Determination of Suspended Solids in Circulating Water by Membrane Filtration: Principles, Procedure

Suspended solids determination by membrane filtration is the standard gravimetric method for quantifying particulate contamination in industrial circulating and cooling water systems. This technical article explains the testing principle, sample types, operating procedure, calculation, quality control requirements, and typical applications in accordance with recognized standards such as GB/T 11901, ISO 11923, and ASTM D5907. Engineers, water treatment specialists, and laboratory personnel can use this guide to control fouling, corrosion, and heat-transfer efficiency risks in recirculating water circuits.

Technical Overview

Suspended solids (SS), also referred to as total suspended solids (TSS), are the undissolved particulate matter retained on a filter when a water sample is passed through it. In recirculating cooling water systems, suspended solids originate from make-up water impurities, airborne dust and debris ingested at cooling towers, corrosion products, scale fragments, biological flocs, and precipitated treatment chemicals. Because these particles deposit on heat exchanger surfaces, tube bundles, and tower fill, they directly accelerate under-deposit corrosion, reduce heat transfer efficiency, and increase microbiological fouling.

The membrane filtration method is the internationally accepted gravimetric technique for determining suspended solids. A well-mixed, measured volume of sample is filtered through a pre-conditioned and pre-weighed membrane filter with a nominal pore size of 0.45 μm. The filter with its retained residue is dried at 103–105 °C to constant weight, cooled in a desiccator, and reweighed. The increase in mass, divided by the filtered volume, yields the suspended solids concentration in mg/L.

The method is aligned with several recognized standards, including GB/T 11901 (Water Quality—Determination of Suspended Substance—Gravimetric Method), ISO 11923 (Water Quality—Determination of Suspended Solids by Filtration Through Glass-Fibre Filters), and ASTM D5907 (Standard Test Methods for Filterable and Nonfilterable Matter in Water). For industrial recirculating cooling water management, design and operation codes such as GB/T 50050 typically require circulating water suspended solids to be controlled at low levels, commonly below 10–20 mg/L depending on the heat exchanger configuration, with stricter limits applied to make-up water.

Testing Samples

Circulating Cooling Water

The primary sample matrix is water drawn from the recirculating loop of open or closed cooling systems, including cooling tower basins, hot water return lines, and cold water supply headers. Samples must be collected at representative points with sufficient flow turbulence and homogenized immediately before filtration, since suspended particles settle rapidly.

Make-Up and Replenishment Water

Make-up water from municipal supplies, wells, surface water sources, or reclaimed water is tested to evaluate the particulate load entering the system. This establishes baseline data for mass-balance calculations and validates pretreatment performance such as coagulation, sedimentation, and multimedia filtration.

Side-Stream Filter Inlet and Outlet Water

Paired samples taken upstream and downstream of side-stream filters, sand filters, or disc filters quantify filtration removal efficiency and support backwash cycle optimization.

Blowdown and System Drain Water

Blowdown samples are analyzed to verify compliance with discharge permit limits for suspended solids and to confirm that concentration cycles are being managed within the designed range.

Testing Items

The core determination is the suspended solids concentration, expressed in mg/L, covering the following measurable parameters:

• Total suspended solids (TSS/SS) retained on a 0.45 μm membrane filter after drying at 103–105 °C
• Filterable (dissolved) residue, determined from the filtrate by evaporation, when a complete solids profile is required
• Total residue (total solids), calculated as the sum of suspended and dissolved fractions
• Volatile and fixed (ignitable residue) fractions, optionally determined by igniting the dried residue at 550 °C to estimate the organic and inorganic proportions of the suspended matter

Testing Methods

Principle

A known volume of homogenized sample is vacuum-filtered through a membrane filter of defined pore size and known constant weight. Particles larger than the pore size are retained on the membrane. After drying to constant mass at 103–105 °C, the mass difference between the loaded filter and the blank filter represents the suspended solids in the filtered volume.

Apparatus and Materials

• Membrane filters, 0.45 μm nominal pore size, 47–50 mm diameter, mixed cellulose ester or equivalent material (glass-fibre filters are acceptable under ISO 11923)
• Membrane filter holder and vacuum filtration assembly with receiving flask
• Forced-draft or gravity convection drying oven capable of maintaining 103 ± 2 °C to 105 ± 2 °C
• Analytical balance with 0.1 mg readability
• Desiccator with indicating silica gel, and flat-tipped forceps
• Graduated cylinders or wide-bore volumetric pipettes for sample measurement

Procedure

The determination follows a controlled gravimetric sequence to minimize systematic error:

