Water resources
"Water is the driving force of all nature."
(Leonardo da Vinci)
In an animal or a plant, 99 molecules in 100 are water… An organism is a pool in a stream of water along with metabolites and energy move through ecosystems.
(W.V. Macfarlane)
At any single moment, 94% of the earth’s water is found in the oceans, 4% is in inaccessible aquifers, and 1,5% is locked up in polar ice and glaciers. This leaves about 0,5% of the earth’s water available for human use, but most of this water is hard to reach and much too costly to be of any practical value.
Three out of every five people in the developing nations do not have access to clean, disease-free drinking water. According to the World Health Organization, 80% of all disease in these countries results from the contaminated water that people drink and bath in.
Water shortage arises from two principal problems: First, water is not evenly distributed across the face of the earth. Tropical rain forests are drenched with rain whereas much of the US deserts receive less than 25 cm a year. The second reason is that human civilization has pushed well beyond the earth’s carrying capacity, exceeding the readily available supply of this renewable resource, even in the areas with abundant rainfall.
One of the methods, how to increase water supply is saltwater desalination, the removal of salts from seawater and brackish (slightly salty) water. The two main methods are evaporation and reverse osmosis. In evaporation, or distillation, the salt water is heated and evaporated, leaving behind the salts and minerals. The steam produced is then cooled, and pure water condenses out. In reverse osmosis, water is forced through thin membranes whose pores allow the passage of water molecules but not the salts and minerals.
Since 94% of the water on earth is in the oceans, desalination might seem like the best answer to water shortages. Unfortunately, water produced by desalination is four to ten times more expensive than water from conventional sources.
We can also do many things to cut down on water use. For instance: in showers - by installing flow restricters or taking shorter showers, in toilets - by installing low-flush toilet, in faucets - by installing low-flush faucets or flow restricters.
While brushing teeth and shaving - do it with faucets off, followed by brief rinse.
Recycling and conservation can make important contributions to increasing water supplies in the near future. Since agriculture uses a large percentage of our water, devices to reduce irrigation losses that are inexpensive and easy to install are needed. Individuals can assist by reducing water consumption.
The hydrologic cycle (fig.1)
The hydrologic cycle is the chain of the history of the water. The global water cycle is driven by solar energy, and a little bit by the rotation of the earth and tide forces. The cycle involves the total earth system comprising the atmosphere, the hydrosphere, lithosphere and biosphere. The hydrosphere includes all the water at and near the near the surface of the earth. All of the water in the hydrosphere is caught up in the hydrologic cycle. The largest single reservoir in the hydrologic cycle, by far, consists of the world’s oceans, which contain 97,5% of the water in the hydrosphere. Lakes and streams together contain only 0,016% of the water. The main processes of the hydrologic cycle involve evaporation into, and precipitation out of the atmosphere. Precipitation onto land can re-evaporate, infiltrate into the ground, or run off over the ground surface. Water that sinks in the ground may also flow back to the oceans. The oceans are the major sources of evaporation water because of their vast areas of exposed water surface. Most of the fresh water is locked up as ice, mainly in the large polar ice caps. Even the groundwater beneath continental surfaces is not all fresh. Groundwater exists wherever water penetrates through the surface. Groundwater has passed through the rock on aquifer and has been naturally filtered to remove some impurities. Usually, groundwater is found, at most, a few kilometers into the crust. Groundwater is by far the largest reservoir of unfrozen freshwater. This extractable body of groundwater occurs in openings between grains in alluvial and sedimentary rocks – pore water; in fractures usually abundant only near the ground surface – fissure water, and in tubular openings in soluble rocks; and openings in lava formed by flow and gas expansion during solidification – cavern water. The water table is not always below ground surface. Where the water table locally intersects the ground surface, the result may be a lake, a stream, or a spring. Groundwater can flow laterally through permeable soil and rock, from higher elevations to lower, from areas of abundant infiltration to drier ones, or from areas of little groundwater use toward areas of heavy use.
