Workpackages & deliverables

Collection of different substrates available in central Macedonia and their analysis in terms of their nitrogen content. Combined digestion of these substrates in batch anaerobic reactors. The combination will simulate real conditions based on their seasonal variation and based on their availability in the area, the ammonia level will be measured during anaerobic digestion and ammonia toxicity control will be performed. At the same time, samples will be taken from Lagada biogas plant and ammonia toxicity will be tested in batch anaerobic reactors. Its impact on the reduction of biogas production will be calculated.

The WP aims to cultivate mesophilic methanogenic consortia that will then be used as bioaugmentation vaccines. In samples collected from the Lagada biogas plant, the ammonia level will be gradually increased (to the ammonia levels estimated to exist under real conditions in the WP 1) using NH4Cl in batch anaerobic reactors. The goal is to create mixed microbial populations that can be tolerant to high concentrations of ammonia without inhibiting methanogenesis. Parameters such as methane production, VFA concentration and maximum growth rate of microorganisms will be taken into consideration for the optimal selection of the most suitable microbial consortium. At the end of each batch experiment, samples will be collected to characterize the microbial population with 16S sequencing. Comparison of microbial profiles from reactors that have undergone a gradual increase in ammonia levels will lead to useful conclusions about the effect of ammonia on microorganisms (eg, comorbidity between specific bacteria and hydrotrophic methanogens).

The microbial population acclimatized to high levels of ammonia will be used as a vaccine in CSTR reactors. Procedure to be followed: The reactors will initially be fed with waste from dairies, ammonia will be gradually increased using NH4Cl to the ammonia levels estimated to exist under real conditions in the WP 1, at this point the reduction in production of methane will approach the real situation. At this point, bioaugmentation will begin with the inoculation produced in the WP 2. The effect of the following operating parameters on the bioaugmentation process will be evaluated: a) the rate of organic charge (2-6 g VS / L reactor / day), b) the HRT (20-30 days, c) free ammonia, d) pH e) the level of “critical biomass” required (no. cells / mL reactor) to achieve bioaugmentation; and f) the combination of these parameters.

The aim is to find the required “critical biomass”, and the correlation of operating parameters that will allow a stable and efficient production of biogas at high levels of ammonia after the process of bioaugmentation. Samples will be collected under steady conditions to compare the microbial population with the population in the original inoculation. Population change under different reactor operating conditions will give us information on selecting the strongest microbial population that can be used in the WP5.

The whole system will be optimized by controlling the operating parameters (ammonia concentrations, hydraulic retention time, organic load rate, etc.) for anaerobic methane production and development of a predictive controller and evaluation of the controller performance in real conditions. The neural network will simulate process behavior and the neural predictive controller will be installed on a platform to perform real-time control. The sensors, actuators and subsystems of the predictive controller will be selected and installed in the pilot unit (WP5). The control and adjustment software will be installed in the bioreactor pilot control system and will be extensively tested for good operation and performance.

A prototype 800 L continuous stirred tank reactor will be designed and built at the biogas plant in Lagada which will operate with real substrates. The bioaugmentation method will be carried out in the reactor using microbial population vaccine that will be cultured in batch anaerobic reactors. This population will have been selected according to the procedure described in WP3. The purpose of the WP is to verify the possibility of bioaugmentation on a larger scale reactor and to identify possible technical problems during its pilot operation. Later, the prototype reactor will be transferred to the unit of Bioenergy Nigrita SA. where different substrates will be used to ensure the reliability of the system under different conditions (different substrates and different operating conditions).

This WP will examine the further utilization of the residue resulting from anaerobic digestion with increased concentration of ammonia nitrogen, for biomass production, to reduce the environmental impact of the overall unit. For this purpose, samples from the effluent of the anaerobic system will be collected and will be fully characterized in terms of their content of ammonia nitrogen but also in terms of their concentration in organic charge and nutrients. These samples will then be fed to a photovoltaic reactor system for the development of photosynthetic organisms (microalgae). The photoreactor system is already available from the partner TETRO and consists of a 10 L reactor with a submersible light source consisting of low power LEDs. The system is equipped with all the necessary pumps for the continuous supply of the inlet, sensors for the regulation of temperature and pH, but also a device for the supply of a gas inside the reactor in desired concentrations. In this system, the growth rate of microalgae will be studied as a function of the rate of consumption of ammonia nitrogen under the influence of different charging conditions, retention time, temperature, and wavelength of the light source. In addition, the operation of the reactor, under two different conditions, will be investigated: continuous supply, batch supply. The growth rate of the microalgae will be measured based on the optical density, while at the same time the possibility of increasing the removal rate of ammonia nitrogen by supplying carbon dioxide gas to the reactor contents will be investigated to accelerate the growth of the microalgae.

The microalgae in the reactor contents will be tested for biogas production potential, with the aim of increasing the methane produced by the unit. The study of anaerobic digestion of microalgae will be carried out either with the microalgae from the reactor, or in combination with the initial feeding of the full-scale anaerobic digester in algae ratio: initial feeding ranging from 100: 0 to 50:50. In this way it is expected that a system of zero impact will be developed with full utilization of all currents and their respective energy content.

Workpackage 7 (WP7) includes the assessment of the economic and environmental viability of the full-scale application of the proposed anaerobic digestion performance improvement technologies for the substrates under two case studies (Nigrita Unit, Lagada Unit).

Specifically, to determine the environmental impact, a Life Cycle Analysis (LCA) will be performed, which will be based on the collection and detailed analysis of input and output data per step of the process, including the stages of process improvement by limiting ammonia in energy, mass balances, chemical consumption and the environmental impact associated with them. The specialized EASETECΗ software will be used for the implementation of the LCA, through which it is possible to develop complex waste management processes, while at the same time alternative LCA tools will be used. Ecoinvent 3.3 will be used as the software database.

For the assessment of financial viability, evaluation of two cases will be carried out in financial terms considering the results of the operation of the pilot unit in each case (Nigrita Unit, Lagada Unit). In this context, the cost of investment, the cost of waste treatment and energy production, and the economic benefits of the produced energy will be determined, in order to evaluate the proposed improvements and economic prospects through the application of the evaluation criteria of Net Present Value and Internal Degree of Performance.