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Very few places remain on Earth where water can be consumed directly without filtration or chemical disinfection. These freshwater sources rely on natural filtration processes that have taken millennia to develop on our planet. As rain falls, it is filtered down through many layers of soil, sand and rock before emerging at or near potable water quality.
Most human-made water treatment methods follow similar steps, using natural or artificial media to simulate the natural processes. These approaches can be scaled to fit a variety of applications, from small in-building systems to large, municipal-scale wastewater treatment plants.
Three main types of filtration processes can be used to purify water. Each serves a different purpose and function as it is up to the engineer or designer to decide when to use each process to create an effective wastewater treatment system.
1. Mechanical. These processes use membranes or other small orifice filters to screen out impurities and allow only water molecules to pass through. Good for larger debris but less effective against ultra-fine contaminants.
2. Biological. This treatment method relies on bacteria and other small organisms to break down organic wastes, similar to processes occurring within the body. The bacteria grow on media, either floating within the tank or in a filter cartridge.
3. Chemical. This method uses chemical elements to completely inactivate or destroy pathogens in wastewater streams, often as the final treatment step before water is discharged to natural waterways in municipal systems or reused in a building as recycled water.
Plumbing engineers often specify filters without truly understanding the fundamental process by which the filters function. It is easy to focus on the results of filtration; however, by knowing the ins and outs of different filtration methods, engineers can make better-informed decisions and better guarantee public health and safety when water is reused within a building.
Rainwater and greywater reuse systems need to ensure that large debris does not enter the system; mechanical filtration is used throughout the water treatment process.
• Vortex filters. These are first flush-type devices, which use the surface tension of water to separate solids from the clean water. The water clings to the stainless-steel mesh inside the filter and passes through, while leaves and other debris are flushed to the storm sewer. These function equally well for greywater as well as rainwater harvesting systems.
• Activated carbon filters. These are highly effective filters for removing chemicals such as chlorine as well as carbon-based organic matter. They also neutralize odors and are critical in greywater systems to remove many chemicals found in soaps, shampoos and makeup.
The media is created by using high temperatures and oxygen to remove contaminants, followed by a steam treatment that causes the granules to fracture and create pores. The filter functions by an adsorption process, in which pollutants become trapped in the carbon granules, similar to how magnets attract small pieces of iron. The raw material for these filters typically comes from bituminous coal, peat or coconut shells. Coconut shells are the most common source and are highly renewable.
• Zeolite filters. These filters have largely replaced sand-based filters and can be used as a direct replacement media for existing sand filters. Due to their increased surface area, zeolite media can handle higher flow rates and remove a wider variety of contaminants.
Zeolites can form in nature through the interaction of seawater and volcanic rock, but most commercially available filters use synthetic zeolites. Similar to activated carbon, these filters work by adsorption, in which chemical impurities such as ammonium and hydrocarbons are captured in the pores of the media along with sediment that can discolor the recycled water.
• Ultrafine filters. These filters come in various sizes and flow rates, with filtered particles ranging from 0.5 to 20 microns. They have no moving parts, which makes them extremely reliable at removing sediment. Most UV-based disinfection requires a 5-micron filter upstream to remove particles and prevent shadowing. This is discussed in the UV section.
Typically, these filters are made of polypropylene strings wound in a precise pattern around a polypropylene core and are easily replaceable. These filters do not entirely remove viruses, bacteria and many other
The other required filtration function of a water reuse system is to remove, neutralize or inactivate typical waterborne pathogens such as cryptosporidium, e Coli, giardia and Legionella. There are various ways to accomplish this; however, the two primary methods are membrane bioreactors (MBRs) and moving bed bioreactors (MBBRs).
Both are used at the municipal scale as well as the district or within individual buildings. This type of biological filtration is not needed for rainwater harvesting systems.
Both types of systems are considered aerobic — they include aeration/oxygenation within the tanks to ensure that the biological treatment process is stable and promotes biofilm growth and the removal of solids and sludge from the filters.
