Очистка воды

Диспетчерская и схемы водоочистной станции Лак-де-Брет , Швейцария

Очистка воды - это процесс удаления из воды нежелательных химикатов, биологических загрязнителей, взвешенных твердых частиц и газов. Цель состоит в том, чтобы производить воду, пригодную для конкретных целей. Большая часть воды очищается и дезинфицируется для потребления человеком ( питьевая вода ), но очистка воды также может выполняться для множества других целей, включая медицинские, фармакологические, химические и промышленные применения. Используемые методы включают физические процессы, такие как фильтрация , осаждение и дистилляция ; биологические процессы, такие как медленные песчаные фильтры или биологически активный уголь ; химические процессы, такие как флокуляция ихлорирование ; и использование электромагнитного излучения, например ультрафиолетового света .

Очистка воды может снизить концентрацию твердых частиц, включая взвешенные частицы , паразитов , бактерии , водоросли , вирусы и грибки, а также снизить концентрацию ряда растворенных и твердых частиц.

Стандарты качества питьевой воды обычно устанавливаются правительствами или международными стандартами. Эти стандарты обычно включают минимальную и максимальную концентрации загрязняющих веществ в зависимости от предполагаемого использования воды.

Визуальный осмотр не может определить, надлежащего качества вода. Простых процедур, таких как кипячение или использование домашнего фильтра с активированным углем , недостаточно для обработки всех возможных загрязнений, которые могут присутствовать в воде из неизвестного источника. Даже природная родниковая вода, считавшаяся безопасной для всех практических целей в 19 веке, теперь должна быть проверена, прежде чем определять, какой вид лечения, если таковой требуется, необходим. Химический и микробиологический анализ , хотя и дорогостоящий, является единственным способом получить информацию, необходимую для принятия решения о подходящем методе очистки.

Согласно отчету Всемирной организации здравоохранения (ВОЗ) за 2007 год, 1,1 миллиарда человек не имеют доступа к улучшенным источникам питьевой воды; 88% из 4 миллиардов ежегодных случаев диарейных заболеваний объясняются небезопасной водой и несоответствующими санитарно- гигиеническими условиями , а 1,8 миллиона человек ежегодно умирают от диарейных заболеваний. По оценкам ВОЗ, 94% этих случаев диарейных заболеваний можно предотвратить путем изменения окружающей среды, включая доступ к безопасной воде. ^[1] Простые методы обработки воды в домашних условиях, такие как хлорирование, фильтры и солнечная дезинфекция, а также ее хранение в безопасных контейнерах могут ежегодно спасать огромное количество жизней. ^[2] Снижение смертности отБолезни, передаваемые через воду, являются одной из основных целей общественного здравоохранения в развивающихся странах.

Источники воды

1. Подземные воды : вода, выходящая из некоторых глубоких грунтовых вод, могла выпасть в виде дождя много десятков, сотен или тысяч лет назад. Слои почвы и горных пород естественным образом фильтруют грунтовые воды с высокой степенью прозрачности и часто не требуют дополнительной обработки, кроме добавления хлора или хлораминов в качестве вторичных дезинфицирующих средств. Такая вода может поступать из родников, артезианских источников или может добываться из скважин или колодцев. Глубокие грунтовые воды обычно очень высокого бактериологического качества (то есть патогенные бактерии или патогенные простейшие обычно отсутствуют), но вода может быть богата растворенными твердыми веществами, особенно карбонатами и сульфатами.из кальция и магния . В зависимости от слоев, через которые протекала вода, также могут присутствовать другие ионы, включая хлорид и бикарбонат . Может потребоваться снизить содержание железа или марганца в этой воде, чтобы сделать ее пригодной для питья, приготовления пищи и стирки. Также может потребоваться первичная дезинфекция . Где подпитка подземных вод (процесс, при котором речная вода закачивается в водоносный горизонт для хранения воды в периоды изобилия, чтобы она была доступна во время засухи), грунтовые воды могут потребовать дополнительной обработки в зависимости от применимых государственных и федеральных правил.
2. Высокогорные озера и водохранилища : обычно расположенные в верховьях речных систем, горные водохранилища обычно располагаются над любым человеческим жилищем и могут быть окружены защитной зоной, чтобы ограничить возможность загрязнения. Уровни бактерий и патогенов обычно низкие, но присутствуют некоторые бактерии, простейшие или водоросли . Там, где возвышенности покрыты лесом или торфяником, гуминовые кислоты могут окрашивать воду. Многие горные источники имеют низкий уровень pH, который требует корректировки.
3. Реки , каналы и низинные водоемы: поверхностные воды с низкой сушей будут иметь значительную бактериальную нагрузку и могут также содержать водоросли, взвешенные твердые частицы и различные растворенные компоненты.
4. Производство атмосферной воды - это новая технология, которая может обеспечить питьевую воду высокого качества за счет извлечения воды из воздуха путем охлаждения воздуха и, таким образом, конденсации водяного пара.
5. Сбор дождевой воды или сбор тумана, которые собирают воду из атмосферы, можно использовать, особенно в районах со значительными засушливыми сезонами и в районах, где бывает туман даже при небольшом дожде.
6. Опреснения из морской воды с помощью перегонки или обратного осмоса .
7. Поверхностные воды : пресноводные водоемы, открытые для атмосферы и не относящиеся к грунтовым, называются поверхностными водами.

