GB National Standard Microbiological Testing of Water Quality: Methods, Parameters and Applications

GB national standard microbiological testing of water quality is the core procedure for verifying the hygienic safety of drinking water, packaged water, recreational water, and wastewater in China. This article explains the governing standards such as GB 5749 and GB/T 5750, the key indicator organisms, the mainstream culture-based and rapid detection methods, and typical application scenarios. Whether you manage a water utility, a beverage production line, or require testing by accredited third-party laboratories, this guide summarizes the essential technical points of national standard water microbiology testing.

Technical Overview

National standard (GB) microbiological testing of water quality refers to the detection and enumeration of microorganisms in water samples using methods and limits established by Chinese national standards. Because most waterborne pathogens are difficult and slow to culture directly, testing relies on indicator organisms, principally total coliforms, Escherichia coli, thermotolerant (fecal) coliforms, and the heterotrophic plate count, to evaluate fecal contamination and the overall performance of treatment and disinfection processes.

The national framework consists of product standards that define hygienic limits and method standards that prescribe how samples are collected, cultured, and reported. The main references are summarized below:

• GB 5749-2022, Standards for Drinking Water Quality, which sets the hygienic limits for municipal and centralized drinking water supplies.
• GB/T 5750-2023 series, Standard Examination Methods for Drinking Water, with a dedicated part covering microbiological indicators such as total coliforms, E. coli, and total plate count.
• GB 8537-2018 and GB 8538-2022, which cover the product requirements and examination methods for drinking natural mineral water.
• GB 19298-2014, the national food safety standard for packaged (bottled and barreled) drinking water, including limits for total coliforms and Pseudomonas aeruginosa.
• HJ 347.1-2018 and HJ 347.2-2018, environmental protection industry standards for the determination of fecal coliforms in surface water and wastewater by membrane filtration, multiple-tube fermentation, and enzyme substrate methods.
• GB 8978-1996 and GB/T 19923-2005, which govern integrated wastewater discharge and reclaimed water reuse, respectively.
• International equivalents such as ISO 9308-1, ISO 9308-2, ISO 6222, ISO 11731, and ISO 19458, which are widely used for comparison, export, and method validation purposes.

Results are expressed as colony-forming units (CFU) per 100 mL for filterable methods or as the most probable number (MPN) per 100 mL for statistical multi-tube and multi-well methods. For drinking water, the hygienic target for indicator bacteria is absence in a defined sample volume, which is why rigorous aseptic sampling and laboratory quality control are essential.

Testing Samples

National standard microbiological methods are applicable to virtually all water categories, each with its own regulatory driver and sampling considerations.

Municipal and Piped Drinking Water

Treated water at treatment plant outlets, distribution network points, and consumer taps is tested for total coliforms, E. coli, total plate count, and turbidity-related microbiological stability. Utilities must monitor indicator bacteria at frequencies proportional to population served.

Packaged Drinking Water and Natural Mineral Water

Bottled purified water, mineral water, and other packaged drinking water products are tested under GB 19298 and GB 8537, typically with five-unit sampling plans (n=5) in which indicator organisms must be absent. Pseudomonas aeruginosa is a product-specific requirement for these categories.

Secondary Water Supply Systems

Rooftop tanks, basement storage, and booster systems in residential and commercial buildings are common recontamination points. Periodic testing of these systems verifies tank cleaning, disinfection residuals, and sealing integrity.

Groundwater and Private Wells

Untreated groundwater sources are tested to confirm whether disinfection is required and to detect fecal intrusion from septic systems, livestock, or surface runoff.

Surface Water and Recreational Water

Rivers, lakes, reservoirs, swimming pools, and spa water are monitored for fecal coliforms, E. coli, and Enterococci to classify water quality and protect bathers. Cooling tower and decorative fountain water may additionally require Legionella testing.

Wastewater and Reclaimed Water

Discharge compliance under GB 8978 and reuse compliance under GB/T 19923 rely heavily on fecal coliform enumeration, particularly after chlorination or UV disinfection of treated effluent.

Healthcare and Industrial Water Systems

Hospitals test hemodialysis water, dental unit waterlines, and central sterile supply water, while food and pharmaceutical plants test process water, purified water, and clean-in-place rinse water for microbiological control.

Testing Items

The core microbiological parameters under the national standards, together with their typical hygienic significance, are as follows:

• Total coliforms: the primary indicator of water hygiene; must not be detected in 100 mL of drinking water.
• Escherichia coli: definitive evidence of recent fecal contamination; must not be detected in 100 mL of drinking water.
• Thermotolerant (fecal) coliforms: a sub-group confirmed at 44.5 degrees Celsius, used for wastewater discharge and surface water classification.
• Total plate count (heterotrophic plate count): a general measure of microbial load and treatment efficiency, limited to 100 CFU/mL for centralized drinking water.
• Enterococci and fecal streptococci: supplementary fecal indicators that survive longer in the environment and validate coliform results.
• Pseudomonas aeruginosa: a hygiene indicator specific to packaged water and recreational water; absence is required.
• Legionella pneumophila: tested in cooling towers, hot water systems, and aerosol-generating equipment, generally by ISO 11731 culture or quantitative PCR screening.
• Giardia and Cryptosporidium: protozoan parasites addressed as extended indicators for source water risk assessment under GB 5749-2022.

