Feasibility study for a demonstration plant for the sustainable and economic conversion of liquid manure into specific plant additives in the circular economy – Project Innovative Liquid Manure Management (PIGM)

Challenge

Manure from animal husbandry is a valuable fertilizer. It supplies agricultural soils with organic matter and important nutrients, helping to meet the nutrient needs of plants and maintain soil fertility. On the other hand, liquid manure, just like digestate, is a non-specific fertilizer: the phosphates and nitrogen compounds needed by plants are present in undefined proportions and different concentrations. Thus, if nitrogen is needed, liquid manure fertilization also applies phosphate, which is not absorbed by the plant, but rather pollutes soil, groundwater and surface water – and vice versa.

In addition, the intensification of livestock farming and the regional concentration of farms are resulting in significant amounts of surplus manure that cannot be efficiently used as a resource in the producing regions alone. In these regions, manure is becoming a waste stream that needs to be treated or disposed of, thereby consuming energy, e.g. for transport and disposal.

At the same time, across the EU, annual excretion of nitrogen and phosphorus from livestock is sufficient to meet the EU's needs for mineral fertilizers. Overuse of synthetic fertilizers and corresponding crop yields obscure the fact that soil organic matter is being lost and not replaced. This results in a loss of soil fertility, biodiversity, lower water retention capacity and disruption of natural nutrient cycles.

Objectives: To determine the feasibility of an industrial demonstration plant for the valorization of manure through specific processing of its constituents

The aim of PIGM was to achieve a continuous, sustainable and economical conversion of liquid manure into specific plant additives, such as natural fertilizers. To this end, the resource- and energy-efficient BioEcoSIM process, which was successfully developed at Fraunhofer IGB on a laboratory scale and tested on a pilot scale in follow-up projects, was to be transferred to an industrial demonstration plant. To this end, PreZero planned a full-scale single-purpose demonstration plant for the complete energetic and material processing of pig manure and other agricultural residues into natural plant additives as part of a feasibility study. Fraunhofer IGB provided scientific support for the study.

In PIGM, a biogas plant was installed upstream of the BioEcoSIM process in order to metabolize disruptive accompanying substances from the raw manure, increase the energy yield and thus cover the energy requirements of the plant, and increase the economic efficiency of the overall project by selling surplus biomethane gas and electricity (CHP).

The fermentation residue from the biogas plant is broken down into individual plant supplement fractions using the BioEcoSIM process:

• Ammonium sulphate and struvite for use as a fertilizer for plants
• The soil conditioner consisting of the solid components for humus formation
• The aqueous residual fraction of the liquid manure as irrigation water for soil irrigation

Fraunhofer IGB provided the technical specifications (requirements specification) for the fermentation residue treatment processes (natural fertilizer plant). These processes were as follows: acidification of the fermentation residue, solid-liquid separation, ultrafiltration, phosphorus recovery, and nitrogen recovery. The process chain is shown in the following diagram:

In close cooperation with PreZero, work was also carried out to determine the optimal substrates for the demonstration plant. This included analyzing the composition of the substrates and simulating their conversion. In the first phase, the following substrates were investigated: pig manure, dry chicken manure, and husks from a grain mill.

Simulations of material conversion and economic efficiency showed that operating the plant with these substrates would not be economically viable under the given conditions at the Aschersleben site. The substrate mix was therefore adjusted.

Horse manure and beet pulp from a sugar factory were added to the substrates already investigated. At the same time, it was decided that the mixture should consist of one third pig manure, another third dry chicken manure and horse manure, and one third beet pulp and cereal husks. These substrates were used to carry out shredding and mixing tests in order to find out how appropriate mixtures could be produced for further processing and to obtain a starting material for further investigations. An important factor that emerged was the mashing of the husks with water in order to achieve swelling of the husks before mixing.

The existing Excel-based stationary simulator was optimized. Up to six feed substrates were taken into account in order to consider complex mixtures. The biogas yield is calculated for each substrate. The composition of the fermentation residues is balanced depending on the defined mixed substrate. The Excel simulator was implemented in the Matlab/Simulink simulation environment in order to model the dynamic behavior of the PIGM plant more precisely with a time step of one hour.

Outlook

Even with the new substrate mix, the analysis with the simulator showed that, based on current laws and regulations on the application of farm manure and the still low costs of artificial fertilizers based on fossil raw materials, a plant based on the PIGM concept cannot currently be operated economically in Germany. Therefore, the construction of the planned demonstration plant will be postponed for the time being. However, there is still great interest in this technology, so that based on the results of this project, possibilities for implementation will continue to be explored.