PRIMARY TREATMENT PLANT (PT PLANT)

Overview of PT Plant

All water sources contain different inorganic and organic substances that must be removed during water treatment to produce water that is fit for domestic use. An integral part of the treatment train is the treatment and disposal of the substances that are removed from the water in the most cost effective and safest manner. To achieve this goal, a variety of treatment processes are utilized which employ various physical and chemical phenomena to remove or reduce the undesirable constituents from the water. Selection of appropriate and effective treatment processes and proper design of the individual processes and process combinations are essential for the successful performance of a water treatment plant. Selection and design decisions made during the design stage can have a major impact on process and plant performance and the total project cost. Errors in process selection and design may have a large impact on the quality of the final water that can be produced, and may require extensive changes during operation to satisfy treated water quality standards.

A Combination of unit Process in PT Plant

Appropriate unit processes from Table A1.2 can be combined into a process train for the desired level of treatment to meet the required water quality. The level of treatment may range from conventional treatment to remove turbidity, taste and odour and to disinfect the water to complete demineralization of the water. The following considerations generally influence the selection of a treatment process train:

• Ability of the process combinations to meet final water quality objectives, considering both seasonal and long-term changes in raw water quality.
• Topography and site conditions, existing treatment facilities, land area available and hydraulic requirements.

Unit process for PT plant

Unit process	Description and application
Trash rack	Provided at the intake gate for removal of floating debris
Coarse screen	Mechanically cleaned screens provided at the intake gate or in the sump well ahead of pumps. Remove small solids
Microstrainer	Removes algae and plankton from the raw water
Aeration	Strips and oxidises taste- and odour-causing volatile organics and gases and oxidises iron and manganese. Aeration systems include gravity aerator, spray aerator, diffuser and mechanical aerator
Mixing	Provides uniform and rapid distribution of chemical and gases into the water
Pre-oxidation	Application of oxidising agents such as chlorine, potassium permanganate and ozone in raw water and in other treatment units to limit microbiological growth and to oxidise taste, odour and colour causing compounds as well as iron and manganese compounds.
Coagulation	Coagulation is the addition and rapid mixing of coagulant with the water to destabilise colloidal particles and form small flocs
Flocculation	Flocculation causes aggregation of destabilised colloidal particles to form rapid-settling flocs
Sedimentation	Gravity separation of suspended solids or floc produced in treatment processes. It is used after coagulation and flocculation and chemical precipitation
Sand filtration	Removal of flocculated and particulate matter by filtration through granular media (normally filter sand). Multimedia may also be used (sand and anthracite, or sand and activated carbon, or a third layer may also be incorporated)
Slow sand filtration	Removal of colloidal matter, micro-organisms and colour by means of slow rate filtration through a sand bed on which a layer of colloidal matter and micro-organisms is allowed to form.
Chemical precipitation	Addition of chemicals in water precipitates dissolved solids with low solubility into insoluble form. Removal of hardness, iron and manganese and heavy metals is achieved by chemical precipitation
Recarbonation	Addition of carbon dioxide to reduce pH of water after addition of lime for coagulation or softening
Activated carbon adsorption	Removes dissolved organic substances such as taste and odour causing compounds and chlorinated compounds. It also removes many metals. It is used as powdered activated carbon (PAC) at the intake or as a granular activated carbon (GAC) bed after filtration
Disinfection	Destroys disease-causing organisms in water. Disinfection is achieved mainly by chlorine, but ultraviolet radiation and other oxidising chemicals such as ozone and chlorine dioxide are also used
Chloramination	Ammonia converts free chlorine residual to chloramines. In this form, chlorine is less reactive, lasts longer and has a smaller tendency to combine with organic compounds, thus limiting taste and odours and THM formation
Fluoridation	Addition of sodium fluoride, sodium silicofluoride or hydrofluosilicic acid to produce water that has optimum fluoride level for prevention of dental caries
Desalination	Involves removal of dissolved salts from the water supply. Desalination may be achieved by membrane processes, ion exchange and distillation
Reverse osmosis (RO)	High-quality water permeates very dense membrane under pressure while dissolved solids and some organics are prevented from permeating the membrane. RO is also used for nitrate and arsenic removal
Nano filtration (NF)	Less dense membranes (than RO) are used for removal of divalent ions (softening), micro-organisms and organics from water under pressure
Ultrafiltration (UF)	Removal of colloidal material and some micro-organisms from water by membranes under pressure
Microfiltration (MF)	Removal of all particulate matter and some colloidal matter
Ion exchange (IX)	The cations and anions in water are selectively removed when water is percolated through beds containing cation and anion exchange resins. The beds are regenerated when the exchange capacity of the beds is exhausted. Selective resins are available for hardness, nitrate and ammonia removal.
Electrodialysis (ED/EDR)	An electrical potential is used to remove cations and anions through ion-selective membranes to produce desalinated water and brine
Distillation	Used mostly for desalination of seawater

