Wastewater treatment plant parameters
Wastewater treatment takes place in several stages within a treatment plant. The most commonly used parameters in treatment plants are BOD, COD, and TSS. When degrading receiving environments, it is common to analyse nitrogen (NGL, NH4, NO3) and phosphorus parameters.
To control environmental risks caused by wastewater, an analysis is necessary. This involves determining and quantifying the substances and microorganisms contained in this water. These in order to :
- find ways to remove them
- reduce them to an acceptable level for release to the environment.
Micro-organism analyses are performed when the discharge area is located near a bathing area. The parameters to be analyzed are defined in the regulations of each country and can be adjusted by the local authorities.
Dissolved oxygen
To sustain life in an aquatic environment, it is essential to maintain a sufficient oxygen level. Indeed, this last one is part of one of the necessary parameters for the continuity of life and its evolution. It is essential for photosynthesis and the alteration of organic components.
The more the water is exposed to open air, the more it is agitated, and the more it becomes supersaturated with oxygen. However, when there is an excess of soluble organic matter, it is considered undersaturated. Indeed, these organic materials serve as food for many micro-organisms. These microorganisms consume a lot of oxygen to grow and degrade this pollution. This is notably what explains the lack of oxygen in wastewater. The temperature also affects this parameter. The colder it is, the more soluble oxygen is in water.
In general, it is the analysis of the dissolved oxygen concentration. It is measured with an oxymeter.
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Chemical Oxygen Demand COD
Chemical Oxygen Demand (COD) is a measure of all oxygen-consuming substances. It is about :
- those that can be removed by wastewater treatment
- those that are not suitable for biological treatment.
This measurement of the amount of oxygen consumed by a water sample is performed with strong oxidizing reagents. For example, potassium dichromate can be used for this measurement. This parameter is expressed as the mass of oxygen consumed in relation to the sample volume. Practically, the oxidation measurement is performed using a COD test to quantify the amount of oxidizable matter. The amount of reagent consumed for the oxidation of the organic matter present, reported in mgO2/L, corresponds to the COD.
In a wastewater treatment plant, COD is an essential parameter for evaluating water quality. It allows to:
- determine the effect of an effluent on the receiving environment
- determine the Biochemical Oxygen Demand (BOD).
Special attention must be paid to the test tubes containing the oxydation agents. These are particularly toxic pollutants and must be processed in a specialized subsidiary.
Biochemical Oxygen Demand BOD
BOD is a parameter indicating the amount of oxygen essential for the elimination or alteration of biologically degradable organic matter contained in wastewater.
The water sample is placed for five days at 20°C, without light and hermetically sealed. We speak of BOD5 because the analysis is carried out over 5 days. Some countries use other variants such as BOD7 or BOD21, called ultimate BOD.
However, BOD5 is mainly used worldwide. The darkness prevents the risk of photosynthesis and the temperature of 20°c favors the propagation of micro-organisms fond of O2.
The study involves 2 samples:
- The first one will be used to know the initial quantity in O2
- The second will be used to measure BOD at the end of the study period
The degradation of organic pollutants by micro-organisms, or self-purification, consumes oxygen. It is this decrease in oxygen in the environment that is measured by BOD5.
Like COD, BOD5 is also expressed in mg/L of oxygen (mgO2/L). It allows to determine the impact of an effluent on the receiving environment.
Indeed, BOD5 represents the portion of naturally biodegradable organic matter, thus mobilizing oxygen from watercourses.
Suspended solids
Suspended Solids (TSS) are the suspended matter in wastewater treatment plants. That is, they are not in colloidal or dissolved form. As the name suggests, these are particles suspended in the liquid. They can be filtered and are composed of organic and mineral particles. The term TSS is commonly used, although it actually refers to Total Suspended Solids (TSS).
TSS analysis involves passing a sample volume to be analyzed through a filter membrane. This membrane will then be placed in an oven at 105°C for at least one hour. The difference in weight before/after filtration is used to determine the amount of suspended solids. This is measured in mg/l.
TSS are among the parameters commonly used to determine wastewater quality because they pose a danger to the receiving environment.
