Raw water preparation.
Concern about water quality brought about new and numerous Federal regulations promulgated by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act (SDWA) and its subsequent amendments. These requirements cover allowable tolerances of bacterial life, suspended solids levels, likely chemical contaminants, metals, and other parameters in the water distributed to the public. Such regulations are termed "primary."
Secondary regulations, which cover chloride content, iron and manganese levels, color, and taste and odor, are also desirable to obtain, but are not enforceable by the Federal government of this country.
Regulations issued by the EPA have been challenged from time to time, particularly those relating to organics that have been labeled toxic through research work. Numerous rules have been established in response to legislation and Congressional mandates over the past few years; more are certain to follow. The interaction between EPA and the overseeing state agencies is a relationship that is still seeking the right measure of balance. The bottom line is, that the water supply and distribution field will continue to be an interesting place to work. Hopefully the regulation adoption procedure will be somewhat flexible, but the standards proposed and applied appear to err wisely on the side of overprotection.
Where groundwater supplies have adequate capacity, they often provide a more dependable supply from a quality standpoint than those from impoundments, lakes, or rivers. Much of the purification necessary for domestic and industrial water use can be accomplished through percolation to groundwater aquifers.
Some soils, in addition to providing the benefit of natural filtration and removal of suspended matter, have adsorbent and ion exchange capabilities, and can consequently change the characteristics of dissolved impurities, often for the better. There is concern over the contamination of groundwater sources by toxic chemicals in landfill leachate or from uncontrolled hazardous waste disposal. Therefore, monitoring and observing the proximity of such landfills is necessary, as well as monitoring local groundwater systems.
Under the SDWA the EPA received broad, new roles in protecting the public from contaminated drinking water. In particular it is directed to:
* Control specific disease-causing organisms and indicators of their presence in drinking water.
* Require public water-supply systems that use surface water sources such as lakes to filter their water unless it is established that their sources are very clean and well-protected.
* Require public systems to disinfect their water, with allowance for variances if the water comes from sources that are determined not to be at risk from microbiological contamination.
Complying with the new rules will be costly. Many small systems will be hard pressed to meet the filtration and disinfection requirements of the Surface Water Treatment Rule. Many systems may opt to protect their watersheds from development, either by outright ownership or working with surrounding communities and landowners to implement sound land-use practices.
Some useful EPA definitions include:
Public Water System - a system that pipes water for human consumption to at least 25 people or has 15 or more service connections.
Community Water System - a public water system serving at least 25 year-round residents or that has 15 or more connections used by year-round residents.
Non-Community Water System - a public water system that does not meet the definition of a community water system. Some schools, factories, campgrounds, motels, and restaurants are examples of non-community water systems.
Surface Water - sources of water such as lakes, reservoirs, rivers, and streams found on the earth's surface.
Groundwater - water sources found below the surface of the earth.
Raw Water - untreated surface or groundwater.
Bacteria - minute one-celled organisms such as total coliforms, the vast majority of which do not require a host organism for survival or do not cause disease.
Pathogens - microbes such as salmonella and Shigella that cause disease.
Protozoa - one-celled animals that are larger and have a more complex structure than bacteria. A few types, such as Giardia and Cryptosporidium, cause disease.
Microbe - an organism too small to be seen without a microscope. Microbes include bacteria; protozoa, and viruses.
Viruses - the smallest and simplest form of microbial life. Viruses can only reproduce inside a host cell. Examples of viruses include Hepatitis A Agent and Norwalk Agent.
Some useful acronyms include:
BAT - Best Available Technology
BTGA - Best Technology Generally Available
IOCs - Inorganic Chemicals
MCL - Maximum Contaminant Level
MCLG - Maximum Contaminant Level Goal
NPDWR - National Primary Drinking Water Regulations
RPDWR - Revised Primary Drinking Water Regulations
SDWA - Safe Drinking Water Act
SOCs - Synthetic Organic Chemicals
VOCs - Volatile Synthetic Organic Chemicals
Additional Information. For more information on the Safe Drinking Water Act, its amendments, and other aspects of drinking water call EPA's Drinking Water Hotline: 1-800-426-4791; in Washington, D.C. and Alaska call (202) 382-5533.
The EPA also has a Public Information Center (for all environmental programs) located at U.S. EPA, 410 M Street, S.W., Washington, DC 20406, tel-(202) 260-7751, fax-(202) 260-3923. You can also contact your state water agency or regional EPA office.