OVERVIEW OF CONVENTIONAL WATER TREATMENT PROCESSES

The term conventional water treatment refers to the treatment of water from a surface water source by a series of processes aimed at removing suspended and colloidal material from the water, disinfecting the water, and stabilising the water chemically.

Conventional treatment of water for domestic use involves a number of treatment steps aimed at achieving the following objectives:

• Removal of suspended and colloidal matter to an acceptable level by means of coagulation-flocculation, sedimentation and sand filtration
• Disinfection to produce water that is safe to drink
• Chemical stabilisation of the water to prevent corrosion of pipelines, attack on concrete pipes and structures or the formation of chemical scale in distribution systems and fixtures

The conventional treatment methods for removal of suspended and colloidal material from water include chemical coagulation of small colloidal particles, flocculation of the small particles to form larger flocs or aggregates, followed by sedimentation and sand filtration. When the water contains a large amount of suspended material, larger suspended particles such as sand particles can be removed by means of settling without coagulation and flocculation.

Other methods that can be used include slow sand filtration, flotation, micro-filtration and ultra-filtration.

• The selection of the best combination of processes to treat water from a particular source depends on a number of factors. These factors include:
• the amount of suspended solids;
• the turbidity of the water;
• the nature of the suspended material;
• the chemical properties of the water (alkalinity and pH);
• the volume of water to be treated, and š the availability of facilities, trained operators and supervisors.

Simple settling of water is often used as a pre treatment step to remove larger suspended particles from water without coagulation-flocculation. Settling requires that the water remains stagnant for a period of time to allow the larger particles to settle to the bottom of a tank or holding reservoir. After settling of the particles clear water can be decanted from the container. Settling can be performed as a batch process (filling a tank with the water, allowing sufficient time for settling, and decanting of the clear water) or as a continuous process. In a continuous process the water flows through the reservoir at a slow rate that allows time for settling while clarified water is withdrawn continuously.

Simple settling is mostly used as a pre-treatment step at a water treatment works when the raw water contains relatively course suspended material. The suspended material is removed in a large holding dam through which the water flows at a slow rate to allow sufficient time for the particles to settle. The clear water then flows to the coagulation section if further clarification is required. The sediment must be removed from the dam at regular intervals to prevent the dam from silting up.

Coagulation 

Coagulation is the process by means of which the colloidal particles in water are destabilised (i.e. the nature of the colloidal particles is changed) so that they form flocs through the process of flocculation that can be readily separated from the water. Destabilisation is achieved through the addition of chemicals (called coagulants) to the water.

Different chemicals can be used as coagulants. The most common coagulants are:

• Aluminium sulphate, also known as alum Al2(SO4)3.16 H2O. The alum is dissolved in water and the aluminium ions, Al3+ that form, have a high capacity to neutralise the negative charges which are carried by the colloidal particles and which contribute to their stability. The aluminium ions hydrolise and in the process form aluminium hydroxide, Al(OH)3 which precipitates as a solid. During flocculation when the water is slowly stirred the aluminium hydroxide flocs enmesh the small colloidal particles. The flocs settle readily and most of them can be removed in a sedimentation tank.
• NOTE Since aluminium may be harmful at high concentrations it must be allowed to precipitate completely as the hydroxide. Complete precipitation is a function of the pH of the water and the pH must therefore be closely controlled between 6,0 and 7,4.
• Hydrated lime is also used as coagulant, but its action is different to that of alum and ferric chloride. When lime is added to water the pH increases. This results in the formation of carbonate ions from the natural alkalinity in the water. The increase in carbonate concentration together with calcium added in the lime results in the precipitation of calcium carbonate, CaCO3. The calcium carbonate crystals also enmesh colloidal particles and facilitate their removal.