The processes of infiltration and migration through which groundwater is replaced are called water recharge. A rock that holds enough of water and transmits it rapidly enough to be useful as a water source is an aquifer. Many of the best aquifers are sandstones or other coarse, clastic sedimentary rocks, but any other type of rock may serve if it is sufficiently porous and permeable – a porous limestone, fractured basalt, etc. An aquiclude may be capable of absorbing water slowly, but it has extremely low permeability, so the water cannot flow rapidly enough and that’s why it cannot serve as a water source.
Major reservoirs of natural water are:
atmosphere, the atmospheric moisture;
underground layer which supports surface water in streams, lakes, ponds, the ocean, and the solid water as snow and ice;
soil zone acting as a reservoir of soil moisture which is held against the pull of gravity and is thus made available for industrial consumption;
groundwater reservoir;
Rainwater as it falls to the earth has ample opportunity to dissolve gasses from the air and also may dissolve particles of dust or other airborne materials. Ammonia and various nitrogen compounds are present in general. Dust particles are added locally in industrial areas, large population centers, and desert areas. The sources of these constituents are the oceans, fresh water and saline lakes, landmasses, vegetation, man-made industries and volcanic emanations.
Mineral waters – are waters different from fresh underground waters by their composition or by their temperature. Mineral water is water, in which the content of dissolved minerals or gasses is at least 1g/l. Thermal water is water with a temperature surpassing the mean annual air temperature near a thermal spring. The limit temperature varies within 20-30 degrees. Water quality
Majority of the water in the hydrosphere is in very salty oceans, and almost all of the remainder is tied up in glaciers. The rest of the water - very small amount – is surface or subsurface water for potential freshwater sources. Even the rainwater contains dissolved chemicals of various kinds, especially in industrialized areas with substantial air pollution. Once precipitation reaches the ground, it reacts with soil, rocks, and organic compounds, dissolving more and more chemicals naturally, aside from any pollution generated by human activities.
Certain dissolved substances that alter water properties may influence water quality.
In areas, where water supplies have passed through soluble carbonated rocks, like limestone, the water can be described as hard water. Hard water contains substantial amounts of dissolved calcium and magnesium. If Ca and Mg concentration reaches or exceeds the range of 80 to 100 ppm, the water hardness becomes objectionable.
Groundwater quality is highly variable. It may be nearly as pure as ideal rainwater or saltier than in the oceans. Water contamination is, however, not stable. Contaminant concentration is lowered by dilution, absorption in the permeable rock substrates, or by biological processes. The most important of natural purification processes are those biochemical changes that decompose organic compounds.
Anthropogenous interactions with water
Anthropogenous interactions with water occur in three main fields:
household area;
husbandry / agriculture industry
industry.
The main sources of water pollution are:
building activities (erosion, fertilizers, etc.);
diary farming, agricultural activities (fertilizers, herbicides, pesticides, erosion, animal wastes concentration);
automobile industry expansion (erosion, deicing chemicals, acid rain, other products connected with cars);
power generation (fossil fuels – acid rain, spilling oil during transportation, warming up the wastewater, construction of the dams, spilling the radioactive materials into waterways);
landfills (contamination of groundwater, illegal waste burial sites);
household products (paint removers, bathroom cleansers, improper disposal of containers, washing powders);
not existing canalization system in villages;
There are also problems as a result of a bad agricultural management in the past years:
Extensive production of the cattle and pigs caused the total damage of soils by animal wastes in heavy concentrations;
Overdose of chemical fertilizers;
Agricultural lands were placed very closely to the pollution sources.
