Anaerobic Digestion Processes in Industrial Wastewater Treatment

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:

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].

Microbial cells exist in a range of sizes, shapes and phases of growth, individually or aggregated into various microstructures. These conditions are of practical significance in anaerobic digestion since the form of the biomass is likely to have a significant effect on organism survival and nutrient transfer, and thus the efficiency of the overall digestion process. In a turbulent system, attached biomass can persist whilst cells in suspension are lost with the effluent [1]. Abiotic suspended particles may be utilised as adhesion sites for bacteria, aiding their persistence by enhanced sedimentation and hence their avoidance of washout in the effluent. Microstructural forms of biomass are shown in Fig. 11; these can be major determinants of mass transfer. Formation of a particular structural aggregate depends on several factors including the size range of cells within the microbial population and the location of each individual cell relative to other cells and the medium, for example at a gas/liquid interface. Non-uniform gradients of organic compounds, ions, enzymes and conductivity (due to bacterial metabolism) exist as the aggregates are non-homogeneous, filamentous forms sometimes predominating.

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.

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].

Several anaerobic reactor types are utilised for waste treatment by biological means; these can be broadly divided into two groups, namely the fixed-film reactors and the non-attached growth systems. The biomass of the former comprises bacteria attached as films to inert supportive media; the latter depend for their operation on the metabolic activity of microorganisms suspended as flocs or granules in the reactor vessel. The bacteria in suspended growth processes must form flocs to remain in the reactor, and the efficiency of non-attached biomass systems is to a great extent dependent upon the floc-forming and settling abilities of the sludge inoculum used to initiate the anaerobic digestion process. The microflora of anaerobic reactors are almost exclusively bacterial; although protozoa may be present, they are generally introduced with the influent and play no active part in the degradation reactions [1].

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.

The effective and reliable treatment of industrial and other wastestreams by biological means necessarily requires a system which demonstrates wide-ranging tolerance of fluctuations in operational conditions. Temperature, influent COD and pH increases and decreases will all, to varying degrees, affect the activity and viability of anaerobic bacterial populations within the reactor systems. The CH₄- producing bacteria for example, are extremely intolerant of even minor fluctuations in environmental conditions. Immediate cessation of gas formation results from the application of a sharp temperature decrease of around 20 °C to the microbial system and this technique is in fact used in other anaerobic digester processes to aid biomass retention in settler units (see Chaps. 6 and 7). Fixed-film processes are in general considered more resistant than alternative systems to hydraulic and other shocks because of their capacity for biomass retention within a film with certain protective characteristics.

Anaerobic systems tend to be subject to a number of operational limitations. More sophisticated reactor designs such as expanded and fluidised beds and the sludge blanket configuration often require careful monitoring and management. A number of difficulties arise from deficiencies in the knowledge of the microbiological and biochemical reactions of anaerobic systems; these include biofilm build-up, pH and micronutrient levels, temperature effects, sensitivity to toxic compounds and acclimation characteristics of digester organisms. Additional problems occur due to the lack of knowledge of many aspects of the digester system as a whole, primarily with reference to the start-up and reliability of the various reactors. Despite the research in progress, practical experience of pilot or full-scale systems has been limited. Anaerobic digester operations vary widely in complexity and configuration and no invididual design can be regarded as ideal, as each application requires a particular system to fulfil specific criteria. Efficient on-line monitoring of process parameters must likewise be developed for efficient operation.

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.

There are several difficulties at once apparent in attempting a useful comparison of the basic reactor types discussed in the preceding chapters: one problem pertains to the characterisation of system design, the demarcation between various bioreactor types being unclear and overlap of many features occurring. The major difference, for example, between the contact and the carrier-assisted contact processes is the presence of a small quantity of inert media as support particles in the latter, a situation easily produced in the former by the introduction of nonbiodegradable suspended solids with the influent; the floc structures of both reactor configurations are practically indistinguishable. The anaerobic sludge blanket and the contact reactor have likewise many similar features, the design differences themselves being in the placement of the settling unit, i.e., internal or external to the reaction vessel. In the case of the fixed-film processes, the borderline between expanded and fluidised bed reactors is indefinite and dependent almost exclusively upon the degree of fluidisation of the carrier particles, which in turn is governed by particle density, porosity and size, and by the fluid flow rate. Other elements must necessarily enter into any qualitative comparisons made between reactor design and mode of operation: these include the strength and complexity of the waste to be treated, the influent flow rate, temperature and pH, and diurnal, seasonal or other temporal variations of these factors. Differences between reactor performances and efficiencies therefore, are not as straightforward as may be expected from design and operational variations.