Process parameters - Anaerobic Digestion

Process parameters

Efficient AD requires the development and maintenance of an optimised internal environment to facilitate biological activity. This is of particular importance in the commercial setting and a number of both physical and chemical factors must be taken into account to achieve it, of which the most important are:

•temperature;

•retention period;

•agitation;

•wetness;

•feedstock;

•loading rate;

•pH and volatile fatty acids concentration.

Temperature

As mentioned previously, in commercial systems, digesters are operated at around 35 ◦C (mesophilic) or 55 ◦C (thermophilic). Irrespective of which approach is adopted for any particular application, a relatively constant temperature is essen-tial for the process to run at its greatest efficiency.

Retention period

Although the amount of biowaste degraded depends on its character, the avail-ability of bacteria and the time allowed for processing, temperature governs both the rate of breakdown itself and the particular bacterial species present in the digester. Hence, there is a direct relationship between temperature and the reten-tion period. Some AD technologies have attempted to shorten the retention period by separating the stages of the process within the digester. The separation of the acidogenic and methanogenic stages permits each to be optimised and this has been well demonstrated at laboratory scale using a completely mixed digester, with phase-isolation being achieved by pH manipulation. Despite the greater effi-ciency, higher biogas yield and enhanced process stability claimed, it has seen little large-scale use, probably as a result of the higher cost implications of such a system.

Agitation

The agitation of the digester contents has a number of benefits, one of the most obvious being that it helps to mix up material, evening out any localised con-centrations, thus also helping to stop the formation of ‘dead zones’ or scum. In addition, it increases the waste’s availability to the bacteria, helps remove and disperse metabolic products and also acts to ensure a more uniform tem-perature within the digester. There have been some suggestions that efficient mixing enhances methane production, but the evidence is inconclusive, so it seems likely that this may only be of noticeable benefit for some systems or operational regimes.

Wetness

Anaerobic digestion is a wet process and any biowaste which is too dry in its natural state will require the addition of a suitable liquid, typically water, recycled AD process liquor or slurries, either sewage or agricultural, before processing can begin. In order to minimise digester size, so-called ‘dry’ systems have tended to dominate the commercial world, but the relatively thicker contents inevitably demand more energy to mix effectively, off-setting much of the advantage. Com-parisons of ‘wet’ or ‘dry’ approaches, like those of mesophilic or thermophilic processes, generally yield no clear winner. Each system has particular advantages and applications for certain kinds of biowaste, and selecting the right one for any given use is almost always best done on this basis.

Feedstock

As with composting, the particle size and nature of the material to be treated play an important role. The ease of breakdown is largely defined by the characteristics of the biowaste material to be treated, but generally finer particles allow for better processing and a homogeneous slurry or suspension is the ideal feedstock for AD. It must be stressed, however, that some biowaste types, particularly the likes of lignin-rich, woody material, are relatively resistant to this process.

Loading rate

Loading depends on the characteristics of the waste, its degree of wetness, digester volume, the expected retention period and similar system design param-eters. It is typically expressed as the chemical oxygen demand per cubic metre of digester void-space (COD/m3) or, for continuous or semi-continuous process, per unit time (COD/day, COD/hr).

pH and volatile fatty acids concentration

These are interdependent factors which need to be considered together. Adequate process control and digester optimisation requires suitable pH monitoring, since many of the bacteria involved are pH sensitive. In particular, acidogens, having better tolerance to acidity, may produce acids faster than the increasingly inhibited methanogens can use it, in conditions of low pH, leading to spiralling acidity and the potential for process collapse. A number of acid – base reactions exist within the typical AD process, which lead to a measure of natural, inbuilt resistance to major pH swings. However, under certain circumstances, the need for external interference may arise and the amount of such intervention necessary to maintain proper equilibrium will depend on the nature of the material. For some wastes, pH control may only be necessary during start-up or in overload conditions; for others where acidity is habitually shown to be a problem, continuous control may be necessary.

 Volatile fatty acid concentration is one of the most important process indica-tors. Elevated VFAs are characteristic of AD instability and thus they may be the first indication of a developing problem, though the actual cause may be less immediately obvious. Inadequate mixing, excessive loading, poor temper-ature control or bacterial inhibition can all lead to an increase in VFAs and a decrease in pH. Considering the inconvenience and cost of being forced to empty a sick reactor, commercial AD operations rely greatly on routine monitoring of this kind.