Posted by : Saurabh Gupta Saturday, April 27, 2013
Introduction to Environmental Engineering
What is Environmental Engineering?
It is the application of scientific and engineering principles to the environmental issues and their solutions. Generally, it includes supply of water, disposal and recycling of wastes, drainage of communities, control of water, soil, atmospheric pollution and environmental impacts of different activities carried out on earth.
The practice and application of engineering laws in compliance with the safety of environment and the code of ethics prescribed as standards. Some of those are as below
Environmental engineering is the application of science and engineering principles to improve the natural environment (air, water, and/or land resources), to provide healthy water, air, and land for human habitation and for other organisms, and to remediate polluted sites. It involves waste water management and air pollution control, recycling, waste disposal, radiation protection, industrial hygiene, environmental sustainability, and public health issues as well as a knowledge of environmental engineering law. It also includes studies on the environmental impact of proposed construction projects.
Environmental engineers conduct hazardous-waste management studies to evaluate the significance of such hazards, advise on treatment and containment, and develop regulations to prevent mishaps. Environmental engineers also design municipal water supply and industrial wastewater treatment systems as well as address local and worldwide environmental issues such as the effects of acid rain, global warming, ozone depletion, water pollution and air pollution from automobile exhausts and industrial sources. At many universities, Environmental Engineering programs follow either the Department of Civil Engineering or The Department of Chemical Engineering at Engineering faculties. Environmental "civil" engineers focus on hydrology, water resources management, bioremediation, and water treatment plant design. Environmental "chemical" engineers, on the other hand, focus on environmental chemistry, advanced air and water treatment technologies and separation processes
Population Forecasting Methods
|Population is one of the
most important factors for design of the water systems, so it should be
estimated, so as to know the increasing demand and ensure continuous supply
Population data is obtained by previous records and the rate of increase is found out and this used for further analysis, which may be by using the methods described below
1. Arithmetic growth method
2. Geometric growth method
3. Curvilinear method
4. Logistic method
5. Decline growth method
6. Ratio growth
dp / dt = Ka
dp / dt is the rate of change of population
Ka = the constant arithmetic incrementKa can be determined by finding the slop of the graph of population against time. The population in the future is thus estimated.
dP /dt ∝ P => dp / dt = Kg where Kg = Geometric Growth constant.
If P0 is the population at any time t0 and Pf is the population at time tf then
∫Pf P0 dp/p = Kg ∫ tf t0 dt = Ln (Pf/P0 = Kg (tf/t0)
=> Ln (Pf/P0 = Kg Δt
=> (Pf/P0 = (e) Kg Δt and Pf = P0 (e) Kg Δt
According to this method
P = P sat / (1+ ea+ bΔt), where P sat is the saturation population, of the community and a, b are constants. P sat, a and b can be determined from three successive census populations and the equations are
Psat = 2 P0 P1P2 - P12 (P0 + P2) / (P0 P2 - P12)
Decline growth method:
Projection of the trend line using any of the technique and application of projected ratio to the estimated required population of projected ratio to the estimated required population in the year of interest. This method of forecasting does not take into account some special calculations in certain area but have the following advantages.
Consumption of water
Domestic use of water:Domestic uses of water include the consumption of water for drinking, washing, cooking, toilets, livestock etc. the domestic average use per capita per day is 50 – 90 gallons (70 – 380 liters per capita per day). This use is increasing by 0.5% - 1.0% per year and at this time comprises 50% of all the uses of water.
Water uses are for drinking, cooking, meeting of sanitary needs in houses and hotels, irrigating lawns etc. Residential water use rates fluctuate regularly.
Commercial and industrial:This is the amount of water used by the shops, markets, industries, factories etc. It contributes 15 – 24% of total use of water.
It includes factories, offices and commercial places demand. It is based on either having a separate or combined water supply system. Demand of water based on unit production: No. of persons working and floor area
Public use:The public use of water is that one which is used by city halls, jails, hospitals, offices, schools etc. This consumes 9% of total use of water. Its water demand is 50 – 75 liters per capita per day. Fire protection's need of water is also fulfilled by this sector. The fire demand does not greatly affect the average consumption but has a considerable effect on peak rates. Schools, hospitals, fire fighting etc
Loss and wastes:: Unauthorized, connections; leakage in distribution system, Hydrant flushing, major line breakage and cleaning of streets, irrigating parks. Total consumption is sum of the above demands. The water which is not intended for specific purpose or use is also called "Un-accounted for". Loss and wastage of water is due to:
- Errors in measurements
- Leakages, evaporation or overflow
- Un-metered uses e.g. fire fighting, main flushing
- Un-authorized connections
Factors affecting the use of water
- Size of the city
- Industry and commerce
- Time of the day
- Day of the week or month
Estimation of Water Demand
- Calculate design flow
- Determine the pumping power of machines to be used
- Reservoir capacity
- Pipe capacity
- Average daily
water consumption: It is based on complete one year supply
of water. It is the total consumption during one year, divided by the
q = (Q / P x 365) lpcd (liters per capita per day)
- Maximum daily
consumption: It is the maximum amount of water used during one
day in the year. This amount is 180% of the average daily consumption
MDC = 1.8 x Avg. daily consumption. It is usually a working day (Monday) of summer season.
- Maximum weekly
amount of water used by a population during a whole single week in a
study span of 1 year.
