Fig. 6. Relationship between pH and removal efficiency of heavy metals at different sludge solids concentrations: (a) Zn, (b) Cu, (c) Cr, (d) Pb, (e) Ni, (f) Al. Symbols: •, 20 g L-1; 
, 50 g L-1; 
, 90 g L-1; 
, 130 g L-1.
Fig. 6. Relationship between pH and removal efficiency of heavy metals at different sludge solids concentrations: (a) Zn, (b) Cu, (c) Cr, (d) Pb, (e) Ni, (f) Al. Symbols: •, 20 g L-1; , 50 g L-1; , 90 g L-1; , 130 g L-1.