She knows that once the tank is full, the sensor detects the water level and stops the motor, preventing overflow. But since the sensor has malfunctioned, the motor shows no sign of stopping. Riya switches off the electric supply to the filter. After the motor stops, the dripping gradually ceases, as the water level drops by then.
Riya’s father goes for a morning walk very early. Since the door has a self lock, it shuts automatically when pulled. He carries the key with him. Hearing the sound of the key turning in the door, Riya rushes to the entrance. Seeing her father, she says, “You know, Dad, the water filter has broken down. I switched it off, and that’s why the water stopped overflowing.”
Taking off his walking shoes, Riya’s father, Professor Samiran Sen, smiles faintly. Then he says, “Did it have to break down today of all days? Your Minu Aunty is arriving today by the afternoon flight. I have an urgent meeting. I’ll give you the mobile number of the mechanic who services the R.O., call him and get it fixed. Do you have college today?”
“There aren’t any important classes today. Arka is also at home. He said he might return to Kolkata next week. We were planning to go out somewhere. But Dad, who is Minu Aunty? Have I seen her before?” There is a trace of concern in Riya’s voice.
Riya’s father walks toward the living room, looking slightly worried himself. Sitting on the sofa, he says, “You saw her when you were little. She is a distant relative of your mother, Minudi. She’s older than me. Her only son lives in Australia. She’s going there via Delhi. She’s recently become a grandmother.”
“Who will go to receive Minu Aunty? Will Ma go? You said you have work.”
The professor’s face grows dim. “That’s the problem! Your mother’s office is undergoing an audit. She can’t skip work. And if none of us go, Minudi will face difficulties.”
“No worries, Professor sahab. I’ll go to the airport. I can take your car,” says Arka, who has quietly come out of the bedroom.
“No, Arka. I need the car for a few places. And don’t mix up Delhi’s roads with Kolkata’s, it’s tricky if you’re not used to them. Do one thing, you two go together and take a cab.”
Riya protests, “No way. Instead of a cab, let’s take the metro. Which terminal will the flight arrive at?”
“Terminal One. Yes, it has good metro connectivity, it will save time. But take a cab while returning. Minudi is very particular.”
Hearing this, Riya frowns. “Particular? You mean a perfectionist?”
Arka explains wisely, “It may mean a perfectionist, or not. Some picky people find faults in everything, they dislike whatever they see. I have a feeling your aunt might be that type.”
Ignoring Arka’s remark, Riya’s father shares the mechanic’s number on her phone and says, “Get the filter repaired before you go to pick her up. Arka is at home, so it won’t be a problem.”
Even before the mechanic arrives, Arka inspects the filter and tells Riya, “It’s been six months since the cartridges were due for replacement. Tell the mechanic to bring new cartridges as well. The water you’ve been drinking all this time was contaminated.”
Riya gets a little frightened. “I didn’t know! I knew cartridges are changed once a year, but I didn’t notice that the replacement date was written on the filter.”
“Usually, no one pays attention to that. Mechanics call on their own and come by, it helps them earn a bit. Anyway, these cartridges you see inside the transparent casing, do you have any idea what they are or what they do?”
Riya shakes her head awkwardly, indicating she doesn’t know. Arka reassures her, “That’s okay, not knowing isn’t a crime. You surely know that R.O. stands for Reverse Osmosis. Basically, in this process, water gets purified inside the machine. First, notice that unfiltered tap water enters a filter placed outside the machine. This is called a sediment filter, which traps dirt and other impurities. It can block particles larger than about 5 microns. After passing through this, the water goes inside the machine into a carbon filter. The reverse osmosis process happens here.”
“What is inside this carbon filter? It contains powdered activated charcoal. Osmosis is a natural process, and it is taught in school.”
Riya interrupts Arka, “If a semi-permeable membrane, meaning a thin layer, is placed between two solutions of different concentrations, the liquid from the lower concentration flows toward the higher concentration.”
