The last 1-2 decades have provided the wastewater sector with unprecendented high quality and trustwrthy sensors. Though some
of the sensors still require close performance monitoring, the sector has experienced a great leap forward due to development efforts in the sensor companies. The next leap forward is the responsibility of future Smart Water Utilies, applying the sensors intelligently.
1. Dissolved oxygen
It is hard to imagine that it was once a difficult job to convince wastewater treatment plant operators to apply online dissolved oxygen sensors in their plants. Dissolved oxygen measures the amount of oxygen in the water, which is a pre-requisite for aerobic biological activity for the degradation of organic matter as well as nitrification.
The saturated (maximum dissolved) concentration of dissolved oxygen in water depends primarily on temperature and is in the range of 8–10 mg/l. Maintaining a DO setpoint around 1.5–3 mg/l in the aerobic tanks ensures aeration adjustment according to load. Today many plants have several DO sensors in the aerobic line as well as sensors in parallel lines to ensure proper aeration at all times.
DO is traditionally measured by the Clark principle, whereby a reaction chamber is isolated from the water by a membrane that allows oxygen molecules to penetrate.
The generated current between a silver anode and a gold cathode in an electrolyte solution of potassium chloride gives a signal proportional to the oxygen concentration dissolved in the water.
New sensor developments allow for membrane free DO sensors. These sensors are based on luminescent principles and require no calibration for years of operation or any maintenance (besides cleaning).
2. Total suspended solids
Total suspended solids (also named TSS or SS) are measured in milligrams per litre and refer to the content of solids that are not fast settling out of the water. In a laboratory, this parameter is measured by filtering a water sample on a pre-weighed filter. After filtering, the filter is dried and weighed again with the suspended solids on top of it. The difference in the weight divided by the amount of water filtered is the measurement of the suspended solids.
This laboratory procedure is obviously not well suited for implementation in an online sensor. Instead, the same method as for turbidity is used, i.e. reflected light from the sample is measured. For suspended solids measurements, various reflection angles are measured and infrared light is used, i.e. light with longer wavelengths than visible light.
When selecting the actual sensor it is important to have a clear idea about the measurement range required for the sensor to be able to give data during all conditions.
Air bubbles and temperature are known to interfere with the measurement; some sensors have different ways of compensating for this interference.
72 sMart water utilities: Complexity made simple
3. Ammonia
Ammonia content is central to the
understanding of the process of nitrification which is part of the overall nitrogen removal process. In nitrification, ammonia is transformed to nitrate while reacting with (and hence consuming) dissolved oxygen.
Some people speak of ammonia (NH3) and others of ammonium (NH4+). pH determines the actual distribution between the two; the expressed value is the sum of the two. This makes good sense because in this context the interesting part is the nitrogen atom that the nitrification and denitrification process is removing by locking it in an N2 molecular structure, which will leave the water phase as gas. Often ammonia is expressed as mg/l NH4-N. This means that only the nitrogen content is calculated in the milligrams per litre unit. This is a convenient way of expressing the content because it means that 1 mg/l NH4-N is converted into 1 mg/l NO3-N by nitrification. This would not the case if the concentrations were expressed as NH4 and NO3 respectively because of the difference in molecular weight. In laboratories, ammonia is usually measured by means of colorimetric methods (FIA). However the sensor arena today is dominated by the ion sensitive electrode (ISE), converting the concentration into an electrical potential by means of specialised electrodes. The clear advantage of this method is that it does not require any small scale pumping, which has previously been prone to failure.
The disadvantage is that no electrodes are completely selective, so the signal may be interfered with by other substances. In the case of ammonia, potassium may have an effect on the accuracy.
4. Nitrate
Nitrate is the other critical substance in the nitrogen removal process. Nitrate is formed during nitrification and removed during denitrification. Denitrification leadsto the gaseous N2 that leaves the water and hence essentially removes the nitrogen from the water. This removal makes the water less potent as a eutrophication agent (eutrophication is the addition of too much nutrients to the aquifers with detrimental effects on the aquatic quality).
Nitrate sensors are either based on ion selective electrodes (see ammonia) or UV (Ultraviolet) absorbance. Visual light/
colours are electromagnetic waves with wavelengths in the range 390–700 nanometres (nm). UV is defined as the spectrum in the range of wavelengths from 185 to 380 nm. Nitrate molecules absorb UV light intensively at a wavelength of around 220 nm; hence, measuring the absorbance at this wavelength gives a good indication of the nitrate concentration in the water. Unfortunately, some commonly present organic matter molecules absorb light at the same wavelength, which may confuse the measurement somewhat. To counteract this interference, a reference measurement of 275 nm is used to give an idea of this interference. Using both results in the calculation provides a “fairly”
precise and very easy way to perform online measurement of nitrate.
5. Phosphate
Phosphate is the third of the three nutrient ions usually measured online in wastewater.
