Sludge treatment reed beds (STRBs) are an alternative low energy, lower cost solution to managing sludge compared to traditional mechanical dewatering processes such as belt presses and centrifuges.
Sludge treatment reed beds comprise a series of gravel/sand basins that are planted with wetland plants/reeds. Many types of sludge can be treated by this technology including digested or undigested activated sludge, and water treatment works sludge. Experience by our partners Orbicon has shown that sludge reed beds are capable of treating most types of sludge with a dry solid content of the inlet sludge up to a concentration between 0.1 and 5 percent.
The operation involves cycling through a number of reed beds basins (e.g. 8–12), with each basin loaded several times per day over a one to two week period, followed by five to ten weeks resting when no sludge is added. The length of the loading and rest periods depends on the sludge characteristics, climate, and the age of the system/basin, the dry solid content and the thickness of the sludge residue. The key to successful treatment is monitoring these parameters and modifying the process to ensure correct operation. Our automated system continually updates and makes the necessary changes, ensuring trouble free operations.
The sludge residue remains on the surface of the basins and mineralises through the natural biophysical interaction of plants, microbes and air, whilst water is removed from the sludge by both evapotranspiration and by drainage through the gravel/sand substrate.
As the organic matter mineralises, overall volume reduces, and it becomes part of a soil layer in which the reeds grow. The mineralised soil layer deepens with time, with the accumulation rate dependent on the solid characteristics of the sludge. Every eight to twelve years the biomass is removed, to be reused offsite.
Sludge accumulation and emptying
Mineralised sludge typically accumulates within the system at a rate of about 10–15 cm per year. Once 1.5m of material has been deposited in a basin, it is removed and can be disposed to agriculture or elsewhere as a stabilised organic material. When a basin is emptied, the bottom 10–30cm of biomass is left in place with rhizomes. This avoids the need to replant, as rhizomes will reshoot.
Reed bed life cycle stages – establishment; operation, and biosolid disposal – are shown below. Final Biosolids have a dry solid content of 25–40%.
Although sludge treatment reed beds are simple in operation, their success is dependent on managing loading cycles so that basins are not overloaded. This can only be achieved by accurately monitoring the hydraulic characteristics throughout the loading and drying cycle, and ensuring that systems are actively managed to reflect the dynamic conditions of each basin. For example, winter cycles will differ from summer, and older basins with deeper biomass deposits will differ from new basins.
Sludge ‘capacity’ of a basin for a single load cycle is determined by measuring how well the basin can be loaded whilst maintaining suitable drainage. This is because drainage characteristics are gradually reduced during a loading cycle. When drainage efficiency reduces to a certain point, sludge loading is moved to another basin, allowing the loaded basin to rest and recover before another loading cycle begins.
Managing loading cycles of sludge treatment reed beds is done through a customised control/SCADA system that comprises an operation module and a data collection module. The operation module displays the status of the system, including flows in and out. This control system continually makes adjustments as required, and needs little involvement from local operators.
The data collection module monitors and records data from the sludge loading in individual basins. Sludge flow and dry matter content are also recorded before the sludge is directed into the basins for dewatering.
The sludge is distributed to basins with free capacity according to a loading plan. In this way, dewatering is optimised, thereby securing a high dry matter percentage, and prolonging the life of the sludge reed bed system. The loading rate of the system and basin level is computed on an ongoing basis.
This data is used to monitor the system’s function and dewatering capacity and provide a basis for future loading plans.
Effective reduction of sludge residue
Sludge with solids content of 0.5–3% processed by the Sludge treatment reed bed system typically has a dry solids content of 30–40% after the dewatering and mineralisation process has been completed. In addition, mineralisation removes up to 25% of the organic matter in sludge.
No chemicals needed
A sludge treatment reed bed system uses no chemicals to treat sludge. This removes the workplace health and safety risks associated with use of chemicals, as well as reducing chemical residue in the treated biosolids that pass into the environment.
A sludge treatment reed bed system uses passive physical and biological processes to reduce and treat sludge. The only power consumption is pumping the sludge and the return filtrate water back to the treatment plant. This is a significant reduction in energy compared to mechanical dewatering systems, such as belt presses or centrifuges.
No odour problems
As the sludge treatment reed beds use an aerobic process no odour problems are created.
Better CO2 balance
Sludge treatment reed bed systems have a far lower carbon footprint than other dewatering methods, especially anaerobic lagoons.
Improved sludge quality
Biosolids produced by the sludge treatment reed beds are not only dewatered, but also treated, which dramatically reduces the pathogens and vector attraction.
Options for beneficial reuse
After treatment, beneficial reuse options for biosolids include application as a fertiliser on agricultural land. The quality of the biosolids is better suited for use as a natural fertiliser than that produced by a mechanically dewatered process.
Reduction of transport and spreading costs
The cost of transportation and land application of the treated biosolids is fifty percent lower than a comparable mechanical dewatering system, since the final product is reduced to approximately two thirds of its original volume.
High quality filtrate Increases plant capacity
Another significant advantage of sludge treatment reed beds is their ability to provide wastewater treatment facilities with a much higher quality of filtrate water than mainstream mechanical dewatering technologies.
The filtrate water from the sludge reed bed is treated as it percolates through the mineralised sludge layers and gravel, removing much of the organic matter present. Consequently, clean filtrate water is recirculated from sludge treatment reed beds back to the plant, reducing process loading within the main treatment plant, and increasing infrastructure capacity.
Existing treatment plants that have switched to sludge treatment reed beds have typically experienced a capacity increase of 5–15 percent.