KompaGG-N - Complete Treatment of Manure and Digestate: Process Developement in Consideration of Regional Substance Flow Concepts for Nutrients and Pollutants

The aim of the KompaGG-N research project was to investigate different processes for the treatment of manure and digestate in order to create a basis of knowledge for deriving robust and economical process recommendations. In the joint project consisting of scientific institutions (ISAH, DVGW-EBI) and private companies (BIORESTEC GmbH, E&P Anlagenbau GmbH), the focus lay on the fractionated evaporation and the process of deammonification. Both processes were investigated in a combined process chain and in part as a stand-alone process. In addition to nitrogen elimination and recovery, the influence of solids separation by membrane filtration (MF) and the retention of antibiotics and antibiotic resistance genes (ARG) by the different processes were investigated as part of the evaluation of the process.

The fractionated evaporation (SEQUESTA) by BIORESTEC produces condensates with different  ammonia-content derived from separated digestate or separated liquid manure. Due to the lower vapor pressure of ammonia compared to water, an aqueous ammonia solution with an ammonium-nitrogen content of about 20 - 50 g/l is produced at the beginning of the evaporation process (condensate 1). The ammonium nitrogen content could be increased to up to 240 g/l by rectification. This first condensate can therefore be used for fertilizer production or outside agriculture (e.g. chemical industry, flue gas cleaning). The other condensates are already significantly depleted in nitrogen. In combination with reverse osmosis, the process is suitable for the complete treatment of liquid manure or digestates.

For investigating the deammonification process (based on the TERRMOAX® process from E&P), laboratory and semi-technical tests showed that this technology from the wastewater sector can be transferred to the direct treatment of liquid manure and digestate. Also the deammonification process can well be used for the residue condensate resulting from evaporation. During the substrate screening, no substrate-specific inhibition of ammonium-oxidizing bacteria (AOB) was detected. IN addition an adaptation of AOB to high NH3 contents was observed, so that the high nitrogen content of manure or digestate does not necessarily lead to any problems when adapted operational management and control strategy are in place. In the project, the deammonification was successfully operated a) for the nitrogen reduction of pig manure as a stand-alone module and b) for the post-treatment of the non-usable condensate fractions resulting from the fractional evaporation. Process concepts have been developed and successfully tested for both applications. Due to the chemical oxygen demand (COD) in manure, which is still present in high quantities at time, depending on the degree of separation, stabling and storage, the thechnical concept for the partial treatment of manure provides for three process stages. The design of the nitritation stage as a flush-reactor has proven to be preferable. Room turnover rates of 1.1 kg NH4- N/(m³*d) can be assumed for the design of a partial treatment. Reduction to discharge quality is possible, but not economically feasible due to the required tank volume. For the treatment of the condensate resulting from evaporation, the single-stage process design as a sequencing batch reactor (SBR) is preferable. With the deammonification, an N-elimination of 88 % could be achieved for the condensate. But it is required to buffer the acid capacity in the reactor. The dissolved COD was reduced by an average of 90 % and was constant at around 40 mg/l in the effluent, so that no restrictions in the discharge capacity due to refractory COD are to be expected.

In the investigation of the solids separation, the deammonification showed no influence on the conversion rate due to solids in the feed. Also, there were no advantages of filtration over conventional press screws for the fractionated evaporation. Due to the high energy requirement, membrane filtration is therefore negligible for the overall process chain. However, a reduction in the total solids content of more than 98 %, a phosphate retention of 82 % and a COD reduction of 80 % were achieved in a pilot scale operation using a ceramic microfiltration membrane. With the combination of microfiltration and vacuum evaporation of pig slurry, a reduction in ARG concentrations of 100 to 104 times was observed and more than 99.99 % of ARGs were removed. The use of fractional evaporation and deammonification as individual process stages also demonstrated a reduction in ARGs, albeit on a much smaller scale than membrane filtration. Additional treatment processes are therefore required to further reduce taxonomic ARGs in the end product.

The KompaGG-N project was funded as part of the “KMU-innovativ” funding program of the Federal Ministry of Education and Research.