An upland reservoir in an area of a sheep farming area which is known to be “soft” and to have unacceptable taste and color with a pH of 5.5.
A network of pumped wells delivering anaerobic groundwater from chalk aquifer. The possibility exists that there may be pathogenic organisms in the supply, together with the significant quantities of iron and manganese.
Compare and contrast the possible water treatment strategies which may be required for the two sources. You should identify other quality test which you would recommend as essential before undertaking a detailed design of either option.
Before recommending an essential water treatment strategy, we should know about the reservoir management.
Water storage before use results in:-
Reduction in Suspended Solids (SS) and color due to natural sedimentation.
Reduction in pathogens due to self-purification.
Minimize the fluctuation in the water quality thus enhance the treatment.
Water storage also may lead to:-
Algae growth which in turn will lead to increase in turbidity, smell, pH and even color.
Increase in Iron and Manganese content present in soil.
Thermal stratification causing variation in water chemistry and characteristics with little intermixing behind the reservoir.
Color, Taste and Odor:-
Hues in water is due to natural materials like iron and manganese; vegetable origins, humus materials, insoluble particles of soil, organic and microorganism. The true color of water is considered to be only that attributable to substance in solution after removal of suspended materials by centrifuging or filtration.
Taste can be affected by inorganic salts or metal ions, a variety of organic chemicals found in nature or products of biological growths. Algae are the most frequent cause of taste and odor problems.
According to Metcalf & Eddy, Inc., the main sources of these are from
(1) Septic wastewater containing hydrogen sulfide and odorous compounds,
(2) Industrial wastes being discharged into the collection system,
(3) Seepage handling facilities,
(4) Screenings and unwanted grit,
(5) Scum on primary settling tanks,
(6) [Bio solids]-thickening tanks,
(7) [Bio solids]-conditioning and dewatering faculties,
(8) [Bio solids] incineration,
(9) Digested [bio solids] in drying beds or [bio solids]-holding basins,
(10) [Bio solids]-composting operations.
pH is used to express the intensity of an acid or alkaline solution. A pH of 7 is neutral, if pH less than 7 it’s acidic, and pH greater than 7 is alkaline. Acidity is the measure of carbon dioxide and other solution. Strong inorganic acid acidity exists below pH 4.5; carbon dioxide acidity (carbonic acid) is between pH 4.5 to 8.3.Water should be close to pH 7 as possible.
A detailed raw water quality analysis (physical, chemical, biological characteristics) of the water should be done to establish a suitable strategy for treatment.
Due to thermal stratification in different seasons, height of draw off point may need to be seasonally adjusted.
Before treatment, the effectiveness of chemical coagulation of waste should be experimentally evaluated in the laboratory by using Jar Test.
To reduce color, chemical coagulation and granular-media filtration is required.
Primary and secondary units
No problem (normal)
More air/less BOD
Chlorine contact tank
Improper chlorine dosage
Adjust chlorine dosage controls
Enforce sewer use regulation
Source:Spellman, F.R.,The Science of Water, Technomic Publ.,1998.
Taste and odor can be enhanced by:
Aeration:-Since the odor compound are often dissolved gases that can be stripped from the solution.
Carbon adsorption:-It’s the most effective way; Activated carbon can be introduced in any stage of processing before filtration where adequate mixing is available to disperse the carbon and where the contact time is 15 min or more before sedimentation or filtration.
Oxidation:-It can be done by chlorination, chlorine dioxide, potassium permanganate, or ozone.
For water less than pH 6, feed pumps inject a neutralizing solution of sodium carbonate or sodium hydroxide at the inlet. The dosage to be provided can be found out using Jar Test.
In The Use of Soda Ash and Caustic Soda to Alter pH (Glenda M. Herman, 2013) it is stated that for water between pH 4 and pH 6, use soda ash mixed with water. Feed this solution in the source at a rate to raise the pH to 7 farthest from the source.
Anyhow the basic system for treatment might be:
Pretreatment – Any process to modify the microbial water quality before the entry to a treatment plant;
Coagulation, flocculation and sedimentation – Process by which small particles interact to form larger particles and finally settle out by gravity;
Ion exchange – Process used for removal of calcium, magnesium & some radionuclides;
Granular filtration – Process in which water passes through a bed of granular materials after coagulation;
Slow sand filtration – Process in which water is passed slowly through a sand filter by gravity, without the use of coagulation.
