A type of treatment where the wastewater and sludge containing microorganisms are mixed together and have air bubbled through them in a treatment tank known as a reactor. The microorganisms use the organic matter in the wastewater as food, breaking it down into smaller molecules. The introduction of air speeds up the process. The sludge that accumulates in the reactor contains billions of actively growing bacteria. It is the bacteria that make the sludge turbid and brown. This sludge is routed to a settling tank (also called a clarifier), where the bacteria are separated from the treated water. A portion of this “activated sludge” is returned to the beginning of the reactor to continue the process as influent wastewater enters the reactor.
This process is used to “make assurance doubly sure.” Hydrogen peroxide, ultraviolet (UV) light, and other processes are used in combination as they form a powerful oxidant that provides further disinfection of the water and breaks down the remaining chemicals and pollutants.
Bacteria are simple, single-celled organisms. Many thousands of different species of bacteria are always around us. They have fundamental and essential roles to play in nature and are an integral part of our lives. The human body contains more bacteria cells than human cells. We are dependent on an important group of bacteria, those that help us digest our food. A few bacteria are pathogens, and some of these can be transmitted in water. In developed countries with drinking water disinfection standards and wastewater collection and treatment systems, deaths from waterborne bacterial diseases, such as typhoid and cholera, are uncommon. Nevertheless, in many parts of the world, these diseases persist because of the lack of water and wastewater infrastructure.
A shallow vertical well used to collect feedwater for a desalination plant that makes use of beach sand as a filter medium. Beach wells have the advantage of minimizing impacts on marine life and reducing pretreatment requirements; however, there is often a limitation on the capacity of water that they can produce and some wells may affect other local groundwater supplies.
The presence or growth of organic matter in a desalination system that can hinder its operation or performance. Biofouling can usually be controlled with proper pretreatment.
Modern BNR plants reduce phosphorus as well as nitrogen and carbon and can produce a very high-quality effluent. The nutrients are reduced by allowing the effluent to flow through different zones in treatment reactors where conditions are made ideal to enable various bacteria to perform their work. The conditions are varied by controlling the amount of oxygen that is available, either dissolved in the water or combined with other molecules.
These ancient microscopic organisms, sharing the characteristics of both algae and bacteria, are abundant and exist naturally in both fresh and salt water. When phosphorus and nitrogen are abundantly present in the right proportions (see eutrophication) and weather conditions are warm, blooms of cyanobacteria may occur. The blooms may cause taste and odor problems in water and produce poisons. Blue-green algae growing in fresh water need nitrogen and phosphorus to grow. The presence or absence of available phosphorus can stimulate or limit their growth.
The word brack is derived from the Dutch brak, which means “salt”. This is a general term for water with a salt content that is too high to drink, but significantly less than that of seawater. As groundwater flows through an aquifer, it reacts with the surrounding soils and rock minerals, resulting in salt concentrations between seawater and fresh water. Most brackish water has a total dissolved solids concentration that ranges from 1,000 to 10,000 milligrams per liter. With treatment (desalination) brackish water can be a source of drinking water.
An obsolete term formerly used to describe the concentrated salt water that remains after desalination process. See also “concentrate.”
The organic matter in wastewater (that is, food) is digested by carbon-eating microorganisms whose activities are speeded up by the introduction of plenty of oxygen. When food becomes available, the bacteria feed, grow, and divide repeatedly to produce millions more of their kind, and they continue to do so until the food is used up. Much of the organic matter is broken down and converted to carbon dioxide, water, and new cell tissue, but a few less biodegradable compounds remain, leaving the water a bit murky or cloudy. After settling out large particles, this effluent may be discharged into waterways that have adequate flow to assimilate the remaining pollutants or used for irrigation in areas where there is no public access. Disinfection is normally done before discharge.
Phosphorus and other unwanted substances that make the water look muddy or murky can be reduced by the addition of chemicals that cause them to settle out of the water. Alum (aluminum sulphate) and ferric chloride are often used for this purpose. This chemical precipitation process is mainly used in water treatment plants (see flocculation).
Particles suspended in water naturally repel other particles. They cannot join to form larger particles that would settle more quickly. A coagulant, such as alum, is added to the water to overcome the repulsion between the particles. Flocculation involves gently mixing the water so that the particles can combine and form larger particles. During sedimentation, larger particles gradually sink to the bottom of the tank where they can be removed.
The siting of a desalination plant near a water or wastewater plant or electric generating plant so both facilities are able share infrastructure such as a seawater intake or concentrate disposal system.
The water that contains the salt removed during the desalination process. In a seawater desalination plant, the concentrate has a salt content that is almost double that of typical seawater. Most modern desalination plants discharge concentrate to the sea in such a way as to ensure its rapid mixing with the seawater, mitigating environmental impacts.
Constructed wetlands are created by converting uplands or by enhancing existing wetlands to a wetland designed to provide specific treatment functions. Constructed wetlands may be an integral part of the wastewater treatment system designed to meet water quality objectives.
A method used to dispose of reverse osmosis concentrate, usually employed in inland areas, where the concentrate is injected through a well into a stable underground geologic formation.