When lime is used as coagulant the pH has to be lowered in order to stabilise the water chemically. Carbon dioxide is normally used for this purpose.

• Polymeric coagulants including Dadmacs and polyamines which form white or brown flocs when added to water.
•  Polyelectrolytes are mostly used to assist in the flocculation process and are often called flocculation aids. They are polymeric organic compounds consisting of long polymer chains that act to enmesh particles in the water.
• Other coagulants are also sometimes used in water treatment. These include:
•  Aluminium polymers such as poly-aluminium chloride that provide rapid flocculation, efficient removal of organics, and less sludge than alum under certain conditions, but at a higher cost.

Activated silica is sometimes used as a flocculant together with alum or hydrated lime as coagulant.

Bentonite and/or kaolin are sometimes added to water when the water to be flocculated contains too few particles for effective flocculation.

Flocculation 

Flocculation follows coagulation (and is often regarded as part of one process: coagulation-flocculation). The objective of flocculation is to cause the individual destabilised colloidal

particles to collide with one another and with the precipitate formed by the coagulant in order to form aggregates that could easily be removed by means of sedimentation or flotation. Flocculation involves the stirring of water to which a coagulant has been added at a slow rate, causing the individual particles to “collide”.

Flocculation is considered to be part of coagulation, although some handbooks treat it as a separate process. Flocculation can take place in different types of equipment. A simple mechanical stirrer can be used for flocculation or a specially designed channel with baffles to create the desired flow conditions can also be used to flocculate the particles in water. The basis of the design of a flocculation channel is that the flow velocity of the water has to be reduced from a high initial value to a much lower value to enable large, strong aggregates to form. If the flow velocity is too high the aggregates may break up again, causing settling of the broken flocs to be incomplete.

Flocculation is controlled through the introduction of energy into the water (through paddles or by means of baffles in the flocculation channel) to produce the right conditions (required velocity gradient) for flocs to grow to the optimum size and strength. The velocity gradient (or G-value) is an extremely important factor that determines the probability of particles to collide and form flocs. If G values are too low, the probability of collisions is low and poor floc formation results. If too high, shear forces become large and this may result in break-up of aggregates.

Aggregates and flocs are removed from water by means of separation processes, i.e. sedimentation and sand filtration; or flotation and sand filtration.

Sedimentation

Sedimentation is the process in which the aggregates that have been formed during coagulation and flocculation are allowed to settle from the water. The flocs collect as sludge at the bottom of the sedimentation tank from where it must be removed on a regular basis. The flocs settle to the bottom of the tank and the clean water leaves the sedimentation tank through collection troughs located at the top of the tank.

There are a variety of designs for sedimentation tanks available. These include large rectangular tanks in which the water enters one side and leaves at the other end. This type is normally used at large conventional treatment works. Circular tanks with flat or cone shaped bottoms are also used, especially at smaller works. Flocculated water enters the tank at a central distribution section and clarified water leaves the tank at collection troughs at the circumference of the tank. The design and flow conditions in a sedimentation tank must be such that the minimum amount of flocs leaves with the clarified water.

Sedimentation is a suitable process for removal of flocs formed from silt and clay particles that settle readily. However, certain flocs are relatively light and do not settle readily and a process such as flotation must be used for their removal. Light flocs are formed when algae or organic matter is flocculated.

The flocs that settle in the sedimentation tank collect at the bottom of the tank as sludge from where it must be removed on a regular basis to prevent accumulation in the tank. If sludge is not withdrawn regularly according to operating schedules, the quality of the clarified water may deteriorate due to re-entrainment of sludge.

Flotation

Flotation is an effective process for removal of relatively light types of flocs. Flotation involves the formation of small air bubbles in water that has to be flocculated. The bubbles attach to the flocs causing them to rise to the surface where they are collected as a froth that is removed from the top of the flotation unit.

Air is dissolved under pressure in a small amount of water in a device called a saturator. This water that is saturated with dissolved air is added to the main stream of water that is to be treated. When the pressure is released after the saturated water is mixed with the water to be treated, the dissolved air comes out of solution in the form of very fine bubbles.

Both sedimentation and flotation remove the bulk of the flocs from the water. However, most of the time a small amount of (broken) flocs or non-flocculated colloidal material remains in the water. This material has to be removed to ensure a low enough turbidity in the water. A sufficiently low turbidity level is required for effective disinfection of the water and to remove all traces of murkiness from the water. Removal of turbidity to low levels is achieved by means of sand filtration.

