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ENVIRONMENTAL ENGINEERING SCIENCE Volume 25, Number 9, 2008 © Mary Ann Liebert, Inc. DOI: 10.1089/ees.2007.0071 Influence of High Organic Loading Rates on COD Removal and Sludge Production in Moving Bed Biofilm Reactor Ahmet Aygun, Bilgehan Nas, Ali Berktay* Department of Environmental Engineering Selcuk University, Campus 42031 K
  ENVIRONMENTAL ENGINEERING SCIENCEVolume 25, Number 9, 2008 ©Mary Ann Liebert, Inc.DOI: 10.1089/ees.2007.0071 Influence of High Organic Loading Rates on COD Removal and Sludge Production in Moving Bed Biofilm Reactor Ahmet Aygun, Bilgehan Nas, Ali Berktay* Department of Environmental Engineering Selcuk University, Campus 42031 Konya, Turkey  Received: March 15, 2007Accepted in revised form: June 16, 2007  Abstract A moving bed biofilm reactor (MBBR), where biomass is attached to small carrier elements that move freelyalong with the water in the reactor, has been tested for organic matter removal at five different organic load-ing rates. A lab-scale reactor with a volume of 2L was built and fed continuously with synthetic wastewater.The reactor was filled with the Kaldnes biomedia K1 which is used in the patented Kaldnes Moving Bed TM  biofilm process at 50% of the volume of empty reactor. Hydraulic retention times (HRT) in the reactor and inthe settler were adjusted to between 8 and 4 hours, respectively. A start-up period of about 4 weeks for biofilmgrowth on the carrier was followed by 10 weeks of testing period. By changing the wastewater composition,the operation of the system was adjusted, one after the other, to five different organic loading rates: 6, 12, 24,48 and 96 g COD/m 2 .d. Organic removal efficiency decreased with increasing organic loading rate, rangingfrom 95.1%, 94.9%, 89.3%, 68.7% and 45.2% as the organic loading rate was increased form 6 to 96 g COD/m 2 .d.In the MBRR reactor, the biofilm reached an average concentration of 3.28 kg TSS/m 3 at the highest organicloading rate. The ratio between the TSS production and the total COD removal was 0.12 kg TSS/kg total CODat an influent total COD of 500 mg/l.  Key words: wastewater treatment, moving bed biofilm reactor, organic loading rate, COD removal, sludge pro-duction, biomass yield 1311Introduction T he patented moving bed biofilm reactor (MBBR) was de-veloped in Scandinavia in the late 1980s by the Norwe-gian company Kaldnes MiljØteknologi (KMT), in coopera-tion with the SINTEF research organization. Presently, thereare more than 400 large-scale wastewater treatment plantsoperating in 22 different countries of the world based on thisprocess. In addition there are several hundred small, on-sitetreatment units based on the MBBR (Rusten et al. 2006).The Kaldnes MBBR is a completely mixed continuouslyoperating biofilm reactor where the biomass is grown onsmall carrier elements that move along with the water in thereactor. The interest for MBBR is justified by the followingadvantages: high sludge age, no sludge recycling, and less bulking problems. In the case of an upgrade in existing plant,the MBBR solution does not increase head loss significantly,allows a continuous feed, does not present clogging prob-lems and does not require backwashing. MBBRs allow ap-plying higher loads compared to those applied to the con-ventional activated sludge, in case the specific surface is suf-ficiently high (Andreottola et al., 2002; Andreottola et al.,2003; Rusten et al., 2006).The biofilm carrier elements are made of polyethylene(density 0.92–0.96 g/cm 3 ) and shaped like small cylinderswith a cross inside the cylinder and longitudinal fins on theoutside. In order to keep the biofilm carriers in the reactor,a sieve is placed at the outlet of the reactor. The filling ratioand the specific area of the biofilm carriers are the two maindesign parameters. The filling ratio ranges from 30–70% of the total reactor volume. Ødegaard (2000) recommends thefilling fractions to be under 70% for the carriers the carriersto be free.MBBRs have already been used in the treatment of dairywastewater (Rusten et al., 1992; Andreottola et al., 2002), for-est industry wastewater (Dalentoft and Thulin, 1997), cheesefactory wastewater (Rusten et al., 1996), newsprint millwastewater (Broch-Due et al., 1997), thermo mechanicalpulping whitewater (Jahren et al., 2002), municipal waste-water (Ødegaard et al. 1993; Orantes and González-Mar-tinez, 2002), and for nitrification (Rusten et al., 1995; Walen-der et al., 1997) and denitrification (Rusten et al., 1995;Aspegren et al., 1998; Maurer et al., 2001; Walender and Mat-tiasson, 