Although _______ can be added to lime-soda ash rapid-mix basins, better results are usually obtained if they are added _______ in a separate rapid-mix basin.

Although _______ can be added to lime-soda ash rapid-mix basins, better results are usually obtained if they are added _______ in a separate rapid-mix basin.

• A. Disinfectants, Upstream
• B. Disinfectants, Downstream
• C. Coagulants, Downstream
• D. Coagulants, Upstream

Answer: (D)

Coagulants should be added upstream in a separate rapid-mix basin. Coagulants work by neutralizing the negative charges on particles so they can come together to form larger flocs. If the coagulant is mixed in the lime-soda ash rapid-mix basin, the intense reactions and sudden pH rise from lime and soda ash can hinder proper coagulant hydrolysis and destabilization, yielding smaller, weaker flocs and poorer removal later on. Adding the coagulant in a dedicated upstream rapid-mix basin gives it a controlled environment with optimal mixing and contact time before the water enters the lime-soda ash stage. The result is better-formed flocs that settle more readily and filter more efficiently downstream. Disinfectants aren’t normally added in rapid-mix basins for coagulation purposes, and adding coagulants downstream would reduce effectiveness due to inadequate mixing and contact time.

Coagulants should be added upstream in a separate rapid-mix basin. Coagulants work by neutralizing the negative charges on particles so they can come together to form larger flocs. If the coagulant is mixed in the lime-soda ash rapid-mix basin, the intense reactions and sudden pH rise from lime and soda ash can hinder proper coagulant hydrolysis and destabilization, yielding smaller, weaker flocs and poorer removal later on.

Adding the coagulant in a dedicated upstream rapid-mix basin gives it a controlled environment with optimal mixing and contact time before the water enters the lime-soda ash stage. The result is better-formed flocs that settle more readily and filter more efficiently downstream. Disinfectants aren’t normally added in rapid-mix basins for coagulation purposes, and adding coagulants downstream would reduce effectiveness due to inadequate mixing and contact time.