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Dry phase anaerobic digestion of organic waste: Resource recovery

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The source segregated mixture of green and food waste is processed inside a building with slight negative air pressure and exhaust air treatment. In the pretreatment and intermediate storage areas the organic waste is shredded to a particle size of 50-60 mm, sieved and stored. From this intermediate storage the digester module is fed continuously with the pretreated material.

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The organic material is fed to the horizontal AD reactor with a plug-screw or pump at one end. A longitudinal agitator device turns slowly inside the digester, degasifying the digesting substrate. At the same time, layering within the substrate and sedimentation of heavy particles is prevented, thanks to the specially positioned agitator paddles. The substrate passes through the reactor in a plug-flow regime. After anaerobic degradation, biomass (digestate) is pumped out of the opposite end of the module for further processing.

Natural anaerobic thermophilic micro-organisms efficiently degrade the organic material inside the digester, releasing over 95% of its energy potential in the form of biogas. Biogas is a mixture of roughly 55% methane and 45% carbon dioxide, with some trace gases.

Two principal methods are used for downstream digestate processing:

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1. Dewatering to produce a liquid fertilizer accounting for normally around 50% of the throughput, and producing an anaerobic compost. The stabilized compost may have to be screened for foreign matter depending on feed organic waste quality. The liquid digestate can be stored on-site until fertilization season.

2. Mixing the digestate directly with structurant, which includes bypassed organics, fibers, excess green waste and recycled residuals. In this case, no dewatering or liquid storage is required. The composting platform has, however, to account for proper curing and maturation time as well as any seasonal storage. This post mix strategy is very common in Europe on existing composting sites with odour and/or capacity challenges.

The AD processing facility can stabilize fresh organics faster in a controlled environment and double the capacity of composting platforms that act as final maturation and storage platforms. This can be an upgrade strategy for composting sites, which have similar issues and want to minimize residual liquid management.

Frequently asked questions

There are a few misconceptions about AD of organics waste in general and plug-flow thermophilic AD specifically. The most common ones are:

Q Are thermophilic dry AD processes sensitive to biological upsets?

A The process is designed to handle significant variation of waste and loads, as system feed can be adjusted over an extended period of time. For instance, the process can accept a significant ratio of yard waste as expected in Canada.

Most AD upset risks in source separated organic waste applications relate to ammonia inhibition. An ammonia concentration limit of 2.8 g/l in the reactor is recommended, but there are systems operating successfully at higher concentrations due to micro-organism adaptability.

Excess meat or fish protein waste may introduce a higher nitrogen input. Since it normally takes weeks to impact the overall system, volatile fat acids measurements and biogas quality give an early indication of trouble. The solution is to balance the nitrogen to carbon ratio in the process, for example by increasing plant matter waste.

Toxicity is very uncommon when feedstock is household waste. A small fraction of contaminants is typical and may include batteries, low/high pH liquids, residual cosmetics or cleaning products. In the case of institutional, commercial and industrial inputs, heavy metals, biocides and acids must be avoided.

Q Can anaerobic digestion destroy organics and therefore reduce mass to compost?

A Mass reduction in an anaerobic reactor is generally around 15%. AD degrades organic matter to biogas at a rate of 110 – 150 Nm3/tonne). The digestate has low biological activity, meaning nearly all of the anaerobically degradable matter has been converted into biogas. The main function of the composting platform downstream (or dewatering) is to transfer the material from anaerobic to aerobic conditions and to reduce the water content which, in turn, will significantly reduce final mass.

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