Wastewater treatment plant parameters
- find ways to remove them
- reduce them to an acceptable level for release to the environment.
To maintain life in an aquatic environment, it is essential to maintain a sufficient level of oxygen. 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 the air, the more it is stirred, and the more it is 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 micro-organisms consume a lot of oxygen to develop and degrade this pollution. This explains the lack of oxygen in the wastewater. The temperature also affects this parameter. The colder it is, the more oxygen is soluble in water.
In general, it is the analysis of the dissolved oxygen concentration. It is measured with an oxymeter.
Chemical Oxygen Demand COD
Chemical oxygen demand (COD) is a measure of all oxygen consuming substances. It is about :
- those that can be eliminated 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 measurement of oxidation is done by a COD test to quantify the amount of oxidizable material. The amount of reagent consumed for the oxidation of the organic matter present, reported in mgO2/L, corresponds to the COD.
COD is useful in terms of water quality because it allows :
- 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 measure of the amount of oxygen required to remove or alter biologically degradable organic matter in wastewater.
The water sample is stored for five days at 20°C, without light and covered tightly. We talk about BOD 5 because the analysis is done over 5 days. Some countries use other varieties such as BOD 7 or BOD 21, called ultimate BOD.
However, BOD5 is mostly used around the world. 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 time
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 the BOD5.
Like the COD, the BOD 5 is also expressed in mg/l of oxygen (mgO2/L). It allows to determine the impact of an effluent on the receiving environment.
Indeed, the BOD 5 represents the proportion of organic matter that is naturally biodegradable, and therefore mobilizes oxygen in the waterways.
Suspended solids (SS) are the materials in the transient phase 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. TSS is a commonly used term although it is actually Total Suspended Solids (TSS).
TSS analysis consists of passing a volume of sample through a membrane filter. 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 is one of the parameters commonly used to determine the quality of a wastewater because it represents a danger to the receiving environment.
- First of all, this TSS will clog the gills of the fish, which can asphyxiate them
- Then, the oxygen of the receiving environment is mobilized to eliminate the organic part and TSS
- Finally, MESTs limit light penetration, thus limiting photosynthesis and O2 production during the day. This phenomenon occurs in summer when the receiving water is warm and cannot retain as much dissolved oxygen as in winter.
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Total nitrogen Kjeldhal NTK or NK
Domestic wastewater is made up almost exclusively of organic nitrogen (Norg) and ammoniacal nitrogen (NH4+). This is generally the case for industrial waters, although there is a wide variation in input 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+ is determined by 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 parameter NTK represents the sum of ammoniacal and organic nitrogen 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 (gas) dissolves in water to form the ammonium cation NH4+. The ammonium/ammonia distribution depends on the pH and temperature of the medium. In the water field, the term ammonia is incorrectly used to refer to ammonium (NH4+).
Ammonium NH4+ is the most common nitrogen component 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 is converted back into NH4+ in the wastewater system by the action of micro-organisms. This is the main contribution of ammonium in domestic wastewater, about 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).
- It is necessary to filter the samples before the analysis because the TSS disturb the staining.
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.
Like NTK, NH4+ is a good chemical indicator of direct pollution of river water. This may involve the discharge of wastewater directly into the natural environment (mainly from storm drains), but also pollution linked to the runoff of animal manure 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. Moreover, NH4+ becomes toxic when the pH is higher than 8 because it is transformed back into gaseous NH3 which remains dissolved in the water.
In aquaponics, aquaculture or fish farming, this parameter is very closely monitored with the pH. Indeed, a poor concentration of NH3, even of the order of 1 mg /l, can lead to mortality.
Fry are highly sensitive, and the quality of their water must be carefully monitored. 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 volatile substances that do not remain in this form for a long time. During regular analysis of wastewater treatment plants, nitrite represents only a small part of the nitrogen fraction, even after a nitrification step. 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.
Nitrites and nitrates are naturally found in the natural environment. Nitrites are found in very small quantities. However, it is common to find nitrates. They come 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 are formed during the oxydation of ammonium (in an aeration tank for example, during the nitrification process). They are extremely toxic for aquatic fauna as they disrupt oxygen fixation, especially when the pH is below 7, and quickly cause fish asphyxiation. Nitrites may also cause public health problems if they are present in drinking water (infant blue disease) but fortunately drinking water networks are very controlled and it is unlikely that a sufficiently high concentration of NO2 would be formed in drinking water 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 very closely monitored in association 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.
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 will cause environmental eutrophication (seaweed propagation).
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
A significant part of our phosphorus in wastewater is caused by human activity. The origin of excessive phosphorus in water is varied, but on average we can say that :
- 2/3 of phosphorus pollution comes from agricultural activities, especially due to runoff from croplands and pastures into water. 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 emissions from non-collective sanitation facilities (ANC).
Urine accounts for about 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, Pt concentration in wastewater has been decreasing for several years.
Use of phosphorus
Phosphorus is essential for the development of all living organisms. It occurs naturally in water courses. They are 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). The wastewater is mostly composed of PO4 phosphates.
As for 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 participates in water pollution by promoting excessive seaweed growth, especially in slow moving waterbody. Indeed, Pt is an essential component of fertilizers, the famous NPK. So imagine a 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 by allowing the seaweed to grow so abundantly, it dies from lack of light and its decomposition mobilizes the 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.