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11. Treatment processes

The processes part of a treatment plant can be divided into different categories according to the final result to be achieved and to the technology used . Usually a division between conventional treatment and not conventional treatment is often present (See Number of treatment plants definition). Furthermore the disinfection step is usually compulsory to fulfil drinking water standard but not always done in case of wastewater reuse for other purposes (such as agriculture).

a) Preliminary treatment
b) Primary treatment
c) Secondary treatment
d) Tertiary treatment and/or Advanced treatment
e) Natural/Biological Treatment
f) Disinfection
g) Effluent Storage
h) Others

Treatment process and technology classifications

a) Preliminary Treatment: treatment meant to remove coarse solid and other large materials often found in raw wastewater. It includes coarse screening, grit removal and sometimes comminution of large objects.

Processes included and codes:

Coarse Screening (CS), Grit Removal (GR), Comminution large Objects (CO).

b) Primary Treatment: meant to remove settleable organic and inorganic solids by sedimentation and remove the materials that will float by skimming. Large fractions of the biochemical oxygen demand (BOD5), total suspended solids, oil and grease are then removed. Some organic nitrogen, organic phosphorous and heavy metals associated with those solids are also removed, but not colloidal and dissolved constituents.
Primary sludge (settled solid) is usually produced in primary sedimentation tank or clarifiers.

Processes included and codes:

Sedimentation Tank (ST): also called clarifiers .Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units;

Skimming (SK): it's scum removal. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units;

Chemically Enhanced Primary Treatment (CEPT): CEPT is the coagulation/flocculation of raw wastewaters with lime or aluminium sulphate or ferric chloride or sulphate (often with polyelectrolytes) followed by primary sedimentation (ie, as commonly used in water treatment but with much higher chemical doses). The chemicals cause the suspended particles to clump together via the processes of coagulation and flocculation, so at the end there are practically no residual metals in the supernatant. CEPT may be implemented using a dedicated "CEPT tank" (i.e. a settling tank specially designed for CEPT), or by retrofitting a conventional primary treatment facility, or stabilization ponds.

c) Secondary Treatment: meant to remove residual organics and suspended solids after primary treatment. It involves the removal of biodegradable dissolved and colloidal organic matter using aerobic/anerobic biological treatment process. Several aerobic biological processes are used for secondary treatment, differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which microorganisms metabolise the organic matter.

Processes included and codes:

Aereated Lagoon (AL): a shallow, artificial treatment pond to which oxygen is introduced (e.g., via a bubble aerator) to speed up the natural process of biological decomposition of organic wastes. The main difference with Waste Stabilization Pond is the mechanical introduction of oxygen trough aerators into the system and not natural aeration of the wastewater. According to this the biodegradation time, and consequently the retention time, will be shorter gaining in efficiency of the system. as negative aspects the energy cost will be increased by the required presence of aerators and also the pathogens die-off will be lessened by the reduced duration of the treatment.

Activated Sludge (AS): product that results when effluent is mixed with bacteria-laden sludge and then agitated and aerated to promote biological treatment. The term 'activated sludge' refers to sludge in the aeration tank of an activated sludge treatment process. It consists of flocs of bacteria, which consume the biodegradable organic substances in the wastewater. Because of its usefulness in removing organic substances from wastewater, the sludge is kept in the process by separating it from the treated wastewater and re-circulating it.

Up-flow Anaerobic Sludge Blanket (UASB): in UASB process settled wastewater is passed upward through a sludge blanket. The sludge blanket consists of anaerobic bacteria, which have developed into granules. Because of the high settling velocity of the granules, the granules are not carried over in the upflowing wastewater. A high concentration of bacteria is therefore retained in the tank. The tank itself has no internal moving parts. If wastewater is distributed evenly at the base of the tank, mixing between the wastewater and the granules of bacteria is promoted by the carbondioxide and methane gases produced by the anaerobic treatment process and the upward moving flow of the wastewater.

Trickling Filters (TF): a trickling filter or biofilter or percolating filter consists of a basin or tower filled with support media such as stones, plastic shapes, or wooden slats. Wastewater is applied intermittently, or sometimes continuously, over the media. Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolized. Oxygen is normally supplied to the film by the natural flow of air either up or down through the media, depending on the relative temperatures of the wastewater and ambient air. Forced air can also be supplied by blowers but this is rarely necessary. The thickness of the biofilm increases as new organisms grow.

Rotating Biological Contactors (RBCs): RBCs are fixed-film reactors similar to biofilters in that organisms are attached to support media. In the case of the RBC, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor. Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs' rotation. Sloughed pieces of biofilm are removed in the same manner described for biofilters.