- First, these TSS clog fish gills, which can asphyxiate them
- Next, the oxygen in the receiving environment is mobilized to eliminate the organic matter and TSS
- Finally, TSS limit light penetration, thus limiting photosynthesis and O2 production during the day. This phenomenon is evident in summer when the receiving water is warm and cannot retain as much dissolved oxygen as in winter.
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Total Kjeldahl Nitrogen TKN or NK
Domestic wastewater contains almost exclusively organic Nitrogen (Norg) and ammoniacal Nitrogen (NH4+). This is generally the case for industrial wastewater, although there is a wide disparity in influent Nitrogen values from one company to another.
Organic nitrogen is a component of living cells (amino acids, proteins) while ammoniacal nitrogen NH4+ comes from :
- direct effluents from living beings (urine)
- from the decomposition of organic nitrogen by micro-organisms.
The ratio between Norg and NH4+ depends, among other things, on the length of the collection network. The longer the time spent in the sewer line, the more microorganisms have time to transform the organic nitrogen into NH4+.
The Kjeldahl Nitrogen TKN parameter corresponds to the sum of ammoniacal and organic Nitrogen contained in the water, expressed in mg/L. This is a complicated analysis to perform, so it is usually calculated as follows:
NTK = total nitrogen NGL – nitrite NO2 – nitrate NO3
For domestic wastewater, nitrates and nitrites are almost non-existent. Thus it is common practice to do only a total NGL analysis and to consider that NTK = NGL.
When a high concentration of Kjeldahl nitrogen is detected in a river, it indicates pollution of human origin. Organic nitrogen must be removed because it significantly reduces the oxygen concentration of an environment. This is why discharge standards are often strict for this parameter, and even more so when the receiving environment is considered a sensitive area.
It is historically named after the Danish chemist who discovered the method in 1883.
NH3 ammonia and NH4+ ammonium
Ammonia NH3
The dissolution of ammonia NH3 (gaseous) in water forms the ammonium cation NH4+. The ammonium/ammonia distribution depends on the pH and temperature of the environment. In the water sector, the term ammonia is mistakenly used to refer to ammonium (NH4+).
Ammonium NH4+ is the most widely represented Nitrogen compound in domestic wastewater. Effectively, animals (including humans) absorb the ammonium that is formed in their bodies by transforming it into urea. Once discharged, this urea decomposes back into NH4+ in sewer networks, primarily due to the action of microorganisms. This is the main source of ammonium in domestic wastewater, accounting for approximately 80% of NH4+.
In general, ammonium is measured by means of an NH4+ test with a spectrophotometer.
- The color is used to indicate the concentration, reported in mg/L of nitrogen (mgN/L).
- Samples must be filtered before analysis because TSS interfere with coloration.
However, experience shows that, for municipal wastewater, the difference with and without filtration is not significant. These analysis tubes, although not as hazardous to the environment, must also be recycled in a specialized channel.
Ammonium NH4
Like TKN, NH4+ is a good chemical indicator of direct pollution of river water. This can be due to wastewater discharge directly into the natural environment (mainly from storm overflows) but also to pollution related to runoff from animal waste spread on fields.
NH4+ is dangerous for aquatic fauna because it contributes to the reduction of oxygen concentration, in particular because of the bacterial proliferation that it promotes. Furthermore, NH4+ becomes toxic when the pH is above 8 because it transforms back into gaseous NH3 which remains dissolved in the water.
In aquaponics, aquaculture, or fish farming, this parameter is closely monitored along with pH. Indeed, a poor concentration of NH3, even of the order of 1 mg /l, can lead to mortality.
Fry are extremely sensitive, and their water quality must be very carefully controlled. Trout are also quite sensitive to NH3, the 96 LC50 (median lethal concentration over 96 hours which expresses the acute toxicity on fish – mortality of 50% of trout) is only 0.4mg/l. So imagine trout fry !
