During anaerobic digestion, only a part of the available organic matter is transformed into biogas. The digestate hence still contains a large fraction of organic matter that potentially can be converted.
By application of a disintegration technique (i.e. AOP, ultrasound, ozone) on the digestate and subsequent recirculation to the digester, the overall degradation potential can be increased significantly. Furthermore, high ammonia concentration in the digester can cause problems towards system stability, which leads to inhibition of the anaerobic microorganisms. The applied disintegration techniques can cause an oxidation of the present ammonia (NH3) to nitrogen-containing compounds that are not or less toxic, and which can be degraded or removed during further digestion. Next to microbial conversion to nitrate (i.e. anammox process) when the treated digestate is fed back to the digester (and nitrogen is neutralised to N2), the possible recovery of ammonia-nitrogen as a valuable mineral is also investigated. For this purpose, we will use LTL (liquid to liquid) transfer systems applied on raw extracted digestate (or liquid fraction after separation). The aim is to supply N-depleted digestate to the digester, leading to a reduced ammonia-inhibition effect and increased biogas production.
There is an increasing trend towards energy recovery from waste streams such as sewage sludge, animal manure and crop residues. In this regard, the use of anaerobic digestion is considered as very positive (from an economic and environmental point of view). During digestion, an energy-rich biogas is produced (55-75% CH4), which can be fed to gas engines to produce heat and electricity. Due to the presence of rigid and recalcitrant structures, the digestion process is slow, leading to long residence times, typically 15-30 days. Even with these long residence times, the total degree of conversion is limited: only ca. 50% of the total organic dry matter is converted to gas. This means that the digestate still contains a considerable amount of organic matter, and hence a potential for further conversion to biogas. Treating this digestate provides the ability to respond to this problem, and furthermore allows to reduce the ammonia concentration in the digester, leading to an improved system stability.
1. Increasing the conversion efficiency
As indicated above, the digestate leaving the digester still contains a substantial amount of digestable organic material. In this part of the project we will examine the extent to which the conversion efficiency of biogas potential of digestate is affected when it is subjected to disintegration techniques such as ultrasound, ozone and peroxidation. A parameter study will be performed to determine the optimum treatment conditions. Previous studies already have shown that disintegration leads to the formation of more biodegradable organic material, which must have a positive impact on the biogas production potential. Towards implementation on an industrial large scale, it is important to estimate the economic potential of this application. An extensive energetic analysis of the experimental runs will therefore be carried out.
2. Issues concerning nitrogen containing compounds
Some organic streams such as animal manure, containing high amounts of ammonia, but this has an negative effect on the activity of the microorganisms involved, and can lead to very low biogas yields. This part of the project will examine the effectiveness of ultrasound, ozone and peroxidation on the oxidation of NH3, which oxidation products will be formed (N2, NOx, NO2- and NO3-), and how much of the available radicals are consumed during this conversion. The influence on the operation of the digester will also be examined. More specifically, we will look at possible denitrification of the formed NO2- and NO3- to N2 and how the biogas composition is influenced. By treating and recycling the digestate, the fresh feed to the digester is diluted, and hence a dilution of the ammonia concentration is established. Next to oxidation, a reduction of NH3 will also be examined by means of stripping and recovery. The extent to which ammonia-nitrogen can be recovered as a valuable mineral, will be examined, by using LTL (liquid to liquid) transfer systems on raw extracted digestate or liquid fraction after separation. The balance between water-soluble NH4+ and volatile NH3 in function of process parameters (T and pH) during this transfer, will be investigated. Increasing either or both of these parameters favours the gaseous form and hence an escape from the substrate (digestate or liquid fraction) is increased.
The target group of this research project is primarily technology providers who design and build biogas installations, environmental consulting bureaus, suppliers of ultrasonic technology and oxidation technologies (ozone, AOP, ...), waste water treatment companies, end users (companies that posses biogas installations and who wish to further optimise them). Your partnership will give you the opportunity to co-determine the precise content of this research project as well as to monitor the project closely and make adjustments, when you are part of user committee. Since IWT-Tetra projects are not completely funded by the Flemish government, a limited co-financing contribution is required for this, which is 7.5% divided over all the participating partners.
KU Leuven/PETLab, Chemical Engineering Department, Prof. Lise Appels
KU Leuven/KULAB, Department of Microbial and Molecular Systems, Prof. Boudewijn Meesschaert
UGent, Department of Applied Analytical and Physical Chemistry, Prof. Erik Meers
Are you interested in participating?
Please contact prof. Lise Appels.