Уход

Цели

Цели обработки - удалить из воды нежелательные составляющие и сделать ее безопасной для питья или пригодной для определенных целей в промышленности или медицине. Доступны самые разнообразные методы удаления загрязняющих веществ, таких как мелкие твердые частицы, микроорганизмы и некоторые растворенные неорганические и органические материалы, или стойкие фармацевтические загрязнители, загрязняющие окружающую среду . Выбор метода будет зависеть от качества обрабатываемой воды, стоимости процесса очистки и ожидаемых стандартов качества обработанной воды.

Приведенные ниже процессы обычно используются на водоочистных установках. Некоторые или большинство из них могут не использоваться в зависимости от масштаба предприятия и качества сырой (исходной) воды.

Предварительная обработка

1. Перекачивание и локализация - большая часть воды должна откачиваться из источника или направляться в трубы или сборные резервуары. Чтобы избежать добавления загрязняющих веществ в воду, эта физическая инфраструктура должна быть изготовлена ​​из соответствующих материалов и построена таким образом, чтобы не происходило случайного загрязнения.
2. Просеивание ( см. Также сетчатый фильтр ). Первым этапом очистки поверхностной воды является удаление крупного мусора, такого как палки, листья, мусор и другие крупные частицы, которые могут помешать последующим этапам очистки. Большинство глубоких подземных вод не нуждаются в проверке перед другими этапами очистки.
3. Хранение. Вода из рек также может храниться в прибрежных водохранилищах от нескольких дней до многих месяцев, чтобы обеспечить естественную биологическую очистку. Это особенно важно при обработке медленными песчаными фильтрами . Резервуары для хранения также служат буфером против коротких периодов засухи или позволяют поддерживать водоснабжение во время кратковременных инцидентов загрязнения в исходной реке.
4. Предварительное хлорирование - на многих заводах поступающая вода хлорировалась, чтобы минимизировать рост организмов-обрастателей на трубопроводах и резервуарах. Из-за потенциального неблагоприятного воздействия на качество (см. Хлор ниже), это было в значительной степени прекращено. ^[3]

регулировка pH

Чистая вода имеет pH, близкий к 7 (ни щелочной, ни кислый ). Морская вода может иметь значение pH от 7,5 до 8,4 (умеренно щелочной). Пресная вода может иметь широкий диапазон значений pH в зависимости от геологии водосборного бассейна или водоносного горизонта и влияния поступления загрязняющих веществ ( кислотные дожди ). Если вода кислая (ниже 7), можно добавить известь , кальцинированную соду или гидроксид натрия для повышения pH во время процессов очистки воды. Добавление извести увеличивает концентрацию ионов кальция, тем самым повышая жесткость воды. Для сильно кислой воды, принудительная тягадегазаторы могут быть эффективным способом поднять pH, удаляя растворенный диоксид углерода из воды. ^[4] делают воду щелочной способствует коагуляции и флокуляции процессы работать эффективно , а также помогает свести к минимуму риска свинца , растворенным из свинцовых труб и от свинца припоя в трубопроводной арматуре. Достаточная щелочность также снижает агрессивность воды к железным трубам. Кислота ( угольная кислота , соляная кислота или серная кислота) может быть добавлен в щелочную воду в некоторых случаях для понижения pH. Щелочная вода (pH выше 7,0) не обязательно означает, что свинец или медь из водопроводной системы не растворятся в воде. Способность воды осаждать карбонат кальция для защиты металлических поверхностей и снижения вероятности растворения токсичных металлов в воде зависит от pH, содержания минералов, температуры, щелочности и концентрации кальция. ^[5]

Коагуляция и флокуляция

Одним из первых шагов в большинстве обычных процессов очистки воды является добавление химикатов, способствующих удалению взвешенных в воде частиц. Частицы могут быть неорганическими, такими как глина и ил, или органическими, такими как водоросли , бактерии , вирусы , простейшие и природные органические вещества . Неорганические и органические частицы способствуют мутности и цвету воды.

