Application of BIOLAK Process in the Upgrading of a Wastewater Treatment Plant to Quasi-Class IV Standards
Introduced to China in the early 21st century, the BIOLAK process gained wide application in municipal wastewater treatment due to its simple structure and low investment costs. In recent years, with the tightening of discharge standards and increasing automation, most existing BIOLAK plants face upgrades. Enhancements like adding suspended carriers, retrofitting tanks, and redefining functional zones are implemented to improve nitrogen and phosphorus removal. While newly built plants predominantly adopt A²/O and oxidation ditch processes, there are few reports on the actual performance of BIOLAK, especially under stringent emission standards. The BIOLAK process utilizes swinging aeration chains to create temporal anoxic and aerobic zones, essentially functioning as a multi-stage A/O process. Through operational optimization, effluent quality can stably meet the quasi-Class IV surface water standard.
1 Project Background
A wastewater treatment plant in Hebei Province uses the BIOLAK process as its core technology. The inflow ranges from 18,000 to 22,000 m³/d, averaging 19,000 m³/d, treating primarily urban domestic sewage and a small amount of agricultural processing wastewater. The designed influent and effluent qualities are shown in Table 1. The original discharge standard was the Grade A standard of *"Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002)*. After an upgrade that included partitioning an anaerobic zone to enhance denitrification and dephosphorization, the plant now complies with the key control area limits of *"Water Pollutant Discharge Standards for the Daqing River Basin" (DB13/2795-2018)*. Except for total nitrogen, all other indicators meet the Class IV standards specified in *"Environmental Quality Standards for Surface Water" (GB 3838-2002)*. The process flow is shown in Figure 1.
The plant uses sodium hypochlorite for disinfection. Sludge is dewatered by high-pressure plate and frame filtration to below 60% moisture content before being transported for co-processing in cement kilns.
The contribution of each treatment unit to pollutant removal was calculated based on mass balance, with specific methods referenced from the literature.
2 Operational Control Optimization Measures
Multiple optimization measures were implemented during operation to enhance effluent stability and achieve energy and cost savings.
2.1 Enhanced Dissolved Oxygen (DO) Control
Existing BIOLAK retrofit projects often note its weak zoning as a multi-stage A/O variant, leading to low denitrification efficiency. In this project, while ensuring effluent ammonia nitrogen compliance, the maximum DO at the end of the aeration zone was maintained at 0.5–1.0 mg/L, lower than conventional DO control requirements.
2.2 Increased Process Data Monitoring
To guide DO control and external carbon source dosing, nitrate nitrogen and ammonia nitrogen were monitored at the end of the anaerobic zone and the BIOLAK tank to determine optimal control ranges. During operation, external carbon source dosing was reduced or stopped when nitrate nitrogen at the end of the anaerobic zone was <2 mg/L, and increased when it was ≥2 mg/L. Similarly, blower output was reduced to lower DO to 0.5 mg/L when ammonia nitrogen at the end of the BIOLAK tank was ≤0.5 mg/L, and increased to raise DO to 1.0 mg/L when it was ≥0.5 mg/L. Adjustments to carbon source dosage and blower frequency were made every 8–16 hours, with each adjustment ranging from 5% to 15%.
2.3 Setting Internal Effluent Control Targets
To ensure stable compliance, internal control targets were set at 30%–80% of the discharge limits, based on the difficulty of controlling each pollutant. Exceeding these internal limits triggered immediate process parameter adjustments to return effluent concentrations to an acceptable range. The annual internal control targets for COD, ammonia nitrogen, total nitrogen, and total phosphorus were 15 mg/L, 0.5 mg/L, 12 mg/L, and 0.12 mg/L, respectively.
2.4 Maintaining Appropriate Sludge Concentration
Sludge wastage was adjusted based on flow, load, and season. The sludge retention time (SRT) was maintained at 15–25 days, and the mixed liquor suspended solids (MLSS) concentration at 2,500–4,500 mg/L. Specifically, MLSS was controlled at 2,500–3,500 mg/L in summer and autumn, with a sludge load of about 0.06 kgCOD/(kgMLSS·d), and at 3,500–4,500 mg/L in winter and spring, with a sludge load of about 0.04 kgCOD/(kgMLSS·d).
2.5 Adjusting Operation of Advanced Treatment Units
Low temperatures in winter affected flocculation and sedimentation. Untimely backwashing of V-type filters could lead to elevated effluent suspended solids and COD. Therefore, during winter operation, backwashing frequency was increased based on coagulation performance, and sludge discharge from the coagulation-sedimentation tank was intensified to reduce effluent suspended solids concentration.
3 Treatment Performance
Annual influent COD ranged from 109 to 248 mg/L, averaging 176 mg/L. Effluent COD ranged from 9.5 to 20.1 mg/L, averaging 12.1 mg/L. When effluent COD exceeded the internal control target (15 mg/L), filter backwashing frequency was increased to reduce suspended solids. It is recommended to upgrade the coagulation-sedimentation tank to a high-density or magnetic coagulation-sedimentation tank for better coagulation efficiency.
Annual influent ammonia nitrogen ranged from 17.8 to 54.9 mg/L, averaging 31.9 mg/L. Effluent ammonia nitrogen ranged from 0.12 to 1.30 mg/L, averaging 0.5 mg/L. When it exceeded the internal control target, aeration was adjusted per the optimization measures. Effluent quality stably met the key control area limits of *DB13/2795-2018* throughout the year.
Due to low influent carbon source concentration, the focus was on optimizing process conditions to enhance nitrogen and phosphorus removal, aiming for energy and cost savings.
