Anaerobic Digestion – Introduction to Hybrid AD - HyAD

The majority of AD's are based on the Continuously Stirred Tank Reactor (CSTR) method of bacterial digestion of wastes. The anaerobic digestion principle is straightforward whereby large insulated tanks are heated and the materials held over time until digested to release a biogas, an analogue of a cow's stomach. This CSTR technology was typically used to treat farm waste and sewage slurries, which have a low dry matter (DM) content, around 3-5%, and are readily pumped. As this popular technology became commonplace AD naturally migrated to the treatment of food wastes, such wastes are typically more viscous and contain a higher volatile solids content, some 20-25%. To enable such materials to be better handled they are usually liquid diluted to reduce fed DM percentage, circa 12-15%, and viscosity. The liquid becomes a carrier through the 'wet AD' digestion process with little or no contribution to biogas yield.

Typical Wet AD

An alternative to CSTR is a plug flow method where a 'plug' of organic feedstock is fed in one end of a digester, still insulated and heated but now being more like a tube than a tank, and then slowly 'flows' to the other end during which time it is broken down by bacteria to releases biogas. The plug-flow typically handles sludge-like higher DM content and relies on the viscosity of the materials to remain together, such technology is also known as 'dry AD'. The digestate is usually not stirred or agitated but may be paddle lifted to keep the materials from stratifying and in sludge suspension. Whilst dry AD can digest efficiently at a 25% DM its performance falls off when asked to digest less viscose or diluted feedstocks. As an example source segregated food-wastes may have sufficient DM% but when packaged food-waste is processed the liquid used to wash the organics from the packaging is likely to dilute the feedstock to a level not readily suitable for dry AD.

Plug-flow Dry AD

HyAD in Action

The latest CSTR technology has evolved from the earlier application in farm and sewage and can now be seen as being a hybrid of wet and dry AD (HyAD). The HyAD can efficiently digest a broad range of feedstocks and is fed at comparably higher dry matter content of 18-22%, and is highly suitable for all food-waste and can also digest co-mingled with garden-wastes. This is perfect for organic wastes from source segregated residential collections along with the packaged and residue wastes from commercial and industrial processes.

The key to how these HyAD systems work is in the ability to operate at higher end of temperatures and the ability to efficiently mix the incoming materials with the digester biomass biome.

Digestate Recirculate & Chop Macerator
Pulls from sides and delivers to centre tank

Digester mixing has always been recognised as a key process and whilst a variety of mixing technologies have been recently implemented on AD sites, the primary objectives of a digester mixing system remain the same and are namely:

  1. Provide contact between the feed input and the active biomass to maximise gas production.
  2. Provide physical, chemical and biological uniformity within a digester in order to maintain a satisfactory environment for both acid and methane forming bacteria.
  3. Distribute organics and dilute inhibitory substances by efficient movement.
  4. Prevent biomass stratification and temperature gradients within digester.
  5. Utilise the digester volume effectively to maintain the required biomass residence time, i.e. minimise short-circuiting meaning the fed material does not get washed through with the digestate output
  6. Minimise the deposition of solids, as this reduces effective digester volume, meaning keeping all material in the sludge moving by side recirculation and delivery to tank centre to keep in suspension.
  7. Minimise foaming and the formation of a surface scum layer that impacts on the biogas yield.

The first three mixing objectives are related to blending the feed with the digesting organic materials. The rate of blending is characterised by a time (the blend time) taken to reach a specified degree of uniformity. The biomass residence time is influenced by the flow pattern and residence time distribution (RTD) in the digester. Significant stagnant or dead regions reduce the effective or 'active' volume and hence reduce residence time and the HyAD technology is optimised to meet such an objective.

The HyAD achieves this by the MAMS approach, Maceration of the materials both by preparation of the feedstock and by biomass recirculation and chopping, Agitation during the stirring mix to ensure all materials are in the suspension and available to the bacterial culture, Measurement of materials and the digester environment (in terms of DM, viscosity, temperature, pH, gas generation etc), and using the overall Systems approach to continuously monitor and feedback accordingly.

HyAD – Optimised from input to gas output

HyAD brings enhanced performance