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A technical comparison of different pH neutralization
system designs used in wastewater treatment.
By Ed Simpson
Burt Process Equipment
The requirement for pH adjustment or neutralization is a common demand that spans various industries, including manufacturing facilities, research laboratories, food processing and bottling plants, and schools and universities.
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 in some cases. 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 Neutralization Tank
Diagram shows how a simple limestone ph adjustment tank operates. These tanks can be installed above or below ground.
Limestone chip tank systems function on a flow-through basis and generally involve a vertical cylindrical tank, which is filled with calcium carbonate, more commonly known as limestone. The limestone chips will raise the pH of acidic waste streams.
The chip tank has an inlet fitting and downpipe to direct the 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. Often, the discharge tank fitting will be plumbed to a U-trap assembly with a pH sensor for effluent monitoring.
These systems are most commonly seen in lab settings, and they are sized based on the number of sinks connected to the system. As a general rule of thumb, the tank volume (gallons) is 3-4 times the number of sinks. For example, if you have 50 sinks, the tank would be 175 gallons. Limestone is available in 50-pound bags, each occupying 0.5 cuft. Knowing that 1 cuft. is 7.481 gallons of water, we can determine that 13.4 lbs of limestone are required per gallon. In our example, that would result in over 2,300 lbs of limestone!
Unfortunately, the limestone breaks down over time and needs to be replaced. There are advantages and disadvantages of this, the most basic pH adjustment system.
The main advantages of these systems are that they are inexpensive and can be designed to handle multiple low-flow waste streams. Disadvantages include that it is 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 (cleaning the tank or changing out the limestone) is costly and requires a system shutdown.
The preferred method for pH neutralization of waste streams is by the automatic addition of acid or caustic. The ideal setup is a batch neutralization module. A basic batch system includes a treatment tank with a mechanical agitator (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. This is the ideal scheme in that you are sure the discharged waste has reached the desired pH range and eliminates the chances of pH excursions.
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 that the tank can be drained upon completion of a batch, it may take between 15-45 minutes on average to process a batch. Looking at an example with a 25 gpm flow rate, this would require a surge or collection tank that would be over 1,125 (25 gal/min x 45 min) gallons just to keep up with the batch system. In addition, the surge tank would require automated pumps/valves and a level control to interlock with the batch tank to transfer waste on demand. Not only are additional controls required, but also the system as a whole requires a large amount of floor space, which is generally at a premium.
Batch Neutralization System
A batch pH neutralization system uses a single treatment tank with control valves to fill or drain the tank depending on whether optimum pH levels have been achieved. Shown above is a batch pH adjustment system custom-engineered by Burt Process Equipment.
Continuous Flow pH Neutralization
Shown here is the Burt Process Equipment pH Plus continuous flow pH adjustment system. It accommodates flows as high as 25 GPM and wastewater pH levels anywhere between 4-12. The pH Plus can also be run with automatic batch controls as an option.
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 is automatically adjusted as it flows through the tank. This setup uses the same mixer, pH sensor, and metering pumps but does not 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 upon the make-up of the waste stream. For example, a 100 gpm flow would require 100 gpm x 25 minutes = 2500 gallon tank.
Often flow-through systems are designed in multiple stages to reduce the tank size. Using the same 100 gpm example, a dual-stage system would employ two 1250 gallon tanks, each with a 12.5 minute retention time. Multiple stages are also used if the pH of the influent 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 at varying pH or if some streams are at elevated temperatures. The equalization tank provides some self-neutralizing of the pH (reducing chemical usage and cost) and also equalizes the temperature of varied incoming streams.
The main disadvantage of a flow-through system is that the effluent pH is not guaranteed to be within specification. If a careful analysis is not done up front and the system is 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 reagents, will not prevent the discharge of out-of-specification waste as it would in a batch system.
Now that the basic system configurations have been reviewed, what are the 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 the temperature is not 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 degrees F, polypropylene is suitable up to 180 degrees F, and fiberglass can be used with solutions seeing spikes of over 200 degrees F with the appropriate resin. Whichever material is used, the tanks share the same features.
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.
Continuous Dual Stage Neutralization
A continuous dual-stage pH adjustment system by Burt Process pumps or gravity drains wastewater into a treatment tank to be automatically adjusted as it flows through the tank. This system uses a mechanical mixer, an in-tank pH sensor, and metering pumps for acid and caustic injection, but does not require an automated discharge valve/pump or a level control to run the system.
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 the incoming waste and the reagent being added. Failure to achieve proper mixing can result in stratification in the tank and effluent excursions. The mixer should also be positioned facing the inlet.
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 degree 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. In addition, mixers are available with sealed flanged connections for applications where odor may be an issue.
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 reccomended.
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 the setpoint. This prevents an overshoot on the pH and also prevents the system from 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.
Metering pumps are available in various sizes, from units in the gpm output range to the gph output range. Ideally, the metering pump sizing should be based on titration with the actual waste and the reagents to be used. Oversizing a metering pump is just as dangerous as under-sizing due to the danger of overdosing of the pump(s).
pH sensors are generally mounted on the top of the tank and
should be inserted to approximately 60% of the depth of the tank. They should
include an automatic temperature compensation circuit and be mounted in such a
way that 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 wet well 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 switchover if one of the probes fails. On a single pH probe system, a failed probe can cause the system to possibly overdose on 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. Limestone systems are still used in low-flow acidic lab waste streams where inexpensive one-way treatment is required. Batch systems are employed where concentrated or widely varying pH streams exist and where space allows. Higher flow systems utilize continuous flow units designed in one, two, or three treatment stages in series. 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.