Oxidation Ditch (OD): the oxidation ditch is a sort of equipment used for a long-term aeration. It consists of a long channel of an elliptical or circular shape equipped with an aeration equipment called a rotor for generating a water flow and stirring water in the channel to supply oxygen. Thought it requires a relatively large area, it has a simple structure and can be easily operated as well as being able to remove nitrogen easily.

Settling Basin Digester (SBD): the Settling Basin Digester (also referred as Imhoff Tank, Emscher Tank) is an anaerobic sewage treatment tank in which solids are withdrawn from the bottom of the tank. The SBD, an improvement over the ordinary septic tank, is a two-story structure with the upper compartment used for settling the sewage, the lower one for the anaerobic disintegration of sludge. A sloping floor enables solid material to slide to the lower compartment, where, since the sludge is separated from the material in the sedimentation compartment, the action is more rapid.

d) Tertiary and/or advanced treatment: to remove specific wastewater constituents couldn't be removed by the secondary treatment. individual processes are nitrogen removal , phosphorus removal, additional suspended solids removal, refractory organics removal, heavy metals removal, dissolved solids removal.

Processes included and codes:

Infiltration/Percolation (IP): it's a low technology process used to treat primary and secondary effluents. It consists in the intermittent application of sewage on buried sand filters or permeable native soils. The infiltrated water percolates through unsaturated porous medium. The treated water is collected by a drainage system or percolates down to the underlying aquifer. When percolating through the filter, water is treated by aerobic biological processes resulting in the mineralization of organic matter and the oxidation of nitrogen compounds;

Membrane Filtration [Micro-; Nano-; Ultra- and Reverse Osmosis] (MF): membrane filtration is a new technique that allows use of at high quality water. It can be considered both a tertiary treatment since it's meant to remove water organic and inorganic constituents as disinfection step for the efficiency in pathogens removal, in many cases at higher level than conventional methods. Mainly the idea behind is filtration through special filters. A membrane filter is typically a polymer with billions of microscopic holes that allows water to pass through but restricts the passage of unwanted materials. With pore sizes ranging from 0.1 to 10 µm, these membranes will retain turbidity and all but the smallest pathogens. The most common configuration for these membranes is hollow fibre. During operation, water is passed through the porous fibre walls under pressure or in a vacuum, and the solids are retained on the fiber wall. To maintain optimal performance over time, the system is backwashed periodically to remove solids from the membrane surface.

Reverse Osmosis: the process is called "reverse" osmosis since it requires pressure to force pure water across a semipermeable membrane, leaving the impurities behind. It is capable of removing 95%-99% of the total dissolved solids (TDS) and 99% of all bacteria, most inorganic salts (including asbestos, lead and other heavy metals, radium) and also non-ionic organic compounds with Molecular Weight<100 (not effective for light organic compounds). This membrane is called membrane. It's a complementary process of Activated Carbon.
Nanofiltration: the particle about 10Å can be eliminated.
Ultrafiltration: the particles about 20Å - 1000 Å can be eliminated.
Microfiltration: the particles about 1000-10000 Å can be eliminated.

Activated Carbon (AC): it is an adsorption process. Adsorption is a process where a solid is used for removing a soluble substance from the water. In this process active carbon is the solid. Active carbon is produced specifically so as to achieve a very big internal surface (between 500 - 1500 m2/g). This big internal surface makes active carbon ideal for adsorption. It is most effective in removing organic contaminants from water. AC filtration will also remove chlorine. AC filtration does not remove microbes, sodium, nitrates, fluoride, and hardness. Lead and other heavy metals are removed only by a very specific type of AC filter. It's a complementary process of Reverse Osmosis.

e) Natural-biological treatment technologies: all the methodologies and processes not included in "Conventional treatment", but used as well to treat wastewater. The final results achieved by the specified non conventional techniques can be compared, on the base of the characteristic of the outcoming water, to the treatment level in conventional processes, i.e. secondary-tertiary level. The methodologies included in this part are characterised by lower cost but higher land area need.

