(562aj) Treating Liquid Effluents from Anaerobic Digestion: Semi-Empirical Modelling of a Multi-Stage Flash Evaporator

Anaerobic digestion is a widespread method for the treatment of waste biomass and energy crops. In 2016, almost 18,000 biogas plants with a total installed capacity exceeding 9 GW were operating in Europe. The main product of anaerobic digestion is biogas, which is subsequently combusted for the generation of electricity. A liquid by-product of anaerobic digestion is the so-called digestate, whose annual output ranges between 10,000 and 30,000 tons per MW of installed capacity. Digestate is an organic fertilizer, but with a very low nutrient concentration (N, P, K). Up to 95% of the digestate volume is represented by water. Storage, transportation, and application of this fertilizer are very costly. This is particularly true in areas with intensive agricultural production (e.g. North-western Europe), where there is a long-term problem with the accumulation of fertilizers (especially nitrogen) in soil and groundwater. Consequently, there are many legislative constraints and operators are forced to export digestate to distant areas. Transport distance may even exceed 100 km in some areas.

Vacuum evaporation is one of the technologies for digestate volume reduction. The evaporators allow separation of water from digestate and reduce its volume several times. The advantage of vacuum evaporation is its reliability and the ability to use low-potential waste heat (<90 ° C), which is generally in excess in biogas plants. Integrating thickening technology into a biogas plant assumes a reasonable return on investment. The material and energy balance of the technology is the key information for a techno-economic evaluation. However, digestate evaporation modelling is complicated for many reasons. The main problems in this regard include the complicated properties of the digestate (e.g. non-newtonian character, variable composition, high viscosity), the complex nature of the evaporation process (phase conversion, foaming, release of dissolved gases), and lack of verified operational data in general.

This paper presents the current state of a model, which aims to reliably predict the mass and energy balance of a multi-stage flash evaporator when concentrating digestate from anaerobic fermentation. The model is based on semi-empirical relationships derived from a series of experiments on a pilot plant evaporator. The acquired knowledge is partly transferable to other types of evaporation devices. The most important modelled parameters include boiling point elevation and overall heat transfer coefficient in the process of digestate evaporation and heating respectively. The model also considers input parameters such as acid dosage and anti-foaming agent addition.