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Influence of different parameters on membrane flux and nutrient retention of digester effluent filtrate in a single-shaft-disk-filter
Einfluss unterschiedlicher Parameter auf den Membranfluss und den Nährstoffrückhalt von Gärrestfiltrat in einem Rotationsscheibenfilter
Digestion of renewable raw materials with other by-products from agriculture (e. g. manure) is a standard technology in practice. Due to legislative regulations, laws or lack of space it is sometimes necessary to process the digester effluent instead of storing it onsite. After a mechanical separation process to eliminate solids from digester effluent the remains are well usable for agricultural recycling on fields. These consist mainly of not separated solids and soluble and non-soluble nutrients which can be utilised as fertilizers on agricultural fields. Ultrafiltration is a further processing step to reduce solids content to a minimum and to concentrate the remaining nutrients. First applications of ultrafiltration of digester effluents were realised with tubular or hollow fibre membranes. While that process is established with a high pressure pump on the feed side this process is realised by vacuum issued on the permeate side. One major drawback of the first process is thehigh energy demand for pumping, whereas the second one is sensitive to pore blocking due to solids content as well as concentration polarisation due to slow fluid flow on feed side. Rotation of membranes seems to be a possible solution to these problems. It can be realised with a rotating single-shaft-disk-filter. Two different effluents from anaerobic treat ment of renewable raw materials were mechanically dewatered by two separation techniques. The remaining liquid from this process was used as a feed for the ultrafiltration step. Due to the special properties of the digester effluents a pilot scale single-shaft-disk-filter was used. The ultrafiltration unit was operated in batch mode. In this paper we present results on the influence of transmembrane pressure (TMP), rotational speed of disks and dry matter content as well as viscosity on permeate flux. The experiments show no influence of TMP on permeate flux whereas the flux decreases with an increase in non-newtonian viscosityand dry matter content respectively. A linear correlation between permeate flux and rotational speed of disks was found. Retention of all nutrient components was measured and correlations with pore diameter are presented. Due to the nature of the substrate used fouling of membranes is a major drawback for the economical use of the process. Post mortem analysis of membranes by SEM revealed the governing mechanism of fouling.
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