WASTE WATER TREATMENT
There are 4 main categories of wastewater and thus wastewater sewage systems:
1. Domestic sewage – wastewater from households, commercials, services, schools, administrative, etc.
2. Industrial wastewater – wastewater produced in various industrial branches, in agro-industries and forestry.
3. Municipal wastewater – sewage system, in which we cannot distinguish domestic wastewater from industrial WW; they are not in separated sewage systems
4. Rainwater – water polluted into some extent.
Rainwater carries dust particles from the atmosphere to the ground, it dissolves also a little oxygen, nitrogen, and carbon dioxide as it falls through the atmosphere. During electrical storms, lighting causes nitrogen, oxygen, and water vapor to form nitric acid. Traces of this are also found in rainwater. Rainwater also dissolves and washes various chemical compounds from roads, streets and fertilizers from fields.
There are 2 types of wastewater treatment:
1. Pre-treatment – treatment of wastewater before its discharging onto the sewer (this is used for strongly polluted industrial wastewater)
2. Full wastewater treatment – treatment of wastewater before its discharging into the recipient (flow, lake, sea)
Biological wastewater treatment
Final aim is to remove organic matter from wastewater
Main components or actors in biological WWT are substrate – an organic matter; and a biomass – microorganisms.
Substrate - organic material contained in domestic WW, in agro-industrial WW, in food industry WW (all these WW are compatible with biological WWT) and can be decomposed by microorganisms. Substrate contains 3 main element that are crucial for bacteria life: N- nitrogen, P- phosphorus, and K-potassium. The organic matter biodegradation can be either aerobic or anaerobic. Aerobic oxidation occurs in the presence of dissolved oxygen. A measure of the amount of oxygen needed for this degradation is the biochemical oxygen demand (BOD). The higher the quantity of degradable organic wastes the higher the BOD.
Flowing streams (thus wall aerated) are able to regenerate themselves. In rapids, the moving water dissolves oxygen. Lakes with little or no flow can remain dead for decades.
Eutrophisation – a problem that can arise in still waters, which is fed by nutrients (nitrates and phosphates). This may serve as nutrient for the growth of algae. These algal blooms are stimulated by the runoff of agricultural fertilizers and by phosphates in detergents. The result of the eutrophisation is that water is overloaded with algae, their relics after their die-off – this causes the bad odor and turbidity of the water.
The problem contaminants most likely to affect water quality are:
Turbidity: silt and fine particles suspended in the water
Bacterial or organic pollution from sewage and as decay products, e.g. E. coli, disease organisms and viral or protozoan pathogens, parasitic worm eggs and so on.
Metallic pollutants such as chromium, cadmium, lead, mercury
Biocides, e.g.
dioxin, PCB, organophosphates, halogenated hydrocarbons
Excessive fertilizers, especially nitrogenous compounds, phosphates, sodium and potassium salts.
Acids or acid-forming compounds (a pH less than 5,5 increases metallic pollution
Many of these factors interact. Acid rain dissolves out of rocks and soil poisonous forms of aluminium, mercury, lead, cadmium, and selenium, or other metals such as copper, nickel and lead, cadmium and selenium, or other metals such as copper, nickel, and lead from drinking tanks, tea urns, and hot water tanks. Organisms may convert inorganic mercury to organic forms, which are readily absorbed by the body
Water treatments commonly used
Aeration (oxygenation) by wind, mechanical aeration, or by increasing turbulence in flow. Aeration is also achieved by trickle columns and vegetation, phytoplancton, or injected air.
Settling: spreading flow in still water ponds or rush beds to allow particles to fall out, filter out, or flocculate
Skimming and sieving to remove large organic particles.
Filtration via sand beds or charcoal-fiber columns, soils, the roots of aquatic plants.
Coagulation or flocculation by using chemical additives (lime, salt, ferric sulphates) or organic (bacterial) gels.
Biological removal by bacteria, phytoplancton, and higher plants.
pH adjustment by adding calcium (as lime) or sulphur compounds as needed.
Filtration
A classical and widely used filter is sand. Many cities used sand filters followed by chlorinating to clean settled and treated raw sewage water sedimentation. Filtration by slow drip through 1,2 m of sand (top half fine, bottom half-coarse) is used even in temporary rural camps for water filtration.
Activated charcoal, often from bones or plant such as willow or coconut husks, is also used as a fine filter in homes and where purity is of the essence. Fine dripstone (fine-pored stone) is used in water cleaners and coolers to supply cool water in homes.
