13.2 Filtration Process Control: Loading Rates, Headloss, Breakthrough & Backwash Triggers

13.2 Filtration Process Control: Loading Rates, Headloss, Breakthrough & Backwash Triggers

Exam Focus: "Tertiary treatment processes — Filtration processes; Media filtration" sits in the 38-item Treatment Process content area, and this is calculation territory. Filter loading rate is a straightforward area-based calculation of exactly the kind the exam presents in both US Standard and metric units.

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1. Filtration Rate

Filtration rate (also called hydraulic loading rate or surface loading rate) is the flow applied per unit of filter surface area:

Filtration rate (gpm/sq ft) = Flow (gpm) / Filter surface area (sq ft)

Worked example. A plant filters 1.44 MGD through two gravity filters, each 10 ft by 12 ft, with both in service.

1. Convert flow: 1,440,000 gal/day divided by 1,440 min/day = 1,000 gpm.
2. Surface area: 10 ft x 12 ft = 120 sq ft per filter, x 2 filters = 240 sq ft.
3. Filtration rate: 1,000 gpm / 240 sq ft = 4.17 gpm/sq ft.

Now take one filter offline for backwash. Area halves to 120 sq ft and the rate on the remaining filter doubles to 8.33 gpm/sq ft. This is the operational point behind the calculation: taking a filter out of service raises the loading on every filter that remains, and backwashing during peak flow can push the in-service filters past the rate at which they can hold solids.

Typical tertiary filtration rates run in the low single digits of gpm/sq ft for granular media, with the exact design value set by the media, the configuration, and the permit.

2. Headloss and the Course of a Filter Run

A clean bed offers a small initial (clean-bed) headloss. As solids accumulate in the pores, the water must be pushed harder to maintain flow, so headloss rises through the run. When it reaches the terminal headloss the filter can no longer pass design flow and must be backwashed.

The core distinction. Headloss-limited runs are normal. Breakthrough-limited runs are a problem, because it means solids are reaching the effluent before the bed is full. A filter should end its run because it is full, not because it has stopped working.

Air binding is a third run-ending fault. When headloss through the upper bed becomes high enough that pressure falls below atmospheric within the media, dissolved gases come out of solution and form bubbles in the bed. The bubbles block pore space, headloss spikes abruptly, and the following backwash may boil violently. Keeping the water level above the bed and avoiding excessive terminal headloss prevents it.

3. Backwash

Initiation triggers — a filter is normally set to backwash on whichever of these comes first:

1. Terminal headloss reached
2. Effluent turbidity exceeds setpoint
3. Maximum run time elapsed — a time limit matters even on a lightly loaded filter, because biological growth in a bed left in service too long causes odors and solids sloughing

Backwash sequence for a granular bed:

1. Close the influent valve and drain down to just above the media.
2. Air scour (and/or surface wash) to break up the surface layer.
3. Backwash water upward at the rate needed to fluidize and expand the bed — commonly on the order of 20 to 50 percent expansion, verified with a bed expansion gauge.
4. Continue until spent backwash runs clear.
5. Refill, and ripen or filter-to-waste before returning the filter to service.

Getting the rate wrong, in both directions:

• Too low: the bed does not fluidize, grains do not scour against one another, and deposits stay put. Mudballs follow.
• Too high: media is carried over the backwash troughs and lost. Falling bed depth measured over time is the evidence. Excessive rate can also cause gravel upset, disturbing the graded support layers so that media migrates into the underdrain.
• Water temperature matters — cold water is more viscous and fluidizes a bed at a lower flow rate than warm water, so a fixed backwash rate set in summer may over-expand the bed in winter.

Ripening and Filter-to-Waste

Immediately after backwash a filter is too clean: the removal mechanism relies partly on previously captured particles, so effluent quality is momentarily poorest right after the wash. The ripening period is the interval during which quality improves to steady state. Where the permit demands it, the first filtered water is sent to waste (filter-to-waste) rather than to the effluent until turbidity settles.

4. Chemical Addition Ahead of Filters

Filters perform better when the particles arriving at them have been destabilized:

• A coagulant (alum or a ferric salt) neutralizes particle charge, and is also the mechanism for chemical phosphorus precipitation ahead of the filter.
• A filter aid polymer, dosed at very low concentration just ahead of the filter, strengthens the captured floc so it resists shearing through the bed.
• Overdosing a filter aid is a real hazard: too much polymer blinds the bed surface and headloss climbs almost immediately, cutting run length drastically. Dose changes should be small and evaluated over a full run.

5. The Backwash Return Stream

Spent backwash water is a high-solids side stream, and it goes back to the head of the plant.

• Return it through an equalization or backwash holding tank at a controlled rate. Dumping an entire backwash volume into the headworks in a few minutes creates a hydraulic and solids shock that can wash solids out of the primary clarifiers and disturb the secondary process.
• Never schedule multiple backwashes in immediate succession during peak flow.
• Track the backwash volume as a percentage of plant flow; a filter using an unusually high fraction of production for backwash is a filter that needs attention.