What are the Design criteria for Sludge drying beds?
When land is available, this method is employed, and the dried sludge is used for soil conditioning. The sludge is spread on a sand substrate that is supported by gravel. The majority of the liquid drains away in the first few hours, following which evaporation causes drying. As the sludge cake shrinks, cracks appear, speeding up evaporation from the sludge surface. In a dry region, the sludge will usually dry in two weeks. Apart from sludge characteristics, the drying period will be determined by sunlight, rainfall, wind velocity, and relative humidity. It might take up to four weeks if the weather is bad. To minimize odour problems, sludge drying beds should be at least 100 meters away from dwellings.
Sludge Drying Bed _ Design Criteria
1: Dimensions of the beds
It is determined by the volume of sludge to be dried, the cycle time required to retain sludge for dewatering, the drying and removal of dried sludge, and the preparation of the sand bed for the following application. The length of time it takes to dewater and dry sludge is determined by the depth of the sludge applied. The cycle time is affected by climatic factors such as temperature, humidity, dry or wet weather, wind velocity, and sludge properties.
Under unfavorable conditions, the land requirement for an aerobically digested sludge can be significant, ranging from 0.1 to 0.25 m2/capita. In most cases, the cycle period is up to 2 weeks in warm climates and 3 to 6 weeks in cold climates.
The following are the requirements for preparing sludge drying beds:
Underdrains: Underdrains are built of open-jointed vitrified clay pipe or tiles with a minimum diameter of 10 cm. Pipes should not be laid further than 6 meters apart. It should be arranged for the drained water to be returned to the primary sedimentation tank.
Gravel: Gravel is used to cover the drainage system under the surface. Gradated gravel is placed in layers up to 30 cm around the under drains, with a minimum of 15 cm above the drains. The top 3 cm of gravel should be 3 to 6 mm thick.
Sand: Sand is employed, with an effective size of 0.5 to 0.75 mm and a uniformity coefficient of no more than 4. The sand depth can range from 20 to 30 centimeters.
Sludge drying beds are typically 6 to 8 meters wide and 30 meters long. With a 0.5 percent bed slope, the length of a single sludge application point should not exceed 30 meters. Multiple sludge entry should be employed for longer sludge beds to shorten the time of wet sludge travel. A minimum of 0.4 m of free board is utilized.
3: Sludge Inlet
Sludge inlet pipes should have a minimum diameter of 20 cm. Sludge should be discharged from this pipe at a height of at least 0.3 m above the sand bed. At discharge stations, splash plates should be installed to disseminate the sludge evenly throughout the bed and avoid erosion of the sand bed.
4: Removal of sludge
When the moisture content of the dried sludge cake is less than 70%, it is removed with a shovel or fork. Sludge is acceptable for grinding when the moisture content is less than 40%. A small amount of sand is always stuck to the bottom of the sludge cake. When the bed's depth is decreased to 10 cm as a result of the loss, fresh sand of the appropriate specification is added to restore the original depth. After removing the dried sludge, the sand bed is levelled in preparation for the next sludge application cycle.