Water quality standardization and requirements for the protection of water bodies

The protection of water bodies from pollution is carried out in accordance with the "Sanitary Rules and Norms for the Protection of Surface Waters from Pollution" (SanPiN 4630-88) and the "Hygienic Requirements for the Protection of Surface Waters" (SanPiN 2.1.5.980-00). The rules include general requirements for water users regarding the discharge of wastewater into water bodies. The rules establish two categories of water bodies: 1 – water bodies for drinking and cultural-domestic purposes; 2 – water bodies for fishery purposes. The composition and properties of water in the first type of water bodies must comply with standards at cross-sections located in watercourses at a distance of at least one kilometer upstream from the nearest downstream water intake point, and in non-flowing water bodies – within a radius of at least one kilometer from the water intake point. The composition and properties of water in type II water bodies must comply with standards at the point of wastewater discharge in the case of a diffusing outlet (if currents are present), and in the absence of a diffusing outlet – no further than 500 m from the discharge point.

The rules establish regulated values for the following parameters of water in water bodies: content of floating impurities and suspended particles, odor, taste, color and water temperature, pH value, composition and concentration of mineral impurities and dissolved oxygen, biochemical oxygen demand, composition and maximum permissible concentration (MPC) of toxic and harmful substances and pathogenic bacteria. The maximum permissible concentration is understood as the concentration of a harmful (toxic) substance in the water of a water body which, upon daily exposure over a long period to the human body, does not cause any pathological changes or diseases, including in subsequent generations, detectable by modern research and diagnostic methods, and also does not disturb the biological optimum in the water body.

Harmful and toxic substances are diverse in their composition, which is why they are regulated based on the principle of the limiting indicator of harmfulness (LIH), which is understood as the most probable adverse effect of a given substance. For the first type of water bodies, three types of LIH are used: sanitary-toxicological, general sanitary, and organoleptic; for the second type of water bodies, two more types are used: toxicological and fishery.

The sanitary condition of a water body meets the requirements of the standards when the following inequality is fulfilled

i =1 MPC i for each of the three (for type II water bodies – for each of the five) groups of harmful substances, the MPCs of which are established respectively by the sanitary-toxicological LIH, general sanitary LIH, organoleptic LIH, and for fishery water bodies – also by the toxicological LIH and fishery LIH. Here n is the number of harmful substances in the water body belonging, for example, to the "sanitary-toxicological" group of harmful substances, Ci is the concentration of the i-th substance from this group of harmful substances; m is the number of the group of harmful substances, for example, m = 1 – for the "sanitary-toxicological" group of harmful substances, m = 2 – for the "general sanitary" group of harmful substances, etc. – a total of five groups. In this case, the background concentrations Cbg of harmful substances contained in the water body before the discharge of wastewater must be taken into account. With the predominance of one harmful substance with concentration C in a group of harmful substances of a given LIH, the requirement must be fulfilled

C + C bg ≤ MPC. (7.2)

The use of relation (7.1) is based on the assumption of the applicability of the principle of additivity of the harmfulness of different substances belonging, for example, to the fourth, "toxicological" group of harmful substances. That is, it is assumed that the integral "harmfulness" of a multi-component system of harmful substances can be defined as the arithmetic sum of the "harmfulness" of individual components. Meanwhile, the phenomenon of synergism is known, where two or more harmful (toxic) substances can produce an effect of harmful action on the body many times exceeding the sum of the action of each of them. Thus, it has recently been established that halogenated hydrocarbons and, possibly, other chemicals (the first factor) weaken the immune system, as a result of which the body becomes more susceptible to the effects of infections and parasites (the second factor). It is assumed that this circumstance was the cause of the catastrophic die-off of seals in the North Sea in the 70s and 80s. However, the quantitative effect of synergism has not been sufficiently studied, so today the use of the principle of additivity of harmful components belonging to any LIH is permitted.

MPCs have been established for more than 400 basic harmful substances in water bodies for drinking and cultural-domestic purposes, as well as more than 100 basic harmful substances in water bodies for fishery purposes. Table 7.4 shows the MPCs of some substances in the water of water bodies.