Nitrites NO2 and nitrates N03
In the nitrogen cycle, ammonium is transformed into nitrites NO2 and then into nitrates. Nitrites are unstable compounds that do not remain in this form for long. In routine analyses performed at wastewater treatment plants, nitrites represent a tiny fraction of total Nitrogen, even after nitrification. That’s why they are generally not monitored. On the other hand, NO3- nitrates are more stable and the final form of oxydation. Therefore, they are used to determine the nitrogen pollution’s oxygen fraction.
Like NH4+, nitrite and nitrate are usually analyzed in tubes by colorimetry using a spectrophotometer, and their concentration is also expressed in mgN/L. The samples must also be filtered and the NO3 and NO2 tubes must also be recycled in a specialized channel.
In a wastewater treatment plant, nitrites and nitrates are naturally found in the natural environment. Nitrites are present in tiny quantities, while nitrates are more frequent and originate from:
- Agricultural waste (fertilizers)
- Domestic and industrial waste. It should be noted that waste standards for NO2 and NO3 are rare. Therefore, most of the wastewater treatment plants are not equipped with a denitrification stage to eliminate nitrates into nitrogen gas N2.
- In the natural environment for a few mg/L: this low concentration is perfect to maintain a balance between the aquatic fauna and flora.
Nitrite NO2 and nitrate N03 composition
Nitrites NO2
Nitrites are formed during the oxydation of ammonium (in an aeration tank for example, during the nitrification process). They are very toxic to aquatic fauna because they disrupt oxygen uptake, especially when the pH is below 7, and quickly cause fish asphyxiation. Nitrites can also pose public health problems if present in tap water (blue baby syndrome), but fortunately, drinking water networks are very well controlled, and furthermore, it is unlikely that a sufficiently high concentration of NO2 could form in drinking water supply networks. To maintain a certain balance, keep in mind that nitrites are commonly used in charcuterie for preservation.
In aquaponics, aquaculture or fish farming, this parameter is closely monitored in correlation with pH. NO2 concentrations below 1 mg/l can cause stress and kill fish. For this reason, it can be said that the NO2 concentration should not exceed 1 mg/l in plant discharges. There is some question as to the relevance of this value given the concentration of wild fish at the outlet of a plant, knowing that on average the concentration of nitrite is close to 2 mg/L for treatments with short residence times such as biofilters.
Nitrates NO3
In terms of regulations, it is extremely rare to have a rejection standard for nitrites. In reality, it is almost impossible for an operator to influence this parameter.
Nitrates N03-, on the contrary, do not represent any danger. However, if they are too numerous, they cause eutrophication of the environment (algal proliferation in the water).
In aquaculture or fish farming, the nitrate parameter is not that important (except in aquaponics for plants). Actually, nitrates are hardly toxic for adult fish. However, be careful with fry, which can be subject to variable mortality depending on the species, sometimes as low as 20 mg/L.
Total Nitrogen NGL
NGL is a measure of the total nitrogen pollution of an effluent.
In contrast to NTK, this analysis is easy to perform using tubes by colorimetry with a spectrophotometer, and their concentration is also expressed in mgN/L.
NGL is the sum of all the different forms of nitrogen contained in a sample, i.e. Total Kjeldhal nitrogen (TKN) and oxidized nitrogen (nitrite + nitrate).
NGL = NTK + NO2 + NO3
Total Phosphorus TP
Phosphorus origin
A large part of the Phosphorus found in wastewater originates from human activity. The origin of excess Phosphorus in water is varied, but on average we can say that:
- Two-thirds of phosphorus pollution comes from agricultural activities, particularly from runoff from cultivated land and pastures into water bodies. The effect is even more significant when grounds have been amended with fertilizer or after spreading.
- The third comes from the wastewater treatment plants of municipalities and industries.
- A very small part of this phosphorus pollution is also due to urban runoff (stormwater) and diffuse discharges from non-collective sanitation systems (ANC).
Urine accounts for approximately 60% of the phosphates in domestic wastewater. Soft drinks are usually the primary source of phosphates. Some commonly used products such as household detergents contain polyphosphates, but it must be recognized that manufacturers have made great efforts to reduce phosphorus content. This is not the case for industrial detergents, which often benefit from a lack of regulation concerning the PO4 content. In Europe, the concentration of Pt in wastewater has been steadily decreasing for several years.
