of treatment in an ABR were shown to be through solids retention and anaerobic digestion of retained solids. It was shown that the baffled design assisted in solids retention and ensured good treatment of soluble components from the influent as a result of filtration through and contact with sludge beds in the compartments. It was observed that significant COD reduction could be achieved, and that the extent of treatment depended on the upflow velocity and A-HRT of the system. Free and saline ammonia and alkalinity concentrations were observed to increase as a result of treatment in the ABR. It was also observed that although treatment in the ABR resulted in significant removal of pathogen indicator organisms, especially of helminth eggs, the load of pathogens in the outflow stream was nevertheless too high for the effluent to be discharged to a water course or to surface irrigation. 169
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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
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most compartments was exactly 6.5,
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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 188 and 189: compartments (Section 6.1.1) pH val
Page 190 and 191: generally about anaerobic digestion
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
Page 238 and 239: Substituting Eq. A6- 1 in Eq. A6- 7
Page 240 and 241: 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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