INTAKES & SCREENS
Waters derived from natural lakes, impoundments, or rivers are considered surface sources. The water treatment system begins with intake facilities at the source. The intake system can take many. forms. In lakes and impoundments (reservoirs) towers or other forms of multi-level intake structures can be used to select the highest quality water. In other cases the water inlet can be placed along the bank side or offshore and connected to the pump structure by piping.
Inlet structure selection should be made with consideration of such factors as available water depth, water level variation, and ice formation. Protection of pumps, force mains, and other structures is also a necessary design item. Another item of concern is that of zebra mussels getting into intake structures and fouling the treatment works.
The selection process includes consideration of where the primary screening should take place. Factors involved in this decision include:
* Debris handling and screening to minimize accumulation and entry into the system, or choosing screens designed to collect debris and dispose of it on a land site.
* Environmental constraints such as protection of aquatic life near the intake.
* Capital and operating cost of the screen system; including cost considerations in other parts of the treatment process that are to some degree influenced by screen selection (i.e., smaller screen openings reduce debris loading in downstream components of the treatment process).
Zebra Mussels. The infestation of the Great Lakes (and subsequently many other North American freshwater systems) with this small bivalve mollusk has enormous implications for water supply operators. Thought to have been brought to North America in the ballast water of a cargo ship from the Black Sea, there are several problems with this exotic species. Its most unendearing quality is that it spreads both rapidly and prolifically - its veliger stage allows the larva to travel great distances before attaching to a structure (such as an intake pipe or screen). Within a year the adult version is capable of producing tens of thousands of offspring. The colonies of mussels may reach densities of thousands per square yard (often crowding out native mollusk species). They filter tremendous amounts of water, which does act to remove some pollutants, but at the same time reduces available food resources for competing organisms. The mollusk is easily spread by floodwaters or by attaching itself to boats, plants, bait buckets, or other submerged equipment (under the right conditions they can survive up to seven days out of water); when the owner takes the craft to different water bodies, so goes the mollusk. In North American waters they have no major predators and consequently have exploited their new environment. Huge masses of zebra mussels can quickly clog a water treatment or power plant intake line. Dead mussels can detach and flow into the treatment works causing equipment problems as well as imparting tastes and odors.
Controlling zebra mussels will probably be an expensive endeavor (by some estimates, up to $5 billion just for the Great Lakes). Treatment systems using chlorine, potassium permanganate, ozone, and other chemicals as well as electric fields and heated water have been used with varying success. Many of those colleges and universities affiliated with the Sea Grant College Program can provide information and the latest in research on zebra mussels.
The two basic screening options - screen design to minimize debris accumulation and screen design to collect and dispose of debris - are discussed below.
Passive Screening for Minimum Debris Accumulation. This approach to intake design is characterized by relatively low intake velocities and screening at the point of water withdrawal and can be separated into three general types.
Type 1 includes cylindrical shaped wire screening designed for maximum velocity through the screens (the screens can be placed on towers, along the bank, or offshore and connected to the pump well with pipes). Screen cleaning, if required is generally accomplished by releasing a burst of air through the screens to lift debris away from the screen surface. This can be done through manual valve operation or an automatic system.
Type 2 includes flat panel screens placed flush with the bank and parallel to the direction of the current. Air lift debris removal can also be used with these screens.
Type 3 consists of screen arrays buried under the bed of the water source as part of a filter bed or infiltration gallery.
For these types of screens, screen openings are generally in the range of 1/32 in. to 3/8 in. with openings between 1/10 in. and 1/4 in. being the most common. The screens protect the pumps and process equipment and can reduce the loading of downstream facilities. The screen opening sizes and the low through-screen velocities generally used with this type of screening result in a minimal potential for adverse environmental damage.
Powered Mechanical Screening for Debris Collection. This approach to intake screening is characterized by the use of mechanically driven screening equipment. Generally the system involves a dual set of screens - a trash rack arrangement with relatively wide screen openings (1 in. to 3 in.) and a second screen set with smaller screen openings (usually about 3/8 in. or less). The trash rack can be cleaned manually or with automatic rakes.
They provide coarse screening and a degree of protection of the secondary screens. The secondary screens can be manually cleaned when constructed of perforated plate or woven wire mesh. More often, they consist of an array of screen panels attached to a rotating drum or a set of moving belts or chains.