Testing Methods

National standard microbiology is predominantly culture-based, complemented by rapid defined-substrate techniques. The choice of method depends on sample turbidity, expected bacterial density, required turnaround time, and reporting format.

Multiple-Tube Fermentation (MPN Method)

The classical three-stage procedure uses lauryl sulfate tryptose broth for the presumptive test, brilliant green lactose bile broth for confirmation of total coliforms, and EC broth incubated at 44.5 degrees Celsius for thermotolerant coliforms and E. coli. A standard five-tube or fifteen-tube dilution series is inoculated, and gas-positive tubes are converted to an MPN index from statistical tables. The method tolerates turbid samples and low bacterial densities but requires 48 to 96 hours for completion.

Membrane Filtration

A measured volume, typically 100 mL, is filtered through a 0.45 micrometer sterile membrane that retains bacteria. The membrane is transferred to a selective medium such as m-Endo agar or a chromogenic coliform medium and incubated at 36 plus or minus 1 degree Celsius for 18 to 24 hours. Typical metallic-sheen or distinctly colored colonies are counted and confirmed biochemically. Membrane filtration provides direct CFU/100 mL results and is the preferred method for treated, low-turbidity water.

Enzyme Substrate Method

Defined-substrate technology uses chromogenic and fluorogenic reagents such as ONPG and MUG that are cleaved by coliform and E. coli enzymes. After 18 to 24 hours of incubation, color change indicates total coliforms and fluorescence under ultraviolet light indicates E. coli. Results can be reported qualitatively (presence/absence) or quantitatively using multi-well trays that yield an MPN value. This method is included in GB/T 5750 and HJ 347.2 because of its speed, minimal confirmation workload, and ease of use in field laboratories.

Heterotrophic Plate Count

Plate count agar pour plates or spread plates are incubated at 36 plus or minus 1 degree Celsius for 48 hours, and colonies are reported as CFU/mL. For purified and high-quality process waters, low-nutrient media such as R2A with longer incubation per ISO 6222 may be used to recover slow-growing organisms.

Sampling, Transport and Quality Control

Microbiological results are only as reliable as the sample. Sampling must use sterile containers; for chlorinated water, sterile sodium thiosulfate must be added to neutralize residual disinfectant at the moment of sampling. Samples should be held at 2 to 8 degrees Celsius, protected from light, and analyzed as soon as possible, normally within 24 hours. Laboratories maintain media quality control, positive and negative control strains, sterile blanks, incubator and autoclave validation, and participate in interlaboratory proficiency testing in accordance with accreditation requirements.

Applications

National standard microbiological testing serves a wide range of compliance, quality, and investigative purposes across industries:

• Compliance monitoring by water utilities and government hygiene surveillance programs.
• Commissioning and periodic verification of new pipelines, secondary supply systems, and storage tanks.
• Factory quality control and batch release testing for bottled water and beverage producers.
• Infection control water sampling in hospitals, dialysis centers, and dental clinics.
• Swimming pool, spa, and public bathing water management.
• Cooling tower and building water system safety programs targeting Legionella control.
• Environmental impact assessment, pollution source tracing, and wastewater discharge compliance.
• Independent verification and dispute resolution testing performed by accredited third-party laboratories.
• Export documentation where buyers require testing aligned with ISO or GB methods.

FAQ

What is the hygiene limit for drinking water under the national standards?

For centralized drinking water, total coliforms, E. coli, and thermotolerant coliforms must not be detected in 100 mL, and the total plate count must not exceed 100 CFU/mL. Packaged water categories apply similar absence criteria under their respective product standards.

How long does microbiological testing take?

Coliform and E. coli results by enzyme substrate or membrane filtration methods are typically available in 24 to 48 hours, while the multi-tube MPN confirmation workflow takes 48 to 96 hours. Heterotrophic plate counts require 48 hours, and Legionella culture may take 7 to 14 days due to slow colony development.

Which method is more accurate: MPN, membrane filtration, or enzyme substrate?

All three are recognized national standard methods when performed correctly. Membrane filtration gives direct counts best suited to low-turbidity water; the MPN method handles turbid samples and low densities; enzyme substrate methods combine speed with quantitative MPN output. The laboratory selects the method based on sample type and reporting requirements.

Can we collect samples ourselves?

Self-sampling is possible for internal monitoring, but the sample collector must be trained in aseptic technique, use sterile containers with sodium thiosulfate for disinfected water, and deliver samples chilled within the holding time. For regulatory or certification purposes, sampling by trained personnel from accredited third-party laboratories is strongly recommended.

What is the difference between total coliforms and E. coli?

Total coliforms are a broader group of bacteria found in environmental water, soil, and vegetation as well as in feces, so they indicate general hygienic breakdown. E. coli is found specifically in the intestinal tract of humans and warm-blooded animals, so its detection is direct evidence of fecal contamination and a substantially higher health risk.

How often should water systems be tested?

Monitoring frequency depends on the regulation and system type: utilities sample proportionally to the population served, secondary supply systems are typically tested every six months or after cleaning, bottled water producers test each production batch, and cooling towers are tested at least quarterly or according to the building water safety plan.