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Abstract
Sludge Reed beds have been used for dewatering and mineralisation of sludge in Denmark since 1988. The effect on the environment of the establishment and operation of Sludge Treatment in Reed beds system is seen as quite limited compared to alternative sludge treatment systems such as mechanical drying and dewatering, with its accompanying use of chemicals; incineration; direct deposition on landfill sites, etc. Experience has shown that the quality of the final product with respect to pathogen removal and mineralisation of hazardous organic compounds after treatment make it possible to recycle the biosolids to agriculture as an Enhanced Treated Product.

Raw sludge contained large amounts of pathogenic bacteria including Salmonella, Enterococci and E. Coli. Analysis of the levels of pathogens in the sludge residue 3-4 months after the last loading indicated that the pathogen content was reduced by a factor of log 6-7.

Linear alkylbenzene sulphonates (LAS) and nonylphenolethoxylates (NPE) may be detrimental to the environment if spread in large concentrations. Mineralisation of LAS and NPE in mesophilic digested sludge was observed during a 9 month monitoring programme where three separate treatment methods were investigated. Treatment in a sludge reed bed was shown to be far more effective than in containers and sludge piles with mineralisation levels of 98% for LAS and 93% NPE being observed.

Keywords
Biosolids, Emptying, Enhanced treated product, Hazardous organic compounds, mineralisation, Pathogen removal, Quality of sludge residue, Reed beds, Sludge dewatering, Sludge drying.

Introduction
Sludge Reed Beds (Figure 1) have been used for dewatering (draining and evapotranspiration) and mineralisation of sludge in Denmark and other countries in Europe since 1986/1988 when the first sludge processing systems were introduced. Sludge from wastewater treatment plants (2,500-125,000 pe) is treated in sludge reed bed systems with 1-18 reed bed basins with loading rates of 25-2,200 tonnes dry solids (TDS) per annum for 10 years. In 2004, approximately 110 systems were in operation in Denmark [1,6].

Long-term sludge reduction takes place in reed-planted basins, partly due to dewatering (draining, evapotranspiration) and partly due to mineralisation of the organic solids in the sludge. Sludge from wastewater treatment plants sludge is pumped onto the basin surface. The dewatering phase results in the dry solids content of the sludge remaining on the basin surface as sludge residue, whereas the majority of its water content continues to flow vertically through the sludge residue. The water content is further reduced through evapotranspiration.

In addition to dewatering, the organic solids in the sludge are mineralised, thereby minimising the sludge volume. Oxygen diffusion via filter aeration and through the cracked sludge surface and oxygen diffusion from the roots into the sludge residue enable aerobic micro-organisms to exist close to the roots and in the sludge residue (Figure 1). The overall reduction of the sludge volume occurs without the use of chemicals. The process involves only a very low level of energy consumption for pumping the sludge and reject water (Figure 2). Experience from reference plants is that this type of system is capable of treating many types of sludge having a dry solids content in the range of approximately 0.5-5% [1,6].

The sizing and design of reed bed systems depends on the sludge production (TDS per annum), sludge type, quality and regional climate. A treatment period of 8 - 12 years means that the depth of the basin above the filter needs to be at least 1.80m. The operation of the system may be divided into a number of phases related to different periods in the lifetime of a system. A system generally runs for a total of at least 30 years: this period being divided into two to three phases each lasting around 8-12 years. Each phase consists of commissioning, full operation, emptying and re-establishment of the system.

In general, the sludge type is surplus activated sludge and surplus activated sludge mixed with anaerobically digested (mesophilic) sludge. The sludge loading amounts to a maximum of 50 - 60 and 40 - 50 kg DS/m²/year, respectively. Loading cycles are related to the sludge type and the age of the sludge. The sludge residue will, after approximately 10 years of operation, reach an approximate height of 1.20-1.50 meters with an approximate dry solids content of 30 - 40% [1,6].

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