Water Practice & Technology Vol 5 No 1 © IWA Publishing 2010 doi:10.2166/WPT.2010.009

Degasification of mixed liquor improves settling and biological nutrient removal M. Maciejewski*, J. A. Oleszkiewicz **, A. Golcz***, A. Nazar*** *CH2M HILL, Suite 1500, 800 – 6th Avenue S.W., Calgary, AB T2P 3G3 CDN (E-mail: Matthew.Maciejewski@ch2m.com) **University of Manitoba, Civil Engineering, 15 Gilson Street, Winnipeg, Manitoba R3T 5V6 CDN (E-mail: oleszkie@cc.umanitoba.ca) ***BIOGRADEX Holding Sp. z o.o., Robotnicza 55/10; 82-300 Elblag, PL (E-mail: nazar@biogradex.pl) Abstract: Degasification of mixed liquor by subjecting it to vacuum is a physical process used in biological nutrient removal (BNR) to improve settleability and allow for achieving higher mixed liquor suspended solids (MLSS). Vacuum degassing installation is located between the last cell of the bioreactor and secondary clarifiers. In this process two operations are performed: gas bubbles contained in mixed liquor leaving the bioreactor are removed and concentration of gasses (mainly nitrogen gas) dissolved in the liquid is reduced. Lack of gas bubbles and concentration of dissolved nitrogen gas below saturation in mixed liquor significantly improved sludge settling in secondary clarifiers and eliminated floating scum formation. Presented settleability tests of degasified MLSS and return activated sludge (RAS) from various BNR facilities showed continued settling and/or thickening for over 3 h at room temperature, without exhibiting any solids separation. Settleability tests of biomass that was not degasified typically led to flotation of portion of the sludge after about 1.5 h. Plants equipped with vacuum degasification consistently operate at larger than typically recommended final clarifier sludge surface loading rates. Rates as high as 180-220 kg TSS/m2d and deep sludge blankets have been employed. Such plants were shown to maintain operational levels of MLSS at 4500 to 6000 mg/L and higher.

Keywords: MLSS; settleability; vacuum degasification; mixed liquor suspended solids; denitrification; foaming and bulking 1. Introduction Activated sludge mixed liquor suspended solids (MLSS) enter the secondary clarifiers from a well aerated and turbulent environment in the aeration basins. Sludge flocs contain micro-bubbles of gas, which make sludge settling difficult or even cause it to float. This buoyancy is further aggravated by hydrophobic foaming microorganisms such as Nocardia often present in biological nutrient removal (BNR) plants operating at long solids residence times SRT (Jenkins et al 2004, WEF 2006). The concentration of nitrogen gas dissolved in the water fraction of mixed liquor is at the saturation level as the result of oxygen consumption from the introduced air and simultaneous nitrification-denitrification. Because of this saturation nitrogen gas produced during denitrification processes occurring in the sludge blanket of the secondary clarifiers cannot dissolve in the surrounding water and forms gas bubbles which affect sludge settleability and create floating sludge or scum (Metcalf & Eddy, 2003). Operators of wastewater treatment plants optimize the plant performance balancing between sufficiently high MLSS concentration in the bioreactors required to achieve the design SRT and the ability of secondary clarifiers to effectively separate activated sludge from treated wastewater. Sludge bulking and foaming (scum formation) in bioreactors and secondary clarifiers are typically dealt with by reducing the MLSS through increased and often excessive, sludge wasting and chlorination of return activated sludge (RAS) to reduce the number of filamentous organism in sludge. Some plants strive to maintain a low sludge blanket in the secondary clarifier. As a result of these remedial measures plants may operate at MLSS concentrations lower than required to achieve the expected treatment results and have to be re-rated below its design capacity. Other plants may work well during dry weather flow conditions but encounter significant solids washouts from secondary clarifiers during wet weather flows. Removal of gas bubbles from mixed liquor and reduction of dissolved nitrogen gas concentration below saturation level can reduce sludge settling problems related to these two factors. Activated sludge was found to readily separate from degasified mixed liquor and to settle well without formation of a layer of partially settled or floating solids. Denitrification processes occurring in settled sludge should not affect sludge settling since produced nitrogen

