SINGLE & MULTISTAGE SYSTEMS
Single stage systems can come in many designs, including continuously stirred tank reactor (CSTR) and plug-flow digesters, each with different modes of operation and differences in design and operation. Generally, single stage systems are simpler than two-stage systems, and cheaper to construct and operate.
Potential limitations of single stage systems are that conditions within the reactor will not be optimal for the various trophic groups of microbes. While it is true that two-stage systems can offer more optimal conditions to the methanogenic bacterial population, this is not to say that single stage systems are unreliable. The methanogenic bacterial population can be managed by controlling the feeding rate, by thorough mixing of incoming wastes to avoid peak concentrations of potentially harmful contaminants, by co-digestion with other organic wastes to provide essential water content, buffering, nutrients and trace elements or by the addition of these through the use of chemicals and nutrients.
Although single stage digesters maintain environmental conditions in which all the anaerobic trophic groups can function, different anaerobic trophic groups perform better under different environmental conditions. This is the key concept behind two (or multi) stage digesters, where digestion is separated into stages allowing the provision of optimum environmental conditions for each bacterial group.
In multi stage systems two digestion stages are normally used. In the first, hydrolysis and acidification (and some degree of acetogenesis) take place, and in the second stage the main biological process is methanogenesis, with some degree of acetogenesis also occurring.
In the first stage, hydrolysis of complex substrates is normally the rate-limiting step. Methanogenic bacteria have the longest doubling time of all the bacterial groups involved in AD and this slow microbial growth rate is normally the rate limiting step in the second stage.
As well as providing optimal environmental conditions in the second stage, some kind of biomass retention scheme is often designed, in order to keep as many active, well adapted methanogens in the digester as possible although it is possible to employ two-stage system designs that resemble two completely mixed reactors in series, or two plug-flow reactors in series. Methanogens can be retained to form higher cell densities in two ways. The first is to raise the solids retention time in the digester by separating hydraulic retention time from solids retention time. One way to do this is to employ upflow systems, where the waste flows upwards, through a layer or ‘bed’ of bacteria, and exits at the top of the reactor. The liquid waste exits at the top, while the heavier sludge layer (containing high concentrations of bacteria) is retained (by gravity) towards the bottom of the reactor. Another way is to filter the effluent from the second stage and re-introduce the solids to the reactor. The second way to retain biomass is to design a reactor with ‘support material’, which allows attached bacterial growth and thus retained biomass. This extra biomass retention provides more efficient biological operation per unit volume of reactor, and greater resistance to potentially inhibitory substances. The disadvantages of multi-stage systems are that they are often more complex, and usually more expensive than single stage AD systems. The implementation of digestion systems with ‘support material’ is however only advised for effluent type feedstocks with low suspended solids, otherwise digesters are prone to clogging.
Advantages of Multi Stage Systems
• Greater biological stability
• Greater ability to cope with fluctuating feedstock volume and quality
• Potentially higher throughputs due to optimal conditions
Disadvantages of Multi Stage Systems
• More complex control and operational requirements
• Potentially higher capital costs
It is worth noting that with a number of easily biodegradable wastes, some of the early stages of AD (hydrolysis, acidogenesis) may have already been carried out naturally in the bins/containers, in the collection vehicles and in the storage/mixing tanks. This is especially the case in the summer when ambient temperatures are higher.