Reuse of water. Reuse and recycling of water in industry

Kaftanchikovo is a village in the Tomsk district of the Tomsk region, the administrative center of the Zarechny rural settlement. Population 1323 people. The village is located on the left bank of the Tom, 15 km from Tomsk, near the village there is the M53 highway. In the 16th century, several groups of Tatars led by Prince Toyan lived on the Tom River. Prince Toyan submitted a petition to Tsar Boris Godunov, in which, on behalf of the “Tomsk residents,” he asked to build a fortress in the lower reaches of the Tom River and accept the Tomsk Tatars into Russian citizenship. To which Boris Godunov gave his consent and in 1604 a detachment was formed to build a Russian fortress. In the summer of 1604 the fortress was built. Subsequently, the population of Tomsk grew. Russian peasant farmers settled here. In 1626, there were already 531 families living. The residents had to be supplied with bread; in 1605, the first grain crops appeared, and people took up agriculture. The villages of the Zarechny rural settlement are one of the oldest at the mouth of the Tom River, which arose in the period 1627 to 1630. The location for the villages was chosen well: close...

2.3 Recycled water in agriculture
Recycled water in agriculture provides significant savings in water consumption. Indeed, water consumption in the agro-zootechnical sector significantly exceeds consumption in the civil sector and industry. For Italy these figures are 60%, 15% and 25% respectively. In pursuance of European regulations (recognition of the provisions of European Directive 91/271), preference is currently given to recycled water, and connection to the main water supply - if the water is not intended for drinking purposes or ichthyogenic purposes - is limited to cases where it is not possible to use treated wastewater or when these economic costs are clearly prohibitive. Wastewater is supplied free of charge, and capital costs for organizing treatment systems are deducted from the tax base.
It should be taken into account that the use of recycled water in agriculture is not always possible, but only, for example, if the agricultural land where such technology is supposed to be used is located in a very remote area or at a lower altitude level.
Wastewater cannot be used when its chemical composition is incompatible with agriculture (excess sodium and calcium content compared to potassium and magnesium). It is important to note that the ridiculously low current price of ordinary tap water supplied for irrigation (determined by the cost of a license to connect to a source or drill a well) does not encourage the transition to using treated wastewater for these purposes. Agricultural wastewater treatment technology varies depending on the types of crops it is intended for. To irrigate crops intended for raw consumption, water must undergo clarification by flocculation, filtration and disinfection (sometimes lagooning). For irrigation of gardens and pastures - only clarification by flocculation (or biological sedimentation) and disinfection; for irrigation of fields with non-food crops - biological sedimentation (and, if necessary, reservoir baths).

2.4 Rainwater regeneration
In individual residential buildings, condominiums, and hotels, rainwater collected in storage tanks can be successfully used in the operating circuits of sanitary appliances, washing machines, for cleaning, watering plants, and car washing. In the private sector, it is estimated that up to 50% of daily water demand could be converted to using reclaimed rainwater.
Due to its characteristics, (very soft) rainwater, compared to tap water, gives the best results when used for watering plants and washing clothes. In particular, such water does not form deposits on pipes, cuffs and heating elements of washing machines and allows you to reduce the amount of detergent, not to mention the fact that you don’t have to pay anyone for it. In the public sector, it can be recommended for watering gardening areas and washing streets. In industry, rainwater can also be used in many production areas, which provides significant savings in water costs and has a significant impact on the cost of processes.
It should be borne in mind that rainwater does not require any special treatment at all: just simple filtering is enough as it flows down the roofs of buildings and ends up in storage tanks.
In a rainwater regeneration system, depending on where exactly the storage tank is located (for example, buried in the ground), a water pressure pump may be required. In Fig. Figure 5 shows a diagram of such a system.
Rainwater is considered undrinkable, therefore the supply pipeline and water supply points (water taps, connection points to household appliances) must be marked with a clearly visible warning sign: “the water is not suitable for drinking.”