Water bodies Category I Water bodies Category II Substance MPC, MPC,

Sanitary Benzene 0.5 Toxicological 0.5 toxicological Phenols Organoleptic 0.001 Fishery 0.001 Gasoline,

The same 0.1 The same 0.05 kerosene

Sanitary Cd2+ 0.01 Toxicological 0.005 toxicological Cu2+ Organoleptic 1 The same 0.01 Zn 2+ toxicological Cr6+ Organoleptic 0.1 - 0

For wastewater itself, MPC (maximum permissible concentration) is not regulated, but rather the maximum permissible discharge quantities of harmful impurities, MPD (maximum permissible discharge), are determined. Therefore, the minimum required degree of wastewater treatment before discharge into a water body is determined by the state of the water body, specifically, the background concentrations of harmful substances in the water body, water flow rate of the water body, etc., that is, the capacity of the water body to dilute harmful impurities.

It is prohibited to discharge wastewater into water bodies if it is possible to use more rational technology, waterless processes, and systems of repeated and recirculating water supply – repeated or constant (multiple) use of the same water in the technological process; if the effluents contain valuable waste that can be utilized; if the effluents contain raw materials, reagents, and production output in quantities exceeding technological losses; if the wastewater contains substances for which MPC has not been established.

The discharge mode can be single, periodic, continuous with variable flow rate, or random. In this case, it is necessary to take into account that the water flow rate in the water body (river discharge) varies both seasonally and annually. In any case, the requirements of condition (7.2) must be met.

The method of wastewater discharge is of great importance. With concentrated outlets, the mixing of effluents with the water of the water body is minimal, and the contaminated plume can have a great length in the water body. The most effective application is dispersive outlets at depth (at the bottom) of the water body in the form of perforated pipes.

In accordance with the above, one of the tasks of water quality regulation in water bodies is the task of determining the permissible composition of wastewater, that is, the maximum content of a harmful substance (substances) in effluents, which, after discharge, will not yet result in exceeding the concentration of the harmful substance in the waters of the water body above the MPC of this harmful substance.

Equation of the balance of a dissolved impurity during its discharge into a watercourse (river), taking into account the initial dilution at the outlet section:

C cm = n0 С f)] + С f, (7.3) Here Ccm, Cr.s., Cf are the concentrations of the impurity in wastewater before discharge into the water body, in the calculated section, and the background concentration of the impurity, mg/kg; no and nr.s. are the dilution factors of wastewater at the outlet section (initial dilution) and in the calculated section.

Initial dilution of wastewater at the section of their discharge

q where is part of the watercourse flow rate flowing over the dispersive outlet, which, let's assume, is in the form of a perforated pipe laid on the bottom, m3/s; q – wastewater flow rate, m3/s; L – length of the dispersive outlet (perforated pipe), m; H, V – average depth and flow velocity above the outlet, m and m/s.

After substituting (7.4) into (7.3)

f)] + С f, (7.5)

f)] + С f, (7.6)

Along the watercourse, the wastewater plume expands (due to diffusion, turbulent and molecular), as a result of which the mixing of wastewater with the water of the watercourse occurs in the plume, an increase in the dilution factor of the harmful impurity, and a constant decrease in its concentration in the wastewater plume, or rather, in the now mixed water. Ultimately, the section (cross-section) of the plume will expand to the section of the watercourse. At this point of the watercourse (where the section of the contaminated plume coincides with the section of the watercourse), the maximum possible dilution of the harmful impurity for the given watercourse is achieved. Depending on the values of the initial dilution factor, width, velocity, sinuosity, and other characteristics of the watercourse, the concentration of the harmful impurity (Cr.s.) can reach the value of its MPC in different sections of the contaminated plume. The sooner this happens, the smaller the area (volume) of the watercourse will be contaminated with the harmful impurity above the norm (above the MPC). It is clear that the most suitable option is when condition (7.2) is ensured right at the point of discharge, and thus the dimensions of the contaminated section of the watercourse will be reduced to zero. Let us recall that this option corresponds to the condition of effluent discharge into a second-type watercourse. Regulatory dilution to the MPC at the outlet section is also required for first-type watercourses if the discharge is carried out within the limits of a settlement. This option can be ensured by increasing the length of the perforated outlet pipe. In the limit, by blocking the entire watercourse with an outlet pipe and thus including the entire flow rate of the watercourse in the process of effluent dilution, taking into account that for the outlet section nr.s. = 1, and also setting in (7.5 MPC V and Cf = 0 we get

C cm = ⋅ MPC = ⋅ MPC, (7.7)

q q where B and H are the effective width and depth of the watercourse, and the water discharge of the watercourse.