Debris is removed from the screens using high pressure water sprays to wash debris into a trough for subsequent collection and disposal. These screens provide pump and system protection and in some cases, have optional devices that can incorporate mechanisms to protect aquatic organisms.
In addition to the screening products discussed above, several other mechanical screening approaches that are generally used for process screening and separation are sometimes applied to water intake applications.
Stationary or traveling screens are used to remove fine particles. The former are usually of perforated metal plate or wire screens, trays or baskets, in frames, which slide vertically in slots and may be raised for cleaning.
A stream of water can be continuously exposed to a clean screen surface by using drum and disc screens. In the drum screen, stainless steel or other screen fabric is mounted on a rotating drum, one end of which is open and the other sealed. Water passes into the open end and through the walls. At the upper end of the rotating cycle, accumulated debris is flushed from the screening surface by nozzles. Disc screens operate on a similar principal, with a disc covered by a screen fabric, which is placed perpendicular to the direction of flow in a channel.
Screens are made of any desired mesh to 100 but 1/8 in. to 3/8 in. is the most common; and in a variety of widths. Corrosion-resistant materials are obtainable, either wire or perforated metal. PVC is also offered as a screen material.
Slotted well screens have also been used for fixed intakes in place of submerged cribs. The screen is attached to the end of the intake pipe with the assembly placed on a foundation in the stream bed. It may be covered with gravel or crushed rock, but should not be fastened so securely that it cannot be removed for inspection.
A microstrainer is a continuously working self-cleaning filter screen of high mesh stainless steel or other corrosion resistant fabric incorporating automatic washing and wastewater disposal. Applications include: a) microstraining as a sole filtration process; b) primary filtration ahead of both rapid and slow sand filtration; c) treating industrial water supplies; d) the final clarification of secondary sewage effluent and industrial wastes.
A rotating drum with one open and one closed end is used. Water entering the open end must pass through the mesh filter screen sides. As the drum revolves it carries the removed solids above the water line where water jets wash them into discharge hoppers. Fabrics are of synthetic media, bronze, or stainless steel.
Microstraining can remove as much as 95 percent of common algae. Wash water consumption may run from 1 to 3 percent of the flow through the unit. Blinding of the fabric rarely occurs, but may do so from inadequate wash water pressure or the presence of bacterial slime. Cleansing is accomplished by applying a solution of sodium hypochlorite.
Traveling water screens provide for automatic solids removal by flushing with water sprays. This occurs at the top of the cycle, where accumulated debris falls into a trough or other container for disposal. Overload protection to take the unit out of service in the event of potential damage, is provided in most commercially available models.
Automatic differential head control can be furnished to permit automatic starting of mechanical rakes for stationary screens, when intermittent operation is desired.
Turbidity, color, and other suspended materials, some of which are colloidal in nature and so finely divided that they will pass through a filter, are removed by coagulation. Many of these constituents carry negative charges and are therefore readily attached to the positive trivalent ions of aluminum and iron. Other materials are removed through mechanical entanglement with the floc. The dosages of coagulants required for optimum floc formation vary widely with waters of different characteristics. The jar test is frequently used to determine proper dosages. However, more precise judgment is possible by measuring zeta potential or streaming current.
With the development of such special methods of water treatment as high-rate filtration or high-rate water treatment and factory-built control components for treatment, coagulation control is somewhat interlocked with the process. Consequently the subject of coagulation control is discussed under process control.
Floc formation and coagulation, in addition to being influenced by the chemical impurities present (both anions and cations) and pH, are also affected by the degree of mixing and flocculation, temperature, and the presence of suspended matter that can act as nuclei.
Mixing and flocculation periods must permit time for the reactions to take place, and in the flocculation stage provide gentle conditions to permit floc growth. Generally, the higher the mineral content of the intake water, or the higher the coagulant dosage, the less time required for mixing and flocculation. Reaction times, of course, vary inversely with temperature, and usually more coagulant should be added during periods of lower temperatures.
Aluminum Sulfate. Also known as filter alum, it may be obtained in lump and in granular form packed in 100-lb or larger paper bags and in carload bulk lots if desired.
Liquid alum is a solution of dry alum. Production plants are located in the paper-making areas, since the paper industry is a big consumer and large water plants in these areas are in the best position to utilize this product.
Ferric Sulfate. This compound is widely used in the partially hydrated granular form, in which it is known by the trade name "Ferrifloc." It is a free.flowing chemical that may be fed dry form. Shipments-are made in bulk, barrels, and bags. The dry form has a tendency to pick up moisture and should be in tight packages.