Conclusion
Both domestic, municipal and industrial wastewater can be sent for recycling. Reuse is permitted provided that complete environmental safety is ensured (i.e. such use should not cause damage to the existing ecosystem, soil and cultivated plants), and any risk to the local population in terms of sanitary and hygienic conditions is eliminated. It is therefore essential that any such project carefully adheres to applicable health and safety regulations, as well as applicable industry and agricultural codes of practice.
In most cases, in order for water to be recycled, it must be pre-treated. The choice of the degree of such cleaning is determined by established sanitary and hygienic safety requirements and cost parameters. To organize the supply of secondary reclaimed water after treatment, a dedicated distribution pipeline is required.
According to Regulation 185/2003, there are three main categories regarding the use of reclaimed water:
– irrigation systems: watering cultivated plants intended for the production of food products for human consumption and domestic animals, as well as non-food products, watering landscaping areas, gardening areas and sports facilities;
– civil purpose: washing pavements and sidewalks in populated areas, water supply to heating networks and air conditioning networks, water supply to secondary water distribution networks (separate from the drinking water supply) without the right to directly use such water in civil buildings, with the exception of drainage systems for toilets and bathrooms;
– industrial use: supply of fire extinguishing systems, production circuits, washing systems, thermal cycles of production processes, with the exception of applications involving contact of secondary reclaimed water with food, pharmaceutical and cosmetic products.
Before reusing reclaimed water, a certain level of quality must be ensured, especially with regard to hygiene requirements. Traditional methods of treating discharged water are insufficient to ensure this quality. Today, new alternative cleaning and disinfection technologies are emerging, with the help of which it is possible to reduce the level of microbes, nutrients, and toxic substances in water and achieve the required level of water quality at a relatively low cost. The regulatory documentation presents the minimum acceptable quality parameters that water must have after regeneration if it is intended to be sent for recycling. The specified requirements (chemical-physical and microbiological) for reclaimed water intended for reuse for irrigation purposes or in civil installations are given in the table in the annex to regulation 185/2003. For water intended for industrial use, maximum permissible values ​​are established depending on specific production cycles. The construction of wastewater recovery systems and their subsequent use must be authorized by the competent authorities and subject to periodic inspection controls. Reclaimed water distribution networks must be specifically marked and distinguished from drinking water supply networks in order to completely eliminate any risk of contamination of the drinking water distribution network. The water points of such networks must be appropriately marked and clearly distinguished from drinking water points.
At the same time, with all the advantages that modern technology provides, in addition to direct benefits, the implementation of measures to save hydro resources may also entail certain risks.

Literature
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A huge amount of water is consumed for industrial and household needs. The situation is worsened by the discharge of contaminated liquid into water bodies. Paying attention to environmental protection and economic aspects of business, many enterprises are switching to recycled water supply. This method involves the repeated use of water resources. Reducing fresh water consumption and wastewater discharge leads to cheaper water supplies.

How does a closed water supply system work?

The most promising option for reducing water consumption is the creation of closed systems. Wastewater is treated with special equipment and reused. The components of a recycling water supply system depend on the volume of wastewater and the requirements for the quality of the purified liquid. A progressive installation can be found in production workshops, nuclear and thermal power plants, car washes, and country houses with autonomous sources.

P – production; OS – wastewater treatment, PS – pumping station, OX-cooling

Depending on the production processes, water may become contaminated the first time or may not require purification for a long time. A closed system is necessary in several cases:

1. The source used does not have enough water to meet the needs of the enterprise.
2. The source is located at a great distance from production workshops (up to 4 km), located at a significant altitude (25 m and above).

It is indispensable in regions with high water costs, excessive hardness or pollution of the source, in case of a real danger of poisoning nature by wastewater. Treatment complexes, depending on their purpose, include from one to six stages. Among them: pre-treatment in settling tanks, electroflotation, filtration, adsorption, reverse osmosis.

An electroflotator is a unit whose operation is based on the principles of electrolysis. It ensures the removal of chemical compounds and suspended particles from water. Its cleanup rates for pollution with petroleum products range from 75 to 90%, and for PVA residues – from 50 to 70%.

Cooling structures include settling ponds, cooling towers and splash ponds. In waterproof pits, water is cut into splashes using special nozzles and cooled by air currents.

The structural parts of the closed network are supply and return pipelines, circulation pumps, treatment facilities and filters, and cooling units. For reservoirs suffering from the discharge of poorly treated wastewater or hot water, such a system becomes a real salvation.

Installation of circulating water supply in production

Information. In addition to open cooling systems, there are closed structures in which water does not come into contact with air. The temperature decrease occurs due to heat exchangers.

Benefits of Reuse

High costs for the purchase and installation of equipment for recycling water supply do not become an obstacle to the introduction of modern technology at enterprises.

• Water requirements are reduced by 10 times.
• Significant financial savings.
• Responsible attitude towards the environment and rational use of resources.
• No fines for dirty wastewater.

