Stabilization of tannery sludge through brick production

Abstract

Chromium-rich Tannery Sludge (TS) has the potential to become a serious environmental burden for Bangladesh in future with very limited safe disposal options. One potential avenue for the disposal and stabilization of TS is by incorporating it in clay bricks and if deemed viable, such application can be practiced in a large scale in the country's huge brick-making industry. However, it must be ensured first that the essential engineering properties of bricks are not diminished and leaching of harmful materials do not occur as a result of incorporation of sludge as a brick constituent. In this study, laboratory experiments have been performed to assess the engineering properties of sludge-amended clay bricks as well as to determine the leaching potential of toxic heavy metals from such bricks. The goal was to explore the viability of chromium-rich TS as a brick constituent and determine the effectiveness of such bricks as a safe option for sludge management. Clay bricks were prepared with different proportions of sludge (10%, 20%, 30% and 40% by weight) in both laboratory-controlled conditions and field conditions (i.e. in a brick kiln) and its suitability as a construction material was assessed based on its strength, water absorption, firing shrinkage, weight loss on ignition and bulk density. Results of the tests indicated that the sludge proportion and firing temperature are two key factors in determining the brick quality. Increasing the sludge content in bricks resulted in a decrease in compressive strength. On the other hand compressive strength increased with the increase of firing temperature. The compressive strength of TS bricks reduced considerably from 376 kg/cm2 (36.83 MPa) to 165 kg/cm2 (16 MPa) when TS content increased from 0% to 40% fired at 1000°C. On the other hand compressive strength increased with an increase of firing temperature for all brick samples. The addition of 40% sludge into the mixture reduces strength approximately 56% compared with the control bricks though all the TS bricks have been found to meet both the Bangladesh and ASTM requirements for building construction material. Water absorption was increased with the increase of sludge addition but decreased with increasing firing temperature. 10% TS bricks burnt at 1000°C temperature absorbed the minimum amount of water and regarded as Grade S bricks according to BDS 208 (2009) standards. Moreover, it was estimated that an energy saving of 26% and USEPA regulatory limits. The leaching concentration of Cr from 10% TS bricks fired at 1000°C was found to be reduced by 90% compared to the original raw sludge. Bricks firing at higher temperatures generally yielded improved engineering properties with lesser tendency of Chromium mobilization. Moreover, air quality may be an issue which was not analyzed in this study. The recommended proportion of sludge in brick making can be 10% (by dry weight) with 20 to 22% moisture content and fired at 950°C to 1000°C for producing good quality bricks. Results from this study indicate that tannery sludge can be sustainably stabilized in clay bricks and large-scale application of this technique can be envisaged for Bangladesh where both the leather and brick industries continue to be important economic pillars of the country.