Specialist Group on ANAEROBIC DIGESTION Newsletter - IWA

Anaerobic digestion in sustainable biomass chains
Institution:
Sub-department of Environmental Technology
Wageningen University
Wageningen, The Netherlands
Supervisors:
Jules van Lier
Rudy Rabbinge
by Claudia P. Pabon Pereira
Abstract:
This thesis evaluates the potential contribution of anaerobic digestion (AD) to the sustainability of biomass
chains. Results provide insights in the technological potential to recover energy and valuable by-products from
energy crops and residues, and evaluate biomass cascades involving AD technology for their feasibility and
desirability. Embedding AD in biomass chains addresses current constraints towards increased use of biomass
for energy production considering land competition and environmental pollution. Within this panorama so far
major advantages of AD to improve energy efficiency and closing material cycles have received a limited
attention. As part of the experimental research an Oxitop® protocol was refined for screening plant material
suitable for anaerobic digestion based on their energy content. Environmental factors influencing the test
outcome are the use of NaOH pellets for CO 2 scavenging, substrate pretreatment, microbial culture, and type of
buffer. The use of NaOH pellets and substarte pretreatments were most influential on the results. By means of
the developed Oxitop® protocol the relationship between plant ligno-cellulosic composition and the biomethane
potential (BMP) and hydrolysis rates was researched. The Acid Detergent Fibre (ADF) and the Neutral
Detergent Fibre (NDF) as analyzed by the van Soest method were proposed as suitable plant characterization
techniques for predicting the biodegradability and hydrolysis constants. The biodegradability model developed
was further used to predict the biodegradability of 114 European plant samples identifying interesting crops and
crop residues suitable for anaerobic digestion. Batch experiments on digestate quality showed an increase of 20-
26% and 0-36% in solublised NH 4 + and PO 4 3- , respectively, after 2 months of digestion. The largest fractions of
the inorganic nutrients were found in the liquid fraction of the digestate, i.e. 80-92% NH 4 + and 65-74% PO 4 3- .
Increase in manure content in the mixture showed a positive effect in the methane production rate and the total
amount of nutrients in the digestate.
The added value of AD within different biomass cascades was evaluated using the sustainable development
concept in a specified system boundary. The sensitivity analysis of the energy balance of an AD facility shows
that heat losses become the most important energy loss when high value substrate such as energy maize are
employed. In contrast, when a low energy substrate such as manure is used, the indirect energy of the
infrastructure facilities, become the most important energy input. A sustainability framework was proposed for
evaluating the potential role of AD in biomass cascades applied for the Colombian situation. Results show that
production of bio-ethanol from cassava is only sustainable from an energy and greenhouse gas (GHG)
perspective when energy recovery from the process residues, using AD, is part of the process. The exact
outcome of the evaluation largely depends on variables like substrate drying, type of fuel used and type of
applied AD system. During the study of other biomass cascades the contribution of by-products was shown to be
crucial, constituting 41-68% of the sum of all energy flows. For oil palm, sugarcane, panelacane and cassava,
the energy contribution of the byproducts to the different biofuel systems fluctuates between 51-158, 122-290,
71-170, and 36-71 GJ.ha -1 yr -1 , respectively. AD had also a positive impact on nutrient recovery and water
savings in the studied chains. The energy, nutrient and water benefits were set in perspective by giving an
indication on the economic benefits and land savings potentially attainable under Colombian conditions.
IWA <strong>Specialist</strong> <strong>Group</strong> on Anaerobic Digestion
24 June 2009 Newsletter