Closed system principle

Turnover complexes in industry

Owners of enterprises who care about the environment and know how to count profits are switching to a progressive method - recycling water supply. The scope of its application is quite wide:

Energy

Enterprises in the energy industry - thermal and nuclear power plants - need water to cool turbines or as a working fluid - steam. Technical water supply to facilities occurs through two systems:

• direct flow;
• negotiable.

The process occurs as follows: steam is supplied to cooling towers, cooled and condensed. Using a pump, water is used to cool turbines and auxiliary mechanisms. Water is taken from their natural source to replenish losses inevitable in technological processes.

Cooling tower diagram

Metallurgy

In many technological processes, water is used exclusively for cooling. It does not become dirty, but only heats up, so after cooling it can be used again. At metallurgical enterprises, the recycling water supply scheme is more complicated. The liquid heats up and becomes contaminated with various impurities. Further use in gas purification will require cooling ponds or cooling towers and mechanical cleaning filters.

Oil refining

In modern oil refineries, 95-98% of all water used is in a closed cycle, including filtration and local treatment. For the chemical industry, closed systems are being developed that do not require the discharge of wastewater into water bodies.

Food industry

Recycling water supply is popular at industry enterprises. The systems for washing containers, packaging and raw materials operate on this principle. It is used in refrigeration units.

Mechanical engineering

Machinery factories use water in the processes of galvanizing parts. The closed system reduces its consumption by 90%. The use of an evaporation plant in a closed system allows the salt concentrate to be sent for processing. The purified liquid is used for washing parts, and the products from the concentrate are used for preparing electrolytic solutions.

The progressive method is being implemented in paper and pulp production, in the mining industry, in vehicle washing, and in laundries.

It is impossible to avoid water loss in industrial environments. A partial decrease in its volume occurs due to evaporation. The level of mineralization in the remaining liquid increases. This leads to negative consequences: active corrosion and salt deposition. Adding fresh water is important to restore the quantity and composition of the circulating fluid.

Schemes of circulating water supply systems

Attention. Liquid losses in a closed network are 3-5%. They are replenished with fresh water from the source.

Installation of a reverse system for a car wash

Technological processes associated with washing cars are accompanied by the consumption of large volumes of water and pollution of wastewater with petroleum products and PVA. To reduce the risk of hazardous compounds entering the natural environment, a wastewater reuse system is being introduced. Installing a closed water supply system at sinks allows you to save up to 90% of water and 50% of detergents.

Closed system at a car wash

Attention. To wash 10 cars, 1 m3 of water is required; when using a recirculating system, up to 50 cars can be washed with this volume of liquid.

Technical wastewater at a car wash goes through several stages of cleaning:

1. The wastewater ends up in a sump, a storage tank. Mechanical filtration removes large particles of contaminants from water.
2. The liquid is supplied by a pressure pump to the membrane flotator. Here, pressurized air is passed through ceramic membranes to saturate the effluent with bubbles. As a result, foam is formed that absorbs the remaining oil products and detergents. Pressure flotation removes fine sludge and suspended matter. These particles end up in a storage tank, from where they are periodically removed for further processing.
3. After the flotator, the water enters containers with filters to remove remaining particles. The installation is designed for repeated use; the filters are regularly washed with a reverse flow of water, which ends up in a wastewater storage tank.

Washing re-water supply scheme

For final treatment of the liquid, chemical (adding reagents) and biological treatment are used. Complete removal of contaminants occurs by microorganisms.

The car wash premises are equipped with two water circuits. They power powerful vehicles cleaning devices. One circuit is filled with fresh water, and the second is filled with recycled water. The liquid used after processing is used in the primary wash. It is used when applying detergents and pre-rinsing foam. The final rinsing of the machines is carried out with fresh water.

Attention. Rinsing with direct tap water will help prevent white streaks from appearing on the surface of your vehicle.

Recycled water supply for car washes is 90%, and fresh water for rinsing accounts for 10%. Wastewater treatment plants have different capacities - from 3 to 40 m 3 /hour. Low power systems are the most popular and are used in most manual and automatic car washes. High-performance installations are designed for large washing complexes with portal and tunnel type systems. Their basic equipment:

• settling tanks;
• filters;
• flocculation system;
• sensors and pressure gauges;
• pumps.

