Treatment Plant Set-up

Drinking water treatment plants involve several consecutive processes, which remove solids, organic and inorganic pollutants, some metals and pathogenic microorganisms. The treatment plant set-up should be designed to take into account the quality of water being treated. In general, water treatment can be divided into: primary treatment to remove solids; secondary treatment to remove organic compounds as well as nitrogen and phosphorous; and tertiary treatment for disinfection.

Different sizes of treatment plants exist like centralised plants for big cities requiring a developed distribution system and semi-centralised plants adapted to provide drinking water to smaller communities at the point of use (POU).

(Adapted from EPA 1999)

There is no precise method for treating surface water because of the various qualities of water that exists. Groundwater typically requires less treatment than water from lakes, rivers and the sea. Nevertheless, a series of conventional processes can be identified; they are presented in the figures below.

In general the treatment of fresh water involves the following key steps:

Primary Steps

• Aeration: The water is mixed to liberate dissolved gases and to suspend particles in the water column. Sometimes a pre-oxidation step is also performed at this stage.
• Coagulation-Flocculation: In this step coagulants are added to remove the suspended particles (clay, organic material, metals, microorganisms), which stick to the coagulants forming heavy particles.
• Sedimentation: The heavy particles (flocs) settle to the bottom leading to clear water.

Secondary Step

• Filtration: The water is run through a series of filters, which trap and remove particles still remaining in the water column. Typically, slow or rapid sand filtration and more recently membranes or reverse osmosis are used to accomplish this task. Due to growing concerns regarding micro-pollutants, additional treatments such as advanced oxidation processes, activated carbon adsorption alone or combined with ozonation or H2O2 are sometimes used to remove these trace organic compounds.

Tertiary Step

• Disinfection: The water is now largely free of particles, organics and microorganism and is now treated to destroy any remaining disease-causing pathogens. This is commonly done with chlorination, ozonation, hydrogen peroxide, or UV radiation (similar to the point of use UV tubes). The water is then sent to the pumping station for distribution to homes and businesses. Chlorination is the most widely used disinfection method because it permits maintaining residual chlorine at a level efficient enough to guarantee the absence of microorganism until the water has reached its point of use.

When drinking water is produced from the sea, the water is desalinated by electrolysis or membranes such as reverse osmosis or by a solar process desalination.

(Adapted from CWEA 2004)

Drinking water treatment plant operation and maintenance includes the following tasks:

• Operate and adjust equipment controls to purify and clarify water.
• Inspect equipment and monitor operating conditions, meters, and gauges to determine load requirements and detect malfunctions.
• Add chemicals, such as ammonia, chlorine, and lime, to purify and disinfect water and other chemicals, such as ferric chloride, peroxide, and polymers, to enhance water treatment.
• Collect and test water samples, using test equipment.
• Record operational and laboratory data, observations of processes, and meter and gauge readings on specified forms.
• Clean and maintain tanks, basins and filter beds, using hand tools and power tools.
• Maintain, repair, and lubricate equipment, using hand tools and power tools.

Many of these tasks can be automated in modern treatment plants but trained operators and engineers are still required to control and maintain the system. Moreover, operation of the treatments also requires a significant amount of chemicals, which might not be available in little industrialised or remote areas.

Required energy input is significant. In Switzerland, required energy input for treatment and distribution of drinking water, including abstraction, is relatively low (around 0.4kWh/m³) due to the excellent quality of groundwater. In case of polluted or seawater treatment in semi-centralised plants, the required input is higher (around 1-2 kWh/m³). Smaller drinking water treatment plants may be installed at semi-centralised level or point of use in order to minimise energy requirements for distribution.

The cost of high quality municipal drinking water including transport is between less than 1 $/m³ in the US and more than 2 $/m³ in Denmark and Germany (CLARK 2007). The cost of drinking water from semi-centralised treatment plants operating in developing countries depends on water source quality and capacity. In the case of seawater and polluted sources it is typically around 1-2 $/m³. An important initial investment is required to build the treatment plant, which should be designed properly taking into account water composition to minimise the costs.

Conservation or restoration of ecosystems is a good strategy to simplify and reduce the costs induced by drinking water treatment plants. If the water body has a good quality, drinking water can be obtained without any treatment. Therefore, wastewater treatment and integrated water resource management are crucier treatmenal to sustainable drinking water supply.

Another interesting alternative to improve the quality of influent water and lower the costs of treatment is bank filtration. This is done for instance in Basel, New York or Vienna.

In the city of Basel (166,000 inhabitants), the quantity of groundwater is insufficient to meet the city’s needs. Some 120,000 m³ of water per day are treated from the River Rhine through a mechanical filtration installation equipped with a quartz sand filter to remove the sediment. It then runs into small channels through a forest where it is infiltrated into the subsoil and thereby cleaned before reaching the groundwater (VERMONT 2004).

The natural systems of upstate New York's Catskill/Delaware watershed provide most of the City's drinking water. Rather than spending $8 billion on a water treatment facility in New York City, New York State opted to spend $1 billion to restore the watershed that provides the City’s drinking water (ESA 2012).