Potable Water Distribution System

1 Overview of the Article

This article presents a general overview of a water treatment and distribution system. The system described comprises source well pumps, settling tanks, filter charging pumps, multimedia filters, carbon activated filters, potable water storage tanks, backwash pumps, water distribution pumps, chemical injection systems, potable water storage tanks, and piping systems.

The water treatment system described in this article does not cover saline water such as seawater. The focus is on freshwater extracted from boreholes or other open sources.

2 Introduction

Water is an essential commodity required in our daily activities. Water has a wide range of applications, including irrigation in agriculture, domestic use such as washing dishes and equipment, and, most importantly, human consumption.

There are two categories of water; Freshwater and Saline water; the other, described as brackish water, is a transition from freshwater to saline water. There are different types of Saline Water; slightly saline water, moderately saline water, and highly saline water.

Below is a brief description of the categories of water

2.1 Fresh Water

Any naturally occurring water containing low concentrations of dissolved salts (usually Sodium chloride) but may contain other dissolved minerals and particles is termed freshwater.

While some sources place the dissolved salt concentration in freshwater at 500 parts per million (ppm), some other sources estimate the salinity of fresh water at lesser than 1000 ppm.

Freshwater can be obtained from ponds, lakes, swamps, underground etc.

Freshwater is, most time, not potable water. Much of the fresh water on the earth is contaminated and polluted; therefore, substantial treatment is required to make the water fit for drinking. The contamination may result from human activities such as sewage contamination or naturally occurring processes, including erosion, landslide, etc.

When freshwater appears physically clean, it may contain pollutants such as minerals, particles and other suspensions that are not visible. These pollutants must be eliminated by subjecting the water through a treatment process before usage. The degree of treatment depends on the application of the water; water for general utility application such as the washing of farm equipment etc. may undergo lesser treatment than water required for drinking and washing of dishes. Note that the treatment process specified should be tailored to the water analysis results. The analysis gives details of mineral concentration, particles concentrations, colour etc.

2.2 Saline Water

Saline water, popularly known as saltwater, contains a high salt concentration (Usually Sodium Chloride). Depending on the salt concentration, saline water is categorised into

2.2.1 Slightly Saline Water

Water with salt concentrations ranging from 1000ppm to 3000ppm is slightly saline water.

2.2.2 Moderately Saline Water

Water with a concentration ranging from 3,000 ppm to 10,000 ppm is termed slightly saline water.

2.2.3 Highly Saline Water

Water with salt concentrations ranging from 10,000ppm to 35,000ppm is highly saline.

Note that ocean water contains about 35,000 ppm of salt.

2.3 Brackish Water

Brackish water occurs in a natural environment with salinity higher than freshwater but not as high as seawater. Brackish water may result from the mixing of freshwater and seawater as in Estuaries, may also occur in brackish fossil aquifers, or may be formed due to human activities such as civil engineering projects like dikes etc.

Though there is no actual defined salinity range of brackish water, a salt content between the ranges 0.5 to 30grams of salt per litre of water is used to describe brackish water.

3 Water treatment processes

In simple terms, Potable water means water that is safe to drink.

For water to be potable, in most cases, treatment is required.

This section describes a standard freshwater treatment process. The system may vary slightly across facilities due to the varying degree of treatment required.

The best form of water treatment is tailored to the results obtained from the water analysis performed. The major difference is the chemical injection and filtration required to bring the water composition to a specified standard such as EPA, WHO or local standards.

The treatment process described does not cover saline water, employing two main treatment processes: distillation or reverse osmosis.

The below sub-section describes a typical water treatment process.

3.1 Coagulation/Flocculation

This process aims to cause the tiny particles (dirt) in the water to coagulate or stick together. Liquid alum sulphate Al₂(SO4)₃  and polymer is introduced into the untreated water to coagulate the particles. The coagulation causes the tiny particle to stick together to form larger, heavier particles called flocs. Due to their weight, the flocs may settle to the bottom, forming sludge that can be easily removed or trapped in the water filters.

