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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Water and Wastewater Treatment Processes › Lesson 5
AGR-WATE71-03 · Lesson 5 · 12.5 clock hours · demo

Filtration: Rapid Sand and Membrane Processes

Objective. Students will explain the mechanisms of filtration in a rapid sand filter and in membrane systems (microfiltration/ultrafiltration), identify conditions that indicate a filter needs backwashing, and describe the backwash sequence.

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This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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How Filtration Works in Water Treatment

Filtration is the physical process of passing water through a porous medium to remove suspended particles, turbidity, microorganisms, and floc that remain after coagulation and sedimentation. Two major filtration technologies are used in drinking-water and reuse systems: rapid sand filtration and membrane filtration.

Rapid Sand Filtration: Mechanisms

A rapid sand filter holds a bed of graded media—commonly a layer of sand, sometimes with an anthracite coal layer on top (dual-media) or garnet on the bottom (multimedia). Water flows downward through the bed under gravity or pressure.

Particles are removed by three overlapping mechanisms:

Proper coagulation and flocculation upstream are essential. Without adequate coagulant dose, particles carry a negative surface charge and repel media grains instead of attaching to them, causing poor filtered-water quality.

As the filter run continues, captured solids accumulate. Two things happen simultaneously: head loss (the pressure difference across the bed) rises because pores become clogged, and eventually turbidity breakthrough occurs when the bed is overloaded and particles pass through to the effluent. Both conditions signal that the filter needs to be taken offline for backwashing.

Conditions requiring backwash initiation:

1. Head loss reaches the facility's predetermined maximum (commonly 8–10 feet of water, but each plant sets its own limit based on design).

2. Filtered-water turbidity rises above the plant's turbidity limit or regulatory threshold.

3. Filter run time reaches the maximum allowable duration set in the operating plan.

The Backwash Sequence

Backwashing reverses flow through the filter bed to dislodge and flush out accumulated solids. A properly executed sequence typically includes:

1. Close the influent valve and drain to the wash-water trough level. The filter is taken out of service and the water level is lowered so backwash water and dislodged solids can overflow into the waste troughs.

2. Surface wash or air scour (or both). Rotating surface-wash nozzles or air injected at the bottom of the bed agitate the media surface and break up mud balls before backwash water flow begins. Air scour uses relatively low airflow to scrub grain surfaces. This step prevents clumping of solids that would resist cleaning.

3. Upward backwash water flow. Treated water (or finished water) is pumped upward through the bed at a rate sufficient to expand and fluidize the media—typically 15–20 gallons per minute per square foot, though design values vary. Fluidization lifts and separates the grains, releasing trapped solids into the wash water that overflows to waste.

4. Collapse-pulse phase (where used). Some facilities alternate air and water pulses to improve cleaning efficiency without fully fluidizing the bed, saving water.

5. Rinse and return to service. After backwash, a brief rinse-to-waste period allows the filter to stabilize before effluent is sent to the clearwell. Filtered-water turbidity is monitored; the filter is not returned to service until it meets the plant's turbidity standard.

Membrane Filtration: Microfiltration and Ultrafiltration

Membrane systems use semi-permeable synthetic membranes with very small pore sizes instead of a granular bed.

The key operating parameter for membrane systems is transmembrane pressure (TMP)—the pressure difference across the membrane from the feed side to the permeate side. As the membrane surface accumulates fouling (biofilm, particulates, organic matter), TMP rises for a given flow rate, or permeate flow drops for a given pressure. Operators track TMP trends on the operating log.

Cleaning triggers: Manufacturers and system designers set TMP thresholds at which cleaning must occur. When TMP rises to the defined limit, a chemically enhanced backwash (CEB) is performed using dilute chlorine, acid, or caustic solution to restore membrane permeability. If CEB is insufficient, a clean-in-place (CIP) procedure using stronger chemical solutions is performed. Operating beyond the TMP limit stresses the membranes and can cause irreversible fouling or fiber damage.

Safety Points

Key terms10

Turbidity breakthroughThe point during a filter run when accumulated solids overwhelm the filter bed's capacity and filtered-water turbidity rises above acceptable limits, indicating the filter must be backwashed.
Head lossThe pressure drop across the filter bed from influent to effluent, measured in feet of water or psi; it increases as solids accumulate and is a primary indicator that backwashing is needed.
BackwashingThe process of reversing water flow upward through a rapid sand filter bed to fluidize the media and flush accumulated solids to waste, restoring filter capacity.
Air scourThe injection of compressed air into the filter bed before or during backwash to scrub grain surfaces and break up compacted solids and mud balls.
Transmembrane pressure (TMP)The pressure difference between the feed/concentrate side and the permeate side of a membrane; rising TMP at constant flow indicates fouling and the need for cleaning.
Microfiltration (MF)A membrane process with pore sizes in approximately the 0.1–0.45 micrometer range that removes particulates, protozoa, and most bacteria by size exclusion.
Ultrafiltration (UF)A membrane process with smaller pores than microfiltration (approximately 0.005–0.1 micrometers) capable of rejecting viruses and larger macromolecules in addition to particulates and protozoa.
Chemically enhanced backwash (CEB)A routine membrane cleaning procedure that adds dilute chemical solutions (such as chlorine, acid, or caustic) to the backwash cycle to dissolve foulants and restore membrane permeability.
Mud ballsDense, compacted clumps of media and accumulated solids that form in a filter bed when backwashing is inadequate; they reduce filter efficiency and media life.
Media fluidizationThe expansion of granular filter media grains during upward backwash flow so they separate and move freely, allowing solids to be carried away in the waste water.

