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
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
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
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
- ^ European Environment Agency. Copenhagen, Denmark. "Indicator: Biochemical oxygen demand in rivers (2001)."
- ^ 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
- ^ 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.
- ^ a b c d Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834.
- ^ Water and Wastewater News, May 2004 <http://wwn-online.com/articles/50898/>
- ^ American Petroleum Institute (API) (February 1990). Management of Water Discharges: Design and Operations of Oil-Water Separators (1st ed.). American Petroleum Institute.
- ^ a b Beychok, Milton R. (December 1971). "Wastewater treatment". Hydrocarbon Processing: 109–112. ISSN 0818-8190.
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