Water treatment

Water Treatment

Question 1)

The sustainable management of water demands that adequate supplies are available to meet the basic human and economic needs, without damaging aquatic systems, including their support of life and natural waste disposal capabilities. Currently, like many other countries, Britain is reliant upon an increasing demand for water. This is a result of economic growth and demographic change, resulting in an increase consumption of water. This along with a changing climate is altering the certainty of water resource replenishment. The higher temperate and decreased precipitation rates have led to reduced water supplies, but have also led to increased water demand (Enviroweb, 2010).

The reclamation and recycling of water are used for a variety of purposes and offer attractive alternatives to the development of new natural water sources or the expansion of old ones. Municipal waste flows are being widely used and is particularly attractive in areas where rainfall is low, evaporation is high, irrigation of water use is intense, and inter-basin transfers of water are in practice (Viessman, et al., 1998).

Water treatment technologies are organised into three categories: Physical Methods, Chemical Methods, and Biological methods. Physical methods of wastewater treatment represent a body of technologies that we refer largely to as solid-liquid separations techniques, of which filtration plays a dominant role. Filtration technology can be broken into two general categories - conventional and non-conventional. There are a variety of equipment and technology options to select from depending upon the final goal of the treatment (Cheremisinoff, 2001).

Chemical methods of treatment rely upon the chemical interactions of the contaminants being removed from water, and the application of chemicals that either aid in the separation of contaminants from water, or assist in the destruction or neutralization of any dangerous effects related with contaminants (Cheremisinoff, 2001).

Biological treatment is used for the removal of dissolved substances, involves the destabilisation and flocculation of the particulate matter which is then eliminated by adsorption onto the biomass. Biological wastewater treatment involves the biotechnological application of processes that occur in nature as well; although they occur very slowly. These are the processes known as the “self purification of surface water” (Wiley, 2009).

The wastewater treatment facility of BASF Aktiengesellschaft in Ludwigschafen, Germany, processes wastewater from 350 chemical plants located around the city Ludwigschafen. The BASF Group produces oil and natural gas, chemicals, fertilizers, plastics, synthetic fibres, dyes and pigments, potash and salt, inks and printing accessories, electronic recording accessories, cosmetic bases, pharmaceuticals, and other related equipment and products (BASF, 2010).

The industrial wastewaters are first neutralized by introducing milk of lime (a calcium hydroxide suspension) into the main sewer. After passing a coarse screening facility that collects large material, the wastewater is sent through two pressurized lines; delivered by a pumping station and then through a fine screen into four circular activated sludge clarification tanks. At this stage, because of a short retention time, only fast sedimenting solids are separated (Wiley, 2009).

A second section of the activated-sludge step consists of five activated-sludge carrousel type tanks operated in parallel; capable of processing 300000m3 of wastewater. These are accompanied with 110 surface aerators for supplying oxygen. The activated sludge tanks are sealed using glass-fiber reinforced plastic elements to reduce odour emissions (Wiley, 2009).

In addition to the decomposition of organic matter, a degradation of nitrates introduced with the raw water also occurs in the activated-sludge unit. To ensure denitrification, one section of the circulation-tank is starved of oxygen, such that microorganisms there utilize oxygen from the nitrates for respiration. Consumption of roughly 50 tonnes/day of nitrate in this way leads to a considerable power reduction for aeration (Wiley, 2009).

The secondary clarification system is composed of fifteen circular tanks with a horizontal liquid flow. After the sludge has settled out in the secondary clarification system, purified wastewater flows into the river Rhine and to domestic wastewater treatment plants for potable use in residential areas. At high-water periods in the absence of a natural gradient for outflow, a wastewater pumping station is available (Wiley, 2009).

A central safety system is employed on this plant for capturing toxic and poorly degradable batches of wastewater as well as water released for fire protection. The entire inflow can be sent into two special aeration tanks as required, which serve as buffer volumes. An extra storage tank is also accessible for wastewater buffering purposes. This tank is equipped with a special treatment system in which the wastewater can be purified with activated carbon (Wiley, 2009).

Excess sludge and primary sludge, which accumulates to the extent of 30000m3/day containing up to 320 tonnes of solids, materializes from the sludge-pumping station into thickening tanks. Here about 77% of the sludge water separates by gravitational compression. After the addition of coal slurry, ash, and polymeric flocculants the concentrated sludge is de-watered in a chamber filter press at pressures of up to 15 bar, to a moist-solid filter cake containing about 45% solids (Wiley, 2009).