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gas dissolves in surrounding water instead of forming gas bubbles. The enhanced ability to settle and thicken in the final clarifier would lead to maintenance of normal solids surface load above 150 kg TSS/m2d. This in turn would allow for larger MLSS concentrations in the reactors leading to increased capacity of the plant without physical increase of the reactor size. Mixed liquor degasification process has been developed some time ago (Golcz 2005) and is being applied in situations where plants have difficulties maintaining an effective year-round BNR performance. There are some forty plants presently using this process (Maciejewski & Timpany 2007, 2008). This paper will present the principles of the degasification technology and its performance in full scale case studies conducted under a variety of conditions. 2. Process description In mixed liquor vacuum degasification technology, a vacuum tower is located between the last cell of the bioreactor and the secondary clarifiers – Fig. 1. The top of the vacuum tower is about 10 m above the water level in the bioreactor; the pressure in the top section of the tower is about 0.04 to 0.05 bar. The top of the tower is connected to a vacuum pump system that maintains low pressure in the tower and provides the “siphon” conditions necessary for the mixed liquor to flow through the tower. Liquid ring vacuum pumps are used to achieve the high vacuum requirements. Airlift pump assistance is often used to meet the plant hydraulic requirements and overcome tower’s internal hydraulic losses. Small openings drilled at the base of the tower suction piping provide site-specific airlift pump assistance. The required energy input of the degasification process is in the range of 0.02–0.03 kWh/m3.

Figure 1: Principle of the degassing vacuum tower operation (not to scale). There are three phases of the degassing process. In Phase 1 mixed liquor flows from the last cell of the bioreactor to the top of the vacuum tower. Gas bubbles contained in the mixed liquor expand and are removed; floc structure containing gas micro-bubbles is destroyed. In Phase 2 partial removal of dissolved gases occurs in the top section of the tower under vacuum conditions. In Phase 3 mixed liquor flows down to the secondary clarifier. In this phase, floc recombines to form sludge without the embedded gas micro-bubbles. Degasification of mixed liquor under low pressure conditions partially removes nitrogen and other gases dissolved in wastewater during treatment conducted under atmospheric pressure conditions (1 bar). The degasification process is based on Henry’s law of gas solubility which

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is a function of the partial pressure of gases above a liquid surface. Using Henry’s gas law one can calculate that, for example at 15oC under normal atmospheric pressure of 1 bar, the solubility of N2 gas in water is approx. 17 mg N2/L. Under the vacuum conditions of 0.05 bar, existing in the degasification unit, that solubility is only 1 mg N2/L. When such mixed liquor enters the final clarifier the dissolved nitrogen gas will be well below the saturation levels under atmospheric pressure. Any denitrification-induced nitrogen gas generated in sludge blanket in the secondary clarifiers can dissolve in the water rather than form gas bubbles buoying the floc. Due to the limited liquid residence time in the low pressure zone at the top of the vacuum tower, theoretical steady state values of solubility at low pressure are not achieved. As dissolved nitrogen probes are not available the actual amount of degasification can only be estimated. A conservative estimate, based on water capacity to dissolve nitrogen gas from denitrification processes in final clarifiers, was that about 25% to 50% of the dissolved nitrogen is removed in the process. 3. Sludge settling after degasification Secondary clarifiers at the plants equipped with vacuum degasification typically operate with surface solids loading (mass flux) in the range of 180-220 kg/m2d (Maciejewski & Timpany, 2008). Settled sludge can form very deep blankets in secondary clarifiers without the danger of solids washout or floatable scum formation and compacts well to concentrations in the range of 1.2–2% total solids (TS). Denitrification, which is typically the cause of floating sludge/scum in clarifiers with appreciable sludge blanket (WEF 2006) has not been found to be a problem. Based on Henry’s law and the example above at 15oC, settling/thickening biomass in the final clarifier can easily absorb 5 to 10 mg N2/L before the liquid becomes saturated and the N2 bubbles start to form. Plants using mixed liquor degasification typically operate at MLSS concentrations in the range of 6 000–8 000 mg/L (Golcz 2005).