Maximum weekly demand = 1.48 x Avg. D. C
Maximum monthly demand = 1.28 x Avg. D. C
Maximum hourly demand = 1.5 x Avg. D. C
Maximum daily demand = 1.8 x Avg. D. C
- Fire water
demand | Fire Demand: Theamount of water usedfor fire
fighting is termed as fire demand. Although, the amount of water used in
fire fighting is a negligible part of the combine uses of water but the
rate of flow and the volume required may be so high during fire that it
is a deciding factor for pumps, reservoirs and distribution mains.
Minimum fire flow should be 500 gpm (1890 L/m)
Minimum fire flow should be 8000 gpm (32, 400 L/m)
Additional flow may be required to protect adjacent buildings.
Chemical Characteristics of water
Acidity:Acidity or alkalinity is measured by pH. PH measures the concentration of Hydrogen ions in water. Ionization of water is
HOH H+ + OH-In neutral solutions [OH] = [H] hence pH = 7
If acidity is increased, [H] increases and pH reduces from 7 (because H is log of [H]). The value of pH of water is important in the operations of many water and waste water treatment processes and in the control of corrosion.
Alkalinity:The values of pH higher than 7, shows alkalinity. The alkaline species in water can neutralize acids. The major constituents of alkalinity (or causticity) are OH-, CO32- and bicarbonates HCO3 ions. Alkalinity in water is usually caused by bicarbonate ions.
Hardness of water: Definition of hard waterHardness is the property that makes water to require more soap to produce a foam or lather. Hardness of water is not harmful for human health but can be precipitated by heating so can produce damaging effects in boilers, hot pipes etc by depositing the material and reducing the water storage and carriage capacity.
Absolute soft water on the other hand is not acceptable for humans because it may cause ailments, especially to heart patients.
Hardness in water is commonly classified in terms of the amount of CaCO3 (Calcium Carbonate) in it.
Table 1 - Degree of Hardness
Low level of hardness can be removed just by boiling but high degree of hardness can be removed by addition of lime. This method has also the benefit that iron and manganese
contents are removed and suspended particles including micro-organisms are reduced.
SedimentationWaters exiting the flocculation basin may enter the sedimentation basin, also called a clarifier or settling basin. It is a large tank with slow flow, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between does not permit settlement or floc break up. Sedimentation basins may be rectangular, where water flows from end to end, or circular where flow is from the centre outward. Sedimentation basin outflow is typically over a weir so only a thin top layer—that furthest from the sediment—exits. The amount of floc that settles out of the water is dependent on basin retention time and on basin depth. The retention time of the water must therefore be balanced against the cost of a larger basin. The minimum clarifier retention time is normally 4 hours. A deep basin will allow more floc to settle out than a shallow basin. This is because large particles settle faster than smaller ones, so large particles collide with and integrate smaller particles as they settle. In effect, large particles sweep vertically through the basin and clean out smaller particles on their way to the bottom.
As particles settle to the bottom of the basin, a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. The amount of sludge that is generated is significant, often 3 to 5 percent of the total volume of water that is treated. The cost of treating and disposing of the sludge can be a significant part of the operating cost of a water treatment plant. The tank may be equipped with mechanical cleaning devices that continually clean the bottom of the tank or the tank can be taken out of service when the bottom needs to be cleaned.
FiltrationAfter separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc.
Rapid sand filters
Cutaway view of a typical rapid sand filterThe 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.
To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing or backwashing) to remove embedded particles. Prior to this, 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.
- 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.
Membrane filtrationMembrane 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 um—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 phosphorus, nitrates and heavy metal ions.
Slow sand filters
Slow "artificial" filtration (a variation of bank filtration) to the ground, Water purification plant Káraný, Czech RepublicSlow sand filters may be used where there is sufficient land and space as the water must be passed very slowly through the filters. These filters rely on biological treatment processes for their action rather than physical filtration. The filters 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 pre-treatment 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 system 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.
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 major streams where bank filtration is often used.
Removal of ions and other dissolved substancesUltrafiltration 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.
Ion exchange: 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 nitrate, nitrite, lead, mercury, arsenic and many others.
Electrodeionization: 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 with a little worse degree of purification in comparison with ion exchange treatment. Complete removal of ions from water is regarded as electrodialysis. The water is often pre-treated with a reverse osmosis unit to remove non-ionic organic contaminants.
Other mechanical and biological techniquesless (or non)-polluting techniques are also being used to treat polluted water. These techniques include those based on mechanical and biological processes. An overview:
- mechanical systems: sand filtration, lava filter systems and systems based on UV-radiation)
- biological systems:
DisinfectionDisinfection is accomplished both by filtering out harmful microbes and also by adding disinfectant chemicals in the last step in purifying drinking water. Water is disinfected to kill any pathogens which pass through the filters. Possible pathogens include viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoa, including Giardia lamblia and other cryptosporidia. In most developed countries, public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days before reaching the consumer. Following 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.
Main article: ChlorinationThe 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 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 exists that releases chlorine on contact with water. Handling the solid, however, requires greater 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 both inexpensive and safer than the use of gas or solid chlorine. 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 trihalomethanes (THMs) and haloacetic acids (HAAs), both of which are carcinogenic in large quantities and 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 protozoa that form cysts in water (Giardia lamblia and Cryptosporidium, both of which are pathogenic).