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The effect of adding position for tannery sludge in a co-processing cement rotary kiln on the chromium leachability of the cement produced was assessed in laboratory scale. Cement clinkers and pastes were produced and evaluated based on analysis of the cement quality, characterization of the clinker crystalline phases, and extraction toxicity of heavy metals in the resulting cements (using the toxicity characteristic leaching procedure (TCLP) and the Chinese GB/T 30810-2014 standard). It was found that the amount of sludge added (up to 0.7% of the feed composition) and use of different addition points for the sludge had no detrimental effect on the resulting cement quality. The retention ratios of heavy metals during sintering were related to their volatilities and the sludge addition points. The Cr concentration in leachate generated by the TCLP was far below the regulatory limit (5mg/L); however, when using the GB/T 30810-2014 procedure, the Cr concentration in the leachate exceeded the regulatory limit (200μg/L) when the sludge addition was greater than or equal to 0.5% and it was added at the kiln main burner position. Based on our laboratory study, tannery sludge might be added at the decomposing furnace addition point to reduce the Cr leachability of the resulting cement., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Chromium-rich tannery sludge generated from tanneries has the potential to become a serious environmental burden in Bangladesh and a promising avenue for disposal of this sludge is by stabilizing it in clay brick products. But for sustainable industrial application of such technique it needs to be ensured first that the engineering properties of bricks as a building material are not diminished by addition of sludge, the process becomes energy efficient compared to alternatives and the use of such bricks do not pose any harmful environmental effects in the long run. In this study, clay bricks were prepared with different proportions of sludge (10%, 20%, 30% and 40% by dry weight) in both laboratory-controlled and field conditions and their suitability as a construction material was assessed based on their strength, water absorption, shrinkage, weight-loss on ignition and bulk density. For the sludge incorporated bricks, the compressive strength ranged from 10.98MPa to 29.61MPa and water absorption ranged from 7.2% to 20.9%, which in most cases met both the Bangladesh and ASTM criteria for bricks as a construction material. Volumetric shrinkage, weight loss and efflorescence properties of sludge-amended bricks were found to be favorable and it was estimated that an energy saving of 15-47% could potentially be achieved during firing with 10-40% tannery sludge-amended bricks. The quality of sludge-amended bricks made in the brick kiln was relatively inferior compared to bricks produced in the laboratory due to operating in a less-controlled environment with respect to maintaining adequate compaction and optimum moisture content. The leaching behavior of several heavy metals (Cr, As, Cu, Ni, Cd, Pb and Zn) from sludge-amended bricks has been found to be insignificant and far below the Dutch regulations and USEPA regulatory limits. 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 in the context of Bangladesh where brick remains a dominant building material., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

A stabilization/solidification (S/S) process by using cement was applied to tannery sludge in order to find a safer way of landfilling this waste. The effects of three parameters on the process effectiveness were analysed in terms of leachate toxicity and chromium retention (%). The parameters studied were the relative amount of added water (30-50 wt.%), cement (10-60 wt.% in the solid components), and the use of three different types of cement (clinker with additions of limestone, with additions of limestone and fly ashes, and with additions of pozzolans). Statistical analysis performed by variance analysis and categorical multifactorial tests reveals that all the studied parameters significantly influence the effectiveness of the process. Results showed that chromium retention decreases as the relative amount of cement and water increases, probably due to additional chromium provided by cement and increased in the porosity of the mixtures. Leachate toxicity showed the same minimum value for mixtures with 30% or 40% cement, depending on the type of cement, showing that clinker is the main material responsible for the process effectiveness, and additives (pozzolans or fly ashes) do not improve it. The volume increase is lower as less sludge is replaced by cement and the relative amount of water decreases, and for the cement without additions of fly ashes or pozzolans. Therefore, the latter seems to be the most appropriate cement in spite of being more expensive. This is due to the fact that the minimum toxicity value is achieved with a lower amount of cement; and moreover, the volume increase in the mixtures is lower, minimizing the disposal cost to a landfill., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Chromium (Cr) accumulation in soil and plants has been reported after successive applications of tannery sludge, which is a matter of concern because Cr can promote environmental contamination and affect the food chain. In this study, we evaluated the growth of and Cr accumulation in maize and cowpea after seven years of consecutive applications of composted tannery sludge (CTS) under field conditions. The experiment consisted of application of CTS at five levels: 0 (control), 2.5, 5, 10, and 20 Mg ha-1 (dry basis). The growth of and Cr accumulation in maize and cowpea were evaluated at 75 and 65 days after plant emergence, respectively. CTS application increased the shoot biomass of maize and cowpea. Accumulation of Cr was similar for both plant species, with higher accumulation in roots. However, Cr accumulation in grains differed as the CTS doses increased; cowpea accumulated more Cr while maize did not accumulate more Cr in grains with higher CTS doses. In conclusion, application of CTS led to higher growth of maize and cowpea plants, and both species exhibited similar Cr accumulation in roots and shoots.