Figure 2: Example of settling curves at room temperature: Dzierzgon PL full scale BNR plant using MLSS degasification. RAS = return activated sludge. A number of tests were performed at full scale plants using degasification. Figure 2 presents a study at a 2 900 m3/d municipal wastewater biological nutrient removal facility in Dzierzgon PL. The plant operates at MLSS concentration of 7 000 mg/L and secondary clarifier surface solids loading of approx. 220-240 kg/m2day. The mixed liquor settling and return activated sludge (RAS) thickening curves were obtained in standard 1 liter calibrated settling cylinders. The test showed that sludge from both the degassed and non-degassed mixed liquor initially settled in a similar pattern. After 1.5 to 2 h, sludge from the non-degasified samples of mixed

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liquor floated, while sludge from the degasified MLSS samples continued to settle and thicken for 3.5 hours – the duration of the extended settling test. The samples of degassed RAS continued to thicken and did not float during the test. Other tests, on this and other plants conducted on degasified RAS samples achieved continuous settling and thickening of RAS for more than 24 hours (Golcz 2005). The feasibility of degasification can be tested on-site using a simple calibrated settling cylinder and a portable vacuum pump. Both degasified and non-degasified samples are placed in identical thick-walled glass settling cylinders. Following are three examples and photographs of such tests conducted at BNR plants not equipped with degasification.

Figure 3: Settling test of mixed liquor samples from three BNR plants: A) Saskatoon CDN; B) Elblag PL; C) Okotoks CDN (Maciejewski & Timpany 2008). Figure 3A shows results from a 120 000 m3/d MUCT biological nutrient removal plant in Saskatoon SK, Canada. At the time of testing, the plant was operating at MLSS concentration of 3 500 mg/L. Sludge in the sample on the right, which was not degasified, floated after approximately two hours from collection. Sludge in the degasified sample on the left remained settled until the end of testing, about four hours after degasification. Figure 3B shows a settling test for a new 25 000 m3/d BNR plant in Elblag Poland. The MLSS concentration was 4 500 mg/L; the photograph was taken 2 h after degasification of the sample on the left. It was apparent that, without degasification, erratic settling and flotation from the sludge blanket occurred in the sample on the right. Figure 3C shows samples tested at the 10 000 m3/d BNR plant in Okotoks AB, Canada. At the time of testing, the MLSS concentration at the plant was close to 8 000 mg/L due to a temporary bottleneck in the WAS dewatering system during construction. The sample that was not degasified (on the right) did not settle at all. The photograph was taken about 30 minutes after degasification of the sample on the left. 4. Full scale case studies 4.1 Biological nutrient removal plant Qinghe in Beijing, China The Qinghe BNR plant consists of two parallel sections, each containing two treatment trains for a total of four trains of 100 000 m3/day design capacity, each. Influent concentrations of total nitrogen (TN) and ammonia have been much higher than the design values - Table 1 presents the design and actual influent values. Table 2 contains average plant results from January 2006 to June 2007. Section 1 was built first – the process consists of anaerobic-anoxic-aerobic sequence, however without internal MLSS recycle. Section 2 has a typical A2O configuration. The plant consistently exceeded the permitted ammonia and total nitrogen (TN) concentrations in the effluent. The MLSS concentrations of 3 100–3 300 mg/L, achieved during the operation, have been significantly below the design value of 5 000 mg/L.

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Table 1: Qinghe BNR plant – Design parameters versus actual performance before installation of biomass degasification.

Influent Design Performance 01/2006-06/2007

Flow (m3/day) 400 000 437 000 BOD5 (mg/L) 200 235 COD (mg/L) 400 469 TSS (mg/L) 250 285 TN (mg/L) 40 65 NH3-N (mg/L) 25 48 TP (mg/L) 8 6.6 MLSS (mg/L) Section 1 MLSS (mg/L) Section 2

5000 5000

3 122 3 304

Table 2: Qinghe plant - average plant results January 2006 – June 2007, before the degasification tests Effluent Design Section 1 Section 2 BOD5 (mg/L) 20 10.8 10.08 COD (mg/L) 60 46.3 43.4 TSS (mg/L) 20 13.8 12.4 TN (mg/L) 20 25.5 19.8 NH3-N (mg/L) 1.5 11.4 6.6 TP (mg/L) 1.0 0.38 0.54 In December 2007/January 2008 two vacuum degasification towers were installed on two bioreactors of the North train of Section 2 of the Qinghe plant – Fig.4. Identical South train of Section 2 was not modified and was operated under reduced flow conditions, as a reference. Results from both trains were compared to evaluate vacuum degasification process performance and effectiveness in Table 3, which also lists TN and NH3-N effluent concentrations and MLSS concentration in February and March 2006/2007 - before the modification.