If necessary, the complexes are supplemented with water softening devices, aerators, reagent dispensers and other devices. The number of reuse cycles depends on the capabilities of the equipment. It ranges from 50 to 70 revolutions with cleaning. The cycle ends with the collection and disposal of the liquid.

Reversible system for a country house

In private homes, where it is possible to separate the sewerage and water supply networks, it is practiced to install a closed system that reduces the volume of fresh water consumed several times. Its implementation is an effective way to save resources. The system operates on the principle of reverse osmosis. One of its features is the need to periodically replace old water.

Equipment for water recycling system

Attention. One of the advantages of recycled water supply to a country cottage is an increase in the service life of an autonomous well.

The installation of special equipment makes it possible to ensure the operation of the circulating water supply. It includes multi-stage filters, various reagents and coagulants that bring the chemical composition of the liquid to sanitary standards. A powerful treatment plant combines three types of processes:

• mechanical;
• chemical;
• biological.

Network monitoring is carried out automatically; indicators are checked for compliance with specified parameters. To maintain efficient operation of the complex, certain climatic conditions are required:

• installation of a ventilation system for air circulation;
• temperature is not lower than +5 0.

A closed structure can have heating and plumbing. In the latter case, the development of biocenoses—a collection of microorganisms—occurs. Periodic washing of containers and pipes will help prevent components from biological fouling. Special substances polyalkylene guanidines provide protection against several destructive factors: corrosion, salts and biofouling.

Metal pipes are used for water supply installation. This material is strong and durable, but under the influence of changes in the composition of water, corrosive processes occur. Using plastic is the best way to create effective recycling. Polymers are neutral to moisture, chemicals and biological substances, therefore they are recommended for creating closed networks.

With industry's increasing demand for high-quality water, water conservation and reduction of water consumption and waste are being achieved through reuse and recycling.

Repeated or sequential use means the use of water in an open system for two sequential but different processes, in some cases with intermediate water pumping or purification. The second process usually has lower water requirements than the first and may therefore use lower quality water. The most common example is using water first for heat exchangers or condensers and then for flushing. Another example: wastewater from toilets and laboratories is collected, subjected to biological treatment, neutralized and then, after additional treatment, used as make-up water in open cooling systems. Special measures are taken to control the physical characteristics of the water, such as temperature and suspended solids, as well as any factor that may promote bacterial growth.

Circulation means the unlimited reuse of the same water for the same process, with water added only to replace losses that cannot be avoided: system blowdown or evaporation losses.

The circulation ratio can be very high, resulting in the concentration of inorganic or organic salts or the gradual accumulation of suspended solids and the need for continuous water treatment. Therefore, the following indicators of the quality of circulating water should be monitored:

• content of sulfates and carbonates of alkaline earth metals - to prevent their precipitation;
• the amount of all dissolved inorganic salts - to prevent an increase in the electrical conductivity of water and increased corrosion;
• the amount of decomposing organic matter, ammonium salts and phosphates that promote the growth of aerobic and anaerobic bacteria;
• detergent content - to prevent foaming and other undesirable phenomena;
• the amount of settling and suspended substances - to prevent fouling of equipment;
• temperature to avoid intermediate cooling or discharge of excessively hot water into the river.

Circulation ratio

Depending on whether evaporation of water occurs during the circulation process, the circulation ratio can be expressed in two ways. Concentration ratio:

where C is the ratio of the amount of make-up water a to the sum of water loss due to droplet entrainment and the flow rate for purging the system p.

In cooling systems with open cooling towers, provided that the surrounding air is clean, C is approximately equal to the ratio of the salinity content of the circulating water in the system S to the salinity content of the make-up water s:

C = S/s = a/p.

In cooling systems for condensers and heat exchangers, C usually varies from 1.5 to 6, but in extreme cases reaches values ​​from 20 to 40.

Since carbonates can be easily removed during make-up water treatment, sulfates are usually the main limiting factor.

When cleaning exhaust gases, the concentration due to evaporation is supplemented by the dissolution of some gases and salts. In this case, the concentration ratio no longer reflects the increase in salt content, which may be much greater in the presence of some of the indicated compounds or, conversely, less in the presence of precipitating or adsorbed compounds.

Circulation ratio R. If there is no evaporation or it is practically negligible, then R is the ratio of the circulating water flow Q to the make-up water flow:

When designing an industrial circulation system, special attention should be paid to uncontrolled conditions that limit the circulation ratio, most notably temperature rise. The presence of sulfates in water due to the use of inorganic coagulants in water preparation should also be taken into account.