Добавление неорганических коагулянтов, таких как соли сульфата алюминия (или квасцов ) или железа (III), таких как хлорид железа (III), вызывает несколько одновременных химических и физических взаимодействий на частицах и между ними. За секунды отрицательные заряды на частицах нейтрализуются неорганическими коагулянтами. Также в течение нескольких секунд начинают образовываться осадки гидроксида металла из ионов железа и алюминия. Эти осадки объединяются в более крупные частицы при естественных процессах, таких как броуновское движение, и в результате индуцированного перемешивания, которое иногда называют флокуляцией.. Аморфные гидроксиды металлов известны как «хлопья». Крупные аморфные гидроксиды алюминия и железа (III) адсорбируют и связывают частицы в суспензии и облегчают удаление частиц с помощью последующих процессов осаждения и фильтрации . ^{[6] : 8.2–8.3}

Гидроксиды алюминия образуются в довольно узком диапазоне pH, обычно от 5,5 до примерно 7,7. Гидроксиды железа (III) могут образовываться в более широком диапазоне pH, включая уровни pH ниже, чем эффективные для квасцов, обычно: от 5,0 до 8,5. ^{[7] : 679}

В литературе существует много споров и путаницы по поводу использования терминов «коагуляция» и «флокуляция»: где заканчивается коагуляция и начинается флокуляция? На водоочистных установках обычно используется высокоэнергетический процесс быстрого смешивания (время выдержки в секундах), при котором добавляются химические вещества-коагулянты, за которыми следуют резервуары для флокуляции (время выдержки колеблется от 15 до 45 минут), где малые энергозатраты вращают большие лопасти или другие устройства для бережного перемешивания для улучшения образования хлопьев. Фактически, процессы коагуляции и флокуляции продолжаются после добавления коагулянтов на основе солей металлов. ^{[8] : 74–5}

Органические полимеры были разработаны в 1960-х годах как добавки коагулянтов и, в некоторых случаях, как заменители коагулянтов на основе неорганических солей металлов. Синтетические органические полимеры - это высокомолекулярные соединения, несущие отрицательный, положительный или нейтральный заряд. Когда органические полимеры добавляются в воду с частицами, высокомолекулярные соединения адсорбируются на поверхности частиц и через образование мостиков между частицами сливаются с другими частицами с образованием хлопьев. PolyDADMAC - популярный катионный (положительно заряженный) органический полимер, используемый в установках очистки воды. ^{[7] : 667–8}

Седиментация

Вода, выходящая из бассейна флокуляции, может попадать в бассейн отстойника , также называемый отстойником или отстойником. Это большой резервуар с низкой скоростью воды, позволяющий хлопьям оседать на дно. Отстойник лучше всего расположен рядом с бассейном флокуляции, чтобы переход между двумя процессами не допускал оседания или распада хлопьев. Отстойники могут быть прямоугольными, где вода течет из конца в конец, или круглыми, когда поток идет от центра наружу. Отток из отстойника обычно проходит через водослив, поэтому выходит только тонкий верхний слой воды, наиболее удаленный от ила.

В 1904 году Аллен Хейзенпоказали, что эффективность процесса осаждения зависит от скорости осаждения частиц, потока через резервуар и площади поверхности резервуара. Отстойники обычно проектируются в диапазоне скоростей перелива от 0,5 до 1,0 галлона в минуту на квадратный фут (или от 1,25 до 2,5 литров на квадратный метр в час). В целом, эффективность бассейна отстойника не зависит от времени задержания или глубины бассейна. Тем не менее, глубина бассейна должна быть достаточной, чтобы потоки воды не нарушали ил и способствовали взаимодействию осевших частиц. Поскольку концентрация частиц в отстоявшейся воде увеличивается вблизи поверхности ила на дне резервуара, скорость осаждения может увеличиваться из-за столкновений и агломерации частиц. Типичное время выдержки для седиментации варьируется от 1.От 5 до 4 часов, а глубина бассейна варьируется от 10 до 15 футов (от 3 до 4,5 метров).^{[6]:9.39–9.40[7]:790–1[8]:140–2, 171}

Inclined flat plates or tubes can be added to traditional sedimentation basins to improve particle removal performance. Inclined plates and tubes drastically increase the surface area available for particles to be removed in concert with Hazen's original theory. The amount of ground surface area occupied by a sedimentation basin with inclined plates or tubes can be far smaller than a conventional sedimentation basin.

Sludge storage and removal

As particles settle to the bottom of a sedimentation basin, a layer of sludge is formed on the floor of the tank which must be removed and treated. The amount of sludge generated is significant, often 3 to 5 percent of the total volume of water to be treated. The cost of treating and disposing of the sludge can impact the operating cost of a water treatment plant. The sedimentation basin may be equipped with mechanical cleaning devices that continually clean its bottom, or the basin can be periodically taken out of service and cleaned manually.

Floc blanket clarifiers

A subcategory of sedimentation is the removal of particulates by entrapment in a layer of suspended floc as the water is forced upward. The major advantage of floc blanket clarifiers is that they occupy a smaller footprint than conventional sedimentation. Disadvantages are that particle removal efficiency can be highly variable depending on changes in influent water quality and influent water flow rate.^[7]:835–6

Dissolved air flotation

When particles to be removed do not settle out of solution easily, dissolved air flotation (DAF) is often used. After coagulation and flocculation processes, water flows to DAF tanks where air diffusers on the tank bottom create fine bubbles that attach to floc resulting in a floating mass of concentrated floc. The floating floc blanket is removed from the surface and clarified water is withdrawn from the bottom of the DAF tank. Water supplies that are particularly vulnerable to unicellular algae blooms and supplies with low turbidity and high colour often employ DAF.^[6]:9.46

Filtration

After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc.

