In today's society, with the continuous rise in public demand for water quality, the development of water treatment technology has increasingly attracted attention. Mechanical filters, as widely applied water treatment equipment, play an indispensable role in numerous fields. This article will explore the working principle, characteristics, application scope, and methods to improve their ability to treat heavy metal contaminants, helping readers gain a comprehensive understanding of mechanical filters.
Mechanical filters, also known as pressure filters, are essential components in the pretreatment stage of pure water production and water purification systems. They are usually made of rubber-lined steel or stainless steel. According to the type of filtration media, they can be divided into natural quartz sand filters, multimedia filters, activated carbon filters, manganese sand filters, and other types. Additionally, according to the water inlet method, mechanical filters can be classified as single-flow or dual-flow filters. In practical applications, they can be used alone or in combination based on specific needs.
Single-flow Mechanical Filters: The pipeline design of single-flow mechanical filters is simple, and their operation is stable and reliable. The filtration flow rate is generally between 4–50 m³/h, and the operating cycle is usually 8 hours. This type of filter has a simple structure, is easy to operate, and is suitable for situations where filtration accuracy is not particularly high. However, because its filtration area is relatively small, the water processing capacity is limited, so it is less commonly used in large water treatment systems.
Dual-flow Mechanical Filters: Dual-flow mechanical filters have a unique design. They feature water inlets at both the top and bottom and a water outlet in the middle. This design gives dual-flow filters advantages such as larger filtration capacity, higher contaminant removal capability, and longer operating cycles (generally 20 hours). However, their pipeline systems are relatively complex, operational stability is slightly lower, and backwashing or media replacement is more challenging. Despite this, dual-flow filters still have significant advantages when treating large volumes of water with impurities, making them suitable for situations that require high water quality and large treatment capacity.
The core working principle of a mechanical filter is to use one or more filtration media, under a certain pressure, to allow raw water to pass through these media, thereby removing impurities and achieving filtration. Common filtration media include quartz sand, anthracite, porous ceramic particles, manganese sand, and others. Users can select appropriate media based on different water quality conditions and treatment requirements.
When water flows through a mechanical filter, suspended solids, organic matter, colloidal particles, microorganisms, chlorine odor, and some heavy metal ions are intercepted by the filtration media. In this way, mechanical filters can effectively reduce water turbidity and provide purified water. This traditional water treatment method is widely used in various water treatment processes due to its simplicity and high efficiency.
Mechanical filters have a very wide application range, covering almost all fields requiring water treatment. They are mainly used for turbidity removal in water supply treatment and serve as pre-treatment equipment for reverse osmosis, ion exchange softening, and desalination systems. Additionally, mechanical filters can be used for silt removal in surface water and groundwater.
In practical applications, the influent turbidity of mechanical filters is required to be less than 20 NTU, while the effluent turbidity can reach below 3 NTU. Due to characteristics such as low equipment cost, low operating expenses, simple management, reusable filter media through backwashing, long media lifespan, effective filtration, and small footprint, mechanical filters have become the preferred equipment in many processes.
For example, in electrodialysis projects, raw water must pass through a mechanical filter to remove suspended solids and particulate impurities before purification. Additionally, processes such as brackish water desalination, space water production, drinking water purification, groundwater defluoridation, pharmaceutical injection preparation, fertilizer production, and industrial water use all require mechanical filters. Mechanical filters play a crucial role in the water treatment field.
With increasing public concern about water safety, heavy metal pollution has become a major focus. How do mechanical filters perform in removing heavy metal contaminants?
Mechanical filters are highly effective at removing particulate heavy metal contaminants. For example, common solid particles in water, such as iron oxide and manganese oxide, are effectively intercepted as water passes through the filter media layer. This is because the filter media in mechanical filters (such as quartz sand or manganese sand) can form an effective filtration layer to physically block particulate matter. During pre-filtration and coarse filtration stages, mechanical filters can effectively remove particulate heavy metal contaminants, protecting downstream treatment equipment, reducing its load, and prolonging its service life.
