pH Adjustment - A Primer
A brief review of the definition of pH, the pH scale, and some of the chemistry involved in pH Adjustment systems is provided below. For some this may be trivial, yet for many others this may be useful. The information provided below is typical of the background information we provide in our training classes / seminars.
By Definition pH is the measure of free hydrogen activity in water and can be expressed as:
In more practical terms (although not technically correct in all cases) pH is the measure of acidity or alkalinity of water. Measured on a scale of 0-14, solutions with a pH of less than 7.0 are acids while solutions with a pH of greater than 7.0 are bases. In very simple terms bases are used to neutralize acids, while acids are used to neutralize caustics (the term caustic and base, although not truly synonymous, are often used interchangeably ). The byproducts are normally salts (which may or may not be soluble) and water.
The task of any pH adjustment system is to adjust the pH of the process stream into the defined acceptable discharge range. In the case of an acid neutralization, caustic (NaOH) is added to the effluent stream to pH Neutralize the solution. This neutralization, or titration can be expressed as follows:
HCl + NaOH ==> NaCl + H2O
In the example shown above hydrochloric acid (HCl) is neutralized with caustic (NaOH) and yields ordinary table salt (NaCl) and neutral water. NaCl, being very soluble in water, remains dissolved in solution, and very little or no solids are generated. It is the task of the pH adjustment system to add just the right amount of caustic to achieve the end point of the neutralization that is desired. This is not quite as simple as it may seem because of the logarithmic nature of the pH titration curve (remember the definition of pH above). The titration curve shown below depicts graphically the neutralization process of HCl with NaOH as the neutralizing agent.
The curve above is the actual titration results of the pH Adjustment of an acid (HCl) with a beginning pH of 2.0 and an ending pH of 11.0. Dilute Sodium Hydroxide (NaOH) was used as the neutralizing agent. A brief glance at the curve indicates that the process of pH neutralization or pH adjustment is not particularly easy. In the above example 10 ml or NaOH was added to increase the pH from 2.0 to 3.0. An additional 1 ml was added to further raise the pH to 4.0, and another 0.1 ml added to raise the pH to 5.0. An increase of 1 pH unit requires 1/10th the amount of caustic required to achieve the previous increase.
Descending from 7.0 to 0.0, each unit on the pH scale represents a ten fold increase in acidity. In simple terms, very large volumes of a neutralizing agent are required to achieve very small results at high or low pH values. Yet at or near neutral incredibly small volumes must be added to achieve neutralization. This means that the pH adjustment system must be capable of delivering large volumes of reagent at extremely high levels of precision. Standard industrial methods for chemical addition and blending do not suffice.
The steep portion of the curve, which is the area near neutral (pH 7.0) is the range that most industrial effluents must be held to. In some cases, such as de-ionized (DI) water, this curve is nearly vertical. In the case of DI water, atmospheric CO2 mixed in with only mild surface agitation is sufficient to notably lower the pH. The point here is that the pH adjustment process is not a linear process and does not lend itself to conventional linear control mechanisms (i.e. PID). The control system and the injection mechanism must be carefully thought through and well designed.
There are two basic system architectures in use in industry today. They are batch and continuous batch systems. A brief overview of these architectures is provided below.
Continuous Batch pH Adjustment Systems uses large volumes of water, controlled to a specific pH range, to adjust the pH of an influent flow. Since these tanks are always full, and since water flows on a continuous basis, these systems have no way of shutting off effluent flow. Since there is no way to shut off effluent flow there is no guarantee that the effluent flow will be in range. However, when properly applied continuous flow through systems work very well and are simpler in design than a batching system.
Use Continuous flow through systems when:
- Influent flow is relatively constant and not characterized by large surges in flow.
- Influent chemistry is relatively constant and not characterized large swings in incoming pH
- Effluent chemistry is not critical, such as the first stage of a multi-stage system.
Example of Effluent Flows that are candidates for treatment with Continuous Batch pH Adjustment:
- Laboratory Wastes Water Treatment.
- Semi-conductor Acid Wastes Neutralization.
- Heavy Metal Reduction system where metals are precipitated at a high pH as metal hydroxides.
Batch pH Adjustment Systems actively treat industrial waste water in batches of specific volumes. A batch is contained, treated, tested for effluent compliance and then discharged. Discharge cannot occur unless the tank contents are stable and in range. Therefore the effluent stream is always within the acceptable discharge band (assuming the equipment is properly maintained). These systems are more complex, and in some cases larger, than continuous flow through systems.
Use Batch pH Adjustment systems when:
- Influent waste water flow is discharged in large batches or when there are normally large fluctuation in influent flow.