Ferrous Sulfate. This material is shipped in bulk, bags, or barrels. It picks up water in storage; for that reason, a product with slightly less water or crystallization is preferred. Ferrous sulfate provides a bivalent iron ion when added to water, which should be oxidized to the trivalent form through chlorination or aeration to avoid excessive solubility. Chlorinated copperas is ferrous sulfate converted to the ferric form by adding chlorine during the feeding process.
Ferric Chloride. This is available in crystal form, which melts at 99 [degrees] F in wood barrels; as anhydrous material in steel drums; or as liquid in tank cars or drums. It is corrosive and should be handled and stored in contact with inert materials, such as fiberglass, rubber lined receptacles, glass, or crockery.
Sodium Aluminate. This substance is highly alkaline and in commercial form contains sodium carbonate and sodium hydroxide. It is usually fed with aluminum sulfate in treating turbid and colored waters, available in crystalline or liquid form.
Magnesium Carbonate. This is particularly adaptable to water softening systems in connection with lime. It is reusable if a sludge recovery arrangement is employed.
Poly Aluminum Chloride. Poly aluminum chloride is a type of inorganic polymer that strongly coagulates substances either suspended or colloidally dispersed in water, to produce large, rapid settling floc that forms an easily filterable sludge. It is easily handled and stored and its use does not essentially change the pH value of the water, thus overdosing is usually not a problem.
Coagulant Aids. Activated silica is perhaps the most widely known coagulant aid. It is prepared by partially neutralizing a dilute solution of sodium silicate with acid or an acid reacting salt or gas. This produces minute colloidal silica particles, micelles, which grow during an aging period. Once proper micelle size is reached, further dilution arrests growth and prevents jelling. Very close control of the aging period is essential. Automatic equipment preparing activated silica soils is available.
Decades ago, the term "polyelectrolyte" was coined to describe various materials that are both polymers and electrolytes.
The polymers act as binders, attaching themselves upon collision with suspended particles with adsorptive ability. Numerous particles may be collected within a flocculent mass, depending on the nature of the flocculating motion and on the degree of adsorption. Anionic polyelectrolytes function only as flocculents, but cationic polyelectrolytes are both flocculents and coagulants. In fact, a stoichiometric relationship exists in destabilizing a clay suspension, between the initial clay concentration and the optimum dosage of polymer needed for destabilization. These can also act as filter aids in that they absorb on the filter media and increase filtration efficiency.
There are several coagulant aids on the market sold under proprietary names. Where water is treated for drinking purposes, toxicity of the aid must be considered. The Office of Air and Water Programs of the EPA, Cincinnati, Ohio, evaluates coagulant aids from the toxicity standpoint. A listing of the various products this committee has approved is available from state health departments or other regulatory agencies.
Other materials such as activated carbon, pulverized limestone, starch, clay and bentonite, while long used in water treatment can also function as coagulant aids. Bentonite is a colloidal clay-like mineral that flocculates physically if there is sufficient concentration of dissolved minerals, or any electrolyte.
Lime is used in coagulation to provide artificial alkalinity in waters that are treated with aluminum or ferrous sulfate; or with some waters it may be used alone. It is used either as quicklime or hydrated. Information on using lime in water treatment and a list of representative producers of quick lime and hydrated lime may be obtained from the National Lime Association.
Soda ash, also known as sodium carbonate, is used to give artificial alkalinity to water to be treated with aluminum sulfate. In small plants it is often preferred to lime for this purpose, although more expensive. Soda ash finds more application in water softening.
All chemicals should be handled carefully to prevent potentially hazardous and dangerous situations. Heavy containers should be handled by mechanical equipment, i.e. fork-lift trucks or hoists and monorail. A toxic atmosphere may be created by damage to containers of liquid chlorine, sodium hypochlorite, anhydrous ammonia, and sulfur dioxide. Burns may be caused by sulfuric acid, sodium hypochlorite, and ferric chloride. Cylinders containing liquefied gases under pressure should not be stored in operating rooms or where they may be heated above 100 [degrees] F.
Granular aluminum sulfate, lime sodium carbonate, and activated carbon create dust problems. Bulk handling with slurry or solution storage minimizes the dust problem.