The biological organisms in the water or wastewater are called the pathogens. They are organisms, capable of transmitting diseases in humans. These waterborne pathogens include bacteria, viruses, protozoa, and parasitic worms.
Points of Origin
Resistance to Disinfection
Humans and animals; water;
Type specific-bacterial spores typically have the highest
resistance whereas vegetative bacteria have the lowest resistance
Humans and animals; polluted water; contaminated food
Generally more resistant than vegetative bacteria
Humans and animals; sewage; decaying vegetation; water
More resistant than viruses or vegetative bacteria
Source:Spellman, F.R.,The Science of Water, Technomic Publ., 1998.
There are various processes for removal of microbes from water. In particular, it discusses:
Pretreatment oxidation -Process in which oxidants are added to water in the treatment process. This helps in:
Maximize the contact time with the oxidant;
Oxidize the compounds for subsequent removal by treatment process(e.g. iron or manganese);
Provide initial treatment in sufficient time for water to be further treated if necessary (e.g. oxidation of taste and odor compounds);
Control growth of microorganisms and higher organisms on intake structures and treatment basins;
Improve the particle removal in clarification and filtration processes.
Primary disinfection – Process is a component of primary treatment of water and important because filter media do not remove all pathogens from water. Different types of disinfectant are –
Secondary disinfection – This process is used to maintain the water quality which we got at the treatment plant throughout the distribution system.
These are found in groundwater, industrial waste, and as by-products of pipeline corrosion.. They don’t cause health related problems, but are not good because they cause aesthetic problems.
Aesthetic problems associated with iron and manganese is:
1. Staining of plumbing fixtures
2. Discoloration of water (iron – red water, manganese – black or brown water)
3. Stimulates the growth of microorganisms.
4. Impart a bitter taste to the water
Some of the economic problems caused are damage to textiles, dye, paper, and food. It may clog pipes and corrode through them. Iron residue in pipes decreases carrying capacity & increases pumping head.
Iron and Manganese Removal Techniques
Precipitation: Precipitation (or pH adjustment) of these from water in their solid forms can be done in treatment plants by adding lime adjusting the pH of the water. Some of the precipitate will settle down by time, while the rest is easily removed by sand filters. This process requires pH level of the water to between of 10 to 11.
Oxidation: It is the common methods of removing these, usually followed by settling and filtration. Air, chlorine, or potassium permanganate can be used for oxidizing.
Ion Exchange: The ion exchange process is mostly used to soften hard water, it will remove all soluble iron and manganese. The water is passed through a bed of resin which adsorbs the undesirable ions, replacing them with less troublesome ions.
Sequestering: Sequestering or stabilization can be used when the water contains low concentration of iron, & when the volumes needed are comparatively small. This process does not remove the manganese and iron from the water, but binds it chemically with other ions in soluble form that will not li come out of solution
Aeration: The physical process uses air to oxidize the manganese and iron. The water is pumped up to the air or allowed to fall directly over an aeration device. The air oxidizes and this is then removed by use of a filter. The lime is often added to raise the pH.
Q2. Secondary wastewater treatment may involve biological treatment using either fixed film or free-film systems. Discuss, using diagrams where appropriate, which type of system might be more appropriate for treating the wastewater from a small market town with a population of 20,000. The town is situated alongside a “sensitive” lowland river from which a major water supply is abstracted further downstream. Your answer should explain, among others:
the advantages and disadvantages of fixed-film and fixed film systems for secondary biological treatment;
the typical effluent characteristics of each type of treatment system when operating in a standard, conventional mode;
the specific needs of sensitive rivers with regards to nutrients loading, especially nitrogen and phosphorous compounds;
the implications of discharging high nutrients into receiving streams being used as water supply sources;
the implications of discharging high nutrients, especially nitrogen and phosphorus compounds to receiving rivers from a water quality management viewpoint;
the details of any proposed modifications to the conventional systems which you would consider might be adopted in these circumstances to reduce (manage) the nutrients and the cost implications for these.
The purpose of secondary treatment (biological treatment) is to provide removal of BOD beyond what is achieved by primary treatment. Secondary treatment process (biological treatment process) can be separated into two categories: fixed film systems and free film system (suspended growth systems).
