Dry anaerobic digestion can be seen as a well-kept secret. It is seldom mentioned in most AD industry circles. In the rush to develop anaerobic digestion plants so far, most biogas project promoters have stuck to the most familiar methods of wet anaerobic digestion.
However, in many countries the profitability of biogas projects will soon depend on utilising the organic feedstocks which others cannot accept.
Those businesses that send their easily to digested feed materials to the most common AD Continously Stirred Tank Reactors (CSTRs), are rapidly becoming aware that their waste has a value.
These companies won't be slow to start demanding higher prices for their organic waste output to be taken away, as time goes on and more AD plants are commissioned.
Therefore, as waste feed materials become more sought after, we contend that there are some very valuable Dry Anaerobic Digestion secrets out there which extend the range of digestable biogas feedstocks.
In particular, value can be gained from solid organic waste, including the organic fraction of household waste (Municipal Solid Waste). Companies such as Kompogas even have innovative continuous Dry AD technology to offer.
If you ignore this, now and stick to wet feedstock AD in CSTRs only, you may later live to regret that you didn't discover them here and now, for yourself.
1. Dry AD plants offer a number of benefits, including wider flexibility in the feedstock source which can be used, reduced retention times relative to wet AD processes, reduced water demand, and more opportunities for producing a unique end-product.
A main distinction between anaerobic digestion (AD) technologies for treatment of municipal and industrial biodegradable wastes is the operating process solids content. Wet AD systems operate at low total solids (<10–20% TS) and dry systems have high operating solids (20–>40% TS).
The performance of wet and dry AD systems was quantified in relation to: technical operation (footprint, capacity, feedstock characteristics, pretreatment and post-treatment, retention time, water usage), energy balance (biogas digester productivity, parasitic energy, methane [CH4] content, utilization of biogas and produced energy), digestate management and economic performance (capital and operational costs, revenues, specific capital costs [per t of waste and per m3 biogas]).
Wet AD plants had improved energy balance and economic performance compared to dry AD plants. However, dry AD plants offered several benefits, including greater flexibility in the type of feedstock accepted, shorter retention times, reduced water usage and more flexible management of, and opportunities for marketing, the end product. via Bentham Science Journal
2. Recent works on dry anaerobic digestion (AD) show that not only methane but also hydrogen, volatile fatty acids (VFAs), and ethanol can be produced from municipal solid waste (MSW), dewatered sewage sludge, animal manure or crop residue by dry AD processes.
Up to now only methane production from household wastes has already been commercialized by using dry AD technology. Single-stage dry AD processes with semi-continuous or continuous operation mode dominates the commercialized dry AD plants. To get enhanced biogasification efficiency, naturally microbial pretreatment methods (like stack-pretreatment and aerobic or facultative composting) and co-digestion are practically useful for dry AD, especially for the treatment of carbon-and-nitrogen-rich organic solid wastes, i.e. crop residue and animal manure.
Dry AD could achieve comparable production efficiency to wet AD systems, yielding 121 – 340 L of CH4 from per gram volatile solids (VS) of organic fraction of MSW (OFMSW) and 51 – 55 ml H2/g- VS reduced from OFMSW, sewage sludge, and paper and food wastes.
Still, future researches are necessary and demanding for dry AD to better challenge with other low-cost treatment and disposal methods, which are also proposed in this review mainly relating with its longer solids retention time, feedstocks collection, inhibitory substances, online process monitoring, and establishment of process assessment index system. via Biofuels Scotland
3. Dry anaerobic digestion technology [will open up the biogas industry, making it capable of producing energy from] solid manures and other dry crop residues on a continuous basis with solids content in the digester up to 45%.
[Until now] anaerobic digestion of animal manure has generally been limited to wet digestion where total solids are less than 12%. This has restricted adoption of biogas production primarily to dairy and hog operations, or required substantial water usage to dilute manures with higher solids content.
