R. Kirchmayr

Enhancement options for the utilisation of nitrogen rich animal by-products in anaerobic digestion

This study focuses on the enhancement of an Austrian anaerobic digestion plant at a slaughterhouse site which exclusively uses animal by-products as substrate. High ammonia concentrations from protein degradation cause severe inhibitions of anaerobic microorganisms. For improving the current situation the COD:TKN ratio is widened by (a) ammonia stripping directly out of the process and (b) addition of a C source to the substrate. Different OLR and HRT were tested in continuous experiments to simulate new operating conditions. The results show that the addition of carbon cannot improve fermentation capacity. The reduction of ammonia boosts the degradation: After reduction of TKN from 7.5 to 4.0 g kg(-1) the initially high VFA concentration decreased and the COD degradation was improved by 55.5%. Hence, the implementation of the new N reduction process facilitates either the increase of the OLR by 61% or the reduction of the HRT by 25%.

New data on temperature optimum and temperature changes in energy crop digesters

As a result of self-heating in anaerobic digesters when using energy crops in the feedstock, the influence of temperature on the digestion process came back into focus. The aim of this study was to investigate the impact of such temperature increases on process stability. Furthermore, different strategies for the transition from mesophilic to thermophilic conditions and the resulting methane yields at different temperature levels were evaluated. Two main effects were identified with different bio-slurries from agricultural biogas plants: (1) a failure of methane production connected to changes in the microbial community; and (2), a slow but continuous accumulation of propionic acid, though without an immediate effect on methane production. All strategies for increasing the operating temperature showed negative effects on digester performance, some with serious economic consequences for the operator. It was shown that methane yields at different temperature levels in the mesophilic and sub-thermophilic ranges are similar.

Recent Developments in Bio-Energy Recovery Through Fermentation

Bio-energy recovery through fermentation is gaining importance because of limited fossil resources. Especially, bio-ethanol and biogas production are applied worldwide and have reached industrial scale. Other options such as bio-hydrogen, microbial fuel cells, and higher alcohol and acid fermentations are still in development. Current fields of intense research are the utilization of lignocellulose substrates, intensification of the recovery of wastes and industrial byproducts for bio-energy recovery, optimization of process control (especially, in anaerobic digestion), and the optimal utilization scenario of byproducts from microbial bio-energy processes (e.g., stillage, digestate). For lignocellulose utilization, the optimal pretreatment technologies (heat, acid, enzyme, steam explosion, mechanical treatment) are being investigated. For methane production, benchmarks for fermentation parameters are presented, and current bottlenecks and deficiencies of the technology are discussed.

The influence of energy crop substrates on the mass-flow analysis and the residual methane potential at a rural anaerobic digestion plant

In a full scale anaerobic digestion plant exclusively operating on solid energy crops the mass-flows were analysed for two different substrate compositions over 583 d. The mono-fermentation of maize whole crop silage was compared to a mixture of maize and grass + clover silage. The two stage system required the input of dilution liquid guarantee digestion and agitation in the high loaded first stage (OLR: 5.50 kg VS.m(-3).d(-1)). Grass + clover demanded the double mass of process dilution liquid, which reduced SRT from 65 d to 34 d for each stage and leaded to an increased generation of Solid Digestion Residues by separation. Experiments showed that 70% of the Residual Methane Potential are caused by the 7% mass fraction of SDR. For maize and maize + grass + clover RMPs of 6.34% and 11.80% were observed, respectively. RMP can also be expressed as additional substrate input required for full granted operation. Thus, the mass stream analysis is used to determine mitigation strategies for RMP.

Anaerobic digestion of stillage fractions – estimation of the potential for energy recovery in bioethanol plants

Stillage processing can require more than one third of the thermal energy demand of a dry-grind bioethanol production plant. Therefore, for every stillage fraction occurring in stillage processing the potential of energy recovery by anaerobic digestion (AD) was estimated. In the case of whole stillage up to 128% of the thermal energy demand in the process can be provided, so even an energetically self-sufficient bioethanol production process is possible. For wet cake the recovery potential of thermal energy is 57%, for thin stillage 41%, for syrup 40% and for the evaporation condensate 2.5%. Specific issues for establishing AD of stillage fractions are evaluated in detail; these are high nitrogen concentrations, digestate treatment and trace element supply. If animal feed is co-produced at the bioethanol plant and digestate fractions are to be reused as process water, a sufficient quality is necessary. Most interesting stillage fractions as substrates for AD are whole stillage, thin stillage and the evaporation condensate. For these fractions process details are presented.

Comparing centralised and decentralised anaerobic digestion of stillage from a large-scale bioethanol plant to animal feed production.

A comparison of stillage treatment options for large-scale bioethanol plants was based on the data of an existing plant producing approximately 200,000 t/yr of bioethanol and 1,400,000 t/yr of stillage. Animal feed production--the state-of-the-art technology at the plant--was compared to anaerobic digestion. The latter was simulated in two different scenarios: digestion in small-scale biogas plants in the surrounding area versus digestion in a large-scale biogas plant at the bioethanol production site. Emphasis was placed on a holistic simulation balancing chemical parameters and calculating logistic algorithms to compare the efficiency of the stillage treatment solutions. For central anaerobic digestion different digestate handling solutions were considered because of the large amount of digestate. For land application a minimum of 36,000 ha of available agricultural area would be needed and 600,000 m(3) of storage volume. Secondly membrane purification of the digestate was investigated consisting of decanter, microfiltration, and reverse osmosis. As a third option aerobic wastewater treatment of the digestate was discussed. The final outcome was an economic evaluation of the three mentioned stillage treatment options, as a guide to stillage management for operators of large-scale bioethanol plants.