Use of phosphorus
Phosphorus is essential for the development of all living organisms. It is naturally present in watercourses. It is also present in industrial and domestic wastewater in much higher proportions. Total phosphorus Pt is composed of:
- of organic phosphorus from the decomposition of living matter,
- of phosphates PO4, the mineralized part (mainly in the form of orthophosphate ions). Wastewater is almost entirely in the form of PO4 phosphates.
As with COD and nitrogen parameters, Pt is analyzed with tubes by colorimetry using a spectrophotometer, and its concentration is expressed in mgP/L.
Phosphates are essential for plants and animals; however, Pt contributes to water pollution by promoting excessive algae growth, particularly in weakly agitated water bodies. Indeed, Pt is an essential component of fertilizers, the famous NPK. So imagine water polluted with phosphorus and NO3! The perfect combination for a quick eutrophication! Some lakes that are relatively clear in the spring may look like a green soup in late summer. The worst part is that by allowing algae to grow so abundantly, they die from lack of light, and their decomposition mobilizes dissolved oxygen in the water. As a result, aquatic species will die of asphyxiation. This explains why phosphorus is restricted. It only applies to the Pt parameter, as it includes PO4.
Wastewater treatment plant
In a wastewater treatment plant, wastewater parameters such as BOD, COD, total suspended solids (TSS), and pH are essential for controlling the various treatment processes. These indicators allow real-time monitoring of the pollutant load of effluents and adjustment of key steps, such as aeration in biological basins or decantation. For example, a high BOD indicates a high concentration of organic matter, requiring sufficient oxygen supply to optimize microbial activity. Similarly, monitoring TSS is crucial for evaluating the efficiency of solids separation. By combining these analyses with modern control technologies, wastewater treatment plants ensure effective treatment that complies with environmental standards, while adapting their operations to variations in effluent flows and loads.
FAQ
Why Monitor Wastewater Quality before Treatment?
Measuring parameters at the inlet of a plant allows anticipating the pollutant load to be treated. This makes it possible to adjust processes (aeration, dosing, hydraulic retention) according to the concentration of pollutants. For example, a high inlet BOD indicates an expected overconsumption of oxygen in the biological basins. It is also essential for detecting unusual pollutants (undeclared industrial effluents) likely to disrupt biological functioning.
What are the Impacts of Improper Aeration Adjustment in Biological Basins?
Insufficient aeration leads to a deficit in dissolved oxygen, which slows down the biological degradation of organic matter. Conversely, excessive aeration unnecessarily consumes energy and generates high operating costs. A good balance optimizes efficiency while minimizing costs, especially through automatic regulations based on dissolved oxygen or real-time measured BOD.
Is Total Nitrogen Always a Parameter to Measure? Why?
Yes, because it offers a complete overview of nitrogen pollution by integrating all its forms (ammoniacal, nitrite, nitrate, and organic). This is particularly useful for evaluating the needs for nitrification/denitrification treatments. Total nitrogen is often used to calculate the pollutant load of a territory and guide water management policies, even when sub-forms are monitored separately.
What is the “Historical Evolution of Wastewater Treatment Plants”?
Initially focused on simple decantation of suspended solids, plants evolved with the appearance of biological treatments (activated sludge, trickling filters) in the 20th century. Today, they also integrate tertiary treatment steps (disinfection, micropollutant removal), in a context of increasing regulatory pressure and environmental concerns such as water reuse.
How to Interpret an Abnormal Parameter in a Wastewater Treatment Plant?
An out-of-norm parameter is not just a regulatory problem: it is a symptom of malfunction. Like, for example:
- Increased BOD ➝ organic overload ➝ insufficient aeration or mixing defect.
- Increased NO₂⁻ ➝ nitrification blockage ➝ toxic shock or lack of oxygen.
- Unstable TSS ➝ poor decantation ➝ clogging or sludge aging.
Each deviation must be placed within a global diagnosis to adjust processes or trigger targeted maintenance.
