Application of Modified AAO Process in Coal Mine Domestic Wastewater Treatment
Domestic wastewater in coal mines primarily originates from staff canteens, dormitories, offices, laundry rooms, and bathrooms, with bathing drainage accounting for over 55% of the total volume. The discharge of bathing water is relatively concentrated, leading to significant flow fluctuations. Characterized by lower organic concentration and higher suspended solids (SS), bathing drainage differs considerably from typical domestic sewage. Its staggered discharge pattern with other wastewater streams contributes to substantial water quality variability.
Most coal mines in China are located in remote areas where sludge transportation costs are high. Therefore, treatment processes with lower sludge yield should be selected. As mines develop and staff numbers increase, wastewater flow often exceeds original design capacity, necessitating processes with strong adaptability to Changes in water quality and quantity within the same footprint. Under increasingly stringent environmental policies requiring full reuse of treated effluent with zero discharge, processes must deliver high and stable effluent quality.
Currently, the AAO (Anaerobic-Anoxic-Oxic) process is the preferred choice in municipal wastewater treatment. This article analyzes the application effectiveness of a Modified AAO Process (AAO + Suspended Carrier Process) for coal mine domestic wastewater, based on its unique characteristics.
1. Modified AAO Process
The AAO process is the simplest flow configuration for simultaneous nitrogen and phosphorus removal. Filamentous bacteria cannot proliferate extensively under alternating anaerobic, anoxic, and aerobic conditions, preventing sludge bulking. It requires no chemical addition, only mild mixing in anaerobic and anoxic tanks, resulting in low operating costs. The sludge has high phosphorus content, giving it good fertilizer value.
However, nitrogen removal and phosphorus removal in the AAO process are interdependent and often conflicting. Nitrifying bacteria require a long sludge age, while phosphorus removal needs a short sludge age. Limited by the sludge age required for simultaneous脱氮, enhancing phosphorus removal, especially in low-carbon wastewater, is challenging. Denitrification efficiency relates to the internal recycle ratio; excessive ratios offer limited improvement, while insufficient ratios reduce effectiveness. Typically requiring >200%, this internal recycle consumes significant energy. Effluent entering the secondary clarifier must maintain a certain dissolved oxygen (DO) level to prevent anaerobic conditions and phosphorus release, yet not too high to avoid interfering with denitrification in the anoxic tank via the recycled mixed liquor.
The Modified AAO Process (AAO + Suspended Carrier Process) effectively mitigates these drawbacks. It increases microbial mass in the biological tanks, enhances volumetric loading, achieves complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), strengthens resilience to hydraulic and organic shock loads, delivers good effluent quality even with low carbon sources, produces less and more stable sludge (reducing downstream sludge handling capacity requirements). The effluent can meet the water quality standards of "The Reuse of Urban Recycling Water-Water Quality Standard for Urban Miscellaneous Water" (GB/T 18920-2020) and "Code for Design of Coal Preparation Engineering" (GB 50359-2016) for coal washing. Hou Feng et al. applied the AAO+suspended carrier process in an underground wastewater treatment plant, achieving Grade 1A standards per "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002), with key indicators (COD, BOD5, NH3-N, TP) reaching Class IV standards per "Environmental Quality Standards for Surface Water" (GB 3838-2002). Hao Ruigang et al. used "A/O Bio-contact Oxidation + Perforated Swirl Flocculation + Inclined Tube Sedimentation + Active Sand Filtration" in the expansion of a coal mine domestic wastewater plant, achieving effluent quality better than Grade 1A. Yan Ziyu et al. also achieved good results using biofilm processes to retrofit existing coal mine domestic wastewater treatment. The Modified AAO Process allows for capacity increases and effluent quality improvement in existing plants with minimal modifications.
This process involves adding suspended carriers to the anoxic and aerobic tanks, combining advantages of activated sludge and biofilm processes. It features high volumetric loading, large biomass, high treatment efficiency, strong adaptability to水质和水量的变化, enhanced process stability, and good nutrient removal. It forms highly specialized active biofilms, increasing efficiency per reactor volume and stability, allowing for smaller reactors. Sloughing biofilm sludge contains more protozoa/ metazoa, has higher density and larger particle size, resulting in good settleability and easy solid-liquid separation. It enables complete SRT-HRT separation, eliminates sludge bulking, and is suitable for wastewater rich in soluble organics.
2.1 Case Study
A coal mine in Yan'an City, approximately 16 km from Zichang City, has a domestic wastewater treatment plant with a design capacity of 1200 m³/d. The process is: "Screen + Equalization Tank + AAO with Suspended Carriers + Advanced Treatment (Coagulation-Sedimentation-Filtration) + Disinfection". Sludge is treated via "Gravity Thickening + Screw Press Dewatering". The effluent meets the stricter limits of *GB/T 18920-2020* and GB 50359-2016 for coal washing water. The treated water is reused for mine Greenery and as makeup water in the coal preparation plant. Design influent/effluent quality is in Table 1. The process flow is shown in Figure 1.
Wastewater passes through a screen (5 mm gap, 75° installation angle) into an equalization tank (L×B×H=14.0 m×6.0 m×6.0 m, effective depth 2.95 m, volume 247.8 m³, HRT 4.13 h), meeting GB 50810-2012 requirements. Two mixers prevent settling. Three submersible pumps (2 duty +1 standby, Q=32.5 m³/h, H=17 m, N=4 kW) lift water to the biological tanks.
The biological system consists of two parallel trains. Per train:
- Anaerobic Tank: L×B×H=2.0 m×5.0 m×5.0 m, effective depth 4.5 m, HRT 1.5 h.
