water treatment systems
water systems: MATERIALS ENCYLOPEDIA
Water treatment systems INTRO:
Water may require more than filtration in order to meet high standards. There are several types of treatment systems that subject source water to a process that removes or sterilizes pathogens. There are two main categories of water treatment: those that introduce chemicals to the water and those that use an energy input. Chemical treatment systems, while effective, have too many environmental and health issues to be considered sustainable. Energy input systems also carry impacts that must be considered in the production of the required energy, but as this energy can be provided in renewable, non-polluting ways, these treatments are given consideration.
Most water treatment systems are used in conjunction with filters, as they do not remove particulate, metals or VOCs.
Types of water treatment
Ultravilot (UV) treatment
Concentrated amounts of ultraviolet (UV) light is effective at sterilizing pathogens, killing their DNA. These treatment systems employ a UV light bulb in a flow chamber. When water passes through the chamber it is exposed to the light for enough time to ensure all pathogens are rendered sterile.
UV systems require the bulb to be lit at all times, bringing an energy cost that can be considerable. Bulbs for whole-house systems range from 60–100 watts, consuming 1.4–2.4 kilowatt hours daily. This is not prohibitively expensive at today’s low electricity rates, but adds a significant burden to renewable energy systems and used widely would increase overall energy demand noticeably.
One of the advantages of UV filters is that they can process water in “real time” as the water passes through the filter at regular flow and pressure rates. The water does not need to be stored, pumped or regulated.
UV filters require relatively clean water, as particulate could provide shading that may allow some pathogens to pass through the light chamber without enough exposure to sterilize them.
Bulbs in UV filters need to be replaced fairly frequently, with most manufacturers recommending annual changing. Some regulations require an alarm system to be built into the filter as the light is not visible from the outside, making it impossible to verify functionality.
Reverse osmosis technology uses several layers of a thin, semi-permeable membrane with tiny pores. Water is put under pressure (0.35–0.48 MPa / 50–70 psi for most household systems) to force it through this membrane, catching any molecules larger than water on the filter while relatively pure water passes straight through.
These systems cannot treat water on demand, and work with a pressure tank that is filled with filtered water, ensuring supply at expected pressure and flow.
Reverse osmosis units keep the membrane clear of built-up minerals by washing water over it. The amount of wash water required can be quite high, with ratios of two parts wash water to one part filtered water not uncommon, so be sure to check efficiency rates before purchasing. In some systems, this wash water is sent to sewer, wasting a lot of water; there are others systems that send wash water to the hot water tank or other uses, helping reduce overall water consumption. In systems where inline water pressure is not sufficient to force water through the membrane, additional pumping capacity will need to be added.
Reverse osmosis filters require thorough pre-filtration to prevent excess clogging of the fine membrane.
In general, reverse osmosis is used on a household scale as a point-source filter for drinking water. A large system would be needed to provide filtered water for high-flow uses like showering or dishwashing.
Ozone is trioxygen, an unstable grouping of three oxygen atoms. It is a powerful and highly reactive oxidant, as one of the oxygen atoms is ready to break away and combine with other atoms. As a water treatment system, ozone is generated with an electrical arc or high-UV light in the presence of oxygen. The resultant ozone bubbles are released into water, where their oxidizing effect is used to react with and kill pathogens and alter or precipitate metals and other contaminants.
The systems use a pump to compress air, which is exposed to the UV or the electrical arc. The resulting ozone enters a water reservoir through a diffuser tube that causes the ozone to break into small bubbles that rise through the water and react with contaminants. The process is very effective and thorough, removing or neutralizing the majority of water-borne contaminants.
The systems do not purify water on demand, relying instead on a storage tank to hold the water as it is purified. Pre-filtration is recommended, and particle filtration is also used after the ozone treatment to remove precipitate caused by oxidation. There are no filters or membranes that need cleaning or replacing, but the air compressor has moving parts and the UV light or “spark plug” will need occasional replacement.
The power consumption of ozone systems is the main drawback, with household units consuming from 55–200 watts continuously, or 1.3–4.8 kilowatt hours per day. This is not prohibitively expensive at today’s low electricity rates, but adds a significant burden to renewable energy systems and used widely would increase overall energy demand noticeably.