Abstract
To assess the use of returned sludge in water treatment, samples were brought from the Tigris River water during the summer and fall of 2002. A chemical treatment was carried out on it using alum as a coagulant, and after adding different percentages of sludge deposited at the bottom of the clarification ponds, it was found that returning 50% of the sludge improves the removal efficiency by an amount that can reach 14%. This applies to models with low and medium turbidity; otherwise, the reduction in removal efficiency is due again. The aluminum sulphate and hydroxides deposited at the bottom of the pond and recycled with the reflux sludge contributed to the improvement of the removal efficiency.
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