Rapid sand filters

The most common type of filter is a rapid sand filter. Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds, which contribute to taste and odour. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. Effective filtration extends into the depth of the filter. This property of the filter is key to its operation: if the top layer of sand were to block all the particles, the filter would quickly clog.^[9]

To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing or backwashing) to remove embedded or unwanted particles. Prior to this step, compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process; this is known as air scouring. This contaminated water can be disposed of, along with the sludge from the sedimentation basin, or it can be recycled by mixing with the raw water entering the plant although this is often considered poor practice since it re-introduces an elevated concentration of bacteria into the raw water.

Some water treatment plants employ pressure filters. These work on the same principle as rapid gravity filters, differing in that the filter medium is enclosed in a steel vessel and the water is forced through it under pressure.

Advantages:

• Filters out much smaller particles than paper and sand filters can.
• Filters out virtually all particles larger than their specified pore sizes.
• They are quite thin and so liquids flow through them fairly rapidly.
• They are reasonably strong and so can withstand pressure differences across them of typically 2–5 atmospheres.
• They can be cleaned (back flushed) and reused.

Slow sand filters

Slow sand filters may be used where there is sufficient land and space, as the water flows very slowly through the filters. These filters rely on biological treatment processes for their action rather than physical filtration. They are carefully constructed using graded layers of sand, with the coarsest sand, along with some gravel, at the bottom and finest sand at the top. Drains at the base convey treated water away for disinfection. Filtration depends on the development of a thin biological layer, called the zoogleal layer or Schmutzdecke, on the surface of the filter. An effective slow sand filter may remain in service for many weeks or even months, if the pretreatment is well designed, and produces water with a very low available nutrient level which physical methods of treatment rarely achieve. Very low nutrient levels allow water to be safely sent through distribution systems with very low disinfectant levels, thereby reducing consumer irritation over offensive levels of chlorine and chlorine by-products. Slow sand filters are not backwashed; they are maintained by having the top layer of sand scraped off when flow is eventually obstructed by biological growth.^[10]

A specific "large-scale" form of slow sand filter is the process of bank filtration, in which natural sediments in a riverbank are used to provide a first stage of contaminant filtration. While typically not clean enough to be used directly for drinking water, the water gained from the associated extraction wells is much less problematic than river water taken directly from the river.

Membrane filtration

Membrane filters are widely used for filtering both drinking water and sewage. For drinking water, membrane filters can remove virtually all particles larger than 0.2 μm—including giardia and cryptosporidium. Membrane filters are an effective form of tertiary treatment when it is desired to reuse the water for industry, for limited domestic purposes, or before discharging the water into a river that is used by towns further downstream. They are widely used in industry, particularly for beverage preparation (including bottled water). However no filtration can remove substances that are actually dissolved in the water such as phosphates, nitrates and heavy metal ions.

Removal of ions and other dissolved substances

Ultrafiltration membranes use polymer membranes with chemically formed microscopic pores that can be used to filter out dissolved substances avoiding the use of coagulants. The type of membrane media determines how much pressure is needed to drive the water through and what sizes of micro-organisms can be filtered out.^{[citation needed]}

Ion exchange:^[11] Ion-exchange systems use ion-exchange resin- or zeolite-packed columns to replace unwanted ions. The most common case is water softening consisting of removal of Ca2+ and Mg2+ ions replacing them with benign (soap friendly) Na+ or K+ ions. Ion-exchange resins are also used to remove toxic ions such as nitrite, lead, mercury, arsenic and many others.

Precipitative softening:^{[6]:13.12–13.58} Water rich in hardness (calcium and magnesium ions) is treated with lime (calcium oxide) and/or soda-ash (sodium carbonate) to precipitate calcium carbonate out of solution utilizing the common-ion effect.

Electrodeionization:^[11] Water is passed between a positive electrode and a negative electrode. Ion-exchange membranes allow only positive ions to migrate from the treated water toward the negative electrode and only negative ions toward the positive electrode. High purity deionized water is produced continuously, similar to ion-exchange treatment. Complete removal of ions from water is possible if the right conditions are met. The water is normally pre-treated with a reverse osmosis unit to remove non-ionic organic contaminants, and with gas transfer membranes to remove carbon dioxide. A water recovery of 99% is possible if the concentrate stream is fed to the RO inlet.

Disinfection

Disinfection is accomplished both by filtering out harmful micro-organisms and by adding disinfectant chemicals. Water is disinfected to kill any pathogens which pass through the filters and to provide a residual dose of disinfectant to kill or inactivate potentially harmful micro-organisms in the storage and distribution systems. Possible pathogens include viruses, bacteria, including Salmonella, Cholera, Campylobacter and Shigella, and protozoa, including Giardia lamblia and other cryptosporidia. After the introduction of any chemical disinfecting agent, the water is usually held in temporary storage – often called a contact tank or clear well – to allow the disinfecting action to complete.

