If you have ever taken care of plants you know that they need to be watered so that they don't wilt. That's because plants take water up into their cells and this water pressure helps keep them upright. But have you ever seen on a really hot day, plants wilting even though they appear to have enough water? That's because a plant's respiration includes transpiration of moisture out of their leaves. On a hot day this transpiration can occur faster than plants can take up more water causing them to wilt even when they do have access to water. Following this module you will be able to explain treatments processes and operation of planted drying beds, discuss operation and maintenance requirements and name important design parameters. Over the last 20 years Sandec has collaborated with researchers in Thailand, Cameroon and Senegal to transfer the knowledge of operating planted drying beds in Europe with waste water and bio-solid sludge to the treatment of faecal sludge. The technology has now been successfully proven but there is still not enough full scale operating experience to provide precise design and operational parameters for different contexts and so implementation of this technology still requires special care and attention. Planted drying beds are similar to unplanted drying beds. They consist of a gravel and sand filter bed. Sludge is loaded onto the top and the leachate percolates through the bed and is drained away in a perforated pipe. The differences are in planted drying beds the filter bed is used for growing plants. This means that in addition to evaporation de-watering is occurring through plant transpiration. Both of these together are called evapotranspiration. The plant roots also function to help keep the bed from clogging and provide a more complex environment for the growth of bacteria within the bed. Planted drying beds are also operated in continuous mode; not as a batch so sludge is continually loaded onto the beds but only removed after a period of years. One of the most important considerations for the design and operation of planted drying beds are the plants. They need to be able to grow in relatively hostile faecal sludge conditions and be tolerant to fluctuating water levels and salinity. They need be fast growing, have high transpiration rates, deep-growing rhizomes and roots and be non-invasive. In Europe, reeds or Phragmites and cattails or Typha are the most commonly used plants. In Africa, Asia and Latin America, indigenous plants are being explored for adaption such as Antelope Grass or Echinochloa and papyrus or Cyperus. An example is this paper identifying 3 new indigenous species in Senegal that are good candidates for use in planted drying beds. Echinochloa crus-galli, Paspalidium geminatum and Pasplaum veginatum. This is a drying bed following harvesting of plants also showing the plants starting to grow back. Plants are harvested by cutting them at the surface of the bed not by pulling them out which could damage the bed and prevent regrowth. Plants are harvested when sludge is removed but can also be harvested more frequently for maintenance or to sell as fodder. For example, this paper found that in Cameroon, sale of the plants as fodder could offset 7% of the annual operating costs. Another important design and operating parameter is the aclimazation phase required by plants prior to growing in full-strength faecal sludge. The start-up phase takes on average around 6 months and in arid climates it should start during the rainy season for best growth. Examples of how to do this are going gradually from a loading rate of 50 to 200kg total solids per meter square per year with a feeding frequency of of at least twice a week. Another way would be starting out with dilute waste water or for example effluent from a settling thickening tank and gradually moving to full-strength faecal sludge. During this period, visual indicators of plant stress such as yellowish color or slow growth rates should be carefully looked out for. Ponding to retain liquid in the bed can also be used to prevent wilting in tropical climates and potentially increase treatment performance but the bed should then be left to fully drain before reloading. This is sludge on a bed without loading for 1 week. Sludge loading rates can be based on hydraulic loadings or the amount of total solids. Loading frequencies are also going to depend on the types of plants, climate and treatment goals. For example, this paper found that increased loading frequency can increase plant growth, liquid lost to evapotranspiration, and the nitrification of leachate. If you base loading rates on total solids, to determine the required surface area you need to know the total volume of sludge that will be treated on an annual basis. The average total solids concentration of the sludge; then we just convert the units here divided by the loading rate that you're going to use and this tells you the required surface area. Desludging frequencies are going to depend on factors such as loading rate and climate. Experimental data from Cameroon suggests that a loading rate of 200kg total solid per square meter per year had 50 to 70cm per year sludge accumulation. That means if you had a free board of 2m, this could result in 3 to 4 years for desludging. However other studies have had lower accumulation rates and desludging intervals can even be up to 10 years long. This is a hole looking down into the sludge layer. After the last loading, sludge should be then left for a few months for increased moisture and pathogen reduction. These long sludge retention times also mean that following removal, the sludge is much more stabilized than with the short retention times of unplanted drying beds. This table from the Fecal Sludge Management book summarizes observed treatment performances with loading rates. Percent removals meaning what percent stays with the sludge layer versus the leachate are quite high but what is important to remember with percent removals is that starting with very high concentrations that are in faecal sludge, you still end up with concentrations in the leachate that are higher than discharged standards. Ways to treat dewatering liquid from sludge are discussed in Co-treatment and Leachate Treatment modules. In summary, important design and operating parameters are loading, feeding frequency, resting periods, plant density, plant acclimatization, and plant harvesting. Conclusions from Sandec's collaborative research on adapting planted drying beds for the treatment of faecal sludge include, planted drying beds are viable for faecal sludge treatment; they're resistant to highly variable hydraulic loadings; higher loading rates can be used in tropical climates; plants need to be selected for the local context; fodder production can offset some of the operation costs; sludge can be stabilized on the beds however it needs further treatment for safe use in agriculture. Desludging periods can be up to 5-10 years long. Pre-treatment with settling tanks results in better de-watering on planted drying beds. The leachate requires further treatment and research as it remains one of the biggest challenges. In this module we learned about treatment processes and operation of planted drying beds, operation and maintenance requirements and design parameters. Thanks for joining and see you next time.
