A water purification system may or may not utilize pre-treatment procedures. In truth,the need for different pre-treatment processes depends primarily on the quality of the source water, although the scale of the water treatment operation and the type of purification equipment that would be used in the conventional treatment processes down the line also figure into the decision.
Why Use Pre-treatment?
When we talk about the quality of the raw water, we refer to the concentrations of various suspended matter and dissolved solids in the feed water. Water sources which can be ground water or surface water may contain colloidal particles, inorganic particles, and biological slime like algae and certain microorganisms. Others may have soluble salts, organic metals, silt, clay, and other suspended contaminants in the mix.
Whatever type of dissolved matter and suspended particles may be contained in the source water,this will ultimately affect the performance of the downstream equipment. This is because as the volume of feed water reduces through its gradual clarification (in the different treatment processes like flocculation,sedimentation, filtration,etc.), the concentration of the contaminants present increases. If the water purification system is utilizing a reverse osmosis membrane or an activated carbon filter for instance, the chances of the suspended particles settling on the surface of the membrane is therefore higher, given the greater concentration of the substances in the water.
While filtering out contaminants from the water is the main purpose of the filtration process and equipment, the quality of feed water that reaches this point of the treatment system should still be maintained to ensure the continued reliability of the equipment. If membrane fouling(defined as the formation of a deposit layer on the surface of the membrane or even within the membrane pores) continues unchecked, degradation and scaling of the membrane will most certainly be unavoidable. Once this happens, there will be a decline in the performance of the equipment, and consequently, the quality of the treated water.
The concentration of suspended particles and the saturation levels of the sparingly soluble salts in water are the widely-accepted standards in determining the feed water quality. These values are expressed as turbidity (in NTU) and Silt Density Index (SDI). Membrane fouling is considered to be significant when turbidity and SDI values breach or are close to the maximum limits which are 1NTU and 4 (SDI) respectively.
Proper pretreatment therefore plays a vital role in optimizing the efficiency of the water treatment system as well as extending the life of the equipment used in the processes.
The Different Pre-treatment Processes
As earlier emphasized, different purification systems have different pre-treatment requirements. One system may utilize all and another system, just one or two ofthe many possible procedures. That said, here are the pre-treatment steps commonly practiced.
Pumping and Containment
For large scale water supply systems, surface water including lakes, rivers, and reservoirs are the usual source of water. Smaller water systems on the other hand, tend to rely on ground water for source water. Whatever the source may be, the first step of the process is pumping the raw water from the source.
Water pumped is then stored in holding tanks while awaiting treatment or directed into pipes that takes it to the next phase of the process. While at this point none of the water contaminants have been removed as yet, care must still be taken to avoid adding any more contaminants into the raw water as the addition of other particles may affect the succeeding treatment processes. The use of appropriate materials and design for the storage tanks and distribution system should therefore be observed to prevent unintentional contamination.
Screening of the water comes next. This process is essentially the first cleaning station in the system and facilitates the removal of large debris carried along by the raw water such as leaves, branches, other plant matter, and basically, all solid particles that are large enough to create obstructions to the equipment in the later stages.
Screen filters that keep these bigger solids out can be made of stainless steel as in a mesh, polypropylene, nylon and polyester. The spacing between screen bars determines the efficiency of the process, although for coarse screening during pretreatment operations, spacing of 20 to 40 mm and above is often used.Although screen filters and screen bars may be cleaned manually, automatedcleaning is recommended for facilities that treat water with a high volume ofcoarse matter.
Where applicable, raw water, usually that sourced from rivers and lakes, is held in storage in designated reservoirs for days and even months. This period during which the water is stored temporarily is called the retention time.During this period, solid particles and silt settle out, acting as a starting point for the process of water clarification. At the same time, the stored water also serves as a buffer for the occasional brief bouts of drought or as aready source of raw water in case of isolated pollution incidents in the surface water.
Pre-conditioning is needed for raw water that contains a significant volume of hardness salts.While hard water is not necessarily harmful to health, it can cause the formation of scale and subsequent deterioration of any equipment handling water.
Hardness in a water supply system can be assessed by determining the content of calcium and magnesium salts. These salts are formed when the elements calcium and magnesium combine with bicarbonates, sulfates, chlorides, and nitrates. When the level of calcium carbonate in the source water is high enough to warrant pre-conditioning,soda-ash (sodium carbonate) is used as treatment.
Chlorination is the process of adding chlorine to water for the purpose of disinfection.When chlorine is added to the raw water, this is termed as pre-chlorination.
Pre-chlorinationas part of the pre-treatment stage is crucial when the raw water is known to be extremely contaminated. In addition, when chlorine is incorporated early on in the purification system, its longer contact time in the water helps to preven talgae formation in the treatment basins and distribution pipes, aids in the oagulation and flocculation of the solid matter, and controls foul odors in the water.
On the other hand, the longer contact time of chlorine in raw water also increases the possibility of the chlorine reacting with certain particles to produce the highly-carcinogenic trihalomethanes. For this reason, prechlorination is no longer used as much in most water treatment plants.
Pre-treatment systems can be chemical or mechanical, or a combination of both. Because these processes have corresponding implementation and maintenance costs, proper assessment of the raw water source is crucial to determine whether the use o fpre-treatment procedures is applicable and necessary.