Sand filtration 

Sand filtration normally follows sedimentation or flotation as the final ‘polishing’ step in conventional water treatment. Conventional sand filtration is also termed rapid sand filtration to distinguish it from slow sand filtration as discussed below.

Sand filtration is a simple process in which the water is allowed to filter through a layer of sand in a specially constructed container. In the filtration process the small remaining floc particles are removed by the sand grains and are retained in the bed of sand, while clean water flows out from the bottom of the sand bed.

There are two types of sand filtration processes:

• rapid gravity sand filtration, and
• Slow sand filtration.

Rapid Sand Filteration

Rapid sand Filtration is used in conventional water treatment following sedimentation or flotation. The filters are open to the atmosphere and flow through the filter is achieved by gravity. Flow is normally downward at rates of about 5 m/h and the filters are cleaned by backwashing at intervals that vary from 12 to 72 hours. Some sand filters are not open to the atmosphere, but operate under pressure. These types of filters are often used in package treatment plants.

During filtration, solids are removed from the water and accumulate within the voids and on the top surface of the filter medium. The filter medium normally consists of a layer of graded sand with a size of about 0,7 mm. and a depth of about 0,8m. Dual media filters are a variation of single-layer sand filters. In these filters a layer of anthracite is placed on top of the layer of sand. This has the advantage of longer filter runs.

The fact that flocs are retained in the filter bed means that the filter will become saturated or clogged with the retained flocs at some stage. The sand has then to be cleaned by means of back washing to remove the accumulated flocs in order to restore the filtering capacity of the sand. The frequency of back washing is determined by the amount of flocs that has to be removed. Backwashing can be controlled on a time basis or on the basis of the pressure drop across the filter.

Slow sand filtration (SSF) on the other hand, has a very slow rate of filtration (compared to rapid sand filtration) and is a process that can be employed as standalone treatment process. The filter media in SSF is not back-washed at all, but the filter is cleaned by removal of the top layer of sand at long intervals of weeks.

Disinfection

Disinfection a large fraction of bacteria and larger micro-organisms are removed during clarification processes, especially by sand filtration. However, many bacteria and viruses still remain in clarified water even at low turbidity levels. It is therefore, essential to disinfect water to prevent the possibility that water-borne diseases are spread by pathogens (disease-causing micro-organisms) in water.

Disinfection of water entails the addition of the required amount of a chemical agent (disinfectant) to the water and allowing contact between the water and disinfectant for a pre-determined period of time (under specified conditions of pH and temperature). Physical methods of disinfection of water include irradiation with ultra-violet light and boiling.

The most commonly used disinfectant is chlorine gas, Cl2 that is dissolved in the water at a certain concentration for a certain minimum contact time. Other disinfectants include ozone, chlorine dioxide and other chlorine compounds such as calcium hypochlorite (HTH), sodium hypochlorite (bleach) and monochloramine.

Chlorine is a strong oxidising agent and it reacts and oxidises some of the essential systems of micro-organisms thereby inactivating or destroying them. The different forms in which chlorine is used for disinfection, have different oxidising powers and this must be taken into account to ensure effective disinfection.

Chlorine can be added to water in different forms.

Chlorine gas, Cl2 is delivered to the plant in gas cylinders and the chlorine is introduced into the water by means of special dosing devices (chlorinators).

Calcium hypochlorite, Ca(OCl)2 (commonly known as HTH) is available in granular or solid (tablet) form and is therefore a very convenient form in which to apply chlorine, especially for smaller or rural plants. It contains between 65 and 70% of available chlorine, it is relatively stable and can be stored for long periods (months) in a cool dry environment.

Sodium hypochlorite, NaOCl (commonly known as household bleach under different brand names) is available as a solution. Water treatment sodium hypochlorite contains 12 to 13% of hypochlorite, which is equivalent to 10 – 12 % available chlorine. Sodium hypochlorite is relatively unstable and deteriorates fairly rapidly, especially when exposed to sunlight. It also forms HOCl and OClupon dissociation.

Monochloramine (so-called combined available chlorine) is also used for water disinfection. It is formed when HOCl is added to water that contains a small amount of ammonia. The ammonia reacts with HOCl to form monochloramine, NH2Cl. It is much less effective as a disinfectant than HOCl (the same order of effectiveness as chlorite ion). However, it has the advantage of being much more stable in water than free available chlorine. For this reason it is often used to provide residual protection in larger distribution systems.