Conveying Liquids. Among materials used are plastic pipe; hard rubber pipe; rubber-lined pipe; plastic and resin-lined pipe; vitrified clay for either acids or alkalis; iron alloys, nickel alloys, bronze, aluminum for a few less corrosive chemicals; asbestos-cement or PVC pipe for chlorine; and copper or PVC pipe for alum.
Plastic pipe in different sizes is produced by extruding thermoplastic materials and is available in flexible, rigid, and semi-rigid form. These materials resist corrosion and are suitable for use with most liquids. A complete list of firms may be obtained from the Plastics Pipe Institute.
Pumping Chemical Solutions. Liquid ferric chloride is withdrawn from tank cars by siphoning through rubber hose or forcing through such hose by air pressure supplied to the tank; or, if the lift is more than 40 ft, by pumping with acid-proof pumps. Other liquid chemicals can be handled by pumps, which can be designed for any acid or alkali, as can valves constructed of corrosion-resistant materials. Gear pumps are well-suited to handling viscous solutions such as polymers.
Chemical solution lines need be equipped with corrosion-resistant valves, such as rubber or plastic lined, constructed of bronze, PVC, etc.
Conveying Solids. Plants generally obtain chemicals in containers weighing under 500 lb, and these are handled by hand trucks, overhead trolleys, barrel elevators, hand cranes, lift trucks, chain hoists, etc. For small plants, the 100-lb multi-walled paper bag is the most convenient for handling.
Alum, soda ash, ferric sulfate, and lime are commonly bought in bulk by large plants and are generally unloaded to elevated bins by bucket conveyors; by belt, scraper or screw conveyors; or by pneumatic conveyors. Dust gives a great deal of trouble, especially that from lime and activated carbon, and for this reason pneumatic conveyors are used in many plants.
Where pneumatic conveying systems are not employed, ventilating systems should be used.
Bins. Solid materials received in bulk must be stored in bins in a dry place. Any of them is likely to cake or arch, and provisions for stirring in bins or jarring with a vibrator are desirable.
Scales. Dial, electronic, and other indicating scales, with or without tare and capacity beams, are available.
Tanks for Chemicals
Tanks used for mixing and storing chemicals must be composed of or lined with materials unaffected by chemicals. Rubber, glass, stoneware, stainless steel, and plastics serve for most chemicals.
Extensive use is made of fiberglass reinforced polyester resin tanks for chemical solution storage. The National Sanitation Foundation has established standards for thermoset plastic tanks. Stainless steel tanks and those certified as STI-P3 tanks are useful.
For ferric chloride (among the most corrosive chemicals used in water purification) the tank may be lined with acid-proof brick, laid in acid-proof cement. A steel, concrete, or wood tank may be rubber lined.
Epoxy resin formulations provide a corrosion resistant coating for concrete containers for chemical storage and handling and for concrete conduits for chemical solution conveyance.
Aluminum sulfate solution and liquid alum are generally stored in lead-lined or rubber lined tanks, although plastic tanks and 20-mil PVC plastic bag-lined wood tanks have been used.
Hazardous materials in bags or 55-gallon drums can be housed in a chemical storage container. Steel containers should be at least 10-gauge in thickness and coated with a corrosion resistant liner. Other features would be spill containment devices, safety door locks, Ventilation system, safety shower/eye wash station, dry chemical fire system, and related appurtenances.
Many chemicals can be fed either dry (in granular or powdered form) or in solution. Ferric chloride, polyphosphates, and sodium silicate must be fed in the solution form; this procedure is acquiring popularity.
Accurate solution feeding involves maintaining continuous uniformity solution strength. If the chemical does not dissolve readily, but is carried partly or wholly in suspension (such as hypochlorite or activated carbon), it should be kept in suspension by continuous stirring by a paddle mixer.
Dry Feeders may be either volumetric, measuring the chemical by volume; or gravimetric, measuring it by weight. The latter is the more accurate, but the equipment is considerably more expensive. A dry feeder consists of 1) a storage hopper, contracting to a throat at the bottom through which the chemical falls by gravity to 2) a proportioning and measuring device adjustable to give different feed rates, and 3) generally a mixer for making a strong solution to be applied to the water.
Gravimetric feeders measure chemicals by weight instead of by volume. This furnishes greater accuracy, within a 1/2- to 1-percent range, and makes possible automatic and remote control, the use of recording and totalizing gauges, and automatic over- and under-feed alarms.
