The Nordkanal wastewater treatment plant, owned and operated by Erftverband, a German public utility institution for water management, discharges effluent to nearby canal and needed to improve effluent quality to meet the new effluent standard. At the same time, treatment capacity increase was required to relieve the chronic hydraulic overloading. The old WWTP was dismantled and a new MBR plant was built 2.5 km away. The new MBR plant needed only 50% of the land that the comparable CAS process would require. Existing rain water storage tank with 2,000m3 capacity in the old facility was converted to a wastewater holding tank. Mechanical screens with 1.0 mm pore size were used to remove solids and fibrous materials from raw wastewater, but no primary clarifier was used. When this plant was commissioned in 2004, hollow fiber clogging by fibrous materials was observed. The screen was replaced with a different type of mechanical screen with a self cleaning function using water jet.
Modified MLE process was adapted targeting nitrogen removal while chemical phosphorus removal was performed using FeCl3 as shown in Fig. 1. The mixed liquor in oxic/membrane tank was recycled back to anoxic tank, where nitrate is denitrified consuming readily biodegradable COD contained in the raw wastewater. Nitrogen removal rate was calculated at 85%. FeCl3 was dosed to the anoxic tank to enhance phosphorus removal. Effluent TP was maintained around 0.5 mg/L while the discharge limit was 1.5 mg/L. The molar ratio of Me/P was between 0.22-1.30. Due to the inorganic solids generated from FeCl3, apparent biosolids yields were estimated at 0.55 g solids/g COD at a SRT of ~25 days, which were much higher than that without chemical phosphorus removal. Process parameters are summarized in Table 1. Contaminant loading to the MBR was summarized in Table 2.
Fine pore membrane plate diffusers made of silicone were used for aeration tank and the effective aerator depth was 4.0 m. SOTR in clean water was measured at 15.6 g O2/m3/min and Î±SOTR at 8.3 g O2/m3/min from which Î±-factor is calculated at 0.53. Average MLSS in oxic/membrane tank was at 12 g/L while VSS at 7 g/L. The F/M ratio averages at 0.05 g COD/gMLSS/day.
While the membrane train is in operation, the respective blower operates constantly. The intermittent aeration at 10s on/10 off mode is achieved by channeling the air to different parts of the train. Lag time of the shifting valves and control equipment may actually lead to up to 50% longer aeration cycles.
It was found that the time lag between ammonia loading increase and ammonia discharge increase was significantly shorter than HRT. During dry weather flow, time lag was observed at 2-4 hours while HRT was longer than 9 hours. Likewise, during wet weather flow, time lag was at 0.5-1.5 hours while HRT was 4.7-6.0 hours.
In this plant, a periodic permeability swing was observed depending on water temperature. Permeability follows a sine curve as shown in Fig. 2 proportional to water temperature.
Fig. 1. Process diagram of one of the four parallel MBR systems in Nordkanal, Germany, USA.
Table 1 . Design parameters of Nordkanal MBR (Brepols, 2011)
|Membrane type||–||HF modules
ZW500c (GE Water)
|4 aeration tanks
2 trains/aeration tank
|Total membrane area||m2||84,480 (Total)|
|10,560 (per train)|
|Aeration rate||m3/hr/trains||4,250||Scouring air only|
|Flow rate||m3/d||24,600 (dry)||Net flux = 12 LMH (0.29 m/d)|
|m3/d||45,100 (wet)||Net flux = 22 LMH (0.53 m/d)|
|Tank volume||m3||2,600 (anoxic)||9,200 m2 total
|hrs||2.5/1.4 (anoxic)||9.0 / 4.9 hrs total|
Table 2. Daily loading (quantile Q.85 and average values of sample data) (Brepols, 2011)
Fig. 2. Weekly average permeability and temperature values at Nordkanal MBR in 2004-2009 (Brepols, 2011).
Â© Seong Hoon Yoon