2. Water differs from other liquids in that it expands when cooled from 0°C, contracts when heated from 0° to 4°C, & reaches its maximum density at 4°C. No other liquid possesses this property.
3. Pure water is rarely found in nature. This is because water is able to dissolve so many substances from the air, the soil & the rocks. The saltness of sea water is caused hy the mineral substances which are dissolved from the Earth’s surface by rivers & carried down to the sea. The Sun’s heat causes *fhe surface sea water to evaporate, or change into vapour, leaving behind the salt & other minerals. This explains why the seas are'so much more salty than rivers flowing into them.
39. а/ Переведите третий абзац текста /38/ в письменной форме, б/ Скажите по-английски, каковы причины солености воды -/10 мин./.
40. /20 мин./ Прочтите следующий текст и напишите название каждого абзаца
Waste-Water Treatment
1. In this age of pollution one of the more encouraging developments is the work that is being done in sewage treatment. For treatment purposes, the contaminents generally are considered as groups such as suspended solids, dissolved inorganics, & dissolved organics. A property of sewage that particularly degrades natural waters is its ability to consume oxygen. A somewhat arbitrary but useful measure of this property is known as biological oxygen demand. /BOD/.
2. Primary treatment consists simply of allowing the sewage to settle & separating the water from the sludge at the bottom & the scum on the top.
3. The most widely used methods of secondary treatment now in operation are the trickling filter & activating sludge processes. Well-operated activated sludge tanks can remove up to 90% of the suspended solids & BOD, & good trickling filters remove 80 to 85%. In practice, figures closer to 75% are more common. The use of pure oxygen in the activated sludge process has been called the most significant recent advance in sewage treatment.
4. For about 20 years engineers have known that with pure oxygen, more bacteria could be supported in a smaller space & less pumping would be required; but an economically competitive system in which pure oxygen is used has emerged only within the past few years. A system has been developed in which oxygen is circulated in closed tanks. The system achieves 90% utilization of oxygen as opposed to 5-10% in conventional system & can support more bacteria than their system. Many sanitary engineers think that in the future physical-chemical methods of secondary treatment will begin to replace the conventional biological methods.
