Backwashing of Greensand Filter


Oliver Zhae

              Backwashing should occur when the head loss reaches about 69 kpa (10 psi.) and the duration of the backwash should be around 10 to 15 minutes allowing the system to unclog the settled insoluble iron and manganese oxides trapped in the filter. Filter cracking can occur which will affect apparent head loss. Filters should be backwashed everyday, but no less than every 2 days to prevent cracking. It is very important not to underfeed the amount of permanganate added to the pretreatment process or else the greensand filter will lose its oxidative properties. However, if the potassium permanganate charge is somehow lost in the filter, the operator can regenerate the greensand manually. The filter must be first shut down. Then, a saturated solution of potassium permanganate (around 5%) is poured into the filters and left to sit for 24 hours.
After 24 hours, the system is backwashed and restarted. Another way the system can be recharged without shutting down is by increasing the potassium permanganate dosage until pink water flows out of the bottom of the greensand filter. When the pink water flows out of the filter, the filter is recharged and regular doses of potassium permanganate can continue.
The operator should perform iron, manganese, pH and chlorine residual tests on a daily basis in order to determine if there are any problems in the system. Remember, the above is only meant as a guide. Specific backwash requirement are site and equipment specific. Refer to manufacturer specification and procedures as they relate to your plant.

For your safety
When mixing, always add chemicals to water. Never add water to chemicals

The recommend backwash rate for manganese greensand is 12 gpm/sq. ft. of filter area at 60 degrees Farenheit. This rate is sufficient to expand the bed 35–40 percent. Please note that backwash rates versus filter loading rates can cause serious problems in smaller treatment units. For example, a small installation with one 12-inch inside diameter filter, will require the well pump to deliver 12 gpm to properly backwash. However, if high levels of iron are present, that same unit may only be capable of filtering 2 gpm.


The backwash cycle is used to remove impurities that have collected in the media bed. When the backwash cycle is initiated, the backwash inlet valve must not open instantaneously. With the high flow rates used in the backwash cycle, "water hammer" will occur if the valve is opened quickly. "Water hammer" can disrupt the support layer of the greensand filter. The control system must be able to control the opening speed of this valve to eliminate "water hammer."
During the backwash cycle, the valves are oriented to reverse the flow of water from normal operation. With sufficient flow, impurities are loosened from the media bed and carried out of the bessed through the inlet distributor and service inlet. The media bed must be expanded by 30% for the backwash to be effective. To prevent filter media particles escaping from the vessel, the inlet distributor must be sufficiently higher than the top of the expanded bed. The valve configuration used during backwash cycle of the greensand filter system is shown in below Figure.
  • Service Inlet valve closed (to prevent incoming water from flowing against the backwash flow)
  • Service Outlet valve closed (to prevent dirty washback water from contaminating downstream equipment)
  • Backwash Inlet valve open (to provide a supply of water from the hub/lateral underdrain to backwash the media bed)
  • Backwash Outlet valveopen (to set the flow rate and carry away the dirty backwash water from the inlet distributor to drain)
  • Rinse Outlet valve closed (to prevent water from the wrong part of the vessel going to drain)
Backwash continues for a specified time (usually 15 minutes). After the backwash cycle is complete, the vessel is rinsed and can then return to normal service.

Backwash Flow
The backwash flow rate is equal to the flow rate required to increase the bed depth by 30%. The flow rate depends on temperature, since the force pushing the particles up is function of the viscosity of the water, which decreases with increasing temperature. The sub-surface wash uses the same flow rate as the backwash. The table below shows the flow rate based on temperature.
  Backwash Flow = Backwash Flow Rate x Diameter2 x π/4
Greensand Filter Backwash Rates Table
Temperature (°F)
Flow (gpm/ft2)
32 to < 40
7.2
40 to < 50
8.4
50 to < 60
9.7
60 to < 70
11.0
70 to < 80
12.8
80 to < 90
14.3
90 to < 100
16.0
100 to < 110
17.8
110 to 120
19.6