Processes included and codes:

Waste Stabilization Ponds (WSP): are also called 'waste stabilisation lagoons, because the organic substances in the wastewater are converted to more stable (less degradable) forms. Ponding or lagooning is effective in treating wastewater and can reduce Biological Oxygen Demand and Suspended Solids to the same levels as mechanical treatment plants (e.g. Activated Sludge Treatment). In addition because of the longer residence time of wastewater in the lagoon (in the order of days), removal of pathogenic bacteria and viruses by natural die-off is greater than in an AS treatment plant (residence time of the order of hours). Cysts of parasites and helminth eggs are also usually removed through sedimentation in the lagoons.
A lagoon is a shallow excavation in the ground (1 to 2 m deep). It is generally unlined and percolation of wastewater into the soil and groundwater takes place. With time the percolation rate will reduce, because of formation of a sediment layer. Evaporation loss of water can be significant in arid climate regions. The soil itself is, however, not involved in the physical and biochemical wastewater treatment processes taking place in the lagoon. A lagoon can therefore be lined with a layer of clay or with an impermeable plastic membrane if protection of groundwater is desired, without affecting the performance of the lagoon. WSP are designed to achieve different forms of treatment in up to 3 stages in series, depending on the organic strength of the input waste and the effluent quality objectives. (first stage anaerobic pond, primary facultative pond , secondary facultative pond, facultative maturation pond).
Depending on the oxygen demand of the bacteria in the lagoon, the following conditions occur:

Overland Treatment (OT): effluent distribution over gently sloping grassland on fairly impermeable soils. Ideally, the wastewater moves evenly down the slope to collecting ditches at the bottom edge of the area. Suspended and colloidal organic materials are then removed by sedimentation and filtration through the surface grass and organic layers. Overland flow systems also remove pathogens from sewage effluent at levels comparable with conventional secondary treatment systems, without chlorination. This form of land treatment requires alternating applications of effluent (usually treated) and resting of the land, to allow soil reaction and grass cutting.

Soil Aquifer Treatment (SAT): SAT relies on natural processes to polish treated wastewater. Where soil and groundwater conditions are favourable for artificial recharge of groundwater through infiltration basins, a high degree of upgrading can be achieved by allowing partially-treated sewage effluent to infiltrate into the soil and move down to the groundwater. The unsaturated or "vadose" zone then acts as a natural filter and can remove essentially all suspended solids, biodegradable materials, bacteria, viruses, and other micro-organisms. Significant reductions in nitrogen, phosphorus, and heavy metals concentrations can also be achieved.

Macrophytes treatment: maturation ponds which incorporate floating, submerged or emergent aquatic plant species. They have been used to upgrade effluents form stabilization ponds, thus acting as a maturation ponds.
Macrophytes take up large amounts of inorganic nutrients (especially N and P) and heavy metals (such as Cd, Cu, Hg and Zn) as a consequence of the growth requirements and decrease the concentration of algal cells through light shading by the leaf canopy and, possibly, adherence to gelatinous biomass which grows on the roots.

Floating Aquatic Macrophytes System (FAMS): floating macrophyte species, with their large root systems, are very efficient at nutrient stripping. Among the floating macrophytes, having large root systems and very efficient nutrient extraction, are the water hyacinth, Eichornia crassipes, able to double in mass about every 6 days, and the coontail, Ceratophyllum demersum. Fly and mosquito breeding is a problem in floating macrophyte ponds but this can be partially alleviated by introducing larvae-eating fish species such as Gambusia and Peocelia into the ponds.

Constructed Wetlands (CW): they are also known as Emergent Macrophyte Treatment Systems, Gravel Hydroponics Beds "rooted macrophyte". In recent years, natural and artificial wetlands and marshes have been used to treat raw sewage and partially-treated effluents. Natural wetlands are usually unmanaged, whereas artificial systems are specially designed to maximize performance by providing the optimum conditions for emergent macrophyte growth. The main species of emergent macrophytes (reeds) are Phragmites communis and Scirpus lacstris.
Constructed wetlands are in-between lagoons and land based treatment systems. A CW consists of a gravel bed in which wetland species, such as reeds, are planted Wastewater (usually after settling of solids) passes through the gravel bed, and organic substances are degraded by bacteria attached to the surfaces of the bed and plant roots. The removal of Biological Oxygen Demand and Suspended Solids in beds with and without plants does not appear to differ by very much. Wetland plants take up nutrients (nitrogen and phosphorus) when water residence time is long. Long-term nutrient removal requires harvesting of the plants. Constructed wetlands need to be designed to minimise problems with insects (mosquitoes and midges).

Nutrient Film Techniques (NFT): the nutrient film technique is a modification of the hydroponics plant growth system, in which plants are grown directly on an impermeable surface to which a thin film of wastewater is continuously applied. Root production above the impermeable surface is high and the large root surface area traps and accumulates matter. Plant growth produces nutrient uptake, and provides shading for protection against the development of algae, while the large mass of roots and accumulated material serve as living filters.

