Potable Water and Treating Waste Water
Potable water is safe to drink but can contain dissolved substances. Treatment depends on the source, its contaminants and cost. Wastewater treatment separates solids and uses microbes to break down organic waste in the liquid and sludge.
What a learner can do afterwards
- Describe the steps that turn fresh water into potable water and say what each step removes
- Explain how the energy needed for desalination affects its use where fresh water supplies are limited
- Distinguish the aerobic and anaerobic stages of sewage treatment by what each one is for
1 · Read
A common treatment sequence for river water is screening, settling, sand filtration and disinfection. Screens remove large debris. Settling lets suspended solids such as silt sink. Sand filters remove more particles. Chlorine can inactivate many bacteria and viruses. Some microbes resist chlorine, so treatment and testing must match the source.
Potable means safe to drink: harmful microbes and chemicals must be controlled to safe levels. It does not mean sterile or chemically pure. Potable water can contain dissolved minerals. Chemically pure water contains only water. A clear appearance cannot show whether water is safe.
Desalination removes dissolved salts. Distillation evaporates water and collects it after condensation, leaving salts behind. Reverse osmosis pumps water through a membrane under pressure; most salts stay behind. Heating or pumping needs energy. Desalination usually uses more energy than treating fresh water, so cost and available supplies affect the choice. Removing salt alone does not establish drinking-water safety.
Screening removes large debris as a separate waste stream. Settling then separates suspended solids, including silt, as sludge. Dissolved organic waste remains in the liquid. In an aerobic tank, air supplies oxygen to microbes that break down this waste. Further settling separates microbial solids from the treated liquid. The liquid must meet discharge requirements before release; it is not automatically drinking water.
Separated sludge can be treated in an anaerobic digester. Unlike the aerobic liquid-treatment tank, this vessel works without oxygen. Microbes break down organic material and produce biogas containing methane, which can be collected as fuel. Treated sludge may be used on land only if it meets the required safety limits. Other sludge needs further treatment or disposal.
Potable means safe to drink. Remove particles and disinfect as needed. Desalination removes salts. Aerobic treatment uses oxygen; anaerobic sludge digestion can produce biogas.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked TED-Ed lessons feature Mia Nacamulli, Francis de los Reyes and Manish Kumar.
More to read
What makes water safe to drink?
A glass of water can look clear and still contain harmful microbes or chemicals. Potable water meets drinking-water safety requirements. Dissolved minerals can remain, so potable water need not be chemically pure.
For river water, screens can remove large debris. Settling and sand filtration remove suspended particles. Disinfection controls many disease-causing microbes. These stages have different jobs. Chlorine does not remove dissolved salt, and an ordinary sand filter is not a substitute for disinfection. Some pathogens resist chlorine, so treatment must match the source.
Dissolved contaminants, such as nitrates or industrial chemicals, may need additional treatment. Operators check water quality and the treatment process. A school demonstration that removes visible dirt cannot establish that the result is safe to drink. Do not drink demonstration samples.
The liquid and sludge take different routes
Screening removes large material from sewage. Settling separates more suspended solids into sludge. The remaining liquid still contains organic waste that needs treatment.
One liquid-treatment method supplies air to a tank. Aerobic microbes use oxygen while breaking down organic waste. A later settling stage separates microbial solids from the liquid. Before entering the environment, the treated liquid must meet discharge requirements. Ordinary sewage treatment alone does not make drinking water. Potable reuse requires further treatment and checks.
Separated sludge can be sent to an anaerobic digester. Microbes there work without oxygen and produce biogas containing methane. Collected gas can be used as fuel. Sludge may need further treatment, and its contaminants affect how it can be used or disposed of. It is suitable for land application only when the relevant safety requirements are met.
Separating water from dissolved salts
Dissolved salt passes through an ordinary sand filter. Desalination therefore needs another separation method.
Distillation collects water separately by evaporation followed by condensation. Dissolved salts remain behind as the water evaporates. Reverse osmosis uses pressure to drive water through a membrane that retains most dissolved salts. Distillation needs energy for heating; reverse osmosis needs energy for pressure pumping.
A town choosing a water supply considers both its available sources and treatment costs. Desalination usually needs more energy than treating fresh water, but it can provide a useful supply where fresh water is limited. It is not restricted to places with no fresh water at all.
Salt removal is only part of producing potable water. Other contaminants and the quality of the collected water also need checks. Do not drink water from a classroom or homemade desalination demonstration.
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