3.2 Sedimentation

As the flocs flow through the treatment process, they move into settling or sedimentation tanks. In some designs, the water flows slowly through the sedimentation tanks to allow the flocs to gradually settle to the bottom, while in some systems, there are dedicated settling tanks. When the water flows into the settling tank, a settling period is allowed for the heavy flocs to fall to the bottom.

The flocs that settle at the bottom are called sludge and are piped out to sludge drying beds or processing units. In some systems, such as the direct filtration system, the sedimentation step is excluded, and the flocs are removed by filtration.

3.3 Filtration

The filtration process removes suspended particles or flocs in the case of direct filtration. Perfect or near-perfect filtration is usually achieved in two stages of filtration. The water flows through multimedia filters and carbon-activated filters. Note that other filters may be applicable; however, all filters must be routinely cleaned by backwashing.

3.4 Disinfection

Disinfection is performed to ensure disease-causing microorganisms are destroyed. Chlorine or chlorine compounds such as hypochlorite are used to achieve disinfection because they have proven to be very effective. The disinfectant is introduced into the water before entering the potable water storage tanks or distribution systems. In some systems, a UV disinfection unit is used against chemical disinfection. UV system major advantage is that there is no chemical injection into the water.

3.5 Sludge Drying or Sludge Processing

A sludge processing unit is an external unit to the water treatment plant. The flocs collected at the bottom (Sludge) are piped into a sludge drying bed or sludge processing unit. The dried sludge can also be used for other purposes, such as a sand filling.

3.6 Fluoridation

Fluoridation entails increasing the Fluoride concentration in the water to an optimum level of approximately 0.7 ppm, or 0.7milligrams of fluoride per litre of water. At this optimal level, tooth decay is prevented. Usually, Fluorides are present in natural water but at different concentrations, but in cases where the concentration is below 0.7ppm, fluoridation is required. Fluoridation is an optional treatment process; in some cases, calcium compounds may also be introduced into the water.

3.7 pH Correction

pH correction is achieved by introducing Sodium Hydroxide (NaOH) into the filtered water. The Sodium Hydroxide raises the pH of the potable water to a level that minimises metal corrosion. This method is one of the most effective ways of reducing lead corrosion and minimising lead levels in potable drinking water.

4 Components of portable water distribution system

The components of a portable water treatment unit may vary; however, some major equipment such as storage tanks, treatment units, source well pumps, piping, and end-user features are common to all systems.

The pressure to sufficiently deliver water to the most remote location in the network is key; therefore, a detailed network analysis must be performed. The analysis is performed using software such as Pipeline. The software determines the pressure and flow rate available at each node within the network. Also, note that software calculated pressure and flow rate is compared to the required pressure. Refer to the link below for details of the required flow rate and pressure for water distribution system features.

https://www.engineeringtoolbox.com/fixture-water-capacity-d_755.html

See below the Process flow scheme of a water distribution system.

P – 01A & P – 01B: Source Well Pumps

ST – 01A: Inline Static Mixer

T – 01A & T – 01B: Settling Tanks

P – 02A & P – 02B: Filter Charging Pumps

S – 01A & S – 01B: Multimedia Filters

S – 02A & S – 02B: Activated Carbon Filters

T – 02A: Ground Storage Tank

P – 03A & P – 03B: Potable Water Pumps

P – 04A & P – 04B: Backwash Water Pump

01: Alum Feed System (Pump and Storage Tanks)

02: NaOH Feed System (Pump and Storage Tanks)

03: Hypochlorite Feed System (Pump and Storage Tanks)

04: Polymer Flocculant Feed System (Pump and Storage Tanks)

05: Calcium Chloride Feed System (Pump and Storage Tanks)

4.1 Source Well Pump

The source well pumps are the first treatment and potable water distribution systems. In most cases, the pumps are submersible pumps installed in underground boreholes, while surface pumps may be used when the pump takes water from an open-source. Note that there are cases that surface pumps are used to transfer water from boreholes. Source well pumps should generate good heads to transfer the water to the settling tanks efficiently.

