In many countries directives have been issued concerning the quality of waste to be discharged directly into a main drain or indirectly into a central treatment plant. For small and medium sized enterprises an independent treatment plant is uneconomic and a system of discharge into central treatment plant is preferable. The minimum requirement for the substance to be discharged differs considerably from one country to another. The average values indicated refer to Germany.
Settling substances
Max. 0.5 mg per liter.
A distinction is made between biodegradable and non-biodegradable solids. Measures for reduction: mechanical separation (sieves, coarse filter). Add highly polymerized sedimentation agents or flocculating agents such as iron sulphate, or aluminum chloride.
Aluminum
Thought to inhibit growth in plants and to be one of the factors causing Alzheimer disease, however this has not yet been proven. Measures for reduction: flocculation of the residual liquors, washing and rinsing floats by adding alkalies.
Ammonia nitrogen
Max. 10 mg per liter.
Is produced through the use of nitrogen-containing products and disturbs biological degradation in the wastewater treatment plant by nitrification and high chemical oxygen demand. Measures for reduction: use nitrogen-free products, especially in the deliming process.
AOX
Max. 5mg per liter
AOX =refers to substances containing adsorbable, organically bound halogen. They exit particularly in chlorinated fatliquoring agents. Measures for reduction: use AOX- free products.
Chloride (salt)
A limit value does not exit yet. Very high concentration may promote destructive attack of cement building material or inhibit biological degradation. Measure for reduction:
Do not introduce the very large amount of curing salt into the production process. Reduce the amounts employed in pickling by using non-swelling acids.
Chromium (III) compounds
Max. 1 gm per liter
Measure for reduction:
Use tannage methods with a high degree of exhaustion, short-float methods, recirculation of residual and washing floats and recycling by precipitation and redissolution, good fixation of chromium salts in the leather.
Chromium (VI)
Discharge not allowed: do not use oxidizing agents if processing chrome-tanned leathers in the beamhouse.
COD and BOD values
COD = Chemical oxygen demand. Max. 160 mg/O2 per liter.
BOD = biological oxygen demand in 5 days.
Max. 25 mg/O2 per liter.
Measure for reduction: do not use oxygen –demanding products, cut down oxygen demand by flocculation, sedimentation and biological degradation.
Iron content
Max. 3 mg per liter
Substances extractable with petroleum ether (fatty substances)
Max. 20 mg per liter.
Natural and synthetic fatty substances are not bio-degradable. Measures for reduction: remove adherent fatty substances to a large extent by preliminary fleshing before processing, dispose of fatty connective tissue from fleshing (furrier`s waste) separately after liming, collect degreasing floats separately, allow them to settle down and remove suspended fat by decanting. Remove fat and oils discharged into the collecting or settling basin by means of a separator.
Halogen organic solvents
Discharge not allowed: if their use is necessary in degreasing systems they should recycled by redistillation
Hydroxyl ethylated phenols (APEO)
In Germany no longer produced owing to voluntary renunciation on the part of the manufactures.
APEO surfactants are hardly biodegradable and their metabolites are toxic to fish. Can be replaced by fatty alcohol polyether hydroxylates without problems.
Phosphorus (total)
Max. 2 mg per liter.
Phosphate ions can cause eutrophication of the waters if they exit in large amounts. In most causes limit values are insignificant when the phosphate is discharged indirectly because larger waste water treatment plants are equipped with phosphate precipitators. Mainly contained in detergents and cleaning agents and in some synthetic fatliquoring agents on the basis of phosphoric esters.
Free phenols
Max. 10 mg per liter
The products may contain different amounts of free phenols depending on the manufacturing processes of synthetic tanning agents based on phenols. In the past years industry has succeeded in producing tanning agents containing small amount of free phenols or not containing free phenols at all.
pH value
A pH value of 6.0 -9.0 is demanded
The limit values can be met in most cases by mixing acid and alkaline waste water floats. If the pH value is below or above this limit, acid or alkali should be added for neutralization.
Sulphates
Max. 200 mg pr liter
If present in high concentrations, cement building material may be attacked and destroyed. Measure for reduction; use products having a low sulphate content for beamhouse processes and replace sulphuric acid by other inorganic or organic acids.
Sulphides
Max. 1 mg per liter
Waste water containing sulphides should be collected separately in a collection basin as a split stream. Discharging it directly in treatment plants causes the formation of dangerous hydrogen sulphides by the presence of acid waste water floats.
Measure for removal of sulphides:
1. Catalytic further oxidation by ventilation and addition of manganese sulphate or manganese chloride (180- 200 g per m3)
2. Treatment by introducing flue gas. To dispose of large quantities of sulphide-containing floats, or if the amounts of flue gas are insufficient. Sulphuric dioxide should be additionally introduced.
3. Precipitation of sulphide by adding iron (II) sulphate. The iron (II) sulphide which is formed is deep black. It can be converted into brown iron (III) hydroxide by intensive ventilation. The disadvantage of this method is that very large amounts of sludge are produced.
Sulphite
Max. 1gm per liter
Measure for reduction
Reduce the use of products containing sulphite.
Temperature
Max. Discharge temperature 35 0 C
Toxic substances
Discharged not allowed
Biological degradation in treatment plants is disturbed by the presence of toxic, organic compounds and may be inhibited completely. The waste water is tested by the fish test. A dilution ratio of 1: 5 must not be toxic.
Water polluting substances
According to the German Water Conservation Law water-pollutants are solid, liquid or gaseous substances which are capable of changing the physical, chemical and biological properties of water unfavorably.
The water –polluting substances have been classified in four classes of pollutants (WGK) and listed in a catalogue:
WGK 3 = highly water-polluting substances
WGK 2 = water-polluting substances
WGK 1= slightly water –polluting substances
WGK 0 = substances which in general have no water- polluting effect.
The assessment of water-polluting potential is based on specific properties of the substances:
Acute toxicity for mammals, aquatic toxicity for fish dolphins, algae and bacteria, biological degradability a biotic degradability (hydrolyses, photolysis, oxidation)
Soil mobility ability to accumulate biologically, carcinogenic effect, teratogenic effect
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.
[edit] Mines and quarries
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.
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 wastewaters.
Treatment of industrial wastewater
The different types of contamination of wastewater require a variety of strategies to remove the contamination.[2][3]
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.[4]
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).[3][5]
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.[3]
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[6]
Parallel plate separators[6] 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.[2][3] 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, schematic 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.
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 Processes.
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