Note on legislation: This section describes Czech permitting practice and typical monitoring parameters. Limits, sampling frequency and reporting duties are always determined by the specific Czech permit and current Czech regulations.
Quality control can be understood in different ways. One thing is a sensory inspection – we open the treatment plant and make sure by looking whether the activation has a brown sludge color that is "just right" and the tank is aerated, that there is no carpet of sludge on the settling tank, but that it is beautifully transparent, that the return sludge is returned to the activation, there is no smell anywhere... The second view is purely analytical and consists in checking the quality of the outflow, sometimes also the inflow, by laboratories, usually accredited. In the vast majority of analyses, the indicators defined in the discharge permit are set. It can be these:
- BSK5
- CHSKCr
- NL105
- N-NH4
- NTotal
- Ptotal
In addition to the above, it is also advisable to monitor the pH, water temperature and, if total nitrogen is limited, also N-NO3, possibly also the dry matter of the activated sludge and its organic fraction.
What the individual abbreviations mean
BSK5
The abbreviation BSK5 means Biological S the need K of zinc, and the index 5 indicates that the determination takes 5 days. The determination takes place in such a way that a sample of purified water is saturated with oxygen and sealed in bottles that are stored in the laboratory in the dark at a temperature of 25°C. After the determination time has passed, the oxygen concentration is measured and from its decrease it is calculated how much oxygen was consumed by the microorganisms in the water for the oxidation of organic substances. The result is given in mg of oxygen per liter of sample. The result of the determination thus tells us how many biodegradable substances are in the effluent. A well-functioning treatment plant is characterized by the fact that the BOD5 value in the effluent is very low, at most tens of mg/l.
CHSKCr
A If it doesn't seem like it, the abbreviation COD is very similar to the abbreviation BOD. It means the Chchemical Sneed for Koxygen. The index Cr then indicates that it is a method using dichromate. There is also a method using permanganate, with an index of Mn, but it is only used in the field of drinking water and is not suitable for wastewater. The method consists in the chemical oxidation of organic substances in a water sample using potassium dichromate in a strongly acidic environment (sulfuric acid) using silver nitrate as a catalyst and mercury compounds to mask chlorides that affect the determination. The conditions of the determination (everything is also boiled) are quite drastic, so the vast majority of organic substances in the sample are oxidized. At the end of the determination, the amount of dichromate left unreacted is determined either by titration or spectrophotometrically. The result is again given in oxygen equivalents, i.e. in mg of oxygen per liter of water. This is the amount of oxygen that is needed to oxidize the organic substances in the sample. The number then corresponds to the sum of organic substances, not only those that are biodegradable. The results are always higher than BOD, and COD is always higher than BOD even in the effluent from the WWTP. Usual values are again in the tens of mg/l, but rather 40 or more, especially in compartmental sewers, where the non-biodegradable proportion of organic substances in wastewater is more pronounced.
NL105
Ndissolved Lsubstances dried at 105°C. The water sample is filtered through a filter with a defined pore size and what is caught on the filter is dried at 105°C to a constant weight. The result is expressed as mg of suspended solids per liter of sample. In wastewater, suspended solids can be anything. But, as with sewage effluent, it is mostly substances that cause water turbidity. In the case of drains, in most cases it is the remains of sludge or microorganisms. Undissolved substances also carry other analytes. If they are of organic origin, they increase BOD and COD indicators as well as nitrogen and phosphorus.
Nitrogen Forms
Nitrogen is an important nutrient, i.e. an element necessary for life, which increases the nutrient value of water - the so-called eutrophication, the manifestation of which is, for example, the excessive growth of aquatic organisms such as algae and cyanobacteria. A person produces approximately 11 g of nitrogen per day. Nitrogen occurs in water in several forms. It enters the water mainly as urea, which undergoes hydrolysis to form ammonia or ammonia (NH3 or NH4+). This can be oxidized to nitrites, which are unstable, and oxidation to nitrates (NO3-) is more likely. In addition, nitrogen may occur as organically bound. The sum of all forms is expressed as total nitrogen (Ntotal). It is customary to express the results as nitrogen - for example, total, ammonia, nitrate... The main reason is the different molecular weight of the individual forms, and their weight would then not be comparable.
If we express the results as nitrogen, then it can be argued that the oxidation of one gram of nitrogen produces one gram of nitrate nitrogen. If we only have 1 g of ammonia nitrogen and 1 g of nitrate nitrogen in the sample and nothing else, then we have a total of 2 g of total nitrogen in the sample. In contrast, the oxidation of 1 g of ammonia produces approximately 3.6 g of nitrate ions. Adding the mass of ammonia and nitrate ions is like adding apples and tires and we get complete nonsense.