Chenal algal à haut rendement (CA): also known in english as High-Rate Algal Pond (HRAP). An innovative treatment method, the high rate pond, is cost effective and capable of producing effluent that is low in dissolved organic material, metals, nutrients, and hazardous bacteria that meet secondary treatment requirements. High rate ponds are shallow (20-50 cm) ponds designed in a racetrack configuration and is continuously mixed to reduce land requirement and maximize algae production. High rate ponds rely on the process of photosynthetic oxygenation to treat wastewater (Oswald 1988). In comparison to conventional treatment methods, high rate ponds require half as much land as oxidation ponds, have shorter residence times and lower evaporation losses, lower aeration costs than mechanical systems, and require little maintenance and no chemical disinfectants (Esen 1991). The technology is potentially extremely interesting for the hot countries.

f) Disinfection

Processes included and codes:

Chlorination (CL): it involves the injection of a chlorine solution at the head end of a chlorine contact basin. The chlorine dosage depends upon the strength of the wastewater and other factors, but dosages of 5 to 15 mg/l are common. Chlorine contact basins are usually rectangular channels, with baffles to prevent short-circuiting, designed to provide a contact time of about 30 minutes. However, to meet advanced wastewater treatment requirements, a chlorine contact time of as long as 120 minutes is sometimes required for specific irrigation uses of reclaimed wastewater.

Ozonation (OZ): also called Ozone Disinfection, a common method of disinfecting wastewater that uses ozone (O3), an unstable gas that can destroy bacteria and viruses. Ozone is generated onsite at the treatment facility by an electrical discharge through dry air or pure oxygen. After generation, the ozone is fed into a down-flow contact chamber containing the wastewater to be disinfected. From the bottom of the contact chamber, ozone is diffused into fine bubbles that mix with the downward flowing wastewater. Ozone disinfection is generally used at medium-to large-sized plants after at least secondary treatment. Another common use for ozone in wastewater treatment is odour control.

UV TReatment (UV): a final treatment method in which partially treated effluent is exposed to ultraviolet light to kill pathogens and microorganisms. It's the natural method of disinfection given by the sun, unfortunately UV light doesn't penetrate very far into water.

Membrane Filtration [Micro-; Nano-; Ultra- and Reverse Osmosis] (MF): membrane filtration is a new technique that allows use of at high quality water. It can be considered both a tertiary treatment since it's meant to remove water organic and inorganic constituents as disinfection step for the efficiency in pathogens removal, in many cases at higher level than conventional methods. Mainly the idea behind is filtration through special filters. A membrane filter is typically a polymer with billions of microscopic holes that allows water to pass through but restricts the passage of unwanted materials. With pore sizes ranging from 0.1 to 10 µm, these membranes will retain turbidity and all but the smallest pathogens. The most common configuration for these membranes is hollow fibre. During operation, water is passed through the porous fibre walls under pressure or in a vacuum, and the solids are retained on the fiber wall. To maintain optimal performance over time, the system is backwashed periodically to remove solids from the membrane surface.

Reverse Osmosis: The process is called "reverse" osmosis since it requires pressure to force pure water across a semipermeable membrane, leaving the impurities behind. It is capable of removing 95%-99% of the total dissolved solids (TDS) and 99% of all bacteria, most inorganic salts (including asbestos, lead and other heavy metals, radium) and also non-ionic organic compounds with Molecular Weight<100 (not effective for light organic compounds). This membrane is called membrane. It's a complementary process of Activated Carbon.
Nanofiltration: the particle about 10Å can be eliminated.
Ultrafiltration: the particles about 20Å - 1000 Å can be eliminated.
Microfiltration: the particles about 1000-10000 Å can be eliminated.

g) Effluent Storage

It's not properly a treatment step but a storage facility, that can help to handle the excess production to be used in dry periods or during high irrigation peak demand on one side, on the other side it can also help to provide insurance against the possibility of unsuitable reclaimed wastewater entering the irrigation system and to provide additional time to resolve temporary water quality problems.

Codes: (YES) In case the treatment is performed.

h) Others

In this column should be indicated any other treatment specified that does not correspond to any given code. If possible give a definition of it as added comments.

Sources: FAO Irrigation and Drainage Paper 47; EPA terminology; "Copying with water scarcity" International Hydrological Programme IHP-VI, Technical Documents in Hydrology No. 58, UNESCO, Paris, 2002; Mottier V., Brissaud F., Nieto P. and Alamy Z. (2001). "Wastewater treatment by infiltration percolation: A case study." Water Science and Technology, 41 (1). 77-84;
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