Upgrading and retrofitting testing for municipal wastewater treatment plants provides the analytical evidence needed to confirm that modified processes reliably meet stricter discharge limits such as Class IA of GB 18918-2002 or even more demanding local standards. This technical article explains the scope of testing samples, key parameters, recognized testing methods, and typical applications throughout an upgrade project. Engineers, plant operators, and project owners can use this guide to plan commissioning verification, performance acceptance, and long-term compliance monitoring.
Technical Overview
Many municipal wastewater treatment plants originally designed to Class IB or Class II discharge limits must now be upgraded to Class IA requirements, or to local standards approaching Class IV surface water quality, in order to protect sensitive receiving water bodies. A retrofit project typically combines capacity expansion, enhanced biological nutrient removal (modified A2O processes, MBBR, IFAS, or membrane bioreactors), advanced tertiary treatment (coagulation, deep-bed filtration, ozone oxidation, activated carbon adsorption), disinfection upgrades, and improved sludge handling such as thickening, dewatering, anaerobic digestion, or thermal drying.
Testing is the objective basis for every phase of such a project. Before design, baseline characterization of influent quality defines the treatment targets. During commissioning, performance testing verifies that each process unit achieves its design removal efficiency. After completion, acceptance testing and compliance monitoring demonstrate conformity with the applicable discharge standard, support pollutant discharge permit reporting, and provide evidence for environmental protection acceptance. Accredited third-party laboratories operating under ISO/IEC 17025 quality systems carry out this work using nationally and internationally recognized methods.
The overall scope of an upgrading and retrofitting testing program generally covers influent and effluent water, intermediate process streams, sludge at multiple treatment stages, biogas and odor emissions, verification of online monitoring instruments, and evaluation of unit energy consumption. Together, these results allow the plant owner to quantify the actual performance gain delivered by the retrofit.
Testing Samples
Influent and Effluent Water Samples
Composite influent samples are collected at the plant inlet or after screening and grit removal to establish the pollution load entering the facility. Effluent samples are taken after the final disinfection unit at the outfall, using time-proportional or flow-proportional composites over a 24-hour cycle in accordance with HJ 91.1 and the HJ 493 series for water quality sampling. Grab samples are additionally collected for parameters unstable during compositing, such as pH, temperature, oil and grease, and sulfide.
Intermediate Process Streams
Retrofit verification requires sampling along the treatment train, including mixed liquor from anaerobic, anoxic, and aerobic zones, secondary clarifier effluent, filter or membrane influent and permeate, return activated sludge, internal recycle flows, and sidestreams from sludge dewatering. These samples are used to calculate stage-by-stage removal efficiencies for COD, nitrogen, and phosphorus, and to diagnose whether the retrofitted biological process is properly balanced.
Sludge and Biomass Samples
Sludge testing covers mixed liquor suspended solids, return and waste activated sludge, thickened sludge, digested sludge, and dewatered cake. Characterization includes dewaterability indices and, where disposal routes such as land application, co-incineration, or use as construction material are planned, contaminant and nutrient profiling in line with GB 24188 and sludge determination methods in the CJ/T 221 series.
Biogas, Odor, and Exhaust Samples
Where anaerobic digestion is part of the retrofit, biogas is analyzed for methane, carbon dioxide, hydrogen sulfide, and oxygen content to confirm energy recovery value. Air and odor samples are collected at screening rooms, grit chambers, sludge dewatering areas, and deodorization system outlets for hydrogen sulfide, ammonia, and odor concentration assessment against GB 14554 emission limits.
Reclaimed Water Samples
For plants delivering reclaimed water after the upgrade, samples from the reuse supply line are tested against the applicable reuse standards, such as GB/T 18920 for urban miscellaneous use, GB/T 18921 for scenic environmental use, and GB/T 19923 for industrial reuse, covering turbidity, pathogens, and residual pollutants.
Testing Items
The core parameter list for Class IA acceptance under GB 18918-2002 includes COD, BOD5, suspended solids, ammonia nitrogen, total nitrogen, total phosphorus, pH, and fecal coliform, with typical Class IA limits of 50 mg/L for COD, 10 mg/L for BOD5, 10 mg/L for suspended solids, 5(8) mg/L for ammonia nitrogen, 15 mg/L for total nitrogen, 0.5 mg/L for total phosphorus, and 1000 MPN/L for fecal coliform. In addition, first-class toxic pollutants such as total mercury, total cadmium, hexavalent chromium, total arsenic, and total lead are monitored at the outfall.
- Physicochemical parameters: pH, temperature, color, turbidity, total dissolved solids, conductivity, oil and grease.
- Organic load indicators: COD, BOD5, TOC, and the BOD/COD ratio for biodegradability assessment.
- Nutrient parameters: ammonia nitrogen, nitrate, nitrite, total nitrogen, total phosphorus, and total Kjeldahl nitrogen.
- Microbiological parameters: fecal coliform, total bacteria, and parasite eggs where reuse or land application applies.
- Process and biomass indicators: MLSS, MLVSS, 30-minute settling volume, sludge volume index, sludge age verification, specific oxygen uptake rate, nitrification and denitrification rates, and filamentous organism microscopy.
