Self-cleaning ability of water source

 Concept of self-cleaning ability of water source

Self-cleaning ability is the ability to self-regulate environmental activities through a number of special mechanisms to reduce pollution from the outside or turn toxic substances into non-toxic substances.

Self-cleaning of water sources is the process of restoring the original quality state thanks to hydrodynamic, physical, chemical, biochemical processes, etc. taking place in the water environment.

  Types of water sources:

All types of water in nature come in the form of rainwater. Water evaporates from the ocean, condenses into clouds and then falls back onto the continent as rain and snow. Then the water concentrates into rivers and lakes and then flows back to the sea and ocean.

The hydrological cycle includes 3 water sources: Rainwater, surface water, and groundwater.

       – Rainwater:In terms of hygiene, microbiology and chemistry, rainwater is the cleanest, the only drawback is that the salt concentration in it is too low.

       Groundwater:In terms of hygiene, ground water is inferior to rainwater, but cleaner than surface water. Many times you don’t have to process it and you can still use it. In fact, groundwater is caused by surface water seeping into the ground. The physicochemical composition of groundwater depends on the geological structure and surface water composition.

For groundwater, bacterial contamination is very diverse. Usually shallow groundwater is more contaminated than deep groundwater. The deeper it penetrates into the ground, the less bacteria there is because the upper layers of soil are also able to retain most of the bacteria. Many data show that in manure pits, bacteria cannot penetrate to a depth of 30-40cm from the ground. However, sometimes at a depth of 1.5m and beyond, bacteria are detected and the groundwater is contaminated.

       – Surface water:When rainwater falls to the ground and flows into rivers and lakes, it is called surface water. Surface water is the dirtiest in terms of microorganisms, organic and inorganic substances. Surface water is rich in nutrients – a good environment for many microorganisms to grow, including fungi and lower animals.

Self-cleaning processes of rivers

The self-cleaning process includes two basic processes: The process of mixing and diluting between wastewater and source water; The mineralization process of organic substances with the participation of microorganisms and aquatic organisms.

In practice, these two processes take place simultaneously, but their intensity depends on the location of the wastewater discharge outlet, flow hydrodynamic factors such as velocity, water level, flow, roughness coefficient, turbulence diffusion, river and lake morphology, flow meandering… and other environmental conditions.

Thanks to the above two processes, the concentration of pollutants introduced into the water source will decrease to a certain level after a period of time. The end result of these processes is partial or complete restoration of the original state of the water source.

To simplify research and evaluation of the self-cleaning process of water sources, people divide the self-cleaning stages according to space in the river flow.

– Zone 1: area where wastewater is mixed with source water. Here, the process of diluting river water with wastewater mainly takes place;

– Zone 2: zone of complete dilution of river water with wastewater. The boundary point between zone 1 and zone 2 is only a convention because in reality there is no point of complete mixing of wastewater with river water. However, the point at which about 80% of the river water participates in diluting wastewater is considered the point of complete disturbance. In this area, physical and biochemical processes mainly take place to metabolize dirt;

– Zone 3: zone to restore the original state of the water source

Self-cleaning with Dilute Turbulence Process:

Dilution is one of the main factors that reduces the concentration of contaminants when discharged into water sources. During the dilution process, the total amount of contaminants is assumed to remain unchanged for the case of persistent and non-persistent pollutants.

Purely physical dilution (disturbance) process between wastewater and water source. When determining the level of disturbance between wastewater and river water, do not take the entire river water flow to calculate because at the sewer aspect, the disturbance process cannot be completed completely, only at a certain distance away. drain discharge. On the other hand, the greater the ratio between wastewater flow and source water flow, the greater the distance from the discharge sewer to the calculation point (where the complete disturbance process is performed).

The correlation between source flow and wastewater flow is an important factor in the self-cleaning process which is the mixing coefficient n:

In there:

Q: Source water flow participating in the disturbance process, m³/s

q: Wastewater flow discharged into the source, m³/s

C: Contaminant content of wastewater, g/l

C_ng: Contaminant content of source water, g/l

C_gh: Contaminant content of wastewater mixed with source water after thorough mixing, g/l.

Thus, we see that the mixing and dilution process only helps reduce the concentration but does not reduce the pollutant load. Material balance equation for mixing and dilution process:

qC_nt + QC_s = C_gh(q +  aQ)

+ q: Discharge flow; m³/h

+ Q: River water flow; m³/h

+ C_nt: Concentration of impurities in wastewater; g/m³

+  C_s: Dirty concentration of river water above the discharge point; g/m³

+ C_gh: The allowable limit concentration in river water depends on the purpose of use; g/m³

+ a: Disturbance coefficient; %

In fact, not all of the source water flow participates in the disturbance process, but only a part. The portion of source water participating in the process is characterized by the disturbance coefficient.

Diffusion disturbance is mainly due to wind and wave forces chasing in all directions and due to pressure differences, temperature and river characteristics: flow velocity, water flow, river width, depth. … create.

The disturbance coefficient a is calculated according to the following formula:

With L: length along the river channel from discharge point to calculation point (end of disturbance zone) (m)

α is a coefficient that takes into account hydraulic factors of the river and is calculated according to the following formula:

ϕ: coefficient that takes into account the meandering of the river and ϕ = L_l/L_t (ratio of length along creek and direction long in a straight line)

ξ: coefficient depends on discharge point location:

+ Discharge immediately to shore: ξ =1

+ Discharge in the middle of the river: ξ = 1.5

K: diffusion coefficient; m/s and K  = (V_tb x H_tb) / 200

With V_tb ; H_tb is the average velocity and average depth of the river section being calculated.

