Removing or decreasing Chromium concentration of wastewater to the permitted levels before discharging to the environment is important because its bio accumulation and non-biodegradation. These pollutants are introduced into the aquatic systems significantly as a result of various industrial operations. The pollutants of concern include lead, chromium, mercury, uranium, selenium, zinc, arsenic, cadmium, gold, silver, copper and nickel. These toxic materials may be derived from mining operations, refining ores, sludge disposal, fly ash from incinerators, the processing of radioactive materials, metal plating, or the manufacture of electrical equipment, paints, alloys, batteries, pesticides or preservatives. Heavy metals such as zinc, lead and chromium have a number of applications in basic engineering works, paper and pulp industries, leather tanning, organochemicals, petrochemicals fertlisers, etc. Removal of metal ions from wastewaters has been performed by chemical precipitation, membrane filtration, ion exchange, reverse osmosis, evaporation, electrolysis and adsorption processes. The best treatment choice among these methods depends on heavy metal concentration in the wastewater and treatment costs. Adsorption is widely used for removal of heavy metals from wastewater because of its simplicity and low cost. Many natural and synthetic adsorbents are used for this method. Recently, natural adsorbent have gained importance, and the choice of these materials depends on their availability, cost and adsorption capacity. The process of heavy metal removal by biological materials is known as biosorption and the biological materials used are called biosorbents. Biosorption can be defined as the ability of biological materials to accumulate heavy metals from wastewater through metabolically mediated or physico-chemical pathways of uptake. There are tree advantages for biological method; first: they could be apply in situ, second: the biological methods are environmental friendly and they don`t creat secondary pollutant, third: they are cost efficient. Various biosorbents like bacteria, fungi, yeasts, agricultural by products, industrial wastes, etc have been used for biosorption. The biosorption mechanisms are various and are not fully understood. In this study, removal of Cr(VI) from aqueous solutions by Excess Municipal Sludge was investigated as a function of initial metal concentration (in the range of 5-90 mg/l), initial pH (in the range of 2-8), stirring speed (in the range of 50-200 rpm), adsorbent dosage (in the range of 2-10 g/l) and agitation time (in the range of 5-480 min) in a batch system. The optimum conditions were found by full factorial design approach. The result show that the equilibrium time for adsorbent is 120 minutes. Also, sorption data of metals are described by Langmuir, Freundlich and BET models and the equilibrium process is described well by Freundlich isotherm model. .
