Automation of Water Production Why the Modernization of Water Production Plants is Necessary
Water –Although seemingly available in abundancy - is a depleting ressource. Endress+Hauser (India) has carried a case study at Bauda water treatment plant, that indicates where the challenges and opportunities of tomorrow's water treatment system lie...
Water production has followed the same basic method for several decades even though the source of the water can be various. The water may be drawn from a lake, a river, subterranean aquifers or simple boreholes. It varies from close to brackish from boreholes up to clear mountain streams where it can be consumed immediately. Consequently some plants are equipped with complex treatment process and some have just simple sedimentation and disinfection processes.
If the source of the water is groundwater, dissolved impurities are likely to be present. These arise from the local soil chemistry and some of the impurities can be the salts of calcium, iron, magnesium, manganese and sodium plus nitrates, arsenic and fluorides. Calcium and magnesium salts affect the hardness, sodium salts cause scaling and corrosion problems in pipes and plant equipment, fluorides can enhance tooth resistance to decay but can also cause bone fluorosis and arsenic is a poison.
Water Treatment Plants, Tailored for Local Needs
Plant designs, therefore, vary based on local conditions and several different treatment processes have been developed. These include aeration, mixing, flocculation, filtration and disinfection processes. The design and complexity of any plant is therefore very much chemistry and local condition driven, but even within the many variations there is a basic overall scheme.
The source water is usually blended with chemicals in mixers and flocculators. These ensure that any microparticles that may be present can be coagulated, enabling easier sedimentation and separation later in the treatment. Once the primary treatment is done the water passes into sedimentation tanks where primary clarification takes place.
This is merely simple settling of the heavy particulate matter and the collection of any precipitants from the chemicals addition. The secondary clarification is in the form of filtration and here the size of the filter determines the water quality. Finally the water is disinfected so that water-borne organisms, which pass through the filters, are killed. Again a variety of techniques are used in disinfection. The water then passes into storage tanks and from there into supply.
How to Optimise the Deign of Water Plants
The whole design process includes flow rate calculations, requirements for the size, number and shape of the various plant components, the general plant layout considerations, hydraulic levels within each unit and the correct sizing of pipes, pumps and instruments to ensure an optimized design and minimized power and energy loss.
In Fig 1, water from a river is blended with groundwater. This latter source may be contaminated or brackish and water blending ensures a more efficient process. In the mixing tanks a variety of chemicals can be added.
These may include alum, milk of lime, colloidal precipitants, acids or alkalis for pH balance or specialty chemicals for toxin removal. Following pre-treatment, the raw water passes through a number of processes aimed at reducing turbidity to around 1NTU. Instrumentation designed for this purpose is well accepted and has shown the benefits of automation in water production. Various disinfection methods have been tried and new methods are under testing.
Alternatives for Water Treatment with Chlorine
It is now recognized that chlorine, although effective, is becoming environmentally unacceptable to kill bacteria and other organisms harmful to health. Indeed many plants in third world countries either over-chlorinate (thereby damaging plant equipment) or underchlorinate resulting in health problems and higher mortality rates particularly with children, the sick and the weak. Proper plant control is therefore urgently required in many places.
Aeration – The First step of Organic Water Treatment
The first operation in a water treatment process is usually aeration. The purpose here is to remove any undesirable dissolved gases such as CO2 or H2S, which are more commonly found in ground water and arise from biological degradation of organic matter. The air or oxygen that is added converts these into more manageable forms. H2S even in the smallest of concentrations adds an unpleasant taste and odor to the water and excess CO2 causes corrosion to metallic parts of the plant and also affects the chemical efficiency.
Following aeration, the raw water is then passed through a long open channel and into flash mixers. The plant inlet flow rate is usually measured between the aerator and the mixer using either weirs or flumes.
Flow rate can be measured by ultrasonic open channel flowmeters or magnetic flowmeters. At Bauda, inlet flow is measured by insertion type magnetic flowmeter at the aerator inlet. In the inlet mixing tank, flocculants are added, so that they are quickly dispersed through the inlet flow to create a homogeneous mixture. This helps to prevent the formation of hydroxides that reduce the effectiveness of the process. The correct amount of flocculants and chemicals is added by full feedback control, but is also influenced by using sensors in the outlet channel for aluminum content as well as turbidity.
Sedimentation (Clarification) and Filtration
Following the addition of reagents and flocs, the mixture passes into large tanks or ponds. This is the sedimentation (or clarification as these terms are used interchangeably) process, where gravity is used to separate the particulate matter from the water. In some plants the clarifier and flocculator may be combined, but hydraulically they are best kept separate, with a proportion of the flocs being returned to the inlet to reduce the addition of new chemicals.
The sediment that gathers at the base consists of three types of particles. The largest are the calcium carbonate particles with a specific gravity of 2.5 and a size between 10 and 20mm. Next comes the fine silica, clay and sand grains less than 1mm in size and finally there are the alum flocs with a specific gravity around 1.3 and sizes down to almost the micron range.
Being of different size and specific gravity, they naturally settle at different rates, so the quality of the sludge depends critically on the water chemistry. The sludge is removed from the base with scarpers for treatment and re-use. The tanks may be circular or square, with the inlet usually in the center and the launder round the periphery.
Circular tanks are the most common and large plants can have several of these depending on the plant influent. Care must be taken to ensure the flow to each tank is identical. The clarifier is usually sized to give a retention time between 2-5 hours.
Filter Design for Water Cleaning
Once the water has been pre-treated and cleaned it is filtered, to reduce the turbidity and remove the very fine particle that may not settle. In most plants sand filter beds are used and these may be open or closed. The sand is periodically backwashed to remove the collected sediment. The flocs gather in the top layers of these filters so the lower parts of the bed remained clean. The main parameter measured here is the turbidity, as this directly shows the quality of the entire process thus far. A typical value is 1 NTU (or less) for the polished water.
Disinfection – Messy, but Necessary
The final process prior to distribution is disinfection. The purpose is to kill living organisms so that the water becomes fully potable. There have been many reported instances where polluted water has caused major health problems and the water supply company is almost always legally obliged to produce water of well-defined standard quality. In order to ensure this requirement is met, instrumentation has to be used to measure the chemistry of the finished waters.
Disinfection has until recently been accomplished solely through the use of chlorine. This was first used almost 100 years ago in the US. Today, alternative processes are being increasingly used. These include chloramination, UV, ozone, and reverse osmosis (RO) methods.
The instrumentation required varies slightly for each process used. The normal injection point is after the sand filters and for this reason the process is generally termed post-chlorination.
Distribution – Bring the Water to the Consumer
With respect to the processes described above the water is usually placed in holding reservoirs prior to a final disinfection and then distribution. The final chemical adjustments, pH balancing and residual chlorine measurements are made here, but the most important measurement is high accuracy metering, as this is the amount of water actually put into supply.
The value is crucial in determining the unaccounted-for-water figures and identifying leakage. The flow measurements are usually made after the water has been split into the respective zones.
Water accounting with telemetry
It is now common practice to measure water flow at various distribution points and transmit the flow rate, totaliser and pressure value of distribution line to a central monitoring room with the help of Telemetry. This helps in unaccounted-for-water measurement and leakage detection. It also, determines consumption patterns in different zones. This helps utility service providers make investment decisions, conduct resource planning and thereby helping them maintain a modern water management system.
Use Modern Communication Technologies
A utility service provider can get quick return on investment because of proper billing that the calibrated metering system provides. Plant operation and distribution personnel can also receive alarm SMS when the flow/pressure exceeds a limit value in the distribution lines. These and many other technology features can enable better and efficient management of the fast depleting important resource – Water.