The process of removing dissolved salts and minerals from seawater or brackish water. It is also called desalinization or desalting. Desalination is an advanced technical process involving many different technologies, including distillation, reverse osmosis, and membrane filtration. Desalination produces drinking water and concentrate (the water that contains the salts that were removed in the desalination process, which used to be called brine).
An obsolete form of “desalination.”
The process of desalinating water. See “desalination.”
The location where a desalination plant concentrate is discharged into a receiving water body; also referred to as an “outfall.” The term “discharge” can also be applied to the concentrate itself.
is carried out to inactivate pathogens (bacteria, viruses, and protozoa—see wastewater). Commonly used disinfectants include chlorine (and its derivatives), ultraviolet (UV) light, and ozone. Chlorine and its derivatives are used to disinfect drinking water because they provide residual disinfection that protects the water as it goes through the pipes to homes and businesses. There are some concerns about the potentially health-damaging byproducts from chlorine disinfection. This risk is minimal, however, if the water is filtered and clear before it is disinfected. UV light (which occurs naturally in sunlight or is generated from special lamps) and ozone are increasingly being used to disinfect treated water. They do not provide residual disinfection, so chlorine is normally added after treatment with UV light or ozone.
A two-step evaporative process used to desalinate seawater. Distillation separates dissolved salts from seawater by boiling the water, then capturing and cooling the resulting vapors to produce pure water. Distillation is still the most common method of desalinating seawater in Middle Eastern countries.
In our homes, we use water for washing our food, dishes, clothes, and bodies, and for toilet flushing. The used water that goes down the drain or is flushed down the toilet is called sewage. Because a considerable amount of water is used to carry away only a small quantity of waste, domestic sewage is mostly water.
is water that has been treated sufficiently for human consumption, whether used for drinking or in the preparation of food. Potable is another word for drinkable. In the United States, potable water conforms to the Safe Drinking Water Act. Risk management is now being applied to ensure water quality. An example of a risk analysis procedure is the Hazard Analysis and Critical Control Point (HACCP), developed and applied in Australia.
Highly treated water can be used to augment drinking water supplies from other sources. It may also be recycled back into the drinking supply without prior discharge to the environment (see environmental barrier). In the United States and Australia, regulators have discussed the need for guidelines to be developed for this purpose.
is a liquid that flows out of something, particularly from a wastewater treatment process.
The capture of small organisms, including the eggs and larvae of fish and shellfish, into a desalination intake system. Intakes for modern desalination plants are located in areas to minimize entrainment, and the size of the screen openings may be reduced to 1 to 3 mm to prevent the passage of most small organisms.
The recovery of pressure-energy from RO concentrate to reduce the overall electrical energy consumed by a desalination plant.
If highly treated water from water purification processes is discharged to a reservoir or groundwater, the time spent in storage before use is said to form an environmental barrier. Some believe this step is needed to put the water “back to nature” where it can mix with other sources and lose its identity because the public lacks knowledge of the water cycle and the word “nature” is helpful to enhance acceptance. Others suggest that blending the water into an existing water supply source is required in case there is an operational upset resulting from plant breakdowns or operator error. Some consider that the insistence on an environmental barrier indicates that something is wrong with the water and it needs further treatment in the environment or the utility lacks faith in its technology and operators. However, no consensus exists whether the environmental barrier attenuates or amplifies public concern. The notion of an environmental barrier may have come from a prior time when technology was not as robust as it is today and dilution and delay were needed to improve the quality of the effluent. There have been no studies to determine if, in fact, dilution and delay are needed for safety or whether proper treatment, by itself, without an environmental barrier can reliably produce water of appropriate quality. Modern-day technologies with a number of process safeguards produce a reliably high quality of water . Some reason that the environmental barrier could well be replaced with a simple storage tank that serves to dampen out flows to the downstream system as well as serve as a final quality measuring station.
A natural or artificial pond that converts solar energy to heat to evaporate water from desalination plant concentrate. Evaporation ponds can be an effective method of concentrate disposal in arid regions for small- to mid-sized plants.
This word comes from the Greek, meaning “well fed.” If a stream or water body becomes overloaded with nutrients, plants and algae grow excessively, then die, and gradually accumulate in the water. The process is known as eutrophication. In some lakes and rivers eutrophication can occur naturally over a long period of time, but human activity speeds it up! Reservoirs can also be subject to eutrophication. Nutrients are critical for life, and insufficient nutrients can lead to water bodies with little life in them. These water bodies would require the addition of nutrients to reestablish productive fisheries. The objective is to maintain the nutrient balance appropriate to life in the water. Eutrophication is more likely to occur in warmer water. Eutrophic water bodies generally have the following:
• Lower overall oxygen levels and big differences between day and night time levels
• Lower biological diversity than desired
• Different species from non-eutrophic water
• Higher nutrient levels
• Overall poorer water quality
Drinking water is most often filtered by allowing it to pass through a thick bed of sand. Other media of varying particle size and density are available—some filters use a mixture of media ( for example, sand and anthracite [coal]) depending on the water source and the types of particles to be removed. Permeable membranes made from plastics are also used for filtration at some locations.