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Pumps & Systems, October 2007

The requirement for pH adjustment or neutralization is a common demand that spans various industries, including manufacturing facilities, research laboratories, food processing, bottling plants, 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. A number of different treatment schemes and equipment can be applied to prevent effluent excursions, including limestone chip tanks, batch treatment systems, and single-, dual-, and three-stage continuous flow systems.

Limestone Chip Tanks

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 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. Often, the discharge tank fitting is plumbed to a U-trap assembly with a pH sensor for effluent monitoring. 

These systems are most commonly seen in lab settings and are sized based upon the number of sinks connected to the system. As a general rule of thumb, the tank volume (gallons) is three to four times the number of sinks. For example, if you have 50 sinks, the tank would be 175-gal. Limestone is available in 50-lb bags, each occupying 0.5-ft³. Knowing that 1-ft³ is 7.481-gal of water, we can determine that 13.4-lbs of limestone are required per gallon. In our example, this 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 to this 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 maintenance (cleaning the tank or changing out the limestone) is costly and requires a system shutdown.

Batch Treatment Systems

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, when 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, 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 ideal scheme ensures that the discharged waste reaches the desired pH range and eliminates chances of pH excursions.

A typical batch system consists of a treatment tank with a mechanical agitator, 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.

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, this may take between 15 minutes to 45 minutes on average to process a batch.