Chlorine disinfection

The most common disinfection method involves some form of chlorine or its compounds such as chloramine or chlorine dioxide. Chlorine is a strong oxidant that rapidly kills many harmful micro-organisms. Because chlorine is a toxic gas, there is a danger of a release associated with its use. This problem is avoided by the use of sodium hypochlorite, which is a relatively inexpensive solution used in household bleach that releases free chlorine when dissolved in water. Chlorine solutions can be generated on site by electrolyzing common salt solutions. A solid form, calcium hypochlorite, releases chlorine on contact with water. Handling the solid, however, requires more routine human contact through opening bags and pouring than the use of gas cylinders or bleach, which are more easily automated. The generation of liquid sodium hypochlorite is inexpensive and also safer than the use of gas or solid chlorine. Chlorine levels up to 4 milligrams per liter (4 parts per million) are considered safe in drinking water.^[12]

All forms of chlorine are widely used, despite their respective drawbacks. One drawback is that chlorine from any source reacts with natural organic compounds in the water to form potentially harmful chemical by-products. These by-products, trihalomethanes (THMs) and haloacetic acids (HAAs), are both carcinogenic in large quantities and are regulated by the United States Environmental Protection Agency (EPA) and the Drinking Water Inspectorate in the UK. The formation of THMs and haloacetic acids may be minimized by effective removal of as many organics from the water as possible prior to chlorine addition. Although chlorine is effective in killing bacteria, it has limited effectiveness against pathogenic protozoa that form cysts in water such as Giardia lamblia and Cryptosporidium.

Chlorine dioxide disinfection

Chlorine dioxide is a faster-acting disinfectant than elemental chlorine. It is relatively rarely used because in some circumstances it may create excessive amounts of chlorite, which is a by-product regulated to low allowable levels in the United States. Chlorine dioxide can be supplied as an aqueous solution and added to water to avoid gas handling problems; chlorine dioxide gas accumulations may spontaneously detonate.

Chloramination

The use of chloramine is becoming more common as a disinfectant. Although chloramine is not as strong an oxidant, it provides a longer-lasting residual than free chlorine because of its lower redox potential compared to free chlorine. It also does not readily form THMs or haloacetic acids (disinfection byproducts).

It is possible to convert chlorine to chloramine by adding ammonia to the water after adding chlorine. The chlorine and ammonia react to form chloramine. Water distribution systems disinfected with chloramines may experience nitrification, as ammonia is a nutrient for bacterial growth, with nitrates being generated as a by-product.

Ozone disinfection

Ozone is an unstable molecule which readily gives up one atom of oxygen providing a powerful oxidizing agent which is toxic to most waterborne organisms. It is a very strong, broad spectrum disinfectant that is widely used in Europe and in a few municipalities in the United States and Canada. Ozone disinfection, or ozonation, is an effective method to inactivate harmful protozoa that form cysts. It also works well against almost all other pathogens. Ozone is made by passing oxygen through ultraviolet light or a "cold" electrical discharge. To use ozone as a disinfectant, it must be created on-site and added to the water by bubble contact. Some of the advantages of ozone include the production of fewer dangerous by-products and the absence of taste and odour problems (in comparison to chlorination). No residual ozone is left in the water.^[13] In the absence of a residual disinfectant in the water, chlorine or chloramine may be added throughout a distribution system to remove any potential pathogens in the distribution piping.

Ozone has been used in drinking water plants since 1906 where the first industrial ozonation plant was built in Nice, France. The U.S. Food and Drug Administration has accepted ozone as being safe; and it is applied as an anti-microbiological agent for the treatment, storage, and processing of foods. However, although fewer by-products are formed by ozonation, it has been discovered that ozone reacts with bromide ions in water to produce concentrations of the suspected carcinogen bromate. Bromide can be found in fresh water supplies in sufficient concentrations to produce (after ozonation) more than 10 parts per billion (ppb) of bromate — the maximum contaminant level established by the USEPA.^[14] Ozone disinfection is also energy intensive.

Ultraviolet disinfection

Ultraviolet light (UV) is very effective at inactivating cysts, in low turbidity water. UV light's disinfection effectiveness decreases as turbidity increases, a result of the absorption, scattering, and shadowing caused by the suspended solids. The main disadvantage to the use of UV radiation is that, like ozone treatment, it leaves no residual disinfectant in the water; therefore, it is sometimes necessary to add a residual disinfectant after the primary disinfection process. This is often done through the addition of chloramines, discussed above as a primary disinfectant. When used in this manner, chloramines provide an effective residual disinfectant with very few of the negative effects of chlorination.

