Nitrogen control in source segregated domestic food waste anaerobic digestion using stripping ...

194 Pages·2015·3.47 MB·English

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UNIVERSITY OF SOUTHAMPTON

Faculty of Engineering and the Environment

NITROGEN CONTROL IN SOURCE SEGREGATED DOMESTIC FOOD WASTE
ANAEROBIC DIGESTION USING STRIPPING TECHNOLOGIES

by

Alba Serna-Maza

Thesis for the degree of Doctor of Philosophy

November_2014

UNIVERSITY OF SOUTHAMPTON
ABSTRACT
FACULTY OF ENGINEERING AND THE ENVIRONMENT
Doctor of Philosophy
NITROGEN CONTROL IN SOURCE SEGREGATED DOMESTIC FOOD WASTE
ANAEROBIC DIGESTION USING STRIPPING TECHNOLOGIES
Alba Serna-Maza
Anaerobic digestion of source segregated domestic food waste (SS-DFW) offers
a sustainable management route for reclaiming potential energy in the form of
a fuel gas, and nutrients which can be recycled back to land. However, the
biochemical characteristics of SS-DFW can lead to free ammonia nitrogen (FAN)
concentrations that are inhibitory to the digestion process causing unstable
operation and in some cases complete process failure, particularly in
thermophilic systems.
With the purpose of reducing the total ammoniacal nitrogen (TAN) in the
digester, side-stream and in situ biogas stripping technologies were tested.
Mesophilic and thermophilic temperatures were evaluated under moderate and
complete biogas mixing rates (0.4 l min-1 – 2.6 l min-1) in a batch system.
Laboratory investigations showed that TAN reductions in an in situ bubbling
reactor with moderate and complete gas mixing rates were non-existent at
mesophilic temperatures and minimal at thermophilic temperatures. For this
reason, it is unlikely that in situ biogas stripping would be adequate to prevent
TAN concentrations greater than 2500 mg N l-1 in a food waste digester and
thus will not mitigate ammonia inhibition in a thermophilic system.
Semi-continuous trials carried out on SS-DFW in laboratory-scale digesters, fed
daily at an organic loading rate (OLR) of 2 kg VS m-3 day-1 and coupled to
stripping columns at low bleed rates (2 – 3.5 % digester volume per day treated
in the stripping process) were effective in reducing ammonia concentrations to
below thermophilic toxic levels (TAN concentration of 2500 – 3500 mg N l-1).
The experiments also confirmed that removal of a proportion of the digester
contents and their exposure to thermophilic conditions with pH adjustment to
10 had no adverse effects on performance in terms of biogas production (0.83
± 0.03 l g-1 VS without stripping, 0.84 ± 0.05 l g-1 VS with stripping) or VS
destruction (81.8 % without stripping, 88.5 % with stripping). The process
required high pH and temperature (≥70 ⁰C) to achieve a TAN concentration
below the toxic threshold for thermophilic digestion, and it is unlikely that
stripping at 55 ⁰C and pH 10 would achieve the target reduction.
The research showed the way forward for the application of side-stream
stripping to prevent the build-up of ammonia under thermophilic conditions, if
the digester is started up with a non-inhibitory FAN concentration in the
inoculum.
Keywords: Anaerobic digestion; source segregated domestic food waste;
ammonia removal; in situ stripping; side-stream stripping