The two most important factors that determine the effectiveness of disinfection by means of chlorine are the chlorine concentration and the chlorine contact time. The pH of the water also plays an important role as well as the turbidity of the water, exposure to sunlight and the water temperature. The chlorine concentration is the most important control factor to ensure effective disinfection. However, since chlorine can exist in different forms in water with different degrees of effectiveness as is described above, the concentration of the actual chlorine species used for disinfection must be taken into account. It is normally accepted that sufficient chlorine must be added to water to give a free chlorine residual of not less than 0,5 mg/l after 30 minutes contact time.

One of the problems associated with chlorination is the formation of chlorinated by-products. Some of these (so-called trihalomethanes or THM’s) have been shown to have negative health effects and for this reason the concentration of THM’s is controlled at very low levels in drinking water. It is important therefore to control chlorination dosages and to pretreat the water before chlorine contact to remove organic material in the water (so-called precursor material) to low levels.

An important factor that affects disinfection, is the turbidity of the water to be disinfected. The reason is that when water contains colloidal particles, they may “shield” the micro-organisms from the action of the disinfectant, or alternatively react with the chlorine and in this way prevent effective disinfection. It is therefore important to optimise the clarification processes to produce water for disinfection with as low as possible turbidity levels (<1, but preferably <0,5 NTU).

Disinfection by means of ultra-violet (UV) irradiation is becoming more and more popular because no by-products are formed in the process. UV radiation kills or inactivates micro-organisms provided each organisms receives a minimum amount of irradiation. UV irradiation functions on the principle that each unit of water must be exposed to the irradiation for a minimum amount of time at a minimum dosage intensity (fluence).

It is important that the water to be disinfected by UV is properly pre-treated to ensure a low turbidity, preferably lower than 0,5 NTU. If the water contains high turbidity levels the colloids either absorb some of the radiation or shield the microorganisms against radiation which reduces the effectiveness of the process.

A further important aspect is that the UV tubes are prone to the formation of layers of scale or other fouling material. This also reduces the effectiveness of radiation. It is therefore important that the tubes are regularly inspected and cleaned to prevent formation of scale or accumulation of other material on them. Prolonged use of UV tubes reduces the effective radiation output. This must be compensated for by increasing the power applied to the tubes or accelerating the tube replacement program

Stabilisation

Stabilisation of water refers to the chemical stability (specifically with respect to CaCO3) of water. Chemical stability affects the tendency of water to be corrosive or to form chemical scale in pipes and fixtures. Stabilisation of water involves the addition of chemicals to the water to adjust its chemical properties in order to prevent corrosion or scale formation.

Water that is not chemically stable may be:

• corrosive towards metal pipes and fittings causing leaks in distribution systems with substantial cost implications,
• Scale-forming, causing a layer of chemical scale to form in pipes and on heating elements. This also has substantial cost implications because the carrying capacity of pipes is reduced and the heat transfer in kettles and geysers is impaired. From a cost point of view, it is very important to ensure that water for domestic use is chemically stable.

Stabilisation of water involves the addition of chemicals to the water to produce water with a calcium carbonate precipitation potential (CCPP) of about 4 mg/l. This means that the water should be slightly supersaturated with calcium carbonate. The effect of this is that a very thin layer of calcium carbonate will form on surfaces protecting it against corrosion. At the low super-saturation value excessive scale formation is avoided.

Sludge treatment and disposal

Sludge from a sedimentation tank has a large pollution potential because it contains all the suspended material removed from the water together with the chemicals used for coagulation. It must therefore be disposed of in a proper manner to prevent contamination of water sources.

The sludge is withdrawn from the sedimentation tank in a diluted form (2-5% m/v solids) and is sometimes thickened (excess water removed) before disposal. At smaller treatment works sludge is disposed of in sludge lagoons. The lagoons are large holding dams in which the sludge compacts and clear water accumulates on top of the sludge. The clear water may be recycled to the inlet of the plant. Water from the backwashing of sand filters have similar characteristics (although much more dilute) and must be treated and disposed of in the same manner as sludge from sedimentation tanks.