The coal introduced into the clarifier sludge tank as a filter aid provides fuel for subsequent incineration. Five fluidized-bed furnaces are capable of burning up to 1000 tonnes of filter cake daily, and any organic constituents in the sludge decompose at temperatures of 1000℃. The heat generated during incineration is recovered and used in waste-heat boilers for steam production, air preheating, and boiler-feed preheating (Wiley, 2009).

1.0: Schematic diagram of the BASF Ludwigshafen wastewater-treatment plant a) Neutralization; b) Wastewater pumping station; c) Fine screen; d) Activated-sludge treatment; e) Clarification; f ) High-water pumping station; g) Sludge pumping station; h) Static thickener; i) Filter press; j) Steam generation; k) Two-stage steam turbine; l) Generator; m) Fluidized-bed furnace; n) Condensation; o) Electrostatic filters; p) Wet off-gas treatment (Wiley, 2009)

The BASF wastewater treatment plant processes a variety of pollutants but pharmaceuticals (i.e., female hormones, tranquilizers, and diuretics) are continually detected in coupled public water supplies. The hazard posed by these compounds in low but chronic levels is not fully understood at present. However, the number of medications entering into and persisting in water resources is increasing (Sullivan, et al., 2005).

Currently, public water supplies do not monitor or report to the United States Environmental Protection Agency (USEPA) the occurrence of pharmaceuticals in drinking water. This raises questions as to the degree to which pharmaceuticals pollute water resources, as well as their ultimate effect on the consumer. This means that chemicals that are excreted into the environment have the potential to bioconcentrate. (Sullivan, et al., 2005).

Many drugs are also designed to be persistent, making their release into the environment extremely dangerous. The polar nature of the majority of drugs/metabolites leads to leaching from land disposal areas into groundwater or wet weather runoff into surface water. The remnants (largely those designed to pass the blood-brain barrier) have lipophilic character, making them prone to bioconcentration from consumption of water or bioaccumulation from the consumption of tissue (Sullivan, et al., 2005).

In addition to pollution arising from the inadequate filtration of pharmaceutical products, agricultural use of antibiotics is also a significant source of pollution. According to a report from the Environmental Defense Fund to the USEPA, approximately 40% of all antibiotics in the United States are used in animal production. In addition, as much as 80% of the antibiotics administered orally pass through an animal unchanged. Subsequently, antibiotics pollute both surface and groundwater resources (Snow, 2003).

Most people believe, at a gut level, that environmental factors, such as the exposure to chemicals, exposure to airborne smoke and particulates, stress, drug and alcohol use, electromagnetic fields, and radiation, can contribute to or cause health problems. These fears are reinforced by the fact-based films “Erin Brockovich” which highlights that chemical pollution of water supplies can have severe consequences. In this film, individuals in two communities suffered from illnesses caused by chemical pollution of their drinking water (Hexavalent Chromium in this case). As a result of these illnesses, individuals prosecuted the companies responsible for the pollution, under the naive use of chemicals act (Sullivan, et al., 2005).

The delay in recognizing the impact of chemicals on human health is another cause for concern to a “standard-based” pollution control policy. For example, such time delays were characteristic of Hexavalent Chromium which was the chemical of concern in “Erin Brockovich.” Chromium has been known as a hazardous chemical since the 1850s, but not until the 1930s were there widespread reports of the toxicity of chromium to both aquatic life and humans known. By the1940s, the pollution of both surface water and groundwater by chromium was a frequent occurrence. Even though chromium was a known toxic compound, after the first drinking water standards were set in 1925, it took another 21 years to add chromium to the list (Sullivan, et al., 2005).

Occurrences like the above and the incompetence of authorities to set solid drinking standards only fuel continued fears about what can happen when sewage water is mixed into a drinking water supply. However with new advances in wastewater treatment technology such as reverse osmosis, ozonation and ion exchange, re-purified water is actually “cleaner than the water that comes out of the tap” (Philadelphia, 2007)

In addition to the practical steps of treating liquid wastes by the construction of suitable treatment plants, there is a need to regulate the discharge of effluents and to control activities which may take place within a water catchment area and could contribute to water pollution (Helmer, et al., 1997).

