There appears to be a clear link between the presence of forests and the quality of water coming out of acatchment. A large bottler of mineral water in Europe,draws its main water supplies from heavily-farmed watersheds, where the run-off of nutrients and pesticides threatens the aquifers upon which the company depends. In response, the company has found that reforesting sensitive infiltration zones and switching to organic farming practices is proving to be cheaper than building water filtration plants.

Some water authorities also have made the link between protecting for water supply and protecting for nature. The city of New York applies a catchment protection approach to maintaining its high quality drinking water.
A land and forest resource protection strategy is being implemented that will result in substantial savings for New York City, as compared to putting in a new water treatment plant. The start-up cost for this strategy is estimated to be US$1 000–1 500 million over 10 years, as compared to the cost of US$6 000–8 000 million, plus US$300–500 million annual operating costs, otherwise required for a treatment plant. In Ecuador, about 80 per cent of Quito’s drinking water comes from two protected areas, the Cayambe Coca Ecological Reserve and the Antisana Ecological Reserve. A nominal water use fee on citizens of Quito, together with one per cent of revenues of hydroelectric companies, are used to finance conservation of the reserves.
On a global scale, about one-third of the largest cities obtain a significant proportion of their drinking water directly from protected areas.

Constructed wetlands or phytoremediation: This is a technology for treating stormwater or wastewater. A constructed wetlandconsists of a gravel bed on which suitable wetland plants are grown. As water passes through the substrate, it is purified through the activity of bacteria attached to the gravel, plant roots, soil and other particles.

Soil and grass filtration for purification of wastewater: Soil filtration relies on filtration by soil particles and bacteria growing on the surfaces of the soil particles and plants (usually grasses) which take up water and nutrients (nitrogen and phosphorus), reducing the concentration of these nutrients. Soil filtration is used during the dry season. Grass filtration is used in the rainy season when the soil is saturated by water. Grasses can be grazed by livestock.

Soil aquifer treatment for purification and storage of treated wastewater: Soil aquifer treatment relies on filtration through the soil and purifying action of bacteria attached to soil particles. Purified water is stored in the groundwater aquifer and can be drawn for agricultural purposes, for example.

Symbiotic activity between algae and bacteria in a wastewater lagoon to purify sewage: As wastewater flows through a lagoon, bacteria consume the organic carbon in the waste. In assimilating the organic carbon, bacterial respiration produces carbon dioxide (CO2) and takes up oxygen (O2). Algae utilize the carbon dioxide for photosynthesis and in turn produce oxygen required by the bacteria, hence the symbiotic activity. Solids settle to the bottom of the lagoon, and pathogenic bacteria die-off in competition with other bacteria in the lagoon. Several lagoons in series are more effective and the last lagoon can be used for aquaculture.