Equation (7.7) means that with maximum utilization of the diluting capacity of the watercourse (watercourse discharge), the maximum possible concentration of a harmful substance in the discharged wastewater

Q+q Q water can be assumed to be equal to ⋅ MPC ≅ ⋅ MPC. If, for the purpose

q q of diluting the effluent, only a part of the watercourse discharge can be used, for example, 0.2Q, then the requirements for treating the effluent from this harmful substance increase, since the maximum permissible concentration of the harmful substance in the effluent decreases by 5 times: 0.2Q

⋅ MPC. In this case, the value qCcm, which in the first case is equal to q

Q 0.2Q q⋅ ⋅ MPC = Q ⋅ MPC, and in the second q ⋅ ⋅ MPC = 0.2Q ⋅ MPC, must

q q be considered as the maximum permissible discharge (MPD) of this harmful substance into the watercourse, g/s. If these MPD values (Q ⋅ MPC and 0.2Q ⋅ MPC, g/s) are exceeded, the concentration of the harmful substance in the watercourse will exceed the MPC. In the first case (MPD = Q MPC), turbulent (and molecular) diffusion will no longer reduce the concentration of the harmful substance along the watercourse, since the initial dilution cross-section coincides with the entire cross-section of the watercourse – there is nowhere for the stream of polluted water to diffuse. In the second case, along the watercourse, there will be further dilution of the effluent and a decrease in the concentration of the harmful substance in the water body below the MPC, and at a certain distance S from the discharge point, the concentration of the harmful substance may decrease to 0.2 MPC.

In the general case, the distance S from the discharge cross-section to the design cross-section, that is, to the cross-section with a given dilution ratio value, nр.с, or – which is effectively the same – with a given concentration of the harmful impurity, for example, equal to its MPC,

d 7.8)

When regulating the discharge of industrial and drainage waters into the river network, it is necessary to accurately calculate the degree of their dilution in the watercourse. The expansion of the polluted stream along the river occurs mainly due to turbulent diffusion. The coefficient of this diffusion is calculated using the expression 2m = 0,7C − 6, where C is the Chézy coefficient (m^0.5/s), and g is the acceleration due to gravity (m²/s).

To determine the permissible concentration of substances in the effluent before discharge, the hydraulic and geometric parameters of the watercourse are taken into account. The calculations include the channel width B (m), the width of the zone without discharge x (m), the channel sinuosity coefficient φ (the ratio of the distance between cross-sections along the fairway to the straight-line distance), and the Reynolds diffusion criterion Re_d. The main condition of the calculation is that at a given distance S from the discharge point, the concentration of the harmful substance, taking into account the background pollution C_b, does not exceed the maximum permissible concentration (MPC).

• Channel proportionality coefficient (A) — 0,9–2,0
• Share of river flow for initial dilution — 0,2 Q
• Dilution coefficient at the discharge point (S = 0) — 1

To convert the permissible concentration into a maximum permissible discharge (MPD) indicator, the obtained value is multiplied by the wastewater discharge q. In the case where the MPC level must be ensured directly at the discharge point (at S = 0), the dilution indicator n_r.s. is equated to 1. Under such conditions, for initial dilution, it is permitted to use not the entire river flow, but only one-fifth of the watercourse discharge (L H V = 0,2 Q).

Features of discharge into standing water bodies and regulation of impurities

The stated principles form the basis for regulating water quality when discharging suspended and organic substances into watercourses. The same requirements apply to the discharge of waters heated in the cooling systems of industrial and agricultural enterprises. The calculation allows for establishing the permissible load on the watercourse without the risk of exceeding water quality standards.