Trickle filters through sand and gravel columns actually feed resident bacteria, which remove the surplus nutrient. In less polluted environments, a similar task is carried out by freshwater mussels.
Carbon is essential for the removal of nitrogen or for its conversion by bacteria, and it is generally added as carbohydrate, which can be liquid such as methanol, ethanol, or acetic acids, many or all of them derived from plant residues. This is to encourage bacteria to work. Surplus nitrogen is released by bacteria to air.
Unless bacteria are encouraged and allowed to work, nitrates move easily through sub-soils, in which no plants or bacteria can live, and can emerge in wells and streams.
Filtration can be combined also with pH adjustment. This is as simple as placing crushed marble or limestone as a layer in a tank, or casting burnt lime over a pond before filling and, if necessary, after filling. Crushed shells or even whole shells in water tanks and ponds have the same effect. Lime flocculates particles, causing them to settle out of the water.
There are several techniques for filtration, some or all of which can be used in series. First, trickle filters of loose pebbles (2,5-10cm) can be used to form an active bacterial surface layer to absorb nutrients, then a sand filter can be used to absorb bacterial pollution. Water rising through a sand column is fairly clear.
Water, now fairly clean, can be passed through a bed of watercress to remove dyes and nitrates, and the cress cut and fed to animals or dried and burnt to ash. As a final process, the water can be trickled through a column (a concrete pipe on end) of active carbon (10%) and silicon dioxide (90%), otherwise known as burnt rice, oat, or wheat husks.
The results should be clear, sparkling, safe water to drink. No machinery is involved if the system is laid out downslope to permit gravity flow.
Lime (freshly burnt) is often used to remove phosphorus and sludge in a primary settling lagoon, and then water is passed to a trickle tower for ammonia removal by bacteria. In small towns, the water can be passed from filter towers to sewage lagoons, which in fact may become rich waterfowl and forest sanctuaries. It can then be routed to field crop such as forest, pasture, and to crops to be distilled or burnt, which does not directly re-enter the food chain.
Sewage treatment using natural processes
Raw sewage is a mixture of nutrients, elements, heavy metals, and carbon compounds; it also contains quite dangerous levels of bacteria, viruses, and intestinal worm eggs. Raw sewage runs into the pond, where it settles out. This settling pond is anaerobic, and gives off biogas, a mixture of methane, carbon dioxide and ammonia gas, with traces of nitrous sulphide or nitrogen dioxide. Biogas is, of course, a useful fuel gas for engines, or a cooking gas for homes.
However, this is also a gaseous component of the atmosphere that is creating the “greenhouse effect” and thus should be used, not released to air.
What are BOD and COD?
WW do not contain a single organic compound; it is a mixture of organic compounds, each in various oxidation state – that is why a non specific test must be used
to determine the concentration of organic carbon, the collective parameters are needed: BOD5 a COD. COD – chemical oxygen demand – it is the quantity of the biodegradable (biomass + consumed oxygen) and non-biodegradable organic carbon. BOD5 – biochemical oxygen demand – the concept of BOD depend on assessing the potential of a WW that contains an available organic carbon as a source for aerobic organisms by measuring the amount of oxygen utilized during growth of the microorganisms. There’s difference between COD and BOD that BOD indicates just the quantity of biodegradable organic carbon in WW. BOD5 is then the difference in oxygen content at the beginning of the experiment and at its end after five days. Very important parameter is then ratio BOD5 / COD, this is the indicator of convenience of biological WWT. For the utilization of this method the ratio has to be higher than 0,5.
Physical and chemical methods for WWT
Equalization or homogenization
Sieving and micro-sieving
Sedimentation
Filtration
Flotation
Clearing and coagulation
Adsorption
Extraction
Deaeration
Distillation
Gamma radiation
Oxidation and reduction
In biological WWT we use 2 basic methods:
Activation pools
Biological filters
WATER RESOURCES
Water is the driving force of all nature.
