Write note on sludge drying beds.
Answers
Sludge drying bed (SDB) is the most widely used method for sludge dewatering. Sludge drying involves natural ways of drying to mechanical ways of removing water content. SDB is generally used for small and medium sized communities (TCHOBANOGLOUS et al. 2003). The selection of the technology will depend upon land availability, climatic factors, the quantity and composition of the sludge. The construction of this is usually undertaken with civil structure.
Advantages
Sources: TCHOBANOGLOUS et al. 2003; SANIMAS 2005; TILLEY et al. 2008 ; GHAZY et al. 2009
Easy to operate.
No electrical energy required.
Organic content can be used as fertiliser.
Disadvantages
Sources: TCHOBANOGLOUS et al. 2003; SANIMAS 2005; TILLEY et al. 2008; GHAZY et al. 2009
Requires stabilised sludge to prevent nuisance and odours.
Technology is land intensive.
Climatic fluctuation may cause disturbance.
Clogging of sand bed.
Only applicable during dry seasons.
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Design and Construction Principle
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Design of a Sludge Drying Bed
The prediction of the drying rate is an empirical part of the designing the system. Drying of the sludge can be divided in to two different stages, namely drainage and evaporation (CHAI 2007). The design of SDB is based mainly on site specifications, as well as environmental and climatic factors. The sizing of the sludge drying beds should be done considering the worst case scenario (CERONIO et al. 1999). The loading is based on per capita basis or on a unit loading of pounds of dry solids per square foot per year (TCHOBANOGLOUS et al. 2003). There are mainly five types of sludge drying methods viz. conventional sand beds, paved drying beds, artificial media beds, vacuum assisted and solar. The typical conventional SDB has dimensions of 6 m width, 6 - 30 m length, with sand layer ranging from 230 – 300 mm depth. The sand should have a uniformity coefficient of not over 4.0 and effective size of 0.3 to 0.75 mm. The piping to the sludge drying beds should be designed for velocity of at-least 0.75 m/s (TCHOBANOGLOUS et al. 2003). The sludge is placed on the bed in 20–30 cm layers and allowed to dry. Sludge cake removal is manual by shovelling into wheel-barrows, trucks, scraper, or front-end loader. The drying period is 10–15 days, and the moisture content of the cake is 60 – 70%. Sludge loading rate is 100–300 kg dry solids/ m2 /year for uncovered beds (AL-MALACK et al. 2002).
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Operation and Maintenance
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The application of sludge treatment methods differs from country to country due to differences in operating conditions and energy prices (GHAZY et al. 2011). Conventional sludge drying is very simple, but trained staff for operation and maintenance is required to ensure proper functioning. This method doesn’t require electrical energy (TILLEY et al. 2008). The O & M also includes application of sludge, desludging, control of drainage system and the control of the secondary treatments for percolate or dried sludge.
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Cost Considerations
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This conventional sludge treatment technology is more land intensive rather than energy intensive. Based on the quantity of wastewater to be treated the sizing of the sludge drying bed changes. The capital investment for this treatment unit is the highest in terms of the land requirement followed by the construction costs. The O & M requirements include the labour hours, fuel energy, back washing as well as the annual materials and maintenance parts.
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Experiences in Europe and other Cities of the World
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SDBs are being used throughout the world especially in United States since the beginning of the 20th century, but over the years its applicability is limited due to the environmental and climatic factors (CARPENTER 1938). In the United States, the majority of Waste Water Treatment Plants (WWTPs) with capacities less than 5 MGD (equal to 18.93 m3/day) use SDBs. Similarly, Russia and other Eastern European countries use SDBs in more than 80% of the WWTPs (TUROVSKIY & MATHAI 2006). By 2009, Egypt had approx. 303 WWTPs handling 11.85 x106 m3 /day of sewage. Most of these WWTPs use natural sludge drying beds (GHAZY et al. 2009).