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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Aquaculture & Marine Science Tech › Water Quality Monitoring and Management for Aquaculture › Lesson 7
AGR-AQUA50-03 · Lesson 7 · 16 clock hours · lab

Turbidity, Solids, and Filtration Management

Objective. Students measure turbidity in NTU and total suspended solids (TSS) gravimetrically, interpret results against discharge permit benchmarks and fish health thresholds, and evaluate the effectiveness of a mechanical filter in a RAS or pond setting.

⚠ Pending professional review

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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Why Turbidity and Suspended Solids Matter in Aquaculture

Water clarity is more than an aesthetic concern. In a recirculating aquaculture system (RAS) or a flow-through pond, fine particles carry pathogens, consume dissolved oxygen as they decompose, clog fish gills, and smother eggs on spawning substrates. Regulatory agencies also set effluent limits for suspended solids, so understanding how to measure and control them protects both your fish and your discharge permit.

Two measurements describe the particle load in water: turbidity and total suspended solids (TSS). They are related but not identical, and a competent aquaculture technician uses both.

Turbidity: The Optical Measurement

Turbidity measures how much light is scattered by particles in a water sample. The standard unit is the Nephelometric Turbidity Unit (NTU). A turbidimeter (such as the HACH 2100Q) shines a light beam through the sample and detects scattered light at a 90-degree angle to the beam. More scattering means higher NTU.

General fish health guidance suggests that prolonged turbidity above roughly 25 NTU can stress salmonids and other sensitive species, though exact thresholds vary by species and life stage. Many state discharge permits require effluent turbidity to stay below a specified limit — check your facility's permit for the exact number.

A Secchi disk is a low-tech alternative used in open ponds and outdoor tanks. The black-and-white disk is lowered on a measured line until it disappears from view; that depth is the Secchi depth. Clearer water produces a greater Secchi depth. Secchi readings are quick and require no power, but they integrate visibility across the entire water column, cannot give an NTU value directly, and depend on sunlight and the observer's eyesight. Always record both methods and their limitations in your field notes.

TSS: The Gravimetric Measurement

TSS is the mass of particles retained on a filter, expressed in milligrams per liter (mg/L). It is the gold-standard measurement used by regulators and follows the procedure in Standard Methods for the Examination of Water and Wastewater (APHA). Unlike turbidity, TSS accounts for particle size, density, and actual mass — not just optical properties.

TSS Procedure Summary

1. Label and pre-condition a 47 mm glass fiber filter (0.45 µm nominal pore size) by rinsing with distilled water, then drying at 105 °C for at least one hour.

2. Cool in a desiccator and weigh to the nearest 0.1 mg. Record as the tare weight.

3. Filter a precisely measured volume of sample (typically 100–1,000 mL depending on particle load) through the filter using a vacuum filtration apparatus.

4. Return the loaded filter to the oven at 105 °C for one hour, cool in the desiccator, and reweigh. Record as the final weight.

5. Calculate TSS using the formula below.

TSS (mg/L) = [(Final weight − Tare weight) in mg] ÷ [Volume filtered in liters]

Record all weights immediately after removal from the desiccator because filters re-absorb moisture from air within minutes.

Mechanical Filtration in RAS and Ponds

A drum filter (rotary microscreen filter) is common in RAS. Water flows through a fine mesh drum; solids collect on the screen and are rinsed off by a backwash spray into a waste stream. A settling basin relies on gravity: water slows, and denser particles sink to the bottom before the clarified water flows on.

Filter performance degrades over time. As solids load the screen, flow rate drops and particles begin to bypass. Tracking TSS before and after the filter at regular intervals reveals when the filter is no longer removing solids effectively. Plotting TSS effluent readings against backwash intervals on a time-series graph lets you identify the inflection point where performance collapses. Use that point to set a preventive maintenance trigger — for example, initiating backwash when effluent TSS exceeds a defined threshold.

Interpreting Results

When your TSS reading approaches or exceeds your discharge permit limit, you must act: increase backwash frequency, inspect the filter screen for damage, reduce feeding rate, or add a secondary settling stage. Document every intervention with date, time, and outcome.

Safety in the Lab and Field

Key terms9

TurbidityThe optical clarity of water, measured by how much suspended particles scatter a beam of light; expressed in Nephelometric Turbidity Units (NTU).
NTU (Nephelometric Turbidity Unit)The standard unit of turbidity measurement; determined by detecting scattered light at 90 degrees to the incident beam inside a turbidimeter.
Total Suspended Solids (TSS)The mass of particles retained on a glass fiber filter after a known volume of water is filtered, expressed in mg/L; determined gravimetrically following Standard Methods.
Secchi DiskA black-and-white disk lowered into a water body on a measured line; the depth at which it disappears indicates water clarity but cannot be directly converted to NTU.
Drum FilterA rotary microscreen mechanical filter used in RAS that removes suspended solids by passing water through a fine mesh and periodically backwashing accumulated solids away.
Tare WeightThe dry weight of a clean, pre-conditioned glass fiber filter before any sample is filtered through it; subtracted from the final weight to determine mass of solids retained.
Standard Methods for the Examination of Water and WastewaterThe joint publication of APHA, AWWA, and WEF that provides standardized, peer-reviewed procedures for water and wastewater analysis, including the TSS gravimetric method.
Backwash IntervalThe scheduled or triggered frequency at which a filter is cleaned by reversing or spraying water through it to remove accumulated solids and restore flow capacity.
Discharge PermitA regulatory authorization (commonly a NPDES permit under the Clean Water Act) that specifies maximum allowable concentrations of pollutants, including TSS and turbidity, in a facility's effluent.

