Liming Pretreatment Reduces Sludge Build-Up on the Dryer Wall during Thermal Drying

In the course of sludge thermal drying, the sludge that sticks to the heated surfaces of drying equipment causes deterioration of drying efficiency and safety. The ability of alkaline pretreatment to reduce sludge build-up on dryer walls during convective drying processes was investigated. The sludge build-up was measured as the residual mass on an iron plate after parts of the sludge were scraped off the plate; the device used simulated the scraping process in drying equipment. The results showed that the sludge build-up on the plate increased at first and then decreased as the sludge water content decreased during thermal drying. The maximum amount of sludge build-up occurred when the sludge water content was about 55–60%. Liming pretreatment enhanced sludge cohesion, hydrolyzed adhesive organic substances, increased the inorganic mineral content of the sludge and, accordingly, reduced sludge build-up. When the dose of Ca(OH)2 was 5% of dry solids (5% DS), the sludge build-up was almost negligible and the average drying rate improved by about 30%. CaO pretreatment was also very effective in preventing sludge build-up, but its effect on the sludge drying rate was not obvious at the same dosage. Compared with Ca(OH)2 and CaO, NaOH did not reduce sludge build-up effectively or improve the sludge drying rate.