The purpose of treating all or part of the circulation flow is to limit the accumulation of the above harmful compounds.

Depending on the properties of the connections to be removed, one of the following processes may be applied:

• general desalting using ion exchange or reverse osmosis; the latter process is commonly used to treat water in electroplating applications;
• clarification of water by settling to remove dust that gets into the water during gas purification, or particles that get into the water during the destruction of various materials;
• filtering through a granular bed to remove oxide particles and various crystalline particles.

If contaminants are contained in the circulating water in small quantities and a partial reduction in their concentration is required, only part of the water in the system is subjected to purification (from 5 to 50%); the bypass portion of the circulation flow is treated to reduce the alkalinity and hardness of the water, and atmospheric dust captured by the cooling water is removed by filtration.

Mineral coagulants should not be used in the above cleaning processes; Instead, it is better to use various polyelectrolytes. Purification of circulating water is often accompanied by anti-corrosion treatment or treatment to prevent the formation of deposits and biofouling.

WATER AND ENERGY SAVING IN URBAN ECONOMY
APPLICATION OF MODERN MEMBRANE TECHNOLOGIES

The problem of energy and resource conservation in housing and communal services is one of the most discussed today. The engineering infrastructure and, in particular, the city’s water management have a great potential for energy and resource conservation, which is already quite well covered in the literature. In our article we would like to consider a number of areas directly related to the use of wastewater and its energy potential, its purification and reuse.

The real source of energy is wastewater. According to University of California professor George Chobanoglus, almost 42 MJ of thermal energy can be obtained from 1 m 3 of wastewater when its temperature is reduced by 10 ° C, and the processing of organic substances contained in wastewater can be from 3 to 6 MJ per 1 m 3. In addition, in high-rise buildings it is possible to use the potential energy of water flowing down in sewer risers to partially reimburse the energy costs for its rise, but this is associated with a number of objective difficulties and is not currently being seriously considered.

Thermal energy of waste water

The idea of ​​extracting thermal energy from wastewater arose quite a long time ago, but the technologies are still in the process of development and testing. Depending on climatic conditions and the season of the year, wastewater has a temperature from 6-12 to 20-30 °C, i.e. it is a source of low-grade heat, and additional equipment is required to generate electricity or high-grade heat for thermal power plants, heating systems or hot water supply - as a rule, these are heat pumps. The resulting heat is most rationally used for primary heating of water at thermal stations or in heating and hot water supply systems of buildings.

It is interesting that heat exchange installations installed on domestic sewerage serve not only to heat buildings in winter, but also to effectively remove excess heat from air conditioning systems in warm seasons (Fig. 1).

In Russia, this technology was tested as an industrial experiment at the district thermal station (RTS) No. 3 of Zelenograd. The heat recovered from domestic wastewater from the main pumping station of the Zelenogradvodokanal plant was used to heat tap water in front of the steam boilers. To transfer heat, two coolants were used sequentially: an intermediate one - water and the main one (in heat pumps) - freon. The need for an intermediate coolant arose due to the fact that the pumping station was located half a kilometer from the RTS-3 territory. The thermal power of recycling was 1100-1400 kW with a wastewater flow rate of 400 m 3 /h with a theoretically possible power of about 2000 kW. The power consumed by the heat pump installation and circulation pumps was 550-680 kW.

An obvious way to increase the efficiency of heat recovery equipment by bringing the heat source and consumer as close as possible has led to the emergence of original solutions for private houses and apartments using local water heaters (Fig. 2). In fact, the device is a heat exchanger of a simple design: a smooth copper pipe insert into a sewer pipeline and a thin copper tube wound onto it, through which cold water flows to the water heater. Obviously, the contribution to water heating and energy savings will be no more than 30%, however, the simplicity of the design and low cost may be of interest to consumers.

The greatest success has been achieved in the field of producing biogas from sewage sludge. As noted above, 1 m 3 of waste liquid, depending on the BOD and COD values, contains from 3 to 6 MJ of potential thermal energy. To purify the same amount of wastewater, 1.2 to 2.4 MJ is required (aeration, pumping and dewatering of sludge, heating of digesters, etc.), therefore, the energy contained in the wastewater is 2-4 times more than is necessary for its cleaning. It should be noted that the indicated amount of energy can be extracted through complete anaerobic decomposition of all organic substances contained in domestic wastewater. In reality, in sewerage facilities, a significant proportion of organic matter is mineralized in biological treatment facilities, and sludge from primary and secondary settling tanks is used to “produce” biogas in digesters. In digesters, the sediment also decomposes only partially - no more than 40-50% of the mass of organic matter is mineralized, and a significant increase in the degree of decomposition of ash-free matter requires significant costs. Therefore, it will not be possible to completely convert aeration stations to self-sufficiency.