Worked examples4

1. A 500 mL sample of RAS effluent is filtered through a pre-weighed glass fiber filter. Tare weight = 1.4832 g. After drying at 105 °C and cooling, the final weight = 1.5107 g. Calculate TSS in mg/L.
  1. Step 1 — Find mass of solids retained: Final weight − Tare weight = 1.5107 g − 1.4832 g = 0.0275 g
  2. Step 2 — Convert grams to milligrams: 0.0275 g × 1,000 mg/g = 27.5 mg
  3. Step 3 — Convert sample volume to liters: 500 mL ÷ 1,000 = 0.500 L
  4. Step 4 — Apply TSS formula: TSS = 27.5 mg ÷ 0.500 L = 55.0 mg/L
Answer: TSS = 55.0 mg/L
2. A 250 mL effluent sample yields solids of 18.2 mg after drying. What is the TSS in mg/L?
  1. Step 1 — Convert volume to liters: 250 mL ÷ 1,000 = 0.250 L
  2. Step 2 — Apply TSS formula: TSS = 18.2 mg ÷ 0.250 L = 72.8 mg/L
Answer: TSS = 72.8 mg/L
3. A drum filter is removing solids from a RAS. Effluent TSS readings (mg/L) are recorded hourly: 0 h = 12, 4 h = 15, 8 h = 18, 12 h = 22, 16 h = 31, 20 h = 48, 24 h = 65. The discharge permit limit is 30 mg/L. At what hour does effluent TSS first exceed the permit limit, and what percentage increase occurred between the 12 h and 16 h readings?
  1. Step 1 — Scan readings for first value above 30 mg/L: 0 h=12, 4 h=15, 8 h=18, 12 h=22, 16 h=31. The reading at 16 h (31 mg/L) is the first to exceed 30 mg/L.
  2. Step 2 — Calculate percentage increase from 12 h to 16 h: Change = 31 − 22 = 9 mg/L
  3. Step 3 — Percentage increase = (9 ÷ 22) × 100 = 40.9%, which rounds to approximately 41%.
Answer: TSS first exceeds the permit limit at the 16-hour mark (31 mg/L); TSS increased approximately 41% between the 12 h and 16 h readings, indicating filter degradation. A backwash or maintenance event should be triggered before the 16-hour point.
4. A technician measures effluent turbidity before a settling basin as 87 NTU and after as 23 NTU. What is the percentage removal of turbidity by the settling basin?
  1. Step 1 — Calculate turbidity removed: 87 NTU − 23 NTU = 64 NTU
  2. Step 2 — Percentage removal = (64 ÷ 87) × 100 = 73.6%, which rounds to approximately 74%.
Answer: The settling basin achieved approximately 74% turbidity removal.

The math in this lesson1

Learners convert filter weight gain to mg and sample mL to L to compute TSS in mg/L.
N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Collect effluent samples before and after a drum filter or settling basin; measure turbidity with a turbidimeter and compare with a Secchi disk reading in an outdoor pond tank, documenting both methods and their limitations. · Filter a measured volume of turbid tank water through a pre-weighed glass fiber filter, dry at 105°C in a laboratory oven for one hour, reweigh, and calculate TSS in mg/L following Standard Methods for the Examination of Water and Wastewater procedure. · Graph filter backwash intervals against TSS readings over a simulated 24-hour data set; identify the point at which filter performance is degrading and write a maintenance recommendation.
Materials: Turbidimeter (HACH 2100Q or equivalent), Secchi disk, vacuum filtration apparatus, glass fiber filters (47 mm, 0.45 µm), drying oven, analytical balance (0.1 mg resolution), Standard Methods for the Examination of Water and Wastewater (APHA), graph paper or spreadsheet
Check: TSS result within ±5 mg/L of instructor-prepared sample of known concentration; turbidity reading within turbidimeter's stated accuracy; written filter maintenance recommendation includes specific TSS trigger value; pass = all three criteria met.

Practice check8

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

Water Quality Monitoring
1. A turbidimeter measures water quality by detecting:
Laboratory Procedures
2. Which drying temperature and duration are specified in the Standard Methods gravimetric TSS procedure?
Water Quality Monitoring
3. A 750 mL effluent sample is filtered; the filter gains 30.0 mg of solids after drying. What is the TSS in mg/L?
Water Quality Monitoring
4. When comparing turbidity measured with a turbidimeter to a Secchi disk reading taken in the same pond, the Secchi disk method is limited because it:
Aquaculture Systems Management
5. A technician notices that effluent TSS after a drum filter has risen sharply over the last four hours even though feed rate has not changed. The most likely explanation is:
Workplace Safety
6. Which personal protective equipment is most important when collecting and filtering aquaculture effluent samples in the laboratory?
Aquaculture Systems Management
7. A technician plots hourly effluent TSS data from a settling basin and identifies a sudden upward inflection in the data trend. The correct next action is to:
Laboratory Safety
8. Before placing a loaded glass fiber filter back into the drying oven, a technician should verify that the filter is on:
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