Over 2 million people in 28 developing countries use Solar Disinfection for daily drinking water treatment.^[15]

Ionizing radiation

Like UV, ionizing radiation (X-rays, gamma rays, and electron beams) has been used to sterilize water.^{[citation needed]}

Bromination and iodinization

Bromine and iodine can also be used as disinfectants. However, chlorine in water is over three times more effective as a disinfectant against Escherichia coli than an equivalent concentration of bromine, and over six times more effective than an equivalent concentration of iodine.^[16] Iodine is commonly used for portable water purification, and bromine is common as a swimming pool disinfectant.

Portable water purification

Portable water purification devices and methods are available for disinfection and treatment in emergencies or in remote locations. Disinfection is the primary goal, since aesthetic considerations such as taste, odour, appearance, and trace chemical contamination do not affect the short-term safety of drinking water.

Additional treatment options

1. Water fluoridation: in many areas fluoride is added to water with the goal of preventing tooth decay.^[17] Fluoride is usually added after the disinfection process. In the U.S., fluoridation is usually accomplished by the addition of hexafluorosilicic acid,^[18] which decomposes in water, yielding fluoride ions.^[19]
2. Water conditioning: This is a method of reducing the effects of hard water. In water systems subject to heating hardness salts can be deposited as the decomposition of bicarbonate ions creates carbonate ions that precipitate out of solution. Water with high concentrations of hardness salts can be treated with soda ash (sodium carbonate) which precipitates out the excess salts, through the common-ion effect, producing calcium carbonate of very high purity. The precipitated calcium carbonate is traditionally sold to the manufacturers of toothpaste. Several other methods of industrial and residential water treatment are claimed (without general scientific acceptance) to include the use of magnetic and/or electrical fields reducing the effects of hard water.^[20]
3. Plumbosolvency reduction: In areas with naturally acidic waters of low conductivity (i.e. surface rainfall in upland mountains of igneous rocks), the water may be capable of dissolving lead from any lead pipes that it is carried in. The addition of small quantities of phosphate ion and increasing the pH slightly both assist in greatly reducing plumbo-solvency by creating insoluble lead salts on the inner surfaces of the pipes.
4. Radium Removal: Some groundwater sources contain radium, a radioactive chemical element. Typical sources include many groundwater sources north of the Illinois River in Illinois, United States of America. Radium can be removed by ion exchange, or by water conditioning. The back flush or sludge that is produced is, however, a low-level radioactive waste.
5. Fluoride Removal: Although fluoride is added to water in many areas, some areas of the world have excessive levels of natural fluoride in the source water. Excessive levels can be toxic or cause undesirable cosmetic effects such as staining of teeth. Methods of reducing fluoride levels is through treatment with activated alumina and bone char filter media.

Other water purification techniques

Other popular methods for purifying water, especially for local private supplies are listed below. In some countries some of these methods are also used for large scale municipal supplies. Particularly important are distillation (de-salination of seawater) and reverse osmosis.