The greensand media has a fixed amount of iron and manganese it can remove before it gets exhausted and needs to be regenerated. It depends on the amount of iron and manganese you have and if there is hydrogen sulfide (“rotten-egg odor”) present.
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Industrial Wastewater Treatment


Oliver Zhae

Industrial wastewater treatment
Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use.
Most industries produce some wet waste although recent trends in the developed world have been to minimise such production or recycle such waste within the production process. However, many industries remain dependent on processes that produce wastewaters.
Sources of industrial wastewater
Agricultural waste
Main article: Agricultural wastewater treatment
Iron and steel industry
The production of iron from its ores involves powerful reduction reactions in blast furnaces. Cooling waters are inevitably contaminated with products especially ammonia and cyanide. Production of coke from coal in coking plants also requires water cooling and the use of water in by-products separation. Contamination of waste streams includes gasification products such as benzene, naphthalene, anthracene, cyanide, ammonia, phenols, cresols together with a range of more complex organic compounds known collectively as polycyclic aromatic hydrocarbons (PAH).
The conversion of iron or steel into sheet, wire or rods requires hot and cold mechanical transformation stages frequently employing water as a lubricant and coolant. Contaminants include hydraulic oils, tallow and particulate solids. Final treatment of iron and steel products before onward sale into manufacturing includes pickling in strong mineral acid to remove rust and prepare the surface for tin or chromium plating or for other surface treatments such as galvanisation or painting. The two acids commonly used are hydrochloric acid and sulfuric acid. Wastewaters include acidic rinse waters together with waste acid. Although many plants operate acid recovery plants, (particularly those using Hydrochloric acid), where the mineral acid is boiled away from the iron salts, there remains a large volume of highly acid ferrous sulfate or ferrous chloride to be disposed of. Many steel industry wastewaters are contaminated by hydraulic oil also known as soluble oil.
Mines and quarries
Mine wastewater effluent with neutralized pH from tailing runoff. Taken in Peru.
The principal waste-waters associated with mines and quarries are slurries of rock particles in water. These arise from rainfall washing exposed surfaces and haul roads and also from rock washing and grading processes. Volumes of water can be very high, especially rainfall related arisings on large sites. Some specialized separation operations, such as coal washing to separate coal from native rock using density gradients, can produce wastewater contaminated by fine particulate haematite and surfactants. Oils and hydraulic oils are also common contaminants. Wastewater from metal mines and ore recovery plants are inevitably contaminated by the minerals present in the native rock formations. Following crushing and extraction of the desirable materials, undesirable materials may become contaminated in the wastewater. For metal mines, this can include unwanted metals such as zinc and other materials such as arsenic. Extraction of high value metals such as gold and silver may generate slimes containing very fine particles in where physical removal of contaminants becomes particularly difficult.
Food industry
Wastewater generated from agricultural and food operations has distinctive characteristics that set it apart from common municipal wastewater managed by public or private wastewater treatment plants throughout the world: it is biodegradable and nontoxic, but that has high concentrations of biochemical oxygen demand (BOD) and suspended solids (SS).[1] The constituents of food and agriculture wastewater are often complex to predict due to the differences in BOD and pH in effluents from vegetable, fruit, and meat products and due to the seasonal nature of food processing and postharvesting.
Processing of food from raw materials requires large volumes of high grade water. Vegetable washing generates waters with high loads of particulate matter and some dissolved organics. It may also contain surfactants.
Animal slaughter and processing produces very strong organic waste from body fluids, such as blood, and gut contents. This wastewater is frequently contaminated by significant levels of antibiotics and growth hormones from the animals and by a variety of pesticides used to control external parasites. Insecticide residues in fleeces is a particular problem in treating waters generated in wool processing.
Processing food for sale produces wastes generated from cooking which are often rich in plant organic material and may also contain salt, flavourings, colouring material and acids or alkali. Very significant quantities of oil or fats may also be present.
Complex organic chemicals industry
