Sandra M. Stronach

Anaerobic digestion processes in industrial waste water treatments

1 The Biochemistry of Anaerobic Digestion.- 1.1 Kinetics of Substrate Utilisation and Bacterial Growth.- 1.1.1 COD Fluxes and Mean Carbon Oxidation State.- 1.1.2 Bacterial Growth and Biokinetics.- 1.1.2.1 Growth and Single Substrate Kinetics.- 1.1.2.2 Multisubstrate Systems.- 1.2 Kinetics and Biochemistry of Hydrolysis.- 1.3 Kinetics and Biochemistry of Fermentation and ?-Oxidation.- 1.4 Kinetics of Methanogenesis.- References.- 2 The Microbiology of Anaerobic Digestion.- 2.1 Nutrient Balance in Anaerobic Digesters.- 2.2 Origin and Nature of Digester Bacteria.- 2.3 The Hydrolysing Bacteria.- 2.3.1 End-Product Inhibition During Hydrolysis.- 2.4 Intermediate Metabolism.- 2.4.1 The Fermenting Bacteria.- 2.4.2 The Bacteria of ?-Oxidation.- 2.5 The Methanogenic Bacteria.- 2.6 Other Bacterial Conversions.- 2.7 Anaerobiosis.- References.- 3 Forms of Biomass.- 3.1 Adhesion.- 3.1.1 The DVLO Theory.- 3.1.2 Interfacial Free Energy and Adhesion.- 3.1.3 Deformation in Relation to Adhesion.- 3.2 Biofilm Formation.- 3.3 Floc Formation.- 3.4 Pellet Formation.- 3.5 Entrapment in Natural Polymers.- 3.6 Estimation of Microbial Mass and Activity.- References.- 4 Influence of Environmental Factors.- 4.1 Temperature.- 4.2 Hydrogen Ion Concentration (pH).- 4.3 Physical Parameters.- 4.4 Nutrients.- References.- 5 Toxic Substances in Anaerobic Digestion.- 5.1 Volatile Acids Inhibition.- 5.2 Sulphide Inhibition.- 5.3 Ammonia-Nitrogen Inhibition.- 5.4 Heavy Metals.- 5.4.1 The Effect of Heavy Metal Speciation in Anaerobic Digestion.- 5.4.2 The Effect of Heavy Metals on the Bacterial Flora of Anaerobic Digesters.- 5.5 The Effect of Cyanide.- 5.6 Anthropogenic and Recalcitrant Compounds in Anaerobic Digestion.- 5.6.1 Response of Digester Systems to Complex Organics.- 5.6.2 Response of Anaerobic Bacteria to Hazardous Organic Molecules.- References.- 6 Single-Staged Non-Attached Biomass Reactors.- 6.1 The Continuously Stirred Tank Reactor.- 6.1.1 Design and Operation.- 6.1.2 Process Efficiency.- 6.2 The Contact Process.- 6.2.1 Design and Operation.- 6.2.2 Process Efficiency.- 6.3 The Upflow Anaerobic Sludge Blanket Reactor.- 6.3.1 Design and Operation.- 6.3.2 Process Efficiency.- References.- 7 Single-Stage Fixed-Film Filter and Contact Processes.- 7.1 Anaerobic Filters.- 7.1.1 Design and Operation.- 7.1.2 Process Efficiency.- 7.2 Rotating Biological Contactors.- 7.2.1 Design and Operation.- 7.2.2 Process Efficiency.- 7.3 Carrier-Assisted Contact Reactors.- 7.3.1 Design and Operation.- 7.3.2 Process Efficiency.- 7.4 Hybrid Reactors.- References.- 8 Single-Stage Fixed-Film Expanded Processes.- 8.1 Expanded Bed Reactors.- 8.1.1 Design and Operation.- 8.1.2 Process Efficiency.- 8.2 Fluidised Bed Reactors.- 8.2.1 Design and Operation.- 8.2.2 Process Efficiency.- References.- 9 Developments in Reactor Design.- 9.1 Improvements.- 9.2 Multi-Stage Operations.- 9.3 Two-Phase Digestion.- References.- 10 Start-Up of Anaerobic Bioreactors.- 10.1 Nutrient Balance and Inhibition at Start-Up.- 10.2 Seeding and Loading Regimes.- 10.3 Washout.- 10.4 Turbulence and Shear.- References.- 11 Economic Considerations.- 11.1 Comparisons of Reactor Types and Efficiencies.- 11.1.1 Overloading and Intermittent Operation.- 11.1.2 Tolerance to Toxic Shocks.- 11.2 Wastewater Characteristics.- 11.3 Cost Analyses.- References.- 12 List of Abbreviations.- 13 Subject Index.

Influence of Environmental Factors

Several environment factors can affect anaerobic digestion, either by enhancing or inhibiting parameters such as specific growth rate, decay rate, gas production, substrate utilisation, start-up and response to changes in input.

