Nutrient recovery from digestates of agricultural biogas plants - an adaptation of wastewater treatment processes
In terms of a sustainable resource and energy economy it is to state that the potential of organic material or biomass prevails those of other renewable energies.
With the aim of an economic operation management of anaerobic digestion plants using a wet fermentation procedure the handling of digestates come more and more to the fore. Caused in a limitation of agricultural land and an increasing biogas plant density the question came up to what extent the accumulated amount can be brought to nearby areas. By the maximum charging of nutrients partly long-distance transportations and extensive storage conditions are necessary which raise the expense factor of the output. This gains reinforcement by using strongly nitrogenous substrates in the fermentation process itself. Because of the high content of liquids within the digestate a long-term safe application can be only managed by a volume reduction and the implicit cost reduction (transportation). Besides these decreased application costs the following benefits can be stated for treating the digestate:
• improved fertilizing effects,
• concentration of nutrients particularly phosphate and
• better storage conditions and decreased final storage capacities respectively.
On the other hand there are increased investment and operating costs. At the current time for a 500 kW-plant it seems to be not an efficient way to install such sophisticated systems like reverse osmosis or ultrafiltration. Thus the presented project dealing with possibilities for an adaptation of established wastewater treatment technologies. Primarily this includes a simplification of the high-engineered and complicated procedures, namely regarding:
• solid-liquid separation,
• nutrient recovery in the liquid phase (enrichment of N, P, and K as well as organic solids) and
• effluent treatment (clarification for recirculation or disposal).
Central component of the project is the part of nutrient recovery especially phosphorus and nitrogen. First investigated experiments concerning struvite as well as calcium phosphate precipitation in laboratory scale as low-cost variants show the general feasibility. Phosphate removal rates between 35 % and 88 % could be achieved. The addition of polymers has a positive effect but they have to bring into solution first and this causes a high increase of the liquid phase. A detected problem for general handling (with view on practical realisation) and the plant efficiency is the adjustment of high pH values. So a huge amount of alkaline solution had to be added to set the prerequisite for the process:
• sodium hydroxide for struvite precipitation and
• lime slurry for calcium phosphate precipitation.
One cause is seen in different buffer systems within the digestate. A carbonate buffer system can be disabled only by calcium carbonate precipitation and seems to be not applicable. In contrary an upstream ammonia stripping stage should lower the NH4 +/NH3 buffer system, reduce the consumption of alkaline solution and represents therefore a possibility for optimisation.
Hence the next project step will be ammonia stripping followed by phosphate precipitation. It is expected to achieve consolidated expertises regarding process suitability, effluent parameters and efficiency as well as further necessary process stages. Finally estimations regarding cost-effectiveness and generalisation will take place what meets the current demand on small biogas plants.