4.2 Inline Static Mixer

The inline mixer is installed on the piping between the source well pump and the raw water settling tank. The mixer ensures that the chemical dosed into the raw water, such as Alum, etc., is properly mixed into the water stream before entering the settling tanks. As the appropriate chemical quantity is dosed into the pipe, the chemical is mixed with the water utilising the energy of the flow stream.

4.3 Raw Water Settling Tanks

The settling tanks are primary treatment units in a water treatment system, primarily used to settle particles collected from the water source. When flocculating agents such as alum are introduced into the water, they aid aggregation of smaller particles that settle to the bottom of the tanks forming sludge. The sludge from the tanks is disposed of at intervals.

When a sedimentation basin is used, the water flows through the basin very slow while the flocs fall to the bottom to form sludge.

Raw water tanks may be made of corrosion-protected steel materials or non-metallic materials.

4.4 Filters Charging Pumps

The filter charging pumps transfer untreated water from the settling tanks into the water treatment unit. The flow rate of the filter charging pumps is aligned to the filtration unit’s flow rate, i.e., the flow rate should not be more than the maximum specified flow rate of the filters; else, the filtration beds will not function properly.

Minimum two fully rated filter charging pumps should be installed.

4.5 Water Treatment Unit

The water treatment unit is the heart of the water distribution system. It is responsible for providing potable water. It consists of different filters, chemical injection tanks and chemical dosing pumps. A typical water treatment unit will be discussed in the next session.

4.6 Potable Water Storage Tanks

The treated water is stored in the tanks. Sometimes, a ground tank (Tank installed at grade level), an overhead tank or both ground and overhead tanks are installed. The decision to install a ground tank is a function of the required water volume; a larger quantity of water is stored on ground tanks. Also, suppose the water distribution system is pressurised. In that case, there might be no need for overhead storage because the pressure generated by the pump will be sufficient to distribute water to the entire network.

Most water storage tanks are Braithwaite Steel or GRP, galvanised tanks with appropriate lining or non-metallic storage tanks.

4.7 Piping

Piping is a key component of every potable water distribution system. Piping connects all the equipment and transports the treated water to the end-users. Piping material selection is a function of pressure, temperature and corrosion resistance. Below are some piping materials used for transporting potable water.

• Galvanised carbon steel pipes. Should be used for temperature range below 60^oC
• Stainless steel piping materials
• Copper pipes. Should be used for temperature range below 60^oC may be up to 90^o
• Glass Fibre Reinforced Plastic (GRP) Pipes. Should be used for temperature range below 100^oC
• Polyvinyl chloride (PVC) Pipes. Should be used for temperature range below 60^oC

Due to cost, stainless steel, copper and GRP pipes are not as popular as PVC and Galvanised carbon steel pipes.

Since water distribution systems are usually low-pressure systems, non-metallic piping materials are the primary choice; however, metallic pipes are preferable for increased pressure systems and large diameter piping systems.

Kindly refer to the individual material pressure rating to select appropriate materials.

4.8 Potable Water Distribution Pumps

These pumps transfer water from the potable water storage tanks to the end-users. When overhead tanks are used, the pressure generated by the tank’s elevation may be sufficient to transfer the water to the end-user; however, in most cases, a pump is required to supply the desired water flow rate to the end-users.

5 Components of A Water Treatment Skid

This section briefly gives the details of a typical water treatment unit. The specified treatment unit is usually tailored to the result of the water test analysis performed. In most systems, there are two filtration trains, one running while the other is on standby to be used during the maintenance of the other unit.

5.1 Filters

The filters are the main components of the treatment unit. Two types of filters are usually installed.

5.1.1 Multimedia Filters

Unlike Single media filters that utilise only sand or other loose materials, Multimedia filters utilise three layers of filtration media in a single vessel. The layers are made of anthracite, sand and garnet. The basis for selecting these media is the obvious differences in their densities. Anthracite is the lightest filtration media, followed by sand and then garnet. Anthracite has the largest particle size, followed by sand and then garnet. The largest particle media (Anthracite) naturally stratifies at the top of the filter, followed by sand, while the heaviest media (Garnet) settles at the bottom.

