Effects of calcined aluminum salts on the advanced dewatering and solidification/stabilization of sewage sludge

The high moisture content (80%) in the sewage dewatered sludge is the main obstacle to disposal and recycling. A chemical dewatering and stabilization/solidification (S/S) alternative for the sludge was developed, using calcined aluminum salts (AS) as solidifier, and CaCl ₂, Na ₂S0 ₄ and CaS0 ₄ as accelerators, to enhance the mechanical compressibility making the landfill operation possible. The properties of the resultant matrixes were determined in terms of moisture contents, unconfined compressive strength, products of hydration, and toxicity characteristics. The results showed that AS exhibited a moderate pozzolanic activity, and the mortar AS ₀ obtained with 5% AS and 10% CaS0 ₄ of AS by weight presented a moisture contents below 50%-60% and a compressive strength of (51.32 ± 2.9) kPa after 5-7 days of curing time, meeting the minimum requirement for sanitary landfill. The use of CaS0 ₄ obviously improved the S/S performance, causing higher strength level. X-ray diffraction, scanning electron microscopy and thermogravimetry-diiferential scanning calorimetry investigations revealed that a large amount of hydrates (viz., gismondine and CaC0 ₃) were present in solidified sludge, leading to the depletion of evaporable water and the enhancement of the strength. In addition, the toxicity characteristic leaching procedure (TCLP) and horizontal vibration (HJ 557-2009) leaching test were conducted to evaluate their environmental compatibility. It was found that the solidified products conformed to the toxicity characteristic criteria in China and could be safely disposed of in a sanitary landfill.