Most introductory-level water treatment facility operator exams do not contain problems on feed rate/flow proportionality. Nevertheless, all introductory-level water treatment facility operators need to know this information to effectively operate a water treatment facility.
Feed rate/flow proportionality is the mathematical procedure by which a water treatment facility operator increases or decreases chemical feed rates whenever the plant's flow increases or decreases. If a water treatment facility operator is unable to accurately perform this procedure, he or she runs the risk of encountering serious water-quality problems.
To increase or decrease a chemical feed rate based on an increase or decrease in plant flow, multiply the existing feed rate by the new flow, and then divide this answer by the old flow. Below are several examples to fully illustrate this procedure. The first two examples are based on gpm flow rates. The last two examples are based on MGD flow rates.
1. A water treatment facility is feeding 840 ml/min of alum for a raw flow rate of 2350 gpm. What alum feed rate is needed if the raw flow rate is increased to 2950 gpm?
Answer: 840 ml/min x 2950 = 2,478,000 divided by 2350 = 1054 ml/min
2. A water treatment facility is feeding 980 ml/min of sodium hydroxide for a raw flow rate of 4730 gpm. What sodium hydroxide feed rate is needed if the raw flow rate is decreased to 3975 gpm?
Answer: 980 ml/min x 3975 = 3,895,500 divided by 4730 = 824 ml/min
3. A water treatment facility is feeding 780 ml/min of alum for a raw flow rate of 4.2 MGD. What alum feed rate is needed if the raw flow rate is increased to 5.5 MGD?
Answer: 780 ml/min x 5.5 = 4290 divided by 4.4 = 1021 ml/min
4. A water treatment facility is feeding 1240 ml/min of alum for a raw flow rate of 5.7 MGD. What alum feed rate is needed if the raw flow rate is decreased to 4.8 MGD?
Answer: 1240 ml/min x 4.8 = 5952 divided by 5.7 = 1044 ml/min