Public works departments now have available to them a selection of dependable online instrumentation that yields the accuracy and repeatability managers can rely on for critical process decisions. These high-quality, low-maintenance analyzers are proving to be an efficient means of controlling wastewater treatment in plants.
Wastewater treatment plant operators can rely on continuous flow monitoring to detect infiltration and inflow in pre- and post-rehabilitation sanitary sewer evaluations, to identify sanitary load rates of pretreated and commercial and domestic discharges, and to characterize treatment-facility loading.
Ultrasonic level-sensing flow meters automatically calculate flow at a primary device such as a flume or weir using microprocessor-controlled pulses reflected off the surface of the flow. The noncontacting ultrasonic sensor doesn't become fouled by suspended solids and grease or corroded by chemicals.
The area velocity technique calculates flow based on cross-sectional area of the flow, average velocity of the flow stream, and the basic channel or pipe characteristics. It eliminates the need for a primary device like a weir or a flume, making it appropriate for open-pipe monitoring in the collection system or other free flow, surcharged, and reverse-flow conditions.
Automated sampling is the surest way to collect pollutant loading information from key points in the collection system, to collect pretreated-discharge data for evidence of permit compliance or violation, and to collect treatment-plant discharge for compliance profiles. New autosampler technologies are capable of time-composite, discrete, flow-weighted, and flow-proportional sampling.
Set-point collection, triggered by external or internal inputs of flow, pH, rainfall, dissolved oxygen (DO), oxidation-reduction potential (ORP), and conductivity reduces sampling frequency, yet increases probability of upset detection, making it one of the most cost-effective tools in industrial discharge monitoring.
SLUDGE AND SOLIDS
Throughout settling and pre-thickening, sludge-level monitoring assists operators in determining sedimentation characteristics and managing sludge recirculation volume during normal daily fluctuations or when unexpected conditions such as sludge discharge or inadequate sedimentation occur. Monitoring the sludge level also improves the safety of the treatment operation, especially with variable sewage loads, and reduces effluent solids discharge. Ultrasonic sludge-level monitors are preferred because they define both height and depth of the sludge blanket without the sludge contact that fouls the sensor.
Solids management is one of the most critical aspects of operating an activated-sludge wastewater treatment plant. Output from online suspended-solids sensors sent to variable-speed pumps, allows operators to automatically slow down or speed up the return activated sludge (RAS) flow to control the biomass of beneficial microorganisms returned to the aeration basin. Monitoring suspended solids in the mixed liquor of primary clarifier effluent and RAS helps to achieve aeration basin solids concentration of 0% to 1.2% that results in efficient air-blower operation and effective removal of organic pollution.
Real-time readings of suspended-solids content of thickener feed and filtrate or centrate allows on-the-spot adjustments to incoming volume and polymer dosage that can make solids removal more effective and consistent. Sludge pumping into an aerobic or anaerobic digester based on solids concentration prevents costly over-pumping and prevents hydraulic or organic overload from occurring.
Today's advanced suspended-solids sensors measure the backscatter of dual infrared light beams with two detectors—one at 90 degrees and another at 140 degrees—to measure suspended solids accurately in heavily loaded streams. These sensors are immune to color interference and feature integral wipers to reduce maintenance and error due to fouling.
The activated sludge treatment plant must maintain sufficient DO for the microorganisms that break down organic suspended solids. Yet DO levels that are too high can result in pin floc in clarifiers, severe sludge bulking in some instances, and large amounts of wasted electricity. An in-basin DO measurement system with output to a variable frequency drive or the plant control system provides efficient control.
Traditional DO sensors use electrodes, electrolytes, and a DO-selective membrane that facilitates oxygen migration to the cell from the sample. These cells can suffer measurement error due to contamination through the membrane and, at low oxygen levels, error due to low signal-to-noise ratio. A luminescent dissolved oxygen (LDO) probe eliminates electrochemical components by relying on excited luminescent material in its sensor that emits light as it relaxes, at a rate proportional to oxygen concentration. With this probe, operators see significantly less probe maintenance and calibration.
Online nutrient monitoring at the plant influent allows feed-forward control of chemical dosing. Throughout aeration and at the exit of the anaerobic zone, monitoring identifies microbial uptake of phosphorus (P) and release of P from phosphorus-accumulating organisms, respectively. Monitoring helps operators optimize biological nutrient removal, control ammonia returning to the head of the plant in dewatering supernatant, and avoid unintentional denitrification and floating sludge in settling tanks.