In the developed countries it is customary to provide some degree of treatment for water from any source, whereas for rural schemes in developing countries treatment is not always possible. Thus it is necessary to consider water sources in relation to what is likely to be the most important quality parameter (Tebutt, 1998).

With reasonably consistent rainfall the collection and storage of runoff from roofs can give a reasonable source of water provided that the first flush of water from a storm, which is likely to be contaminated by bird droppings, and so on, can be diverted away from the storage tank. Spring water is another valuable water source and is usually of good quality provided that it is derived from an aquifer and is not simply the discharge of a stream which has gone underground for a short distance (Tebutt, 1998).

Because ground waters are usually of good bacteriological quality tube wells can be employed in many developing areas. Care must be taken to ensure that sanitation practices do not cause groundwater contamination. Driven wells in suitable ground conditions are relatively cheap although they often have a limited life due to corrosion of the tube and blocking of the perforations (Tebutt, 1998).

In developing countries water treatment should not be adopted unless its use is unavoidable. It is important to keep the treatment as simple as possible to try to ensure low cost, ease of construction, reliability in operation and to enable operation and maintenance to be suitably undertaken by local labour (Tebutt, 1998).

The use of chemicals for coagulation brings a level of complexity to the treatment process and should only be adopted if the necessary supplies and skills are available locally. The use of natural coagulants like Moringa Oleifera can make coagulation feasible in situations where conventional coagulants are unaffordable or unavailable. Chemical coagulation will only be successful if the correct dose can be determined and then applied as to ensure adequate mixing and flocculation (Tebutt, 1998).

Although some simplified types of rapid sand filter are available, the slow sand filter is likely to be the most suitable form of treatment process for many developing country installations. Slow filtration is able to provide high removals of many physical, chemical and bacteriological contaminants from water whilst still being simple to operate. No chemicals are required and no sludge is produced (Tebutt, 1998).

If disinfection is necessary, the previously stated problem regarding chemical dosing must again be faced. Chlorine is the only practicable disinfectant, but the availability of the gaseous form is likely to be restricted and in any event the hazards of handling chlorine gas make it undesirable for rural supplies. A more suitable source of chlorine for such installations is bleaching powder which is about 30% available chlorine and is easy to handle (Tebutt, 1998).

The fact remains that water is a precious resource, yet less than 10% of the world's urban and industrial water is recycled. Much of this is for uses that do not require potable water, and could be substituted with recycled water. Water recycling is a socially, environmentally and economically viable solution to help preserve our drinking water supplies. It reduces the demand on fresh water and makes use of a precious resource that currently goes to waste. Water recycling schemes protect the environment by reducing the discharge of treated effluent to rivers, bays and oceans (Helmer, et al., 1997).

Question 2)

Hardness in water is caused by the ions of calcium and magnesium. Although ions of iron, manganese, strontium, and aluminium also produce hardness, they are not present is significant quantities in natural waters (Viessman, et al., 1998).

Water hardness is largely the result of geological formations of the water source. Public acceptance of hardness varies from community to community, depending on the degree of hardness to which the consumer is familiar with. Hardness of more than 300-500 mg/l as CaCO3 is considered excessive for public water supply and consequently leads to high soap use as well as unwanted scale in heating vessels and pipes. A reasonable is around 60-120 mg/l (Viessman, et al., 1998).

Scales formed in boilers and other exchange equipment act as insulation, preventing efficient heat transfer and causing boiler tube failures through overheating of the metal. Free mineral acids in the scale cause rapid corrosion of boilers, heaters, and other metal containers and piping (Cheremisinoff, 2001).

The soda-lime water-softening process uses lime (Ca(OH)2 and soda ash (Na2CO3) to precipitate hardness from solution. Carbon dioxide and carbonate hardness (calcium and magnesium bicarbonate) are complexed by lime. Noncarbonate hardness (calcium and magnesium sulphates or chloride) requires the addition of soda ash for precipitation (Viessman, et al., 1998).

There are several advantages of lime softening in water treatment. The most obvious is that the total dissolved solids can be significantly reduced; hardness is taken out of solution; and the lime added is also removed. Lime also precipitates any soluble iron and manganese often found in groundwaters and any excess lime treatment can act as a disinfectant. However this produces a precipitate in the form of a sludge, which must be settled out and the clarified water filtered in a sand filter. The problems of handling solid chemicals and of sludge disposal have largely made the processes obsolete (Snow, 2003).