- Anoxic Tank: L×B×H=4.0 m×5.0 m×5.0 m, effective depth 4.25 m, HRT 2.83 h.
- Aerobic Tank: L×B×H=15.0 m×5.0 m×5.0 m, effective depth 4.0 m, HRT 10.0 h.Total system HRT is 15.75 h. Suspended carriers (filling ratio 80%, specific surface area 600 m²/m³) are installed in the aerobic tank. Design Air-to-Water Ratio is 13.7:1. Three Roots blowers (2 duty +1 standby, Q=6.84 m³/min, N=11 kW, P=44.1 kPa) are used. Sludge Recycle Ratio is 100%, Mixed Liquor Recycle Ratio is 200%.
Two rectangular peripheral-inlet/outlet secondary clarifiers (L×B×H=5.0 m×5.0 m×3.5 m each) have a surface loading rate of 1.2 m³/(m²·h) and HRT of 2.5 h.
An integrated water purifier (combining coagulation, sedimentation, filtration) provides advanced treatment for further SS and phosphorus removal.
Sludge treatment includes gravity thickening (Φ2.5 m×5.0 m carbon steel tank) followed by screw press dewatering. Polyacrylamide (PAM) is dosed at 3.0–5.0 kg/t dry solids before dewatering. Daily dewatered sludge cake is ≤150 kg with moisture content ≤80%, transported off-site.
Disinfection uses an on-site ClO2 generator (effective chlorine dosage 120 g/h) dosed at the clear well inlet. The clear well has an effective volume of 250 m³, providing a contact time of 4.2 h.
The plant is equipped with extensive online monitoring (flow meters,residual chlorine, pH, DO, COD, turbidity, sludge level/concentration) and automated control systems for pumps, blowers, backwashing, chemical dosing, and mixing, ensuring intelligent, unattended operation.
2.2 Performance Analysis
The plant completed commissioning in 2021 and has operated for over two years. Actual 2024 influent/effluent quality is shown in Table 2.
The influent BOD5/N ratio is 5.5, indicating low carbon-to-nitrogen (C/N) ratio wastewater, which decreases further in summer due to rainfall infiltration and habit changes. Extreme winter temperatures in Yan'an can reach -21°C. Actual effluent quality is better than design, with removal rates reaching: COD 97.8%, BOD5 99.7%, SS 99.7%, NH3-N 93.5%, TP 87.10%, meeting standards for绿化 and coal washing.
The active biofilm mass in the anoxic/aerobic tanks is as high as 125 g/m² carrier, equivalent to MLSS of 13 g/L-four times that of conventional activated sludge. Microorganisms are in the endogenous respiration phase, resulting in daily sludge production about 1/3 of conventional methods, with better settleability, allowing for smaller sludge treatment equipment.
Although bio-contact oxidation can operate without sludge recycle, research by Xiong Ren et al. shows that systems with recycle achieve higher removal rates for COD, TN, NH3-N, SS and reduce sludge yield by 29.6%. This design incorporates mixed liquor recycle, with operational flexibility based on effluent quality.
The plant (1200 m³/d) occupies 1350.3 m², with a capital investment of 20 million CNY and operating cost of 1.05 CNY/m³.
Compared to conventional AAO, which requires extended SRT for effective low-temperature operation, this modified process retains the simplicity of simultaneous nutrient removal while enriching the biological community with carriers. The SRT-HRT separation enhances bio-stability, ensuring reliable operation under low C/N and low-temperature conditions. Stable effluent can be maintained with little or no sludge recycle, enabling in-situ sludge reduction and lower sludge handling costs. Its simplicity and lack of bulking make it highly suitable for coal mine domestic wastewater treatment.
3. Optimization Research for AAO Process
Modified AAO processes are typically designed per parameters in "Standard for Design of Outdoor Wastewater Engineering" (GB 50014-2021). However, optimization of operational parameters (HRT, SRT, aeration, recycle ratios, MLSS) specific to coal mine wastewater is needed to identify optimal conditions for future design and operation.
In conventional AAO, sludge is recycled from the aerobic to the anaerobic tank, carrying nitrate and high DO, which can impair biological phosphorus removal. The University of Cape Town (UCT) process can be considered, where sludge is recycled to the anoxic tank, nitrified liquor to the anoxic tank, and an additional recycle from anoxic to anaerobic tank is added to enhance bio-P removal.
Sludge treatment can account for 50–60% of a plant's operating cost. In-situ sludge reduction technologies should be adopted. The high MLSS in modified AAO bio-tanks leads to a high F/M ratio, where discoupling metabolism can occur, promoting sludge reduction and lowering sludge handling costs. Future focus should be on applying in-situ reduction technologies like cryptic growth via micro-lysis, Oxic-Settling-Anaerobic (OSA) process, and discoupling metabolism in coal mine wastewater treatment.
This process is suitable for retrofitting existing AAO plants at coal mines. Adding carriers to anoxic/aerobic tanks can improve effluent quality, increase capacity, and enhance system stability. For plants with stricter effluent requirements, replacing the secondary clarifier with an MBR system can further upgrade water quality.
4. Conclusion
- The Modified AAO Process is suitable for upgrading existing AAO systems in coal mines to enhance stability, increase capacity, or meet stricter standards.
- When treating coal mine domestic wastewater, the effluent can simultaneously meet *GB/T 18920-2002* standards for road watering/greenery and GB 50359-2016 standards for coal washing water, demonstrating strong adaptability to changes in water quality and quantity.
- The process produces stable sludge with good settleability and easy separation, generates less sludge, and reduces sludge treatment costs.