It should be noted that the water to be filtered flows from the top of the filter to the bottom. The largest contaminants are trapped on the Anthracite layer as the water flows downward while smaller contaminants sift further downward. The three media filtration results in more efficient turbidity (suspended solids) removal and allows the filters to be operated longer before backwash than single media filters. Usually, sand filters can remove contaminant particle sizes down to 25 – 50 microns, while multimedia filters can eliminate 10-25 microns.

After using the filter for some time, maintenance should be performed to remove the trapped contaminants through a process called backwash. Backwash may be required when there is a significant pressure drop across a filter. In some units, the pressure drop across a clean filter may be between 3–7 psi, while filters with a pressure drop of about 10psi should be backwashed. In general, a backwash should be performed when the differential pressure exceeds 1.5 to 3psi over the differential pressure of a clean filter.

Backwash means reversing the flow through the multimedia filters resulting in loosening the beds to remove trap contaminants in the beds. The backwash flow rate is usually higher than the water filtration flow rate; the flow rate should be sufficient to loosen the entire beds, thereby driving the contaminants upward above the filtration media. Also, note that the flow rate should not be sufficient to drive the filter media out of the vessel; therefore, the backwash flow rate/velocity should not exceed the vendor specified parameters. During backwash, both the suspended contaminants and the water exits the top of the filter.

5.1.2 Carbon Activated Filters

In most filtration processes, carbon is a commonly used medium. They are used to reduce or remove organic chemicals and chemicals that give odours or tastes to water, chlorine, herbicides, etc., through adsorption. Adsorption means the contaminants are attracted and held to the surface of the activated carbon in the same way a magnet attracts and holds iron filings. Note that a carbon filter cannot remove chemicals like iron and nitrates; therefore, other filters such as greensand filters may be required. Kindly contact the filter manufacturer for specific advice on the removal of chemicals.

Filters made from carbon for water filtration are produced by grinding up a carbon source such as bituminous coal, coconut shells etc. though coconut shells are the most widely used.

Making the filter material entails heating the material to 1000^oC without oxygen to bake off impurities. The material is later subjected to steam at about 1600^oC to activate the carbon; the effect of the steam leaves carbon granules filled with cracks and pores, which enables them to store many chemicals and contaminants. As the water from the multimedia filters flows into the activated carbon filter, contaminants are attracted to the carbon to perform the filtration. The service life of the filters depends on the concentration of the contaminants; therefore, the manufacturer’s recommendation is key.

Carbon filters may be of different types, including; Granular Activated Carbon (GAC) Filters, Carbon Blocks Filters etc.

5.2 Chemical Storage Tanks

Based on the water analysis result, different chemicals may be required to be injected into the water, such as Sodium Hydroxide (NaOH), Flocculants, Hypochlorite, etc. Each of the required chemicals is stored in a separate tank. The size of each tank is a function of the number of chemicals to be dosed into the water.

5.3 Pumps

5.3.1 Chemical Dosing Pump

Each dedicated chemical dosing pump delivers the appropriate required quantity of chemicals into the filtration system. Refer to Fig 2 for typical examples of chemicals dosed into the water treatment system. Chemical dosing pumps are mostly positive displacement pumps.

5.3.2 Backwash Pumps

The backwash pumps are integral components of any water treatment plant. They are only used during maintenance. They provide the required flowrate to loosen the filter beds to eliminate the trapped contaminants in the filters. Centrifugal pumps are used as backwash pumps.

5.4 Other Components

In some water treatment processes, chemical injection is minimised by introducing an aeration unit and UV disinfection Box. Water aeration can precipitate dissolved iron and manganese, raise water pH, and eliminate dissolved gases such as carbon dioxide, hydrogen sulfide, and volatile organic compounds. Aeration is performed before the water flows into the filtration unit.

The UV disinfection process eliminates micro-organisms such as bacteria, algae, viruses etc. this is achieved by exposing the micro-organism to the ultra-violet rays of light for a specified period. The UV treatment replaces chlorination; however, UV treatment does not eliminate all micro-organisms.

Note: The water treatment process must be tailored to the water analysis results.