Helsinge sludge reed bed system: reduction of pathogenic microorganisms

Reduction of pathogenic bacteria: Salmonella, enterococci and Escherichia coli was investigated using the sludge reed bed system. The system at Helsinge was established in 1996 and has a capacity of 630 tonnes dry solids per year and consists of 10 basins. Since 2000 the individual basins have been subjected to an average area-specific loading rate of 46–56 kg dry solids/m²/year. The total sludge residue height in April 2006 was approximately 1.40 m. The sludge (approximately 0.5–0.8% dry solids), with which the individual basins were loaded, contained a large number of bacteria. Salmonella, enterococci and E. coli were found in the sludge in the following quantities: 10–300/100 g (wet weight), 7,000–25,000 CFU/g (wet weight) and 800,000–10,000,000 CFU/100 g (wet weight), respectively. The analysis of the reduction in pathogens in the sludge residue through a period of 3–4 months after the last loading indicated that the pathogen content was reduced down to <2/100 g (Salmonella), <10 CFU/g (enterococci) and <200 number/100 g (E. coli). For enterococci and E. coli the reduction was approximately log 5 and log 6–7, respectively. In the same period the sludge residue achieved a dry solids content of approximately 20–35%.