In the United States, before wastewater can be discharged into surface waters such as lakes or rivers, it is mandatory to meet the National Pollutant Discharge Elimination System (NPDES) effluent guidelines. These federal regulations set technology-based limits on parameters like pH (potential hydrogen), TSS (total suspended solids), and BOD (biological oxygen demand) to control pollutants and ensure compliance with water quality standards. Controlling algal growth is one of the treatment and control technologies used to lower these parameters. This blog is the first in a three-part series to dive deeper into these effluent limitations and how the NPDES program controls water pollution by regulating point sources of discharge pollutants.
An Introduction to pH
pH measures the movement of free hydrogen (H+, acid) and hydroxyl (OH−, base) ions within a solution. Analysts frequently measure pH to assess water quality, using a scale from 0 to 14. Scientists classify a pH of 7.0 as neutral, values below 7.0 as acidic, and values above 7.0 as alkaline. Under the National Pollutant Discharge Elimination System (NPDES) effluent guidelines issued by the Environmental Protection Agency (EPA), facilities involved in wastewater discharges must monitor and manage pH levels to ensure compliance with federal standards and protect public health and water sources.
Why is pH important in the wastewater treatment process?
Determining pH is an essential part of the wastewater treatment process to optimise procedures like coagulation and flocculation to ensure the final effluent meets discharge quality standards under the NPDES permits. Coagulants and flocculants operate within specific pH ranges. Bacteria digesting organic material die when the pH becomes too high or too low. Alkaline conditions reduce the potency of chlorine-based disinfectants, and varying pH levels cause certain suspended solids to precipitate. Treated effluent that is too acidic may corrode pipes, leach toxic pollutants such as heavy metals, and impact marine and aquatic life. Conversely, effluent that is too alkaline can increase water hardness and cause scale build-up and mineral deposition in pipes. Consistent pH management helps facilities remain in compliance with the NPDES effluent guidelines, which set the national standards for safe wastewater discharge and effluent reduction, considering factors such as energy requirements and treatment technology.
Measuring pH for NPDES effluent compliance
You can measure pH in three ways: the electrode method, the colorimetric method, and the hydrion paper method. The most common is the electrode method that utilizes a meter and a probe. The meter measures the minute voltage differences between a reference electrode and a measuring electrode. The meter measures the resultant voltage in millivolts (mV) and converts it into a pH reading. When properly calibrated with a fresh sample using approved analytical methods, the electrode method provides the most accurate results of the three. Calibrating an electrode meter is possible through standardized buffer solutions: commonly, these have a pH of 4, 7, and 10.
The colorimetric method involves indicator reagents such as bromthymol blue and phenol red to produce a coloured solution which indicates its pH level. Red colouration suggests a solution is acidic, whereas blue colouration signifies a solution is alkaline. Using a set of colour standards, it is possible to determine the concentration strength of the solution.
The hydrion method uses litmus paper, a specially developed test paper. The user dips the litmus paper into the solution and compares the resulting colour with the colour standards, as in the colorimetric method. An acidic solution turns the litmus paper red, whereas an alkaline solution turns it blue.
Calibration and accuracy in effluent monitoring
Buffering a solution with a standardized pH 7 provides the meter with a reference point to determine a balanced solution. Including solutions with a pH of 4 and 10 within your calibration provides the meter with a reference point for acidic and alkaline solutions. A meter’s ability to determine the target buffer value is known as accuracy and is referred to as the meter’s slope.
pH electrodes wane in accuracy over time. A new pH electrode immersed in a fresh pH buffer is typical across this slope and can closely identify pH values. Older probes may be less accurate and take longer to provide a reading.
Maintaining pH probe performance over time
A new pH meter and probe assembly that is very accurate may have a slope percentage of 98%, whereas an older probe might have a slope value of less than 60%. For example, when calibrating a pH meter to meet NPDES effluent guidelines, the user may find that the meter and probe accept the pH 7.0 buffer calibration value. However, when the user places the probe into a pH 4 or pH 10 buffer, the meter often reads values significantly different from those stated for the buffer. In some cases, the meter displays an error code or rejects the calibration. Most pH probes operate accurately for about 12 to 18 months before they lose precision and require replacement.
Users must bracket the anticipated pH reading with the proper buffers if executing a two-point calibration. For instance, if the normal reading in plant effluent from industrial categories is 7.5, the meter should be calibrated with at least a pH 7 and a pH 10 buffer. If the reading turns out to be pH 6.8, you should recalibrate the meter with pH 7 and pH 4 buffers.
Users can acquire different pH buffers to use with their meter. Users may like to use an additional standard buffer, such as pH 6.76, to double-check the meter for accuracy and calibration buffers. Using an extra pH buffer, other than the calibration buffers, provides the user more confidence in the meter’s accuracy and compliance with pretreatment standards under the Clean Water Act.
Quality Control
Quality control when measuring pH is vital as it ensures both accuracy and accountability. There is a tenfold value difference between each pH unit; for example, a pH 8 is ten times higher in hydroxyl ions than a pH 7. By extension, a pH of 12 is 100,000 times more alkaline than pH 7. On the other hand, a pH reading of 5 would be 100 times more acidic than pH 7.
For optimum quality control, operators must use a pH 7 buffer as a reference point for the meter during the calibration process. Using only buffer solutions with pH 4 and 10 alone will not produce accurate results as the difference between these readings is significant.
Algal growth and its impact on pH compliance
Algal abundance is affected by pH. An increase in algal abundance is often observed when a pH is lowered, and algal abundance decreases when pH is raised. Installing real-time water quality monitoring equipment allows businesses to predict and prevent harmful algal bloom events in a cost-effective way. An example of this is the Ultrasonic Algae Control method, an eco-friendly technology to improve water quality that has demonstrated international success. By ensuring accurate pH measurement and compliance with the NPDES effluent guidelines and best practicable control technology requirements, facilities can improve water quality, prevent algal blooms, and protect aquatic ecosystems from contaminants, nutrients, and other pollutants regulated under the program.