A striking example of the implementation of this technology in Russia is a thermal power plant with a capacity of 10 MW, operating on biogas from the Kuryanovsky wastewater treatment plant (Fig. 3). As a result of the implementation of this project, 70 million kWh, or 50% of electricity and heat, began to be received by the WWTP through its own production.

To directly generate electricity from wastewater, microbial fuel cells have been developed in recent years, in which microorganisms are used to convert the energy of chemical bonds of organic substances into electricity. Such elements perform a dual function, since they simultaneously partially purify wastewater from organic contaminants.

Reuse of wastewater

Around the world, the next step in water conservation is the reuse of domestic wastewater. Treated wastewater is used for artificial replenishment of ground and surface water, replenishment of drinking water supply sources, for irrigation and agriculture, for technical water supply to industrial enterprises, fire-fighting and household (non-drinking) water supply, and even for drinking water supply!

Reuse of wastewater can be divided into several categories (according to the degree of water purification and purpose).

1. Technical water supply and irrigation.
Here, municipal (domestic) wastewater is used that has undergone complete biological treatment and simplified post-treatment. The post-treatment scheme usually includes mechanical screens with fine slots, rapid filters and disinfection. However, when membrane bioreactors are used at main treatment facilities, post-treatment is not required at all.
The resulting process water can be used at the enterprise to produce demineralized water. In this case, what follows is a standard scheme, including preliminary purification (deep clarification and disinfection), one or two stages of reverse osmosis.

2. Household water supply (cleaning, watering, washing cars, flushing toilets, etc.).
For these purposes, it is convenient to use the so-called “gray drains” - from bathtubs and washbasins. In this case, they are processed according to a simplified scheme, including mechanical cleaning (removal of debris and clarification) and disinfection.
For general household waste, complete biological treatment is required, supplemented by tertiary treatment described in paragraph 1.

3. Drinking water supply.
It is in turn divided into indirect (replenishment of natural water reserves in drinking water supply sources) and direct. This requires complete biological treatment and deep tertiary treatment, usually including reverse osmosis in the final stages.

We can partly observe the reuse of wastewater for indirect drinking water supply on any large river, where upstream settlements discharge treated wastewater, which is mixed with river water and subsequently, after additional treatment under natural conditions, goes to water intakes located downstream. In our article, we mean by this the targeted replenishment of water reserves in non-flowing water supply sources - reservoirs, lakes and underground horizons.

As for direct drinking water supply, the psychological factor plays a big role here, and only serious reasons can motivate people to accept the fact that they will drink water that recently flowed through the sewer.

There are few such examples in the history of water supply; most of them remained within the framework of experiments conducted abroad in different years. Here are a few of the most typical ones.

“Classic” example: Windhoek, Namibia. The first municipal wastewater treatment station for drinking water supply with a capacity of 4,800 m 3 /day. was built back in 1968, and in 1997-2002 it was reconstructed with an increase in water supply to 21,000 m 3 / day. The decisive factor was the lack of accessible sources of water supply - all possible resources were either already exploited or their development was not economically viable, including the collection of rainwater in this arid and hot region.

The purification scheme was very complex and included dosing of powdered activated carbon (PAH), primary ozonation, dosing of coagulant and flocculant, flotation, dosing of potassium permanganate (KMnO4) and caustic soda (NaOH), filtration on a double-layer granular charge, secondary ozonation , treatment with hydrogen peroxide (H2O2), biosorption on granular activated carbon (GAC), sorption on GAC, ultrafiltration and disinfection with liquid chlorine. The cost of water purification was $0.76/m3. The resulting water was mixed with drinking water obtained from traditional water supply sources directly in the city's distribution network.

Example 2. In 1976-1982, the American company Pure Cycle Co. installed systems for complete treatment of domestic wastewater in private homes in Colorado to create a closed cycle and produce drinking water. The installation included a mesh for mechanical cleaning, a bioreactor with immobilized biofilm, a fabric (bag) filter, ultrafiltration membranes, an ion exchange filter, a GAC ​​filter and a bactericidal lamp. Due to financial difficulties, the company soon stopped servicing its installations and their use was discontinued, but residents continued to operate them for some time and demanded permission from the state authorities for their continued use.