Figure. 4: Qinghe WWTP, Section 2, North Train - overall view of bioreactors, secondary clarifiers and the vacuum towers. Table 3: Qinghe plant - Section 2. Average TN and NH3-N effluent concentrations before modification and the two trains operated in parallel without degasification.

Both Trains Feb/Mar 06/07

Prior to modification

South Train (not modified)

Feb/Mar 08

North Train (modified)

Feb/Mar 08 Q (m3/day) 200 075 90 000 120 000 NH3-N(mg/L) 8.4 1.8 0.9 TN (mg/L) 20 17 11 MLSS (mg/L) 3191 3441 4475

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The objectives of the modification were to obtain TN effluent below 20 mg N/L and ammonia below 1.5 mg N/L, while at the same time increasing the hydraulic loading of the North Train by 20%, to 120 000 m3/d. Both objectives were accomplished in the North Train equipped with degasification. South Train, working under a reduced load of 90 000 m3/d, without degasification, achieved the permitted TN effluent but still failed to achieve the required effluent ammonia level of 1.5 mg N/L. 4.2 Upgrade of Gorzow BNR plant in Gorzow, PL The 27 000 m3/day Gorzow PL biological nutrient removal municipal wastewater treatment plant was unable to meet the TN effluent requirement of 10 mg N/L. One reason was poor design of the two existing (old) radial final clarifiers which forced the operators to overload the third (newly constructed) suction-equipped horizontal clarifier. Fig. 5 illustrates the vacuum degasification system installed between the aeration zone and that final clarifier. Start-up of the degasification system was completed in May 2007, with the objective to improve TN removal below the 10 mg/L. The improved settling allowed an increase in biomass concentration from 3 500 mg MLSS/L to 6 000 mg/L. One should note that the influent TN concentration fluctuated significantly above the 80 mg N/L. The effluent TN requirement of 10 mg/L was however maintained throughout the fluctuations – Fig.6. The total solids content in the year preceding the installations of degasification averaged 15 mg/L. After the upgrade the annual average for year 2008 was 7 mg TSS/L.

Figure 5: Degasification facility at a 27 000 m3/d plant in Gorzow PL.

Figure 6: Gorzow WWTP influent and effluent TN concentrations before and after installation of the degasification

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5. Summary Mixed liquor vacuum degasification process achieved improved settleability of the biological nutrient removal plant biomass in the final clarifier. The removal of gases, particularly nitrogen from MLSS, to levels well below saturation in the liquid allowed prolonged thickening of the BNR sludge in the clarifier hopper without any problems of floating sludge – the latter being the result of denitrification and flotation of sludge by bubbles of nitrogen in systems without degasification. The process is simple, lends itself to the upgrade of existing facilities as it does not impact the hydraulic profile of the treatment train and is characterized by low operating and capital costs. The increase in MLSS in the bioreactor can be translated to smaller reactor size or larger rated capacity of the existing facility. The full scale plant degasification upgrades presented here have converted to operational MLSS of 4500-6000 mg/L and achieved their goals of consistently meeting effluent nitrogen requirements in conditions of higher than design hydraulic and organic loadings. References Golcz, A. (2005) Biogradex® technology of activated sludge vacuum modification in the

wastewater biological treatment. Proceed. IWA 2005 Special. Conf. Nutrient Management in Wastewater Treatment Process and Recycle Streams, Krakow PL. 1085-1093.

Jenkins D, Richard M, Daigger G T (2004) Manual on the causes and control of activated sludge bulking. 3rd Ed. CRC Press, Boca Raton FL.

Maciejewski M., Timpany P. (2007) Low cost upgrade to BNR and BNR efficiency improvement by mixed liquor vacuum degassing. Western Canada Water and Waste Assoc. Ann. Confer., October 23-26; Edmonton, AB.

Maciejewski M., Timpany P. (2008) New European BNR technology of mixed liquor vacuum degassing debuts for Beijing 2008 Olympics. BC Water and Waste assoc. Annual Conference, April 26–30,Whistler, BC,

Metcalf & Eddy (2003) Wastewater engineering. McGraw-Hill, Boston MA, 1818 p. Timpany P., Maciejewski M. (2007). Fundamentals of Biogradex® process technology.

Manitoba Water and Wastewater Association Ann. Confer. February 20. WEF (2008) Biological nutrient removal (BNR) operation in wastewater treatment plants.

MOP 30, Water Envir. Feder.; Amer. Soc. Civil Engin., McGraw-Hill, 597 p.