However, for dissolved heavy metal ions, such as lead ions or cadmium ions, the removal effect of mechanical filters is relatively limited. This is because dissolved heavy metal ions exist in water in ionic form with very small particle sizes, making them difficult to remove through the physical interception of mechanical filters. If water contains a large amount of dissolved heavy metal contaminants, using mechanical filters alone may not meet water treatment requirements.
To enhance mechanical filters' capability to remove heavy metal contaminants, the following methods can be applied:
By adding chemical agents such as coagulants or precipitants, dissolved heavy metal ions can be converted into particulate form. For example, after adding a coagulant, dissolved heavy metal ions combine with other substances to form larger particles, which can then be effectively intercepted by the mechanical filter. This method can significantly enhance the treatment capacity of mechanical filters for heavy metal contaminants.
Selecting filter media with adsorption capability, such as modified activated carbon or manganese sand, can partially remove dissolved heavy metal ions. These media not only perform physical filtration but also adsorb dissolved heavy metal ions on their surfaces. For example, the porous structure of activated carbon provides a large surface area capable of adsorbing heavy metal ions in water. In this way, mechanical filters can simultaneously remove particulate heavy metals and partially treat dissolved heavy metal ions.
Adding reverse osmosis devices or ion exchange systems after mechanical filters can efficiently remove dissolved heavy metal contaminants. Reverse osmosis uses semipermeable membranes to selectively remove dissolved substances, including heavy metal ions. Ion exchange involves resins exchanging metal ions with harmless ions in water to remove heavy metals. By combining mechanical filters with other water treatment processes, higher quality water treatment can be achieved, meeting stricter water quality requirements.
Regular maintenance and care are critical to ensure proper operation and long-term stability of mechanical filters.
When the system restarts after long-term shutdown, the filter media should be forward washed for about 5 minutes until the effluent is clear. During initial operation or after prolonged shutdowns, the system should be vented. For large filters, air scouring can enhance backwashing, typically using compressed air (10–18 L/s·m²), followed by air-water backwashing. During backwashing, the intensity should be controlled to prevent activated carbon from escaping the system.
Activated carbon media should be replaced regularly, typically every 3–6 months. Replacement intervals for other media depend on actual use and water quality requirements. Regular media replacement ensures filtration effectiveness and prevents aging or contamination from reducing filter efficiency.
Backwashing is a key maintenance step. When the pressure difference between the filter inlet and outlet reaches 1 kg/cm², backwashing should be performed. During backwashing, compressed air can be used to scour quartz sand. Generally, filters with diameters under 2500 mm do not require air scouring, while those above 2500 mm must use compressed air for thorough cleaning. Backwash flow is usually 3–4 times the design capacity of the filter.
Older mechanical filters generally use large pebbles as a base layer, with convex steel plates at the bottom perforated for water distribution. This design can cause uneven flow distribution, with higher filtration rates at the center and lower at the edges. During backwashing, mixing of quartz sand layers may occur, potentially leading to filter media entering downstream pipes or precision filters, posing risks to precision filters and reverse osmosis systems.
Through practice and experimentation, many manufacturers have improved mechanical filters. Water distribution devices now use perforated plates with special ABS water caps. These ABS caps have dual output functions: low output during operation and increased output during backwashing. This design ensures more uniform water distribution during forward washing and thorough cleaning during backwashing, significantly improving effluent quality.
Additionally, to prevent fine sand from passing through the filter during operation or backwashing, the gaps in ABS water caps are very small, generally 0.1–0.2 mm. During media loading, water must be added to prevent large quartz sand from breaking the ABS caps. Care should be taken not to wear hard-soled shoes when installing caps. Mechanical filters also have backwash water inlet limiting butterfly valves to control and adjust backwash flow. Backwash intensity should expand the filter layer by 15–25%, and compressed air intensity during backwash is typically 10–18 L/s·m².
As widely used water treatment equipment, mechanical filters play an important role in removing suspended solids, particulate matter, and some impurities from water. Although their removal capacity for dissolved heavy metal ions is limited, adding chemical agents, selecting adsorptive filter media, and combining with other water treatment processes can effectively enhance their ability to treat heavy metal contaminants. In practical applications, process combinations should be selected according to water quality characteristics and treatment goals to achieve the best water treatment results. Additionally, regular maintenance and care are key to ensuring long-term stable operation of mechanical filters.