Example 3. International Space Station. In 2009, a new system was delivered to the ISS for obtaining drinking water from urine and moisture condensed from the station’s atmosphere (steam and sweat emitted by humans). The urine processing scheme includes multi-stage filtration, distillation, catalytic oxidation and ion exchange.

The scale of wastewater reuse is well characterized by the following examples:

• Vulpin, Belgium. 6850 m 3 /day, post-treatment of municipal wastewater to replenish groundwater reserves used for drinking water supply, the scheme includes microfiltration, reverse osmosis and ultraviolet treatment;
• Ipswich, Australia. 230,000 m 3 /day, post-treatment of municipal wastewater for cooling thermal power plant equipment, the scheme includes microfiltration and reverse osmosis;
• Orange, USA. 265,000 m 3 /day, post-treatment of municipal wastewater for groundwater replenishment, the scheme includes microfiltration, reverse osmosis and ultraviolet and hydrogen peroxide treatment;
• Singapore, project "NEWater". 5 stations with a total capacity of about 450,000 m 3 /day, post-treatment of municipal wastewater to replenish water sources used for drinking water supply, industrial use and as water for non-potable purposes, the scheme includes microfiltration and reverse osmosis;
• Sulaybiya, Kuwait. The world's largest wastewater treatment plant 311,250 m 3 /day. (for purified water), the scheme includes mesh filters, ultrafiltration (8704 X-Flow, Norit devices), reverse osmosis (21,000 Toray devices), CO 2 blowing, chlorination. The treated water is used for industrial purposes, and the reverse osmosis concentrate is discharged into the Persian Gulf. Quality of purified water: suspended solids, BOD, ammonium nitrogen, nitrates (by N) - less than 1 mg/l, phosphates (by PO4) - 2 mg/l, petroleum products - less than 0.5 mg/l, total salt content - 100 mg /l.

It can be concluded that currently the key technology for reusing wastewater is membrane technology - in the vast majority of cases, post-treatment schemes include one or more stages of membrane separation: micro- or ultrafiltration and reverse osmosis. It can be said differently: without reverse osmosis and ultrafiltration, such a large-scale use of wastewater in the water sector would be impossible.

For more than 10 years, membrane bioreactor technology for wastewater treatment has been successfully developing all over the world. Initially, the use of ultrafiltration instead of secondary sedimentation made it possible to reduce the size of structures and increase the efficiency and stability of treatment. Now we can consider membrane bioreactors as a technological solution that allows us to immediately, in the main technological chain, obtain water of technical quality for irrigation, industry, and household needs.

It is interesting to note that the three largest wastewater treatment plants with membrane bioreactors are located in China.

A good example of systematic wastewater management is Australia, a country with limited freshwater resources. One of the large projects was implemented in the Sydney area, where a second, non-potable water supply for household needs was laid parallel to the drinking water supply. The system provides water to more than 60 thousand people and its supply is 13,000 m 3 /day.

The technological chain consists of the following structures:

• main structures: grate, sand trap, primary settling tank, bioreactor (aeration tank), secondary settling tank;
• tertiary treatment facilities: coagulation with aluminum sulfate, settling tank (tertiary), rapid filter. After rapid filters, part of the water is disinfected and released into marshy areas, while the other part goes to membrane microfiltration (0.2 microns) and, after disinfection, is sent to the distribution network.

The fee for using treated wastewater in Sydney is approximately $2,068/m 3 , while the cost of tap water is only slightly higher at $2,168/m 3 . There is also an annual flat fee of $125 for a city water connection and $34 for a non-potable water connection.

The water pipeline through which treated wastewater flows, pipelines and fittings are marked with lilac paint; water points are equipped with warning signs: “reused water, do not drink”, “not potable water”, etc. (Fig. 4). Similar labeling is used in the USA, where non-potable domestic water supply systems based on post-treated wastewater have become widespread.

Water reuse systems can be of completely different sizes - from an entire city to one building and your own apartment. Systems such as the AQUS Gray Water Recycling System (Fig. 5) or the Aqua2use Greywater System (Fig. 6), which represent a small collection tank with a low-power pump and a simple mechanical cleaning system, can be used in apartments. Possible water savings when using such installations are up to 30%.

There are also almost curious designs (Fig. 7).