by Ed Simpson and Dave Carson
The requirement for pH adjustment or neutralization is a common demand spanning various industries. While discharge pH limits vary from region to region, the side effects of discharging outside these limits are the same: possible fines and damage to the environment. There are a number of different treatment schemes and equipment that can be applied to prevent effluent excursions. These include limestone chip tanks, batch treatment systems, and single-, dual-, and three-stage continuous flow systems.
Limestone chip tank systems function on a flow-through basis and generally involve a vertical cylindrical tank, which is filled with calcium carbonate, commonly known as limestone. The limestone chips raise the pH of acidic waste streams. The chip tank has an inlet fitting and downpipe to direct influent to the bottom of the tank. The tank is filled almost to the top with limestone chips and the influent percolates up through the chip bed until it reaches the overflow fitting on the opposite side of the tank.
The main advantages of these systems are that they're inexpensive and can be designed to handle multiple low-flow waste streams. Disadvantages include that it's only a one-way pH system (cannot handle high pH streams), it cannot handle concentrated waste dumps, the tank may foul and generate bacterial growth and odor, and finally, maintenance is costly and requires a system shutdown.
The preferred method for pH neutralization of waste streams is by automatic addition of acid or caustic. The ideal set-up is a batch neutralization module. A basic batch system includes a treatment tank with a mixer, an in-tank pH sensor, an in-tank level control, metering pumps for acid and caustic injection, an automated drain valve or pump for the effluent, and a control panel. In a batch cycle, the treatment tank fills until it reaches its start point where the mixer is energized and acid or caustic is proportionally added until the pH is within the desired range. At this point, the tank goes through a dwell cycle of a few minutes to ensure the pH is maintained and then the tank is dumped via an automatic drain valve or pumped down. Once the tank has emptied, it is ready for a new cycle.
What are the downsides of a batch treatment system? First, unless the waste is generated in batches itself, some provision must be made to store incoming waste when the system is in a batch cycle. Depending upon the speed the tank can be drained upon completion of a batch, it may take between 15-45 minutes on average to process a batch. So additional controls are required, and the system as a whole would consume a large amount of floor space, which is generally at a premium.
The next type of pH adjustment is a continuous or flow-through system where wastewater is pumped or gravity drained into the treatment tank and automatically adjusted as it flows through the tank. This set up uses the same mixer, pH sensor and metering pumps, but doesn't require the automated discharge valve/pump or the level control to run the system. A continuous flow system can handle large flows, up to and over 1,000 gpm. This style system is designed for a retention time of 20-30 minutes, depending on waste stream make-up. Often flow-through systems are designed in multiple stages to reduce tank size. Multiple stages are also used if influent pH is greater than 2 pH units from the desired discharge range (two pH units being the maximum adjustment range per stage on a flow through basis). In this case, the first tank serves as a course adjustment, and the second stage serves as a fine adjustment of the pH.
In addition, an equalization tank may be provided upstream of the pH tanks. This is useful when multiple streams are being treated of varying pH or if some streams are at elevated temperature. The equalization tank provides some self-neutralizing of the pH and also equalizes the temperature of incoming varied streams.
The main disadvantage of a flow through system is that the effluent pH isn't guaranteed to be within specification. If careful analysis isn't done up front and the system not properly sized or the waste profile changes, excursions can occur. In addition, equipment failures such as probes or metering pumps, or even not replacing reagent won't prevent the discharge of out-of-spec waste, as it would in a batch system.
Now that the basic system configurations have been reviewed, what are details of the common components they share? The treatment tanks are constructed from thermoplastics, either HDPE (high density polyethylene) or natural polypropylene, or sometimes stainless steel or fiberglass. In smaller systems where temperature isn't an issue, a molded HDPE tank is most economical for volumes <500 gallons. Larger tanks can be fabricated in polypropylene up to 2,000 gallons. Over that size, fiberglass tanks are recommended due to the tank weight and structural reinforcements required. HDPE tanks have a temperature limit of 140°F, polypropylene is suitable up to 180°F, and fiberglass can be used with solutions seeing spikes of over 200°F with the appropriate resin.
An inlet downpipe or baffle is used to direct flow to the bottom of the tank. Installed in this baffle are the chemical injectors from the metering pumps. Most systems utilize solenoid driven electronic diaphragm metering pumps, which are supplied with their own spring loaded injector valve. These are installed near the bottom of the tank via tubing connections. Installation in the inlet baffle eliminates the chance of the tubing being caught in the mixer prop. The tanks include a cover, either bolt-down or welded with a vent fitting and access manway. The cover also includes guide rails, generally epoxy coated carbon steel for the mixer support. The mixer itself is either a direct-drive or gear-driven unit with T316LSS wetted parts. The mixer should be designed for a tank turnover rate between 1.5-2 turnovers per minute. This ensures rapid mixing of incoming waste and the reagent being added. Failure to achieve proper mixing can result in stratification in the tank and effluent excursions.
In vertical cylindrical tanks, the tank can be vertically mounted with anti-vortex baffles installed inside the tank; or, if it has an angle bracket (generally 10-15° pitch), it can be mounted 1/6 of the tank diameter off center. Various mixer configurations and mixing rates are available to meet the needs of varying tank sizes.
The standard adjustment chemicals are sodium hydroxide (to raise the pH) and sulfuric acid (to lower the pH). Sodium hydroxide can be used at concentrations between 20% and 50% and sulfuric acid is generally used at concentrations of 50% and up. The exact concentrations may be determined by the fact that one or both of the chemicals may already be in use at the facility. If not, a 50% concentration of each is recommended. The metering pumps should include a proportional control input for the frequency, or speed of the pump. This input can be either a proportional pulse or a 4-20mA signal depending upon the pump brand. The input to the pump is set such that the pump runs at full speed as the pH is furthest away from the setpoint and slows down as it approaches this point. This prevents overshoot on the pH and also prevents the system going into an oscillation created by the system itself. Care must be taken not to overlap the control points for the acid and caustic addition just for that reason. Ideally, the metering pump sizing should be based upon a titration with the actual waste and reagents to be used.
pH sensors are generally mounted on the top of the tank and should be inserted to about 60% of the tank depth. They should include an automatic temperature compensation circuit and be mounted so they can be easily removed for cleaning and calibration, which is required at least once per month. On large tanks, pH sensors can be mounted through a sidewall fitting with a wetwell retraction assembly to facilitate maintenance. The retraction assembly allows the sensor to be removed under pressure. Systems may also include two or three probe setups for alarming and automatic switch over if one of the probes fails. On a single pH probe system, a failed probe can cause the system to possibly overdose chemistry and create a pH excursion on the discharge.
The options for pH neutralization systems range from a basic limestone tank to a batch treatment system to a multi-stage continuous flow unit. Whichever treatment mode is used, the more upfront data that can be gathered on the expected waste flow rates, temperatures, and waste stream constituents, the better the final system will operate.
About the Authors: Ed Simpson is manufacturing director and Dave Carson is industrial sales director at Burt Process Equipment, of Hamden, CT. Founded in 1970, it's a worldwide leader in designing, manufacturing and distribution of high purity and corrosion resistant equipment, systems and services that provide solutions for, among others, the pharmaceutical, biotechnology, microelectronics, metal finishing and chemical processing industries. Contact: 877-742-2878, [email protected] or www.phplus.com