Coagulation and membrane filtration
At water treatment plants, contact coagulation is used for clarification: the coagulant solution is fed through a system of perforated pipes, and flocs form directly within the filter bed (process speed is 4–5 m/h). The use of contact clarifiers makes it possible to exclude bulky settling tanks from the process flow diagram.
Modern alternatives include membrane methods (installed after classical treatment facilities):
- Ultrafiltration: membranes with 0.05–0.20 µm pores trap organic matter, reducing water color. This technology enables reagent-free surface water treatment, eliminating sedimentation and the use of coagulants.
- The combination of ultrafiltration and nanofiltration allows for targeted control of the ionic composition of drinking water.
Disinfection methods: chlorine, ozone, and UV
Standard filtration leaves up to 30% of bacteria free. The risk of death from untreated water is hundreds of times higher than the carcinogenic hazard of chemical disinfectants.
- Chlorination is the basic method. Gaseous chlorine reacts with water to form hypochlorous acid. It decomposes to release atomic oxygen, which provides the bactericidal effect. Under normal chlorination, the amount of residual chlorine is 0.3–0.5 mg/L. The chlorine dose is calculated individually: it equals the sum of chlorine demand (consumption for oxidizing organics) and residual active chlorine.
- Ozonation is disinfection by gas. Ozone destroys bacterial and viral proteins in 10 minutes, decolorizes water, and eliminates odors. Disadvantages include high energy consumption and the risk of secondary microbial regrowth in the distribution network.
- Ultraviolet (UV) irradiation is performed using mercury-quartz lamps. It rapidly kills pathogens and does not change the properties of water, but requires it to be perfectly transparent.
Side effects of strong oxidizers
The impact of powerful oxidizers on impurities leads to the synthesis of hazardous substances:
- Halogen-containing compounds (HCCs). Chlorination synthesizes over 100 types of HCCs (e.g., chloroform). They promote tumor development.
- Aldehydes. Ozonation transforms humic substances. When organics are in excess, ozone promotes the appearance of toxic formaldehyde (causing gastric papillomas) and bromates (kidney tumors).
- Chlorites. Replacing free chlorine with chlorine dioxide reduces HCC production, but leads to the formation of chlorates and chlorites, which cause compensated anemia.
UV irradiation is considered the safest method: the formaldehyde level during UV treatment does not exceed 3% of the maximum allowable concentration (MAC).
Mineral composition correction: fluoride
Fluoridation from birth reduces the incidence of dental caries by 50–60%. Sodium fluoride or sodium fluorosilicate is added to the water.
If the fluoride concentration in the source exceeds 1.5 mg/L, fluorosis develops. The disease begins with chalky spots on the enamel, progresses to pigmentation and transverse striation ("tigroid incisors"), and in late stages leads to systemic skeletal destruction. The pathogenesis is associated with the toxic effect of fluoride on ameloblasts.
For defluoridation, alumina coagulation, ion-exchange resins, freezing, or blending with water from another source are used.
Household filters
Due to worn-out infrastructure, many resort to advanced treatment at the point of use.
- Sorption filters (carbon, resins). Reliable only with constant monitoring. The lack of a replacement indicator makes their use risky.
- Track-etched membranes (e.g., Nerox systems). They are a structural analogue of living cell membranes with strictly identical 0.2 µm pores. They retain all larger contaminants. They require manual sponge washing of the membrane when the filtration rate drops by 30%.
Briefly: answers to topical questions
What methods of drinking water chlorination exist at water treatment plants?
The following methods of drinking water chlorination are used at water treatment plants:
- Normal-dose chlorination — the amount of residual chlorine is 0.3–0.5 mg/L.
- Hyperchlorination (high-dose chlorination) — the amount of residual chlorine exceeds 0.5 mg/L; subsequent dechlorination is required.
- Double chlorination — chlorine is added twice: before filtration and after it; primary chlorination improves the water clarification process.
- Chlorination with preammoniation or ammoniation.
- Chlorination with chlorine dioxide.
What clinical forms (stages) does endemic dental fluorosis go through?
Endemic dental fluorosis goes through four degrees of damage, manifesting as specific clinical and morphological forms. According to the classification of odontopathies in fluorosis, the following stages are distinguished:
- Striae form
- Spotted form
- Chalky-mottled form
- Erosive form
- Destructive form
In the later stages of the disease, demineralization processes predominate and enamel permeability increases due to an increase in the volume of micropores.
Which infectious diseases are predominantly waterborne?
A wide range of infectious and parasitic diseases are waterborne. The main groups include:
- Acute intestinal infections — cholera, typhoid fever, paratyphoid A and B, dysentery (specifically caused by S. flexneri), enteritis, and enterocolitis.
- Viral infections — viral hepatitis A and E, poliomyelitis, rotavirus, adenovirus, and enterovirus infections.
- Bacterial zoonotic infections — tularemia, leptospirosis.
- Protozoal infections — amebic dysentery, giardiasis, balantidiasis.
- Helminthic invasions — biohelminthiases and geohelminthiases.
- Contact-transmitted infections (during swimming) — trachoma, scabies, fungal infections.
According to Sanitary Rules and Norms (SanPiN), by which groups of indicators is drinking water quality assessed?
According to sanitary regulations, drinking water quality is evaluated by four main groups of indicators ensuring its safety. These include:
- Organoleptic indicators — odor, taste, color, turbidity.
- Chemical indicators — total hardness, mineralization, content of nitrates, sulfates, chlorides, fluorides, heavy metals (lead, arsenic, mercury, etc.), and aluminum.
- Microbiological indicators — total microbial count, presence of total and thermotolerant coliform bacteria, coliphages, spores of sulfite-reducing clostridia, and Giardia cysts.
- Radiation indicators — total alpha radioactivity and total beta activity.
What toxic compounds are formed during the chlorination of water containing phenols?
In the presence of natural and anthropogenic phenols in water, molecular chlorine leads to the formation of dioxins. Dioxins are highly toxic and highly accumulative compounds that cause suppression of the body's immune defense.
What are the physicochemical requirements for water prior to UV irradiation?
Before water disinfection by ultraviolet irradiation, very high water transparency is required.
Calculation of the chlorine dose: OD (Oxygen Demand + Residual chlorine). The total dose consists of the chlorine used for oxidizing impurities (demand) and the amount remaining to maintain the effect (residual).
Class and exam questions
Why can ozonation pose a bacteriological hazard?
Under the action of ozone, humic substances are transformed into biologically active compounds that become a favorable nutrient medium for the growth of Klebsiella and Pseudomonas in pipes.
What are the hazards of petroleum products and surfactants during disinfection?
Upon contact with chlorine or ozone, there is a sharp increase in the intensity of odor and color of the water, and the resulting transformation products are often more hazardous than the parent substances.
What is the characteristic feature of enteric viruses in water treatment?
Rotaviruses and enteroviruses have a higher resistance to standard chlorine preparations compared to bacteria and can survive in water from several months to several years.
What else to study on this topic
- New-generation guanidine disinfectants (Anavidin, Phlogucid)
- Double chlorination technology for water
- Ozonosorption treatment method at water treatment plants
- Specifics of field hyperchlorination and dechlorination methods
- Waterborne transmission routes of viral hepatitis A and rotaviruses
Master the entire topic in the "Hygiene" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Wastewater Treatment" and 530 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in "Chapter 2. Ecological and Hygienic Problems of the Hydrosphere and Water Supply"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.