2003). *Corresponding Author: Prof. Ali Berktay (MSc., Ph.D.Eng.) De-partment of Environmental Engineering, Campus, Selcuk Univer-sity, 42031, Konya-Turkey. Phone:  90-332-223 2093 Fax :  90-332-241 0635; E-mail:  MBBR is a good process for upgrading current wastewatertreatment systems. Many studies regarding successful oper-ation for new wastewater treatment plants and upgrades of existing wastewater treatment plants have been reported(Ødegaard et al., 1993; Hem et al., 1994; Orantes et al., 2002;Daude and Stephenson 2003; Andreottola et al., 2003). An-dreottola et al. (2000) found that hydraulic retention time(HRT) affected the COD removal in MBBR processes andsuggested that the HRT should be higher than 5 h. Anotherstudy was performed by Andreottola et al., (2003), in whichthey evaluated the application of MBRR system for the up-grading of an overloaded MWWTP. A preliminary experi-mentation at pilot-scale was carried out in order to evaluatethe efficiency of the system in the removal of organic mat-ter using a high loaded MBBR. KMT plastic media was ap-plied with a filling ratio of 50%. The applied surface loadsranged between 5.4 and 32 g totalCOD/ m 2 .d.With the rising costs of sludge disposal, the minimizationof sludge production has become increasingly important.The expense of excess sludge treatment has been estimatedto be 50–60% of the total cost of municipal wastewater treat-ment. Therefore, modification of the aerobic treatment pro-cess in order to reduce biosolids production is promising(Kulikowska et al. 2007).The main objectives of this research were to investigateCOD removal efficiency and biomass yield coefficient in syn-thetic wastewater by using the kaldnes biomedia K1, whichis used in the patented Kaldnes Moving Bed TM  biofilm pro-cess, at various organic loading rates starting from 6 to 96 gtotCOD/ m 2 .d). Material and Methods Carriers  The Kaldnes K1 biofilm carrier elements are made of poly-ethylene and shaped like small cylinders (a nominal diame-ter of 9.1 mm and a nominal length of 7.2 mm) with a crossinside the cylinder and longitudinal fins on the outside. TheKaldnes carriers have a specific gravity of 0.96 with a spe-cific biofilm protected surface area of 500 m 2 per m 3  bulkvolume of carriers. The Kaldnes biofilm carrier element is il-lustrated in Figure 1.Microscopy of the biofilm media from several pilot andfull scale moving bed biofilm plants has shown no sign of  biofilm growth on the outside of the smooth plastic elements.The reason is believed to be the erosion caused by the fre-quent collisions between the pieces. Therefore, the biofilmsurface area has been calculated based on the internal (pro-tected) surface of the plastic elements (Rusten et. al, 1992).With a filling ratio of 50%, the available surface area (referredto the reactor volume) was 250 m 2 /m 3 (considering only theinternal surface of cylinders). Lab-scale reactor and wastewater  A laboratory scale plexiglas reactor with a total liquid vol-ume of 2 l and a final settler (volume equal to 1.2 l) was usedin the study (Figure 2). The reactor was filled with thekaldnes biomedia K1 to 50% of the volume of empty reactorand without recycle. Diffusers were used for oxygen supplyand mixing.Activated sludge was obtained from a local municipalWWTP as a seeding material to the reactor. The MBBR wascontinuously fed with a dosage pump. The theoretical hy-draulic retention time (HRT) in the oxidation tank and in the AYGUN ET AL.1312 Feedt a nkDo sa gepompMBBRCl a rifierEffl u entW as te s l u dgeAir FIG. 1. Kaldnes biofilm carrier FIG. 2. Simplified flow-sheet of the lab-scale MBBR.  settler was adjusted to 8 hours and 4 hours, respectively. Thesettled sludge was removed from the bottom of the settler.Synthetic wastewater was used to provide a source of car- bon, nitrogen, phosphorus and trace metals required for bio-mass growth. It was prepared daily with deionized water ata Chemical Oxygen Demand (totalCOD) of 500, 1000, 2000,4000 and 8000 mg/l. The synthetic wastewater compositionsare shown in Table 1. The calculated COD/N/P ratio of thesynthetic wastewater was 100/5/1.A start-up period of about 4 weeks for biofilm growth onthe carrier was followed by 10 weeks of testing period. Thesteady state condition is defined as the period during whichthe effluent quality was relatively constant at a constant load-ing with regard to the parameters of COD, and SS. Steady-state conditions were resumed for a minimum of one week before the next trial commenced. The dissolved oxygen (DO)concentrations in the MBBR ranged from 0.30 to 3.00 mg O 2 /ldepending