Inner Mongolia WWTP Upgrade: How AAO+MBBR Replaced a Failing DE Oxidation Ditch to Treat 2,000 mg/L COD at 0.85 RMB/m³

In 2004, a wastewater treatment plant in Xilinhot, Inner Mongolia, started treating 38,000 m³/d with a DE oxidation ditch. It was designed for municipal sewage. What it got was slaughterhouse wastewater - pen washings, blood, grease, bone residue, and manure - pushing influent COD to 2,000 mg/L and ammonia nitrogen to 120 mg/L. The oxidation ditch, cycling between aerobic and anoxic modes in a city where temperatures stay low year-round, could not keep up. Dissolved oxygen hovered below 1 mg/L. Nitrifying bacteria never established. Effluent missed Class B more often than it hit it. The upgrade added DAF pretreatment, a new AAO+MBBR biological stage, and advanced treatment - and cut through all four problems at once. Effluent now stably meets Class A. Cost: 0.85 RMB per cubic metre.

1. THE ORIGINAL PLANT - AND WHY IT FAILED

The Phase I plant was commissioned in August 2004 with a design capacity of 40,000 m³/d. The biological section used the DE oxidation ditch process (see Figure 1), with effluent required to meet the Class B standard of GB 18918-2002. Actual flow through the pipe network was approximately 38,000 m³/d. The original design assumed typical municipal sewage characteristics. The reality was different: a large fraction of the influent came from slaughterhouses, and the pollutant concentrations massively exceeded the design basis.

Four distinct problems converged to make the DE oxidation ditch the wrong process for this site:

Problem 1 - COD suppressed nitrification

Influent COD regularly hit 2,000 mg/L. Heterotrophic bacteria, feeding on this organic abundance, multiplied rapidly and consumed dissolved oxygen at a rate the aeration system could not match. DO in the oxidation ditch stayed below 1 mg/L for extended periods. Nitrifying bacteria - autotrophs that need high DO, not high organics - were starved of oxygen. Ammonia nitrogen removal collapsed as a direct consequence of COD overload.

Problem 2 - Slaughterhouse waste resisted biodegradation

Slaughterhouse wastewater is a complex mixture: pen wash water carrying pig manure and undigested feed; blood, grease, meat scraps, bone residue, hair, and faeces from the killing floor. The wastewater is brownish-red with a fishy odour detectable metres away. These high-COD, high-ammonia streams putrefy rapidly. Limited by the biological tank volume, hydraulic retention time was too short for these recalcitrant organics to biodegrade to a low level. Unretained organic suspended solids discharged with the effluent, consuming dissolved oxygen in the receiving water and damaging the ecosystem.

Problem 3 - Alternating aerobic/anoxic mode failed in the cold

Xilinhot has low temperatures year-round. Microbial growth is slow; nitrifying bacteria are the most cold-sensitive of all. The DE oxidation ditch alternates between aerobic (favouring nitrifiers) and anoxic (favouring denitrifiers) conditions. Under aerobic operation, the oxygen-sufficient window was too short for nitrifiers to build a viable population before the cycle switched back to anoxic, suppressing them again. Denitrifying bacteria, adapted to anoxic conditions, suffered equally from the constant cycling. The alternating oxygen regime, tolerable in a warmer climate, became a nitrogen-removal killer in Inner Mongolia's cold.

Problem 4 - Surface aeration bled away heat

Surface aerators work by maximising the wastewater–air contact area to drive oxygen transfer. The same contact area also drives heat loss. In Xilinhot's climate, every unit of oxygen transferred came with a thermal penalty - the wastewater in the oxidation ditch lost limited heat to the cold air, further depressing the already-slow biochemical kinetics.

2. THE UPGRADE STRATEGY: DAF + AAO+MBBR + TWO-STAGE NITROGEN REMOVAL

The upgrade had to solve four constraints simultaneously: high COD, high ammonia, low temperature, and no available land for new tank construction. The solution was built around MBBR technology - selected for its strong cold-weather adaptability, compact footprint, and ability to combine with conventional processes without requiring additional tank volume.

MBBR carriers in the aerobic zone preferentially promote nitrifying bacteria growth, and biofilm that detaches from the carriers continuously inoculates the suspended activated sludge with nitrifiers. This compensates directly for the nitrification deficiency that plagued the original DE oxidation ditch. The process also demonstrates strong tolerance to toxic shocks - important given the variability of slaughterhouse discharge.

Five design decisions defined the upgrade:

1. DAF pretreatment. A dissolved air flotation unit was added to remove oil and SS from the slaughterhouse fraction before it reached the biological stage. High influent SS and grease had been overloading the oxidation ditch from day one.

2. New AAO tank with MBBR carriers. A completely new primary anaerobic/anoxic/aerobic tank was built, with HDPE MBBR carriers (500 m²/m³ specific surface area, cylindrical, ~20 mm diameter × 10 mm length) in both the anoxic zone (3,110 m³ carrier volume) and aerobic zone (6,264 m³ carrier volume). Design MLSS: 4,000 mg/L. Anaerobic HRT: 2.0 h. Anoxic HRT: 6.2 h. Aerobic HRT: 12.5 h.

3. Oxidation ditch converted to secondary AO. The existing DE oxidation ditch was repurposed into a secondary anoxic/aerobic tank (total HRT 12.2 h, 6.1 h each zone). With influent TN reaching 130 mg/L, a single-stage AO could not achieve the required removal - internal recirculation ratios would need to exceed 1,000%. Two-stage AO makes the nitrogen removal mathematically achievable at practical recirculation ratios (300% primary, 200% secondary).

