Unplanted Drying Beds

All organic degradation processes produce sludge (SASSE & BORDA 1998). 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.

The bottom of the drying bed is lined with perforated pipes to drain the leachate away that percolates through the bed. On top of the pipes are layers of gravel and sand that support the sludge and allow the liquid to infiltrate and collect in the pipe. While the solid fraction remains on the filter surface and is dried by natural evaporation the liquid percolates. In planted drying beds, the plants enhance evaporation by transpiration. This process is called evapotranspiration. Sludge is applied in layers on top of the gravel beds and is naturally dried. It should not be applied in layers that are too thick (maximum 20 cm), or the sludge will not dry effectively. The final moisture content after 10 to 15 days of drying should be approximately 60%. When the sludge is dried, it must be separated from the sand layer and transported for further treatment, end-use or final disposal. Furthertreatment for instance by composting| allows to stabilise the sludge and the product can be reused as nutrient-rich soil amendment.[/no-ecompendium The leachate that is collected in the drainage pipes must also be treated properly, depending on where it is discharged. Leachate treatment in waste stabilisation ponds prepares the liquid for reuse in irrigation or aquaculture or for surface water disposal. Infiltration through soak pits or leach fields helps to recharge the groundwater.

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. The drainage pipes are covered by 3-5 graded layers of gravel and sand. The bottom layer should be coarse gravel and the top fine sand (0.1 to 0.5 mm effective grain size). The top sand layer should be 250 to 300 mm thick because some sand will be lost each time the sludge is removed (STRAUSS & MONTANGERO 2004; TILLEY et al. 2008). Indication about the diameter of the medium and the height of the different gravel and sand layers vary depending on the source (SASSE & BORDA 1998; STRAUSS & MONTANGERO 2004). Generally, the coarse gravel layer (grain diameter: 15 to 50 mm) is within 20 to 30 cm of height. A gravel layer (diameter of 7 to 15 mm) of 10 to 15 cm follows this layer. This layer can be followed of a similar one of slightly lower diameter (see picture below, STRAUSS & MONTANGERO 2002), but this is not compulsory (SASSE 1998). There should be a final sand layer of 25 to 30 cm.

The sludge is applied in a batch mode about once per week intervals in layers of no more than 20 to 30 cm.On an annual basis, about 100 to 200 kg TS/m2 can be applied on a drying bed. Drying takes 10 to 20 days. Land requirements are 0.05 m2 per capita for a 10 days cycle (STRAUSS & MONTANGERO 2002). Before fresh sludge is applied, dried sludge needs to be desludged and brought to a composting site, what makes it a rather laborious treatment option. To improve drying and percolation, sludge application can alternate between two or more beds. About 50 to 80 % of the initial volume is removed by percolation (STRAUSS & MONTANGERO 2002), resulting in total solid (TS) content of 20 to 70 % (STRAUSS & MONTANGERO 2004) depending on the local weather conditions and climate. The inlet should be equipped with a splash plate to prevent erosion of the sand layer and to allow for even distribution of the sludge.

Designing unplanted drying beds has to consider future maintenance because ensuring access to people and trucks for pumping in the sludge and removing the dried sludge is essential.

If installed in wet climates, the facility should be covered by a roof and special caution should be given to prevent the inflow of surface runoff. A fence should prevent animals and people from entering the area.

Both the incoming and dried sludge are pathogenic; therefore, workers should be equipped with proper protection (boots, gloves, and clothing). Also beds should be installed in a certain distance to settlements, as odour occur, especially when sludge has been recently applied. The dried sludge and effluent are not sanitized and may require further treatment or storage, depending on the desired end-use. Sludge can be composted before reuse to enhance pathogen removal.

The drying bed may cause a nuisance for nearby residents due to bad odours and the presence of flies. Thus, it should be located sufficiently away from residential areas.

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 (application of sludge, desludging, control of drainage system and the control of the secondary treatments for percolate or dried sludge) is required to ensure proper functioning. Even though experts are not compulsory for the operation and maintenance, a well-organised community group, which has experience in organic fertiliser use and preparation should be involved (SANIMAS 2005).

Dried sludge can be removed after 10 to 15 days, but this depends on the climate conditions. Because some sand is lost with every removal of sludge, the top layer must be replaced when it gets thin. The discharge area must be kept clean and the effluent drains should be regularly flushed.