Deliming

Abstract: This paper presents experimental results from the implementation of two measures aimed at reducing the nitrogen concentration in a tannery wastewater. Specifically, this research has focused on the wastewater from the deliming/bating operations. The proposed measures are the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. The experimental study covered different wastewater pretreatment alternatives and experiments with two membranes (with different separation properties): one in the range of microfiltration (MF) and one in the range of the ultrafiltration (UF). Results of the pretreatment study indicated that neither settling nor protein precipitation were feasible. Only a security filtration prior to membrane filtration was recommended. The tested MF membrane was selected due to the higher flux (around 25 L/(m2 h)) in comparison with the UF membrane. The MF permeate was successfully reused in the deliming/bating process. The delimed leather quality was excellent according to both visual and organoleptic inspection from process technicians and phenolphthalein test, confirming the technical feasibility of the proposal. Globally, the implementation of the above mentioned two measures resulted in 53% total nitrogen reduction.

Abstract: Due to increased legislation geared towards environmental protection and preservation of water quality, tanneries are under constant pressure to implement source reduction technologies that reduce their organic and inorganic wastewater loading. The processing of hides or skins to leathers involves five important stages: raw skins/hides preservation, pre-tanning, tanning, post tanning, and finishing, of which, pre-tanning and tanning processes are known to contribute more than 90% of the total pollution load from leather processing. These include soaking, liming, reliming, deliming, bating, pickling, chrome tanning and basification. Wide variation in pH (2.8–12.0) is also encountered during the entire leather making process. These different operations and the pH variations result in the emission of large pollution loads as biochemical oxygen demand (BOD), chemical oxygen demand (COD), total solids (TS), chlorides, sulphates, chromium and sludge. This article describes an integrated three-step bio-based process, which limits the pH range in leather processing to 4.0–8.0. A sequence of operations namely, dehairing, fibre opening and pickle-basification-free chrome tanning (chrome tanning that does not require pickling or basification), has been designed primarily with the use of enzymes. Application of this source reduction process leads to a substantial decrease in chemical consumption from 380 to 50 kg for tanning one ton of raw skins. Leathers processed using this process sequence matched the performance of conventionally processed leathers. Further, the integration of enzyme technology with a specially designed chrome tanning process presents substantial environmental benefits as observed in the reduction of COD, total solids and chromium by 61, 78 and 92%, respectively. Cost data are presented to demonstrate that the process is economically viable.