Abstract:
Most tanneries in Ethiopia (90%) do not treat and very few (10%) partially treat their effluent
before discharging it into the receiving water bodies, therefore, untreated leather effluent causes
tremendous pollution of water resources in the country, especially due to its high organic loading
and chromium content. Thus, proper treatment to eliminate or reduce the problem has to be
devised. The purpose of this study performed at Addis Ababa Tannery Sh.Co. was to develop an
alternative technical process for removing and recovering trivalent chromium from tannery
wastewater via chemical precipitation. Chromium(III) salts are the most widely used chemicals
for tanning processes, but 60-70% of total chromium salts reacts with the hides. In the other
word, about 30-40% of the chromium amount remains in the solids and liquid wastes (especially
spent tanning solutions). Therefore, the removal and recovery of the chromium content of these
wastewaters is necessary for environmental protection and economic reasons. To this purpose,
three precipitating agents; calcium hydroxide, sodium hydroxide and magnesium oxide were
used. The effects of pH, stirring time, settling rate and sludge volume were studied in batch
experiments. Flame atomic absorption spectroscopy (FAAS) method was used to determine Cr
concentration. The results NaOH(99.973%), Ca(OH)2(99.97% ) and MgO( 99.98%) showed that
removal efficiencies at optimum pH values were not significant. However, there is significant
difference in sludge characteristics with sludge volume of NaOH (590 mL), Ca(OH)2(412 mL)
and MgO(85 mL). The optimum pH is 9.8-10.3 with removal efficiency of 99.98% and the good
sludge with high settling rate and lower volume was obtained by MgO precipitating agent. Hence
the MgO is a good precipitating agent for removal and recovery of chromium from tanning
wastewater. The basic chromium sulfate (BCS) recovered at pH 2.8 is suitable for tanning mixed
with fresh BCS from the market.
Key words: Chromium, removal, recovery, chemical precipitation, tannery wastewater