1. Boiling: Bringing water to its boiling point (about 100 °C or 212 F at sea level), is the oldest and most effective way since it eliminates most microbes causing intestine related diseases,^[21] but it cannot remove chemical toxins or impurities.^[22] For human health, complete sterilization of water is not required, since the heat resistant microbes are not intestine affecting.^[21] The traditional advice of boiling water for ten minutes is mainly for additional safety, since microbes start getting eliminated at temperatures greater than 60 °C (140 °F). Though the boiling point decreases with increasing altitude, it is not enough to affect the disinfecting process.^[21][23] In areas where the water is "hard" (that is, containing significant dissolved calcium salts), boiling decomposes the bicarbonate ions, resulting in partial precipitation as calcium carbonate. This is the "fur" that builds up on kettle elements, etc., in hard water areas. With the exception of calcium, boiling does not remove solutes of higher boiling point than water and in fact increases their concentration (due to some water being lost as vapour). Boiling does not leave a residual disinfectant in the water. Therefore, water that is boiled and then stored for any length of time may acquire new pathogens.
2. Granular Activated Carbon adsorption: a form of activated carbon with a high surface area, adsorbs many compounds including many toxic compounds. Water passing through activated carbon is commonly used in municipal regions with organic contamination, taste or odors. Many household water filters and fish tanks use activated carbon filters to further purify the water. Household filters for drinking water sometimes contain silver as metallic silver nanoparticle. If water is held in the carbon block for longer periods, microorganisms can grow inside which results in fouling and contamination. Silver nanoparticles are excellent anti-bacterial material and they can decompose toxic halo-organic compounds such as pesticides into non-toxic organic products.^[24] Filtered water must be used soon after it is filtered, as the low amount of remaining microbes may proliferate over time. In general, these home filters remove over 90% of the chlorine available to a glass of treated water. These filters must be periodically replaced otherwise the bacterial content of the water may actually increase due to the growth of bacteria within the filter unit.^[13]
3. Distillation involves boiling the water to produce water vapour. The vapour contacts a cool surface where it condenses as a liquid. Because the solutes are not normally vaporised, they remain in the boiling solution. Even distillation does not completely purify water, because of contaminants with similar boiling points and droplets of unvapourised liquid carried with the steam. However, 99.9% pure water can be obtained by distillation.
4. Reverse osmosis: Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. Reverse osmosis is theoretically the most thorough method of large scale water purification available, although perfect semi-permeable membranes are difficult to create. Unless membranes are well-maintained, algae and other life forms can colonize the membranes.
5. The use of iron in removing arsenic from water. See Arsenic contamination of groundwater.
6. Direct contact membrane distillation (DCMD). Applicable to desalination. Heated seawater is passed along the surface of a hydrophobic polymer membrane. Evaporated water passes from the hot side through pores in the membrane into a stream of cold pure water on the other side. The difference in vapour pressure between the hot and cold side helps to push water molecules through.
7. Desalination – is a process by which saline water (generally sea water) is converted to fresh water. The most common desalination processes are distillation and reverse osmosis. Desalination is currently expensive compared to most alternative sources of water, and only a very small fraction of total human use is satisfied by desalination. It is only economically practical for high-valued uses (such as household and industrial uses) in arid areas.
8. Gas hydrate crystals centrifuge method. If carbon dioxide or other low molecular weight gas is mixed with contaminated water at high pressure and low temperature, gas hydrate crystals will form exothermically. Separation of the crystalline hydrate may be performed by centrifuge or sedimentation and decanting. Water can be released from the hydrate crystals by heating^[25]
9. In Situ Chemical Oxidation, a form of advanced oxidation processes and advanced oxidation technology, is an environmental remediation technique used for soil and/or groundwater remediation to reduce the concentrations of targeted environmental contaminants to acceptable levels. ISCO is accomplished by injecting or otherwise introducing strong chemical oxidizers directly into the contaminated medium (soil or groundwater) to destroy chemical contaminants in place. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation
10. Bioremediation is a technique that uses microorganisms in order to remove or extract certain waste products from a contaminated area. Since 1991 bioremediation has been a suggested tactic to remove impurities from water such as alkanes, perchlorates, and metals.^[26] The treatment of ground and surface water, through bioremediation, with respect to perchlorate and chloride compounds, has seen success as perchlorate compounds are highly soluble making it difficult to remove.^[27] Such success by use of Dechloromonas agitata strain CKB include field studies conducted in Maryland and the Southwest region of the United States.^[27][28][29] Although a bioremediation technique may be successful, implementation is not feasible as there is still much to be studied regarding rates and after effects of microbial activity as well as producing a large scale implementation method.

Safety and controversies

The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate. (April 2011) (Learn how and when to remove this template message)

In April, 2007, the water supply of Spencer, Massachusetts in the United States of America, became contaminated with excess sodium hydroxide (lye) when its treatment equipment malfunctioned.^[30]

Many municipalities have moved from free chlorine to chloramine as a disinfection agent. However, chloramine appears to be a corrosive agent in some water systems. Chloramine can dissolve the "protective" film inside older service lines, leading to the leaching of lead into residential spigots. This can result in harmful exposure, including elevated blood lead levels. Lead is a known neurotoxin.^[31]

Demineralized water

Distillation removes all minerals from water, and the membrane methods of reverse osmosis and nanofiltration remove most to all minerals. This results in demineralized water which is not considered ideal drinking water. The World Health Organization has investigated the health effects of demineralized water since 1980.^[32] Experiments in humans found that demineralized water increased diuresis and the elimination of electrolytes, with decreased blood serum potassium concentration. Magnesium, calcium, and other minerals in water can help to protect against nutritional deficiency. Demineralized water may also increase the risk from toxic metals because it more readily leaches materials from piping like lead and cadmium, which is prevented by dissolved minerals such as calcium and magnesium. Low-mineral water has been implicated in specific cases of lead poisoning in infants, when lead from pipes leached at especially high rates into the water. Recommendations for magnesium have been put at a minimum of 10 mg/L with 20–30 mg/L optimum; for calcium a 20 mg/L minimum and a 40–80 mg/L optimum, and a total water hardness (adding magnesium and calcium) of 2 to 4 mmol/L. At water hardness above 5 mmol/L, higher incidence of gallstones, kidney stones, urinary stones, arthrosis, and arthropathies have been observed.^[33] Additionally, desalination processes can increase the risk of bacterial contamination.^[33]

Manufacturers of home water distillers claim the opposite—that minerals in water are the cause of many diseases, and that most beneficial minerals come from food, not water.^[34][35]

History

The first experiments into water filtration were made in the 17th century. Sir Francis Bacon attempted to desalinate sea water by passing the flow through a sand filter. Although his experiment did not succeed, it marked the beginning of a new interest in the field. The fathers of microscopy, Antonie van Leeuwenhoek and Robert Hooke, used the newly invented microscope to observe for the first time small material particles that lay suspended in the water, laying the groundwork for the future understanding of waterborne pathogens.^[36]

Sand filter

The first documented use of sand filters to purify the water supply dates to 1804, when the owner of a bleachery in Paisley, Scotland, John Gibb, installed an experimental filter, selling his unwanted surplus to the public.^[37] This method was refined in the following two decades by engineers working for private water companies, and it culminated in the first treated public water supply in the world, installed by engineer James Simpson for the Chelsea Waterworks Company in London in 1829.^[38] This installation provided filtered water for every resident of the area, and the network design was widely copied throughout the United Kingdom in the ensuing decades.