Lime slakers consist of a feeder, a means of water control, and a slaker. The volume of slaking water is automatically kept proportional to the amount of lime fed.
Polymer Addition. Mixing dry and liquid polymers requires careful control and special mixers and feeders.
Solution feeders are devices of various designs, intended to precisely measure a chemical solution as it is fed to the system. They may be decanting feeders, consisting of a solution tank equipped with a draw-off pipe lowered or raised at a controlled rate by a variable speed motor; a revolving dipper, rotated at a controlled rate; or reciprocating or diaphragm pumps, which are manually adjustable by varying a piston stroke, but still remotely controllable with regard to operating speed. Some are more adaptable to handling slurries than are others.
Automatic Control of Feed
The strength of the chemical additives is carried by mixing different strengths of solutions or by varying the feeder discharge rate. Apparatus is available that automatically varies the dose with the amount of water being treated.
To achieve this, a rate-of-flow sensor must be placed in the intake line. This can be a "differential producer," in which a differential pressure is created within the device, related to the rate of flow. Venturi tubes, flow tubes, venturi nozzles, orifice plates, weirs, measuring flumes, and Pitot tubes are in this category. Other types of flow measuring devices that can be used as a rate-of-flow sensor are the propeller meter and the magnetic flow meter.
Through systems of pneumatic (air operated) or electrical transmission, the feeders may be coupled with the rate-of-flow sensor. Instrument manufacturers generally can provide complete control transmission systems for pacing chemical feeders. Often this is done in conjunction with centralized control of an entire plant, with gauges, switches, and alarms located on a console at a central point. Streaming current measurement can be used to provide monitoring and control of coagulant dosages.
Coupling the rate of chemical feed from such devices as lime slakers and feeders, soda ash feeders, acid proportioning feeders, and other devices to pH sensors for automatic control of pH is feasible with modern technology. Automatic conveyors are often used when solid chemicals are part of the treatment process.
Mixing & Flocculation
The chemicals used must be fed into the water at a determined dosage rate and must then be thoroughly and quickly mixed with water. This is effected by mechanical mixers. After thorough ("flash") mixing, it is desirable to aid agglomeration of the floc by a gentle stirring - "flocculation."
Flash mixers consist of a stirring device and a motor drive, which may be installed in tanks of various sizes and shapes.
Static or motionless mixers are incorporated into pipelines or flow channels and contain fixed baffles that mix additives introduced upstream, such as flocculents or pH modifiers, on a continuous in-line basis.
Flocculation. In addition to rapid, or "flash" mixing, a continual slow rolling motion called flocculation, is desirable to form large floc. Flocculation equipment has several submerged paddles that revolve with a peripheral speed of 0.9 to 1.8 fps about a horizontal or vertical shaft that is generally set at right angles to flow.
Horizontal Flow Tanks
Sediment and coagulated floc are removed by passing the water through a basin at a very slow rate to allow the material to settle and form a sludge. The flow from inlet to outlet should be very slow and uniform without "short-circuiting." The water flow should have a detention time of at least four hours. There should be mechanical devices for continuous or frequent sludge removal.
In all types of tanks, effluent collection system design is important in extracting clarified water for filtration. Long weir lengths enhance this operation.
With precise control of coagulation and filtration, high-rate treatment can be accomplished with substantially reduced coagulation and sedimentation time.
Sludge Removal Equipment. Removal of sludge is best effected by use of special mechanisms. These are mostly of two general types. In one, the mechanism revolves very slowly about a central vertical axis, carrying scrapers that work the sludge slowly toward a central well, from which it is drawn off by gravity or pumped. In the other, the scrapers are drawn in a straight line through a rectangular tank by endless chains to a sump at the inlet end, from which it is drawn off by gravity or pumped. In each the motion is slow to avoid eddies in the liquid or the re-suspension of the sludge.
Another sludge collection arrangement uses a siphon, installed mostly on secondary settling tanks of wastewater treatment units, but applicable to water treatment in units utilizing sludge blankets.
Rotary collectors are ordinarily used in circular tanks, but square tanks can be used with rotary equipment, a supplementary arm and its scraper blades moving radially out to the corners and then back toward the center so as to follow the basin's perimeter.
Rectangular collectors have two endless chains that move close to one side wall of a rectangular tank, which carry scrapers or flights, extend the. width of the tank, and are drawn by the chains along the bottom of the tank's entire length, pushing sludge into an end hopper. In this type the motor is installed at one end of the tank.