Worked examples3

1. A rapid sand filter has an initial head loss of 1.2 feet at the start of a run. After 18 hours of operation the head loss gauge reads 9.7 feet. The plant's backwash trigger is set at 10.0 feet. How much additional head loss can accumulate before backwash must begin, and what percentage of the total allowable head-loss rise does the current reading represent?
  1. Step 1 – Find remaining head loss before the trigger: 10.0 ft (limit) − 9.7 ft (current) = 0.3 ft remaining.
  2. Step 2 – Find total allowable rise: 10.0 ft (limit) − 1.2 ft (initial) = 8.8 ft total allowable rise.
  3. Step 3 – Find actual rise so far: 9.7 ft − 1.2 ft = 8.5 ft rise so far.
  4. Step 4 – Calculate percentage of allowable rise used: (8.5 ÷ 8.8) × 100 = 96.6%.
Answer: Only 0.3 feet of head loss remain before the backwash trigger. The filter has consumed 96.6% of its allowable head-loss rise and should be backwashed very soon.
2. A filter bed is 20 feet wide and 30 feet long. The design backwash rate is 15 gallons per minute per square foot (gpm/ft²). What total backwash flow rate (in gpm) must the backwash pump deliver?
  1. Step 1 – Calculate filter bed surface area: 20 ft × 30 ft = 600 ft².
  2. Step 2 – Multiply area by backwash rate: 600 ft² × 15 gpm/ft² = 9,000 gpm.
Answer: The backwash pump must deliver 9,000 gpm to achieve the required backwash rate across the entire filter bed.
3. A membrane system's operating log shows TMP was 8 psi at startup. After two weeks it reads 22 psi at the same permeate flow rate. The manufacturer's CEB trigger is a TMP rise of 15 psi above the clean baseline. Has the CEB trigger been reached, and by how much?
  1. Step 1 – Calculate TMP rise: 22 psi − 8 psi = 14 psi rise.
  2. Step 2 – Compare to trigger: The trigger is 15 psi rise. 14 psi < 15 psi.
  3. Step 3 – Determine margin remaining: 15 psi − 14 psi = 1 psi below the trigger.
Answer: The CEB trigger has not yet been reached; TMP has risen 14 psi, which is 1 psi below the 15 psi threshold. The operator should continue monitoring closely because the trigger is imminent.

The math in this lesson1

Learners compute the share of allowable head-loss rise used, 8.5 ÷ 8.8 = 96.6%, to time backwash.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Observe or conduct a bench-scale rapid sand filter demonstration: pass a turbid coagulated sample through the filter, monitor head loss and filtered turbidity over time until turbidity breakthrough is observed, and record all data on a filter run log. · Initiate and observe a simulated backwash sequence on the bench-scale unit or using a step-by-step flowchart; identify the surface wash, air scour, and collapse-pulse phases and state the purpose of each. · Review a real or sample membrane system operating log showing transmembrane pressure trends; identify the point at which chemical cleaning would be triggered and explain the reason using manufacturer guidance.
Materials: Bench-scale rapid sand filter (or PVC column model), filter sand, turbid coagulated water, turbidimeter, head-loss gauge, filter run log form, sample membrane system operating data printout, AWWA Microfiltration and Ultrafiltration Membranes manual excerpt.
Check: Student correctly identifies the turbidity breakthrough point on the filter run log and states two conditions requiring backwash initiation; successfully sequences all backwash steps in correct order on a written step-sequence exercise (≥ 80% correct steps).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Water Treatment Operation
1. During a filter run, which TWO conditions most commonly trigger backwash initiation?
Water Treatment Operation
2. What is the primary purpose of the air scour or surface wash step performed before or during backwashing?
Water Treatment Operation
3. A filter is returned to service immediately after backwashing without a rinse-to-waste period. What is the most likely consequence?
Water Treatment Processes
4. Which mechanism explains how a rapid sand filter removes particles that are much smaller than the pore spaces between the media grains?
Water Treatment Processes
5. In a membrane filtration system, transmembrane pressure (TMP) is trending steadily upward at constant permeate flow. What does this trend indicate?
Water Treatment Processes
6. Which membrane process has pore sizes small enough to reject most viruses in addition to protozoa and bacteria?
Health and Safety
7. An operator is about to open the valve on the chemical feed line to perform a chemically enhanced backwash (CEB) on a membrane system. Which personal protective equipment (PPE) is most critical before handling the cleaning chemical?
Water Treatment Operation
8. A bench-scale rapid sand filter run log shows the following turbidity readings in the filtered effluent: hour 1 = 0.08 NTU, hour 4 = 0.11 NTU, hour 8 = 0.14 NTU, hour 10 = 0.52 NTU, hour 11 = 1.8 NTU. At which point has turbidity breakthrough clearly occurred?
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