There are important differences in the environmental perspectives and costs involved in mechanical sludge dewatering followed by disposal on agricultural land compared to reed bed sludge treatment. The background for the evaluation of their influence on the environment is to a large extent the experience gained since 1988 from the operation of the large number of reed beds systems currently operating in Denmark and Europe. The effect on the environment of the establishment and operation of a Sludge Treatment in Reed Beds system (STRB) is seen as quite limited compared to traditional sludge treatment systems such as mechanical dewatering and drying, with their accompanying use of chemicals; incineration; direct deposition on landfill sites, etc. After reduction, dewatering, and mineralisation in a reed bed sludge treatment system, sludge with a solids content of 0.5-3% can attain dry solids content of up to 20-40%. In addition, mineralisation removes up to 25% of the organic matter in sludge. Experience has shown that the quality of the final product in sludge reed beds with respect to pathogen removal and mineralisation of hazardous organic compounds after treatment make it possible to recycle the biosolids to agriculture.
Keywords: Sludge Dewatering, Loading, Reed Beds, Emptying, Sludge quality, environmental impact.
Loading – Operational Strategy
The operation of a reed bed system may be divided into a number of periods relating to the lifetime of the system. A system generally runs for a total of at least 30 years; this period is divided into two or three 8-12 year phases. Each phase consists of commissioning, normal loading, emptying and re-establishment of the system. Full Operation following the commissioning of the plant operations means that the yearly loading is increased to the sludge production from the wastewater treatment plant corresponding to the maximum capacity (tons dry solid/year) of the sludge reed bed system. The loading strategy involves assigning an individual quota to each individual basin. This quota is a sludge volume which generally increases throughout the entire period of operation until emptying, but it may also vary or even decrease to zero for periods. The length of the loading periods and rest periods between loadings depends on the age of the system/basin, the dry solid content, the thickness of the sludge residue and the intensity of partial loadings during the period of loading. On a daily basis, the basins are subjected to a loading of 1-3 partial loadings of approx. 1 hour for a short period (from a few days to a maximum of 2 weeks during commissioning) until the quota is used and loading switches to the next basin.
Mechanical sludge dewatering involves conditioning with chemicals, usually in connection with the dewatering process itself. Either organic polyelectrolytes or inorganic conditioning substances are used (Table 1). In a sludge reed bed system the dewatering process is governed by the sludge quality, the climate, the wind, the gravity and the vegetation. The water in sludge with a dry solid content of 5% can be divided into pore water (66.7%), capillary water (25%), adsorption water and structurally bound water (8.3%). Dewatering the pore water concentrates the sludge to a dry solid content of about 15%. Further dewatering by removal of the capillary water concentrates the sludge up to a dry solid content of about 50%. The remainder of the water in the sludge may be removed by drying. Reed beds have been used for sludge reduction in Denmark and Europe since 1988 when the first sludge processing system was introduced. Long-term sludge reduction takes place in reed-planted basins, partly due to dewatering (draining, evapotranspiration) and partly due to mineralisation of the organic matter in the sludge. From waste-water treatment plants the sludge is pumped onto the basin surface/sludge residue. The dewatering phase thus results in the dry solid 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 and filter layer. The sludge residue water content is further reduced through evapotranspiration. In addition to dewatering, the organic matter in the sludge is mineralised, thereby minimising the sludge volume. The overall sludge volume reduction occurs without the use of chemicals and involves only a very low level of energy consumption for pumping sludge and reject water. Experience from the reference plants is that this type of system is capable of treating many types of sludge with a dry solid of approx. 0.5 to approx. 3-5%.