Sludge Drying Bed

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).

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.

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.

SDBs are being used throughout the world especially in United States since the beginning of the 20^th 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).

In Africa, a STP at Cambérène (Dakar, Senegal) uses SDB for sludge treatment since 2006. The initial design underestimated the sludge volumes to be treated and overestimated the sludge concentrations by 40%. This caused serious problems in operations. After detailed study this issue was resolved and the capacities of the SDBs were increased from 200 kg TS/m²/year to 400 kg TS/m²/year. Currently, the plant is running at 300 kg TS/m²/year, thus allowing for an additional bed-scrubbing period of about ten days (EAWAG 2009).

Many of the cities in India including Chennai, Thane, Pune, Patna, Chandigardh, and Bhopal use the conventional sludge drying beds as a part of sludge treatment process. The quantum of sludge to be treated is generally very high due to the amount of wastewater that is generated in these cities. The climatic conditions in India are also favourable for the use of SDBs as the solar energy is available in ample in Indian subcontinent. In the city of Patna, many municipal wastewater treatment plants are using the SDBs as part of sludge treatment technology. Plants at Beur, Saidpur have installed SDB having total capacity of 405 m³ each. Furthermore, cities including Raipur, Khurd (Chandigarh), Ahemdabad, Vasna, Rajkot, Vadodara, Surat (located in the state of Gujarat) also have multiple sewage treatment plants, which include sludge drying beds as the treatment units (CPCB 2005). Further to that The Central Public Health & Environmental Engineering Organisation under Ministry of Urban Development has published the design guidelines for Sludge Drying Beds in India (CPHEEO 2012).