Vacuum Evaporation Of Wastewater As A Mean For Nutrient Recovery And Waste Heat Utilization In Biogas Plants

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Following significant financial incentives in European countries, biogas plants became a widespread technology for biomass conversion and electricity production. In 2015 more than 17 thousand plants were running in EU with total installed capacity over 8.3 GW,el. Despite undisputed advantages of this technology, there are still several issues that should be solved. One of them is an inefficient use of heat that is produced during the cogeneration process. Typically, only 20-40 % of the heat produced is utilized. A shortage of suitable technologies and low-potential character of the waste heat (around 90 °C) are the main reasons of this situation. Another issue is connected with wastewater (so-called digestate or fermentation residue) treatment and utilization. Digestate is a by-product of anaerobic fermentation of biomass and is especially rich in nutrients such as nitrogen, potassium or phosphorus. Is therefore mainly used as a natural fertilizer. However, nutrients concentration is quite low and water content in digestate ranges from 88 to 98 %. Since the digestate production is significant, typically around 15,000 m3 per MW,el of installed capacity, plant operators have to deal with high costs for storage and transport of this wastewater. Furthermore, together with agricultural land scarcity, high treatment costs can be reasons for nitrogen overload and groundwater pollution in certain areas. Thickening (dewatering) of digestate could not only save fossil fuels consumption for transportation purposes, but also for synthetic fertilizer production. Concentrated digestate is cheaper to transport to remote locations and may be used as a substitute for artificial fertilizers. The presented study focuses on solving both -- waste heat utilization and digestate thickening -- by using vacuum evaporation as the main treatment technology. Vacuum evaporators are robust and reliable devices using low-potential thermal energy as the main energy source. Several authors have already reported successful operation in a lab-scale environment. However, there are still certain problems connected to digestate evaporation such as 1. intensive foaming, 2. non-newtonian character and high viscosity, 3. ammonia volatility and distillate contamination. To answer this issue, a unique prototype of multi-stage vacuum evaporator was constructed and series of experiments were carried out. The prototype is currently producing distillate in the volume of 5 to 10 kg/h. The produced water is free of dry matter, salts, phosphorus, and potassium. On the other hand, a significant concentration of ammonia is still present (up to 30 g/l). Hence, three auxiliary processes and their combination were subjected to testing as well: a system for ammonia expansion and absorption, pH adjustment and ammonia stripping. This paper will further discuss current results and difficulties connected with recent experimental activity.

nutrient recovery; biogas plant; anaerobic digestion; vacuum evaporation; ammonia; waste heat

VONDRA, M.; MÁŠA, V.; TOUŠ, M.; KONEČNÁ, E.

26. 6. 2018

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