A range of industries manufacture or use complex organic chemicals. These include pesticides, pharmaceuticals, paints and dyes, petro-chemicals, detergents, plastics, paper pollution, etc. Waste waters can be contaminated by feed-stock materials, by-products, product material in soluble or particulate form, washing and cleaning agents, solvents and added value products such as plasticisers. Treatment facilities that do not need control of their effluent typically opt for a type of aerobic treatment, i.e. Aerated Lagoons.[2]
Nuclear industry
The waste production from the nuclear and radio-chemicals industry is dealt with as Radioactive waste.
Water treatment
Water treatment for the production of drinking water is dealt with elsewhere. (See water purification.) Many industries have a need to treat water to obtain very high quality water for demanding purposes. Water treatment produces organic and mineral sludges from filtration and sedimentation. Ion exchange using natural or synthetic resins removes calcium, magnesium and carbonate ions from water, replacing them with hydrogen and hydroxyl ions. Regeneration of ion exchange columns with strong acids and alkalis produces a wastewater rich in hardness ions which are readily precipitated out, especially when in admixture with other wastewater.
Treatment of industrial wastewater
The various types of contamination of wastewater require a variety of strategies to remove the contamination.[3][4]
Solids removal
Most solids can be removed using simple sedimentation techniques with the solids recovered as slurry or sludge. Very fine solids and solids with densities close to the density of water pose special problems. In such case filtration or ultrafiltration may be required. Although, flocculation may be used, using alum salts or the addition of polyelectrolytes.
Oils and grease removal
Main article: API oil-water separator
A typical API oil-water separator used in many industries
Many oils can be recovered from open water surfaces by skimming devices. Considered a dependable and cheap way to remove oil, grease and other hydrocarbons from water, oil skimmers can sometimes achieve the desired level of water purity. At other times, skimming is also a cost-efficient method to remove most of the oil before using membrane filters and chemical processes. Skimmers will prevent filters from blinding prematurely and keep chemical costs down because there is less oil to process.
Because grease skimming involves higher viscosity hydrocarbons, skimmers must be equipped with heaters powerful enough to keep grease fluid for discharge. If floating grease forms into solid clumps or mats, a spray bar, aerator or mechanical apparatus can be used to facilitate removal.[5]
However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate. Dissolving or emulsifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat.
The wastewaters from large-scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids. Those industries use a device known as an API oil-water separator which is designed to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API).[4][6]
The API separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises to top of the separator and the cleansed wastewater is the middle layer between the oil layer and the solids.[4]
Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump. The water layer is sent to further treatment consisting usually of a Electroflotation module for additional removal of any residual oil and then to some type of biological treatment unit for removal of undesirable dissolved chemical compounds.
A typical parallel plate separator
Parallel plate separators[7] are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The parallel plates provide more surface for suspended oil droplets to coalesce into larger globules. Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates enhance the degree of oil-water separation. The result is that a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation.
Removal of biodegradable organics
Biodegradable organic material of plant or animal origin is usually possible to treat using extended conventional wastewater treatment processes such as activated sludge or trickling filter.[3][4] Problems can arise if the wastewater is excessively diluted with washing water or is highly concentrated such as neat blood or milk. The presence of cleaning agents, disinfectants, pesticides, or antibiotics can have detrimental impacts on treatment processes.
Activated sludge process
Main article: Activated sludge
A generalized, diagram of an activated sludge process.
Activated sludge is a biochemical process for treating sewage and industrial wastewater that uses air (or oxygen) and microorganisms to biologically oxidize organic pollutants, producing a waste sludge (or floc) containing the oxidized material. In general, an activated sludge process includes:
  • An aeration tank where air (or oxygen) is injected and thoroughly mixed into the wastewater.
  • A settling tank (usually referred to as a "clarifier" or "settler") to allow the waste sludge to settle. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal.