Contents
ABSTRACT ................................................................................................................................ i
Contents ................................................................................................................................... i
List of tables ...................................................................................................................... vii
List of figures ......................................................................................................................ix
DECLARATION OF AUTHORSHIP ............................................................................x iii
Acknowledgements ........................................................................................................ xv
Definitions and Abbreviations ..............................................................................x vii
1. Introduction.................................................................................................................1
1.1 Background ................................................................................ 2
1.2 Aims and objectives .................................................................... 4
1.2.1 Aims ........................................................................................... 4
1.2.2 Objectives ................................................................................... 5
2. Literature review ......................................................................................................7
2.1 Principles of anaerobic digestion................................................. 7
2.1.1 Degradation route ....................................................................... 7
2.1.2 Environmental factors ............................................................... 10
2.1.2.1 Temperature ........................................................................ 10
2.1.2.2 pH and alkalinity .................................................................. 12
2.1.2.3 Nutrients ............................................................................. 14
2.1.2.4 Inhibitors ............................................................................. 15
2.2 Ammonia inhibition of anaerobic digestion ............................... 15
2.2.1 Impact of ammonia toxicity to methanogenesis pathway in
anaerobic digestion ................................................................................. 16
2.2.2 Mechanisms of ammonia inhibition ........................................... 19
2.2.2.1 Free ammonia nitrogen ........................................................ 19
i
2.2.2.2 Ammonium ion .................................................................... 21
2.2.3 Inhibition levels ......................................................................... 22
2.3 Ammonia inhibition mitigation methods ................................... 30
2.3.1 Acclimation of microflora .......................................................... 30
2.3.2 Temperature and pH control ..................................................... 31
2.3.3 Substrate dilution ...................................................................... 31
2.3.4 Trace element supplementation ................................................ 33
2.3.5 C/N ratio adjustment ................................................................ 34
2.3.6 N removal ................................................................................. 35
2.3.7 Stripping ................................................................................... 37
2.3.7.1 Introduction ......................................................................... 37
2.3.7.2 Physicochemical effects of biogas stripping .......................... 40
2.3.7.3 Stripping configurations ....................................................... 44
Pre-digestion ..................................................................................... 44
Post-hydrolysis .................................................................................. 49
In situ ............................................................................................... 51
Post-digestion and side-stream ......................................................... 54
2.3.7.4 Nitrogen recovery: reasons and methodology ....................... 57
3. Methodology ............................................................................................................ 59
3.1 General ..................................................................................... 59
3.2 Monitoring and analytical methods ............................................ 59
3.2.1 Total and Volatile Solids ............................................................ 59
3.2.2 pH ............................................................................................. 60
3.2.3 Alkalinity ................................................................................... 61
3.2.4 Total ammonia nitrogen ............................................................ 62
3.2.5 Total Kjeldahl Nitrogen ............................................................. 63
3.2.6 Volatile Fatty Acids .................................................................... 64
ii
3.2.7 Trace metals extraction and analysis ......................................... 64
3.2.8 [2-14C] Sodium acetate labelled analysis to determine the
methanogenic pathway ............................................................................ 65
3.2.9 Capillarity Suction Time ............................................................ 66
3.2.10 Frozen Image Centrifuge Test ................................................... 66
3.2.11 Gas composition ....................................................................... 67
3.2.12 Gas volume ............................................................................... 67
3.2.13 Rotameter and pump calibration ............................................... 68
3.3 Materials ................................................................................... 69
3.3.1 Feedstocks................................................................................ 69
3.3.2 Trace element solution ............................................................. 70
3.4 Equipment ................................................................................ 70
3.4.1 75-L CSTR digesters .................................................................. 70
3.4.2 35-L CSTR digesters .................................................................. 73
3.4.3 Side-stream stripping columns .................................................. 74
3.4.4 Batch stripping columns ........................................................... 76
3.5 Experimental plan ..................................................................... 77
3.6 Calculation methods ................................................................. 78
3.6.1 VS destruction .......................................................................... 78
3.6.2 Ammonia removal rate .............................................................. 79
3.6.3 Ammonia stripping efficiency .................................................... 79
3.6.4 N mass balance ......................................................................... 81
3.6.4.1 N mass balance: Batch stripping column .............................. 81
3.6.4.2 N mass balance: Continuous digester ................................... 82
Control reactor ................................................................................. 82
Side-stream stripping system ............................................................ 83
4. Results and discussion ...................................................................................... 87
iii
4.1 75-L CSTR digesters for anaerobic digestion of food waste ........ 88
4.1.1 Methodology for food waste anaerobic digestion trial ............... 88
4.1.2 Results of food waste anaerobic digestion trial .......................... 90
4.1.3 Conclusion ................................................................................ 92
4.2 In situ stripping......................................................................... 92
4.2.1 Methodology for in situ stripping experiments .......................... 93
4.2.2 Results of in situ stripping experiments .................................... 94
4.2.2.1 In situ stripping at mesophilic temperature .......................... 94
4.2.2.2 In situ stripping at thermophilic temperature ....................... 96
4.2.2.3 Results of batch stripping test for synthetic and real digestates
............................................................................................ 99
4.2.3 Conclusions ............................................................................ 101
4.3 Batch ammonia stripping experiments .................................... 101
4.3.1 Methodology ........................................................................... 102
4.3.2 Results of comparison of alkaline compounds for pH control .. 102
4.3.3 Overview of results of batch stripping experiments ................. 105
4.3.3.1 Nitrogen mass balances ..................................................... 111
4.3.3.2 Comparison of time constants. ........................................... 116
4.3.4 Analysis of the effects of temperature, pH, biogas flow analysis ....
............................................................................................... 117
4.3.4.1 Conclusions ....................................................................... 119
4.3.5 Ammonia stripping efficiency .................................................. 119
4.3.6 Hydrolysis experiment ............................................................ 122
4.4 Side-stream stripping .............................................................. 125
4.4.1 Methodology ........................................................................... 125
4.4.1.1 Phase 1: Establishing a digestion baseline .......................... 126
4.4.1.2 Phase 2: Ammonia removal by side-stream stripping .......... 127
4.4.2 Results .................................................................................... 128
iv
4.4.2.1 Phase 1: Baseline performance and stability assessment .... 128
4.4.2.2 Phase 2: Side-stream ammonia stripping ............................ 128
Digesters performance ................................................................... 129
Short-term effects of return of stripped digestate ........................... 140
Stripping columns .......................................................................... 143
Digestate solids separation and dewaterability results .................... 145
14C assay for dominant methanogenic pathway ............................... 147
4.4.2.3 Digester N mass balance .................................................... 149
Control reactor ............................................................................... 149
Side-stream stripping system .......................................................... 150
5. Conclusions and further work ................................................................... 153
5.1 Conclusions ............................................................................ 153
5.2 Further work ........................................................................... 155
References ........................................................................................................................ 157

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