134. Read the following passage, then answer the question below.

More than a billion people live in regions where water is scarce, and as populations grow and aquifers shrink, many governments look to the sea. Desalination, the removal of salt from seawater, is hardly new; sailors boiled seawater centuries ago. Modern plants, however, use two main methods. Thermal plants heat seawater until it evaporates, leaving the salt behind, and then condense the vapor into fresh water. Reverse osmosis forces seawater at high pressure through membranes with pores so small that water molecules pass but salt ions do not. Because reverse osmosis uses far less energy, it now accounts for most of the world's new capacity.

Even so, desalinated water remains more expensive than water drawn from rivers or wells, mostly because of the electricity required. Pushing water through membranes at such pressure consumes energy, and a plant's costs follow the price of power. Engineers have therefore concentrated on recovering energy from the high-pressure waste stream, an effort that has cut consumption to a fraction of what it once was. Some plants now operate next to solar farms.

Environmental concerns have not disappeared, though. The waste product, a dense brine nearly twice as salty as seawater, is usually discharged back into the sea. If currents do not disperse it quickly, the brine can sink to the seabed and deprive bottom-dwelling organisms of oxygen. Intake pipes can also trap small marine animals. Designers respond by spreading the discharge through diffusers and by drawing seawater through the seabed rather than directly from the sea.

Critics say the technology tempts cities to avoid harder choices, such as repairing leaking pipes, which in some places lose a third of the water they carry. Supporters reply that conservation and desalination are not rivals. In arid countries like Israel, where desalination supplies a large share of drinking water, the two have grown together, and most wastewater is reused for farming.

The phrase 'an effort' in paragraph 2 refers to