  • As a general process for most of the Industrial waste water the following Technologies are used.
1. ASP : Activated Sludge process 2. SAFF system of Submerged aerobic fixed film system 3. MBBR : Moving bed bio reactor ( Anox invented this now is considered generic technology) 4. MBR : Membrane Bioreactor 5. DAF clarifiers 6. TBR : Turbo bioreactor Technology ( A patented technology of Wockoliver) 7. Filtration technologies More information about above can be found on various commercial manufactures like WOIL
Trickling filter process
Main article: Trickling filter
Image 1: A schematic cross-section of the contact face of the bed media in a trickling filter
A typical complete trickling filter system
A trickling filter consists of a bed of rocks, gravel, slag, peat moss, or plastic media over which wastewater flows downward and contacts a layer (or film) of microbial slime covering the bed media. Aerobic conditions are maintained by forced air flowing through the bed or by natural convection of air. The process involves adsorption of organic compounds in the wastewater by the microbial slime layer, diffusion of air into the slime layer to provide the oxygen required for the biochemical oxidation of the organic compounds. The end products include carbon dioxide gas, water and other products of the oxidation. As the slime layer thickens, it becomes difficult for the air to penetrate the layer and an inner anaerobic layer is formed.
The components of a complete trickling filter system are: fundamental components:
  • A bed of filter medium upon which a layer of microbial slime is promoted and developed.
  • An enclosure or a container which houses the bed of filter medium.
  • A system for distributing the flow of wastewater over the filter medium.
  • A system for removing and disposing of any sludge from the treated effluent.
The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies.
A trickling filter is also often called a trickle filter, trickling biofilter, biofilter, biological filter or biological trickling filter.
Treatment of other organics
Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, coking products and so forth can be very difficult to treat. Treatment methods are often specific to the material being treated. Methods include Advanced Oxidation Processing, distillation, adsorption, vitrification, incineration, chemical immobilisation or landfill disposal. Some materials such as some detergents may be capable of biological degradation and in such cases, a modified form of wastewater treatment can be used.
Treatment of acids and alkalis
Acids and alkalis can usually be neutralised under controlled conditions. Neutralisation frequently produces a precipitate that will require treatment as a solid residue that may also be toxic. In some cases, gasses may be evolved requiring treatment for the gas stream. Some other forms of treatment are usually required following neutralisation.
Waste streams rich in hardness ions as from de-ionisation processes can readily lose the hardness ions in a buildup of precipitated calcium and magnesium salts. This precipitation process can cause severe furring of pipes and can, in extreme cases, cause the blockage of disposal pipes. A 1 metre diameter industrial marine discharge pipe serving a major chemicals complex was blocked by such salts in the 1970s. Treatment is by concentration of de-ionisation waste waters and disposal to landfill or by careful pH management of the released wastewater.
Treatment of toxic materials
Toxic materials including many organic materials, metals (such as zinc, silver, cadmium, thallium, etc.) acids, alkalis, non-metallic elements (such as arsenic or selenium) are generally resistant to biological processes unless very dilute. Metals can often be precipitated out by changing the pH or by treatment with other chemicals. Many, however, are resistant to treatment or mitigation and may require concentration followed by landfilling or recycling. Dissolved organics can be incinerated within the wastewater by Advanced Oxidation Process.

References

  1. ^ European Environment Agency. Copenhagen, Denmark. "Indicator: Biochemical oxygen demand in rivers (2001)."
  2. ^ Tannery Wastewater Treatment by the Oxygen Activated Sludge Process Mamoru Kashiwaya and Kameo Yoshimoto Journal (Water Pollution Control Federation), Vol. 52, No. 5 (May, 1980), pp. 999-1007 (article consists of 9 pages) Published by: Water Environment Federation
  3. ^ a b Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.
  4. ^ a b c d Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834.
  5. ^ Water and Wastewater News, May 2004 <http://wwn-online.com/articles/50898/>
  6. ^ American Petroleum Institute (API) (February 1990). Management of Water Discharges: Design and Operations of Oil-Water Separators (1st ed.). American Petroleum Institute.
  7. ^ a b Beychok, Milton R. (December 1971). "Wastewater treatment". Hydrocarbon Processing: 109–112. ISSN 0818-8190.


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