The practice of water treatment soon became mainstream and common, and the virtues of the system were made starkly apparent after the investigations of the physician John Snow during the 1854 Broad Street cholera outbreak. Snow was sceptical of the then-dominant miasma theory that stated that diseases were caused by noxious "bad airs". Although the germ theory of disease had not yet been developed, Snow's observations led him to discount the prevailing theory. His 1855 essay On the Mode of Communication of Cholera conclusively demonstrated the role of the water supply in spreading the cholera epidemic in Soho,^[39][40] with the use of a dot distribution map and statistical proof to illustrate the connection between the quality of the water source and cholera cases. His data convinced the local council to disable the water pump, which promptly ended the outbreak.

The Metropolis Water Act introduced the regulation of the water supply companies in London, including minimum standards of water quality for the first time. The Act "made provision for securing the supply to the Metropolis of pure and wholesome water", and required that all water be "effectually filtered" from 31 December 1855.^[41] This was followed up with legislation for the mandatory inspection of water quality, including comprehensive chemical analyses, in 1858. This legislation set a worldwide precedent for similar state public health interventions across Europe. The Metropolitan Commission of Sewers was formed at the same time, water filtration was adopted throughout the country, and new water intakes on the Thames were established above Teddington Lock. Automatic pressure filters, where the water is forced under pressure through the filtration system, were innovated in 1899 in England.^[37]

Water chlorination

John Snow was the first to successfully use chlorine to disinfect the water supply in Soho that had helped spread the cholera outbreak. William Soper also used chlorinated lime to treat the sewage produced by typhoid patients in 1879.

In a paper published in 1894, Moritz Traube formally proposed the addition of chloride of lime (calcium hypochlorite) to water to render it "germ-free." Two other investigators confirmed Traube's findings and published their papers in 1895.^[42] Early attempts at implementing water chlorination at a water treatment plant were made in 1893 in Hamburg, Germany and in 1897 the city of Maidstone, England was the first to have its entire water supply treated with chlorine.^[43]

Permanent water chlorination began in 1905, when a faulty slow sand filter and a contaminated water supply led to a serious typhoid fever epidemic in Lincoln, England.^[44] Dr. Alexander Cruickshank Houston used chlorination of the water to stem the epidemic. His installation fed a concentrated solution of chloride of lime to the water being treated. The chlorination of the water supply helped stop the epidemic and as a precaution, the chlorination was continued until 1911 when a new water supply was instituted.^[45]

The first continuous use of chlorine in the United States for disinfection took place in 1908 at Boonton Reservoir (on the Rockaway River), which served as the supply for Jersey City, New Jersey.^[46] Chlorination was achieved by controlled additions of dilute solutions of chloride of lime (calcium hypochlorite) at doses of 0.2 to 0.35 ppm. The treatment process was conceived by Dr. John L. Leal and the chlorination plant was designed by George Warren Fuller.^[47] Over the next few years, chlorine disinfection using chloride of lime were rapidly installed in drinking water systems around the world.^[48]

The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed by a British officer in the Indian Medical Service, Vincent B. Nesfield, in 1903. According to his own account:

It occurred to me that chlorine gas might be found satisfactory ... if suitable means could be found for using it.... The next important question was how to render the gas portable. This might be accomplished in two ways: By liquefying it, and storing it in lead-lined iron vessels, having a jet with a very fine capillary canal, and fitted with a tap or a screw cap. The tap is turned on, and the cylinder placed in the amount of water required. The chlorine bubbles out, and in ten to fifteen minutes the water is absolutely safe. This method would be of use on a large scale, as for service water carts.^[49]

U.S. Army Major Carl Rogers Darnall, Professor of Chemistry at the Army Medical School, gave the first practical demonstration of this in 1910. Shortly thereafter, Major William J. L. Lyster of the Army Medical Department used a solution of calcium hypochlorite in a linen bag to treat water. For many decades, Lyster's method remained the standard for U.S. ground forces in the field and in camps, implemented in the form of the familiar Lyster Bag (also spelled Lister Bag). This work became the basis for present day systems of municipal water purification.

See also

References

Further reading

External links

Wikimedia Commons has media related to Water_purification.

• American Water Works Association
• "Water On Tap: What You Need To Know." – Consumer Guide to Drinking Water in the US (EPA)
• Emergency Disinfection of Drinking Water – Camping, Hiking and Travel (CDC)
• Code of Federal Regulations, Title 40, Part 141 – U.S. National Primary Drinking Water Regulations