Multi-media filter (pressure media filter)

01

Set the filtration rate and redundancy

Ireland's Environmental Protection Agency, in its Water Treatment Manual: Filtration (2020), defines filtration rate as filtered flow (m³/h) per filter surface area (m²). It says rates above 7.5 m/h for mono-media and 10 m/h for dual-media filters are considered inappropriate in the manual when protozoa removal is targeted, while up to 15–20 m/h is acceptable in the manual when only metal removal is targeted. [2]

The manual requires enough filter area that the maximum rate is not exceeded with filters offline: larger plants design on an n − 2 basis (one backwashing, one in maintenance), and facilities with fewer than four filters may use n − 1. The manual gives pressure filter vessels as under 4 m in diameter and 10 m long because of transport, with a maximum media depth of 1–1.5 m; the manual says pressure filtration is more cost-effective at sites under 2 million litres per day, but backwashing cannot be watched. [2]

Editorial calculation: to treat 50 m³/h at 10 m/h with three vertical vessels on an n − 1 basis, the two remaining vessels need 5 m² together, 2.5 m² each, or about 1.78 m diameter. One 2.0 m vessel (3.14 m²) carries about 31 m³/h at 10 m/h. Flow, rate and vessel count are assumed examples; use the rate your water and treatment goal call for.

Give the supplier the raw water analysis (turbidity, suspended solids, iron, manganese, organics), flow range and operating hours, treatment goal (pretreatment to RO, ion exchange, drinking water, process water), required filtrate quality, design pressure, available backwash water and air, number of vessels and redundancy, materials and space.

02

Specify media sizes, depth and backwash

The EPA manual describes modern dual-media installations as using around 500 mm of anthracite (effective size 1.4 mm, uniformity coefficient 1.5) over 500–800 mm of sand (UC 1.4–1.5, effective size 0.4–0.6 mm), on graded support gravel. Its media table gives sand 0.5–0.7 mm ES at 2.6 g/ml, anthracite 1.2–1.4 mm at 1.5 g/ml and garnet 0.2–0.4 mm at 4.2 g/ml; a garnet layer of at least 150 mm was used in tri-media filters, but current consensus finds no benefit worth its cost. [2]

It cites the American Water Works Association's recommended minimum L/D ratio of 1,200 (bed depth over effective size) for Cryptosporidium removal; its example of 600 mm of 0.6 mm sand plus 400 mm of 1.3 mm anthracite gives 600/0.6 + 400/1.3 ≈ 1,308. Graded 16/30 sand (ES 0.54–0.7 mm) needs about 35–40 m/h of upflow for 15–20% bed expansion, the manual says modern plants target at least 20% expansion, colder water expands the bed more at the same rate, and media losses of up to 2–3% a year are expected, so a depth marker plate is advised. [2]

Carbotecnia, a filter maker, specifies its industrial multimedia (tri-media) filter with anthracite of 0.85–0.95 mm effective size, 16 × 35 mesh silica sand of 0.44–0.52 mm and 30–40 garnet of 0.33–0.37 mm, media to AWWA B-100, and a layer of 8–12 garnet over the gravel so the dense garnet and gravel do not mix in backwash. It lists a hydraulic service load of 7–10 gpm/ft², a backwash load of 20–22 gpm/ft², 30–100 psi operation and backwash at a differential above 10 psi or at most every 72 hours. [1] Editorial calculation: 7–10 gpm/ft² is about 17–24 m/h, well above the 10 m/h the EPA manual treats as the limit for dual-media drinking-water filters targeting protozoa, so agree the rate with the purpose of the filter. [2]

Editorial calculation: backwashing a 2.0 m diameter vessel (3.14 m²) at 35–40 m/h needs about 110–126 m³/h of wash water, far above its 31 m³/h service flow at 10 m/h, so size the backwash pump and water store for it. Diameter and rates are assumed examples.

03

Check the regulatory rate limit for pressure filters

California's drinking water regulation, 22 CCR §64660 (operating criteria for surface water treatment), limits pressure filters to filtration rates of 2.0 gpm/sq ft for single-media filters and 3.0 gpm/sq ft for dual, mixed-media or deep-bed filters, against 3.0 and 6.0 gpm/sq ft for gravity filters in conventional and direct filtration plants. Rates up to twice these need a demonstration that the filters meet the performance requirements of §64653; higher rates need 99% Giardia, 90% virus and 99% Cryptosporidium removal. [3]

These limits apply to surface water treatment plants under California rules; process-water filters ahead of membranes or ion exchange are set by the downstream equipment's needs instead. Compare them with the Irish EPA rates above, which are given in m/h. [3] [2]

Editorial calculation: 1 gpm/sq ft ≈ 2.44 m/h, so 2.0 and 3.0 gpm/sq ft are about 4.9 and 7.3 m/h; a 2.4 m diameter dual-media pressure vessel (4.52 m²) at 7.3 m/h treats about 33 m³/h.

State whether the water is regulated drinking water, the jurisdiction's rate limit and turbidity triggers, and ask the supplier for filtration rate, filter area and inspection access for media checks.

04

Compare Texas design rules for pressure and multi-media filters

The Texas Commission on Environmental Quality's public water system rule, 30 TAC §290.42, limits pressure filters to plants with a treatment capacity below 0.50 MGD and caps their design capacity at 2.0 gpm/sq ft with the largest filter off-line. High-rate gravity filters may be designed for 5.0 gpm/sq ft. Filter media must conform to AWWA standards, be at least 24 inches deep and give an L/d ratio of at least 1,000. [4]

Editorial calculation: taking mid-range effective sizes, the minimum Texas multi-media bed gives an L/d of about 76/0.25 + 229/0.55 + 610/1.0 ≈ 1,330, above the rule's 1,000 and the AWWA figure of 1,200 cited by Ireland's EPA. [4] [2] At 2.0 gpm/sq ft (about 4.9 m/h), the 50 m³/h example above would need about 10.2 m² of filter area with the largest filter off-line, against 5 m² at the 10 m/h dual-media rate. Editorial calculation; these rules apply to Texas public water systems, so a process-water filter is set by its downstream equipment instead. [4]

05

Who lists multi-media filter vessels

Listings include multi-media filters alongside sand, deep-bed and carbon media filters, FRP and steel filter tanks within water treatment plants, and multi-media filters within ranges of self-cleaning, bag and cartridge filters. Ask each for vessel area, media specification and backwash design.

06

From underdrain to air-release valve

07

Raw-water analysis and vessel sizing data

Flow, vessel diameter, media and backwash rows lean on Ireland's EPA manual, with Carbotecnia and California's rule for pressure loss and rate limits; shell material and weight come from the supplier.

08

Send the water, flow and backwash data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Water: [turbidity; suspended solids; iron; manganese]
Flow: [normal; maximum; hours]
Goal: [RO pretreatment; drinking water; process water]
Rate: [m/h; redundancy n−1 or n−2]
Media: [layers; ES; UC; depths]
Backwash: [water rate; air scour; source]
Vessels: [number; diameter; pressure; material]

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