The Microbiology of Anaerobic Digestion

The combined and coordinated metabolic activity of an anaerobic reactor population is required for the complete degradation of complex organic matter to CO2 and CH4. The intermediates necessary for certain microorganisms are produced as a consequence of the action of others and therefore consortia of bacteria are frequently involved in these conversions. Despite several analyses of the major non-methanogenic bacteria present in anaerobic digesters, detailed investigations into the generic and specific nature of the hydrolytic and fermentative populations have not generally been reported. The predominant organisms in some waste-treatment systems may not, moreover, participate actively in the process but may merely be components of the wastestream itself; coliforms have been implicated here [1].

Start-up of anaerobic bioreactors on high strength industrial wastes

Abstract Anaerobic filters, anaerobic fluidized beds and upflow anaerobic sludge-blanket (UASB) reactors were started up on two types of pharmaceutical waste; anaerobic fluidized beds were also started up on glucose, fruit processing, soft drink manufacturing and pharmaceutical wastes. Fluidized beds proved superior to UASB reactors and filters in COD removal capacity and pH stability during start-up, although methane production was greatest in the UASB systems. The industrial wastes proved recalcitrant to anaerobic conversion as they contained substances inhibitory to microorganisms but loadings of up to 7·5 kg COD m −3 day −1 could be applied with a COD removal of 78% achieved. The types of volatile acids produced in four of the units were found to relate closely to substrate composition.

The influence of variable organic loading on the start-up of an anaerobic fluidised bed reactor

SummaryA stepped-loading start-up regime utilising variable organic influent concentrations in the range 1650–11600 mgCOD1−1 was applied to an anaerobic fluidised bed bioreactor at 37°C. The reactor was sensitive to variable influent COD concentrations, but the stepped-loading aided rapid recovery from transient organic loading shocks. Variable effluent COD levels were produced but a COD removal efficiency of 76% was obtained at a final HRT of 0.5 d and an organic loading rate of 5.3 kg COD m−3 d−1.

The Biochemistry of Anaerobic Digestion

Anaerobic digestion processes are widely used in the treatment of sewage sludge although the biochemical reactions comprising various stages in the anaerobic degradation of organic materials have not yet been fully elucidated. The overall anaerobic conversion of biodegradable organic solids to the end products CH4 and CO2 was initially believed to proceed in three stages which occurred simultaneously within the digester. These were: 1) the hydrolysis of insoluble biodegradable polymers; 2) the production of acid from smaller soluble organic molecules; and, 3) CH4 generation.

Start-Up of Anaerobic Bioreactors

One of the few major problems that recurs in the application of anaerobic biotreatment to wastewater is the frequent recalcitrance of start-up procedures. It is widely observed in the literature that a significant amount of down-time is involved in the initial start-up of most, if not all, anaerobic reactor systems. The main difficulty appears to be the development of the most suitable microbial culture for the wastestreams in question. Once the biomass has been established, either as a granular particle or floc system, or attached to inert carrier media as a biofilm, the operation of the reactor is generally quite stable. The sensitive nature of the majority of anaerobic bacteria and the extreme oxygen lability of the enzyme systems of obligate anaerobes render the reactor population more susceptible to slight fluctuations than is the case under comparable aerobic conditions; start-up of an anaerobic system is consequently more time consuming than initiation of an aerobic process.

Toxic Substances in Anaerobic Digestion

Inhibition of the anaerobic digestion process can be mediated to varying degrees by toxic materials present in the system; these substances may be components of the influent wastestream, or byproducts of the metabolic activities of the digester bacteria. Inhibitory toxic compounds include sulphides, consequential in the processing of wastes from such sources as molasses fermentation, petroleum refining and tanning industries, and volatile acids, microbial products which can accumulate and exceed the reactor buffering capacity. Inhibition may also arise as the consequence of the levels of ammonia, alkali and the alkaline earth metals, and heavy metals in the system. The latter have been considered the most common and major factors governing reactor failure [1, 2].

The effect of downtime on the operational performance and steady state biomass distribution of anaerobic fluidised beds

Abstract The operational capabilities of four anaerobic fluidised bed reactors restarted after periods of downtime of 39 and 89 days were determined, together with a profile of biomass activity at steady state through the bed length of two reactors. Full operational performance was retained after the shorter downtime period, but restart required manipulation after the 89-day shutdown due to poor COD removal and high volatile acid production. Biomass activity was greatest in the upper third of the reactors, with attached biomass being more active than suspended.

Single-Stage Fixed-Film Filter and Contact Processes

The settling and recycling of biomass present difficulties in the anaerobic treatment systems which depend upon freely suspended bacterial growth. Several techniques have elaborated on these processes by immobilisation of the biomass on or around carrier particles or inert surfaces. The anaerobic filter and rotating biological contactor systems require a relatively large quantity of inert media whereas the carrier-assisted contact process utilises very little.