- Influent waste water chemistry is characterized by large swings in influent pH.
- Influent waste water contains concentrated acids or bases.
- Effluent pH of the waste water stream must be held within a tight band.
Examples of Effluent Flows that should be treated with Batch pH Adjustment Systems:
- De Ionized Water column regeneration.
- Barrel cleaning operations.
- Chemical processing and production operations.
A description of these two designs follows.
Continuous Batch (Flow Through) pH Adjustment Systems
The system depicted below is the simplest of the two basic architectures discussed here. The pH adjustment system shown here is for discussion purposes only and does not accurately depict the systems that are designed and built by Digital Analysis Corp. Our systems are far more sophisticated yet work on the same basic principals. In an effort to limit the scope of the discussion to an overview, a very basic / simple system is used for this discussion.
In the system shown above there is a treatment tank, mixer, acid and caustic metering pumps and a pH probe and controller. The influent flow enters the tank at the bottom and exits the tank, through a distant port at the top. The objective with port placement is to create the longest possible path between the inlet and discharge ports.
In this system the treatment tank remains full all of time. Therefore one gallon entering the influent port displaces one gallon through the discharge port. As the influent flow enters the treatment tank it is thoroughly mixed with the tank contents. If the pH of the influent varies from the tank contents (an obvious likelihood) then the influent flow will be pH adjusted through the resultant chemical reaction that occurs as the influent mixes with the contents. There will obviously be an equal and opposite reaction within the tank contents. This opposite reaction is sensed by the pH probe which provides a continuous pH signal to the pH controller. The controller then paces the appropriate metering pump to bring the body of water within the tank back into range. If the influent flow was alkaline, for example, the result would be a steady rise in the pH of the tank contents as measured by the pH probe at the discharge port. The pH controller would then pace the acid metering pump at an appropriate rate to bring the pH back down into range.
One of the biggest advantages to a system of this basic design is that it is simple and capable of handling relatively high flows. However, since the tank is always full there is no guarantee, regardless of tank size or control system proficiency, that the effluent will always be in range. After all, the pH control uses a feed-back loop, which by design does nothing until an error is sensed. If the influent flow and chemistry are high enough or strong enough then the effluent pH can easily drift out of the prescribed bounds. Therefore pH contraventions are a distinct possibility with this system since there is no mechanism to stop a discharge.
A batch system, however, treats a batch of a fixed volume and then discharges the batch only when it meets the discharge criteria.
In the system shown above there is a treatment tank, mixer, acid and caustic metering pumps, a pH probe and controller, a level sensor, and a discharge valve. The influent flow enters the tank anywhere that is convenient and exits the tank via gravity near the bottom where ever a port can be conveniently located. Although the Flow Through system shown above appears to be almost identical to the batch system shown here they operate in very different modes.
In this system the untreated influent fills the tank to the high level point as measured by the level sensor. Once the tank is full the pH adjustment process begins much in the same way that the Flow Through system performed. The difference, however, is that a large batch volume is treated in one cycle. Once the tank contents are within the acceptable discharge range, and have been for a minimum period of time the effluent Discharge Valve opens thereby draining the tank via gravity. Once the tank is drained the cycle repeats.
The advantage of a batch system is that no effluent can flow to drain until the effluent meets the discharge criteria. No matter how large or well built a Flow Through system may be there is no way to ensure that this is the case. Batch systems are far more suitable for effluents that may be characterized by large swings in influent pH, concentrated discharges, or large swings in flow rate. The majority of the systems that we install are batch systems since they are inherently safer and capable of handling streams of any composition.
It is important to reiterate that the systems shown here are simplified versions of the systems that we design and build for industry. There are many more considerations that are incorporated into system design. We have developed a technology referred to as system 'Optimization' that involves very specific criteria for tank size and geometry, chemical injection methods, probe installation and location requirements, and most importantly, control algorithms. A separate technical article on 'Optimization' is available upon request. All of our systems, whether they be Flow Through or Batch employ system 'Optimization' technology.
'Optimized' pH Treatment Technology
The throughput of a conventionally designed pH neutralization / adjustment system is limited by several major short comings. These short comings pertain to pH probe response time, mixing efficiency, tank design, chemical metering precision, chemical reaction times, and pH control intelligence. 'Optimized' pH control technology addresses each of these weak areas individually and synergistically. With the implementation of 'Optimized' pH control we are able to achieve system throughputs that simply are not possible with conventionally designed systems.
All of our pH adjustment systems employ this technology. A technical paper describing 'Optimization' in detail is available upon request.