N-NH4
The abbreviation N-NH4 stands for ammoniacal nitrogen. The latter is determined by alkalizing the sample and overdoing it with steam. The steam is then captured in a boric acid solution to determine the amount of ammonia by titration. There is also a spectrophotometric determination when a dye is used, which changes the intensity of its color according to the amount of ammonia, and the intensity of the color is then measured. As mentioned, ammonia is produced by the hydrolysis of urea, but also by the decomposition of organic substances, for example proteins. It is toxic to aquatic organisms, especially fish. In higher concentrations, it is characterized by its smell. It has no color and is therefore not visible in water. Even apparently clean water can contain ammonia. It is removed from the water in an aerobic environment using nitrifying bacteria by oxidation to nitrates. In the case of activation treatment plants, practically zero concentrations are usual at the effluent. The exception is the winter period, when the nitrification process is slower. Legislation differentiates between temperatures above and below 12°C. At temperatures below 12°C, the limit for ammonia does not apply. The efficiency of ammonia removal is relatively low at treatment plants with vegetation, anaerobic and earth filters.
Ammonia can also be measured at home using various sets, for example aquarium ones, or from manufacturers specializing in water analysis.
N-NO3
Nitrate nitrogen is created by breaking down ammonia, sometimes part of it can also come from drinking water. It is determined after the evaporation of the sample, when the vapor dissolves again and the reaction with the colored substance creates a shade of color depending on the amount of nitrates, which is measured using the device. Nitrates are not particularly toxic. They are limited as part of the total nitrogen parameter. Their determination makes sense for revealing the possible cause of failure to achieve the required quality in the total nitrogen parameter. It acts as a nutrient in the environment and causes eutrophication. Effluent concentrations range from virtually zero if denitrification is operating to more than 100 mg/l.
Nitrate concentration can also be measured with different sets, as in the case of ammonia.
Ntotal
Total nitrogen consists of inorganic nitrogen (Nanorg), which consists mainly of nitrate and ammonia nitrogen, and organic nitrogen. Total nitrogen is usually limited at larger treatment plants.
Ptotal
Total phosphorus consists mainly of various forms of phosphate phosphorus and organically bound phosphorus, for example in proteins. a person produces approx. 2.5 phosphorus per day. Total phosphorus is determined by converting all the phosphorus in the sample to phosphates, which then react with a molybdenum compound to produce a blue color, the intensity of which is measured using the instrument. In addition to solid faeces, various cleaning and washing agents are also a significant source. Phosphorus is partially removed biologically, higher efficiency is achieved by chemical precipitation, which is a process that is simple in practice, but very complex in terms of chemistry. Phosphorus does not have a direct toxic effect, but causes eutrophication of waters, thereby degrading them. From the point of view of eutrophication, phosphorus is far more dangerous than nitrogen.
Determination of phosphates - not total phosphorus - can also be done using mobile analyzer sets in home conditions. You only need to take into account that some sets give results as phosphates, which need to be converted to phosphate phosphorus (approximately divide the result by 3), but you also need to take into account that total phosphorus can be organically bound. If the drain is not cloudy, we can get the correct number by multiplying the phosphate phosphorus by about 1.5.
pH
Determination of pH indicates how acidic (pH <7) or alkaline/alkaline (pH>7) the water is. The neutral value is 7 and for purified waters the optimal range is 6.5-8. pH is measured using a dedicated glass electrode.
dry matter
It makes sense to determine the dry matter for activated sludge. The determination is made by drying a known volume of sludge at 105°C and weighing the weight of the vapor. Sometimes it is also appropriate to find out the so-called organic dry matter, i.e. the proportion of organic substances in the dry matter. This is determined in such a way that the dried vapor from the determination of dry matter is annealed at 550°C and the weight loss is determined by determining how much has burned and this corresponds to the organic dry matter. The dry matter of the activated sludge tells us how much sludge we actually have in the treatment plant. The determination of V30 will give us an indication of the sludge volume, but its ability to sediment can be different. The dry matter will give us a really accurate amount. The optimal dry matter in activation is 3-4 g/l, then up to 7 g/l in return sludge. A membrane treatment plant usually works with higher solids - 8-10 g/l. The proportion of organic solids roughly tells us how much of the sludge is made up of microorganisms and how much inorganic inert. Values of 60-70% are common.
