Biologo

Technologies/systems for different manure and organic waste treatment options

NITROGEN RECOVERY FROM BIOGAS PLANT DIGESTATES VIA SOLID-LIQUID SEPARATION AND STRIPPING
Anasruron D.F.D, Bade O., Körner I. Hamburg University of Technology, Institute of Environmental Technology and Energy Economics, Bioconversion and Emission Control Group, Harburger Schlossstr. 36; 21079 Hamburg, Germany E-mail:devina.anasruron@members.em-a.eu 1 INTRODUCTION

The installation of anaerobic digestion plants has progressed extensively in Germany. Anaerobic digestion has the benefit of biogas but also produces residues (referred to as digestate). The amount of digestate generated is parallel with the increase of anaerobic digestion treatment for agricultural waste, municipal biowaste or energy crops. Thedigestate has high nitrogen content which is favourably valued for its fertiliser potential. However, approach to apply the digestate directly in agricultural field has adverse impacts, such as odorous effect, nitrous oxide (N2O) emission and nitrate leaching (Dittert et al., 2009; Dorno et al., 2009), due to its high content of ammonia. Another disadvantage is related to its transport to theneeded areas, since anaerobic digestion plants can be located in the city or in agricultural areas with surplus of nitrogen. The transport problem is mostly attributable to the high volume of digestate required. It is reported that transportation costs can be reduced by half by increasing its solid content as much as 1.5% to 5% (Noone, 1990). As transport costs continue to rise, reprocessing digestateto a more concentrated or solid form is inevitable. An alternative of digestate treatments for nitrogen removal and recovery is stripping, thus transforming the digestate into a suitable form of fertiliser and transportable product. Stripping method also is preferred due to the availability of heat in the process. Since the limiting factor for stripping is the energy requirement for hightemperature process (Bonmati and Flotats, 2003), the heat from the biogas engine can be further utilised and therefore makes stripping as the best proposed treatment. The focus of the study is the development of a treatment of digestate that can avoid the adverse environmental impacts, minimise the loss of nitrogen contents and increase the solid contents in its end-product, thus making it technically andeconomically feasible as fertiliser.

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MATERIALS AND METHODS

FIGURE 1

Experimental set up of stripping unit

Technologies/systems for different manure and organic waste treatment options

Experiments were performed with digestate from a biogas plant treating grass mixed with cow manure as the feedstock. Digestate was preserved in the refrigerator at 4-5°C prior to the treatment.The treatment of digestate consists of solid-liquid separation and stripping. The solid-liquid separations of the digestate were performed with filter bag and manual pressing. The filter bag is a standard polypropylene material of 195 micron rating. Stripping was performed for solid and liquid digestate after solid-liquid separation. Stripping with diffuse aeration was chosen due to difficulty infurther separation of the liquid part of digestate. The stripping retention times for liquid and solid digestate were 6 hours and 14 days, respectively. Ammonia was recovered in sulphuric acid solution (0.5 M) with volume of 200 mL. Stripping were performed with the variations of aeration flow rate (50, 150 and 300 L/h), digestate temperature (20 and 35°C) and digestate pH (pH 8 and 12). Samplingand analysis of sulphuric acid solution were performed after stripping. Sampling was performed hourly for liquid digestate and daily for solid digestate. The analyses that were performed on raw digestate are pH, NH4-N, and Total Nitrogen/ Total Kjeldahl Nitrogen (TN/TKN) analysis. NH4-N analysis was performed on post-treatment digestate and the absorbent solution, sulphuric acid. In addition,...