|The MELiSSA compartments|
The liquefying compartment (compartment 1)
This compartment is the collection pool for the waste produced by the consumer (i.e faeces, urine, paper) and also the non-edible output of the higher plant compartment (straw, roots,..) and the non-edible microbial biomass.
Its essential task is to anaerobically transform this waste to ammonium, H2, CO2, volatile fatty acids and minerals. For biosafety reasons, as well as degradation efficiency, the compartment operates in thermophilic conditions (i.e. 55°C). In terms of degradation, three main functions are expected in this compartment: proteolysis, saccharolysis, and cellulolysis.
During the early years of MELiSSA, it was expected that a proper selection of a restricted number of mesophilic bacteria could reach a reasonable level of degradation. However, despite interesting results, among them the identification of a new proteoloytic bacteria (i.e. protelyticus thermocellum), a percentage of degradation higher than 15% has never been obtained.
Therefore it was rapidly decided to extend the number of bacteria and to work with a consortium of autochthonous strains. Now, the overall biodegradation efficiency by the selected inoculum allows the reaching interesting values (i.e.proteolysis 70%, fibre 44%).
Better degradation values are currently limited by two factors: the very slow degradation of fibrous material (i.e cellulose, xylan, lignin) and the mechanical difficulty of extracting these non-degraded compounds for a specific and more adapted treatment.
To improve this degradation level, several technologies are currently being studied (subcritical oxydation, fungi, rumen bacteria, hyperthermophilic bacteria,…)
The photoheterotrophic compartment (compartment 2)
This compartment is responsible for the elimination of the terminal products of the liquefying compartment. In the MELiSSA concept, it is divided into two separate and independent compartments (i.e a photoautotrophic and an photoheterotrophic compartment).
This separation was mainly due to an expectation of high H2 production from the first compartment. Although the specific treatment of hydrogen is still being kept in mind, the very wide results for substrate degradation obtained with R.rubrum lead to simplifcation of the loop to only one second compartment (that is, a photoheterotrophic one).
Regarding the current researches on this compartment, they are driven by two major factors: the demonstration of the validity of the preliminarily established stoichiometries in presence of one or several carbon sources and the improvement of the light transfer model.
As the quality of microbial biomass is always questionable for man, specific studies have been performed with R.rubrum and have demonstrated its acceptance at low level concentrations.The nitrifying compartment (compartment 3)
The nitrifying compartment's main function is to cycle NH4+ evolved from waste to nitrates, which is the most favourable source of Nitrogen for higher plant as well as Arthrospira platensis.
The compartment is composed of a mix Nitrosomas and Nitrobacter which oxidise NH4+ to NO2– and NO2– to NO3– respectively. As this compartment is a fixed bed reactor, the importance of the hydrodynamic factors is slightly more important as well as more complicated.
Therefore the activity on this compartment has been focused on two areas: the physical characterisation of the reactors (i.e. Kla , RTD) and numerous bench tests to establish the proper kinetics and stoichiometries for different configurations.
A pilot reactor has been built and has now beenrunning for several months. Its last update was the insertion of an automatic cleaning system to avoid clogging.The photoautotophic compartment (compartment 4):
As already explained the fourth compartment is split into two parts:
- the algae compartment colonised by the cyanobacteria Arthrospira platensis
- and the Higher Plant(HP)compartment
These compartments are essential for the regeneration of oxygen and the production of food.
The algae compartment(4A) has been extensively studied in the MELiSSA project, stoichiometries have been validated in optimal and several limitation conditions. A specific photobioreactor has been built to support investigations in the MELiSSA pilot plant, and a predictive model has been validated during continuous experiments.
The higher plant compartment (HPC: 4B):
The activities on this compartment have been initiated with eight crops: wheat, tomato, potato, soybean, rice, spinach, onion and lettuce. Simulations of this HPC require a description of biomass production rates as well as their mineral and proximate compositions. An important number of investigations has been initiated with the University of Guelph to obtain these values, first from open-field data, then in greenhouses.
Environmental considerations (light, nutrients, vapour pressure,…) are of course taking into account. A third aspect of the investigations on the HPC is the study of the hardware necessary for higher plants. Studies have been initiated for the light sources and nutrients sensors.
Last update: 22 November 2007