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Graywater Reuse

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To select the optimum treatment technologies for a particular graywater source, it is necessary to identify the particular contaminants in the supply. The following table categorises the classes of these contaminants:.

Treatment Technologies

There is no single technology that will sufficiently and economically remove all classes of contaminants; however, there are treatment technologies that, collectively, are capable of effectively reducing the concentration of virtually any contaminant down to acceptable levels for any water reuse requirement.

I) Suspended solids removal

Cartridge filters: Cartridge filters are replaceable “inserts”,usually cylindrical in configuration, that are inserted into housings and are typically replaced when they have captured so much suspended solids that the pressure drop across the housing becomes unacceptable (usually above 10psig). Offered in many different designs and micron removal ratings (down into the submicron range), they provide an excellent array of choices to the knowledgeable design engineer. They are typically used at flow rates less than 5gpm.

Media filters: These consist of a tank containing granular media such as sand, anthracite and garnet which capture suspended solids and retain them inside the bed until it is taken off line and backwashed. These bed filters are typically capable of removing suspended solids down to 10-20 microns in size and are normally used at flow rates in the 5 to 20gpm range. Media filters are backwashed to remove captured particles

Carbon and ceramic block filters: These are similar in design to cartridge filters. The advantage of the carbon block cartridge is that it also performs the adsorptive function of activated carbon. Ceramic cartridges can be cleaned and reused.

Microfiltration: It is one of the four pressure-driven membrane technologies that are based on a process known as “crossflow” filtration, which allows for continuous treatment of liquid streams. In this process, the bulk solution flows over and parallel to the membrane surface, and because the system is pressurised, water is forced through the membrane and becomes “permeate”. The turbulent flow of the bulk solution over the surface minimises the accumulation of particulate matter. The four major pressure-driven crossflow membrane technologies in use today are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO).These technologies behave differently than filters in that (with some exceptions) the feed stream is continuously pumped at a high flow rate across the surface of the filter media (membrane), with a portion of this stream forced through the membrane to effect separation of the contaminants, producing the permeate. The concentrated contaminant remaining in the other stream (concentrate) exits the membrane element on a continuous basis. Because the concentrate stream is continuously removing contaminants, these technologies require only occasional backwashing or cleaning. Conventional and crossflow filtration are illustrated below.

Microfiltration is the membrane technology designed for suspended solids removal and there are systems available to remove particulate contaminants down into the submicron range, including bacteria. They are capable of operating at virtually all flow rates.

II) Dissolved organics removal

Activated carbon adsorption utilises a specially prepared granular carbon media capable of adsorbing dissolved organic contaminants. It is very effective in removing many taste and odour contaminants, including chlorine, and is usually in housings similar to media or cartridge filters. The activated carbon material normally requires replacement once or twice per year.

Special resin adsorbers are also available for organics removal. They are designed for a particular removal function, such as humic acids, and require occasional replacement.

Ultrafiltration is another membrane technology, with tighter pores than MF, capable of removing dissolved organics. Instead of adsorbing the contaminants, it is continuously removing them in the concentrate stream.

III) Dissolved salts removal

Most graywater contains relatively high concentrations of salts, both from the incoming water supply, as well as soluble contaminants resulting from activities with the facility. These can include both benign and potentially hazardous compounds. The most practical technology for salts reduction is reverse osmosis, (or possibly nanofiltration). These are often designed to operate at a single tap (point-of-use). POU RO systems are very commonly used throughout the US today.

IV) Microorganisms removal/inactivation

Without a doubt, microorganisms represent the most troublesome of the contaminants associated with graywater reuse. For one thing, “pathogens” (microorganisms capable of causing disease in humans) may be released into the graywater and many kinds of microorganisms are “viable”, in that they grow in the moist environment.

Bacteria, fungi and algae, in particular, can proliferate and cause fouling and plugging problems in the system. Bacteria create biofilms (layers of lipopolysaccharide films) that attach to virtually any surface and collect particles and other microorgamisms. Biofilms are capable of shielding microorganisms from disinfectant chemicals and are very difficult to remove. The best way to counter biofilm formation is to reduce the concentration of bacteria in the first place.

Disinfection technologies: Disinfection is the process used to kill or inactivate microorganisms. In graywater applications, since some microorganisms will grow quickly (at least one type of bacteria will double its population every 20 minutes), it is advisable for stored water to be disinfected. Most disinfection technologies involve chemicals as described below:

Chlorine: Judicious use of this chemical has virtually eliminated the typical waterborne diseases of dysentery, typhoid fever and cholera. Chlorine, with its active ingredient, hypochlorous acid, is very effective in inactivating almost all waterborne pathogens, and provides an acceptable residual, but it does have limitations.