(Leonardo da Vinci)
In an animal or a plant, 99 molecules in 100 are water… An organism is a pool in a stream of water along with metabolites and energy move through ecosystems.
(W.V. Macfarlane)
At any single moment, 94% of the earth’s water is found in the oceans, 4% is in inaccessible aquifers, and 1,5% is locked up in polar ice and glaciers. This leaves about 0,5% of the earth’s water available for human use, but most of this water is hard to reach and much too costly to be of any practical value.
Three out of every five people in the developing nations do not have access to clean, disease-free drinking water. According to the World Health Organization, 80% of all disease in these countries results from the contaminated water that people drink and bath in.
Water shortage arises from two principal problems: First, water is not evenly distributed across the face of the earth.
Tropical rain forests are drenched with rain whereas much of the US deserts receive less than 25 cm a year. The second reason is that human civilization has pushed well beyond the earth’s carrying capacity, exceeding the readily available supply of this renewable resource, even in the areas with abundant rainfall.
One of the methods, how to increase water supply is saltwater desalination, the removal of salts from seawater and brackish (slightly salty) water. The two main methods are evaporation and reverse osmosis. In evaporation, or distillation, the salt water is heated and evaporated, leaving behind the salts and minerals. The steam produced is then cooled, and pure water condenses out. In reverse osmosis, water is forced through thin membranes whose pores allow the passage of water molecules but not the salts and minerals.
Since 94% of the water on earth is in the oceans, desalination might seem like the best answer to water shortages. Unfortunately, water produced by desalination is four to ten times more expensive than water from conventional sources.
We can also do many things to cut down on water use. For instance: in showers - by installing flow restricters or taking shorter showers, in toilets - by installing low-flush toilet, in faucets - by installing low-flush faucets or flow restricters. While brushing teeth and shaving - do it with faucets off, followed by brief rinse.
Recycling and conservation can make important contributions to increasing water supplies in the near future. Since agriculture uses a large percentage of our water, devices to reduce irrigation losses that are inexpensive and easy to install are needed. Individuals can assist by reducing water consumption.
The hydrologic cycle (fig.1)
The hydrologic cycle is the chain of the history of the water. The global water cycle is driven by solar energy, and a little bit by the rotation of the earth and tide forces. The cycle involves the total earth system comprising the atmosphere, the hydrosphere, lithosphere and biosphere. The hydrosphere includes all the water at and near the near the surface of the earth. All of the water in the hydrosphere is caught up in the hydrologic cycle. The largest single reservoir in the hydrologic cycle, by far, consists of the world’s oceans, which contain 97,5% of the water in the hydrosphere. Lakes and streams together contain only 0,016% of the water. The main processes of the hydrologic cycle involve evaporation into, and precipitation out of the atmosphere.
Precipitation onto land can re-evaporate, infiltrate into the ground, or run off over the ground surface. Water that sinks in the ground may also flow back to the oceans. The oceans are the major sources of evaporation water because of their vast areas of exposed water surface. Most of the fresh water is locked up as ice, mainly in the large polar ice caps. Even the groundwater beneath continental surfaces is not all fresh. Groundwater exists wherever water penetrates through the surface. Groundwater has passed through the rock on aquifer and has been naturally filtered to remove some impurities. Usually, groundwater is found, at most, a few kilometers into the crust. Groundwater is by far the largest reservoir of unfrozen freshwater. This extractable body of groundwater occurs in openings between grains in alluvial and sedimentary rocks – pore water; in fractures usually abundant only near the ground surface – fissure water, and in tubular openings in soluble rocks; and openings in lava formed by flow and gas expansion during solidification – cavern water. The water table is not always below ground surface. Where the water table locally intersects the ground surface, the result may be a lake, a stream, or a spring. Groundwater can flow laterally through permeable soil and rock, from higher elevations to lower, from areas of abundant infiltration to drier ones, or from areas of little groundwater use toward areas of heavy use. The processes of infiltration and migration through which groundwater is replaced are called water recharge. A rock that holds enough of water and transmits it rapidly enough to be useful as a water source is an aquifer. Many of the best aquifers are sandstones or other coarse, clastic sedimentary rocks, but any other type of rock may serve if it is sufficiently porous and permeable – a porous limestone, fractured basalt, etc.