Fixed film systems: These are processes that use a biomass or slime (biological growth) which is attached to some form of film or media. Wastewater is allowed to passes over or around the film and the slime. When the slime and wastewater are in contact, the organisms oxidize the organic solids. The film may be stone, synthetic materials or any substance that is durable, should provide a huge area for slime growth & an open space for ventilation. Fixed film system includes trickling filters, bio towers and RBCs.
Suspended growth systems: These are processes that use a biomass (biological growth) that is mixed with the sewage. They can be used in smaller space than trickling filter that treats the same amount of water. A typical free film system includes the activated sludge process.
There are a variety of these secondary treatment, we will discuss about the following conventional processes used:
In most wastewater treatment, the trickling filter is used after the primary treatment.
This process is a fixed film method designed to remove suspended solids and BOD.
It consists of a rotating distribution arm that sprays the influent over a circular bed of rocks, synthetic media, or other coarse materials.
The spaces between the film helps to circulate air easily so that aerobic conditions is maintained. The space allows waste to trickle down through and over the media.
Organic matter in the water diffuses into the media, where it is metabolized. Periodically, a portion of the film sloughs off the media material and is collected at bottom of filter.
This is passes on to the secondary settling tank along with the treated wastewater, where it is removed.
The overall performance of filter is depended on hydraulic and organic loading, recirculation and temperature.
Appropriate for small to medium sized communities (15000 to 20000) peoples.
Good Quality(80-90% BOD removal) for 2nd stage efficiency could reach 95%
Moderate operating cost(less than activated sludge)
Withstands shock loads than other processes
Simple and reliable process and can be used were large area not available.
Very efficient in removal of ammonia
High capital costs
Clogging of distributors or beds.
Snail, mosquito and insect problems.
Generate sludge that must be treated
Regular operator’s attention is needed.
Relatively high incidence of clogging.
Additional treatment may be needed for the effluent to meet strict discharge standards.
In this process influent and activated sludge is aerated and agitated. The activated sludge is finally separated from the treated mixed liquor by process called sedimentation and is returned to the reactor as needed. The treated waste flows over the weir of the settling tank in which it is separated from the sludge.
Wastewater is fed into an aerated tank, where the microorganisms (activated sludge) metabolize and flocculate the organics.
These are settled from the aerated mixed liquor in the final clarifier and are returned to the aeration tank.
A portion of the concentrated solids are removed from the bottom of the settling tank from the process.
Clear supernatant fluid from the final settling tank is the plant effluent.
This process is effective to treat large volumes of flow (10000 to 1000000) people.
Flexible, can adapt pH, organic and temperature changes.
Small area required.
Degree of nitrification is controllable.
Relatively minor odor problems.
Low construction cost.
Removes high percent of BOD.
High operating cost.
Generated solids requiring sludge disposal.
Some process alternatives are sensitive to shock loads and metallic or other poisons.
Requires continuous air supply.
For a small population of 20000 and river being so sensitive I recommend using Trickling Filter would be more advisable for the town.
Excess nitrogen and phosphorous compounds can over stimulate the growth of aquatic weeds and algae. Excess growth of these organisms can blue baby syndrome.
Apply two-thirds to three-fourths of the planned fertilizer nitrogen just before the crop enters a period of rapid growth. Proper timing ensures maximum daily nitrogen uptake and minimizes the likelihood of unused nitrogen leaching below the plant roots.
Apply a reasonable amount of nitrogen to your crop. When grain and forage yields are low, less nitrogen will be removed with the grain, silage, or hay crop or by grazing. Because a soil test is not a reliable means of predicting nitrogen response, consider analyzing plant samples collected early in a period of rapid growth. The need for additional nitrogen can be determined and applied before the crop matures.
If your crop will follow peanuts, soybeans, or forage legumes (clover or alfalfa) of average or greater yield, reduce the amount of nitrogen you apply. Soybeans and peanuts may provide 20 to 40 pounds of carryover nitrogen per acre. A “strong” alfalfa stand may provide 80 to 100 pounds of nitrogen per acre for the next crop.
Be sure to analyze animal, municipal, and industrial wastes for nitrogen content when applied to cropland. Guard against “dumping,” as this practice may contaminate water with excess nitrate.
Throughout the sandy soil surfaces of the coastal plain, do not apply nitrogen in the fall for springplanted crops. Piedmont fields may receive some nitrogen (up to one-half of crop needs) for springplanted crops.
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