- Sludge characteristics: moisture content, organic matter and volatile solids, pH, calorific value, heavy metals, total nitrogen, phosphorus, and potassium, dewaterability indices such as capillary suction time and specific resistance to filtration.
- Gas parameters: methane, hydrogen sulfide, ammonia, odor concentration, and greenhouse gas components where carbon accounting is required.
- Instrument verification: parallel laboratory analysis for calibration and comparison of online COD, ammonia, total phosphorus, total nitrogen, pH, and flow analyzers.
Testing Methods
Sampling is performed according to HJ 91.1 for sewage monitoring technical regulation and the HJ 493, HJ 494, and HJ 495 series for sampling program design and technique, complemented by the ISO 5667 series for water quality sampling. Samples are preserved, transported, and documented under full chain-of-custody procedures, with field blanks, duplicates, and spiked recoveries as quality control measures.
- COD: potassium dichromate digestion with titrimetric or photometric finishing per HJ 828, ISO 6060, or ASTM D1252.
- BOD5: dilution and seeding method per HJ 505 or ISO 5815-1.
- Ammonia nitrogen: Nessler or salicylate spectrophotometry per HJ 535 and HJ 536, or continuous flow analysis per HJ 637.
- Total nitrogen: alkaline potassium persulfate digestion with ultraviolet spectrophotometry per HJ 636.
- Total phosphorus: ammonium molybdate spectrophotometry per GB 11893.
- Suspended solids: gravimetric filtration through 0.45 micrometer membranes per GB 11901 or ASTM D5907.
- pH: glass electrode method per GB 6920, ISO 10523, or ASTM D1293.
- Turbidity: formazine methods per ISO 7027-1 or equivalent national methods.
- Fecal coliform: membrane filtration or enzyme substrate techniques per HJ 347.1.
- Metals and metalloids: ICP-MS or ICP-OES per HJ 700, and atomic fluorescence spectrometry for mercury and arsenic per HJ 694.
- Anions: ion chromatography per HJ 84.
- Sludge parameters: digestion and instrumental methods per the CJ/T 221 series.
Performance acceptance testing is normally conducted as a continuous 72-hour or longer test at no less than 75 percent of design hydraulic and pollutant load, with time-weighted and flow-weighted daily composites. Removal efficiency, effluent stability, disinfection performance, and specific energy consumption in kWh per cubic meter and kWh per kilogram of COD removed are calculated to close out the retrofit contract. Online monitoring systems are separately validated through installation, acceptance, operation, and calibration checks in accordance with the HJ 353, HJ 354, HJ 355, and HJ 356 series.
Applications
Upgrading and retrofitting testing is applied wherever a municipal plant must demonstrate improved performance to regulators, financing institutions, or project contractors. Typical scenarios include:
- Acceptance of upgrades from Class IB or Class II to Class IA discharge limits under GB 18918-2002.
- Compliance demonstration against stricter local standards referencing water quality classes of GB 3838.
- Commissioning verification of new or retrofitted processes such as MBR, MBBR, IFAS, moving-bed biofilm retrofit of existing aeration basins, ozone oxidation, biological aerated filters, and advanced phosphorus removal.
- Reclaimed water project qualification against GB/T 18920, GB/T 18921, and GB/T 19923 reuse standards.
- Pollutant discharge permit self-monitoring and reporting, and completion environmental protection acceptance for the retrofit works.
- Sludge disposal route qualification for land application, co-incineration, cement kiln co-processing, and building material utilization.
- Odor control and deodorization system performance evaluation against GB 14554.
- Energy audits and carbon emission benchmarking to quantify the sustainability benefit of the retrofit.
FAQ
What is the main difference between Class IB and Class IA testing requirements?
Class IA imposes significantly stricter limits, for example COD of 50 mg/L instead of 60 mg/L, suspended solids of 10 mg/L instead of 20 mg/L, total nitrogen of 15 mg/L instead of 20 mg/L, and total phosphorus of 0.5 mg/L instead of 1 mg/L. The testing frequency, detection limits, and quality control requirements are therefore more demanding, and tertiary treatment units must be verified in addition to the biological stage.
How long does performance acceptance testing take after a retrofit?
Most contracts require a continuous performance test of at least 72 hours at design load once the biological system has stabilized, often following a two-to-four-week acclimation period. Extended monitoring over several weeks may be required to demonstrate reliable nitrogen and phosphorus removal across seasonal temperature variations.
Can online monitoring data be used directly for acceptance?
Online analyzers can support the record only after they have passed system acceptance, and continue to pass periodic calibration and comparison against laboratory reference methods in line with the HJ 354, HJ 355, and HJ 356 series. Laboratory analysis by an accredited third-party laboratory remains the decisive evidence for final acceptance.
Is historical operating data sufficient to demonstrate retrofit performance?
No. Because the retrofit changes the process configuration, load distribution, and hydraulics, a new testing baseline is required after modification. Historical data is useful only for comparison and for quantifying the improvement achieved.
Who should perform the testing?
Sampling and analysis should be entrusted to accredited third-party laboratories holding appropriate national metrology accreditation or ISO/IEC 17025 laboratory accreditation, ensuring that reported results carry legal validity for permits, acceptance, and regulatory reporting.
