| |
|
| |
|
| |
 |
|
These systems consist of a cylindrical tank filled with limestone chips. An acidic waste stream will react with the limestone to raise the pH. No pH instrumentation is utilized except in the monitoring system. These were the pioneer systems that were first developed until chemical-mixing systems became available. Today, the limestone systems still are applied on small individual streams where multiple chemical mixing type systems are impractical. The disadvantages of limestone systems are that they only adjust in one direction from an acidic waste stream to react with lime to raise the pH. Caustic or basic waste streams cannot be treated. Additionally, the limestone chip beds have been found to retain solids and debris, thereby restricting and stopping flow. In some cases, the fouling has caused severe odor problems. The limestone chips also become depleted requiring attention and replacement.
A good limestone application might be multiple small existing streams where a consolidated system is not possible if only one-way adjustment is required. Small individual tanks could be installed. Tanks are sized on the basis of the peak flow rate and required retention time. Maintenance of the limestone is important to keep the system functioning properly.
|
|
|
|
|
| |
|
| Liquid Chemical Mix Systems: |
|
| |
These systems consist of a cylindrical mix tank, agitator, acid metering pump, caustic metering pump, pH electrode/amplifier, and pH controller. The pH controller provides a proportional signal to pace the appropriate metering pump at a high pumping rate to a low pumping rate as the desired pH point is approached. The pH controllers should be field programmable. The pH electrode requires periodic cleaning, inspection, and calibration. As a neutral pH is approached on the titration curve, very small amounts of reagent will cause a large shift in pH. Therefore, proportional control is very advantageous. Generally, 50-93% sulfuric acid is used as the acid reagent. Hydrochloric acid (10-37%) is sometimes used although is less desirable due to fuming and building corrosion. Caustic soda (25-50%) is used as the base. Some designers have used magnesium hydroxide due to its safe, non-hazardous advantages. The tanks and pumps must be designed to prevent hardening and clogging as a disadvantage. Tank sizing will be considering in the discussion of continuous flow systems. The agitator is sized to provide 1-2 tank volume turnovers per minute. The metering pumps are sized to deliver the volume of chemistry for titration of the waste volume within the desired reaction time. The reagent chemistry should be delivered to the tank bottom in the path of the agitations to assure proper mixing. An injection valve should be used to avoid “over-shooting”. Tanks should be covered, sealed, and properly vented. |
| |
|
| |
These systems consist of a cylindrical mix tank, agitator, gas feed system, pH electrode/amplifier, and pH controller. The pH controller provides a proportional signal to control a proportional gas valve as the pH point is approached. The pH controllers should be field programmable. These systems are unidirectional when using carbon dioxide gas to reduce a pH level. The CO2 gas is delivered to the bottom of the tank through a gas diffuser. The carbon dioxide gas forms carbonic acid to reduce the pH. The advantage of the CO2 gas is that it is safe and non-hazardous without the need for handling liquid sulfuric or hydrochloric acid. Many plants have existing CO2 gas lines within the plant. Otherwise, CO2 gas may be purchased in cylinders. A CO2 gas monitor is recommended for assuring a safe breathing environment. If a bi-directional system is required, magnesium hydroxide may be used to increase pH levels and is also non-hazardous. Tanks should be covered, sealed, and properly vented. |
|