The following are some chemical reactions in soda-lime ash treatment:

Reaction with CO2 and precipitation of Ca2+ from lime;

Eq. 1.0 (Viessman, et al., 1998)

Precipitation of bicarbonate Ca2+ from lime;

Eq. 1.1

Precipitation of noncarbonated Ca2+ by addition of soda ash;

Eq. 1.2

Eq. 1.3

Precipitation softening cannot produce water that is completely free of hardness because of the solubility of calcium carbonate. Additionally, completion of the chemical reaction is limited by physical activity; mixing and limited detention time in settling basins. Thus the maximum limits of precipitation softening produce water with 30mg/l of CaCO3 (Viessman, et al., 1998).

After lime treatment, the water is scale forming and must be neutralised to remove caustic alkalinity. Recarbonation and soda ash are regularly used to stabilise the water (Viessman, et al., 1998):

Eq. 1.4

This reaction precipitates calcium hardness and reduces the pH from 11 to around 10 before further recarbination of the water reduces the remaining carbonate ions into bicarbonate:

Eq. 1.5

The final pH is usually around 8.5, depending on the desired carbonate-to-bicarbonate ratio (Viessman, et al., 1998).

A two-stage system is the preferred method for lime treatment. In the first stage lime is applied and mixed with the water to precipitate calcium and magnesium. Then carbon dioxide is applied to neutralise the excess lime (equations 1.4/1.5), and soda ash is added to reduce noncarbonate hardness. Solids formed in these reactions are removed by secondary settling and subsequent filtration. Recarbonation immediately ahead of the filters may be used to prevent scaling of the medium (Viessman, et al., 1998).

Water can contain varying concentrations of dissolved salts which dissociate to form ions. These ions are the positively charged cations and negatively charged anions that permit the water or solution to conduct electrical currents. Ion exchangers utilise these characteristics with a reversible reaction where an ion from solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins (Cheremisinoff, 2001).

Water to be softened is first passed through a cation-exchange resin which converts the influent salt (e.g., sodium sulphate) to the corresponding acid (e.g., sulphuric acid) by exchanging an equivalent number of hydrogen ions for the metallic cations (Ca, Mg, and Na). These acids are then removed by passing the effluent through an alkali regenerated anion-exchange resin which replaces the anions in solution (Cl1-and SO2-) with a corresponding number of hydroxide ions. The hydrogen ions and hydroxide ions neutralize each other to form an equal amount of pure water. However the exchange capacity of ion-exchange materials is limited; they eventually become exhausted and must be regenerated. During regeneration, the reverse reaction takes place. The cation resin is regenerated with an acid and the anion resin is regenerated with sodium hydroxide (Cheremisinoff, 2001).

Modern ion-exchange resins have high capacities, readily regenerated - allowing their reuse, and can remove unwanted ions preferentially. Ion-exchange designs are well developed units that are durable and reliable, with well-established applications. Unlike soda-lime treatment temperature effects over a fairly wide range (0-35℃) are negligible and the technology can be applied to both small and large installations, from home water softeners to large industrial applications (Cheremisinoff, 2001).

The use of physical conditioners is a relatively new type of water softening and involves the use of magnetic fields or radio waves. The idea is that by passing water through a magnetic field, the calcium and magnesium ion's are altered in such a way that they lose their ability to cause scale (Hardwater, 2001).

This has a number of benefits; although the water is not technically soft, it has the useful properties of soft water; it won't cause scale in your pipes thus increasing heating efficiency and lengthening the lifespan of any clothes washed in the conditioned water. However Calcium is an important dietary element, so the fact that conditioned water still retains its calcium content is an added benefit (Hardwater, 2001).

Another type of conditioner involves the use of chemicals and can be categorised into precipitating and non-precipitating water softeners. Precipitating water softeners include washing soda and borax. These products form an insoluble precipitate with calcium and magnesium ions. The mineral ions then cannot interfere with cleaning efficiency, but the precipitate makes water cloudy and can build up on surfaces. Precipitating water softeners increase alkalinity of the cleaning solution. Non-precipitating water softeners use complex phosphates to sequester calcium and magnesium ions. There is no precipitate to form deposits and alkalinity is not increased. If used in enough quantity, non-precipitating water softeners will help dissolve soap curd for a period of time (Hardwater, 2001).

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