When operating a surface water treatment plant, which laboratory test is critical for establishing chemical dosages for coagulating water?

The critical laboratory test for establishing chemical dosages for coagulating water is alkalinity and pH. Alkalinity measures the water's capacity to neutralize acids and helps in determining the appropriate amount of coagulants needed for effective treatment. It plays a key role in the coagulation process by ensuring that there is enough buffering capacity to maintain an effective pH during treatment.

pH, on the other hand, is essential because it affects the solubility and effectiveness of coagulants such as aluminum sulfate or ferric chloride. If the pH is too high or too low, the coagulants may not work effectively, leading to poor coagulation and clumping of particles. Therefore, assessing both alkalinity and pH is vital for optimizing the coagulation process, ensuring that the correct dosage of chemicals is applied for efficient treatment of the water.

Other tests, while relevant in different contexts, do not specifically address the critical information needed for determining coagulation chemical dosages. For example, turbidity and hardness can provide insights into water quality but do not directly inform the necessary adjustments for chemical addition during coagulation.