Table 4
Concentration of heavy metal in sewage sludge and acceptable limit mg kg−1
	Ni	Pb	Zn	Cr	Cu	Cd
Sewage sludge used	18.6	81.4	355.8	2,404.6	48.4	0.7
Acceptable limita	100	500	2500	500	800	10

^aSources (Moreno et al. [1999]; Chefetz et al. [1996]).

Rezig et al. International Journal Of Recycling of Organic Waste in Agriculture 2012 1:1   doi:10.1186/2251-7715-1-1