Proceedings

Sludge Reed beds have been used for dewatering and mineralisation of sludge in Denmark since 1988. Experience has shown that the quality of the final product with respect to pathogen removal after treatment make it possible to recycle the biosolids to agriculture as an Enhanced treated product. In June 2004 the reduction of pathogenic bacteria: Salmonella, Enterococci and E. Coli was investigated using the Sludge Reed Bed System at Helsinge, which was established and planted with reeds in October 1996. The sludge system has a capacity of 630 tonnes dry solids per year and consists of 10 reed bed modules, or basins, each having an area of 1,050 m2 at the filter surface. Sludge production consists of activated sludge from final settling tanks and constitutes approximately 50% of the loading of the sludge reed bed system. The remaining 50% of the sludge production consists of concentrated activated sludge from four smaller wastewater treatment plants. The two sludge types are mixed before being added to the sludge reed bed system. Since 1998 the individual basins were subjected to an average loading rate of approximately 55 tonnes dry solids per annum after commissioning, resulting in an average area-specific loading rate of 52.4kg dry solids/m2 /year. The total sludge residue height in June 2004 was approximately 1.10 m.

As a general rule, pathogenic bacteria, which are excreted and end in an alien environment only live for a short period of time, depending upon various environmental factors and the bacteria’s own characteristics. The sludge (approximately 0.5-0.8% DS), with which the individual bed modules were loaded, contained a large number of bacteria. Salmonella, Enterococci and E. Coli were found in the sludge in the following quantities: 10-300 per 100g (wet weight), 7,000 – 25,0000 CFU/g (wet weight) and 800,000 – 10,000,000 CFU/100g (wet weight), respectively. Analysis of the reduction in pathogens in the sludge residue (having a dry solids content of approximately 20–30%) through a period of 3- 4 months after the last loading from Helsinge Sludge Reed Bed System (module no. 8) indicated that the pathogen content was reduced down to <2 per 100g (Salmonella),<10 CFU/g (Enterococci) and <200 number/100g (E. Coli). For Enterococci and E.Coli the reduction was approximately log 5 and log 6-7, respectively.