Phosphorus is the second of the two most important nutrients leading to eutrophication (the other being nitrogen). Phosphate is either removed chemically by precipitation or biologically by the micro-organisms taking up the phosphorus in their cells. In both cases the phosphorus is removed together with the sludge. While phosphorus is not in the solid phase it is dissolved in the water primarily as phosphate. While historically nitrate and ammonia measurement methods were based on colorimetric methods, these have been replaced by methods better suited for implementation in sensors (ion selective).
So far however, it has not been possible to find a robust and easier method for the detection of phosphate. Hence, most sensors for phosphate are still based on colorimetric methods. This means the addition of a chemical, which by reaction with the ion, in this case phosphate, colours the water. The intensity of the colour is then detected by a photometer which translates this into the concentration of mg/l PO4-P.
6. Organics
One of the main purposes of traditional wastewater treatment is the removal of organic matter, which otherwise would consume oxygen from the recipient during degradation. The difficulty in measuring organic matter lies in the great multitude of compounds that make up the pool of organic matter.
To evaluate the effect that the organic matter puts on the recipient, two parameters have been developed to quantify the organic matter: the chemical oxygen demand (COD) and the biochemical oxygen demand (BOD). These parameters describe how much oxygen is required to degrade the water sample’s content of organic matter – either through full chemical oxidation by a strong oxidant or through bacteriological degradation. The COD is always higher than the BOD, as the difficult degradable organic matter is not degraded biologically.
Instead some sensor companies are developing parameters that approximate COD and BOD. One method is based on UV measurements as many organic compounds absorb UV radiation. A formula is developed by selecting wavelengths at which different organic compounds absorb light. Then statistical methods are applied to calibrate the sensor towards the COD and BOD content measured in water samples at the site.
74 sMart water utilities: Complexity made simple
7. Sludge level
Sludge level means the level in a sedimentation tank above which there is clear water and below which there is sludge at a high
concentration. If the sludge level moves upwards there is a risk that the sludge will escape out of the sedimentation tank together with the effluent water – which is exactly the opposite of what is the purpose of the sedimentation tank.
Additionally, it should not go too low as that would impair the filtering effect of the sludge – depending on sedimentation tank design.
In reality it is not always so easy to define the sludge level, because the sludge concentration has a vertical gradient all through the
sedimentation tank. Under steady-state conditions the concentration of sludge is largest in the bottom of the settler and decreases all the way up to very low levels in the clarified water at the top. Defining at which shifting point exactly in the sludge concentration the sludge level is located is not trivial. This is further complicated by different compaction characteristics of different types of sludge and the occasionally non-steady state behaviour of the settler.
Ultrasonic measurement is a common online method for sensing sludge level. Ultrasound is sound with higher frequencies than the human ear can hear. The ultrasonic method for sludge level sensing sends an ultrasound impulse into the water; the time it takes for the sound to be reflected by the sludge levels is used as an indicator of the sludge level’s position. By applying different sonar frequencies different levels can be detected in the same situation.
These differences correspond to using different concentration shift points in defining the sludge level.
A new sludge level sensor generation uses a near infrared optical measurement head. As the head is lowered in the clarifier or thickener it will continuously measure the suspended solids versus liquid depth and how the amount of suspended solids varies in the clarification zone.
8. Respirometry
Respirometry received a lot of attention from the research world for some decades without really gaining momentum in the utilities. The idea of respirometry is to make a respirometric experiment with sludge (microorganisms) in a batch reactor with well controlled conditions.
By monitoring the respiration, i.e. the rate of change of dissolved oxygen, various parameters can be estimated. Depending on the conditions, parameters such as easily degradable organic matter, bacteriological growth rate and inhibition due to for example toxicity can be estimated.
In some cases, the full-scale aerobic reactor has been used as a respirometer to obtain the required information. This can be done by monitoring the airflow rate to the system and monitoring the rate of change in the reactor, while controlling the dissolved oxygen concentration. By means of mathematical models it is then possible to calculate fairly precise estimates of the respiration rate.
9. Airflow
Online measurement of airflow in a diffusor type aeration system may be used as part of the controller system to ensure more stable control of the aeration process.
The measurements can also be used to compare the load in various compartments of the biological system, for example, to see if the load is the same in two parallel identical lines – as it should be. If aeration is controlled to the same DO setpoint and the suspended solids concentration is similar, the required air flow should be the same.
There are at least three different ways of measuring air flow in diffusor systems:
By means of differential pressure across a pipe restriction, it is possible to estimate the air flow.
This is probably the oldest measurement method available.
Vortex shedding based air flow meters are based on the physical principle that when an obstacle is introduced in an airflow, whirlpools or vortices are created. As speed increases, the number of formed vortices increases. By measuring these the flow can be calculated.
The most widely applied technology for air flow sensing is based on thermal dispersion.
By applying a heated and a non-heated temperature sensor the amount of energy used to keep the temperature difference between the two constants is proportional to the air flow.
76 Smart Water UtilitieS: Complexity made simple