An ammonia gas sensitive electrode (GSE) offers the wide range of an ammonium ion selective electrode (ISE) and is less subject to interferences. Colorimetric analyzers do not require the periodic electrode cleaning and/or replacement necessary with ISE- and GSE-type analyzers but do consume reagents. Ultraviolet-nitrate analyzers rely on the fact that nitrate in water absorbs ultraviolet light; a second wavelength compensates for interference from suspended solids, and, to some degree, organics.
In many cases, operators can track phosphorus reliably with relatively inexpensive orthophosphate (PO4) analyzers instead of more-complex total phosphorus instruments. In all cases of nutrient monitoring, operators should consider the sample conditioning necessary to eliminate interference from grease or oils, turbidity, color, magnesium, calcium, amines, heavy metals, and pH.
General online monitoring issues
Here are some key issues to consider when selecting monitoring instruments:
Online pH sensors help operators control acid/base addition in primary effluent or mixed liquor, enhance phosphorus removal by alum addition, and maintain optimum conditions for nitrification/denitrification. However, many pH electrodes can provide erroneous readings of up to 2.0 pH units due to reference solution dilution, deposits on the reference electrode, or ground loop current flowing through the reference electrode.
A differential-technique pH sensor overcomes these problems by using a glass measuring electrode and a reference assembly incorporating a second glass pH measuring electrode immersed in a 7 pH buffer solution. A double junction “salt bridge” minimizes dilution in the inner chamber. Precipitates forming on this salt bridge, in series with the high-impedance glass electrode, cause minimal resistance. Ground loop currents, passing through the third “ground electrode” instead of the reference electrode, do not affect pH signal output.
The nonspecific ORP measurement is a positive voltage (mV) value in a sample containing a strong oxidizing agent like chlorine and a negative mV value in a sample containing a strong reducing agent like sodium bisulfate. ORP is well-applied in tightly controlled environments such as a sequencing batch reactor to signal the end of process stage or to control odor.
Operators who establish a relationship between mV readings and pH and total chlorine residual readings in the chlorine feed stream can rely on an ORP sensor installed about two minutes downstream from a well-mixed chlorine feed point for control purposes. In the biological nutrient removal process, operators can correlate a range of ORP values defining the oxidative state of the process, with the lowest ORP values signaling anaerobic areas and the highest values reflecting aerobic conditions in the process.
Online measurement of chlorine as frequently as every 2½ minutes allows the best control of disinfectant dosage in the variable presence of chlorine-demanding substances, hardness, iron, pH, nitrate and nitrite, and total suspended solids. In “feed-forward” dechlorination control, an online total residual chlorine (TRC) signal sends a mass flow signal to the sulfonator to calculate and deliver the proper dechlorinating dose that results in zero TRC concentration. Processes not required to dechlorinate effluent to zero level can use a “feedback” control system where TRC monitoring at a point downstream of dechlorinating agent addition signals—along with flow-rate data—the sulfonator for dosage adjustment.
For processes that must maintain TRC discharge between 0.002 and 0.050 mg/L, operators can use a “zero-shifted” or “biased” analyzer, in which a known concentration, X, of chlorine is added to the effluent sample. The analyzer shifts the “zero” point by the value of X, allowing operators to infer the actual low-level TRC value. For public works facilities that can accommodate a hydraulic control scheme and are not required to demonstrate TRC levels less than 35 parts per billion (ppb), operators correlate ORP readings to chlorine when the TRC levels hover near 0 ppb and to sulfite when its levels are less than 2 ppb.
Colorimetric-technique chlorine analyzers rely on color development directly related to chlorine concentration when DPD (N, N-diethyl-p-phenylenediamine) reagent mixes with a discrete sample. An amperometric total chlorine residual analyzer requires routine cell cleaning, except for those designs that include a ball or beads that continuously clean the surface of the electrode.
— Phil Kiser, Greg DeSantis, and Mike Rousey are all technical managers, wastewater applications, at Hach Co., Loveland, Colo.
The luminescence dissolved oxygen (LDO) probe does not require electrodes, electrolyte, or selective membrane. Instead, blue light from an internal LED strikes a luminescent chemical coating the LDO probe sensor. The luminescent chemical instantly becomes excited and then releases red light. A photo diode detects the emitted red light, and the time required for the chemical to return to a relaxed state—dependent on oxygen concentration—is displayed directly as milligrams per liter (mg/L) DO.