Sources and build up of Zn, Cd, Cr and Pb in the sludge of Gaza

Abstract

A comprehensive monitoring program was conducted for the sludge of Gaza between 2001 and 2006. All 32 tested parameters except zinc and adsorbable organic halogens were within the allowable values for sludge to be applied in agriculture. Average concentrations of zinc (Zn) in the sludge from the Gaza Strip for the 4 years 2002-2005 reached 2,000 mg/kg which represents a major limiting factor for sludge application in agriculture. This study aimed to measure levels of Zn in the wastewater and sludge in December 2006 and to identify the sources and the build up of Zn in the sludge in Gaza. Cd, Cr and Pb were also assessed for their relationship to sources and buildup of Zn. The results showed that there is no significant fluctuation in the concentration of Cd, Cr and Pb in the different stages of wastewater treatment. Zn, however, is concentrated inside the treatment plant by processes of precipitation and/or absorption, particularly in the aerobic facilities. Although the plant receives wastewater with Zn concentrations of only 9 microg/l, this concentration increased 18-fold inside the aerobic lagoon of the treatment plant, before dropping to an average of 14 microg/l in the effluent wastewater. The sludge from the first sedimentation pond showed a Zn concentration of 567 mg/kg and increased in the effluent polishing pond to 1,643 mg/kg. The Zn concentration in 3-month-old sludge averaged 592 mg/kg. There was no correlation between the Zn concentrations in the sludge and the wastewater at the same location. However, there was a strong correlation between Zn and Pb in the sludge. The electroplating and galvanization industries are the major Zn producing industries in Gaza, with an average Zn of 2,995 and 1,557 microg/l, respectively in their effluent wastewater. These values do not represent a significant Zn pollution load to the treatment plant because these industries are limited in size and number, and their effluents are diluted before entering the treatment plant. Industrial activity decreased in 2006 to less than 70% of that in the previous four years. Consequently, the average concentrations of Zn in selected industrial effluents decreased from 1,500 for 2002-2005 to 400 microg/l in December 2006. Sludge from these industries showed 1,300 mg/kg Zn for 2002-2005 and only 400 mg/kg in 2006. Moreover, the Zn in the influent wastewater discharged to the treatment plant decreased from 65 to 9 microg/l for the same period. Exposure of sludge to the sun for few months reduces the Zn concentration. One reason is probably leaching. This simple procedure may solve the problem of sludge application in agriculture. This will be especially important if industrial production returns to its former level.