Plants intended for wastewater recovery allow the re-use of rainwater, storing it in a reserve tank.
These plants allow the creation of the possibility to recover rainwater that, rather than being dispersed, can be used for external and internal uses.
• watering of public or condominium green areas;
• washing paved areas;
• car washes, meaning businesses;
• technological uses and supplying fire extinguishing systems.
• filling toilet flush tanks;
• filling washers (if prepared);
• water distribution for basements and car washing;
• various technological uses such as, for example, passive/active air conditioning systems.
The plant for optimizing rainwater recovery is made up of a filtering and storage part and an actual re-use part.
The plant usually has four intervention phases:
1) Water is collected from the gutters and conveyed to a filter that separates the water from larger suspended material.
2) Water is channelled inside the tank through a pipe so no turbulence is created.
3) Subsequent intake of the water in the tank occurs a few centimetres below the water level in order to collect cleaner water.
4) An electronic control unit controls a feed pump and the entire system.
The characteristic components of a rainwater recovery plant are:
The diverter separates the first flush (generally having more pollutants) from the water intended for storage.
The filter keeps any foreign bodies in the rainwater from entering the tank.
The tank is the heart of the entire rainwater recovery system.
The ideal conditions for water conservation are: oxygenated environment and the absence of light.
The choice of the type to be adopted depends on various factors.
a) Position. The position influences the distribution system (with or without pump) and its uses, the installation and maintenance costs, the form (compact for inside, resistant for being buried) and the materials used. The tank can be placed above ground (it generally contains water intended for irrigating or washing and the like), inside the building (in premises at ground level or underground) and buried (the most expensive due to the excavation necessary, but it has the advantage of keeping the shape of the tank out of view and allows the installation of large capacity items).
b) Capacity: to size these plants and to then determine the volume to assign to the accumulation tank, both the environmental characteristics (local rainfall, size and type of the collection surfaces, etc.) and the performance required (in relation to the number of inhabitants) must be evaluated.
c) Material: The tanks are made of materials that are compatible with the standards. They are generally made of fibreglass, polyethylene, or are made of concrete poured on site.
• Drain pipe
The drain pipe, shaped like a siphon, prevents the backflow of unpleasant odours from the disposal system. It is at a height equal to or lower than the input.
• Check valve
The check valve prevents the contamination of the water stored in the tank, inhibiting backflow of waters from the disposal system.
It is normally equipped with a grating filter that blocks access to the tank and other components upstream and animals and insects that could enter from the drain.
• Delivery pump
The delivery pump, controlled by an electronic control panel, picks up the water stored in the tanks and distributes it to the equipment that reuses it.
To prevent contamination, pipes and terminals on the recycling plant must be clearly marked and separate from those for potable water.
Refinement treatments for recovered water
After primary filtration of the large material, recovered rainwater can only be reused for irrigation with a subirrigation system.
For all of the other non-potable uses, a refinement phase is usually required to improve the quality characteristics of the water, especially regarding SS, BOD5, and bacteria.
Four systems are recommended as refinement treatments:
• disinfection (hypochlorite, UV, dioxide, etc.);
• filtration (sand and carbon);
• membrane filtration;
For the description of the techniques listed, refer to the previous chapters regarding potable water and wastewater.
The advantages from the installation of rainwater collection plants for individual use can reflect positively on both the private level:
• water savings;
• economic savings considering that the re-used water is free;
• absence of limescale deposits in pipes and on heating elements in machines (washer, dishwasher) and subsequent savings on electrical consumption;
• detergent savings (up to 50%) due to lower water hardness.
and the public level:
• they prevent the recurrence of overloading of sewer and disposal systems during heavy precipitation;
• they increase the efficiency of the purifiers (where the white and black sewer systems are not separate) subtracting large amounts of liquid from the outflow that, diluting the amount of slurry to be treated, will reduce the effectiveness of the biological purification phase;
• they retain and/or disperse the excess rainwater on site (for example during heavy storms) that is not absorbed by the ground on the municipal level due to the progressive impermeabilization of the soil, making upgrades to the public collection systems useless.
GREY WATER RECOVERY
Grey water refers to water from sinks, showers, and bath tubs (except for the toilet, bidet, and the entire kitchen) and that, due to their degree of contamination, can be collected, treated, and disinfected to then be returned for domestic use (inside rinse tanks) or reused for irrigation.
Operations regarding “grey water” are fundamentally comprised of:
• separation of the discharge networks of black water (containing toilet discharge) and grey water (all other discharged water);
• creation of distinct water distribution networks (potable water and non-potable water);
• treatment and re-use of purified grey water for non-potable uses such as irrigation of green areas, filling toilet flush tanks, and washing external areas.
Grey water recovery and re-use systems can reduce the consumption of potable water with particular efficiency in medium-large buildings or complexes that produce a consistent quantity of wastewater each day.
These buildings can be: hotels, guest houses, tourist facilities, retirement homes, housing complexes, apartment buildings, campsites, fitness centres, sports halls, swimming pools, hair salons, office buildings, motorway services, and companies which use showers.
Storage of grey waters is through underground tanks or tanks installed inside the building.
Grey water can be treated with a biological oxidation system or with an ultrafiltration system.
Water demand has grown in recent decades and is likely to expand further, due to social and cultural factors and the development of urban conglomerations and production companies. UN estimates predict a strong growth in the world population that, in 2025, will be about 9 billion people, 50% of which will be concentrated in the larger towns.
It is therefore necessary to intervene in order to ensure a water supply for the future, limiting the exploitation of water resources and protecting the receiving bodies.
The re-use of domestic, urban, and industrial wastewater purified with tertiary type innovative systems (membrane plants, ozone, ultra filtration, etc.) is one of the most effective methods for meeting the objectives listed above.
Purified water must meet a certain degree of quality, especially at the health and hygiene level. There is a long list of illnesses (dysentery, gastroenteritis, allergies, etc.) that can be contracted through wastewater and it is more than evident that there is a need for water disinfection for its re-use.
On the disinfection level, three primary objectives must be established: reduction of the microbial load, control of the chemical elements present, and limitation of possible contact between people and the wastewater.
It is important to diversify the complexity of the treatments and the re-use environments depending on the origin of the wastewater. Treatment plants can be set up to treat water from city drains (municipal wastewater) or industrial drains (industrial wastewater).
Based on the type of re-use, a more or less intense treatment will be performed. The complexity of the process used to treat the wastewater call for increasingly elevated quality levels depending upon whether it will be for agricultural, industrial, or potable re-use.
• Reuse for agriculture is one of the most adopted solutions, in various areas. These are mainly:
• direct use (in which the wastewater, which is more or less refined, is directly re-used for irrigation);
• indirect use (where the wastewater is put into a water body destined for irrigation use).
• Even industrial re-use has two possible solutions:
• reuse for general services (essential heating and cooling circuits);
• specific re-uses in various technological cycles (textiles, tanning, paper mills, steel mills, etc.).
• Re-use for potable use which calls for compliance with high quality standards (to prevent contamination with harmful and damaging substances), is divided into two types:
• a “direct” (closed cycle) re-use that calls for direct inflow of the treated wastewater in the water distribution system;
• “indirect” re-use that calls for intermediate storage of the wastewater in an artificial or natural basin before distribution in the network.
• Another category for re-use is for civil, non potable uses such as: irrigation of parks, green areas, and sports fields; domestic use in hygiene services (not in contact with people); commercial uses (e.g.: car washes); ornamental uses (e.g., fountains) that can be supplied by the so-called “dual systems” of distribution (with a network that transports “potable” water and another that contains re-use water for “non potable” use).