WET & DRY SYSTEMS
Anaerobic digestion systems can be broadly classified into ‘wet’ (liquid) digestion or ‘dry’ (solid) digestion.
Wet (Liquid or Low Solid) Systems
Wet digestion systems are designed to process a dilute organic slurry with typically <15% total solids. For substrates with higher than 15% total solids this slurry is created by adding fresh water, re-circulated process water, or another organic waste with a lower total solids percentage to the incoming waste stream (i.e. co-digestion).
Wet systems have a successful track record in treating low solid materials such as sewage sludges and food industry effluents, however, the wet system approach has had to overcome a number of challenges to treat Biodegradable Municipal Waste or Organic Fraction of Municipal Solid Waste. These include:
• The production of a wet slurry from residual MSW can result in the loss of volatile organics;
• Mixing is necessary as a wet slurry inside the digester will tend to separate into layers of material, with a floating layer of scum at the top of the digester that may also require breaking;
• Heaviest particles will settle to the bottom where they can accumulate (and reduce the digester capacity over time), or cause damage to pumps;
• Fibrous material has a tendency to form strings that wind around mechanical stirrers in certain systems
• Minimising the potential for “Short circuiting” (when particles of organic waste are removed from the digester before they have been fully digested) has to be addressed at the design stage.
The pre-treatment necessary to condition solid wastes into a slurry of adequate consistency and devoid of coarse or heavy contaminants can be very complex involving screens, pulpers, drums, presses, breakers, and flotation units. This is especially the case for mechanically-sorted OFMSW.
Wet systems require comparatively larger digesters, more and greater capacity water pumping and piping/valving, more extensive digestate storage and / or de-watering, higher capacity wastewater treatment facilities and more energy required to heat the larger volumes.
Providing that the above points are considered at the design stage, a wet system can provide an effective and robust means of treating low solid content waste, or high solid waste that has been adjusted to <15% total solid content.
Case Studies of operational wet digestion systems can be seen below.
Dry (High Solid) Systems
Dry AD systems digest a waste stream of 15 – 40% total solids. The physical characteristics of the wastes at such high solids content impose technical approaches in terms of handling, mixing and pre-treatment, which are fundamentally different from those of wet systems. Transport and handling of the wastes is carried out with conveyor belts, screws, and powerful pumps especially designed for highly viscous streams (such as cement).
This equipment is more expensive than the centrifugal pumps used in wet systems, however, this additional cost is offset against the smaller vessels required for digesters and reduced storage requirement due to the lower water content of the substrate.
The only pre-treatment which is necessary before feeding the wastes into the digester is the removal of the coarse impurities which can be accomplished either via trommel screens, or via shredders and trommel screens for OFMSW. The heavy inert materials such as stones and glass which pass the screens or shredder need not necessarily be removed from the waste stream.
Due to their high viscosity, the fermenting wastes move via plug flow inside the digesters, contrary to wet systems where completely mixed digesters are usually used. Therefore heat and nutrient transfer and homogeneity in dry AD systems is less efficient than in wet AD systems. Plug flow operations need specialised mixing arrangements, as mixing the incoming wastes with the fermenting biomass is crucial to guarantee adequate inoculation and to prevent localised overloading and acidification.
Dry digesters may be vertical where incoming waste (mixed with treated waste) is pumped to the top of the digester and moves downwards as a plug, or horizontal where plug flow is aided by slowly-rotating impellers inside the reactors, which also serve for homogenization, de-gassing, and re-suspending heavier particles.
A commercial system based on circular plug flow with periodic high pressure gas mixing is also available in the market place. This does not generally require the mixing of treated and incoming wastes although this advantage should be balanced with the potential for blockage of gas injection ports and periodic maintenance required on the injection system within the reactor.
Whilst dry systems may still require additions of water (or co-digestion with low solid wastes) to achieve a total solids content of around 30%, dry systems use considerably less water as part of the process than wet systems. This in turn leads to lower energy requirements for in-plant needs, because less energy is needed for heating process water, and for de-watering AD digestate.
Dry digestion systems normally operate at thermophilic temperatures so that bacterial metabolism is faster in order to deal with such high organic loads.
A case study of an operational dry digestion system can be seen below.