Arka becomes excited, “Excellent! Osmosis is called ‘abhisraban’ in Bengali, though many Bengalis may not know this term. Reverse osmosis is the opposite process, movement from higher concentration to lower concentration. But this doesn’t happen on its own; pressure has to be applied. Unfiltered water contains various dissolved substances. Using a pump, water is forced through the semi-permeable membrane in the opposite direction. As a result, dissolved particles, some germs, and other pesticides present in the water can be easily filtered out. This membrane is made of cellulose materials, like wood fiber.”
“The next filter is the carbon filter. One property of carbon is that it traps relatively heavy metals present in water. For example, lead is a harmful metal that can be present in water. Mercury, another toxic metal, also gets trapped in the charcoal filter. You do remember what charcoal is, right?”
Riya responds eagerly, “Yes. Charcoal is pure carbon, obtained by burning wood in the absence of oxygen. In this case, the carbon doesn’t turn into ash.”
“Correct. Factories that manufacture water filters usually produce activated charcoal by burning coconut shells. When heavy metals like lead come into contact with activated charcoal, a reaction occurs. The metal separates from its salt, this is called ion exchange. Now you might ask if the positive metal ions are removed and trapped in the carbon filter, won’t the purified water have an excess of negative ions. Let’s take an example. Suppose water contains lead sulphate, which dissolves easily in water. When it comes into contact with activated charcoal, the lead separates. What happens to the sulphur then. There are some free positive metal ions or hydrogen ions in the water. The sulphur easily combines with these less harmful ions or hydrogen. As a result, the water becomes electrically neutral again.”
“Now look, after passing through the carbon filter, the water goes into another filter where bacteria and other microscopic organisms are removed. Generally, this filter traps organisms smaller than one micron. Many filters also use ultraviolet rays to kill germs.”
Riya asks, “During the reverse osmosis process, a lot of water flows out through another pipe while purification is happening. Why does that happen?”
“The ions of metals, germs, and other harmful chemicals that get trapped in the membrane need to be washed away with water. Water is pumped to clean the filters this way. As a result, some water is wasted, but it is for human benefit. Earlier, about two litres of water were wasted to produce one litre of purified water in R.O. systems. Now, in advanced modern machines, this has been reduced significantly, down to about half a litre of waste per litre of purified water. Water is extremely valuable, so conserving it is very important.”
“Arka, tell me one more thing, while filtering water, many essential minerals that are important for our body are lost. So how safe is it to drink R.O. water?”
“That’s why another special cylinder is attached to R.O. systems. It already contains essential minerals, which get mixed back into the water. So the water we finally get doesn’t lack minerals.”
Riya comments thoughtfully, “The problem of drinking water on Earth is becoming more serious. The per capita availability of potable water is decreasing. Due to pollution and population pressure, meeting future water demands has become a major challenge.”
“There are several countries where drinking water is obtained by recycling wastewater, for example Israel. Even seawater is purified using advanced R.O. systems to make it drinkable there. They consider it a luxury to waste water from bathrooms and toilets,” Arka says, looking concerned.
Riya wrinkles her nose, “Drinking water from toilet water? Ugh!”
“Yes, from toilet water. Seeing your reaction, it seems explaining reverse osmosis to you was pointless. By now you should understand that all the foul and contaminated substances in such water can be completely filtered out to make it odorless and clean. But remember, the water filter in your kitchen won’t be enough for that. Here you see a five-stage filter, whereas in places like Israel, water is filtered through ten to twelve stages. Minerals are then added to make it suitable for human health. Now I think we should call the filter mechanic, otherwise Minu Aunty will scold both of us in the evening.”
Riya says mockingly, “No worries, I’ll just order R.O. water from the market. It’ll come in those big blue five-litre jars at a cheap price.”
“How do you know that’s really R.O. water. Don’t go down that route. If it’s not from a reliable company, people simply fill those jars with unpurified tap water and sell them. If picky Minu Aunty creates a scene over this, it won’t be good. Come on, don’t delay, we have to go to the airport.” (Continued)