Membrane bioreactors contain a membrane filtration element either suspended in a treatment tank or a pumped side-stream arrangement. The filter membrane can be either flat sheets or tubular and separates solids in the wastewater. The membranes typically include several different sizes of pores ranging from 0.1μm to 10μm. MBRs can produce high-quality discharge, which requires less post-filtration than other methods of greywater filtration.
The solids accumulate as sludge on the membrane in a process known as fouling. This requires frequent backwashing and occasional replacement of the membrane unit. MBRs also require pressure to move water through the membrane, resulting in a higher energy cost than the more passive filtration of an MBBR. Low influent temperatures or large inlet flow swings can have adverse effects on the performance of MBRs.
Moving bed bioreactors use a similar process as MBRs; however, the primary filtration media are small polyethylene carriers free-floating within a holding tank rather than a large membrane. The carriers provide surface area for biofilm growth, and the biofilm absorbs, oxidizes and reduces pollutants in the wastewater.
The carriers are designed to have a density close to water so they mix freely without sinking or floating. The tank is aerated to ensure even biofilm growth and typically includes mechanical mixing. These carriers can be replaced much easier than backwashing a large membrane filter.
The operation of MBBRs is simple, and the process is a reliable way to treat greywater. MBBRs use significantly less energy than MBR systems and have lower capital and operating costs. MBBRs can handle larger fluctuations in greywater influent and are not as susceptible to clogging as MBRs due to their smaller individual filter elements. As a result, MBBRs are more commonly used for onsite water reuse systems.
Ultraviolet (UV) lights rely on the damaging effects of ultraviolet light on living matter to sterilize any pathogens. UV purifiers can deactivate up to 99.99 percent of organisms in water. However, UV lights require extremely clear water to operate effectively; typically, the water stream needs a turbidity level below 1 NTU. This can be accomplished by placing a 5-micron or smaller filter directly upstream of the UV light.
This disinfection method includes several benefits, such as easy maintenance (bulb changes are quick and easy). No water is wasted in the process as all water that passes through is filtered.
UV light treatment is measured based on an energy per area basis with two main categories, Class A and Class B. Both classes deliver UV light at a wavelength of 254nm, but Class A is more powerful at a power level of 40mJ/cm2. Class B UV lights only deliver 16mJ/cm2. Class A lights are more commonly used for potable water disinfection; Class B lights are sufficient for disinfection for onsite water reuse systems.
Once the primary filtration and treatment process is complete, there may still be active pathogens (viruses/bacteria) in the water stream. The standard method for accomplishing this in municipal potable water systems is chlorine injection due to its simple usage and high level of effectiveness.
Chlorine injection provides an immediate dosage to kill viruses and bacteria. A small residual level of chlorine remains in the water stream for up to 24 hours to prevent new pathogens from developing in the distribution system.
Onsite water reuse systems can use chlorine in a similar manner. Prior to distribution to the building, the reclaimed water is dosed with chlorine to ensure that the water is not harmful to the building’s occupants. This is an essential step for greywater harvesting systems, even if the reclaimed water is only provided to nonpotable fixtures, such as toilets and urinals.
The chlorine injection is typically added to the water while it is in the storage tank before being supplied to the building. It is common for this equipment to be provided on a self-contained skid with a small dosage pump and water sensors to measure chlorine levels. Rainwater harvesting systems may not need this extra disinfection, but it is wise to verify the authority having jurisdiction’s requirements.
A newer method of disinfection is ozone water treatment. Ozone (O3) is an extremely powerful oxidizing gas that occurs naturally in the atmosphere but also can be created by an ozone generator. In this process, ozone is injected into the water stream, where it dissolves and neutralizes contaminants such as bacteria and viruses. The ozone gas disrupts the pollutants on a cellular level and destroys their ability to reproduce.
Ozone is not as commonly used for onsite water reuse systems as it is more costly than comparable chlorine systems. However, as the technology becomes more widely available, that cost difference is likely to shrink.
All the filters discussed in this column require a constant maintenance routine to be effective. As the engineer or designer of the system, it’s imperative to consider this while laying out equipment rooms to allow for future replacement or repairs.