Batch and continuous pH control compared and contrasted
Someone recently asked, “What are process controls for adjusting and maintaining pH during wastewater treatment?”
The pH is one of the most important and frequently used measurements in wastewater treatment. Dependence of pH occurs in practically every phase of water supply and wastewater treatment, e.g., acid-base neutralization, water softening, precipitation, coagulation, disinfection and corrosion control.
An explanation for pH is the measure of free acidity or free alkalinity of water. Measured on a numeric 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 and solutions with a pH of 7.0 are neutral.
The definition of pH (Potential of Hydrogen) is the measure of free hydrogen activity in water.
In very simple terms bases are used to neutralize acids, while acids are used to neutralize alkalis and the terms alkali, alkaline, caustic or base are often used interchangeably.
The pH of common solutions vary dramatically such as lime juice having a pH of <2.5 to Milk of Magnesia having a pH of >10.0.
The best pH for a wastewater treatment process depends on the ultimate water use affected by pH.
For example, the pH value for discharging treated wastewater into an environmental water stream (ocean, river or creek) often has requirements for regulated pH ranges between 6.0 and 9.0 and they may be different as compared to discharging pre-treated wastewater into a municipal sewer system having requirements for regulated pH ranges between <6.0 and >9.0.
Additionally, the optimum pH may also vary when wastewater treatment process tanks receiving untreated wastewater from a production process or pumping the wastewater into specific treatment process tanks depending on the explicit requirements for wastewater treatment the optimum pH is tightly controlled at specific values depending on the treatment procedures.
These pH values are tightly controlled for certain wastewater treatment processes maintaining pH as low as 3.0 to other specific processes stabilizing a precise pH value up to and including pH as high as 11.0.
Considerations for adjusting and controlling pH in treatment tanks to sustain unique wastewater treatment during a continuous process or a batch process are covered below:
Continuous pH adjustment system in a wastewater tank
In a continuous pH adjustment system layout the pH treatment tank operates full at all times. Consequently one gallon entering the tank influent location displaces one gallon through the tank discharge location.
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, which is likely, 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 feed-back signal to the pH controller. The controller then signals to operate the appropriate metering pump to bring the body of water within the tank back into set-point 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 tank discharge location. The pH controller would then signal to operate the acid metering pump at an appropriate rate to return the pH down to set-point range.
A major advantage to this type system is its simplicity and capability 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 set-point range. Recognize that the pH control uses a feed-back loop, which by design does nothing until an out of set-point value is sensed. If influent flow and chemistry are high enough or strong enough, then the effluent pH can easily move and remain out of the pre-programmed set-point range. Therefore, a pH control backup measure such as regular monitoring is advisable.
Batch pH adjustment system in a wastewater tank
In a batch pH adjustment structure layout there is a treatment tank, mixer, acid and caustic metering pumps, pH probe and controller, level sensor and discharge valve. Influent flow enters the tank anywhere convenient and exits the tank near the bottom, wherever the tank effluent location can be operational.
In a batch pH adjustment system, the untreated influent enters and fills the tank to the high-tank-level point as measured by the level sensor. For the untreated waste, the pH adjustment process occurs much in the same way as with a continuous system. The difference, however, is that a large batch volume is treated in one batch cycle. Once the tank contents are within the acceptable discharge range and have been for a minimum working period of time, the effluent discharge valve opens, thereby allowing for the tank to be emptied. Once the tank is empty the cycle can repeat.
The advantage of a batch system is that no effluent is removed from the tank until the effluent meets the control discharge criteria. Batch systems are far more suitable for smaller treatment volumes and effluents that may be characterized by large swings in influent pH, concentrated discharges or erratic flow rates.
The throughput of many designed pH neutralization / adjustment systems is limited by several major drawbacks. These flaws pertain to pH probe response time, mixing efficiency, tank design, chemical metering precision, chemical reaction time and pH control interaction.
Advanced procedure pH controls address each of these deficiencies individually and harmoniously. With the use of advanced procedure pH controls, increased consistent reliable results are achievable.
Certain general steps for controlling pH (Potential of Hydrogen) have been described above; however, if you have specific pH or other wastewater queries, please submit a question.
Known in the industry as “Wastewater Dan,” Daniel L. Theobald, proprietor of Environmental Services, is a professional wastewater and safety consultant/trainer. He has more than 24 years of hands-on industry experience operating many variants of wastewater treatment processing units. He is eager to share with others his knowledge about water conservation. (www.Conserve-On-Water.com).
Theobald serves as an active consultant for industries looking to achieve and maintain improved wastewater treatment at reduced cost. He holds numerous certifications from wastewater management regulatory boards and professional organizations. Theobald contributed one chapter to the Water Environment Federation’s (www.wef.org) Manual of Practice # 37 (MOP-37), a technical manual resource guide for biological nutrient removal, published the summer of 2013. He also authors an industry-related blog. (http://TheWastewaterWizardBlog.com/).