3.1 DO Control Optimization and Total Nitrogen Removal
Annual influent total nitrogen (TN) ranged from 20.3 to 55.6 mg/L (see Figure 2), averaging 42.1 mg/L. Effluent TN ranged from 2.5 to 14.2 mg/L, averaging 8.8 mg/L, within the internal control target (12 mg/L). The average TN removal rate was 79.1%. With a sludge recycle ratio of 90% (no internal mixed liquor recycle), the theoretical denitrification efficiency was 47.4%, indicating that denitrification also occurred in other process zones beyond the anaerobic selector. Changes in nitrogen along the treatment train in a typical cycle are shown in Figure 3.
In a typical cycle, influent TN was 42.0 mg/L, with the sum of ammonia and nitrate nitrogen being 35.2 mg/L. After the anaerobic selector, TN was 16.7 mg/L, resulting in a 43.5% removal rate via mass balance, consistent with the theoretical value. The BIOLAK tank contributed a 24.0% TN removal. Effluent TN was further reduced in the secondary sedimentation tank, contributing an additional 11.3% removal, mainly due to its long hydraulic retention time (8.6 hours) allowing for endogenous carbon source-driven denitrification. Other units contributed 1.9% removal. Final effluent TN was 8.1 mg/L, with a total removal rate of 80.7%.
Operational experience shows that DO control is crucial for TN removal in the BIOLAK process. In conventional processes, DO is typically measured at the end of the aerobic zone in a channel structure where DO is relatively uniform across the cross-section. However, in the BIOLAK tank, the aeration zone end is nearly 70 meters wide, with DO increasing from the slope edge to the center, differing by 0.5–1.0 mg/L. Therefore, the location of DO probes requires careful attention.
By strictly controlling the maximum DO at the end of the BIOLAK aeration zone, an anoxic environment necessary for denitrification was effectively ensured. Simultaneous nitrification and denitrification (SND) utilizing endogenous carbon sources was achieved, resulting in effective TN removal.
3.2 Total Phosphorus Removal and Operational Optimization
Annual influent total phosphorus (TP) ranged from 1.47 to 4.80 mg/L (see Figure 4), averaging 2.99 mg/L. Effluent TP ranged from 0.04 to 0.17 mg/L. Phosphorus removal agent dosage was adjusted based on the internal control target (0.12 mg/L). The average effluent TP concentration was 0.07 mg/L, stably meeting the discharge standard, with an average TP removal rate of 98.3%.
Changes in phosphate along the treatment train in a typical cycle are shown in Figure 5.
Influent phosphate was 2.70 mg/L, and return sludge phosphate was 0.58 mg/L, making the theoretical phosphate entering the anaerobic selector 1.70 mg/L. After anaerobic phosphorus release by polyphosphate-accumulating organisms (PAOs), phosphate concentration reached 3.2 mg/L. The phosphate concentration ratio (maximum in anaerobic zone / influent) was 1.9, indicating significant release. The main reason was the effective denitrification under low DO conditions, resulting in low nitrate concentration in the return sludge to the anaerobic zone, maintaining a good anaerobic environment (ORP generally below -200 mV) and promoting phosphorus release.
After the BIOLAK aeration zone, substantial phosphorus uptake occurred, reducing the phosphate concentration at the end to 0.3 mg/L, achieving a biological phosphorus removal efficiency of 88.9%. After the sedimentation and stabilization tanks, phosphate concentration increased to 0.64 mg/L. Analysis suggests this was due to the long HRT in the sedimentation tank and the strictly controlled DO in the BIOLAK tank, creating an anaerobic condition in the sedimentation tank and causing secondary phosphorus release. After chemical dosing in the coagulation unit, effluent phosphate was reduced to 0.06 mg/L. Therefore, considering economic costs and operational complexity, sacrificing some biological phosphorus removal efficiency to enhance denitrification is a viable optimization strategy for similar plants.
4 Operational Costs
Direct operational costs include electricity, chemicals, and sludge disposal. Based on annual statistics, the specific power consumption was 0.66 kWh/m³. With an electricity price of 0.65 CNY/kWh (based on a composite of peak/off-peak rates), the electricity cost was 0.429 CNY/m³. This consumption is on the higher side according to the "Evaluation Standard for Operational Quality of Municipal Wastewater Treatment Plants", mainly due to the slightly lower oxygen utilization efficiency of the aeration system. Chemical costs, including sodium acetate, phosphorus removal agent, PAM, sodium hypochlorite, and dewatering chemicals, totaled 0.151 CNY/m³. Specific usage and costs are shown in Table 2.
Sludge originates mainly from biological and chemical (coagulation tank) sources. High-pressure plate and frame filtration is used with lime and ferric chloride as conditioning agents. Lime dosage is about 25% of the dry sludge weight. Dewatered cake has a moisture content of 60%. Daily dewatered sludge production is about 9 tons, with a specific dry sludge yield of about 0.15%. Sludge transportation costs 250 CNY/ton, resulting in a sludge disposal cost of about 0.118 CNY/m³. Therefore, the total direct production cost is 0.698 CNY/m³.
5 Conclusions
① A wastewater treatment plant in Hebei Province, using the BIOLAK process to treat municipal wastewater, operated continuously for one year with effluent quality stably meeting the key control area limits of *DB13/2795-2018* (Quasi-Class IV surface water standard).
② As a variant of the multi-stage A/O process, controlling the maximum DO at the end of the BIOLAK aeration zone at 0.5–1.0 mg/L resulted in a TN removal rate of 24.0% in the BIOLAK zone and 11.3% in the sedimentation tank. This achieved simultaneous nitrification-denitrification and endogenous carbon source denitrification, demonstrating significant nitrogen removal capability.
③ The direct operational cost for the BIOLAK process was 0.698 CNY/m³. Operational optimization measures, including process data monitoring and setting reasonable internal control targets, can provide references for optimizing operation and achieving energy/cost savings in similar wastewater treatment plants.