Chloramines: These compounds, resulting from the reaction of ammonia with chlorine in water, are commonly used in municipal water supply systems because of the superior stability of chloramine compounds over chlorine, and because they do not form trihalomethanes; however, chloramines are weaker oxidants than chlorine and thus have less ability to kill pathogens. Concentrations of these compounds in the range of 5 to 10ppm are required.

Chlorine Dioxide: Chlorine dioxide exhibits stronger disinfecting characteristics than chloramines, but, as it is more expensive than chlorine, it is not widely used. Chlorine dioxide does not form trihalomethances and exhibits rinsing, corrosion, and handling characteristics similar to those of chlorine. It has to be generated on site. Recommended concentrations are 2 to 5 ppm.

Iodine: This common relative of chlorine (one of the halogens) has been used for years by campers and the military for disinfecting small quantities of drinking water of unknown quality. Unfortunately, certain gram-negative bacteria strains can become resistant to iodine. Much less reactive to dissolved organics than chlorine, it will not form trihalomethanes. The recommended concentration is 0.3 to 0.5 ppm.

Bromine: Another halogen, bromine is often used in swimming pools and spas because it has a lower evaporation rate than chlorine. On the other hand, the trihalomathanes formed with bromine are considered more carcinogenic. Bromine also imparts an unpleasant taste, so is generally not used in potable water applications.

All of the above chemicals can be removed from water supplies by activated carbon; however, it should be noted that not only will the water leaving the activated carbon no longer be disinfected but the carbon bed itself is an excellent medium for growing bacteria.

Ozone: This most powerful disinfectant, which consists of oxygen in a three-atom form (O3), is used to disinfect some municipal drinking water systems, particularly in Europe. It is a very effective against all microorganisms; however, must be generated on site and has a relatively short life (less than 30 minutes), thereby, leaving no residual. When used at the recommended concentration of two to three ppm, ozone will kill bacteria, viruses, spores and cysts. Both ultraviolet irradiation and activated carbon will remove ozone from water.

In the presence of bromine (generally found in extremely small concentrations, if at all) ozone can produce bromates, suspected carcinogens.

The only effective ozone generation technology available for disinfection applications is corona discharge, illustrated below:

Care must be taken in handling any of the above chemicals and their effect on the materials of construction of the system must be considered.

Ultraviolet Irradiation: Ultraviolet (UV) irradiation is a common method of treating relatively low-flow water supplies. In this process, the water is passed over an ultraviolet lamp after it has been filtered. UV is effective in inactivating Cryptosporidium oocysts. Only momentary exposure is required to kill microorganisms, but this condition may not be met if the microorganisms are shielded by particles of sediment in the water. Furthermore, there is some evidence that certain bacteria may merely be inhibited in growth, rather than killed. Such bacteria, after a period of time, may recover and reproduce. Because ultraviolet irradiation does not involve the addition of chemicals, it leaves no residual, and the only costs in this process are the investment in equipment, replacement of ultraviolet bulbs, electrical power consumption, and the occasional cleaning of the bulb surfaces.

Note that neither ozone nor UV impart a residual disinfectant to the water. This is an important consideration when water must be stored for any length of time. Whereas a chlorine residual may be the result of good engineering design in a single residence, it is important to remember that it is mandated for potable application in public buildings.

A suggestion, ozonate the water entering storage tank followed by feeding a low concentration of liquid bleach to maintain a residual of 0.3-0.5mg/L free chlorine. For potable applications, activated carbon adsorption can be utilised at the point-of-use for chlorine removal.

Ozone will inactivate all microorganisms much more effectively than chlorine, and will also break down dissolved organic molecules to a certain extent, allowing activated carbon adsorption to be more effective.

The selection of treatment technologies in any graywater reuse application is dictated by the following factors:

  • Ultimate use of the recovered graywater.
  • Specific contaminants in the graywater to be reduced.
  • Total volume requirements.
  • Regulations.

As regulators acknowledge the many benefits of graywater reuse and mandate quality requirements for specific uses, the opportunities will grow rapidly. It is important that we are aware of the technology and design requirements to take advantage of these opportunities.

Peter S Cartwright
PE Cartwright
Consulting Co USA

 

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