An aquiclude may be capable of absorbing water slowly, but it has extremely low permeability, so the water cannot flow rapidly enough and that’s why it cannot serve as a water source.
Major reservoirs of natural water are:
atmosphere, the atmospheric moisture;
underground layer which supports surface water in streams, lakes, ponds, the ocean, and the solid water as snow and ice;
soil zone acting as a reservoir of soil moisture which is held against the pull of gravity and is thus made available for industrial consumption;
groundwater reservoir;
Rainwater as it falls to the earth has ample opportunity to dissolve gasses from the air and also may dissolve particles of dust or other airborne materials. Ammonia and various nitrogen compounds are present in general. Dust particles are added locally in industrial areas, large population centers, and desert areas. The sources of these constituents are the oceans, fresh water and saline lakes, landmasses, vegetation, man-made industries and volcanic emanations.
Mineral waters – are waters different from fresh underground waters by their composition or by their temperature. Mineral water is water, in which the content of dissolved minerals or gasses is at least 1g/l. Thermal water is water with a temperature surpassing the mean annual air temperature near a thermal spring. The limit temperature varies within 20-30 degrees. Water quality
Majority of the water in the hydrosphere is in very salty oceans, and almost all of the remainder is tied up in glaciers. The rest of the water - very small amount – is surface or subsurface water for potential freshwater sources. Even the rainwater contains dissolved chemicals of various kinds, especially in industrialized areas with substantial air pollution. Once precipitation reaches the ground, it reacts with soil, rocks, and organic compounds, dissolving more and more chemicals naturally, aside from any pollution generated by human activities.
Certain dissolved substances that alter water properties may influence water quality. In areas, where water supplies have passed through soluble carbonated rocks, like limestone, the water can be described as hard water. Hard water contains substantial amounts of dissolved calcium and magnesium. If Ca and Mg concentration reaches or exceeds the range of 80 to 100 ppm, the water hardness becomes objectionable.
Groundwater quality is highly variable. It may be nearly as pure as ideal rainwater or saltier than in the oceans. Water contamination is, however, not stable. Contaminant concentration is lowered by dilution, absorption in the permeable rock substrates, or by biological processes.
The most important of natural purification processes are those biochemical changes that decompose organic compounds.
Anthropogenous interactions with water
Anthropogenous interactions with water occur in three main fields:
household area;
husbandry / agriculture industry
industry.
The main sources of water pollution are:
building activities (erosion, fertilizers, etc.);
diary farming, agricultural activities (fertilizers, herbicides, pesticides, erosion, animal wastes concentration);
automobile industry expansion (erosion, deicing chemicals, acid rain, other products connected with cars);
power generation (fossil fuels – acid rain, spilling oil during transportation, warming up the wastewater, construction of the dams, spilling the radioactive materials into waterways);
landfills (contamination of groundwater, illegal waste burial sites);
household products (paint removers, bathroom cleansers, improper disposal of containers, washing powders);
not existing canalization system in villages;
There are also problems as a result of a bad agricultural management in the past years:
Extensive production of the cattle and pigs caused the total damage of soils by animal wastes in heavy concentrations;
Overdose of chemical fertilizers;
Agricultural lands were placed very closely to the pollution sources.
