The effluent quality of the hotel's reclaimed water reuse system is unstable, with occasional odors and yellowing.

The effluent quality of the hotel's reclaimed water reuse system is unstable, with occasional odors and yellowing.

As a hotel MEP expert, in response to your question—"The effluent quality of the hotel's reclaimed water reuse system is unstable, with frequent odors and yellowing"—I will conduct a systematic, point-by-point detailed analysis based on national standards such as GB/T 18920-2020 "Urban Sewage Recycling—Water Quality Standard for Urban Miscellaneous Water Consumption", GB 50015-2019 "Standard for Design of Building Water Supply and Drainage", and GB 50336-2018 "Code for Design of Building Reclaimed Water", combined with years of practical hotel engineering management and operations cases. The complete answer is as follows:

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I. Diagnosis of Core Causes (Based on GB Standards and Engineering Mechanisms)

Odors and yellowing in reclaimed water effluent are typical combined manifestations of imbalance in biochemical treatment and disinfection by-products/secondary pollution in pipelines. According to Article 6.2 of GB 50336-2018 and Table 1 of GB/T 18920-2020, the key indicators of reclaimed water quality include: chromaticity ≤15 degrees, no unpleasant odor, turbidity ≤5 NTU, residual chlorine ≥0.2 mg/L (at the end of the pipe network), pH 6.0–9.0. Abnormalities are usually caused by the following four major categories of causes:

1. Failure of the biochemical treatment system (most common)

• Insufficient dissolved oxygen in the aeration tank (DO<1 mg/L), leading to the death of aerobic bacteria and the proliferation of anaerobic bacteria, producing hydrogen sulfide (rotten-egg odor) and ammonia (pungent odor).
• Sludge age too long or too short, leading to sludge disintegration or bulking, with the effluent carrying large amounts of microbial metabolites (yellowing, turbidity).
• Article 6.3.3 of GB 50336-2018 explicitly requires that "biological treatment should ensure sufficient retention time and dissolved oxygen."

• Insufficient dosage of disinfectant (sodium hypochlorite/ultraviolet) or insufficient contact time, leading to the regeneration of bacteria and algae and causing biological odors.
• Excessive chlorine dosing reacts with organic matter in the water to form chlorophenols and chloramines (strong medicinal odor), and high residual chlorine can make the water pale yellow.
• Table 1 of GB/T 18920-2020 specifies "total residual chlorine ≥0.2 mg/L (after 30 min contact), and ≥0.05 mg/L at the end of the pipe network."

• Raw water (such as bathing wastewater and laundry wastewater) contains large amounts of surfactants, grease, and hair, which cannot be removed by screens + equalization tanks alone, resulting in excessively high subsequent biochemical loads.
• Lack of activated carbon adsorption or membrane filtration (such as MBR, UF), so fine suspended solids, pigments, and humic acids cannot be removed, and the effluent turns yellow.
• Article 5.3.2 of GB 50336-2018 requires that "reclaimed water treatment should be provided with pretreatment, main treatment, and advanced treatment units."

• The reclaimed water pipe network has not been cleaned for a long time, and biofilm on the pipe walls has sloughed off; the storage tank lacks an anti-pollution partition or is not disinfected regularly, and the mixture of algae and rust leads to yellow water and musty odor.
• Article 4.9.8 of GB 50015-2019 specifies that "reclaimed water pipelines should be provided as an independent system, strictly prohibited from being connected to domestic water supply pipelines, and should be provided with flushing drain outlets."

II. Comparative Analysis of an Actual Case (A Real Retrofit of a Five-Star Hotel)

Case Background:

• Effluent chromaticity: 30–40 degrees (standard ≤15 degrees), showing obvious yellowish-brown.
• Odor: Strong musty and fishy odors (hydrogen sulfide measured at 0.8 mg/L).
• Residual chlorine: 0 mg/L measured at the end of the pipe network.

Diagnosis Process (Item-by-Item Investigation per GB Standards):

Retrofit Measures (Implemented Step by Step, with Standard References):

Post-Retrofit Results:

• Effluent chromaticity: 8–12 degrees (no color perception)
• Odor: no unpleasant sensation (odor threshold below the standard)
• Residual chlorine: stable at 0.2–0.3 mg/L at the end of the pipe network
• Turbidity: <3 NTU
• Operating cost increased by about 0.15 yuan/m³ (mainly from activated carbon replacement and increased disinfectant)

III. Systematic Solution (Detailed Point by Point, Including Parameters and Cycles)

1. Immediate Investigation and Emergency Handling

• Testing items (according to Table 1 of GB/T 18920-2020): chromaticity, odor, turbidity, residual chlorine, pH, BOD₅, NH₃-N, total coliforms.
• Emergency measures: If residual chlorine is 0, immediately manually dose sodium hypochlorite into the contact tank (at 10 mg/L available chlorine, 30 min contact); if the effluent is yellow, suspend reuse, drain the storage tank, and flush at high pressure.

2. Process Optimization (Specific Parameter Recommendations)

3. Strengthened O&M Management

• Daily mandatory checks: Aeration tank DO, effluent residual chlorine, pH; measure color and odor once a week.
• Regular cleaning: Sand filter backwash cycle ≤24h; replace or regenerate activated carbon every 6~12 months; perform air-water flushing of the reclaimed water pipe network every six months (flow velocity ≥1.5m/s).
• Biofilm control: Thoroughly clean and disinfect the storage tank quarterly (soak in 100mg/L sodium hypochlorite for 2h).
• Establish records: Record daily influent volume, effluent quality, chemical consumption, and energy consumption, and compile monthly compliance rate statistics against GB/T 18920-2020.

4. Preventive Retrofit Recommendations (for Long-term Stability)

• Add online monitoring equipment: Turbidity meter, residual chlorine analyzer, pH meter (linked with alarms and automatic dosing).
• Optimize raw water collection: Pre-treat kitchen oily wastewater and laundry high-COD wastewater separately (oil separation + air flotation) to avoid shock loading on the biological system. Per GB 50336-2018, Clause 4.3.2.
• Adopt shock-resistant process: If raw water quality fluctuates greatly (e.g., peak and off-peak tourist seasons), it is recommended to upgrade to an MBR membrane bioreactor (effluent SS≤1mg/L, extremely low color), but note the membrane cleaning costs.

IV. Summary and Key Reminders

1. Root cause: Your system problem is most likely a triple compounding of "oxygen deficiency in the biological system + disinfection failure + insufficient advanced treatment," rather than a single link.
2. Regulatory red line: Reclaimed water effluent must meet all indicators of GB/T 18920-2020; in particular, color ≤15 degrees and no unpleasant odor are basic sensory requirements, otherwise it is judged non-compliant and must not be reused.
3. Actual cost: Solving the odor and yellowing problems usually requires an investment of about RMB 50,000~150,000 (depending on system scale), but can save 30%~50% in water charges annually (based on a hotel monthly water consumption of 5,000 m³), and avoids brand damage and environmental penalties caused by water quality complaints.
4. Immediate action: It is recommended that you commission a qualified third-party testing agency to conduct a full-indicator analysis of influent and effluent (including BOD₅, NH₃-N, total phosphorus, iron and manganese), then implement the above retrofit plan step by step. Do not merely increase disinfectant dosage to mask the problem, otherwise it will accelerate pipe corrosion and produce carcinogenic by-products (such as trihalomethanes).

If you can provide the process flow diagram, design treatment capacity, and actual influent water quality of the current system, I can further provide targeted parameter adjustment recommendations.