Procedures Disinfection of drinking and processed water
Water disinfection is a process with which disease causing microorganisms are destroyed. Disinfection of drinking water prevents spreading of contagious diseases caused by microorganisms which are transmitted by drinking water. For this reason water disinfection is an urgent process to prepare drinking water. Disinfection of processed water prevents the damage which would be done by microorganisms entering the technological process of water.
Procedures of disinfection:
Disinfection using chlorine gas is a bit older but most frequently used process and technology for water disinfection. Through centuries this kind of process has evolved to the extent to provide safety at a very high level. A vacuum process and some additional neutralisation of chlorine in the air contribute to its safety. The gas comes under pressure from a gas bottle or from a tank to vacuum regulator, where vacuum is created which prevents leakage of gas in case of eventual faults in the pipelines. Afterwards, the dosing controller using electromagnetic valve and automatics doses an adequate gas quantity regarding the flow and the estimated chlorine residual in the pipeline.
Sodium hypochlorite is also known as a bleaching agent in the form of solution and it is dosed depending on the active chlorine content proportional to the flow and the estimated chlorine residual. It can be prepared on the dosing spot from NaCl by means of electrolysis. Most of the part sodium hypochlorite is delivered in drums.
Chlorine dioxide is a strong disinfectant which disinfects water using a different method as chlorine. Since it is explosive, we generate it on the dosing spot from NaClO2 and HCl. Even though operational and investment costs are a bit higher than the ones for chlorine disinfection, this process is being more widely used due to its insensitivity to pH which contributes to formation of less harmful side products. It does not react to ammonia.
UV disinfection is carried out without adding any substances into the water, thus it does not form any side products. UV radiation, which is generated by special illuminants, penetrates the cells of microorganisms and prevents their multiplication. UV disinfection is not suitable for water with high content of suspended articles, turbidity and dissolved organic compounds.
Disinfection with ozone
Disinfection with ozone does not leave any side products. If dosed adequately and in timely manner it can be a highly efficient process. Since ozone is a highly reactive gas, it must be produced on the spot of dosing in special generators where strong electrical current splits the molecules of oxygen (O2) into individual oxygen atoms (O), which are then combined into the molecule of ozone (O3). The investment and operational costs are relatively high. Besides, operations and maintenance of this process is demanding as well.
Municipal waste water treatment
Municipal waste waters are generated in households and in public institutions. This type of waste water contains high level of nutritive and organic matter.
Treatment consists of the following phases: pre-cleaning (mechanical phase), primary cleaning, secondary cleaning (biological phase), tertiary cleaning, disinfection, sludge treatment.
Mechanical process is designed for removal of bigger particles (branches, rocks, glass, bottles, and rags) by means of rakes and sieves. Sand, gravel, egg shells, cigarette filters, seeds and grease are removed through grit and grease chambers. There are several types of grit chambers: gravity, ventilation, vortex grit chambers.
In the process of primary cleaning treatment sedimentary particles and a part of organic matter are removed. Lately, primary sludge settling tanks have been replaced with fine sieves in smaller treatment plants.
The biological cleaning treatment includes secondary and tertiary cleaning. Biochemical processes by means of microorganisms remove pollution.
Secondary treatment is carried out under aerobic conditions by means of air injection through membrane diffusers. Organic matter is removed and nitrogen in form of ammonia is transformed into nitrites.
Tertiary treatment is carried out under anoxic conditions where microorganisms consume organically bound oxygen. Nitrogen and phosphorus are removed through this method.
Various technologies are used for water treatment:
- technologies imitating natural systems (lagoons, irrigation systems of fields); simple and cheap for maintenance, suitable for smaller settlements
- classic continuous process of active sludge: tested, moderate investment and operational costs, great possibilities for optimisation, it needs much space
- SBR process of active sludge: smaller place for installation, lower investment costs, more complex operation, problems with floating sludge
- MBR (membrane bioreactor): smaller place for installation, high efficiency of cleaning, high operational costs
- MMBR (biological reactor with fitted biomass): lower investment costs, lower operational costs, more complex operation
The sludge occurring in the process of water treatment is treated through various processes:
- concentration (gravity, mechanically – centrifuges, belt thickeners)
- stabilisation (aerobic, anaerobic)
- dehydration (centrifuges, filter presses)
- drying process
During the operation of treatment plants much energy can be saved if the process is optimised and power consumption minimised:
- optimisation of sludge flow and inflow structures of sludge settling tank
- anaerobic decay, formation of bio gases and electrical power (suitable for larger treatment plants)
Protection against legionella with ultra filtration
It is a modern microbiological procedure for drinking water preparation (free from legionella, coli bacteria and other bacteriological pathogens) with no additional chemicals and no use of electro-chemical or thermal procedures. Ultrafiltration mechanically removes microorganisms and colloids from running water.
Apart from bacteria and particles the process of ultrafiltration removes also viruses and other microbiological ingredients of water. Substances dissolved in water, f.e. metals, remain in the water. Consequently this means that microorganisms are entirely left without food after the process of ultrafiltration is completed. The thickness of biofilms , which are food chain basis for legionella, is reduced. The taste of drinking water does not change.
Modules are composed of capillary membranes with a pore size of 0.01 µm. It is the so called ‘dead end’ filtration principle from the outside toward the inside of the capillary. Reverse rinsing (from inside toward the outside) with water and compressed air. Filtration and rinsing procedures are partially or completely automated, depending on the performance and size of the system.
There are systems available with flow of 1.5 m3/h or more.