4. Advanced treatment. A high-efficiency sedimentation tank (surface loading 7.3 m³/m²·h, with tube settler media) followed by continuous sand filters (48 units, AS 50020, 5.5 m² each, total filtration area 288 m², design filtration rate 5.78 m/h) were added after the secondary clarifier to polish effluent to Class A standards.

5. Smart aeration and chemical dosing. IoT-based precision aeration and automated chemical dosing replaced manual, experience-based control - eliminating the chronic over-aeration and over-dosing that had inflated the original plant's operating costs.

3. CONSTRUCTION WITHOUT SHUTDOWN - THREE CHALLENGES

The plant could not stop treating wastewater during the upgrade. Three challenges emerged:

Challenge 1 - Maintaining compliance during tank reconstruction. Taking one biological tank offline during the rainy season (July–September) increased the load on the remaining tanks. Solution: tanks were reconstructed one by one. The old sludge thickening tank was converted into a temporary sedimentation tank to collect and return secondary circulating water, preventing it from adding load to the biological section. Emergency tanks were emptied before forecast heavy rainfall.

Challenge 2 - Overlapping construction and safety. Civil works and process pipe installation ran simultaneously in the secondary AO tank. Solution: a structured communication and coordination protocol was established, with detailed safety plans and emergency response procedures defining each party's responsibilities.

Challenge 3 - Sludge disposal from the decommissioned tank. The secondary AO tank held 4.5 m of activated sludge water plus 0.5 m of inorganic sediment. Solution: the new sludge dewatering system was commissioned early; part of the sludge was dewatered, and part was transferred to seed the new primary AAO tank.

4. RESULTS: CLASS A EFFLUENT FROM CLASS B+ INFLUENT

4.1 Water quality - before and after

The influent characteristics are severe by any standard - a mixture of municipal sewage and slaughterhouse discharge. Table 1 shows the design and actual values across seven parameters, together with the removal rates achieved after the upgrade.

*NH₃-N effluent limit is 5 mg/L normally; 8 mg/L applies when water temperature is below 12 ℃.

After the upgrade, all effluent indicators stably met the Class A standard of GB 18918-2002, despite influent concentrations far exceeding the original design basis. The biological system's treatment efficiency improved significantly across all parameters (see Figure 3).

4.2 Operating cost breakdown

The annual direct operating cost is approximately 12.45 million RMB, giving a unit cost of 0.8528 RMB/m³ (excluding equipment depreciation and financing). Electricity dominates, accounting for 62.4% of the operating cost - a direct consequence of the high oxygen demand from concentrated influent and the cold climate that slows biological kinetics.

5. ENERGY OPTIMISATION: SMART CONTROLS OVER MANUAL GUESSWORK

With electricity at 62% of operating cost, energy efficiency was not optional. Three measures were embedded in the design:

1. IoT-based precision aeration. An Internet of Things platform collects real-time DO, ammonia, and flow data, then adjusts blower output automatically. Instead of running blowers at a fixed speed based on operator experience, the system delivers exactly the air the biology needs - no more, no less.

2. Intelligent chemical dosing. PAC, PAM, and chlorine dioxide dosing rates are tied to real-time flow and pollutant load, eliminating the safety factor creep that inflates chemical consumption in manually operated plants.

3. Adaptive operation modes. During periods of low influent concentration, the MBBR process can shift to a reduced-energy operating mode - fewer blowers online, lower mixing intensity - without risking effluent compliance. The building design also maximises natural ventilation to reduce HVAC power demand.

KEY TAKEAWAYS

1. DE oxidation ditches fail in cold climates with high-strength wastewater. The alternating aerobic/anoxic cycle prevents either nitrifier or denitrifier populations from stabilising, and surface aeration costs heat. If your plant has both low temperature and high ammonia, the DE oxidation ditch is the wrong biological process.

2. MBBR carriers inoculate nitrifiers into the suspended sludge. This is the key mechanism that compensates for the nitrification deficiency of the original process. The biofilm on the carriers preferentially grows nitrifying bacteria and continuously seeds the mixed liquor - a form of in-situ bioaugmentation that requires no external carbon source.

3. Two-stage AO makes high-strength nitrogen removal practical. With influent TN at 130 mg/L, a single-stage AO would need internal recirculation ratios exceeding 1,000%. Splitting the nitrogen load across a primary AAO+MBBR and a secondary AO tank achieves Class A TN (≤15 mg/L) at manageable recirculation ratios.

4. DAF pretreatment is non-negotiable when slaughterhouse wastewater is in the mix. Oil and grease from rendering operations will foul MBBR carriers and suppress oxygen transfer. Remove them before the biological stage, or pay the penalty in blower electricity.

5. Construction without shutdown is achievable with phased tank sequencing. Build the new primary AAO tank first, commission it, then take the old oxidation ditch offline for conversion to secondary AO. The old sludge thickening tank can serve as a temporary sedimentation buffer during the cutover.

6. Operating cost at 0.85 RMB/m³ for 2,000 mg/L COD influent is competitive. The electricity-dominated cost structure (62%) reflects the real oxygen demand of this wastewater. IoT-based aeration control and adaptive operating modes are the levers that keep it from going higher.