The DRANCO-Farm dry anaerobic digestion technology processes solid manures and other dry crop residues on a continuous basis with solids content in the digester up to 45%.
High net energy yield per hectare is an indispensable prerequisite for an economic operation of an energy crop digestion plant. This includes high biomass yields and low energy requirement for plant cultivation, harvest and processing. From practical experience, on average, about 50% of the total energy requirement is spent for fertilizer production, minor amounts are required for machinery (22%), transport fuel (15%) and pesticides (13%). via Univr – ArticoloRivista – allegato
4. Dry anaerobic co-digestion of seaweed and solid cow manure looks promising as a way to raise biogas production for coastal biogas plants close to a source of seaweed, with additional fertiliser production benefits.
Two-stage, dry anaerobic co-digestion of seaweed and solid cow manure was studied on a laboratory scale. A methane yield of 0.14 L/g VS added was obtained when digesting solid cow manure in a leach bed process and a methane yield of 0.16 L/g VS added and 0.11 L/g VS added was obtained from seaweed and seaweed/solid manure in a two-stage anaerobic process, respectively.
The results showed that it was beneficial to operate the second stage methane reactor for the digestion of seaweed, which produced 83% of the methane, while the remainder was produced in the first leach bed reactor.
Also, the two-stage system was more stable for the co-digestion for seaweed and manure when compared to their separate digestion.
In addition, the initial ammonia inhibition observed for manure digestion and the acidification of the leach bed reactor in seaweed digestion were both avoided when the materials were co-digested. The seaweed had a higher Cd content of 0.2 mg Cd/kg TS than the manure, 0.04 mg Cd/kg TS, and presents a risk of surpassing limit values set for fertiliser quality of seaweed digestate.
Evaluation of the heavy metal content of seaweed or a mixture of seaweed and manure digestate is recommended before farmland application. via AJol – Dry
5. The dry anaerobic digestion of wheat straw and wool textile waste in dry anaerobic digestion (AD) process has been investigated and it was found that the co-digestion of these two substrates showed higher methane yields than expected.
Utilisation of wheat straw and wool textile waste in dry anaerobic digestion (AD) process was investigated. Dry-AD of the individual substrates as well as co-digestion of those were evaluated using different total solid (TS) contents ranging between 6 to 30%.
Additionally, the effects of the addition of nutrients and cellulose- or protein-degrading enzymes on the performance of the AD process were also investigated.
Dry-AD of the wheat straw resulted in methane yields of 0.081 – 0.200 Nm3CH4/kgVS with the lowest and highest values obtained at 30 and 21% TS, respectively. The addition of the cellulolytic enzymes could significantly increase the yield in the reactor containing 13% TS (0.231 Nm3CH4/kg VS).
Likewise, degradation of wool textile waste was enhanced significantly at TS of 13% with the addition of the protein-degrading enzyme (0.131 Nm3CH4/kg VS).
Furthermore, the co-digestion of these two substrates showed higher methane yields compared with the methane potentials calculated for the individual fractions at all the investigated TS contents due to synergetic effects and better nutritional balance.
Akia, M., Yazdani, F., Motaee, E., Han, D., Arandiyan, H., 2014. A review on conversion of biomass to biofuel by nano-catalysts. Biofuel Res. J. 1(1), 16-25. via BiofuelJournal
What is your opinion of these dry anaerobic digestion innovations?
Do you agree that thinking laterally about new biogas feedstocks and co-disposal opportunities, is the way to a sustainable and profitable future for biogas producers, as subsidies for biogas production are reduced?
For AD plant operators making use of other people's waste products, the first to start using using novel waste materials, will inevitably have access to the lowest cost feed materials.
Even farm based AD energy businesses with their own feedstock sources, would do well to look at dry anaerobic digestion for extending their in-house facilities for greater flexibility, and also raising their income through synergistic feedstock co-disposal of the sort discussed in our 5 dry AD “secrets” above.
Terminology: CSR – Completely Stirred Reactor.