Sludge from the end hoppers (or from the central well of a rotary collector) can be removed into a sludge well by means of screw collectors.
In "upward-flow" tanks, water, previously mixed with a coagulant, softening, or other chemical, is passed downward to the tank's bottom where the floc collects. Flow is then directed up through previously accumulated floc or thin sludge to an overflow at the top. In passing through the sludge blanket, the heavier floc settles to the bottom by gravity. Most remaining floc is removed by filtering and other physical and chemical means by the sludge blanket. In most plants this removal is further induced by gradual deceleration of the upward flowing water. The sludge blanket is maintained at the optimum depth and consistency by the continuous automatic withdrawal of a portion of the sludge. In this device, mixing, coagulation, and clarification take place in the same structure.
These are also called "solids-contact" clarifiers. Some models involve an arrangement to recirculate some previously precipitated sludge to either the mixing or flocculating zones. This helps "seed" the floc, economizes coagulation, and helps avoid waste of water that would be discharged with the sludge in a conventional horizontal basin.
Tube & Inclined Plate Settlers
A tube-type clarifier consists of a series of shallow depth tubes operating in parallel, permitting solids to settle in a smaller area than conventional basins. The use of this clarifier with mixed media filter beds of this system may be incorporated in new or existing plants.
Inclined plate settlers (Lamella) consist of a series of inclined plates in close proximity to each other. The feed flow is from the side and exiting upward.
The factor that primarily governs a plant's operation rate is the variation in demand, which in turn can be buffered over a short period of time by distribution reservoirs. Varied plant delivery can be handled by intermittently operating the entire facility, or by intermittently operating the single units of a multiple unit plant. Some latitude is offered in sedimentation basin flow as opposed to flocculation and mixing units where material has to be maintained in suspension. Because filters are usually provided in multiple units, latitude is offered in that phase of operation.
Balancing flow between pumps and different plant units is accomplished by coordinating filter operation; filtered water reservoir levels, mixing, flocculation, and sedimentation basin levels; pump operation; and/or control valve operation. Pump operation may be programmed between several different capacity pumps or by using variable speed pumps.
Water levels throughout the plant should be indicated on the main control panel. This is made possible by using liquid level sensors at strategic points with transmission to the central point to operate gauges, recorders, and alarms.
Drives for sludge collectors can be programmed for intermittent operation as desired. Those for flocculation equipment may be equipped with variable speed motors to obtain optimum rotation speed for the type of floc.
All variable factors are subject to integration in a supervisory control system that provides command and report-back features. Provisions for manual over-riding are necessary to assume continued plant operation in the event of function failure or unusual conditions.
Under the provisions of the Clean Water Act, sludge discharge from water treatment is considered in the same category as an industrial waste. As such, it cannot be discharged to a watercourse without treatment that meets whatever standards the EPA establishes for "best practical control technology."
The coagulation phase contributes a high concentration of solids and lesser concentration exists in the filter wash water. The gelatinous characteristics of sludge from using alum make it difficult to remove by conventional sedimentation processes. Processes include alum recovery by treatment with acid to dissolve aluminum hydroxide, followed by neutralization with lime, followed by dewatering by mechanical processes.
A group of units that can be piped together to handle the successive stages of water treatment necessary for a particular community, is available in a range of sizes.
Several companies not only furnish equipment for both gravity and pressure filters, but also install complete plants and will also do the design for the appropriate treatment system. Some of these units are self-contained in trailers and can be used for emergency water systems.
The power crisis of the 1970s spurred development of alternative sources of energy. One such source was hydroelectric systems, a somewhat neglected area in an era of plentiful fossil fuels, but one with long historical roots. The emphasis, however, was not on large massive projects, but on retrofitting small existing dams and harnessing small rivers. Federal legislation and court rulings encouraged the development of such small hydro systems.
The great number of applications throughout the country to the Department of Energy does have some negative aspects. If all proposed systems were constructed, the damming of innumerable rivers and streams would have a severe environmental impact and a substantial social and economic impact as well.
However, the possibilities of using existing dams, reservoirs, small rivers, and other water bodies for power generation are widespread. Perhaps any facility using a lot of water - including treatment plants, industrial operations, outfalls, etc. - has the potential for "recapturing" some energy.
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|Title Annotation:||purifying untreated groundwater|
|Date:||Apr 15, 1998|
|Previous Article:||Highway safety.|