WASTE WATER TREATMENT
There are 4 main categories of wastewater and thus wastewater sewage systems:
1. Domestic sewage – wastewater from households, commercials, services, schools, administrative, etc.
2. Industrial wastewater – wastewater produced in various industrial branches, in agro-industries and forestry.
3. Municipal wastewater – sewage system, in which we cannot distinguish domestic wastewater from industrial WW; they are not in separated sewage systems
4. Rainwater – water polluted into some extent. Rainwater carries dust particles from the atmosphere to the ground, it dissolves also a little oxygen, nitrogen, and carbon dioxide as it falls through the atmosphere. During electrical storms, lighting causes nitrogen, oxygen, and water vapor to form nitric acid. Traces of this are also found in rainwater. Rainwater also dissolves and washes various chemical compounds from roads, streets and fertilizers from fields.
There are 2 types of wastewater treatment:
1. Pre-treatment – treatment of wastewater before its discharging onto the sewer (this is used for strongly polluted industrial wastewater)
2. Full wastewater treatment – treatment of wastewater before its discharging into the recipient (flow, lake, sea)
Biological wastewater treatment
Final aim is to remove organic matter from wastewater
Main components or actors in biological WWT are substrate – an organic matter; and a biomass – microorganisms.
Substrate - organic material contained in domestic WW, in agro-industrial WW, in food industry WW (all these WW are compatible with biological WWT) and can be decomposed by microorganisms. Substrate contains 3 main element that are crucial for bacteria life: N- nitrogen, P- phosphorus, and K-potassium. The organic matter biodegradation can be either aerobic or anaerobic. Aerobic oxidation occurs in the presence of dissolved oxygen. A measure of the amount of oxygen needed for this degradation is the biochemical oxygen demand (BOD). The higher the quantity of degradable organic wastes the higher the BOD.
Flowing streams (thus wall aerated) are able to regenerate themselves. In rapids, the moving water dissolves oxygen. Lakes with little or no flow can remain dead for decades.
Eutrophisation – a problem that can arise in still waters, which is fed by nutrients (nitrates and phosphates). This may serve as nutrient for the growth of algae. These algal blooms are stimulated by the runoff of agricultural fertilizers and by phosphates in detergents.
The result of the eutrophisation is that water is overloaded with algae, their relics after their die-off – this causes the bad odor and turbidity of the water.
The problem contaminants most likely to affect water quality are:
Turbidity: silt and fine particles suspended in the water
Bacterial or organic pollution from sewage and as decay products, e.g. E. coli, disease organisms and viral or protozoan pathogens, parasitic worm eggs and so on.
Metallic pollutants such as chromium, cadmium, lead, mercury
Biocides, e.g. dioxin, PCB, organophosphates, halogenated hydrocarbons
Excessive fertilizers, especially nitrogenous compounds, phosphates, sodium and potassium salts.
Acids or acid-forming compounds (a pH less than 5,5 increases metallic pollution
Many of these factors interact. Acid rain dissolves out of rocks and soil poisonous forms of aluminium, mercury, lead, cadmium, and selenium, or other metals such as copper, nickel and lead, cadmium and selenium, or other metals such as copper, nickel, and lead from drinking tanks, tea urns, and hot water tanks. Organisms may convert inorganic mercury to organic forms, which are readily absorbed by the body
Water treatments commonly used
Aeration (oxygenation) by wind, mechanical aeration, or by increasing turbulence in flow. Aeration is also achieved by trickle columns and vegetation, phytoplancton, or injected air.
Settling: spreading flow in still water ponds or rush beds to allow particles to fall out, filter out, or flocculate
Skimming and sieving to remove large organic particles.
Filtration via sand beds or charcoal-fiber columns, soils, the roots of aquatic plants.
Coagulation or flocculation by using chemical additives (lime, salt, ferric sulphates) or organic (bacterial) gels.
Biological removal by bacteria, phytoplancton, and higher plants.
pH adjustment by adding calcium (as lime) or sulphur compounds as needed.
Filtration
A classical and widely used filter is sand. Many cities used sand filters followed by chlorinating to clean settled and treated raw sewage water sedimentation. Filtration by slow drip through 1,2 m of sand (top half fine, bottom half-coarse) is used even in temporary rural camps for water filtration.
Activated charcoal, often from bones or plant such as willow or coconut husks, is also used as a fine filter in homes and where purity is of the essence. Fine dripstone (fine-pored stone) is used in water cleaners and coolers to supply cool water in homes.