The process of mixing and diluting river and lake water with wastewater is of great significance in protecting water sources:

– Reduces the concentration of dirt at local points in rivers and lakes;

– Evenly distributes the dirt load throughout the entire water capacity, thus enhancing the self-cleaning process (distributing the dirt load to microorganisms);

– Due to the reduction of local dirt load, consistent with the self-regulating ability of the water ecosystem, the stability of the system is guaranteed;

– Based on the number of dilutions of source water with wastewater, we can determine the necessary level of wastewater treatment and establish other measures to protect rivers and lakes.

Self-cleaning by mineralization of organic matter

For unstable contaminants, when introduced into water sources, under the influence of biological, chemical and physical processes, they will be decomposed or transformed from one form to another. This transformation tends to restore the original state of the water source.

This is a synthesis of many processes taking place in water sources:

– Aerobic decomposition of organic substances with the participation of microorganisms and aquatic organisms (oxygen consumption process).

– Oxygen dissolution process (aeration process).

The rate of pollutant metabolism in each of these processes depends on a series of factors such as the composition and characteristics of the aquatic community in the water body, water temperature, pH, lighting intensity, water depth, composition of suspended sediment and dissolved substances, characteristics of bottom sludge…

Aerobic decomposition process (oxygen consumption process):

The decomposition of dirt has the greatest significance in the self-cleaning process of water sources. When pollutants appear, 70-80% are fermented and oxidized to form CO2, H2O, and NH4 in 2 stages:

– Oxidizes carbon containing compounds to CO2 and H2O.

– Oxidation of nitrogen-containing compounds to nitrite by Nitrosomonas microorganisms and from nitrite to nitrate by Nitrobacter microorganisms.

In there:

+ L_a: BOD content at the initial time; mg/l

+ L_t : BOD content over time period t; mg/l

+ k: oxygen consumption constant with given temperature

+ t: time of oxygen consumption; day

Mechanism: bacteria are the main factors in normal biochemical processes in natural water sources. They convert dissolved organic substances into their cells and inorganic substances. Inorganic substances are used by algae. Lower animals and bacteria make food for small fish and then big fish. Fish are a source of food for humans. Bacteria and fungi decompose organic matter to release CO₂, H₂O and some mineral salts. In this process, they use a very large amount of oxygen, so they often cause oxygen deficiency immediately after discharging wastewater into rivers and lakes. The stronger this process is, the more effective it is at self-cleaning the water source.

Bacteria play a major role in the decomposition of organic matter in nature and are considered effective scavengers of organic matter in dilute solutions. Bacteria oxidize organic matter to provide themselves with energy to synthesize complex organic molecules needed for the formation of new cells. Absorption of food by bacteria takes place over its entire surface.

The amount of bacteria and fungi in water depends on many factors. In clean water there is often not enough organic matter (nutrients) for saprophytes. When organic substances mix with source water, microorganisms grow by leaps and bounds. During the growth process, microorganisms consume more and more oxygen. As a result, the oxygen concentration in the water decreases, or even is completely consumed, creating anoxic conditions in the environment.

Phytoplankton: are plants that live floating in water, can autotroph through photosynthesis, contributing to the decomposition of organic matter and limiting water pollution. It enriches oxygen in water, oxygen is essential for the decomposition of organic matter, reducing nutritional elements in water. It is a food source for plant-eating organisms. However, when phytoplankton grows too strongly, the water source changes a second time.

Waste transformation zones:

• Disintegration zone: this zone discharges at the lower part of the drain, when water flows into the river. Typical of this region is that the water becomes dark and turbid with the formation of sediment. The dissolved oxygen content drops to 40% and the CO₂ content increases, reaeration occurs but is slower than deoxidation. This situation is not favorable for the development of aquatic organisms. Fungi grow at higher points, bacteria grow at lower points, giving rise to small insects that dominate the sewer water.
• Strong decomposition zone: this zone is clearly seen when water is heavily polluted and is characterized by the absence of dissolved oxygen, anaerobic decomposition takes place. As a result of sediment decomposition, air bubbles and sludge may appear on the water surface forming a black scum. The water will be dark gray and have a foul odor of sulfur-containing compounds. Microorganisms, mainly anaerobic bacteria and fungi, have almost disappeared; There are very few species of higher animals, with only a few species of larvae, insects, etc.
• Recovery zone: in this zone many organic substances have settled down in the form of residue. Residue is decomposed anaerobically at the bottom or in moving water. Because the water’s need for oxygen consumption is smaller than the surface aeration rate, the condition improves and the water becomes clearer. The amount of CO2, NH4 decreased and dissolved oxygen, NO2-, NO3- increased. Bacteria tend to decrease in number as the food supply is reduced; they are mainly aerobic. Green fungi and algae appear to use compounds containing nitrogen and CO2 and release oxygen to help clear and dissolve oxygen more strongly. Next, the need for oxygen consumption decreases; Algae species are also less; Protozoa, molluscs, water plants appear, fish populations also gradually stabilize and find food in this area.
• Clear water area: here the flow has returned to its natural state and there are normal plankton species of clean water. Due to the influence of nutrient levels due to previous pollution, plankton species will appear in large numbers. The water returns to a state of oxygen balance – the amount of dissolved oxygen is greater than the amount of oxygen consumed – the original state of the water has been completely restored.

Aeration process

The aeration process adds oxygen to the water source. Two sources of oxygen supply to water: air oxygen through turbulence and photosynthesis of water plants.

The ventilation process is calculated according to the formula:

In there:

D_a: initial oxygen deficiency; mg/l

D_t: oxygen deficiency over time t; mg/l

K₂: constant rate of oxygen dissolution in water

T: time of oxygen dissolution process