Fog Drip

Introduction

(Adapted from MUNICH RE FOUNDATION n.y. and SCHEMENAUER & CERECEDA 1997)

The ever-growing need for freshwater in both developing and developed countries is indisputable and both increasing populations and the contamination (water pollution) of existing supplies will lead to constantly escalating demands. Water can be collected in almost every stage of the water cycle. The use of non-traditional water supplies such as the collection of fog must be considered as a complementary source and not as replacement. As clouds move over hills and mountains, the hilltops and ridge lines are enveloped in fogs.

Just as the leaves and needles of trees can collect some of the water in these fogs, large artificial collectors can produce a flow of potable water. Specially designed nets (fog collectors) are used to capture the moisture carried by the air. Tiny droplets of condensed water form on the mesh and are collected in a gutter and transported to a storage site. Other precipitation harvesting technologies for drinking water purposes are: rural rainwater harvesting, or urban rainwater harvesting).

Basic Design Principles

(Adapted from UNEP 1997 and SCHEMENAUER & CERECEDA 1997)

A fog collector is simply a frame that supports a section of mesh in a vertical plane. The large, operational fog collectors are typically made of two supporting posts, and cables on which the mesh is suspended. In addition, there is a network of guy wires to support the posts, a plastic trough to collect the water, and pipes to move water from the troughs to a reservoir or cistern. The large collectors are usually 12 m long and 6 m high. The mesh covers the upper 4 m of the collector. This gives a collecting surface of 48 m² and typical water production rates of 150 to 750 litres per day depending on the site. Alternatively, the collectors may be more complex structures, made up of a series of such collection panels joined together. The number and size of the modules chosen will depend on local topography and the quality of the materials used in the panels. Multiple-unit systems have the advantage of a lower cost per unit of water produced, and the number of panels in use can be changed as climatic conditions and demand for water vary.

The surface of fog collectors is usually made of fine-mesh nylon or polypropylene netting. Studies of various mesh densities in El Tofo (Chile) showed high efficiency at 35% coverage, mounted in double layers (UNEP 1997). This proportion of polypropylene-surface-to-opening extracts about 30% of the water from the fog passing through the nets (UNEP 1997). As water collects on the net, the droplets join to form larger drops that fall under the influence of gravity into a trough or gutter at the bottom of the panel, from which it is conveyed to a storage tank or cistern. The collector itself is completely passive, and the water is conveyed to the storage system by gravity. If site topography permits, the stored water can also be conveyed by gravity to the point of use. The storage and distribution system usually consists of a plastic channel or PVC pipe approximately 110 mm in diameter, which can be connected to a 20 nun to 25 nun diameter water hose for conveyance to the storage site/point of use (see also water distribution pipes) (UNEP 1997).

Storage facilities should be provided for at least 50% of the expected maximum daily volume of water consumed (UNEP 1997). However, because the fog phenomenon varies from day to day, it may be necessary to store additional water to meet demands on days when no fog water is collected. Chlorination of storage tanks may be necessary if the water is used for drinking or cooking purposes.

Cost Consideration

(Adapted from FOGQUEST n.y.)

Costs are variable depending on location, access and whether all labour costs are donated. The experience of FogQuest (www.fogquest.org) shows that small fog collectors for an evaluation cost 75 to 200 USD each to build. The large 40 m² fog collectors cost about 1000 to 1500 USD each and can last 10 years. A village project producing about 2000 litres per day will cost about 15’000 USD.

Health Aspects

(Adapted from FOGQUEST n.y.)

Fog water chemistry has been studied and found to meet the WHO drinking water standards. Because it is produced in remote areas few sources of potential contamination are present. Normally bacterial contamination would also not be an issue since it is very unlikely that there would be harmful bacteria in the fog. The mesh itself rapidly cleans itself from any dust that may have settled on it during a dry period. Once the water is produced by the fog collector the same precautions and considerations apply as for any other water source (e.g. biosand filter, SODIS, chlorination; or in general HWTS).

At a Glance