Chromium (Cr) accumulation in soil and plants has been reported after successive applications of tannery sludge, which is a matter of concern because Cr can promote environmental contamination and affect the food chain. In this study, we evaluated the growth of and Cr accumulation in maize and cowpea after seven years of consecutive applications of composted tannery sludge (CTS) under field conditions. The experiment consisted of application of CTS at five levels: 0 (control), 2.5, 5, 10, and 20 Mg ha-1 (dry basis). The growth of and Cr accumulation in maize and cowpea were evaluated at 75 and 65 days after plant emergence, respectively. CTS application increased the shoot biomass of maize and cowpea. Accumulation of Cr was similar for both plant species, with higher accumulation in roots. However, Cr accumulation in grains differed as the CTS doses increased; cowpea accumulated more Cr while maize did not accumulate more Cr in grains with higher CTS doses. In conclusion, application of CTS led to higher growth of maize and cowpea plants, and both species exhibited similar Cr accumulation in roots and shoots. [ABSTRACT FROM AUTHOR]

• A recycled concrete was synthesized by tannery sludge, fly ash and construction waste. • The alkali activator prepared by sodium silicate and NaOH was beneficial to the strength development of AA-SFRC. • The addition of alkali activator optimized the pore structure and phase composition of AA-SFRC. • The grey prediction model of strength and pore structure parameters was established. In order to alleviate the environmental degradation and resource waste caused by cement production and construction waste disposal, some cement is replaced by tannery sludge and fly ash, and natural aggregate is replaced by recycled coarse aggregate. The preparation of alkali activated tannery sludge/fly ash composite cement-based recycled concrete (AA-SFRC) is of great significance for the safe disposal and utilization of solid waste. In order to study the mechanism of the effect of alkali-activator on the mechanical properties and microstructure of AA-SFRC, six kinds of AA-SFRC with different alkali equivalents (AE) were designed, and the strength tests, nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were carried out. On this basis, the influence of AA-SFRC pore structure parameters on strength performance was discussed by using grey entropy analysis method, and a grey model GM (1, 5) prediction model of 28d strength was established. The results demonstrated that with the increase of AE, the AA-SFRC strength increased at first and then decreased, and the tension–compression ratio (Ts/Cs) remained in the normal range, between 0.065 and 0.081. When the AE was 9%, the mineral composition of AA-SFRC reached equilibrium, and the hydration reaction was the most sufficient. The compressive strength and splitting tensile strength reached the maximum. Meanwhile, the pore structure showed that the porosity and the proportion of macro pores were the smallest. The 28d strength of AA-SFRC was greatly affected by its 7d strength, first peak ratio, fractal dimension D max and porosity. The average relative errors between the predicted value of GM (1, 5) model and the experimental value were less than 0.03%. This study can provide a reference for the resource utilization of tannery sludge, fly ash and construction waste. [ABSTRACT FROM AUTHOR]

Abstract: We experimentally studied the occurrence of spontaneous self-heating of sludge after drying, to understand its nature, course and remediation. The sludge originates from primary and biological treatment of both municipal and industrial wastewater, the latter largely dominant (approx. 90% total organic carbon, mainly from local tanneries). Dried sludge is collected into big–bags (approx. 1.5m3) and landfilled in a dedicated site. After several years of regular operation of the landfill, without any management or environmental issue, indications of local warming emerged, together with smoke and smelling emissions, and local subsidence. During a two year monitoring activity, temperatures locally as high as 80°C have been detected, 6–10m deep. Experiments were carried out on large quantities of dried sludge (∼1t), monitoring the temperature of the samples over long periods of time (months), aiming to reproduce the spontaneous self-heating, under different conditions, to spot enhancing and damping factors. Results demonstrate that air is a key factor to trigger and modulate the self-heating. Water, in addition to air, supports and emphasizes the heating. Unusual drying operation was found to affect dramatically the self-heating activity, up to spontaneous combustion, while ordinary drying conditions yield a sludge with a moderate self-heating inclination. Temperature values as well as heating time scales suggest that the exothermic process nature is mainly chemical and physical, while microbiological activity might be a co-factor. [Copyright &y& Elsevier]

Knowledge of the behaviour of heavy metals in the combustion process is a most important factor in selecting disposal alternatives for waste materials. Accordingly, in this work, the vaporization behaviour of highly concentrated heavy metals (Pb, Zn, Cu and Cr) in tannery sludge were investigated experimentally. The sludge was spiked with various chlorine compounds (i.e. PVC, FeCl3, CaCl2 and NaCl) and pre-treated with phosphoric acid in order to evaluate the capacity of enhancement and weakening of the volatility of the heavy metals contained in tannery sludge. The experimental results show that the vaporization percentages increased with increasing ratio of Cl/sludge and temperature, and the accelerating and increasing effect of the addition of chlorides on the vaporization percentage of heavy metals was dependent on the release capacity of chlorine radicals. The vaporization percentages of lead and zinc increased by 15-20%, whereas those of copper and chromium increased by only about 3 and 10% at 800 °C. However, heavy metals were not expected to be completely released in the combustion process in spite of the high ratio of Cl/sludge. Alternately, heavy metals contained in tannery sludge can be immobilized effectively by pre-treatment with phosphoric acid. When the 85% phosphoric acid accounted for 10% of dry basis of tannery sludge, the phosphate-treated sludge showed the lowest vaporization percentage of about 3-15% with formation of Ca18Cu3(PO4)14, Ca9Cr(PO4)7, Ca19Zn2(PO4)14 and PbMgP2O7 in the bottom ash. [ABSTRACT FROM AUTHOR]