Trickle filters through sand and gravel columns actually feed resident bacteria, which remove the surplus nutrient.
In less polluted environments, a similar task is carried out by freshwater mussels.
Carbon is essential for the removal of nitrogen or for its conversion by bacteria, and it is generally added as carbohydrate, which can be liquid such as methanol, ethanol, or acetic acids, many or all of them derived from plant residues. This is to encourage bacteria to work. Surplus nitrogen is released by bacteria to air. Unless bacteria are encouraged and allowed to work, nitrates move easily through sub-soils, in which no plants or bacteria can live, and can emerge in wells and streams.
Filtration can be combined also with pH adjustment. This is as simple as placing crushed marble or limestone as a layer in a tank, or casting burnt lime over a pond before filling and, if necessary, after filling. Crushed shells or even whole shells in water tanks and ponds have the same effect. Lime flocculates particles, causing them to settle out of the water.
There are several techniques for filtration, some or all of which can be used in series. First, trickle filters of loose pebbles (2,5-10cm) can be used to form an active bacterial surface layer to absorb nutrients, then a sand filter can be used to absorb bacterial pollution. Water rising through a sand column is fairly clear.
Water, now fairly clean, can be passed through a bed of watercress to remove dyes and nitrates, and the cress cut and fed to animals or dried and burnt to ash. As a final process, the water can be trickled through a column (a concrete pipe on end) of active carbon (10%) and silicon dioxide (90%), otherwise known as burnt rice, oat, or wheat husks.
The results should be clear, sparkling, safe water to drink. No machinery is involved if the system is laid out downslope to permit gravity flow.
Lime (freshly burnt) is often used to remove phosphorus and sludge in a primary settling lagoon, and then water is passed to a trickle tower for ammonia removal by bacteria. In small towns, the water can be passed from filter towers to sewage lagoons, which in fact may become rich waterfowl and forest sanctuaries. It can then be routed to field crop such as forest, pasture, and to crops to be distilled or burnt, which does not directly re-enter the food chain.
Sewage treatment using natural processes
Raw sewage is a mixture of nutrients, elements, heavy metals, and carbon compounds; it also contains quite dangerous levels of bacteria, viruses, and intestinal worm eggs. Raw sewage runs into the pond, where it settles out. This settling pond is anaerobic, and gives off biogas, a mixture of methane, carbon dioxide and ammonia gas, with traces of nitrous sulphide or nitrogen dioxide. Biogas is, of course, a useful fuel gas for engines, or a cooking gas for homes.
However, this is also a gaseous component of the atmosphere that is creating the “greenhouse effect” and thus should be used, not released to air.
What are BOD and COD?
WW do not contain a single organic compound; it is a mixture of organic compounds, each in various oxidation state – that is why a non specific test must be used
to determine the concentration of organic carbon, the collective parameters are needed: BOD5 a COD. COD – chemical oxygen demand – it is the quantity of the biodegradable (biomass + consumed oxygen) and non-biodegradable organic carbon. BOD5 – biochemical oxygen demand – the concept of BOD depend on assessing the potential of a WW that contains an available organic carbon as a source for aerobic organisms by measuring the amount of oxygen utilized during growth of the microorganisms. There’s difference between COD and BOD that BOD indicates just the quantity of biodegradable organic carbon in WW. BOD5 is then the difference in oxygen content at the beginning of the experiment and at its end after five days. Very important parameter is then ratio BOD5 / COD, this is the indicator of convenience of biological WWT. For the utilization of this method the ratio has to be higher than 0,5.
Physical and chemical methods for WWT
Equalization or homogenization
Sieving and micro-sieving
Sedimentation
Filtration
Flotation
Clearing and coagulation
Adsorption
Extraction
Deaeration
Distillation
Gamma radiation
Oxidation and reduction
In biological WWT we use 2 basic methods:
Activation pools
Biological filters.
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