Thanks to advanced Total Suspended Solids (TSS) monitoring technology and pump automation, power plant operators now have the ability to accurately ensure that discharge water is in compliance, enabling them to reduce manpower costs while ensuring all discharge guidelines are being met.
By Dave Fraley, Chris Bauman, and Dave Donahue, Xylem
TSS is the major parameter for verifying effluent quality in regulatory discharge permits for coal ash remediation. The necessity to consistently meet TSS discharge limits and avoid fines and other potential liabilities today requires significant organizational structuring involving field data collection, verification, documenting and reporting.
Testing Lag-Time Raises Risk
Most coal combustion residual (CCR) operations today either retain or release plant effluent based on laboratory TSS test results. These test results require at least a day to complete, but typically take as many as two or three days. The existing TSS testing method for compliance verification is a time-consuming laboratory test that cannot currently be completed in the field, unless by mobile certified field labs. This inherent lag-time between sampling and verification brings unknowns to the remediation process, often necessitating more field sampling and testing (erring on the side of over-compliance) than a plant’s permit requires. It also raises the potential for risky bottlenecks in overall remediation operations.
As part of their permit specifications, most plants have a maximum allowable discharge flow rate – only a certain number of gallons can be discharged in any 24-hour period. The permitted discharge volume depends primarily upon the regulatory designation of the receiving stream and the characteristics of the plant’s wastewater effluent. Once a plant has met its allowable discharge for the day, the plant’s pumps must be turned off until the next day.
These discharge permits typically contain both a daily limit and a monthly limit for TSS. Most remediation operations have also established their own internal limit, or daily high, that is set far enough below the regulatory permit limit to provide a margin of protection against violations.
The inherent lag-time between gathering field samples and final laboratory TSS analysis has necessitated this expensive and burdensome “overcompliance” strategy at many facilities. For example, a lab result from a sample pulled two days earlier shows TSS levels above the facility’s internally established limit or, worse, its permit limit. At this point, operations will stop discharging for an extended period of time to compensate. Because TSS monthly regulatory reporting is based on a timeweighted average, the plant receives a zero for TSS discharge reporting for those days when pumps were off, thereby averaging out daily TSS levels to meet the monthly permit requirement.
However, when the facility is in a position where it needs to discharge but cannot, the retaining basin or pond is still filling up with the water that must be pumped. Due to its daily maximum allowable discharge limit, a plant cannot get those zero days back by increasing the pump schedule other days to make up the difference. It is a vicious cycle. Excessive zero days can lead to high containment levels that can further exacerbate the potential for very serious issues such as a pond breach during hurricanes and other severe weather events.
Depending on their permits, some locations test TSS levels several times per day. Other locations test several times a week. In either case, testing requires manpower to pull samples. In addition, the facility must rely on an outside laboratory or perform the test in-house, which requires having a certified lab. To shave off some lag-time, some facilities contract mobile certified labs to remain onsite full time to handle effluent testing. Regardless of how lab tests are run, it is an expensive and labor-intensive process.
Probe-Based, Real-Time Measurement
Reducing this high cost and compliance burden associated with challenging wastewater operations has served as a key driver to technical innovation in recent years. Some coal fired power plants have successfully adopted the use of recently advanced sensor technology to measure real-time TSS to verify their discharge water is meeting specified requirements.
TSS is a physical laboratory measurement; it defines the actual weight of suspended material in a given volume of water. The measurement derived from the optical sensor is not an absolute unit. However, TSS has a linear relationship with the scattering and backscattering coefficients of light. Light-scattering technology differentiates samples based on refractive index, size, shape and composition. Backscattering is a method of light scattering measurement that correlates with TSS. The field probe reads the light measurements as a suspended solids value based on the calibration against laboratory grab sample analysis.
Scheduled laboratory TSS testing continues to be performed to meet permit requirements, but real-time, probe-based TSS monitoring eliminates the need for frequent extra sampling and laboratory testing. Through rapid and consistent in-the-field TSS monitoring, these facilities are reducing compliance risks and costs. Based on real-time TSS monitoring results, facilities can also automatically adjust pump operations to avoid costly project delays.
TSS in water can be detected by a number of optical sensing techniques, and it has been understood for many years that TSS and the backscattering coefficient correlate well. However, attempts to use early instruments using this method were met with limited success in harsh environments, such as coal ash remediation. It is important to note that these early poor performances were due to shortfalls in electronics, light source, materials of construction, and control algorithm. Difficulty keeping the sensor’s lens clean while operating in real-world environments also contributed to sensing limitations of early units.
The pressures faced by both industries and municipalities to meet new, increasingly stringent limits prompted companies to concentrate their research and development efforts towards further improving and stabilizing the light scattering method for TSS prediction and protecting the probe against interferences brought by severe operating conditions. It is the goal of many technical teams to successfully eliminate and compensate for the most common sources of instability through the use of light-scatter and backscatter measurement methods, advancements in LED technology, the development of a comprehensive algorithm to determine the relationship between measured TSS and predicted TSS, and an advanced sonic lens cleaning system. An example of this new technology is Xylem’s YSI IQ SensorNet VisoTurb 700 IQ® Suspended Solids Probe, which is being used in harsh wastewater streams to consistently and accurately measure TSS in real time with a repeatability of <0.015% or >0.0006% FNU.
The ability to monitor TSS in real time and obtain accurate results that are repeatable and verifiable with random lab tests, also enables users to automate functions such as controlling pumps transporting effluent, or starting and stopping pre-treatment processes designed to keep effluent within permit guidelines.
Real-time TSS measurement, for example, can allow for automated polymer dosage control based on the probe’s continuous TSS measurements. If effluent TSS levels get too high, systems have been designed using the probe to automatically stop the transfer pump and instead recirculate the flow within the retaining pond while automatically dosing a flocculent, thereby making the solids further accumulate and drop out of solution. The same control system serving the TSS probe can also simultaneously measure pH (by adding a pH probe). If effluent pH is too high or too low, the controller can be programmed to turn on a small pump to add acid or base for pH adjustment to ensure upper and lower pH permit limits are being met.
Many coal ash remediation sites have been decommissioned and are now an added expense to owners; much of this cost is attributed to manpower requirements. Real-time TSS monitoring coupled with automation requires fewer employees to be onsite. In addition to the reduction in sample gathering and laboratory testing, additional staff is not needed to go out and turn a pump on or off, or open and close a valve. Real-time TSS measurement allows these functions to be automated, providing owners the necessary oversight without having to be physically present on site.
An ‘Optical Fingerprint’
Accurate online, optical sensor-based TSS monitoring provides real-time measurements of effluent TSS that can be used as a pump operation and management parameter for complying with regulatory requirements.
Continuous TSS measurement can also provide plant owners the potential for significant operational efficiencies and cost reductions. Sampling and laboratory testing exceeding the number of random samples required to meet permit are no longer necessary, freeing up manpower and laboratory costs. The long-term reliability of the measurement also allows for the automation of certain functions such as to control transport pumps and valves, and to start and stop chemical pre-treatment processes.
By integrating next-generation TSS monitoring technology as part of their temporary pre-treatment solutions, power plant operators are now able to meet discharge limits while controlling costs.