• Condition each membrane by rinsing with distilled or deionized water, dry at 103–105 °C for one hour, cool in the desiccator for 30 minutes, and weigh. Repeat the dry-cool-weigh cycle until successive masses differ by no more than 0.4 mg. Record this as the initial filter mass.
• Homogenize the sample by vigorous agitation; avoid introducing air bubbles. Measure a sample volume (typically 100–500 mL for circulating water) selected so that the collected residue falls within an optimal range of approximately 10–50 mg for balance accuracy.
• Filter the measured volume under moderate vacuum. As filtration nears completion, rinse the measuring cylinder, filter walls, and residue with at least three 10 mL portions of distilled water to remove entrained dissolved salts that would otherwise bias the result high.
• Place the loaded filter in the oven at 103–105 °C, dry for at least one hour, cool in the desiccator, and weigh. Repeat drying cycles until the difference between consecutive weighings is no more than 0.4 mg.
• Run reagent blanks and duplicate filters with each batch to verify membrane and rinse-water cleanliness and to assess precision.

Calculation

The suspended solids concentration is calculated as: C (mg/L) = (m₂ − m₁) × 10⁶ / V, where m₁ is the constant mass of the blank filter in grams, m₂ is the constant mass of the filter plus dried residue in grams, and V is the filtered sample volume in milliliters. Report results to two significant figures for concentrations below 100 mg/L.

Quality Control and Interferences

• Positive bias results from residual dissolved solids on the filter; thorough rinsing of the residue is essential, particularly for high-concentration circulating water with elevated dissolved salts.
• Negative bias or poor precision results from inhomogeneous samples, particle settling in the container, or solids adhering to container walls; agitate continuously while subsampling.
• Acid preservation must not be used, as it dissolves carbonates, hydroxides, and some metal particulates.
• Analyze samples promptly, preferably within 24 hours; if delayed, store at 4 °C without freezing and analyze within 7 days.
• Acceptable duplicate precision for circulating water is typically a relative deviation below 10%; matrix spikes or replicate filtration verify method control.

Applications

Membrane filtration suspended solids data support the operation and optimization of recirculating water systems across all major industries:

• Power generation: monitoring condenser cooling loops, cooling tower basins, and closed auxiliary cooling circuits to protect condenser tubing and maintain design heat rejection rates
• Petrochemical and chemical processing: controlling particulate load in cooling water supplying shell-and-tube and plate heat exchangers, where deposition directly causes under-deposit corrosion and energy losses
• Steel and metallurgical plants: tracking suspended solids in indirect cooling circuits and process water recirculation systems exposed to mill scale and oxide fines
• Commercial HVAC and district cooling: routine verification of tower water quality, side-stream filter performance, and make-up water suitability
• Water treatment program management: evaluating the effectiveness of flocculants, dispersants, and side-stream filtration; setting blowdown schedules and concentration cycles; and demonstrating compliance with internal control limits and discharge requirements
• Pretreatment validation: confirming that suspended solids levels are compatible with downstream ion exchange, ultrafiltration, or reverse osmosis protection targets

FAQ

Why is a 0.45 μm pore size specified?

A 0.45 μm membrane is the internationally adopted convention defining the operational boundary between suspended and dissolved matter. It provides reproducible retention of particles that contribute to deposition and fouling, and it harmonizes results with GB/T 11901 and equivalent international standards.

What is the typical detection limit of the method?

With a 0.1 mg readability balance and a 500 mL sample volume, practical reporting limits of approximately 1–2 mg/L are achievable. Filtering a larger volume improves sensitivity, provided the membrane does not clog and the residue mass remains within the recommended range.

How does suspended solids differ from turbidity?

Turbidity is an optical, light-scattering measurement affected by particle size, shape, and color, whereas suspended solids is a direct gravimetric mass measurement. The two parameters correlate loosely within a stable system but not universally; gravimetric SS is required for regulatory and contractual compliance.

How long can samples be held before analysis?

Analysis is best performed within 24 hours of collection. When immediate testing is impractical, samples should be refrigerated at 4 °C, kept sealed, homogenized thoroughly before subsampling, and analyzed within 7 days.

How frequently should circulating water be tested?

Most industrial monitoring programs test circulating water suspended solids daily to weekly depending on system criticality, with additional testing after make-up water source changes, treatment chemical adjustments, tower cleaning events, or filtration upsets. Trends over time are more diagnostic than single results.

Can high dissolved solids in concentrated circulating water affect the results?

Yes. At high concentration cycles, entrained dissolved salts retained in the filter cake can inflate results significantly. Extensive rinsing of the residue with distilled water, plus consistent blank correction, is mandatory for accurate determination in high-TDS matrices.