OVERVIEW OF ADVANCED TREATMENT PROCESSES

The term advanced treatment processes refers to processes other than conventional processes, i.e. coagulation-flocculation, sedimentation, filtration, chlorination and stabilisation. Processes normally considered as advanced processes are membrane processes (reverse osmosis RO, nanofiltration NF, ultrafiltration UF and electrodialysis ED), activated carbon adsorption, ozonation, oxidation processes for iron and manganese removal and processes for removal of specific substances such as fluoride.

Reverse osmosis. The main application of RO is to remove dissolved substances, including ions such as Na+ and Cl- from solution. RO is a general desalination process being used to desalinate seawater, brackish water and high-TDS effluents. The membranes are continuous in the sense that they do not have any pores. The smallest size of dissolved ions and organics that can be removed by RO is in the order of 0.1 nm (nanometer), which is equal to 0,0001 micrometer, or 0,0000001 mm. RO therefore removes all particulate matter including all bacteria and viruses, all organic macromolecules and most organic molecules with molecular mass of larger than about 150 Daltons (mol mass units). RO therefore produces product water of extremely good quality.

Nanofiltration is also a desalination process since it separates dissolved salts from solution. However, NF membranes contain very small pores and therefore allow substances to pass that are retained by RO membranes. Monovalent ions such as Na+ and Cl- readily permeate the NF membrane, while divalent ions such as Ca2+ and SO4 2- are rejected to a larger degree by NF membranes. It is therefore effective to soften water (remove Ca, Mg and other hardness causing ions). The types of membranes and modules are similar to those used in RO.

Ultrafiltration is similar to the above two processes in respect of driving force, but it differs greatly in that the membranes are porous. This means that separation is due to a sieve mechanism and dissolved ions and dissolved organics are therefore not removed. However, particulates and macromolecules are rejected. This means that bacteria and viruses are removed as well as larger organic substances such as the socalled precursors for chlorinated compounds including THM’s. This characteristic has resulted in the application of UF in drinking water treatment as an alternative for conventional coagulation-filtration-disinfection processes. Pore sizes in UF membranes range from 10-50 nm with operating pressures in the range 200-800 kPa

Microfiltration is very similar to UF with the main difference being pore size, operating pressure and permeate quality. Pore sizes are larger than 50 nm and operating pressures around 100 kpa. Only particulate matter is removed by MF.

Electrodialysis is a membrane separation process in which the driving force is an electrical potential across the membrane. In contrast to pressure-driven processes where water is separated from the feed solution, in electrodialysis charged ions are separated from the feed water. This means that the product water contains less dissolved salts but that all non-charged compounds such as organic molecules and all particulates including bacteria and viruses will remain in the product water. This is a disadvantage of ED compared to RO but the process has certain other advantages which makes it competitive with RO in many applications.

Oxidation and removal of iron and manganese

Some inorganic compounds in water must first be oxidised to the chemical form that can readily be removed from water. Examples are iron and manganese that occur in some ground waters and some polluted surface water sources in relatively high concentrations. These substances are soluble and invisible and are not removed by conventional treatment processes. However, during water treatment and in the distribution system, iron and manganese may be oxidised and cause problems in the distribution systems and in the home. The iron and manganese products precipitate and settle in the systems and may cause discolouration of water and staining of clothes. It is therefore necessary to remove iron and manganese by means of specialised processes in the treatment plant.

Dissolved iron and manganese occur in reduced form in some waters (Fe2+and Mn2+). The first step in the removal process therefore involves oxidation of the iron and manganese to forms that can subsequently be precipitated and removed during filtration. Oxidation can be achieved by means of oxidants such as chlorine, ozone, potassium permanganate, oxygen or air. The iron is normally precipitated as ferric hydroxide, Fe(OH)3 , while manganese is precipitated as the oxide MnO2.

Dissolved iron occurs as Fe2+ and is readily oxidised to Fe3+ which can be precipitated as Fe(OH)3 and be removed during sedimentation and sand filtration. Iron can be oxidised by aeration of the water, but sometimes a stronger oxidant such as chlorine may be necessary when the iron occurs in the complexed form.

Manganese is not readily oxidised by air and stronger oxidants are required. Potassium permanganate is an effective oxidant for the oxidation of Mn2+ to Mn4+ that precipitates as MnO2. The sand in a sand filter that is used for the removal of iron and manganese gets coated with a layer of manganese dioxide and this coated sand (green sand) assists in the removal of iron and manganese.