on the influent organic loading rates. The tem-perature and pH in the reactor varied from 18.4 to 23.6 0 Cand 6.72 to 7.88, respectively. Table 2 shows operational con-ditions for the reactor during all the experimental periods. Analytical methods  To measure the performance of the lab-scale MBBR, sam-ples were taken from the biofilm reactor and the final efflu-ent. Total COD, filtered COD (on membrane of 0.45-  mporosity), TSS, VSS, N-NO 3 , N-NH 4 , DO and pH were mea-sured on samples every day.Closed reflux colorimetric method (Method 5220 D) wasused for COD analysis as specified in the Standard Methods(APHA, AWWA, WEF, 2005). TSS and VSS concentrations re-ferred to the fixed biomass in the MBBR were also measured.TSS analysis of fixed biomass was obtained detaching the bio-mass from 10 KMT elements, diluting the biomass in a knownamount of demineralized water, referring the values to thesurface of an element and taking into account the number of elements per liter (Andreottola et al. 2000; Andreottola et al.2003). N-NO 3 and N-NH 4 were analyzed with Orion 710A ad-vanced ion selective meter with Method 4500-NO 3 -D nitrateand 4500-NH 4 -D ammonia-selective electrode method(APHA, AWWA, WEF, 2005). DO and pH measurementswere done by using the WTW Multiparameter 340i. Results and Discussion Experimental work in the laboratory was carried out inorder to evaluate the efficiency of the system for the removalof organic matter and relationship between organic removaland observed yield using a high loaded MBBR. During theexperimental works, five different organic loading rates wereapplied to the reactor.Effluent totalCOD and COD removal rates versus time areshown in Figure 3. At the lowest organic loading rate corre-sponding to 6 gCOD/m 2 .d, average effluent totalCOD con-centration was 24.5 mg/l and average totalCOD removal ef-ficiency was 95.1%. At 12, 24, 48 and 96 gCOD/m 2 .d organicloading rates average effluent totalCOD concentrations were50.8, 213, 1253, 4381 mg/l respectively, while the average to-talCOD removal efficiencies were 94.9%, 89.3%, 68.7%, and45.2% (Figure 3a,b,c,d,e).The DO was measured in the reactor every day at 2 hoursinterval. The average DO levels in the MBBR ranged from0.30 to 3.00 mg O 2 /l depending on the influent organic load-ing rate. Although the amount of air increased to 1.8 and 2.3l/min, DO levels decreased to an average of 0.83 mg/l and0.30 mg/l when 48 and 96 g totalCOD/ m 2 .d surface organicloading rates were applied, respectively. Low dissolved oxy-gen concentrations could affect COD removal efficiency at48 and 96 g totalCOD/ m 2 .d surface organic loading rates.Figure 4 shows the removal efficiency versus volumetricand surface applied loads. By applying a volumetric organicloading rate between 1.5 and 6 kg totalCOD/m 3 .d (corre-sponding to the range 6–96 g totalCOD/m 2 .d), it was possi- ble to obtain the highest removal efficiency of the totalCODin the MBRR. However, removal efficiency of the totalCODdecreased to 45.2% when 96 g totalCOD/ m 2 .d surface or-ganic loading rate was applied.Sludge production ranged from 0.35 gTSS/d at the low-est organic loading rate to 12.25 gTSS/d at the highest or-ganic loading rate. The detachment of the biomass from the biofilm appears to follow a linear relationship with the or-ganic loading rate (Figure 5). Sludge production is in agree-ment with the findings by Orantes and Martinez (2002). HIGH ORGANIC LOADS ON COD REMOVAL1313 T ABLE 1.C OMPOSITIONS OF S YNTHETIC S UBSTRATE (COD  1000 MG / L )Glucose258 mg/lSodium Acetate471 mg/lNH 4 Cl95.5 mg/lKH 2 PO 4 22 mg/lNaHCO 3 295 mg/lMgSO 4 .7 H 2 O100 mg/lCaCl 2 .2 H 2 O50 mg/lFeCl 3 . 6 H 2 O50 mg/lT ABLE 2.O PERATIONAL C ONDITIONS OF MBBR R EACTOR Influent totCOD (mg/L)5001000200040008000Influent NH 4 -N (mg/L)2550100200400Volumetric load (kg totCOD/ m 3 .d)1.5361224Surface load (g totCOD/m 2 .d)612244896TSS (kg/m 3 )2.342.492.753.233.28VSS (kg/m 3 )1.731.892.102.462.39Biofilm Age (days) (l/min) oxygen (mg/l)3.002.832.530.840.30Length of Operation (days)1510141310  AYGUN ET AL.1314 T ABLE 3.E FFLUENT COD AND A VERAGE COD R EMOVAL E FFICIENCIES Effluent COD (mg/l)Number of Influent Organic loading ratesamplesCOD (mg/l)min.max.mean 6 g totCOD/m 2 .d15500193424.5  4.812 g totCOD/m 2 .d101000485450.8  2.324 g totCOD/m 2 .d142000136450213  10248 g totCOD/m 2 .d13400089615221253  18096 g totCOD/m 2 .d108000398243814576  173 FIG. 3. COD removal efficiencies at different organic loading rate FIG. 4. Total COD removal efficiency versus applied organic loading.
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