Planted Drying Beds

All organic degradation processes produce sludge (SASSE & BORDA 1998). Sludges from wastewater collection or treatment units are of variable consistency. Anaerobic sanitation systems (e.g. single pit latrines, septic tanks, biogas settlers, small and large scale anaerobic digesters, anaerobic digesters for organic waste, wastewater stabilisation pond systems) produce less sludge than aerobic treatments (e.g. from trickling filters, activated sludge) and anaerobic sludge also dries better and results in fewer odours as it is more stabilised. Generally, they do not have a higher solids content than 2 to 10 % and can therefore not be transported easily with simple equipment. Apart from this, sludge is contaminated and occupies large volumes for storage (SASSE & BORDA 1998). Therefore it is better to dry or dewater sludge before further use or disposal. Unplanted and Planted Drying beds are one of the simplest and oldest techniques for sludge dewatering (SANIMAS 2005). They are a simple means to reduce the volume of the sludge and prepare reuse as fertiliser.

This technology has the benefit of dewatering and stabilizing the sludge. When plants are present, evaporation is enhanced by transpiration (evapotranspiration) and sludge can be applied more frequently. Also, the roots of the plants create pathways through the thickening sludge that allow water to easily escape. Compared to unplanted drying beds, planted drying beds (also called humification beds), require desludging only once every 5 to 10 years and the removed sludge from is a nutrient-rich soil amendment, that can be directly reused in agriculture.

The appearance of the bed is similar to a Vertical Flow Constructed Wetland. The beds are filled with sand and gravel to support the vegetation. Instead of effluent, sludge is applied to the surface and the filtrate flows down through the subsurface where it is collected in drains. While the solid fraction remains on the filter surface and is dried the liquid percolates. The plants enhance evaporation by transpiration. This process is called evapotranspiration. The leachate collected in the drains may requires further treatment depending on the age and quality of the applied sludge.

In principle the construction of unplanted drying beds is similar to gravel/sand filters. The bed frame is usually made from concrete or a plastic liner with the bottom surfaces slightly sloped in order to facilitate percolation and drainage. Ventilation pipes connected to the drainage system contribute to aerobic conditions in the filter. A general design for layering the bed is: (1) 250 mm of coarse gravel (grain diameter of 20 mm); (2) 250 mm of fine gravel (grain diameter of 5 mm); and (3) 100 to 150 mm of sand (EAWAG/SANDEC 2008) . Free space (1 m) should be left above the top of the sand layer to account for about 3 to 5 years of accumulation. If there is enough space for sludge accumulation, desludging is only required every 5-10 years.

Reeds (Phragmites sp.), cattails (Typha sp.) antelope grass (Echinochloa sp.) and papyrus (Cyperus papyrus)are suitable plants, depending on the climate. Local, non-invasive species can be used if they grow in humid environments, are resistant to salty water and readily reproduce after cutting.

Sludge should be applied in layers between 75 to 100 mm thick and reapplied every 3 to 7 days, depending on the sludge characteristics, the environment and operating constraints. Unlike unplanted beds, planted beds do not need desludging before each new application as the root system of the plants maintains the permeability. Sludge application rates of 100 to 250 kg/m2/year have been reported in warm tropical climates. In colder climates, such as northern Europe, rates up to 80 kg/m2/year are typical. Once the sludge is removed it is well mineralised and has a soil-like structure with a TS content of 40 to 70 % (STRAUSS & MONTANGERO 2002). Therefore, planted sludge drying beds are also called humification beds. It is best to stop applying sludge one or two years before removing it. Two or more parallel beds can be alternately used to allow for sufficient degradation and pathogen reduction of the top layer of sludge before it is removed.

The leachate that is collected in the drainage pipes must be treated properly, depending on where it is discharged. Suited post-treatment for sludge drying bed effluents arefor instance simple waste stabilisation ponds which prepare the liquid for reuse in irrigation, aquaculture or for surface water disposal. Infiltration through soak pits or evaporation beds helps to recharge the groundwater.

Because of the pleasing aesthetics, there should be few problems with acceptance, especially if located sufficiently away from dense housing. Undisturbed plantations can attract wildlife, including poisonous snakes.

Faecal sludge is hazardous and anyone working with it should wear protective clothing, boots and gloves. Also beds should be installed in a certain distance to settlements, as odour occur, especially when sludge has been recently applied. Percolates from sludge drying beds contain also pathogens and need to be further treated. The degree of pathogen reduction of the sludge will vary with the climate. Depending on the desired end-use, further storage and drying might be required. Dried sludge from planted drying beds (if there was no fresh application of sludge during the past 1 to 2 years) is generally free from pathogens and can be used directly in reuse in agriculture (STRAUSS et al. 1997).

The investment costs of sludge drying beds are moderate where land prices are low and filter material (gravel/sand) is locally available (SANIMAS 2005). However, the pond may need to be made impermeable and expert skills are required for design. Operation costs are low as no energy or complicated equipment is required. However, desludging is laborious.

Trained staff for operation and maintenance is required to ensure proper functioning. Even though experts are not compulsory, a well-organised community group, which has experience in organic fertiliser use and preparation should be involved (SANIMAS 2005). The drains must be maintained and the effluent properly collected and disposed of. The plants should have grown sufficiently before applying the sludge. The acclimation phase is crucial and requires much care. The plants should be periodically thinned and/or harvested. After 3 to 5 years the sludge can be removed.